Showing papers in "Ecological Monographs in 1992"

Modeling Survival and Testing Biological Hypotheses Using Marked Animals: A Unified Approach with Case Studies

Abstract: The understanding of the dynamics of animal populations and of related ecological and evolutionary issues frequently depends on a direct analysis of life history parameters. For instance, examination of trade-offs between reproduction and survival usually rely on individually marked animals, for which the exact time of death is most often unknown, because marked individuals cannot be followed closely through time. Thus, the quantitative analysis of survival studies and experiments must be based on capture- recapture (or resighting) models which consider, besides the parameters of primary interest, recapture or resighting rates that are nuisance parameters. Capture-recapture models oriented to estimation of survival rates are the result of a recent change in emphasis from earlier approaches in which population size was the most important parameter, survival rates having been first introduced as nuisance parameters. This emphasis on survival rates in capture-recapture models developed rapidly in the 1980s and used as a basic structure the Cormack-Jolly-Seber survival model applied to an homogeneous group of animals, with various kinds of constraints on the model parameters. These approaches are conditional on first captures; hence they do not attempt to model the initial capture of unmarked animals as functions of population abundance in addition to survival and capture probabilities. This paper synthesizes, using a common framework, these recent developments together with new ones, with an emphasis on flexibility in modeling, model selection, and the analysis of multiple data sets. The effects on survival and capture rates of time, age, and categorical variables characterizing the individuals (e.g., sex) can be considered, as well as interactions between such effects. This "analysis of variance" philosophy emphasizes the structure of the survival and capture process rather than the technical characteristics of any particular model. The flexible array of models encompassed in this synthesis uses a common notation. As a result of the great level of flexibility and relevance achieved, the focus is changed from fitting a particular model to model building and model selection. The following procedure is recommended: (1) start from a global model compatible with the biology of the species studied and with the design of the study, and assess its fit; (2) select a more parsimonious model using Akaike's Information Criterion to limit the number of formal tests; (3) test for the most important biological questions by comparing this model with neighboring ones using likelihood ratio tests; and (4) obtain maximum likelihood estimates of model parameters with estimates of precision. Computer software is critical, as few of the models now available have parameter estimators that are in closed form. A comprehensive table of existing computer software is provided. We used RELEASE for data summary and goodness-of-fit tests and SURGE for iterative model fitting and the computation of likelihood ratio tests. Five increasingly complex examples are given to illustrate the theory. The first, using two data sets on the European Dipper (Cinclus cinclus), tests for sex-specific parameters,

Leaf Life-Span in Relation to Leaf, Plant, and Stand Characteristics among Diverse Ecosystems

Abstract: Variation in leaf life—span has long been considered of ecological significance.Despite this, quantitative evaluation of the relationships between leaf life—span and other plant and ecosystem characteristics has been rare. In this paper we ask whether leaf life—span is related to other leaf, plant, and stand traits of species from diverse ecosystems and biomes. We also examine the interaction between leaf, plant, and stand traits and their relation to productivity and ecological patterns. Among all species, both mass— (Amass) and area—based (Aarea) maximum net photosynthesis decreased with increasing leaf life—span, but the relationship was stronger on a mass (P .25, r2 = 0.01). Specific leaf area (SLA, leaf area/leaf dry mass) and leaf diffusive conductance also decreased with increasing leaf life—span. Decreasing Amass with increasing leaf life—span results from the impact of decreasing Nmass and SLA on Amass. Variation in leaf traits as a function of leaf life—span was similar for broad—leaved and needle—leaved subsets of the data. These leaf—scale data from several biomes were compared to a data set from a single biome, Amazonia. For several leaf traits (e.g., SLA, Nmass, and Amass) the quantitative relationship with leaf life—span was similar in the two independent data sets, suggesting that these are fundamental relations applicable to all species. Amass was a linear function of Nmass (P .001, r2 = 0.74) with a regression similar to previous analyses, while Aarea was not significantly related to Narea. These results suggest that the photosynthesis—leaf N relationship among species should be considered universal when expressed on a mass, but not on a leaf area, basis. Relative growth rates (RGR) and leaf area ratio (LAR, the whole—plant ratio of leaf area to total dry mass) of seedlings decreased with increasing leaf life—span (P < .001, r2 = 0.61 and 0.89, respectively). LAR was positively related to both RGR and Amass (r2 = 0.68 and 0.84, respectively), and Amass and RGR were also positively related (r2 = 0.55). Absolute height growth rates of young trees decreased with increasing leaf life—span (P < .001, r2 = 0.72) and increased with Amass (P < .001, r2 = 0.78). It appears that a suite of traits including short leaf life—span and high leaf Nmass, SLA, LAR, and Amass interactively contribute to high growth rates in open—grown individuals. These traits interact similarly at the stand level, but stands differ from individuals in one key trait. In closed—canopy forests, species with longer lived foliage (and low LAR as seedlings) have greater foliage mass per unit ground area (P < .001, r2 = 0.74) and a greater proportion of total mass in foliage. The aboveground production efficiency (ANPP/foliar biomass) of forest stands decreased markedly with increasing leaf life—span or total foliage mass (P < .001, r2 = 0.78 and 0.72, respectively), probably as a result of decreasing Amass, Nmass, and SLA, all of which were positively related with production efficiency and negatively related to total foliage mass. However, high foliage mass of species with extended leaf life—spans appears to compensate for low production per unit foliage, since aboveground net primary production (ANPP, in megagrams per hectare per year) of forest stands was not related to leaf life—span. Extended leaf life—span also appears to compensate for lower potential production per unit leaf N per unit time, with the result that stand—level N use efficiency is weakly positively related to leaf life—span. We hypothesize that co—variation among species in leaf life—span, SLA, leaf Nmass, Amass, and growth rate reflects a set of mutually supporting traits that interact to determine plant behavior and production, and provide a useful conceptual link between processes at short—term leaf scales and longer term whole plant and stand—level scales. Although this paper has focused on leaf life—span, this trait is so closely interrelated with several others that this cohort of leaf traits should be viewed as casually interrelated. Generality in the relationships between leaf life—span and other plant traits across diverse communities and ecosystems suggests that they are universal in nature and thus can provide a quantitative link and/or common currency for ecological comparisons among diverse systems.

Cross-Scale Morphology, Geometry, and Dynamics of Ecosystems

Abstract: Community ecology and ecosystem ecology seem to have existed in different worlds. Levin (1989) suggests that the gulf between the two is the consequence of the different historical traditions in each. Community ecology, for example, emerged from basic studies, where generalized patterns were sought in the natural interactions among the biota. From the outset, the goal has been to deduce general and simple theory. On the other hand, many of the modelling approaches developed to understand ecosystem dynamics emerged from specific applied problems, where not only biotic but abiotic and human disturbances transformed ecosystem function. That tradition, therefore, is often more complete, but at the price of producing a collection of complex specific examples from which generalization is difficult.

Geostatistical tools for modeling and interpreting ecological spatial dependence

Abstract: Geostatistics brings to ecology novel tools for the interpretation of spatial patterns of organisms, of the numerous environmental components with which they in- teract, and of the joint spatial dependence between organisms and their environment. The purpose of this paper is to use data from the ecological literature as well as from original research to provide a comprehensive and easily understood analysis ofgeostatistics' manner of modeling and methods. The traditional geostatistical tool, the variogram, a tool that is beginning to be used in ecology, is shown to provide an incomplete and misleading summary of spatial pattern when local means and variances change. Use of the non-ergodic covariance and correlogram provides a more effective description of lag-to-lag spatial dependence because the changing local means and variances are accounted for. Indicator transforma- tions capture the spatial patterns of nominal ecological variables like gene frequencies and the presence/absence of an organism and of subgroups of a population like large or small individuals. Robust variogram measures are shown to be useful in data sets that contain many data outliers. Appropriate removal of outliers reveals latent spatial dependence and patterns. Cross-variograms, cross-covariances, and cross-correlograms define the joint spa- tial dependence between co-occurring organisms. The results of all of these analyses bring new insights into the spatial relations of organisms in their environment.

Life history diversity of canopy and emergent trees in a neotropical rain forest

Abstract: To assess the diversity of tropical tree life histories, a conceptual framework is needed to guide quantitative comparative study of many species. We propose one such framework, which focuses on long—term performance through ontogeny and over the natural range of microsites. For 6 yr we annually evaluated survival, growth, and microsite conditions of six non—pioneer tree species in primary tropical wet forest at the La Selva Biological Station, Costa Rica. The species were: Lecythis ampla, Hymenolobium mesoamericanum, Dipteryx panamensis, Pithecellobium elegans, Hyeronima alchorneoides (all emergents), and Minquartia guianensis (a canopy species). The study was based on long—term measurement of individuals from all post—seedling size classes. Trees were sampled from 150 ha of primary forest spanning several watersheds and soil types. To evaluate individuals' microsites we recorded the number of overtopping crowns, forest phase (gap, building, mature), and crown illumination index (an estimate of the tree's light environment). For comparison, we also evaluated the microsites of three species that have been categorized as pioneers (Cecropia insignis, C. obtusifolia) or high—light demanders (Simarouba amara). For the six species of non—pioneers, mortality rates declined with increasing juvenile size class. As a group, these emergent and canopy trees showed a much lower exponential annual mortality rate (0.44%/yr at >10 cm diameter) than has been found for the La Selva forest as a whole. Growth rates increased with juvenile size class for all six species. As adults (trees >30 cm in diameter), all five emergent species showed substantial annual diameter increments (medians of 5—14 mm/yr). Small saplings and adults of all species had significant year—to—year variation in diameter growth, with much greater growth occurring in the year of lowest rainfall. Passage time analysis suggests that all six species require >150 yr for growth from small saplings to the canopy. Evaluation of all nine species revealed four patterns of microsite occupancy by juveniles. Among the non—pioneers, one species pair (Lecythis and Minquartia: Group A) was associated with low crown illumination and mature—phase forest in all juvenile stages. For two species (Dipteryx and Hymenolobium: Group B) the smallest saplings were in predominantly low—light, mature—forest sites, but crown illumination and association with gap— or building—phase sites increased with juvenile size (Simarouba also showed this pattern). Two species (Pithecellobium and Hyeronima: Group C) were strongly associated with gap or building phase as small juveniles (≤4 cm diameter) and again as subcanopy trees (>10—20 cm diameter), but were predominantly in mature—phase sites at intermediate sizes. Juveniles of the two pioneer species (Cecropia: Group D) showed the highest crown illumination and association with gap or building sites. Among the six non—pioneer species, only one aspect of juvenile performance clearly varied according to microsite group. The smallest saplings (≤1 cm diameter) of Groups B and C showed significant mortality differences across a small gradient in crown illumination; neither of the Group A species showed this pattern. Otherwise, juvenile performance was strikingly similar among the six species. All showed a capacity for growth responses to small increases in light, substantial height and diameter increments at higher light levels, equal ability to survive 4—yr periods of no growth, and very low mortality rates at intermediate—to—large juvenile sizes. Species differed significantly in growth rates, but relative differences shifted with tree size and were unrelated to microsite group. These findings do not support prevailing paradigms concerning trade—offs and correlated suites of traits. For non—pioneer tropical trees, life history classification based on generalized concepts such as gap dependence and shade tolerance is inadequate to describe the complex size—dependent patterns of life history differences and similarities that exist among species.

Microbial and Faunal Interactions and Effects on Litter Nitrogen and Decomposition in Agroecosystems

Abstract: We conducted field experiments to test the general hypothesis that the com- position of decomposer communities and their trophic interactions can influence patterns of plant litter decomposition and nitrogen dynamics in ecosystems. Conventional (CT) and no-tillage (NT) agroecosystems were used to test this idea because of their structural sim- plicity and known differences in their functional properties. Biocides were applied to ex- perimentally exclude bacteria, saprophytic fungi, and microarthropods in field exclosures. Abundances of decomposer organisms (bacteria, fungi, protozoa, nematodes, microar- thropods), decomposition rates, and nitrogen fluxes were quantified in surface and buried litterbags (Secale cereale litter) placed in both NT and CT systems. Measurements of in situ soil respiration rates were made concurrently. The abundance and biomass of all microbial and faunal groups were greater on buried than surface litter. The mesofauna contributed more to the total heterotrophic C in buried litter from CT (6-22%) than in surface litter from NT (0.4-1/1%). Buried litter decay rates (1.4-1.7%/d) were -2.5 times faster than rates for surface litter (0.5-O.7%/d). Ratios of fungal to bacterial biomass and fungivore to bacterivore biomass on NT surface litter generally increased over the study period resulting in ratios that were 2.7 and 2.2 times greater, respectively, than those of CT buried litter by the end of the summer. The exclusion experiments showed that fungi had a somewhat greater influence on the decomposition of surface litter from NT while bacteria were more important in the de- composition of buried litter from CT. The fungicide and bactericide reduced decomposition rates of NT surface litter by 36 and 25% of controls, respectively, while in CT buried litter they were reduced by 21 and 35% of controls, respectively. Microarthropods were more important in mobilizing surface litter nitrogen by grazing on fungi than in contributing to litter mass loss. Where fungivorous microarthropods were experimentally excluded, there was less than a 5% reduction in mass loss from litter of both NT and CT, but fungi- fungivore interactions were important in regulating litter N dynamics in NT surface litter. As fungal densities increased following the exclusion of microarthropods on NT surface litter, there was 25% greater N retention as compared to the control after 56 d of decay. Saprophytic fungi were responsible for as much as 86% of the net N immobilized (1.81 g /m2) in surface litter by the end of the study when densities of fungivorous microarthropods were low. Although bacteria were important in regulating buried litter decomposition rates and the population dynamics of bacterivorous fauna, their influence on buried litter N dynamics remains less clear. The larger microbial biomass and greater contribution of a bacterivorous fauna on buried litter is consistent with the greater carbon losses and lower carbon assimilation in CT than NT agroecosystems. In summary, our results suggest that litter placement can strongly influence the com- position of decomposer communities and that the resulting trophic relationships are im- portant to determining the rates and timing of plant litter decomposition and N dynamics. Furthermore, cross placement studies suggest that the decomposer communities within each tillage system, while not discrete, are adapted to the native litter placements in each.

Temporal and Spatial Patterns of Disturbance and Recovery in a Kelp Forest Community

Abstract: This paper addresses questions of community and patch stability as defined by the population biology of dominant plants in the context of different areas within a large kelp forest We ask (1) "Do large—scale episodic events override biological mechanisms as major community structuring processes?", (2) "Are different local areas characterized by different processes?", and (3) "How persistent are the patches or biological structure over decal and local spatial scales?" We evaluate these questions with regard to the effects of various types of disturbance for as much as three decades on the populations of several species of kelp in the large kelp forest off Point Loma, San Diego, California The most sensitive population factors we studied include recruitment, density, and survivorship Patch stability was evaluated with regard to the persistence of patches already well established in 1971—1972 The study sites offer a cross—shore transect through the central part of a large kelp forest at depths of 8, 12, 15, 18, and 21 m; two additional sites at the north and south ends of the forest offer a longshore transect along the 18—m contour There were marked differences among the decades with regard to the intensity of the disturbances Compared with the 1980s, the two preceding decades were relatively benign The 1980s had two extreme disturbance events: the 1982—1984 El Gino—Southern Oscillation (ENSO) was the most severe El Nino event in the last century, which included very warm, nutrient—depleted water, and a short but intense storm in January 1988 appeared to have been the most severe in perhaps 200 yr The storm changed age—specific kelp mortality patterns and caused the first large—scale understory mortality in several decades By sweeping away drift algae it caused intense local urchin grazing The storm was followed by a strong La Nina event marked by cool, nutrient—rich water in 1988—1989 Differences in kelp recruitment and survivorship in different areas of the kelp forest are influenced by gradients in longshore currents, temperature, light, wave energy, floc, planktonic propagules, and physical disturbance The areas are characterized by different plant population patterns and the effects of several species of herbivores The massive disturbances of the 1980s obliterated much of the structure in the kelp forest Certainly the disturbances caused many lag effects including outbreaks of understory algae such as Desmarestia ligulata, intraspecific competition, changes in grazing patterns, etc, which in turn resulted in between—area variation in recovery rates However, in all cases this variation was overshadowed by the overwhelming competitive dominance of Macrocystis pyrifera Most of the understory patches on the transect lines, some of which had persisted for 7 yr, died out by the end of 1990 The population biology of Macrocystis was remarkably similar in most areas, as the cohort longevity and survivorship curves were very similar, and the plant and stipe densities tended to level off in only a few years Thus large—scale episodic events such as El Ninos, La Ninas, and rare storms exert dramatic impacts, but small—scale responses such as density—vague recruitment (neither density dependent nor density independent) and survival allow prompt recovery, often to preexisting patterns The one exception was the southern site, which was marked by sea urchin grazing and poor kelp recruitment through the latter half of the 1980s, but a recent sea urchin disease event has led to kelp recruitment in fall 1991 A seeming paradox to the observed Macrocystis dominance is that in almost all areas, some understory patches of old plants have persisted through the 1980s

Carbon dynamics of rocky mountain douglas-fir: influence of water and nutrient availability'

Abstract: Changes in biomass distribution, canopy dynamics, and above- and be- lowground net primary production were examined in a Rocky Mountain Douglas-fir (Pseu- dotsuga menziesii var. glauca forest in New Mexico. Nutrient and water availability were experimentally altered by: fertilization (F), irrigation (I), carbon in the form of wood chips (WC), carbon + irrigation (WC/I), and control (C). Prior to treatment, aboveground tree biomass ranged from 238 to 369 000 kg/ha, projected leaf area index (LAI) ranged from 5.4 to 8.7 m2/m2 and aboveground net primary production (ANPP) ranged from 9200 to 11 900 kg* ha-I yr-i. Aboveground NPP was correlated positively (R2 = 0.85) with LAI before the treatments. Canopy dynamics were strongly influenced by water and nutrient availability. For trees of similar diameter, irrigated and fertilized trees supported a signif- icantly greater biomass of new twig and new foliage than control trees. During the 2-yr study leaf area index (LAI) increased by 5, 12, 18, and 24% in the C, I, WC/I, and F plots, respectively, and decreased by 3% in the WC plots. Stand level biomass distribution and production patterns were also affected by the availability of nutrients and water. Two years after the treatments were initiated, new foliage masses were 2400 (F), 2300 (WC/I), 2000 (I), 1900 (C), and 1800 (WC) kg/ha. In 1986, aboveground NPP was 33% greater in the F than WC treatment. Irrigation also increased ANPP. Fine root net primary production ranged from 1540 to 4200 kg ha-i yr-i and was significantly greater (P < .1) in the control than in the four treatments. BNPP comprised 46 (C), 32 (WC), 31 (I), 23 (WC/I), and 23 (F) % of total NPP. Total NPP was correlated positively with LAI (R2 = 0.66) and ranged from 15 360 kg ha- Iyr-I in the WC treatment to 21 140 kg ha-l yr-I in the F treatment. Many of the physiological relations between water or nutrient availability and production and carbon allocation reported in this study are consistent with results from studies on lowland Douglas-fir and other conifer forests in the Pacific Northwest. Collectively, these studies provide a mechanistic understanding of how water and nutrient availability govern production and carbon allocation of conifer forests in the western United States.

Comparison of Tropical Tree Plantations with Secondary Forests of Similar Age

Abstract: The structure and dynamics of small plantations of pine (Pinus caribaea; 4 and 18.5 yr old in 1980) and mahogany (Swietenia macrophylla; 17 and 49 yr old in 1980) were compared with those of paired secondary forest stands of similar age and growing adjacent to each other under similar edaphic and climatic conditions. The study was conducted in the Luquillo Experimental Forest between 1980 and 1984. Comparisons included a variety of demographic, production, and nutrient cycling characteristics of stands. Although the small unmanaged plantations had a lower number of species in understory than paired secondary forests, the understory of the older plantations developed high species richness, including many of native tree species. After 17 yr, native tree species invaded the overstory of plantations. After 50 years the species richness in the understory of a mahogany plantation approached that of its paired secondary forest. Plantation un- derstories had important ecological roles, including high nutrient accumulation. Understory plant tissue, particularly leaf litter, had higher nutrient concentration in pine plantations than in paired secondary forests. Understory biomass in plantations accumulated a higher proportion of the total nutrient inventory in the stand than did the understory in paired secondary forests. Plantations had higher aboveground biomass and net aboveground bio- mass production than paired secondary forests. Higher root densities and biomass were found in secondary forests as were greater depth of root penetration, higher nutrient con- centration in roots, and more microsites where roots grow, than paired plantations. These characteristics may improve the capacity of secondary forests relative to that of paired plantations to rapidly recapture nutrients that become available by mineralization and that could otherwise be lost through hydrological or gaseous pathways. Both forest types accumulated nutrients and mass, but secondary forests recirculated nutrients much faster than the plantations, which tended to store the nutrients. Plantations had higher leaf fall and total litterfall, had litterfall with lower nutrient concentrations, accumulated more nutrients in litter, decomposed more litter on an annual basis, exhibited more variation in the spatial distribution of litter mass, and had more month-to-month variation in litter storage than paired secondary forests. Litter of the secondary forests, on the other hand, had a faster nutrient turnover than plantation litter, though plantations retranslocated more nutrients before leaf fall than did secondary forests. Nutrient retranslocation increased with plantation age. Plantations, particularly pine plantations, produced more litter mass per unit nutrient return than did paired secondary forests. Total nutrient storage in soil gave the best correlation with nutrient use efficiency estimated as element: mass ratios in various compartments. Nutrient use efficiency ranked differently among forest pairs, depending upon which nutrient and ecosystem parameters were being compared. Because of high retranslocation of nutrients, and in spite of greater nutrient "need" to produce higher biomass, plantations had nutrient demands on soil similar to paired secondary forests. Among the ecosystem parameters measured, nutrients in leaf fall correlated best with differences in soil nutrients across stands. Nutrient concentrations in understory species appeared to be a sensitive indicator of whole-stand nutrient use efficiency. Some of the observations of the study could be attributed to intrinsic differences between small un- managed plantations and secondary forests, but many could be explained by species dif- ferences (i.e., timing of leaf fall), age of plantation (i.e., accumulation of biomass or species), or the relative importance of angiosperms and gymnosperms (i.e., nutritional quality of litter). The study challenges the conventional dogma with respect to differences between plantations and native successional ecosystems and underscores the dangers of generalizing about all tropical tree plantations or all natural tropical forests, or even extrapolating from one sector of the ecosystem to another.

Northern spotted owls : Influence of prey base and landscape character

Abstract: We studied prey populations and the use and composition of home ranges of 47 Northern Spotted Owls (Strix occidentalis caurina) over 12 mo in five landscapes in two forest types in southwestern Oregon. We measured 1-yr home ranges of 23 owl pairs, 2-yr home ranges of 13 pairs, and 3-yr home ranges of 3 pairs. The landscapes differed in the degree to which old forest had been fragmented by wildfire and logging. Prey populations were measured at 47 sites in southwestern Oregon. Further data on prey populations were gathered on 14 sites on the Olympic Peninsula in northern Washington, where owls use larger ranges than in Oregon. Owls in Washington used - 1700 ha of old forest annually and primarily one prey species; available prey biomass was 61 g/ha. Owls in Oregon Douglas-fir (Pseudotsuga menziesai) forests used 813 + 133 ha (X + SE) of old forest annually and concentrated on two prey species that had a combined biomass of 244 g/ha. Owls in Oregon mixed-conifer forest used 454 ? 84 ha of old forest annually and three primary prey whose availability averaged 338 g/ha. The amount of old forest used by owls studied for 2 yr was 40% greater in the 2nd yr than that used in the Ist yr. No increase in use of old forest was seen in the 3rd yr in Douglas-fir forest; 50% more old forest was used in 3 yr than in the 1st yr in mixed-conifer forest. The most common prey in Washington and Oregon was the northern flying squirrel (Glaucomys sabrinus). In areas where the flying squirrel was the primary prey and where predation was intense (as judged by telemetry), flying squirrel populations were depressed. The addition of medium-sized mammal species, especially woodrats (Ne- otoma spp.), to the prey base appeared to reduce markedly the amount of old forest used for foraging. Owls traversed 85% more Douglas-fir forest and 3 times more mixed-conifer forest in the heavily fragmented areas than in the lightly fragmented areas. Overlap among pairs and separation of birds within pairs in space increased with fragmentation. In the most heavily fragmented landscape, social structure appeared to be abnormal, as judged by the proportion of adult-subadult pairs, instances of adult nomadism, and overlap among the home ranges of pairs. The pattern of fragmentation affected the ability of owls to find concentrations of old forest in the landscapes. Even so, almost all the owls consistently selected old forests for foraging and roosting; only one owl selected a younger type as part of its foraging range. Selection of old forest was significant at three levels: landscape, annual home ranges of pairs, and foraging and roosting sites of individuals. The most important prey species, the northern flying squirrel, was twice as abundant in old forest as in young forest in all areas. Landscape indices (dominance, contagion, variance in density of old forest) had less predictive ability than indices based on owl home ranges because owls selected areas of concentrated old forest and because patterning was complex, reflecting four processes, each operating at a different scale: physiography, human land ownership (259-ha scale), history of catastrophic fires, and history of small-scale fires and timber harvesting.

Episodic hypoxia in Chesapeake Bay : interacting effects of recruitment, behavior, and physical disturbance

Abstract: Physical disturbance can be an important force at the individual, population, and community levels of organization. The effects of disturbance may differ for mobile and sessile organisms, however, because of differences in the potential for escape and postdisturbance recolonization by survivors. I used field sampling and laboratory experiments to examine how episodic movement of severely oxygen—depleted (hypoxic) bottom water into nearshore habitat in the Chesapeake Bay affects population density, recruitment, and reproduction of a mobile species–the naked goby (Gobiosoma bosc), a benthic oyster bed fish. Oxygen depletion is a common physical disturbance in freshwater, estuarine, and coastal aquatic systems. In this study, episodic hypoxia influenced mortality, size structure of the population, reproductive behavior, and spatial distribution. Intrusion of severely hypoxic water occurred in late July and early August during the 2—yr study. These intrusions coincided temporally with peak periods of recruitment, and caused the most severe drops in dissolved oxygen concentrations in deep and mid—depth areas of the oyster reef, where recruitment was highest. Laboratory experiments suggested that newly settled recruits require higher oxygen concentrations for survival than do older individuals. Field samples also indicated that these new recruits are less able to escape to more highly oxygenated shallow water refuges when an intrusion occurs. Thus, the spatial and temporal patterns of recruitment and disturbance, and physiological requirements, combine to result in extremely high mortality of new recruits during severe intrusions. In contrast to effects on new recruits, some large juveniles and adults successfully migrate inshore when oxygen levels decline. In both field samples and laboratory experiments, adult males continued to guard eggs and shelters until dissolved oxygen closely approached lethal levels. Calculations based on size—specific physiological tolerances and swimming speeds suggest that the occurrence of lethal conditions in the fluctuating environment may be more predictable to larger individuals than to new recruits. This predictability may increase the possibility of an appropriate response to low oxygen disturbances by large juveniles and adults. After the disturbance abates, surviving individuals recolonize abandoned areas. This ability of mobile animals to recolonize a disturbed area as adults or juveniles, rather than solely through reproduction, may lead to differences in postdisturbance ecological interactions and differences in selection for colonizing ability between mobile and sessile species.

Diving Behavior and Energetics During Foraging Cycles in King Penguins

Abstract: King Penguins are the second largest of all diving birds and share with their congener, Emperor Penguins, breeding habits strikingly different from other penguins. Our purpose was to determine the feeding behavior, energetics of foraging and the prey species, and compare these to other sympatric species of subantarctic divers. We determined: (1) general features of foraging behavior using time—depth recorders, velocity meters, and radio transmitters, (2) energetics by doubly labeled water, (3) food habits and energy content from stomach lavage samples, and (4) resting and swimming metabolic rate by oxygen consumption measurements. The average foraging cycle was ≈6 d, during which the mass gain of 30 birds was ≈2 kg. When at sea, the birds exhibit a marked pattern of shallow dives during the night, whereas deep dives of >100 m only occurred during the day. Maximum depth measured from 34 birds and 18 537 dives was 304 m, and maximum dive duration from 23 birds and 11 874 dives was 7.7 min. The frequency distribution of dive depth was bimodal, with few dives between 40 and 100 m. Overall, swim velocities when a bird was at sea averaged 2.1 m/s (N = 5), while descent and ascent rates of change in depth averaged 0.6 m/s for dives 150 m (N = 90). Night feeding dives occurred at a rate of ≈20 dives/h, and deep dives occurred at a rate of ≈5 dives/h. The energy consumption rate while resting ashore was 3.3 W/kg (N = 3) or 1.6 times the predicted standard metabolic rate (SMR). The average energy consumption rate while away from the colony was 10 W/kg (N = 8) or 4.6 x SMR, compared to 4.3 x SMR estimated from a time—energy budget. The latter value is based on an average metabolic rate of 4.2 W/kg for three birds while resting in 5°C water and 9.6 W/kg while swimming at 2 m/s, which was extrapolated from the average of three birds swimming at 1 m/s. The average energy intake based on 9 stomach content samples was nearly 24.6 kJ/g dry mass. The main prey by number are myctophid fish of the species Krefftichthys anderssoni and Electrona carlsbergi. It was concluded that: (1) feeding begins ≈28 km from the colony, (2) prey is pursued night and day through its vertical movements, (3) vertical distribution of the prey is reflected closely by diving habits of the birds, (4) deep—diving, for unknown reasons, is an important component of foraging success, (5) diving capacities of King Penguins are remarkable compared to other birds and many pinnipeds, and (6) calculated foraging energetics can be closely estimated from time—energy budgets.

Simple methods for calculating age-based life history parameters for stage-structured populations

Abstract: Stage—classified matrix models are important analytical and theoretical tools for the study of population dynamics; in particular, these models may be appropriate for populations in which survivorship and fecundity are dependent on size or developmental stage, populations in which the age of individuals is difficult to determine, and populations in which there are multiple types of newborns. Nevertheless, methods for analyzing the implications of a population's stage—transition matrix have been limited in comparison to methods available for age—structured models (life tables or Leslie matrices). In this paper we show that all of the standard age—based measures of life history traits can be derived from a stage—transition model. By decomposing the transition matrix into separate birth, survival, and fission matrices we derive simple, direct formulas for age—based life history traits such as the discrete survivorship function, lx, maternity function, fx, mean age at maturity, and net reproductive rate, Ro, and also population parameters, including the stable age distribution, age—specific reproductive value, and generation time. These provide a common set of parameters for comparing age—structured and stage—structured populations or comparing populations with differently structured life cycles. In addition, we define four measures of age and life—span that summarize the relationship between stage and age in a stage—structured population: age distribution and mean age of residence for each stage class, expected remaining life—span for individuals in each stage class, and total life—span conditional on reaching a given stage class. We illustrate the use of our methods to address specific ecological questions by applying them to several previously published demographic data sets. These questions include: (1) what are the demographic effects of crowding on the tropical palm Astrocaryum mexicanum?; (2) how important is the initial rosette size in determining life history of teasel, Dipsacus sylvestris?; and (3) how old are reproducing adults in a stage—classified population of pink lady's—slipper, Cypripedium acaule? Our results may also be useful for evaluating the adequacy of a given stage—transition model.

A 14 300-yr paleoecological profile of a lowland tropical lake in panama'

Abstract: The first paleoecological analysis of a complete sedimentary record spanning the period from the late Pleistocene to the present from lowland Panama, documents changes in lowland vegetation communities through major climatic change and the onset of human disturbance. Past sympatry is found among presently allopatric species, suggesting that tropical forest communities are not species-stable through time. Late Pleistocene floras at Lake La Yeguada (elevation 650 m), Panama, had high relative abundance of montane forest elements, e.g., Quercus and Magnolia, existing some 900 m below their present range, suggesting a climatic cooling of - 50C below present. This descent of montane forest taxa onto lowland hilltops denied the ground to postulated lowland rain forest refugia. The late Pleistocene (14 350-11 050 yr BP) was not uniformly cool and was interrupted by brief phases of near present-day warming. The onset of the Holocene was abrupt, taking < 100 yr, and was almost coincidental with the start of human forest disturbance. Changes in climate at La Yeguada were found to be largely synchronous with those documented at Lake Valencia, Venezuela, but no fine-scale climatic synchrony was apparent with South American or European sites, and significant departures from the predictions of published climatic circulation models are found.

Tolerance of Marine Invertebrate Herbivores to Brown Algal Phlorotannins in Temperate Australasia

Abstract: Estes and Steinberg hypothesized that the intensity of sublittoral marine plant—herbivore interactions among different temperate regions has differed over the past 5—10 million yr, due to differences in the influence of diving mammals that prey on marine herbivores. They argued that because diving predators have not occurred in temperate Australasia, densities of sublittoral herbivores have been historically higher there than in comparable North American systems. This has resulted in marine algae and herbivores in temperate Australasia having strong selective effects on each other. Estes and Steinberg predicted that one result of these strong interactions would be that invertebrate herbivores in temperate Australasia should have evolved greater tolerance to brown algal phlorotannins (polyphenolic compounds that are the dominant secondary metabolites in temperate brown seaweeds) than similar herbivores in North America. We tested this prediction for five invertebrate herbivores in temperate Australasia. Mo...

Defense of Ascidians and Their Conspicuous Larvae: Adult vs. Larval Chemical Defenses

Abstract: Previous investigations, focused primarily on vertebrates, have noted substantial losses of eggs and embryos to predators and questioned why selection has not more commonly resulted in the evolution of chemically defended eggs or embryos Hypotheses regarding the apparent rarity of such defenses have emphasized the potential incompatibility of actively developing tissues and toxic metabolites Alternatively, this apparent pattern could be an artifact of our greater knowledge of vertebrates, which in general show few tendencies for synthesizing defensive metabolites in either juvenile or adult stages In this study, we investigated adult and larval chemical defenses of a group of benthic marine invertebrates, the ascidians, in which the adults are often chemically rich, and we contrast our findings with what is known about chemical defenses of eggs and embryos from terrestrial and aquatic organisms Our findings suggest that there is no fundamental incompatibility of rapidly developing juvenile tissues and bioactive metabolites, and that chemically defended eggs and larval stages may be common among some taxonomic groups Ascidians are benthic invertebrates that often lack apparent physical defenses against predation, yet are common on coral reefs where predation by fishes is intense In contrast to most co—occurring invertebrates, many ascidians also release large, conspicuous larvae during daylight hours when exposure to fish predation would be highest Thus selection by predators might favor the evolution of distasteful larvae In situ observations indicate that many conspicuous ascidian larvae are distasteful to potential consumers We investigated the ability of secondary metabolites produced by taxonomically diverse ascidians from geographically distant locales to deter predation on both adults and larvae Larvae from the Caribbean ascidian Trididemnum solidum were distasteful to reef fishes, and when organic extracts of individual larvae were transferred onto eyes of freeze—dried krill (a good larval mimic in terms of size and color), these eyes were rejected by fishes while control eyes (solvent only) were readily eaten Larvae of the Indo—Pacific ascidian Sigillina cf signifera were also distasteful to coral—reef fishes and contained the unpalatable bipyrrole alkaloid tambjamine C When added to artificial foods at or below their natural mean concentrations and offered to consumers in field and laboratory feeding assays, the secondary metabolites produced by Trididemnum solidum (Caribbean Sea), Sigillina cf signifera (Indo—Pacific), and Polyandrocarpa sp (Gulf of California) significantly deterred feeding by co—occurring fishes and invertebrates Secondary metabolites produced by Trididemnum cf cyanophorum from the Caribbean Sea, Lissoclinum patella from the Indo—Pacific, and Aplidium californicum from the temperate Pacific, and the small stellate spicules common to many tropical didemnid ascidians did not significantly affect fish feeding High—pressure liquid chromatography (HPLC) analyses of six didemnin cyclic peptides in individual colonies of Trididemnum solidum from one patch reef at Little San Salvador, Bahamas found large inter—colony differences in their concentrations The mean concentration of didemnin B was more than double the concentration needed to significantly deter fish feeding in our field assays, and feeding tests with nordidemnin B showed that it deterred fish feeding across the entire range of natural concentrations HPLC analysis of the extract from a combined collection of T solidum larvae found adequate concentrations of didemnin B and nordidemnin B to account for their rejection by foraging fishes We demonstrate that taxonomically diverse ascidians from habitats characterized by intense predation pressure produce secondary metabolites that significantly reduce predation on both adults and larvae, and suggest that this defensive chemistry may be crucial in allowing the release of large, well—provisioned larvae during daylight periods when larvae have the greatest probability of using photic cues to select physically appropriate settlement sites Production of defensive secondary metabolites appears widespread among certain groups of ascidians, some of which are also known to concentrate acid and heavy metals as additional defensive strategies

Patterns in Population Change and the Organization of the Insect Community Associated with Goldenrod

Abstract: The extent to which insect communities are organized is poorly understood because few studies have examined both population-level processes and community-level patterns. Furthermore, our view of phy- tophagous insect populations is probably biased by the more frequent attention given to economic pests whose dynamics might be expected to be somewhat exceptional. Herein, we report on the population dynamics and community-level features in a diverse, native fauna of phytophagous insects associated with goldenrod (Solidago altissima), a native perennial plant that forms a dominant, long-persisting element in old-field succession. The data consist of censuses taken in six consecutive years at 16 old fields in the Finger Lakes Region of New York; five additional stands were followed for four consecutive years. Our analyses address two questions: (a) to what extent is the functional structure of the community (as reflected in such attributes to the herbivore load, guild spectrum, and dominance hierarchy) maintained by compensatory changes in the densities of the member species and (b) are certain traits associated with a species' tendency to fluctuate in density or to dominate the community? In general, populations of goldenrod insects fluctuated less than those of insects reported in the literature. Few were abundant; only 7 of the 138 species in the goldenrod fauna ever reached densities at which their biomass exceeded 0.1% of the leaf biomass. The functional structure of the community, as reflected in the total herbivore load and the proportions of that load contributed by various guilds, varied widely in space and time. A small subset of the fauna remained dominant throughout the study; these species, whose populations occa- sionally irrupted, retained their high rank in relative abundance even during those periods when their populations were waning. There was no evidence for compensatory changes in the densities of species within guilds, as would be expected if a relatively distinct set of limiting resources were available to insects with different feeding styles. Species were no more likely to exhibit negative correlations with their guild mates than they were with members of other guilds. There was also little evidence of sporadic or diffuse competition within the fauna as a whole; even during population outbreaks, the dominant species rarely engendered decreases in the biomass or diversity of the remaining species, and increases in the total herbivore load were seldom associated with losses of species from the community. Several species were positively associated with one another on the basis of similar habitat requirements or the use by one species of conditions created by the presence of another. The community was predictable only in the sense that dominant species remained dominant and rare species rare. The abundance and population variability of species were not correlated with such traits as body size, generation time, or host range. Taxon-related trends, on the other hand, stood out; coleopterans tended to be abundant whereas lepidopterans were rare, and hemipterans tended to fluctuate widely whereas dipterans remained relatively constant. Furthermore, species that were clumped in space (onto stems within fields) tended to be abundant and variable. The "boom and bust" dynamics of these aggregating species may be related to the ability of colonies to survive the inroads of generalist predators. The low degree of regional synchrony in the performance of populations at different sites suggests that large-scale, weather-driven fluctuations are un- common. Our analysis of this well-developed, native insect fauna reveals a community that has a somewhat predictable structure that reflects the idiosyncracies of the component species. These species do not fluctuate in an integrated community-structuring manner, but as an outcome of each species performing at its "individualistic" level, the community displays a characteristic dominance hierarchy. The members of this diverse fauna rarely achieve densities at which they compete with one another. Our results are consistent with Hairston, Smith, and Slo- bodkin's hypothesis (1960) which predicts that terrestrial herbivores rarely deplete plant resources to levels at which there is widespread interspecific competition.

Competition and the Structure of a Benthic Stream Community

Abstract: The mechanisms structuring benthic stream communities composed largely of highly mobile taxa are poorly known. I studied the role of competition in structuring the benthic community of hard substrates in a coldwater stream dominated by mobile insects that grazed upon periphyton and by relatively sessile filter-feeding insects. I expected competitive interactions among periphyton grazers to be significant and chronic because the community was not subject to frequent flow-related disturbances and grazer densities were high despite consistently low periphyton availability. Laboratory and field experiments were performed to determine: (1) the potential for, and relative strengths of, competition within and between two common, widely distributed grazers, the caddisfly Glossosoma nigrior and the mayfly Baetis tricaudatus, (2) the role of Glossosoma in affecting the or- ganization of the community, and (3) the mechanisms of competition. Strong intraspecific effects on Baetis survivorship were observed in laboratory enclosures within the range of natural densities, but interspecific effects were not detected. Glossosoma survivorship was not affected by competition with itself or with Baetis. Individuals of both species experienced significant intra- and interspecific effects on growth, but the relative strengths of these effects differed between species. The effects of Baetis on Glossosoma growth were stronger than the effects of Glossosoma on itself. In contrast, Baetis growth was much less affected by the presence of Glossosoma than by conspecifics. Both species strongly depressed periphyton abundance, indicating that observed competitive effects resulted, in part, from exploitation. Exclusion of Glossosoma from habitats in the field resulted in a marked increase in periphyton biomass throughout a 10 -mo study in which the movements of other taxa were not restricted. Most periphyton grazers responded to this increase in resource availability with increased densities in the Glossosoma exclusion treatment relative to the control. The size distributions of several chironomid grazers suggested that larval growth rates were greater in the absence of Glossosoma than with Glossosoma present. The larvae of two relatively small-bodied filter-feeders (Simulium, Rheotanytarsus) responded positively to the removal of Glossosoma, but larger bodied species (Hydropsyche, Brachycentrus) were not affected. There was not strong evidence for seasonal variation in the intensity of competition. Effects of Glossosoma on filter-feeders appeared to result from a form of interference competition, but there was no evidence for the operation of interference among grazers. Glossosoma had no effect on the colonization of habitats by grazers, and the feeding rates of Baetis and Glossosoma were not affected by the presence of conspecifics or heterospecifics. Contrasting patterns in periphyton abundance between the laboratory and field exper- iments suggest that, when their movements are unrestricted, Baetis departs periphyton patches at higher food densities than does Glossosoma to search for food elsewhere. Thus, strong competitive effects of Baetis on Glossosoma in the laboratory experiments resulted, in part, from limited Baetis dispersal in the enclosures. Baetis and Glossosoma appear to exhibit trade-offs in feeding efficiency and the costs of exploiting food patches that should help to ensure their coexistence on a spatially variable food resource. Results of this study suggest that the structure of benthic communities in coldwater streams composed largely of mobile taxa may often be strongly influenced by Glossosoma. A similar role has been observed for sessile, territorial grazers in other systems. Glosso- soma's dominance does not result from aggressive behavior, but from physiological, be- havioral, and life cycle attributes that allow it to regulate periphyton biomass throughout the year.

Population Cycles of Mammals: Evidence for a Ratio-Dependent Predation Hypothesis

Abstract: Populations of certain mammal species and their predators show cyclic fluctuations in northern latitudes, and the amplitude of cycles in some cases increases towards the north. The evidence reviewed suggests that (1) abiotic factors and intrinsic mechanisms are unable to explain cycles; (2) quantity and quality of food resources of the herbivore population have important effects on population dynamics, although plant- herbivore interaction cannot explain the cycles by itself; (3) predation is another important factor for these populations, and probably essential for cyclicity of herbivore populations. A number of mathematical models have shown that prey-predator models can produce limit cycles, but they have not demonstrated that the cyclic fluctuations observed in natural populations can be explained by the mechanisms they incorporate and the parameters they define. In this study a mathematical model is developed to predict specific patterns of prey- predator cycles observed in nature with independently estimated parameters. This prey- predator model is based on the concept of "ratio dependence": the trophic functions (functional and numerical responses) are modeled as functions of prey-to-predator ratio rather than as functions of prey density only, as in traditional prey-predator models. This approach incorporates the concept of interference in a simple way by describing trophic interactions as functions of per capita resources. The parameters of the model are estimated from studies on the biology of cyclic lynx and hare populations, rather than by fitting time- series data to the model. Parameters of the model give rise to limit cycles when they are changed in the way they are expected to change from south to north, which is consistent with the observations on the latitudinal patterns in cyclicity. The major quantitative pre- diction of the model is the cycle period. The period is predicted to be around 10 yr, which is the observed period of hare-lynx fluctuations.

Twenty years of ecosystem reorganization following experimental deforestation and regrowth suppression

Abstract: Watershed 2 of the Hubbard Brook Experimental Forest was experimentally deforested in the winter of 1965-1966, and subsequent plant regeneration was suppressed by herbicide application until the 1969 growing season. Changes in species structure, plant abundance, aboveground primary productivity and biomass, and aboveground nutrient pools were recorded on 70 permanent plots distributed across the watershed during 1, 2, 3, 5, 11, and 20 yr of subsequent succession. Species richness increased rapidly over time but equitability and the Shannon-Wiener diversity index declined with succession. Plant densities of herb, shrub, sapling, and tree strata increased, then decreased, in successive waves as the vegetation increased in height. Basal area (stems >5.1 cm dbh) initially increased exponentially, then linearly after the 5th yr to attain a total of 18.7 m2/ha by the 20th yr. Primary productivity also increased exponentially at first, but then increased linearly through the 20th yr at which time annual aboveground primary productivity was 20 Mg/ha. Aboveground biomass increased linearly after the 5th yr until by the 20th yr it was 52 Mg/ ha, 38% of the reference forest biomass. Aboveground nutrient pools of some elements accumulated faster than biomass by the 20th yr when calculated as percentages of the 55-yr-old reference forest pools and biomass. These included: potassium-52%, phosphorus-44%, magnesium-42%. Others accu- mulated more slowly than biomass: sulfur-32%, and nitrogen and calcium each-29%. While recovery rate on Watershed 2 was delayed in the first decade of regrowth, and composition was somewhat altered because of the 3-yr suppression, this forest nevertheless displayed a vigorous capacity for regeneration after suppression ceased.