Quantifying the importance of regional and local filters for community trait structure in tropical and temperate zones

The influence of regional and local processes on community structure is a major focus of ecology. Classically, ecologists have used local-regional richness regressions to evaluate the role of local and regional processes in determining community structure, an approach that has numerous flaws. Here, we implemented a novel trait-based approach that treats local and regional influences as a continuum, rather than a dichotomy. Using hylid frogs (Hylidae), we compared trait dispersion among members of local species assemblages to the trait dispersion in the regional assemblage from which they were drawn. Similarly, we compared trait dispersion in the regional assemblages to dispersion in the continental species pool. We estimated the contributions of local and regional filters, and we compared their strength in temperate and tropical zones. We found that regional and local filters explained 80% of the total variation among local assemblages in community body size dispersion. Overall, regional filters reduced trait dispersion, and local filters increased it, a pattern driven by particularly strong antagonistic effects in temperate zones that reduced the realized total variation by more than 40%. In contrast, local and regional filters acted in concert in tropical regions. Patterns within the tropics did not differ from the random expectation based on a null model, but within the temperate zone, local community filtering was stronger than expected by chance. Furthermore, in temperate regions, antagonistic regional and local filtering masked from 76% to 90% of the total variation in trait dispersion. Together, these results suggest that there are fundamental differences in the scale and identity of the processes determining community structure in temperate and tropical regions.     

Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community

Ecological communities are structured by both deterministic, niche-based processes and stochastic processes such as dispersal. A pressing issue in ecology is to determine when and for which organisms each of these types of processes is important in community assembly. The roles of deterministic and stochastic processes have been studied for a variety of communities, but very few researchers have addressed their contribution to insect herbivore community structure. Insect herbivore niches are often described as largely shaped by the antagonistic pressures of predation and host plant defenses. However host plants are frequently discrete patches of habitat, and their spatial arrangement can affect herbivore dispersal patterns. We studied the roles of predation, host plant quality, and host spatial proximity for the assembly of a diverse insect herbivore community on Quercus alba (white oak) across two growing seasons. We examined abundances of feeding guilds to determine if ecologically similar species responded similarly to variation in niches. Most guilds responded similarly to leaf quality, preferring high-nitrogen, low-tannin host plants, particularly late in the growing season, while bird predation had little impact on herbivore abundance. The communities on the high-quality plants tended to be larger and, in some cases, have greater species richness. We analyzed community composition by correlating indices of community similarity with predator presence, leaf quality similarity, and host plant proximity. Birds did not affect community composition. Community similarity was significantly associated with distance between host plants and uncorrelated with leaf quality similarity. Thus although leaf quality significantly affected the total abundance of herbivores on a host plant, in some cases leading to increased species richness, dispersal limitation may weaken this relationship. The species composition of these communities may be driven by stochastic processes rather than variation in host plant characteristics or differential predation by insectivorous birds.     

Neighborhood and community interactions determine the spatial pattern of tropical tree seedling survival

Factors affecting survival and recruitment of 3531 individually mapped seedlings of Myristicaceae were examined over three years in a highly diverse neotropical rain forest, at spatial scales of 1-9 m and 25 ha. We found convincing evidence of a community compensatory trend (CCT) in seedling survival (i.e., more abundant species had higher seedling mortality at the 25-ha scale), which suggests that density-dependent mortality may contribute to the spatial dynamics of seedling recruitment. Unlike previous studies, we demonstrate that the CCT was not caused by differences in microhabitat preferences or life history strategy among the study species. In local neighborhood analyses, the spatial autocorrelation of seedling survival was important at small spatial scales (1-5 m) but decayed rapidly with increasing distance. Relative seedling height had the greatest effect on seedling survival. Conspecific seedling density had a more negative effect on survival than heterospecific seedling density and was stronger and extended farther in rare species than in common species. Taken together, the CCT and neighborhood analyses suggest that seedling mortality is coupled more strongly to the landscape-scale abundance of conspecific large trees in common species and the local density of conspecific seedlings in rare species. We conclude that negative density dependence could promote species coexistence in this rain forest community but that the scale dependence of interactions differs between rare and common species.     

Size-mediated non-trophic interactions and stochastic predation drive assembly and dynamics in a seabird community

Theoretical and empirical evidence suggests that body size is a major life-history trait impacting on the structure and functioning of complex food webs. However, long-term analyses of size-dependent interactions within simpler network modules, for instance, competitive guilds, are scant. Here, we model the assembly dynamics of the largest breeding seabird community in the Mediterranean basin during the last 30 years. This unique data set allowed us to test, through a "natural experiment," whether body size drove the assembly and dynamics of an ecological guild growing from very low numbers after habitat protection. Although environmental stochasticity accounted for most of community variability, the population variance explained by interspecific interactions, albeit small, decreased sharply with increasing body size. Since we found a demographic gradient along a body size continuum, in which population density and stability increase with increasing body size, the numerical effects of interspecific interactions were proportionally higher on smaller species than on larger ones. Moreover, we found that the per capita interaction coefficients were larger the higher the size ratio among competing species, but only for the set of interactions in which the species exerting the effect was greater. This provides empirical evidence for long-term asymmetric interspecific competition, which ultimately prompted the local extinction of two small species during the study period. During the assembly process stochastic predation by generalist carnivores further triggered community reorganizations and global decays in population synchrony, which disrupted the pattern of interspecific interactions. These results suggest that the major patterns detected in complex food webs can hold as well for simpler sub-modules of these networks involving non-trophic interactions, and highlight the shifting ecological processes impacting on assembling vs. asymptotic communities.     

Distinct community assembly processes underlie significant spatiotemporal dynamics of abundant and rare bacterioplankton in the Yangtze River

• Season and landform influenced spatiotemporal patterns of abundant and rare taxa. • Different stochastic processes dominated abundant and rare subcommunity assembly. • River flow and suspended solids regulated assembly processes of rare taxa. The rare microbial biosphere provides broad ecological services and resilience to various ecosystems. Nevertheless, the biogeographical patterns and assembly processes of rare bacterioplankton communities in large rivers remain uncertain. In this study, we investigated the biogeography and community assembly processes of abundant and rare bacterioplankton taxa in the Yangtze River (China) covering a distance of 4300 km. The results revealed similar spatiotemporal patterns of abundant taxa (AT) and rare taxa (RT) at both taxonomic and phylogenetic levels, and analysis of similarities revealed that RT was significantly influenced by season and landform than AT. Furthermore, RT correlated with more environmental factors than AT, whereas environmental and spatial factors explained a lower proportion of community shifts in RT than in AT. The steeper distance–decay slopes in AT indicated higher spatial turnover rates of abundant subcommunities than rare subcommunities. The null model revealed that both AT and RT were mainly governed by stochastic processes. However, dispersal limitation primarily governed the AT, whereas the undominated process accounted for a higher fraction of stochastic processes in RT. River flow and suspended solids mediated the balance between the stochastic and deterministic processes in RT. The spatiotemporal dynamics and assembly processes of total taxa were more similar as AT than RT. This study provides new insights into both significant spatiotemporal dynamics and inconsistent assembly processes of AT and RT in large rivers.     

Quantifying the importance of patch-specific changes in habitat to metapopulation viability of an endangered songbird

A growing number of programs seek to facilitate species conservation using incentive-based mechanisms. Recently, a market-based incentive program for the federally endangered Golden-cheeked Warbler (Dendroica chrysoparia) was implemented on a trial basis at Fort Hood, an Army training post in Texas, USA. Under this program, recovery credits accumulated by Fort Hood through contracts with private landowners are used to offset unintentional loss of breeding habitat of Golden-cheeked Warblers within the installation. Critical to successful implementation of such programs is the ability to value, in terms of changes to overall species viability, both habitat loss and habitat restoration or protection. In this study, we sought to answer two fundamental questions: Given the same amount of change in breeding habitat, does the change in some patches have a greater effect on metapopulation persistence than others? And if so, can characteristics of a patch (e.g., size or spatial location) be used to predict how the metapopulation will respond to these changes? To answer these questions, we describe an approach for using sensitivity analysis of a metapopulation projection model to predict how changes to specific habitat patches would affect species viability. We used a stochastic, discrete-time projection model based on stage-specific estimates of survival and fecundity, as well as various assumptions about dispersal among populations. To assess a particular patch's leverage, we quantified how much metapopulation viability was expected to change in response to changing the size of that patch. We then related original patch size and distance from the largest patch to each patch's leverage to determine if general patch characteristics could be used to develop guidelines for valuing changes to patches within a metapopulation. We found that both the characteristic that best predicted patch leverage and the magnitude of the relationship changed under different model scenarios. Thus, we were unable to find a consistent set of relationships, and therefore we emphasize the dangers in relying on general guidelines to assess patch value. Instead, we provide an approach that can be used to quantitatively evaluate patch value and identify critical needs for future research.     

Nonredundancy in the dispersal network of a generalist tropical forest tree

Plant species with generalized dispersal mutualisms are considered to be robust to local frugivore extinctions because of redundancy between dispersal agents. However, real redundancy can only occur if frugivores have similar foraging and ranging patterns and if fruit is a limiting resource. We evaluated the quantitative and qualitative contributions of seed dispersers for an endochorus mast-fruiting species, Prunus javanica (Rosaceae) in Khao Yai National Park, Thailand, to evaluate the potential redundancy of dispersers. Data were collected from tree watches, seed/fruit traps, and seed transects under and away from fruiting trees, feeding and seed deposition by gibbons (Hylobates lar), and evaluations of seed and first-year seedling survival. We identified three clusters of dispersers within the network. Most (>80%) frugivore species observed were small birds and squirrels that were not functional dispersers, dropping most seeds under or very near the tree crown, where seedling survival was ultimately nil. Monkeys (Macaca leonina) were low-quality, short-range dispersers, but they dispersed large numbers of seeds and were responsible for 67% of surviving first-year seedlings. Gibbons and Oriental Pied Hornbills (Anthracoceros albirostris) handled few fruits, but they provided the highest quality service by carrying most seeds away from the canopy to medium and long distances, respectively. Although there was overlap in the deposition patterns of the functional dispersers, they displayed complementary, rather than redundant, roles in seed dispersal. Satiation of all functional dispersers further limited their capacity to "replace" one another. Redundancy must be evaluated at the community level because each type of disperser may shift to different species in the non-masting years of P. javanica. Our results underscore the need for research on broader spatial and temporal scales, which combines studies of dispersal and plant recruitment, to better understand mechanisms that maintain network stability.     

Spawning migrations and local movements of a tropical sparid fish

Between June 1994 and September 1996, a mark–recapture study was conducted in the Hinchinbrook Channel area of tropical northeastern Australia to investigate the movements of the sparid fish Acanthopagrus berda. A total of 962 A. berda were tagged, 922 within a single estuary system, Deluge Inlet. No movements among estuaries were detected. Of the fish tagged in Deluge Inlet, 12.9% were recaptured. Two movement regimes were detected: local movements (mostly <500 m) during non-spawning times, and longer migrations (up to 3.12 km) during spawning seasons, to and from a spawning site at the estuary mouth. Although there appeared to be a major spawning aggregation in the mouth of Deluge Inlet, ripe female A. berda were present in upstream sites, indicating that spawning probably occurs there also. This contrasts to the situation in South Africa, where A. berda is reported to spawn only close to estuary mouths. Differences such as this highlight our limited knowledge of variations in the reproductive biology of widespread species such as A. berda. Received: 25 November 1997 / Accepted: 10 September 1998     

Planktic community assembly in flowing water and the ecosystem health of rivers

Some of the modern criteria for assessing ecosystem health are compared with current understanding of ecosystem function in rivers. Owing to the predominance of catchment imports over autochthonous primary production, most rivers are naturally heterotrophic. This does not make them unhealthy but the pristine condition is that much harder to determine. The case is put for an index of ecosystem health and sustainability that takes into account the system's capacity for processing its resources, the species richness and its interdependence and its resilience to external forcing. Although these are not easily quantified, the qualitative indicators of healthy ecosystem function are easily checked. The sensitivity of organisms in suspension to fluvial flow may seem to counter the suitability of plankton as a reliable state indicator of river health. On the other hand, the rules governing the assembly of planktic communities in rivers are often strict and quantifiable: this makes them attractive candidates to act as indicators of the ecological condition of rivers.     

Facilitation across stress gradients: the importance of local adaptation

While there is some information on genetic variation in response to competition in plants, we know nothing about intraspecific variation in facilitation. Previous studies suggest that facilitation should increase fitness in stressful environments. However, whether a plant experiences an environment as stressful may depend on prior adaptive responses to stressors at a site. Local adaptation to stress at a site may reduce the likelihood of facilitation. Seeds of Plantago erecta from stressful (serpentine soil) and non-stressful (non-serpentine soil) edaphic environments were reciprocally planted into these two soil types. Although competition did not differ significantly among seed sources, there was evidence for a local adaptation effect on facilitation. Non-serpentine seeds planted into serpentine soil exhibited greater individual plant biomass at higher densities. The interaction between population source and growth environment indicates a role for evolutionary processes such as local adaptation in the expression of facilitation in plants.     

Carbohydrate storage and light requirements of tropical moist and dry forest tree species

In many plant communities, there is a negative interspecific correlation between relative growth rates and survival of juveniles. This negative correlation is most likely caused by a trade-off between carbon allocation to growth vs. allocation to defense and storage. Nonstructural carbohydrates (NSC) stored in stems allow plants to overcome periods of stress and should enhance survival. In order to assess how species differ in carbohydrate storage in relation to juvenile light requirements, growth, and survival, we quantified NSC concentrations and pool sizes in sapling stems of 85 woody species in moist semi-evergreen and dry deciduous tropical forests in the rainy season in Bolivia. Moist forest species averaged higher NSC concentrations than dry forest species. Carbohydrate concentrations and pool sizes decreased with the light requirements of juveniles of the species in the moist forest but not in the dry forest. Combined, these results suggest that storage is especially important for species that regenerate in persistently shady habitats, as in the understory of moist evergreen forests. For moist forest species, sapling survival rates increased with NSC concentrations and pool sizes while growth rates declined with the NSC concentrations and pool sizes. No relationships were found for dry forest species. Carbon allocation to storage contributes to the growth-survival trade-off through its positive effect on survival. And, a continuum in carbon storage strategies contributes to a continuum in light requirements among species. The link between storage and light requirements is especially strong in moist evergreen forest where species sort out along a light gradient, but disappears in dry deciduous forest where light is a less limiting resource and species sort out along drought and fire gradients.     

The vertical structure of a pelagic community in the tropical ocean

The vertical structure of the ecosystem in the euphotic zone of the western Equatorial Pacific has been studied. The studies were based on a continuous sounding of the bioluminescence field, with simultaneous, vertically aimed sampling made with the aid of a 5l water bottle and plankton nets. The bioluminescence field has a two-maxima structure with a more pronounced and permanent lower maximum found in the oligotrophic regions as deep as 60 to 100 m. In the narrow (10 m) layer of maximum bioluminescence, the concentration of zooplankton, as well as the concentration, activity and production of bacteria and phytoplankton, are several times higher than those in immediately adjacent waters at greater or lesser depth. At the same time, the concentration of nutrient salts in that layer diminishes sharply and approaches zero in the overlying water. Analysis of the data obtained shows that the production processes above the lower maximum layer are dependent not only on the turbulent influx of nutrient salts from the underlying layers, but also on the horizontal supply from the zone of upwelling. A pelagic community, in its different time aspects from the moment of water ascending until its sinking in the convergence zone, is suggested as a common system for modelling.     

Temporal turnover in the composition of tropical tree communities: functional determinism and phylogenetic stochasticity

The degree to which turnover in biological communities is structured by deterministic or stochastic factors and the identities of influential deterministic factors are fundamental, yet unresolved, questions in ecology. Answers to these questions are particularly important for projecting the fate of forests with diverse disturbance histories worldwide. To uncover the processes governing turnover we use species-level molecular phylogenies and functional trait data sets for two long-term tropical forest plots with contrasting disturbance histories: one forest is older-growth, and one was recently disturbed. Having both phylogenetic and functional information further allows us to parse out the deterministic influences of different ecological filters. With the use of null models we find that compositional turnover was random with respect to phylogeny on average, but highly nonrandom with respect to measured functional traits. Furthermore, as predicted by a deterministic assembly process, the older-growth and disturbed forests were characterized by less than and greater than expected functional turnover, respectively. These results suggest that the abiotic environment, which changes due to succession in the disturbed forest, strongly governs the temporal dynamics of disturbed and undisturbed tropical forests. Predicting future changes in the composition of disturbed and undisturbed forests may therefore be tractable when using a functional-trait-based approach.     

Exploring the role of local community perceptions in sustainability measurements

Measuring sustainability is an integral part of decision-making processes in order to promote sustainable development. The present paper focuses on sustainability indicators as these are measured on local level and explores two main issues: firstly, the subjective measurement of indicators focusing especially on social dimensions of sustainability, secondly, the incorporation of local perceptions in sustainability assessments. These two issues are explored in the Asopos River basin in Greece, an area where significant environmental degradation has been observed in the past decades and is also under financial pressure due to the ongoing national recession. A large-scale research study was conducted measuring environmental, economic and social indicators while, at a second stage, a model was developed, estimating new indicators that incorporate local communities’ perceptions on what they considered as important for their area. The results of the study reveal that the most important indicators for the sustainable development of the area, according to locals’ perceptions, are environmental quality as well as quality of life. By contrast, trust in local and central institutions and also local enterprises were not considered as important by locals. These results illustrate the importance of combining global and national scale assessment with locally focused social measurements of sustainability in order to better understand what is important for local communities prior to embarking on public policy planning.     

Genetic adaptation to local habitats and speciation processes within the genus Gammarus (Amphipoda: Crustacea)

A recent event of allopatric speciation (a time span of about 4 000 years) is reported from a comparison between two guilds of amphipod gammarids from the central Baltic Sea and from the Limfjord (inner Danish waters), respectively. For at least two of the five species populations involved, a genetically fixed habitat specialization has led to the establishment of reproductive barriers. It has, however, not been possible to correlate this speciation event with electrophoretically determined isozyme changes. It is argued that the lack of correlation is due to the presence of an initial time lag before a speciation event can be detected as isozyme differences between newly formed species populations as predicted by the neutrality hypothesis for evolution at the molecular level.     

Production of tropical copepods in Kingston Harbour, Jamaica: the importance of small species

The copepod community observed during an 18-month period at the mouth of eutrophic Kingston Harbour, Jamaica, was dominated by small species of Parvocalanus, Temora, Oithona, and Corycaeus. Mean copepod biomass was 22.1 mg AFDW m⁻³ (331 mg m⁻²). Annual production was 1679 kJ m⁻², partitioned as 174 kJ m⁻² naupliar, 936 kJ m⁻² copepodite, 475 kJ m⁻² egg and 93 kJ m⁻² exuvial production. All nauplii, most copepodites and many adults, equivalent to half of the biomass and production, were missed by a standard 200-μm plankton net, emphasizing the importance of nauplii and small species in secondary production estimates. The evidence suggests that growth rates and production are generally not food limited, and we speculate that size-selective predation shapes the structure of the harbour community. Biomass and production are higher than previous estimates for tropical coastal waters, but comparable to other eutrophic tropical embayments and many productive temperate ecosystems. Far from being regions of low productivity, tropical zooplankton communities may have significant production and deserve greater research attention than they currently receive. Received: 19 September 1997 / Accepted: 21 October 1997     

珠江三角洲冲积平原种植热带果树的土壤条件探讨

以广州市番禺区万顷沙十五涌的土壤为例，探讨珠江三角洲冲积平原种植热带果树的土壤条件。该地土壤位于珠江三角洲冲积平原，是由工人围海造田而成的，有关部门计划在此大面积种植木瓜、杨桃、番石榴等热带果树。通过在种植果树前进行土壤本底调查，摸清土壤养分含量状况，找出土壤存在问题，并结合果树生长的需要，提出土壤利用改良意见。降低地下水位，合理施肥，建立合理的间套作制度，是提高和稳定土壤肥力的基本措施。     

Perceptions of the local community on the outcome of community fishery management in Krala Peah village,Cambodia

Since fishery resources are regarded as an important source of income and food security, these resources need to be protected and conserved for sustainable use. To do this, the government of Cambodia has revised its policies, shifting management and responsibility to the local level. In response to government policy, Krala Peah village community fishery was established to manage resources within the area. The purpose of this study was to assess the outcome of community fishery management. Moreover, community fishery management was assessed through face-to-face interview and a participatory approach. The major findings of this study indicate that community fishery has led to a more equitable and efficient fishery within Krala Peah village. Although it has not led to improvements in fishing habitats and fish catch, it has reduced some factors that adversely affected sustainability. Furthermore, it has caused a reduction in illegal fishing, which was the main objective of the fishers.     

The importance of fires and floods on tree ages along mountainous gravel-bed streams.

This paper examines the commonly accepted assumption in the riparian literature that areas adjacent to streams do not burn. Using time-since-fire distributions, derived from stand-origin maps for a watershed in the front ranges of the Canadian Rocky Mountains, we found that the areas adjacent to streams and the whole study watershed have similar fire frequencies. In addition, the relative importance of fires and floods is regulated by a change in channel morphology associated with the creation of bars. The results demonstrate that fires solely control tree establishment along straight streams without bars, while the influence of floods is observed at the onset of lateral- and point-bar formation. This occurs because bars are formed in-channel and require smaller discharges in order to be flooded, compared to higher terraces. Consequently, bars are the only surfaces being flooded more frequently than they are being burned. Thus, overall the results indicate that, on this watershed, areas adjacent to streams are not less likely to burn than the uplands, except for lateral and point bars. The generality of these results to other systems should be tested as they have important implications for current forest ecological definition of "riparian zones," which typically include all fluvially derived landforms, from the channel banks to the terraces. Indeed, this study suggests that along smaller, headwater, gravel-bed mountain watersheds, the forests found on terraces are only influenced by fire and not fluvial processes and should therefore not be included in the riparian zone, while the forests on bars are the only surfaces currently being influenced by fluvial processes. Such a change in definition has implications for both ecologists and forest managers aiming to protect areas along streams as they now must take into account the effects of two disturbances on these small gravel-bed streams.     

The relative importance of host-plant genetic diversity in structuring the associated herbivore community

Recent studies suggest that intraspecific genetic diversity in one species may leave a substantial imprint on the surrounding community and ecosystem. Here, we test the hypothesis that genetic diversity within host-plant patches translates into consistent and ecologically important changes in the associated herbivore community. More specifically, we use potted, grafted oak saplings to construct 41 patches of four saplings each, with one, two, or four tree genotypes represented among the host plants. These patches were divided among two common gardens. Focusing first at the level of individual trees, we assess how tree-specific genotypic identity, patch-level genetic diversity, garden-level environmental variation, and their interactions affect the structure of the herbivore community. At the level of host-plant patches, we analyze whether the joint responses of herbivore species to environmental variation and genetic diversity result in differences in species diversity among tree quartets. Strikingly, both species-specific abundances and species diversity varied substantially among host-tree genotypes, among common gardens, and among specific locations within individual gardens. In contrast, the genetic diversity of the patch left a detectable imprint on local abundances of only two herbivore taxa. In both cases, the effect of genetic diversity was inconsistent among gardens and among host-plant genotypes. While the insect community differed significantly among individual host-plant genotypes, there were no interactive effects of the number of different genotypes within the patch. Overall, additive effects of intraspecific genetic diversity of the host plant explained a similar or lower proportion (7-10%) of variation in herbivore species diversity than did variation among common gardens. Combined with the few previous studies published to date, our study suggests that the impact of host-plant genetic diversity on the herbivore community can range from none to nonadditive, is generally low, and reaches its most pronounced impact at small spatial scales. Overall, our findings strengthen the emerging view that the impacts of genetic diversity are system, scale, and context dependent. As the next step in community genetics, we should then start asking not only whether genetic diversity matters, but under what circumstances its imprint is accentuated.