Showing papers in "Ecological Monographs in 1998"

Predicting abundance of 80 tree species following climate change in the eastern united states

Abstract: Projected climate warming will potentially have profound effects on the earth's biota, including a large redistribution of tree species. We developed models to evaluate potential shifts for 80 individual tree species in the eastern United States. First, environmental factors associated with current ranges of tree species were assessed using geographic information systems (GIS) in conjunction with regression tree analysis (RTA). The method was then extended to better understand the potential of species to survive and/ or migrate under a changed climate. We collected, summarized, and analyzed data for climate, soils, land use, elevation, and species assemblages for .2100 counties east of the 100th meridian. Forest Inventory Analysis (FIA) data for .100 000 forested plots in the East provided the tree species range and abundance information for the trees. RTA was used to devise prediction rules from current species-environment relationships, which were then used to replicate the current distribution as well as predict the future potential distri- butions under two scenarios of climate change with twofold increases in the level of at- mospheric CO2. Validation measures prove the utility of the RTA modeling approach for mapping current tree importance values across large areas, leading to increased confidence in the predictions of potential future species distributions. With our analysis of potential effects, we show that roughly 30 species could expand their range and/or weighted importance at least 10%, while an additional 30 species could decrease by at least 10%, following equilibrium after a changed climate. Depending on the global change scenario used, 4-9 species would potentially move out of the United States to the north. Nearly half of the species assessed (36 out of 80) showed the potential for the ecological optima to shift at least 100 km to the north, including seven that could move .250 km. Given these potential future distributions, actual species redistributions will be controlled by migration rates possible through fragmented landscapes.

Effects of plant composition and diversity on nutrient cycling

Abstract: We evaluated the effects of plant functional group richness on seasonal patterns of soil nitrogen and phosphorus cycling, using serpentine grassland in south San Jose, California. We established experimental plots with four functional types of plants: early-season annual forbs (E), late-season annual forbs (L), nitrogen-fixers (N), and perennial bunchgrasses (P). These groups differ in several traits relevant to nutrient cycling, including phenology, rooting depth, root:shoot ratio, size, and leaf C:N content. Two or three species of each group were planted in single functional group (SFG) treatments, and in two-, three-, and four-way combinations of functional groups. We analyzed available nutrient pool sizes, microbial biomass nitrogen and phosphorus, microbial nitrogen immobilization, nitrification rates, and leaching losses. We used an index of “relative resource use” that incorporates the effects of plants on pool sizes of several depletable soil resources: inorganic nitrogen in all seasons, availabl...

Stages and spatial scales of recruitment limitation in southern appalachian forests

Abstract: Recruitment limitation of tree population dynamics is poorly understood, because fecundity and dispersal are difficult to characterize in closed stands. We present an approach that estimates seed production and dispersal under closed canopies and four limitations on recruitment: tree density and location, fecundity, seed dispersal, and estab- lishment. Consistent estimates are obtained for 14 canopy species using 5 yr of census data from 100 seed traps and several thousand mapped trees and seedlings from five southern Appalachian forest stands that span gradients in elevation and moisture. Fecundity (seed production per square centimeter of basal area) ranged over four orders of magnitude, from 10 0 cm 2 basal area/yr (Carya, Cornus, Nyssa, Quercus )t o.10 3 cm 2 /yr (Betula). Mean dispersal distance ranged from , 5m( Cornus, Nyssa )t o.20 m (Acer, Betula, Liriodendron, Tsuga) and was positively correlated with fecundity. Species also differ in the degree of seed clumping at fine (1 m 2 ) spatial scales. Dispersal patterns can be classed in two groups based on dispersal vector: wind-dispersed taxa with high fecundities, long-distance dis- persal, and low clumping vs. animal-dispersal taxa with low fecundities, short-distance dispersal, and a high degree of clumping. ''Colonization'' limitations caused by sizes and locations of parent trees, fecundity, and dispersal were quantified as the fraction of sites receiving seed relative to that expected under null models that assume dispersal is nonlocal (i.e., long-distance) and not clumped (i.e., Poisson). Difference among species in coloni- zation levels ranged from those capable of saturating the forest floor with seed in most stands (Acer, Betula, Liriodendron) to ones that leave much of the forest floor without seed, despite presence of adults (Carya, Cornus, Nyssa, Oxydendrum ). Seedling establish- ment is one of the strongest filters on recruitment in our study area. Taken together, our results indicate (1) that fecundity and dispersal can be resolved, even under a closed canopy, and (2) that recruitment of many species is limited by the density and location of source, dispersal patterns, or both.

The impact of predator-induced stress on the snowshoe hare cycle

Abstract: The sublethal effects of high predation risk on both prey behavior and phys- iology may have long-term consequences for prey population dynamics. We tested the hypothesis that snowshoe hares during the population decline are chronically stressed be- cause of high predation risk whereas those during the population low are not, and that this has negative effects on both their physiology and demography. Snowshoe hares exhibit 10- yr population cycles; during declines, virtually every hare that dies is killed by a predator. We assessed the physiological responsiveness of the stress axis and of energy mobilization by subjecting hares during the population decline and low to a hormonal-challenge protocol. We monitored the population demography through live-trapping and assessed reproduction through a maternal-cage technique. During the 1990s' decline in the Yukon, Canada, hares were chronically stressed-as indicated by higher levels of free cortisol, reduced maximum corticosteroid-binding ca- pacity, reduced testosterone response, reduced index of body condition, reduced leucocyte counts, increased overwinter body-mass loss, and increased glucose mobilization, relative to hares during the population low. This evidence is consistent with the explanation that predation risk, not high hare density or poor nutritional condition, accounted for the chronic stress and for the marked deterioration of reproduction during the decline. Reproduction and indices of stress physiology did not improve until predation risk declined. These findings may also account for the lag in recovery of hare reproduction after predator densities have declined and thus may implicate the long-term consequences of predation risk on prey populations beyond the immediate effects of predators on prey behavior and physiology.

Beetle species responses to tropical forest fragmentation

Abstract: The effects of forest fragmentation on beetle species composition were investigated in an experimentally fragmented tropical forest landscape in Central Amazonia. Leaf-litter beetles were sampled at seven distances from the forest edge (0–420 m) along forest edge-to-interior transects in two 100-ha forest fragments and two continuous forest edges, and at an identical series of distances along two deep continuous forest transects. Additional samples were taken at the centers of two 10-ha forest fragments and two 1-ha fragments. This sampling regime allowed discrimination between edge and fragment area effects. Beetle species composition changed significantly and independently with both decreasing distance from forest edge and decreasing fragment area. Edge effects on species composition were mediated by six important environmental variables: air temperature, canopy height, percent ground cover of twigs, litter biomass, litter moisture content, and an air temperature × distance from edge interaction effect, due to the different temperature profiles of edges with differing edge vegetation density. Population densities of 15 of the 32 most abundant beetle species tested (47%) were significantly affected by forest fragmentation. Species responses were classified empirically into four major categories: (A) edge sensitive, area insensitive; (B) area sensitive, edge insensitive; (C) edge and area sensitive; and (D) edge and area insensitive. Within these categories, trends in density were either positive (deep-forest species), or negative (disturbed-area species), with species showing the full spectrum of responses to fragmentation. The vast majority of species were adversely affected. Estimated species loss rates from forest fragments were: 49.8% of common species from 1-ha fragments, 29.8% from 10-ha fragments, and 13.8% from 100-ha fragments. Declining density was a significant precursor of species loss from forest fragments, but other species that did not show significant population density responses to fragmentation were also absent from some fragments, presumably by chance. The probability of species loss from forest fragments was not correlated with body size or trophic group for the 32 common species, although for the entire beetle assemblage (993 species) proportions of species in different trophic groups changed significantly with fragmentation. Rarity and population variability (in undisturbed forest) were significant predictors of susceptibility to fragmentation. Surprisingly, though, common species were significantly more likely to become locally extinct in small fragments than rarer species. This lends empirical support to models of multispecies coexistence under disturbance that suggest competitively dominant but poorly dispersing species are the first to become extinct due to habitat destruction. Thus, rarer species are predicted to be better dispersers and better at persisting.

Seed dispersal and the holocene migration of woodland herbs

Abstract: The distribution of many woodland herbs extends 1000-2000 km in a north- south direction, yet the majority of these species grow clonally, have little recruitment by seed, and possess no obvious mechanism for long-distance seed dispersal. Although aware that woodland herbs disperse poorly, ecologists have tacitly assumed that, given long periods of time, even small dispersal distances would allow woodland herbs to colonize the vast geographic region they now occupy. We examined this assumption for the understory herb Asarum canadense. To estimate long-term rates of spread by seed, we calibrated seed- dispersal diffusion models with life history data and with data on seed carries by ants. We supplemented our field observations and modeling results forA. canadensewith a literature survey on the dispersal capabilities of other plant species. Ants transported A. canadense seeds up to 35 m, the largest distance ants are known to move the seeds of any woodland herb. Empirically calibrated diffusion models indicated that over the last 16 000 yr A. canadense should only have traveled 10-11 km from its glacial refugia. In actuality, A. canadense moved hundreds of kilometers during this time. Models that examined the tail of A. canadense's seed-dispersal curve indicated that oc- casional dispersal events had to have a high frequency ($0.001 on a per seed basis) and a large magnitude (dispersal distance .1 km) for A. canadense to have traveled over 200 km in 16 000 yr. The literature survey showed that most woodland herbs and many other forest, desert, coastal, and open-habitat plant species have limited seed-dispersal capabil- ities, similar to those in A. canadense. We conclude that woodland herbs, as well as many other plants, disperse so slowly that there is no documented mechanism by which most of these species could have reached their present geographical range since the last glacial maximum. This suggests that occasional events leading to long-distance dispersal dominate the Holocene colonization of northern temperate forest by woodland herbs, and this, in turn, has implications for issues ranging from the importance of genetic analyses to the structure of metapopulation models.

Long-term dynamics of mediterranean frugivorousbirds and fleshy fruits: a 12-year study

Abstract: The relationship between fleshy-fruited plants and their vertebrate seed dispersal agents often has been depicted as subject to important interannual variation, but no study has thus far documented such variation on a long-term basis. This paper presents the results of a 12-yr investigation on fleshy-fruited plants and avian frugivores in a Mediterranean montane locality of southeastern Spain. The main objective was to document patterns and correlates of long-term vari- ation in the composition and abundance of fruits and birds, with particular reference to seed dis- persers. During October-December (''autumn'' period) 1978-1990, abundance of ripe fruits and birds was assessed in a 4-ha plot in dense, well-preserved sclerophyllous scrub, by means of counts in permanent plots and mist-netting, respectively. Diet composition and fruit preference patterns of Erithacus rubecula and Sylvia atricapilla, the two most abundant seed dispersers, were also inves- tigated over the same period, using fecal sample analyses. Possible consequences to the birds of annual variation in fruit supply and diet composition were investigated using data on fat deposition levels and recapture rates of mist-netted individuals. Total fruit abundance (i.e., mean ripe fruit density of all species combined) fluctuated among years between 5.4 6 11.1 fruits/m 2 (mean 6 1 SD; 1986) and 77.1 6 78.0 fruits/m 2 (1989) and was positively related to the amount of rainfall in the preceding spring. Not all fruiting species bore ripe fruits every year, and among those species that did, fruit density fluctuated asynchronously and to variable degrees. Seven out of 13 species exhibited significant supra-annual periodicity in fruit abundance, with fluctuation periods ranging from 2 to 6 yr. Variation in the abundance of each of the six most abundant fruit species was unrelated to annual variation in rainfall. The autumn bird assemblage at the study site was made up of year-round resident species (54.1% of captures, all years combined) that were largely fruit predators (feeding on pulp or seeds without performing dispersal) and of autumn-winter resident species (45.5% of captures) that were largely seed dis- persers. Bird abundance, all species combined, ranged between 27.1 and 61.5 captures/100 net-hours for 1987 and 1986, respectively. Depending on year, seed dispersers made up 25.6-75.4% of the total captures, and fruit predators made up 20.9-69.7%. The relative importance of nonfrugivores was always negligible (1.1-9.9%). No correlation existed across years between total fruit abundance and the capture rates of all bird species combined, seed dispersers, or fruit predators. Annual variation in the abundance of seed dispersers was positively related to November mean maximum temperature. At the individual species level, S. atricapilla capture rates were correlated with the abundance of the fruits of Phillyrea latifolia, a species exhibiting extreme annual fluctuations. Annual variation in the importance of fruits in the diet of S. atricapilla and E. rubecula was not significantly related to changes in fruit abundance. Composition of the fruit diet of these species fluctuated markedly among years, and there was little agreement between composition of the diet and of the fruit supply. Certain fruit species were significantly preferred, and others avoided, by both S. atricapilla and E. rubecula. Ranking of interspecific fruit preferences remained consistent among years and was related to differences in carbohydrate and lipid content of fruit pulp. Fat deposition levels of S. atricapilla and E. rubecula did not vary among years and were not significantly related to fruit abundance, percentage of fruit volume in the diet, or contribution of lipid-rich fruits to the diet. Return rates of individuals of these species to the study locality in successive wintering periods were very low, and not significantly related to diet composition or fruit abundance. Prevalence of abiotic over biotic determinants of annual variations, extensive decoupling of the long-term temporal dynamics of fruits and dispersers, and the remarkable ''indifference'' of frugivores to variations in the fruit supply all point to the non-equilibrial nature of this assemblage of fleshy- fruited plants and their avian dispersers.

Assemblage organization in stream fishes: effects of environmental variation and interspecific interactions

Abstract: We assessed the relative importance of environmental variation, interspecific competition for space, and predator abundance on assemblage structure and microhabitat use in a stream fish assemblage inhabiting Coweeta Creek, North Carolina, USA. Our study encompassed a 10–yr time span (1983–1992) and included some of the highest and lowest flows in the last 58 years. We collected 16 seasonal samples which included data on: (1) habitat availability (total and microhabitat) and microhabitat diversity, (2) assemblage structure (i.e., the number and abundances of species comprising a subset of the community), and (3) microhabitat use and overlap. We classified habitat availability data on the basis of year, season, and hydrologic period. Hydrologic period (i.e., pre–drought [PR], drought [D], and post–drought [PO]) represented the temporal location of a sample with respect to a four–year drought that occurred during the study. Hydrologic period explained a greater amount of variance in habitat availability data...

Intertidal mussel microclimates: predicting the body temperature of a sessile invertebrate

Abstract: To elucidate the determinants of intertidal invertebrate body temperatures during aerial exposure, I developed deterministic models using the environmental inputs of solar radiation, air temperature, ground temperature, and wind speed to predict the body temperatures of intertidal mussels (Mytilus spp.). Combined with field studies, these models were used to determine the effects of body size on body temperature, and to compare the heat budgets of mussels living as solitary individuals vs. those living in aggregations (beds). On average, the model accurately predicted the body temperatures of solitary mussels in the field to within ∼1°C. Steady-state simulations (using constant environmental conditions) predicted that, under conditions where evaporative water loss is limited, smaller (5 cm) mussels experience lower body temperatures than larger (10 cm) mussels exposed to identical environmental parameters. When evaporative cooling is limited only by intolerance to desiccation, the trend in body size rever...

Biomass and co2 flux in wet sedge tundras: responses to nutrients, temperature, and light

Abstract: The aim of this research was to analyze the effects of increased N or P availability, increased air temperature, and decreased light intensity on wet sedge tundra in northern Alaska. Nutrient availability was increased for 6–9 growing seasons, using N and P fertilizers in factorial experiments at three separate field sites. Air temperature was increased for six growing seasons, using plastic greenhouses at two sites, both with and without N + P fertilizer. Light intensity (photosynthetically active photon flux) was reduced by 50% for six growing seasons at the same two sites, using optically neutral shade cloth. Responses of wet sedge tundra to these treatments were documented as changes in vegetation biomass, N mass, and P mass, changes in whole-system CO2 fluxes, and changes in species composition and leaf-level photosynthesis. Biomass, N mass, and P mass accumulation were all strongly P limited, and biomass and N mass accumulation also responded significantly to N addition with a small N × P interaction. Greenhouse warming alone had no significant effect on biomass, N mass, or P mass, although there was a consistent trend toward increased mass in the greenhouse treatments. There was a significant negative interaction between the greenhouse treatment and the N + P fertilizer treatment, i.e., the effect of the two treatments combined was to reduce biomass and N mass significantly below that of the fertilizer treatment only. Six years of shading had no significant effect on biomass, N mass, or P mass. Ecosystem CO2 fluxes included net ecosystem production (NEP; net CO2 flux), ecosystem respiration (RE, including both plant and soil respiration), and gross ecosystem production (GEP; gross ecosystem photosynthesis). All three fluxes responded to the fertilizer treatments in a pattern similar to the responses of biomass, N mass, and P mass, i.e., with a strong P response and a small, but significant, N response and N × P interaction. The greenhouse treatment also increased all three fluxes, but the greenhouse plus N + P treatment caused a significant decrease in NEP because RE increased more than GEP in this treatment. The shade treatment increased both GEP and RE, but had no effect on NEP. Most of the changes in CO2 fluxes per unit area of ground were due to changes in plant biomass, although there were additional, smaller treatment effects on CO2 fluxes per unit biomass, per unit N mass, and per unit P mass. The vegetation was composed mainly of rhizomatous sedges and rushes, but changes in species composition may have contributed to the changes in vegetation nutrient content and ecosystem-level CO2 fluxes. Carex cordorrhiza, the species with the highest nutrient concentrations in its tissues in control plots, was also the species with the greatest increase in abundance in the fertilized plots. In comparison with Eriophorum angustifolium, another species that was abundant in control plots, C. cordorrhiza had higher photosynthetic rates per unit leaf mass. Leaf photosynthesis and respiration of C. cordorrhiza also increased with fertilizer treatment, whereas they decreased or remained constant in E. angustifolium. The responses of these wet sedge tundras were similar to those of a nearby moist tussock tundra site that received an identical series of experiments. The main difference was the dominant P limitation in wet sedge tundra vs. N limitation in moist tussock tundra. Both tundras were relatively unresponsive to the increased air temperatures in the greenhouses but showed a strong negative interaction between the greenhouse and fertilizer treatments. New data from this study suggest that the negative interaction may be driven by a large increase in respiration in warmed fertilized plots, perhaps in relation to large increases in P concentration.

The consequences of changing the top predator in a food web: a comparative experimental approach

Abstract: Changing the top predator in a food web often results in dramatic changes in species composition at lower trophic levels; many species are extirpated and replaced by new species in the presence of the new top predator. These shifts in species composition also often result in substantial alterations in the strengths of species interactions. However, some species appear to be little affected by these changes that cause species turnover at other positions in the food web. An example of such a difference in species responses is apparent in the distributions of coenagrionid damselflies (Odonata: Zygoptera) among permanent water bodies with and without fish as top predators. Enallagma species segregate between ponds and lakes that do and do not support fish populations, with each lake type having a characteristic Enallagma assemblage. In contrast, species of Ischnura, the sister genus to Enallagma, are common to both fish and fishless ponds and lakes. Previous research has shown that Enallagma species segregate because they are differentially vulnerable to the top predators in each lake type: dragonflies in fishless lakes and fish in fish lakes. This paper reports the results of a series of laboratory and field experiments quantifying the mortality and growth effects of interactions in the food webs surrounding Enallagma and Ischnura species in both lake types. These results are compared to determine how features of the food web change to force segregation of Enallagma species between the lake types but permit Ischnura species to inhabit both. The results of experiments conducted in a fishless lake show that damselflies are not food limited in this lake type, but that they do strongly compete via interference mechanisms. Interference effects between the genera are symmetrical. Ischnura species have substantially higher growth rates than Enallagma species under all conditions in fishless lakes. Although both Enallagma and Ischnura experience substantial mortality from predation by dragonflies (Anax and Aeshna species, the top predators in fishless lakes), these dragonflies display a significant bias towards feeding on Ischnura. Mortality rates due to dragonfly predation are not density dependent. The results of experiments done in a fish lake indicate that damselflies are food limited and thus compete for resources in fish lakes. Ischnura growth rates are also substantially higher than Enallagma species in the fish-lake system. Dragonfly species that coexist with fish (Basiaeschna and Epitheca species) do not impose significant mortality on coexisting damselflies, but they do compete for resources with the damselflies, and they may also generate feeding interference in the damselflies. Fish impose significantly higher mortality on Ischnura species than on coexisting Enallagma species, and this mortality is negatively density dependent. The coexistence of Enallagma and Ischnura species is fostered in both lake types by trade-offs in their abilities to avoid predators and to utilize resources. Native Enallagma species are better at avoiding coexisting predators in each lake type, but these abilities come at the expense of the ability to utilize resources effectively and to avoid the predator found in the other lake type. In contrast, Ischnura are better at utilizing resources in both lake types, but these abilities come at the expense of effectively avoiding both fish and dragonflies. Understanding the trade-offs faced by species at similar trophic positions within a food web is critical to predicting changes in food webs following major environmental perturbations such as changing the top predator.

Regional gradient analysis and spatial pattern of woody plant communities of oregon forests

Abstract: Knowledge of regional-scale patterns of ecological community structure, and of fac- tors that control them, is largely conceptual. Regional- and local-scale factors associated with re- gional variation in community composition have not been quantified. We analyzed data on woody plant species abundance from 2443 field plots across natural and seminatural forests and woodlands of Oregon to identify and quantify environmental, biotic, and disturbance factors associated with regional gradients of woody species composition; to examine how these factors change with scale (geographic extent) and location; and to characterize and map geographic patterns of species and environmental gradients. Environmental correlates of species gradients, species diversity patterns, and the spatial pattern- ing of woody plant communities varied with geographic extent and location. Total variation ex- plained (TVE) by canonical correspondence analyses (CCAs) was 9-15% at three hierarchical geo- graphic extents: the entire state, two half-states, and five subregions. Our high level of unexplained species variation is typical of vegetation gradient analyses, which has been attributed to landscape effects, stochastic processes, and unpredictable historical events. In addition; we found that TVE in canonical correspondence analysis is confounded by sample size. Large numbers of plots and spe- cies, as in our study, are associated with lower TVEs, and we propose a; mechanism for this phenom- enon. Climate contributed most to TVE (46-60%) at all locations and extents, followed by geology (11-19%), disturbance (6-12%), and topography (4-8%). Seasonal variability and extremes in climate were more important in explaining species gradients than were mean annual climatic conditions. In addition, species gradients were more strongly associated with climatic conditions during the grow- ing season than in winter. The dominant gradient at the state scale was from the lower elevation, moderate, maritime climate ,along the coast to the higher elevation, drier, continental climate of eastern Oregon. The second canonical axis followed a gradient from the warm, dry, growing seasons of the western interior valleys and eastern Cascade Range to the cooler, wetter mountainous areas. Geologic variables were most strongly correlated with axis 3, and measures of local site and distur- bance with axis 4. For most of the state, our findings on the associations of disturbance factors with species gradients were inconclusive due to confounding of land ownership patterns, disturbance histories, and elevation in our sample. Near the coast, where gradients were not confounded, clear-cutting and stand age accounted for only 2 and 1% of TVE, respectively, in partial CCA. Ordi- nations of our long, regional gradients were influenced more by species presence than by abundance, and few woody species have been totally eliminated from sites by clear-cutting. Within Oregon and for the range of geographic extents we examined, variation in the environ- mental correlates of species gradients was more strongly associated with geographic location than with geographic extent, although topographic factors explained slightly more variation at smaller geographic extents. The greatest subregional contrast in vegetation character was between eastern and northwestern Oregon, and the Klamath subregion was intermediate. In the drier climate of east- ern Oregon, community structure varied at a finer spatial scale, and climatic and topographic mois- ture were more strongly associated with species gradients than in the moister areas of western; Or- egon. Topographic effects were weakest, and climatic effects strongest near. the coast, where climate is moderate. Alpha and gamma diversity were greater in western Oregon, but beta diversity was greater in eastern Oregon and greater for shrubs than for. trees. Our findings supported a conceptual model of multiscaled controls on vegetation distribution, and the related notion that local community structure is the result of both regional- and local-scale processes. Despite strong ecological contrasts within the region, we were able to synthesize species-environment relations at the regional level. This suggests that apparent conflicts among local vegetation studies can be explained by real ecological differences among places

Do similar communities develop in similar sites? A test with zooplankton structure and function

Abstract: McCune and Allen (1985) asked the question “Will similar forests develop on similar sites?” and concluded that dissimilar old-growth forests had developed on similar sites due to historical factors (colonization, disturbance, etc.). We asked “Do similar zooplankton communities develop in similar ponds?” We compared zooplankton community structure and function in 12 newly constructed experimental ponds during 1 yr of natural colonization and analyzed a suite of physical–chemical variables to evaluate the assumption of environmental similarity among ponds. Ponds were similar for the measured environmental variables. However, zooplankton communities were structurally different, as indicated by analyses of species presence/absence, colonization and species accrual curves, and taxa (rotifer, copepod, cladoceran, and Chaoborus) density and biomass. Species varied widely in their colonization abilities. Zooplankton communities also differed in productivity of some taxa and community-level respiration rates. Scale was important in detecting structure and function differences among zooplankton communities. Species- and taxa-level analyses showed clear differences among communities, but community-level analyses of structure (species richness, total density and biomass) and function (productivity, respiration, and ammonia regeneration rates) could not identify distinct sets of communities. Community structure and function may be comparable in sensitivity for detecting change but need to be compared at equivalent scales. Dispersal (as evidenced by colonization history) was a regulator of new zooplankton communities, because it did not occur rapidly or uniformly among similar ponds. All zooplankton do not disperse readily. The extent to which dispersal limits older zooplankton communities is unknown, but genetic studies indicate low dispersal rates among established populations. Dispersal also regulates assemblages of organisms expected to be less vagile than zooplankton and in various ecosystems, indicating that “supply-side” and metapopchulation concepts are valuable for community ecology. Priority effects may have lasting influence on subsequent community structure, depending on colonization rates and sequences. We propose explicit recognition (and careful examination) of a commonly assumed but rarely tested “quorum effect”: local abiotic and biotic processes regulate communities and arrival processes do not, because potential members have already arrived. Given either priority or quorum effects, dispersal may be an important, often-overlooked process regulating community structure and function, especially when it is not rapid.

Habitat structure and regulation of local species diversity in a stony, upland stream

Abstract: Habitat structure may regulate species diversity at local scales, with complex habitats being normally associated with greater species richness than simple ones. We employed a new method of quantifying habitat structure to examine community structure in a stone-dwelling community of mobile macroinvertebrates inhabiting a perennial, upland stream (the Steavenson River) in southeastern Australia. We distinguished between the effects of increasing habitat structure by adding similar physical elements (habitat complexity) from that produced by adding qualitatively different sources of habitat structure (habitat heterogeneity) at spatial scales relevant to the biota. We used a field experiment to ask: (1) Does variation in habitat structure at local scales (i.e., between individual stones) result in variation in species richness (S) and numbers of individuals (N), and if so, are changes in S wrought passively by changes in N, or is there evidence of local regulation of S? (2) Are macroalgae, which are a source of habitat structure for invertebrates, also affected by stone surface structure? (3) What are the effects of habitat structure on faunal composition and body sizes? We used clay bricks as substrata and manipulated three sources of habitat structure in a crossed design: large surface pits and cracks (low density/high density); small pits caused by variation in surface texture (rough/smooth); and the abundance of macroalgae (begun with algae, begun without algae). The bricks were sampled for both fauna and epilithon on days 14 and 28 of colonization, when species richness and densities of individuals were comparable to natural stream stones. Habitat structure altered faunal diversity and abundances, with the majority of common species reaching higher abundances on creviced or rough surfaces. Rough surfaces were additionally associated with shifts in overall faunal composition and markedly smaller body sizes. Each element of habitat structure (large crevices, roughness, and macroalgae) promoted both increased species richness and densities of individuals. Rarefaction indicated that changes in S were disproportionate to changes in N, which suggests that S is regulated by local processes. Overall species richness was highest on day 14 with no difference in S between simple and complex surfaces. By day 28, simple surfaces had lost taxa relative to complex surfaces, suggesting that species richness in this stream community is regulated at a local scale, even though faunal composition changes continually and is contingent upon habitat structure. Habitat structure also affected the epilithon, suggesting that sources of habitat complexity and heterogeneity are interwoven in this system. Furthermore, the epilithon response to surface structure depended on the spatial scale of habitat complexity, with more of the red, filamentous alga Audouinella hermannii being found on rough surfaces than on smooth surfaces, but less on bricks with large crevices than without. These different responses to surface structure at different spatial scales demonstrate the importance of quantifying and manipulating substrate complexity at scales that are comparable with natural surfaces.

Primary succession and forest development oncoastal lake michigan sand dunes

Abstract: Vegetation and soil properties were described across a well-dated sand-dune chronosequence bordering northern Lake Michigan to document patterns and rates of primary succession and forest ecosystem development, and to determine environmental constraints that potentially drive succession and regulate species diversity. The site experienced frequent and continuing formation of 72 shore-parallel dune ridges over the past 2375 yr. Across the chro- nosequence represented by the youngest 13 dune ridges aged 25-440 yr, there were clear patterns of species turnover and community convergence as well as successional changes in species diversity, aboveground biomass, aboveground litter production, net ecosystem produc- tion, nutrient pools, and nutrient cycling. Dune-building species were replaced by evergreen shrubs and bunchgrass within 100 yr, which in turn, were replaced by mixed pine forest within 345 yr. Plant-species richness increased to a peak in developing forest at 285 yr but thereafter decreased as early-successional species disappeared from the communities. Rates of species addition peaked between 95 and 145 yr as forest species invaded, whereas rates of species loss peaked between 345 and 440 yr as early-successional species were lost from the developing forest. Development of the forest ecosystem required ;300 yr (i.e., 145-440 years). Total eco- system carbon increased in a logistic manner to 128 Mg C/ha, with net ecosystem production peaking at 30 g C·m 22 ·yr 21 in developing forest. Aboveground biomass and O horizon mass increased to ;137 Mg/ha and ;79 Mg/ha, respectively, whereas aboveground litter production increased to 3.5 Mg·ha 21 ·yr 21 at 440 yr, but thereafter varied between 175 and 350 Mg·ha 21 ·yr 21 . Total carbon and total nitrogen in the upper 15 cm of mineral soil and O horizon accumulated to ;42 Mg/ha and ;1.36 Mg/ha, respectively. Estimated average rates of carbon and nitrogen accumulation over 440 yr of ecosystem aggradation were 23.2 g·m 22 ·yr 21 for carbon and 0.38 g·m 22 ·yr 21 for nitrogen. Because nitrogen-fixing plants are rare on the upland dune ridges, ecosystem aggradation depends largely on atmospheric nitrogen inputs. Following colonization by conifers, soil acidification resulted in rapid leaching losses of calcium and magnesium, whereas phosphorus and potassium were cycled more tightly. The dune chronosequence represents a complex gradient of changing environmental con- straints that differentially reduce the survival, growth, and reproduction of plant species. Young dune ridges near the lake shore are characterized by strong winds, sand burial and erosion, high insolation, high rates of evaporation, and low availability of nitrogen and phosphorus. These conditions ameliorate with increasing dune age as wind velocities and sand movement diminish with distance from the lake, as accumulating organic matter improves the moisture- holding capacity and nitrogen availability of the soil, and as mineral weathering mobilizes soil phosphorus. However, in developing forest, light and cationic nutrients may become limiting, and decreased light availability, cool soil temperatures, and accumulation of a thick forest floor may limit recruitment from seed for many species. These numerous potential environmental constraints suggest a considerable complexity in this ostensibly simple ecosystem.

Assessing the risk of extinction for the brown bear (ursus arctos) in the cordillera cantabrica, spain

Abstract: The status of the brown bear (Ursus arctos) in Spain has suffered a dramatic decline during the last centuries, both in area and numbers. Current relict populations are suspected to be under immediate risk of extinction. The aim of our model is to attain an understanding of the main processes and mechanisms determining population dynamics in the Cordillera Cantabrica. We compile the knowledge available about brown bears in the Cordillera Cantabrica, northern Spain, and perform a population viability analysis (PVA) to diagnose the current state of the population and to support current management. The specially constructed simulation model, based on long-term field investigations on the western brown bear population in the Cordillera Cantabrica, includes detailed life history data and information on environmental variations in food abundance. The method of individual-based modeling is employed to simulate the fate of individual bears. Reproduction, family breakup, and mortalities are modeled in annual time ...

Sampling-variance effects on detecting density dependence from temporal trends in natural populations

Abstract: Monte Carlo simulations were conducted to evaluate robustness of four tests to detect density dependence, from series of population abundances, to the addition of sampling variance. Population abundances were generated from random walk, stochastic exponential growth, and density-dependent population models. Population abundance es- timates were generated with sampling variances distributed as lognormal and constant coefficients of variation (CV) from 0.00 to 1.00. In general, when data were generated under a random walk, Type I error rates increased rapidly for Bulmer's R, Pollard et al.'s, and Dennis and Taper's tests with increasing magnitude of sampling variance for n . 5y r and all values of process variation. Bulmer's R* test maintained a constant 5% Type I error rate for n . 5 yr and all magnitudes of sampling variance in the population abundance estimates. When abundances were generated from two stochastic exponential growth models (R 5 0.05 and R 5 0.10), Type I errors again increased with increasing sampling variance; magnitude of Type I error rates were higher for the slower growing population. Therefore, sampling error inflated Type I error rates, invalidating the tests, for all except Bulmer's R* test. Comparable simulations for abundance estimates generated from a density-dependent growth rate model were conducted to estimate power of the tests. Type II error rates were influenced by the relationship of initial population size to carrying capacity ( K), length of time series, as well as sampling error. Given the inflated Type I error rates for all but Bulmer's R*, power was overestimated for the remaining tests, resulting in density depen- dence being detected more often than it existed. Population abundances of natural popu- lations are almost exclusively estimated rather than censused, assuring sampling error. Therefore, because these tests have been shown to be either invalid when only sampling variance occurs in the population abundances (Bulmer's R, Pollard et al.'s, and Dennis and Taper's tests) or lack power (Bulmer's R* test), little justification exists for use of such tests to support or refute the hypothesis of density dependence.

Spatially and temporally predictable fishcommunities on coral reefs

Abstract: Spatial and temporal variation in the distribution and abundance of site-attached fish species inhabiting small, isolated patches of coral reef has been attributed to variability in larval recruitment. However, the relative importance of settlement and postsettlement processes in determining the structure of fish communities in general, i.e., including non-site-attached species inhabiting large sections of contiguous reef, remains to be determined. Here, we examine the degree of spatial and temporal variation in community structure and population density of fish inhabiting sections of coral reef varying in size and connectivity. To investigate spatial variability in fish community structure and population density, we surveyed 36 sites on contiguous reef and 39 patch-reef sites varying in size and isolation from neighboring patches. Ordination and regression analyses indicated that the structure of fish assemblages inhabiting contiguous reef varied predictably along habitat gradients. In contrast, intrinsic habitat characteristics, such as the shelter availability and the composition of the substratum, were apparently unrelated to the structure of fish assemblages on patch reefs. For sites on contiguous reef, multiple regression models explained a significant proportion of spatial variation in the population density of 10 site-attached and vagile species (including 90% of variation in the density of Pomacentrus moluccensis, a site-attached damselfish). For patch-reef sites, models of spatial variation in population density were significant for six species, five of which were not site attached. The overall pattern across most species was that patch-reef models were characterized by a reduced r2 relative to corresponding models of contiguous-reef populations, but the reduction was substantially less for vagile species than for site-attached species. Ordination analysis of temporal variability in community structure over two years suggested that none of the fish communities at the sites examined was in a consistent state of community succession. For most sites, community structure varied randomly over time, although at some sites, the structure of resident fish communities tended towards a stable position in multivariate community space. Across all sites, temporal change in community structure was significantly higher during periods of recruitment than at other times of the year, although there was little evidence to suggest that recruitment was the sole source of temporal variation. At most sites, the structure of fish assemblages fluctuated considerably during nonrecruitment periods. Patterns of temporal variation in the population density of four site-attached species indicated that population increases corresponding with pulses of recruitment were modified by postsettlement processes. For site-attached and moderately vagile species, overall temporal variability in assemblage structure increased significantly as sites became smaller and more isolated. Temporal variability in assemblages of highly vagile species was unrelated to survey area and connectivity. Overall, the results of the analyses of spatial and temporal variability in fish community structure suggest that species' vagility and reef connectivity strongly influence the relative importance of recruitment and postrecruitment processes in determining local population density. Individuals of highly vagile species are able to move among isolated patches in response to habitat preferences or resource availability. Conversely, the continuous shelter provided by contiguous reef may allow sedentary species to migrate to more favorable areas. We suggest that for many fish species, including vagile species on patchy reef and site-attached species on contiguous reef, patterns in distribution and abundance established at recruitment are modified by postsettlement migration, which enhances the relationship between population density and habitat structure. Thus, while recruitment patterns may explain much of the spatial and temporal variation in populations of site-attached fish on small, isolated patch reefs, this result cannot necessarily be extrapolated to fish communities inhabiting large sections of contiguous reef.

Dynamics and viability of a metapopulation of the endangered iberian lynx (lynx pardinus)

Abstract: The use of metapopulation models in conservation biology is growing ex- ponentially, but there is a need for empirical studies that support theoretical approaches, especially for species with large and long-lived individuals. In this paper we explore the viability and dynamics of a real metapopulation of an endangered mammal by combining field work and modeling in order to support conservation decisions and evaluate theoretical approaches. The Iberian lynx (Lynx pardinus), considered the most vulnerable felid in the world, is restricted to the Iberian Peninsula in southwestern Europe. The persistence of the species is handicapped by the high fragmentation of its populations. Fewer than 1000 individuals are distributed in nine isolated populations, each of them also fragmented but with their patches connected by dispersers, in what could be called metapopulations. One of these metapopulations, including ;60 individuals, inhabits the Donana National Park (DNP) and its surroundings. Demographic and behavioral data gathered over one decade for this lynx population were employed to develop a spatially realistic structured model with density-dependent fecundity and migration, including demographic and environmental stochasticity. Such a model is used to identify the demographic features that determine the dynamics of this population and to predict its risk of decline under a set of alternative assumptions. A hypothetical lynx metapopulation with values of the parameters such as those observed in Donana, but without stochastic events, could sustain itself over time. Results of this deterministic model show how females occupy all the potential breeding territories, while males are below the carrying capacity. The metapopulation has a source- sink structure, with the sources internal and the sinks external to the national park. Sinks result from reduced survival rather than reduced fecundity, as generally assumed. High mortality in sink patches is deterministic, deriving both from within-patch risks and from factors related to the landscape matrix among patches. The survival rate of adults with territories in the sources was the most sensitive parameter, leading the dynamics of the metapopulation. When we include demographic stochasticity in the model, the population becomes extinct 22% of the time within 100 yr, and this value increases to 33.8% when environmental stochasticity is also considered. Most of the metapopulation extinctions occurred because of the disappearance of males due to sex differences in demographic parameters related to behavioral aspects (e.g., dispersal rate). Different scenarios were simulated as modifications affecting either within- or between-patch dynamics. Changes in the carrying capacity of source and sink patches would have very different consequences in terms of metapopulation persistence: one breeding territory increase in the largest source reduces metapopulation extinction risk from 33.8 to 17.2% in 100 yr, while an increase of three territories in the largest sink does not modify the extinction risk. In this sense, results suggest that the best management strategy for conservation should be restoring habitat at the source patches and reducing mortality at the sinks. The results of our models emphasize the need for empirical studies to characterize metapopulations in nature and distinguish between such terms as source-sink, mainland-island, nonequilibrium, or even ''refuge''

Scale-dependent variation in local vs. regional effects on coral species richness

Abstract: Large-scale geographical and historical factors can strongly affect species richness within ecological communities (local richness), because most communities are at least partially open to immigration from regional species pools. To completely understand local richness patterns, it is important to distinguish local explanations, such as spatiotem- poral heterogeneity, from larger scale explanations for regional enrichment, and to integrate these explanations with traditional niche-based notions (e.g., resource partitioning, limiting similarity, limited membership, etc.). Here we use regression models to evaluate the sen- sitivity of local richness in reef-building corals to two local and six regional variables at multiple geographical scales. The local variables are depth and habitat, and the regional variables are the number of species and genera in a region, average age of genera, distance to the equator, distance to the nearest high-diversity region, and a dummy variable con- trasting the Indo-Pacific and Atlantic provinces. At the largest geographical scale (Indo- Pacific and Atlantic provinces combined), we find that local richness varies significantly with depth, habitat, and five regional variables. The relative sensitivity of local richness to local and regional variables (61 and 39%, respectively) is similar to that at two smaller geographical scales: (1) the Indo-Pacific and (2) the speciose central Indo-Pacific. However, when only depauperate regions of the Indo-Pacific are considered, regional variables con- tribute more strongly to local richness, explaining 95% of the variation in this attribute. The analysis indicates that Indo-Pacific coral assemblages in both speciose and depauperate regions are unsaturated, as local richness consistently increases with the size of the regional species pool. Comparable regional effec'ts are totally absent in the Atlantic province, where local richness is sensitive only to depth and habitat. This result is consistent with the assessment that regional species distributions are relatively homogeneous within this prov- ince. We interpret this lack of regional heterogeneity as a historical consequence of climatic and geologic events, which isolated this province from the Indo-Pacific in the Pliocene and caused major extinctions among predominantly stenotopic species 1-4 X 106 yr BP. In general, our results indicate that geographical scale is an important factor to consider in comparative regional studies of ecological communities. Furthermore, we suggest that pure- ly local spatiotemporal heterogeneity is insufficient to explain the sensitivity of local rich- ness to large-scale geographical and historical variables.

Responses of early successional northern hardwood forests to changes in nutrient availability

Abstract: In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation...

Regeneration of three sympatric birch species on experimental hurricane blowdown microsites

Abstract: Tip-up mounds, pits, and other microsites created by hurricanes may promote diversity in many forests by providing opportunities for different species to regenerate. To see if we could detect differences in microsite preference among closely related species, we studied the regeneration of three sympatric Betula species on five types of microsites on experimental mound–pit complexes. Microsites were created by pulling down canopy trees to simulate damage from past hurricanes in southern New England. Seeds were collected in litter traps and experimentally released over mounds and pits to determine effects of microtopography on fine-scale dispersal patterns. The fate of naturally germinating seedlings was monitored on the disturbed site, and seedlings were also transplanted onto microsites to examine growth patterns, causes of mortality, and leaf-level physiology. Seed rain onto the disturbed site was abundant and spatially heterogeneous because of scattered residual canopy trees and surviving uprooted trees...

Effects of herbivory on arrowgrass: interactions between geese, neighboring plants, and abiotic factors

Abstract: Herbivores may affect plants by removing biomass, altering competitive interactions, and altering the abiotic environment. Changes in the size and quality of forage species and in species composition as a result of herbivory, in turn, affect future herbivory. We investigated the direct and indirect effects of herbivory by Brant Geese (Branta bernicla nigricans) on Triglochin palustris (arrowgrass) in a subarctic salt marsh in southwestern Alaska. In the first experiment, we compared arrowgrass in exclosed plots, unexclosed plots with feces removed, and control plots. In the second experiment, we used a full-factorial design to examine the effects of clipping arrowgrass, clipping neighboring plants, depositing goose feces, and their interactions on arrowgrass size and biomass allocation. In the third experiment, we placed hand-reared goslings on premanipulated plots from the second experiment to examine the effects of arrowgrass size, density, and species composition on the probability of an individual arrowgrass plant being grazed. For unclipped plants, feces deposition resulted in reduced bulb mass, reduced percentage of biomass in bulb and roots, and increased percentage of biomass in leaves, whereas feces deposition had no effect on clipped plants. Clipping neighbors resulted in increased arrowgrass root and stolon mass only for unclipped plants. Feces deposition resulted in increased vegetative reproduction when neighbors were clipped, but had no effect on vegetative reproduction when neighbors were not clipped. Plants in exclosed plots were larger, had greater allocation to leaves, had higher concentrations of C and N, and were more likely to flower than were plants in unexclosed plots. These results indicate an increase in competition for light with neighbor plants under feces deposition, which may be ameliorated by biomass removal of neighbors. Our results predict that an increase in grazing pressure is not necessarily detrimental to arrowgrass, provided that it is accompanied by increases in consumption of neighbor plants. The number of arrowgrass plants completely removed was not related to arrowgrass density, but the number of plants partially grazed increased with arrowgrass density. The probability that an individual arrowgrass plant would be grazed was negatively related to biomass or percent cover of several other species (Potentilla egedei, Chrysanthemum arcticum, Carex spp., Salix spp.). These results suggest that some neighbor species may provide a measure of protection (associational refuge) from herbivory, and that an increase in grazing intensity may have a strong negative effect on arrowgrass populations by reducing this protection and through an increased likelihood of consumption of the arrowgrass remaining after neighbors are removed. We conclude that the way in which we view neighboring plants (as competitors or potential protectors) affects our predictions regarding the effects of changing herbivore populations. Feedbacks from the plant community to herbivores following grazing should be included in studies that aim to extrapolate to the population level the effects of herbivory on individuals.

The effect of density dependence and weather on population size of a polyvoltine species

Abstract: The identification of what factors determine the population dynamics of polyvoltine species has been a difficult problem in ecology because population dynamics can contain intra- and interannual variability, and because the time scale at which factors affect the population is often unknown. We created a comprehensive population model to determine how density dependence (linear, nonlinear, and time-delayed) and weather affected the rate of population growth of white-footed mice (Peromyscus leucopus) in an isolated woodlot. We studied this nonoutbreak, polyvoltine species using a 257-mo data set spanning 23 yr, which incorporated both detailed intra-annual and long-term dynamics, and we used this model to forecast future population size. We then evaluated whether 3-yr spans of monthly data or a 22-yr span of annual data were better able to identify the key determinants that drive population dynamics, and we identified which data type created more accurate forecasts. The 257-mo comprehensive model determined...