摘要:

A spatial and mechanistic model is developed for the dynamics of transition oak—northern hardwoods forests in northeastern North America. The purpose of the model is to extrapolate from measurable fine—scale and short—term interactions among individual trees to large—scale and long—term dynamics of forest communities. Field methods, statistical estimators, and model structure were designed simultaneously to ensure that parameters could be estimated from data collected in the field. This paper documents eight aspects of a three—year study to calibrate, test, and analyze the model for the nine dominant and subdominant tree species in transition oak—northern hardwoods forests: 1) Design and structure of the model. The model makes population dynamic forecasts by predicting the fate of every individual tree throughout its life. Species—specific functions predict each tree's dispersal, establishment, growth, mortality, and fecundity. Trees occupy unique spatial positions, and individual performance is affected by the local availability of resources. Competition is mechanistic; resources available to each tree are reduced by neighbors. Although the model was developed to include light, water, and nitrogen, the version described here includes only competition for light (shading and light—dependent performance) because the field data provide little evidence of competition for nitrogen and water over the range of sites examined. 2) Estimates of the model's parameters for each species. The estimates reveal a variety of "strategic trade—offs" among the species. For example, species that grow quickly under high light tend to cast relatively little shade, have low survivorship under low light, and have high dispersal. In contrast, species that grow slowly under high light tend to cast relatively dark shade, and to have high survivorship under low light and low dispersal. These trade—offs define one of two dominant "axes" of strategic variation. 3) Community level predictions of the model. The model predicts succession from early dominance by species such as Quercus rubra and Prunus serotina, to late dominance by Fagus grandifolia and Tsuga canadensis, with Betula alleganiensis present as a gap phase species in old—growth stands. The model also predicts that old—growth communities will have intraspecifically clumped and interspecifically segregated spatial distributions. 4) An error analysis that identifies community level predictions that are robust given the level of sampling uncertainty in the study. This analysis translates the statistical uncertainty associated with each parameter estimate into statistical uncertainty in the model's predictions. The robust predictions include those mentioned in aspect (3) above. 5) Sensitivity of the model to changes in initial conditions and to changes in the three parameters not included in the error analysis. For example, the model predicts that initial abundances continue to affect community composition well into succession (>300 yr for some species). 6) Tests of the system— and community—level predictions of the model against independent data gleaned from other studies. These tests support the predictions found to be robust in the error analysis, including those predictions mentioned in aspect (3) above. 7) Modeling experiments that determine which aspects of individual performance and inter—neighbor competition are responsible for each of the robust predictions identified in aspect (4) above and tested in aspect (6) above. This analysis reveals a wide variety of causal relationships, with most parameters contributing to at least one community level phenomenon. 8) An explanation of the diversity of individual level causes identified in aspect (7). The two "axes" describing most of the strategic variation among the species (see [2]), provide a simple explanation of community level pattern in terms of individual level processes.

ISSN:0012-9615    

DOI:10.2307/2963479

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Studies across a range of spatial and temporal scales are needed to discern multiple forces structuring communities. Subtidal prop roots of red mangroves host diverse assemblages of sessile marine epibionts that provide a model system for examining community development and maintenance at a variety of discrete spatial scales. During 1991—1992 we twice surveyed 11 sites at four cays in Belize, Central America, to quantify spatial variability and temporal change in distribution and abundance of root—fouling organisms at five sampling scales: (1) fronts and backs of roots (1—cm scale); (2) roots close to and extending away from peat bank (0.5—m scale); (3) along linear transects parallel to shore (1—50 m scale); (4) on leeward and windward shores of cays (0.5—km scale); and (5) among cays (1—10 km scale). Although epibiont community structure differed widely among sites, all cays surveyed had similar seasonal values of water salinity, pH, and temperature. Within cays, windward sites had higher dissolved oxygen levels and water flow rates than leeward sites. At still smaller scales, outer roots and fronts of roots received significantly more light and were subject to higher water flow rates than inner roots and backs of roots. Species richness, diversity, and mosaic diversity patterns indicated that epibiont assemblages were distributed non—randomly in space: leeward sites were more speciose than windward sites, and fronts of roots were more speciose than backs. Jaccard's index of similarity, cluster analysis, and Kendall's coefficient of concordance showed hierarchical patterns of decreasing similarity with increasing sampling distance. Significant spatial autocorrelation among Jaccard values occurred at 2—3 m intervals, possibly reflecting mean larval dispersal distances. Analysis of mosaic diversity among sites indicated the absence of a clear environmental gradient and supported the hypothesis that species distributions may reflect patterns of dispersal from initial source populations. While precise identity of species was unpredictable among roots, species groups based on taxonomy, morphology, and life history showed very consistent distributions among sites that may reflect variability in local root environments: algae were most prevalent in well—lit areas and on windward sites, while sponges and ascidians predominated in leeward areas. Relative importance and dominance of both individual species and species groups changed substantially between 1991 and 1992. Representatives of four species groups were transplanted across three spatial scales to assess whether post—settlement dynamics limit distributions of these taxa. All transplants survived well for the first 6 wk of the experiment. After 6 mo, all transplants exhibited similarly high levels of mortality regardless of treatment. Overall, the results indicate that larval supply may shape epibiont community composition on short time scales and small and very large spatial scales, while variation in physical factors may influence distributions over the long term and at intermediate spatial scales.

ISSN:0012-9615    

DOI:10.2307/2963480

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Most general models do not include herbivory as a major agent of successional change. Potentially, herbivores can affect succession in three ways: accelerating or decelerating the rate of succession, where the sequence of dominant species is unaltered, or deflecting succession onto a new trajectory, where the species composition of dominants becomes substantially different than during ungrazed succession. We examined these alternatives for benthic algae on a coral—reef crest off Oahu, Hawaii. In this system, exposed coral—rock surfaces naturally undergo one of two major grazing regimes: (1) relatively protected inside defended territories of the damselfish Stegastes fasciolatus (Pomacentridae), where the benthos is dominated by filamentous algae; or (2) exposed to abundant schooling parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) outside territories, where the bottom is covered mostly by crustose algae. We compared the effects of this differential grazing on primary succession, relative to ungrazed succession, by distributing on the same date 1332 settling surfaces among three treatments: exposed inside damselfish territories, exposed just outside territories, and within fish—exclusion cages just outside territories. To balance the advantages and disadvantages of different settling surfaces, we used equal numbers of each of three kinds of 50—cm2settling plates: naturally contoured coral rock, coral rock cut into flat plates, and roughly sanded PVC plastic. To follow relative successional pathways, we sampled destructively 63 plates (21 from each grazing treatment) 17 times over 1 yr. Plates placed in the field several months before and after the main experiment suggested no seasonal differences in algal colonization. A concurrent cage—control experiment involving 144 settling plates, combined with measurements of light and water motion inside vs. outside cages, indicated that the secondary effects of cages were minor compared to the primary effect of preventing fish grazing. In the absence of fish grazing within cages, algal succession over the year followed three stages: early dominance by simple green and brown filaments (such as Enteromorpha rhizoidea and Ectocarpus indicus), a midsuccessional stage dominated by thin and finely branched red filaments (such as Centroceras clavulatum and Taenioma perpusillum), and a late stage dominated by blades and coarsely branched thick filaments (especially Tolypoicladia glomerulata). Species diversity followed a unimodal pattern during ungrazed succession, declining as a few species of late—stage algae predominated. Inside damselfish territories, succession was decelerated. The early stage was protracted and the midsuccessional stage, similar to natural assemblages inside territories, still dominated by the end of the year. Here, herbivory was of moderately destructive intensity (as measured by the density of fish bite marks that removed algal holdfasts) and fairly nonselective (as measured by comparisons of the gut contents of damselfish paired with samples of their algal mats). Algal biomass reached only about a quarter of what accumulated during ungrazed succession, but species diversity gradually increased through time. By the end of the experiment, algal species diversity was greatest inside damselfish territories compared to the other two grazing treatments. Outside territories, where grazing was destructively intense, resulting in the removal of all erect algae, succession was strongly deflected. The early stage was quickly replaced by a low—biomass and low—diversity assemblage of crusts (such as Hydrolithon reinboldii) and prostrate blue—green mats (such as Calothrix crustacea), characteristic of natural assemblages outside territories. Besides demonstrating the importance of herbivory during succession and providing insight on the mechanisms involved, these patterns have ramifications for explaining the maintenance of high local species diversity on coral reefs at two spatial scales. Between patches, differential grazing by territorial damselfish vs. schooling herbivores causes succession to follow different trajectories toward different algal assemblages. Within patches defined by damselfish territories, moderate grazing decelerates succession and prolongs a high—diversity mid—successional stage. Both these patterns provide an example of predation maintaining high local diversity in tropical systems, and indicate that territorial damselfish can function as keystone species on coral reefs.

ISSN:0012-9615    

DOI:10.2307/2963481

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

A series of time—specific food webs for the macroinvertebrate riffle community of Duffin Creek, Ontario was constructed using dietary information obtained from the analysis of gut contents. Trophic links were quantified using a dietary index of relative importance. Precision of the analysis was maintained at a high level by: (1) identifying dietary items as accurately as possible via direct gut analysis; (2) identifying web members to the species level, thus avoiding the taxonomic aggregation and lumping of size classes common in food web analyses; and (3) ensuring temporal resolution of the web by determining ontogenic variation in the diets of dominant members of the community. The Duffin Creek webs are heavily detritus—based with a large proportion of top—to—basal, and intermediate—to—basal links. Top—to—basal links, proportions of top and intermediate species, and lower connectance (0.180—0.219) varied temporally. Trophic connectance ranged from 0.090 to 0.109, consistent with values expected for a web consisting largely of specialist feeders. Weak links made up the largest proportion of total links in the webs, whereas very strong links made up the smallest proportion. Omnivory was more common than indicated in other webs and can be attributed to ontogenic diet switching. Comparison of the statistics for a summary web with those generated for the time—specific webs indicated that the total number of links per web, total number of species, number of top and intermediate species, and linkage density were much greater for the summary web. In view of these differences, the importance of temporal resolution when assessing food web structure and dynamics is emphasized. The possibility that some of the observed features in our web are common to other detritus—based webs is considered. Future studies of this calibre are justified.

ISSN:0012-9615    

DOI:10.2307/2963482

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Assessments of ecological risk require accurate predictions of contaminant dynamics in natural populations. However, simple deterministic models that assume constant uptake rates and elimination fractions may compromise both their ecological realism and their general application to animals with variable metabolisms or diets. In particular, the temperature—dependent metabolic rates characteristic of ectotherms may lead to significant differences between observed and predicted contaminant kinetics. We examined the influence of a seasonally variable thermal environment on predicting the uptake and annual cycling of contaminants by ectotherms, using a temperature—dependent model of137Cs kinetics in free—living yellow—bellied turtles, Trachemys scripta. We compared predictions from this model with those of deterministic negative exponential and flexibly shaped Richards sigmoidal models. Concentrations of137Cs in a population of this species in Pond B, a radionuclide—contaminated nuclear reactor cooling reservoir, and137Cs uptake by uncontaminated turtles held captive in Pond B for 4 yr confirmed both the pattern of uptake and the equilibrium concentrations predicted by the temperature—dependent model. Almost 90% of the variance in the predicted time—integrated137Cs concentration was explainable by linear relationships with model parameters. The model was also relatively insensitive to uncertainties in the estimates of ambient temperature, suggesting that adequate estimates of temperature—dependent ingestion and elimination may require relatively few measurements of ambient conditions at sites of interest. Analyses of Richards sigmoidal models of137Cs uptake indicated significant differences from a negative exponential trajectory in the 1st yr after the turtles' release into Pond B. We also observed significant annual cycling of137Cs concentrations, apparently due to temperature—dependent metabolism and its influence on ingestion and elimination rates. However, equilibrium concentrations of the radionuclide in the wild population were predictable from negative exponential models based on average annual temperature and its effects on intake and elimination rates, also suggesting that predicting ectotherm responses to long—lived contaminants (such as137Cs) may be possible without complex ecophysiological modeling.

ISSN:0012-9615    

DOI:10.2307/2963483

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY