Showing papers in "Ecological Monographs in 2000"

Impacts of root competition in forests and woodlands: a theoretical framework and review of experiments

Abstract: Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 ...

Climate, habitat quality, and fitness in northern spotted owl populations in northwestern california

Abstract: A controversy exists in the Pacific Northwest of the United States between logging of old-growth coniferous forests and conservation of Northern Spotted Owl (Strix occidentalis caurina) populations. This species has a strong association with old-growth forests that also have economic value as timber. Research questions relevant to conservation of this species include how temporal trends in Northern Spotted Owl populations are influenced and how spatial configuration of old-growth forests affects these populations. To address these questions, we studied a population of marked Northern Spotted Owls on 95 territories in northwestern California from 1985 through 1994. We examined the mag- nitude of temporal and spatial variation in life history traits (survival, reproductive output, and recruitment), the effects of climate and landscape characteristics on temporal and spatial variation in these traits, respectively, and how this variation affected aspects of population dynamics. We used a components-of-variation analysis to partition sampling from process variation, and a model selection approach to estimate life history traits using capture- recapture and random-effects models. Climate explained most of the temporal variation in life history traits. Annual survival varied the least over time, whereas recruitment rate varied the most, suggesting a ''bet-hedging'' life history strategy for the owl. A forecast of annual rates of population change ( l), estimated from life history traits, suggested that Northern Spotted Owl populations may change solely due to climate influences, even with unchanging habitat conditions. In terms of spatial variation, annual survival on territories was positively associated both with amounts of interior old-growth forest and with length of edge between those forests and other vegetation types. Reproductive output was nega- tively associated with interior forest, but positively associated with edge between mature and old-growth conifer forest and other vegetation types. A gradient existed in territory- specific estimates of fitness derived from these life history estimates. This gradient suggested that a mosaic of older forest interspersed with other vegetation types promoted high fitness in Northern Spotted Owls. Habitat quality, as defined by fitness, appeared to buffer variation in annual survival but did not buffer reproductive output. We postulated that the magnitude of l was determined by habitat quality, whereas variation of l was influenced by recruitment and reproductive output. As habitat quality declines, variation in l should become more pronounced.

What governs protein content of pollen: pollinator preferences, pollen–pistil interactions, or phylogeny?

Abstract: Pollen ranges from 25% to 61% protein content Most pollen proteins are likely to be enzymes that function during pollen tube growth and subsequent fertilization, but the vast range of protein quantity may not reflect only pollen–pistil interactions Because numerous vertebrate and invertebrate floral visitors consume pollen for protein, protein content may influence floral host choice Additionally, many floral visitors pollinate their host plants If protein content influences pollinator visitation, then pollinators are hypothesized to select for increased protein content of host plants We analyzed or gleaned from the literature crude pollen protein concentrations of 377 plant species from 93 plant families Using this database, we compared pollen protein concentration with (1) pollination mode, (2) pollen collection by bees, and (3) distance from stigma to ovule, after accounting for phylogeny through paired phylogenetic comparisons and a nested ANOVA including taxonomic rank We found that pollen pr

Foraging ecology of northern elephant seals

Abstract: Sexual segregation in foraging is predicted from the great size disparity of male and female northern elephant seals, Mirounga angustirostris. Our aim was to test this prediction by measuring diving and foraging behavior, foraging locations, and distribution of the sexes during biannual migrations in the northeastern Pacific Ocean. Daily movements of 27 adult males and 20 adult females, during 56 migrations from Afio Nuevo, California, USA, were determined by Argos satellite telemetry via head-mounted platform transmitter terminals. Diving records were obtained with archival time-depth-speed recorders attached to the backs of seals that were recovered when the seals returned to the rookery. Pronounced sex differences were found in foraging location and foraging pattern, as reflected by hor- izontal transit speed and diving behavior. Males moved directly north or northwest at a mean speed of 90 ? 27 km/d to focal foraging areas along the continental margin ranging from coastal Oregon (534 km away) to the western Aleutian Islands (4775 km away). Males remained in these areas (mean size = 7892 ki2) for 21-84% of their 4-mo stays at sea. The predominance of flat-bottom dives in these areas suggests concentrated feeding on benthic prey. Migration distance and estimated mass gain were positively correlated with male size, and individual males returned to the same area to forage on subsequent migrations. In contrast, females ranged across a wider area of the northeastern Pacific, from 380 to 600 N and from the coast to 172.5? E. Focal foraging areas, indicated by a reduction in swim speed to <0.4 m/s, were distributed over deep water along the migratory path, with females remaining on them a mean of 3.5 d before moving to another one. Jagged-bottom dives that tracked the deep scattering layer prevailed in these areas, suggesting that females were feeding on pelagic prey in the water column. Females took roughly similar initial paths in subsequent migrations, but large deviations from the previous route were observed. We conclude that there is habitat segregation between the sexes. Females range widely over deep water, apparently foraging on patchily distributed, vertically migrating, pelagic prey, whereas males forage along the continental margin at the distal end of their migration in a manner consistent with feeding on benthic prey.

Strong impacts of grazing amphipods on the organization of a benthic community

Abstract: Large brown seaweeds dominate coastal hard substrata throughout many of the world's oceans. In coastal North Carolina, USA, this dominance by brown seaweeds is facilitated by omnivorous fishes, which feed both on red and green algae and on herbivorous amphipods that graze brown algae. When fish are removed in the field, brown seaweeds are replaced by red seaweeds, and herbivorous amphipods are more abundant. Using an array of large (;4000 L) outdoor mesocosms, we tested three mechanistic hypotheses for this pattern: fish feeding facilitates brown algal dominance (1) by removing red and green algal competitors, (2) by removing amphipods and reducing their feeding on brown sea- weeds, or (3) through an interaction of these mechanisms. Our experiments revealed strong impacts of both fish and amphipods, and a key role for the interaction, in structuring this community. When both fish and amphipods were removed (the latter with dilute insecticide), space was rapidly dominated and held for 17 weeks by fast-growing, primarily filamentous green algae. In contrast, when either fish, amphipods, or both were present, green algae were cropped to a sparse turf, and space was more rapidly dominated by larger macroalgae. The impacts of amphipods and fish on late-successional macroalgal assemblages were comparable in magnitude, but different in sign: red seaweeds prevailed in the amphipod- dominated treatment, whereas browns dominated in the presence of fish. Laboratory feeding assays and amphipod densities in the tanks suggested that the significant effects of am- phipods were attributable largely, if not exclusively, to the single amphipod species Am- pithoe longimana, which fed heavily on brown macroalgae. Our experimental removal of red and green algae failed to enhance cover of brown algae significantly; however, the latter reached substantially lower cover in the grazer-removal treatment, where green algae were very abundant, than in the fish-only treatment, where green algae were sparse. Thus, our results support the third hypothesis: fish-mediated dominance of brown algae involves both suppression of grazing amphipods and removal of algal competitors. Although collective impacts of fish and amphipods on this benthic community were generally comparable in magnitude, impacts normalized to each grazer's aggregate biomass were consistently higher for amphipods than for fish, sometimes by 1-2 orders of magnitude. Thus, the impacts of grazing amphipods (specifically A. longimana) on the benthic community were both strong and disproportionate to their biomass. These experimental results imply that grazing am- phipods, which are ubiquitous in marine vegetation but poorly understood ecologically, may play important roles in the organization of benthic communities, particularly where predation pressure is low.

Seed disperser effectiveness: the quantity component and patterns of seed rain for prunus mahaleb

Abstract: In this paper we analyze the seed dispersal stage of the Prunus mahaleb-frugivorous bird inter- action from fruit removal through seed delivery within the context of disperser effectiveness. The effectiveness of a frugivorous species as a seed disperser is the contribution it makes to plant fitness. Effectiveness depends on the quantity of seed dispersed (''quantity component'') and the quality of dispersal provided each seed (''quality component''). For the main frugivores, we studied abundance, visitation rate, and feeding behavior, the major variables influencing the quantity component of effectiveness, and the postforaging microhabitat use and resultant seed shadows, which set the stage for postdispersal factors that will influence the quality component of effectiveness. Legitimate seed dispersers (SD) swallowed fruits whole and defecated or regurgitated intact seeds; pulp consumers (PC) pecked fruits to obtain pulp and dropped seeds to the ground, but some species occasionally dispersed seeds (PCSD species). Overall numbers of fruits removed (i.e., handled) by avian frugivores were similar in the two study years; however, the estimated percentage of seeds dispersed differed significantly, with lower relative dispersal success in the year with greater relative abundance of PC species. Similar numbers of seeds were dispersed in the two years despite nearly a fourfold difference in number of fruits produced. Fruit crop size explained .80% variance in the number of seeds dispersed per tree. A total of 38 species of birds were recorded during censuses, with frugivores representing 68.8% of them; the relative representation of SD, PC, and PCSD species was 42.2%, 17.2%, and 9.4%, respectively. Individual trees showed extensive variation in visitation rates, ranging from 0.3 to 41.6 visits/10 h in any year. The main visitors were the SD species Phoenicurus ochruros (10.8 visits/10 h), Turdus viscivorus (9.2 visits/10 h), Erithacus rubecula(3.5 visits/10 h), and Sylvia communis(2.6 visits/10 h); and the PC species Fringilla coelebs (16.7 visits/10 h) and Parus ater (4.7 visits/10 h). Species with large quantity components of effectiveness typically had either high visit or high feeding rates, combined with high probability of dispersing a handled seed. Variation among species in fruit-handling behavior, however, was the main factor influencing variation in the quantity component. Visit rate in turn was influenced largely by local abundance. No single frugivore trait, however, can adequately estimate the quantity component of disperser effectiveness. A ''gulper''/''masher'' dichotomy helps explain differences in fruit handling among major frugivore types and shows many correlates with other aspects of frugivore activity that ultimately influence the quantity component. Most species showed marked preferences for microhabitats with plant cover, especially P. mahaleb,midheight shrubs, and Pinus (86.1% of the departure flights) and avoided open microhabitats. Most flights were over short distances (77.5% to perches located within 30 m). Among the main frugivores, 40.3% of the exit flights were to perches .15 m away from the feeding tree, but only 18.5% of these flights were to perches .15 m from any P. mahaleb. Covered microhabitats received significantly more seeds (39.3 6 5.0 seeds dispersed/m 2 , 1988 (mean 6 1 SE); 31.7 6 5.9 seeds dispersed/m 2 , 1989) than open microhabitats (2.8 6 0.7 seeds dispersed/m 2 , 1988; 1.8 6 0.4 seeds dispersed/m 2 , 1989). The potential contribution of each bird species to the seed rain in each microhabitat was estimated from the number of visits recorded, the mean number of seeds dispersed per visit, and the proportion of exit flights to each microhabitat. Microhabitats differed strongly in the proportions of seeds delivered by the main frugivores, and bird species also differed in the proportions of seeds delivered to a given microhabitat. The seed rain to covered microhabitats was delivered by a more heterogeneous assortment of species than the seed rain to open sites. The resulting seed shadow was a complex result of the interaction between movement patterns of a suite of bird species differing both in the quantity of seed dispersed and microhabitat preferences and in the landscape distribution of these microhabitat patches. This seed shadow was extremely nonrandom due both to a strong overall preference by most of the birds for the relatively scarce covered microhabitats and to species-specific preferences for particular types of covered microhabitats. Different microhabitat types not only received variable amounts of dispersed seed, but also differed in the number and identity of disperser species contributing to that seed rain.

Vertebrate responses to spatiotemporal variation in seed production of mast-fruiting dipterocarpaceae

Abstract: To examine the interspecific reproductive synchrony of Dipterocarpaceae with vertebrate responses to seed availablity, we monitored the spatiotemporal distribution and phenology of more than 2367 adult dipterocarp individuals of 54 species from March 1985 to January 1993 in the Gunung Palung National Park, West Kalimantan, Indonesia. Seven vegetational formations were sampled along an altitudinal gradient from peat swamp forest (5 m a.s.l. (above sea level)) across lowlands to upper montane zones (1100 m a.s.l.) that encompassed two upland valley complexes in a 15-km 2 area. Four significant reproductive events were documented: (1) a common lowland species reproduced outside of mast events in 1986 and in 1990; (2) a localized lowland ''minor'' mast event in 1986 in which 24.3% of the adult trees (21 spp.) participated; (3) a major community-wide mast event (92.8%, 48 spp.) in 1987, just 6 mo after the minor event; and (4) another major community-wide mast fruiting event after a 4-yr intermast interval (88%, 48 spp.) in 1991. West Kalimantan export records of illipe nut (Dipterocarpaceae: Shorea section Pachycarpae) from 1968 to 1997 were compiled as a baseline measure of the frequency and relative intensity of dipterocarp mast-fruiting events in the region ( CV 5 152%). A ''bumper crop'' occurred about every 5 6 2.6 yr (mean 6 1 SD; range 3-9 yr). Fruit production was significantly associated with El Nino-Southern Oscillation (ENSO) events. The 1987 and 1991 mast events monitored were the third and fourth largest export years in the province since 1968. Because of the disputed role of vertebrate seed predators in causing and maintaining mast-fruiting behavior, the response of seed-eating vertebrates to this spatiotemporal variation in dipterocarp seed production was examined. Seed of a common, but asynchronous, lowland species was largely consumed by vertebrates. In the 1986 minor mast, 21 dipterocarp species that produced 60 000 seeds/ha (dry mass 46 kg/ha) lost all monitored viable seed to a diversity of resident and nomadic vertebrate seed predators. Timed with dipterocarp seed production in all mast events, nomadic vertebrates increased their pop- ulations through both reproduction and regional movement (numerical response). However, in both the 1987 and 1991 mast events, resident vertebrates destroyed only 1.5% and 2.6% of community seed production, and predation was recorded only in the tails of the fruit-fall distribution. During these community mast events, resident vertebrates switched from dipterocarp seed to feed on fruit and seed from other available species. Nomadic vertebrates arrived late in the fruit-fall period during both major mast events and, thus, were able to destroy only seed dispersed in the final 1-3 wk of fruit-fall. Seed escape, and thus regeneration, only occurred in major mast events when all dipterocarp species across large areas participated. Considerable seedling recruitment was recorded in both the 1987 ( ;95 000 seedlings/ha) and 1991 (155 824 6 36 764 seedlings/ha) mast events. Results from this natural experiment indicate that resident and nomadic vertebrates cause stabilizing selection for tight interspecific synchrony of dipterocarp seedfall over vast regions. Although the pattern generally conformed to the predator-satiation hypothesis, the observed mechanisms for seed escape within a mast-fruiting event did not. Resident and nomadic vertebrate foraging and ranging patterns resulted in dipterocarp seed ''escape'' rather than local ''swamping'' with copious seed production per se. To account for these observations, two hypotheses, ''interfamilial satiation'' and ''regional escape,'' were developed. The period of seed availability and length of the regional intermast interval is examined with seed-predator reproduction and behavior to assess the impact of seed availability on seed-predator populations. The importance of rare events and appropriate spatial scale for investigating evolutionary

Seed dispersal, seed predation, and seedling recruitment of a neotropical montane tree

Abstract: Postdispersal fate of seeds from Ocotea endresiana (Lauraceae), a bird-dispersed Neotropical montane tree, was studied in Costa Rica to determine the influence of seed dispersers, seed predators, and microhabitat characteristics on seedling recruitment. Particular emphasis was placed on finding naturally dispersed seeds in order to study the link between dispersal and postdispersal fate of seeds. Four species of birds (Emerald Toucanet, Aulacorhynchus prasinus; Resplendent Quetzal, Pharomachrus mocinno; Three-wattled Bellbird, Procnias tricarunculata; and Mountain Robin, Turdus plebejus) dispersed the seeds by regurgitation, and one species (Black Guan, Chamaepetes unicolor), by defecation. Most seeds (80%) were dispersed within 25 m of parent trees and under high (>92%) canopy cover. Bellbirds deposited 52% of the seeds they dispersed under habitual song perches in standing dead trees on the edges of treefall gaps >25 m from parent trees. In contrast, the other four species dispersed only 6% of the seeds...

Herbivory and plant species coexistence: community regulation by an outbreaking phytophagous insect

Abstract: Most general theories proposed to explain the trophic structure of communities ignore the possibility that insect outbreaks can severely damage vegetation and reduce the abundance of dominant plant species over vast areas. Specialist chrysomelid beetles can irrupt and defoliate goldenrods (Solidago spp.), a group of widespread, long-lived, herbaceous perennials. We examined the long-term effects (10 yr) of suppressing insects, using insecticide in replicated plots on the structure and diversity of an old field dominated by the goldenrod, Solidago altissima. An outbreak of the chrysomelid beetle, Microrhopala vittata, that specializes on S. altissima, occurred during the experiment and persisted several years. Damage caused by this outbreak dramatically reduced the biomass, density, height, survivorship, and reproduction of S. altissima. Herbivore exclusion caused the formation of dense stands of goldenrods with a twofold increase in both standing crop biomass and litter. The understory in these dense stands had significantly lower plant abundance, species richness, flowering shoot production, and light levels; these conditions persisted for years following the outbreak. Thus, M. vittata functioned as a keystone species. Furthermore, insect herbivory indirectly increased the abundance of invading trees, thereby increasing the rate of succession, by speeding the transition of this old field to a tree-dominated stage. We conducted two follow-up experiments to test the hypothesis that insects altered community dynamics by their indirect effect on litter accumulation and light availability in the understory. In the first experiment, we tied back the canopy to increase light into the understory and removed litter in both the insecticide-treated and control plots. We found little effect of removing litter. By contrast, increasing understory light levels significantly increased understory forb abundance and species richness. In the second experiment, we placed rosettes of Hieracium pratense, the dominant understory forb, under nine levels of shade cloth, ranging from 95% shade to full sun. Flowering-shoot production was a linear function of light availability (r2 = 0.92; P < 0.0001). We concluded that insect herbivores indirectly promoted plant species richness and coexistence, primarily by augmenting light availability to suppressed understory species. Insect herbivory may often play a strong role in goldenrod stands, because outbreaks will likely occur at least once, if not more, during the period when goldenrods are dominant. Furthermore, our findings provide compelling evidence for two general mechanisms whereby insect herbivory promotes plant species diversity and coexistence. The first mechanism operates during outbreaks when insects act as keystone species. The second mechanism can operate at less than outbreak levels and occurs whenever insect damage augments light to a sufficient degree to enhance the fecundity of suppressed nonhost species. If this increase in fecundity increases recruitment of subordinate species, then insect herbivory will promote plant species coexistence and diversity. Our data suggest that there is a continuum in the influence of insect herbivory on plant communities from the more subtle, but important, effects of herbivory on the fecundity of nonhost species to the devastating influence of outbreaks. Also, our results demonstrate that long-term experiments are required to elucidate the role of insect herbivores. Finally, we propose that insect outbreaks are common enough in many community types, particularly forests, to warrant explicit consideration in theories of trophic regulation, particularly in terrestrial communities inhabited by long-lived plant species.

Intra- and interspecific variation for summer precipitation use in pinyon-juniper woodlands

Abstract: In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer ''monsoon'' precipitation generally wets only surface layers. Use of these spatially separated water sources was determined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Arizona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this summer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation input. This prediction was largely supported by our observations of tree water source use over a two-year period and from irrigation ex- periments. Hydrogen and oxygen stable isotope ratios ( dD and d 18 O) of tree xylem water were compared to that of precipitation, groundwater, and deep and shallow soil water to distin- guish among possible tree water sources. dD-d 18 O relationships and seasonal xylem water potential changes revealed that trees of this ecosystem used a mixture of soil water and recent precipitation, but not groundwater. During the monsoon period, a large proportion of xylem water in Pinus edulis and Juniperus osteospermawas from monsoon precipitation, but use of this precipitation declined sharply with decreasing summer rain input at sites near the regional monsoon boundary in Utah. Quercus gambelii at most sites along the gradient used only deep soil water even following substantial inputs of summer rain. Pop- ulations of Quercus at sites with the highest average summer precipitation input, however, predominantly used water in upper soil layers from recent summer rain events. Soil tem- perature correlated with patterns of summer precipitation use across the gradient; high soil temperatures north of the monsoon boundary may have inhibited surface root activity for some or all of the three tree species. Irrigation experiments with deuterium-labeled water revealed that Quercus gambeliiin northern Arizona and southern Utah did not use water from surface layers. In contrast, Juniperus osteosperma at these sites responded significantly to the irrigations: between 37% and 41% of xylem water originated from irrigations that wetted only the top 30 cm of soil. Responses by Pinus edulis to these irrigations were variable; uptake of labeled water by this species was greater in September at the end of the summer than during the hot midsummer period. Inactivity of Pinus roots in midsummer supports the hypothesis that root activity in this species is sensitive to soil temperature. Seasonal patterns of leaf gas exchange and plant water potential corresponded to the seasonality of rainfall at different sites. However, no correlation between a species' ability to use summer rainfall and its tolerance to water deficits at the leaf level was found. Midday stomatal conductance (gs) for Pinus needles approached zero at predawn water potentials near 22 MPa, whereas gs in Quercus and Juniperus declined to zero at 22.8 and 23.7 MPa, respectively. The relationship between photosynthesis (A) and gs was similar among the three species, although Quercus maintained higher overall rates of gas exchange and tended to operate higher on the A/gs curve than the two conifers. At sites in eastern Arizona where Quercus fully used moisture from summer rains, leaf gas exchange characteristics were similar to those of Pinus and Juniperus.

Nitrogen cycling in a forest stream determined by a 15n tracer addition

Abstract: Nitrogen uptake and cycling was examined using a six-week tracer addition of 15N-labeled ammonium in early spring in Walker Branch, a first-order deciduous forest stream in eastern Tennessee. Prior to the 15N addition, standing stocks of N were determined for the major biomass compartments. During and after the addition, 15N was measured in water and in dominant biomass compartments upstream and at several locations downstream. Residence time of ammonium in stream water (5–6 min) and ammonium uptake lengths (23–27 m) were short and relatively constant during the addition. Uptake rates of NH4 were more variable, ranging from 22 to 37 μg N·m−2·min−1 and varying directly with changes in streamwater ammonium concentration (2.7–6.7 μg/L). The highest rates of ammonium uptake per unit area were by the liverwort Porella pinnata, decomposing leaves, and fine benthic organic matter (FBOM), although epilithon had the highest N uptake per unit biomass N. Nitrification rates and nitrate uptake lengths and rates were ...

Landscape-scale disturbances and changes in bird communities of boreal mixed-wood forests

Abstract: Bird community response to both landscape-scale and local (forest types) changes in forest cover was studied in three boreal mixed-wood forest landscapes modified by different types of disturbances: (1) a pre-industrial landscape where human settlement, agriculture, and logging activities date back to the early 1930s, (2) an industrial timber managed forest, and (3) a forest dominated by natural disturbances Birds were sampled at 459 sampling stations distributed among the three landscapes Local habitat and landscape characteristics of the context surrounding each sampling station (500-m and 1-km radius) were also computed Bird communities were influenced by landscape-scale changes in forest cover The higher proportion of early-successional habitats in both human-disturbed landscapes resulted in significantly higher abundance of early-successional bird species and generalists The mean number of mature forest bird species was significantly lower in the industrial and pre-industrial landscapes than in the natural landscape Landscape-scale conversion of mature forests from mixed-wood to deciduous cover in human-disturbed landscapes was the main cause of changes in mature forest bird communities In these landscapes, the abundance of species associated with mixed and coniferous forest cover was lower, whereas species that preferred a deciduous cover were more abundant Variation in bird community composition determined by the landscape context was as important as local habitat conditions, suggesting that predictions on the regional impact of forest management on songbirds with models solely based on local scale factors could be misleading Patterns of bird species composition were related to several landscape composition variables (proportions of forest types), but not to configuration variables (eg, interior habitat, amount of edge) Overall, our results indicated that the large-scale conversion of the southern portion of the boreal forest from a mixed to a deciduous cover may be one of the most important threats to the integrity of bird communities in these forest mosaics Negative effects of changes in bird communities could be attenuated if current forestry practices are modified toward maintaining forest types (deciduous, mixed-wood, and coniferous) at levels similar to those observed under natural disturbances

Experimental tests of the spatiotemporal scale of seed predation in mast‐fruiting dipterocarpaceae

Abstract: Mast-fruiting Dipterocarpaceae exhibit highly synchronous, interspecific seedfall at irregular, multiyear intervals. To investigate how the temporal pattern of seedfall affects dipterocarp seed and seedling survival, in both a logged and a primary lowland tropical forest, we planted Shorea stenoptera Burck seeds in the last three weeks of a 12- wk synchronous dipterocarp seedfall during a major community mast-fruiting event in West Kalimantan, Indonesia. As a result of commercial timber harvest of dipterocarp individuals eight years before, total dipterocarp seed production in the logged site was only 23% of that in the primary forest. At both sites, an average of 35 kg of seed was sown across a large area (?1 kM2) to examine the spatial pattern of seed destruction. During the period in which "natural" community mast seed was available to predators, 92% and 99% of experimentally sown seed escaped predators in logged and in primary forest, respectively. After regional seed resources were exhausted, nomadic vertebrates (primarily the bearded pig, Sus barbatus) were observed in both forest areas, and all ungerminated seed was destroyed. Seed predators arrived earlier in the logged area, before most experimentally sown seed had germinated, and the logged site experienced greater seed loss to vertebrates than did the primary forest. Because nomadic seed predators were absent during peak fruit fall of naturally occurring communities at both study sites, there was no evidence of local predator satiation. Rather, experimentally sown seed escaped predation because of rapid germination before predator arrival, as opposed to being ignored by satiated predators. Seed escape was more dependent on the late arrival of pigs than on the amount of local seed production. There was no significant spatial autocorrelation of seed predation. All remaining seed at the scale of the experiment (>1 kM2) was destroyed by predators. These findings suggest that satiation of nomadic predators occurs at the landscape scale. Postdispersal seed predators caused significantly greater seed destruction in the exper- imentally sown seed populations than in naturally dispersed, mast-fruiting communities at both sites. In both logged and primary forests, there was significantly greater loss of experimentally sown seed to predation than was found in the entire natural mast-fruiting Shorea community combined (21 spp.). Moreover, a naturally occurring, but late-fruiting, Shorea species also exhibited greater seed losses to predation than did all other species within each mast-fruiting community, and these proportional losses were similar to those observed in the experimentally sown seeds. Seeds that escaped predation and vertebrate herbivory on post-establishment seedlings dis- played high survival, indicating that the availability of suitable microsites did not limit re- cruitment. In the primary forest, 65% of the germinated experimental seed that survived early causes of mortality was alive 40 mo post-planting, which coincided with the next mast-fruiting event. The spatial distribution of these seedlings was modified primarily by the foraging behavior of vertebrate seed predators in the first two weeks post-planting. The influence of vertebrate predation on seed and seedling survival suggests that foraging behavior by terrestrial vertebrate seed predators may cause directional and/or stabilizing selection for synchronous, interspecific supra-annual dipterocarp seed production across forest regions in Kalimantan.

Fire, soil heating, and the formation of vegetation patterns in chaparral

Abstract: We documented patterns of surface heating associated with chaparral fire to characterize funda- mental scale variation in the intensity of this stand-replacing disturbance. To test how this variation may influence community structure, we studied its effect on the soil seed bank and the distribution of seedlings and resprouts that emerged after fire. To evaluate the long-term significance of initial patterns, we monitored vegetation development for 4-5 yr, thereby encompassing the dynamic portion of the chaparral fire cycle. We studied two stands on level uniform terrain before, during, and after fall fires. Stands were dominated by chamise (Adenostoma fasciculatum), a postfire seeder/sprouter. Nonsprouting Arctostaphylos and Ceanothus spp. were also present. Preburn vegetation, seed populations, soil heating, and postburn plant growth were analyzed along transects of contiguous 1-m 2 plots, so that we could block them together incrementally to identify scale dependence of patterns. In addition, we directly compared heating effects under the fuel array with those just outside by establishing plots in canopy gaps, under the adjacent canopy, and in gaps created and eliminated by reciprocally translocating fuel. Pre- and postburn seed populations were estimated in soil samples collected from all plots. The proportion of seed that survived above and below 2.5 cm in the soil was determined in a subset of plots. The amount and distribution of canopy fuel that collapsed during fire and smoldered on the ground caused pronounced spatial variation in total surface heating. The strength of relationships among patterns of soil heating, preburn canopy, surviving seeds, and seedlings and herbaceous resprouts was consistently most pronounced in blocks 3-5 m long. At this scale, postburn patterns were strongly negatively associated with the amount of preburn canopy and the pattern of soil heating this fuel created. Seedlings or herbaceous resprouts of numerous species were abundant where soil heating was relatively low, most notably in natural and created canopy gaps. Conversely, areas where dense canopy occurred before fire, especially gaps displaced by fuel addition, were barren except for occasional Arctostaphylos and Ceanothus seedlings. These obligate postfire seeders, along with the subshrub Helianthemum scoparium , had more deeply buried seeds, and some of them were able to survive where soil heating was prolonged. However, Helianthemum did not emerge from depth. Seedlings of Arctostaphylos and Ceanothus nearest Adenostoma burls survived significantly better when Adenostoma failed to resprout. This was common in one burn where heating was relatively high and burl size was small. Seed mortality prevented Adenostoma seedling emergence from occurring where its seeds were most abundant prior to fire, which was in proximity to its burls. Adenostoma seedlings did emerge in areas of lower soil heating, but their survival was inversely related to the density of Helianthemum seedlings. No shrub seedlings emerged after the first year following fire because their seed banks were exhausted by fire-induced mortality and/or germination. After 4-5 yr, few young Adenostoma remained. The combination of seedling and resprout regen- eration allowed this shrub to maintain dominance, but to a lesser extent in the older stand. Our results support a vegetation pattern-process model in which local species distributions after fire in Adenostoma chaparral are antecedently linked to the physical and chemical properties of the canopy. These control the nature of combustion, the soil heating that results, and the distribution of seeds and resprout tissues that survive. The vegetation develops entirely from these sources, so fire-induced patterns are manifest in the long-term structure of this vegetation.

Recruitment vs. postrecruitment processes as determinants of barnacle population abundance

Abstract: Determining the relative contributions of recruitment vs. postrecruitment processes to adult populations is an unresolved issue. The “recruit–adult” hypothesis suggests that the density of recruits is a good predictor of adult density when low but not when high. That is, the relative importance of recruitment vs. postrecruitment factors varies inversely with increasing density of recruits. In a rocky intertidal habitat at two Oregon coastal sites, a field experiment was done using two barnacle species to test this hypothesis. The relative impacts of these factors on adult barnacle abundance was determined using a reciprocal transplant design to manipulate both the density of barnacles established by recruitment and the postrecruitment conditions (tidal height, wave exposure) in which they lived. The relative contribution of recruitment to adult densities was strongly context dependent and species specific. While density of recruits clearly influenced density of adults for both species in most combinations...

Forest bird community structure in central panama : Influence of spatial scale and biogeography

Abstract: Historical and biogeographic contexts can play important, yet sometimes overlooked, roles in determining structure of local communities. In particular, few examinations of historical influences on patterns of species richness and relative abundances in tropical communities have been conducted. In part, that gap in our knowledge has been caused by a paucity of data on tropical communities, even for relatively well-studied taxa such as birds. In the Neotropics, only two sites, a 97-ha plot in lowland Peru and a 100-ha plot in French Guiana, have been inventoried on a spatial scale sufficient to estimate population densities for a majority of resident bird species. Results from those studies revealed extremely similar species richness, community biomass, and patterns of relative abundance. A third site in lowland Panama was originally censused in 1968–1969 and has often been compared with many other tropical and temperate sites. Results from Panama suggested an exceptionally different community structure fro...

Thermal ecology and sexual size dimorphism in northern water snakes, nerodia sipedon

Abstract: We used more than 326 000 observations of temperature collected by radio telemetry from 38 individuals over three years to investigate thermoregulation and thermal relations of northern water snakes (Nerodia sipedon) near the northern limit of their distribution in Ontario, Canada. We tested hypotheses concerning the effects of feeding, season, sex, and reproductive condition on thermoregulation of individuals. The mean preferred body temperature (PBT) for captive snakes from the study population was 27.1°C, similar to that reported for other populations, and PBT range (defined as the 25th–75th percentiles of selected temperatures) was 25–30°C. When environmental conditions allowed, the mean and range of body temperature (Tb) of free-living snakes were nearly identical to those observed in captivity. The typical daily pattern in snake Tb was an increase in late morning to a plateau temperature in the preferred range, followed by a decrease in the evening with a nightime plateau at approximately the temper...

Predicting direct and indirect interactions during succession in a mid-littoral rocky shore assemblage

Abstract: Increasing the predictive capabilities of ecological models is important for providing solutions to environmental problems. Progress in this direction relies on the understanding of basic ecological processes. Here, I used interaction web models and natural history information to predict the direct and indirect interactions that regulated succession in a relatively unstudied rocky shore assemblage in the northwest Mediterranean. Natural changes in abundance of organisms and general patterns of succession were examined during March 1991–September 1995. It was predicted that limpets enhanced succession by preventing the monopolization of the substratum by filamentous algae, indirectly facilitating the establishment of other colonists, such as the red alga Rissoella verruculosa, cyanobacteria (Rivularia spp.), and barnacles (Chthamalus spp.). This hypothesis was first tested by comparing succession in artificially denuded patches of substratum maintained at reduced densities of herbivores, with similar patch...

Corridors and mammal community structure across a fragmented, old-growth forest landscape

Abstract: We studied the influence of corridors on the community structure of old- growth forest mammals across a fragmented ecosystem, the Olympic National Forest, Wash- ington, USA. This region of once contiguous forest has been transformed by logging into a mosaic of landscape features including clearcuts, second-growth forest, and old-growth forest patches and corridors. To assess corridor utility, we quantified among- and within- corridor variability in community structure, landscape indices, and habitat descriptors. Discriminant analyses showed that the four corridors studied differed significantly both in species assemblages (P , 0.05) and in habitat characteristics ( P , 0.005). Changes along individual corridors were primarily associated with adjacent habitat. The proportion of adjacent old-growth forest significantly decreased along two of the four corridors, re- flecting this system's fragmentation gradient of an increasingly disturbed landscape matrix. The number of forest species found in corridor sites was significantly correlated with this matrix: less old-growth forest surrounding a corridor site resulted in fewer forest-dependent species. Width of the corridor at a site and site isolation often acted singly or in combination with the fragmentation gradient as an influence on community structure. This was also the case for many individual species. Demographic measures suggested that, while reproduction is occurring along these corridors, levels are lower than in continuous forest. Finally, data showed that, while forest species richness and occurrence of specific forest species were consistently higher in corridors than in the surrounding matrix, these patterns were driven primarily by differences between corridors and clearcuts. These results suggest that, al- though these corridors appear to be effective and may possibly serve as demographic sources of individuals, they should not be considered equivalent to one another. Only by considering corridors individually can their respective value be determined. For the Olympic National Forest, this value is significant, with the use of a combination of different corridors com- prising a viable supplement to maintaining continuous forest for long-term preservation of

The interaction of habitat fragmentation, plant, and small mammal succession in an old field

Abstract: We compared the density and spatial distribution of four small mammal species (Microtus ochrogaster, Peromyscus maniculatus, Sigmodon hispidus, and P. leucopus) along with general measures of an old field plant community across two successional phases (1984–1986 and 1994–1996) of an experimental study of fragmentation in eastern Kansas. During the early phase the plant community was characterized by little spatial or temporal variance across patch size, consistent with spatially neutral models of succession. In contrast, there was a strong, species-specific effect of patch size on small mammal species distribution and abundance. The lack of variance in vegetation structure across patch size during the early seres suggests that small mammal distributions were responding in large part to features of the system other than variance in vegetation structure and composition across patch size. As succession proceeded, the colonization of the system by woody plant species precipitated a series of patch size effect...

Direct and indirect effects of predation by common eiders and abiotic disturbance in an intertidal community

Abstract: Predation and disturbance have been well studied in intertidal communities. However, the impact of vertebrate predators, particularly waterfowl, has been only infre- quently determined in long-term intertidal studies. Using predator exclusion cages and simulated abiotic disturbance, I studied the direct and indirect effects of predation by Common Eiders (Somateria mollissima) on an intertidal mussel bed community in Passa- maquoddy Bay, New Brunswick, Canada. Eiders fed heavily on blue mussels (Mytilus edulis), the dominant invertebrate present, reducing their abundance in "?+duck" (exposed to predation) relative to "-duck" (exclosure) plots by nearly 50% within eight months of initiation of the experiment. Based on counts and estimated consumption rates, eiders appeared to be responsible for most predation observed in this system. Although ducks fed heavily on the dominant intertidal invertebrate, they had little effect on species diversity or richness in the community. However, relative abundance of the most common invertebrates did vary. In particular, exclusion of eiders led to an increase in abundance of dogwhelks (Nucella lapillus) one year into the experiment. Whelks, in turn, fed heavily on mussels under cages and obscured the longer-term effect of ducks in the system. This indirect effect appears to be an example of asymmetric exploitation compe- tition, with ducks influencing the food supply of whelks, but whelks having little effect on ducks. When ducks were excluded, whelks were released from this competition and acted as compensating predators. No other indirect effects developed following duck exclusion, presumably due to the increased whelk effect. Disturbance, in contrast, did lead to an increase in species diversity, which later returned to predisturbance levels as the community recovered. Predation delayed the recovery of disturbed sites, because ducks began feeding in these plots before mussel abundance had completely rebounded. Disturbance, while initially deterring predation somewhat, ulti- mately allowed the effects of predators to persist longer. This interaction of predation and disturbance resulted from compensatory growth of mussels under exclosures in disturbed sites. Mussels protected from eider predation grew quickly after disturbance and rapidly became larger than the preferred prey for whelks. As a consequence, whelks did not feed as heavily on mussels in disturbed sites as in undisturbed sites, where more mussels were of the preferred size, and the observed effect of ducks on mussel biomass persisted. This experiment demonstrates that eiders were significant, and, probably, keystone pred- ators in this system. Eiders directly reduced abundance of mussels, thereby indirectly increasing whelk density in plots where ducks were excluded. Compensatory predation by whelks following duck exclusion blocked other potential indirect effects, thus preventing eiders from having more widespread effects in the system. Such compensatory predation may act to stabilize communities and evidently can occur in relatively simple systems as well as the more species-rich communities with which it is usually associated.

A vitality‐based model relating stressors and environmental properties to organism survival

Abstract: A survivorship curve is shaped by the differential survivability of the or- ganisms within the population, and a change in a survivorship curve with a stressor reflects the differential response of the organisms to the stressor. Quantifying this linkage in a simple, rigorous way is valuable for characterizing the response of populations to stressors and ultimately for understanding the evolutionary selection of individuals exposed to stress- ors. To quantify this stressor-individual-population linkage with as few parameters as possible, I present a simple mechanistic model describing organism survival in terms of age-dependent and age-independent mortality rates. The age-independent rate is represented by a Poisson process. For the age-dependent rate, a concept of vitality is defined, and mortality occurs when an organism's vitality is exhausted. The loss of vitality over age is represented by a continuous Brownian-motion process, the Weiner process; vitality-related mortality occurs when the random process reaches the boundary of zero vitality. The age at which vitality-related mortality occurs is represented by the Weiner-process probability distribution for first-arrival time. The basic model has three rate parameters: the rate of accidental mortality, the mean rate of vitality loss, and the variability in the rate of vitality loss. These rates are related to body mass, environmental conditions, and xenobiotic stress- ors, resulting in a model that characterizes intrinsic and extrinsic factors that control a population's survival and the distribution of vitality of its individuals. The model assumes that these factors contribute to the rate parameters additively and linearly. The model is evaluated with case studies across a range of species exposed to natural and xenobiotic stressors. The mean rate of vitality loss generally is the dominant factor in determining the shape of survival curves under optimal conditions. Xenobiotic stressors add to the mean rate in proportion to the strength of the stressor. The base, or intrinsic, vitality loss rate is proportional to the - 1/3 power of adult body mass across a range of iteroparous species. The increase in vitality loss rate with a xenobiotic stressor can be a function of body mass according to the allometric relationship of the organism structures affected by the stressor. The model's applicability to dose-response studies is illustrated with case studies including natural stressors (temperature, feeding interval, and population density) and xenobiotic stressors (organic and inorganic toxicants). The model provides a way to extrapolate the impact of stressors measured in one environment to another envi- ronment; by characterizing how stressors alter the vitality probability distribution, it can quantify the degree to which a stressor differentiates members of a population.

Paleoecology of submerged macrophytes in the upper chesapeake bay

Abstract: Fossil seed distributions of submerged aquatic vegetation (SAV) from dated sediment cores in tributaries of the upper Chesapeake Bay show prehistoric changes in species composition and abundance and reflect the response of SAV species to human disturbance since European settlement. The interval of time spanned by the cores includes several centuries prior to, and three centuries following, European settlement. Species diversity is greatest in the low-salinity northern and upper tributaries, while areas of higher salinity and extensive salt marshes are characterized by low diversity or absence of SAV. Mapped distributions of seed abundances show the migration from upstream to downstream in some tributaries of the brackish species Potamogeton perfoliatus, Zannichellia palustris, and Ruppia maritima following deforestation. The largest increase in SAV, represented by the highest abundance of fossilized seeds, occurred during the 1700s after Europeans first cleared the land for farms, and the largest and most widespread decline took place in the 1960s and 1970s after most of the watershed had been at one time or another cleared and heavily fertilized for agriculture. Distributions of SAV are highly variable both temporally and spatially, reflecting the dynamic nature of estuarine habitats. Despite high environmental variability, local and regional extinctions occurred only in the most recent decades, indicating a threshold response to land use changes and nutrient loading which had begun at least two centuries earlier and intensified in the mid- to late 19th century.

Evolution of clutch size in the eastern kingbird: tests of alternative hypotheses

Abstract: David Lack proposed that parental feeding ability ultimately limited clutch size in bird species in which the young were dependent upon their parents for food. However, many species can raise broods that are larger than their normal clutch size. Based on nine years of experimental results from an individually marked population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York (USA), I test six hypotheses that have been proposed as explanations for why birds fail to lay larger, seemingly more productive clutch sizes. I modified brood sizes by adding or removing 1-2 nestlings when broods were 1-3 d old and then documented the effects of brood size and manipulated brood size on nestling size and survivorship, offspring recruitment, adult survival, and future adult reproduction. Most first clutches of the season held three eggs (62% of 503 clutches), but the proportion of young to fledge did not vary with brood size (1-5 young), and as a result, broods of five were the most productive. Lack's basic food-limitation model was thus rejected. Although nestling mass and ninth-primary length at fledging declined with brood size, offspring survival during the im- mediate 10-12 d period after fledging was unrelated to nestling mass or lengths of the tarsus or ninth primary. The findings that the underweight young in broods of four and five did not suffer disproportionate mortality and that they were just as likely to appear as recruits in future years led to a rejection of the extended version of Lack's food-limitation model. Comparisons of annual variation in the relationship between productivity and brood size showed that productivity increased with brood size in eight of nine years (significant in six years). Thus, high temporal stochastic variation in conditions for rearing young (the "bad-years" hypothesis) is unlikely to explain the relatively small clutch size of kingbirds. Predictions of two other hypotheses that predict asymmetrically low survivorship of young in large broods (the "cliff-edge" and&"brood-parasitism" hypotheses) were also rejected. On the other hand, evidence suggested that females individualize clutch size such that each female lays a clutch that matches her individual feeding ability. Although fledgling production was not adversely affected by experimental increases in brood size, most enlarged broods lost young during the 10-12 d immediately after fledging. Thus, enlarged broods ultimately produced no more independent young than did control broods that began with the same number of eggs. Fledgling deaths were not related to nestling mass or size, and recruitment was independent of manipulations. Survival and fecundity costs of reproduction also existed for females. Male survival (68%) was independent of the number of young that had been raised (0-5 young), and future breeding efforts were not compromised by elevated effort in the past year. However, females that raised broods of five were less likely to return to breed in the following year (42%) than were females that raised 2-4 young (62%). Among the survivors, females that raised enlarged broods in the preceding year also experienced more hatching failure and fledged fewer young than females that raised reduced broods in the preceding year. I suggest that costs of reproduction probably set the ultimate limit to clutch size in Eastern Kingbirds. I did not test the hypothesis that high rates of nest predation favor the evolution of small clutch size, but given that predators destroyed -50% of nests each year, it is also likely that nest predation has contributed to the evolution of the current clutch size of kingbirds. Whether a female produces a clutch of three or four eggs is probably determined by individual differences in parental ability, which may be related to either intrinsic properties of the female or territory quality.

Metapopulation dynamics of multiple species: the geometry of competition in a fragmented habitat

Abstract: Populations of species living within a spatially fragmented habitat may be affected by interspecific competition on a regional time scale. Metapopulation theory is a potentially powerful tool for understanding the dynamics of such competing species, but metapopulation models with competition are often too complex to be useful as guides for empirical investigation. In this paper I employ two techniques from dynamical systems theory (perturbation expansion and the examination of invariant manifolds) in a qualitative and quantitative analysis of two-species competition within a metapopulation, with the goal of using geometric ideas to provide insight into how competition affects species living in a fragmented habitat. Four-state metapopulation competition models (where habitat patches may be occupied by either of two competing species, both, or neither) can be analyzed as perturbations of simpler three-state models (those without doubly occupied patches as one of the state variables). Symmetric competition f...

Genetic influences on experimental population dynamics of the least killifish

Abstract: When natural populations differ in density or in the dynamic fluctuations of population size, some of those differences may result from their different ecological conditions, and some may originate from genetically based differences in life history expression. Natural populations of the live-bearing poeciliid fish, Heterandria formosa, vary considerably in their population dynamics, with densities that differ between populations by as much as sevenfold. This system offers an excellent opportunity to explore the potential role of genetically based differences in life history expression in creating different dynamic patterns in a common environment. We created five different genetic stocks of H. formosa by carrying out a series of crosses using fish from two North Florida populations (the Wacissa River and Trout Pond) and used them to initiate replicate experimental populations in artificial ponds. The five stocks consisted of two “controls,” which were pure Wacissa River and Trout Pond stocks, and three types of hybrid stocks. The hybrid stocks differed in a regular way in the proportion of genes from one population or the other. The crossing scheme was designed so that each hybrid stock would have the same proportion of heterozygous (or “heterodemic”) loci but would differ in the proportion and/or identity of homozygous (or “homodemic”) loci from the Wacissa River and Trout Pond populations. These populations were chosen because a previous study had found that population densities in the Wacissa River greatly exceeded those of Trout Pond and exhibited a higher range of population fluctuation during the breeding season. We addressed four questions in this experiment: (1) Are there genetically based differences in life history traits of fish from the two populations? (2) If so, do differences in life history expression produce differences in population dynamics in a common environment? (3) Which traits have the greatest influence on population dynamics? (4) How do changes in density affect the phenotypes of individual traits that govern the rates of birth and death in a population? We followed experimental populations of the five genetic stocks from their initiation at low density through 4–6 generations of population growth and decline. The mean offspring size differed among stocks by as much as 50%. At low densities, offspring size exhibited a trade-off with brood size: Trout Pond alleles were associated with more, smaller offspring. At higher densities, offspring sizes were similar among stocks, and the trade-off with offspring number was not evident. Stocks differed in realized population growth rate by as much as 70%; the rank order differences among stocks with respect to population growth rate appeared to match the genetic relatedness among stocks based on the expected percentage of Trout Pond alleles. Differences in population growth rate appeared to be due to differences in brood size among stocks at low density. Stocks did not differ in the equilibrium population size, which indicated the absence of a trade-off between population growth rate and carrying capacity in this environment. Adult survival and recruitment of juveniles into the adult population both declined linearly with increasing density; and stocks did not generally differ in those rates after the effects of density had been taken into account. The stocks differed in their response to the depressant effects of density on life history trait expression. The offspring size of the pure Wacissa River stock was much more sensitive to density than was the offspring size of the pure Trout Pond stock. However, the brood sizes of the Wacissa River stock were reduced much less than those of the Trout Pond stock when exposed to the same high density. These results suggest that life history distinctions among populations, both in the mean values and plasticity of traits, play a role in creating different dynamics. We discuss the ways in which phenotypic plasticity in reproductive traits potentially acts as a mechanism to stabilize population dynamics in this species.