Diane Srivastava

Habitat is an area that allows a species to survive and thrive. Habitat loss and habitat modification are considered two major threats to species persistence. Habitat predictors are environmental conditions which dictate patterns in species occupancy and thereby community distribution. Understanding habitat predictors at multiple spatial scales can aid managers in directing conservation measures, predicting effects of habitat modification and advancing theoretical knowledge about the effects of habitat fragmentation. Habitat predictors of ground-dwelling Neotropical vertebrates at multiple spatial scales, especially landscape scales, are often poorly known. To fill this knowledge gap, I conducted a study in a fragmented forest landscape in Guanacaste Province, Costa Rica. In the 1960s, continuous tropical wet forests in my study area were burnt to clear land for cattle ranching. This process modified swathes of wet forests into smaller fragments surrounded by human-use areas. I chose 19 forests in this landscape situated in and around the Área de Conservacíon Guanacaste (ACG), a UNESCO World Heritage Site, documented vertebrates using camera traps and calculated habitat variables at the three spatial scales of camera trap point, forest and landscape using field measurements and remote sensing imagery. We documented 32 species of ground-dwelling vertebrates and calculated 13 aspects of the vertebrate community as response variables. We tested the ability of 12 habitat variables to explain variation in the community response variables using linear mixed effect modelling in an AIC-based model averaging framework. Our results show that different scales of habitat affect different aspects of the vertebrate community, highlighting a need for examination of multi-scale habitat variables. Habitat predictors at landscape scales were important to the widest range of vertebrate response variables. Our key results highlight that threatened species associated with areas of continuous forests and species richness were highest in forests surrounded by plantation matrix as opposed to pasture. Detections of species at higher trophic levels (e.g. large carnivores) increased with the amount of forested area within 2km of camera traps. Our study thus highlights the need to examine various aspects of a vertebrate community, not just species richness, in order to understand in-depth, the effects of habitat change.

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The extent to which the diversity in habitat of an ecological system, or habitat heterogeneity, affects communities is an important aspect of ecology. Although higher habitat diversity is generally linked to higher species richness, how the increase in specific groups affects trophic dynamics in a system remains elusive. Studying biotic habitat modification, the formation of habitats by species and the subsequent alterations in persistence of other local species, can shed light on this topic. As biotic habitat modification tends to increase abundance and/or richness of specific taxa, the impacts of the increase of these specific groups can be analyzed. Tank bromeliads are common epiphytic biotic habitat modifiers throughout the Neotropics, and are facultatively associated with a variety of predatory arthropods. Here, we examine the community consequences of bromeliad-mediated increase in predator habitat in Costa Rican orange trees. As the effects of predation can attenuated by a variety of ecological processes, such as facilitative interactions, we examined two different aspects of community modification. First, in a manipulative experiment, we tested the hypothesis that bromeliads mediate trophic cascades in their support tree communities. In that view, we manipulated bromeliad densities on trees, and measured impacts on arboreal and bromeliad invertebrate communities, and leaf damage. Second, in an observational survey, we examined if the presence of bromeliads in a tree modified the behaviour of arboreal invertebrates, and interspecific interactions. We found that habitat modification by bromeliads was highly contingent on season and time of the day, and influenced arboreal invertebrate communities in various ways. Predators were more numerous with higher bromeliad densities, but effects on herbivores differed. Bromeliads did not modify cascading dynamics, and only weakly affected community structure, suggesting that invertebrate communities in orange trees can be functionally resilient. Even if bromeliads largely harboured predators, several groups of herbivores were commonly encountered in the epiphytes, plausibly attenuating cascading effects. The bromeliad-associated ants that dominated our system were strongly associated with honeydew-producing homopterans, suggesting less reliance on predation. These results suggest that the impacts of increased habitat heterogeneity on trophic dynamics can be attenuated by functional resilience and facilitative dynamics.

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Habitat loss and fragmentation are the main drivers of biodiversity loss in terrestrial ecosystems, particularly in the tropics. Fragmented habitats can interfere with organism dispersion, population persistence and ecosystem functions but empirical studies report variation in the sensitivity of species and processes to fragmentation and the mechanisms operating behind observed responses remain poorly understood. In this study we examined the effects of forest fragmentation on the colonization of artificial treeholes in northwestern Costa Rica by measuring the responses at the community level. We explored four potential mechanisms driving differences in macroinvertebrate communities between continuous and fragmented forests: dispersal limitation, microclimate changes and bottom up or top down effects. Macroinvertebrate community composition differed significantly between continuous forests and forest remnants but not in the predicted direction. Our results suggest that treeholes in fragmented forest contain higher abundance of detritivores and experience changes in predator species identity consistent with increased nutrient input and a potential relaxation of predation pressure in small forest remnants. An overall resilience of treehole communities to forest fragmentation is interpreted with care as time-delayed responses to fragmentation continue to be a possibility. These findings advance our understanding of the response of biological communities to forest fragmentation and emphasize the value of preserving even small forest remnants for biodiversity conservation.

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A major challenge of ecologists is to discover general mechanisms that explain how climate shapes ecological communities and ecosystems. Efforts have traditionally focused on direct effects, but a growing body of evidence suggests that indirect effects of climate, via altering species interactions, may be more important. Predators are often particularly vulnerable to environmental stress, thus effects of climate may cascade through ecosystems by altering top-down trophic interactions. In Costa Rica, where climate change is predicted to decrease the amount of precipitation, bromeliads contain aquatic insect food-webs largely controlled by the top predator, damselfly nymphs. Community composition varies with bromeliad size. Notably, top predators occur only in large bromeliads, possibly because the probability of drought stress decreases with bromeliad size. Thus, bromeliads are ideal systems to study the relative importance of direct and indirect effects of climate on community and ecosystem responses. To determine whether climate governs community composition directly, I regressed insect drought sensitivity, which I obtained experimentally, against insect habitat size sensitivity, which I calculated from observational data. To examine the importance of indirect drought effects from altered trophic interactions, I experimentally manipulated trophic composition and drought in mesocosms mimicking a single bromeliad leaf well and measured changes in community composition, decomposition, and water quality. Climate directly governed community composition at the scale of the bromeliad, as drought sensitivity strongly predicted habitat size sensitivity. At the scale of the leaf well, drought altered community composition and ecosystem properties indirectly by reducing top-down control from the top predator. Moreover, indirect effects of drought cascaded through the food-web to affect ecosystem functioning (decomposition) and state (water quality). These findings suggest that in complex habitats, such as bromeliads, direct (physiological) effects of climate may sufficiently explain community composition. However, in isolated habitats, such as a single leaf well in which dispersal is hindered, indirect effects of climate, via altered trophic interactions, may emerge and cascade through the ecosystem. The relative importance of direct and indirect effects of climate may thus depend on habitat scale.

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Microarthropod diversity patterns were investigated in southwestern British Columbia, Canada. We surveyed soil microarthropods associated with moss carpets on exposed rocky outcrops. Our survey identified 352 morphospecies in 32 sites spanning a 130 km 60 km area. Previous studies have interpreted strong correlations of species composition with environmental factors as evidence of niche limitation, and strong correlations with spatial factors as evidence of dispersal limitation. Here, we examine 18 ecological variables relevant to either spatial location or environmental aspects of ecological processes, and evaluate their influences on the microarthropod community. We tested whether the relative importance of spatial and environmental factors was concordant between various community attributes including composition, abundance and species richness, and between different taxonomic groups of microarthropods (Oribatida, Mesostigmata, Collembola). We used two different methods (distance-based Mantel and raw data-based ordination methods) to show that spatial variables could not explain composition or compositional turnover for most microarthropod groups, except Collembola. Dispersal limitation of Collembola is surprising given the high dispersal ability of this group. Although environmental factors explained a large amount of spatial variance in composition (raw data-based ordination method) for all microarthropod groups, environmental similarity (distance-based Mantel method) was a poor predictor of compositional similarity for Oribatida and Mesostigmata. Total abundance and species richness could also be explained by combinations of environmental factors, particularly those relating to tree cover and soil-relevant microhabitat variables (i.e, water content/mass, total soil mass and particle mass), but total abundance and richness were themselves only weakly correlated across space. The most important environmental influences on microarthropod communities were tree cover and water mass, followed by distance-to-sea. At the same time, there was a lot of unexplained variance in the composition of microarthropod communities (especially for species incidences) which could not be explained by the available ecological variables. As richness hotspots were dispersed across different habitats for different taxonomic groups, we suggested that species interactions might be equally important as environmental filtering and spatial autocorrelation in shaping microarthropod community structure, especially for patterns in species incidence.

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Forest conversion and fragmentation are major causes of diminished ecosystem function and biodiversity loss at a global scale. Understanding landscape effects on leaf litter decomposition and recycling of available nutrients is critical for conservation, especially in diverse tropical rainforests. Here, we studied the effects of land conversion and forest fragmentation on these two processes in a tropical ecosystem. We examined decomposition and nutrient cycling during the wet season in 22 sites across four habitat types: continuous forests, large fragmented forest patches, small fragmented forest patches, and orange plantations, in the tropical moist forest zone of NW Guanacaste, Costa Rica. The study employed a two-by-two factorial design to explore the effects of leaf litter type (site-specific litter vs. control litter) and mesh size (9 mm vs. 1 mm) on decomposition rate across land cover types. Litter bags with different mesh sizes either included (9 mm) or excluded (1 mm) macroinvertebrates. We removed litter bags from the field after 3 and 8 weeks, and cleaned and weighed the contents. Additionally, we utilized Plant Root Simulator (PRS™) Probes to examine nutrient cycling of 10 cations and anions including N, P, K, Ca, and Mg in all site-specific litter types and both mesh treatments. We found that the rate of decomposition differed depending on habitat type, leaf litter type and mesh size. Decomposition was faster in orange plantations compared to any other forest type, and decomposition was faster in forest fragments compared to continuous forests. We determined that these differences in decomposition rates were due predominantly to differences in site-specific litter quality. Only in forest fragments did the distance to forest edge impact macroinvertebrate feeding behaviour (seen in control litter), presumably due to spatial variation in leaf litter quality. Finally, mineralization of organic nutrients differed across the four habitat types. For example, nitrogen, which is an important compound for plant cell structure and function, mineralized more quickly in orange plantations than in forest habitats. This study highlights the importance of litter quality differences due to fragmentation and land use in altering leaf litter decomposition and nutrient cycling.

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Immigration from surrounding areas is known to benefit community recovery following disturbance. However, whether such rescue effects can increase regional diversity in landscapes subject to patchy disturbance has rarely been investigated experimentally. This study manipulated landscape connectivity and disturbance synchrony in moss patches, and examined the consequences for the embedded microarthropod communities. We assembled metacommunities with two patches connected by 0, 1, or 2 corridors. Patches were left undisturbed or were disturbed asynchronously or synchronously by Tullgren funnel extraction. We found evidence of rescue effects at the local-scale for microarthropods overall, especially for the oribatids and collembolans. However these rescue effects did not result in greater regional diversity for several reasons. First, rescue effects only occurred after one of the disturbances so they did not increase average local-scale diversity and any increase in local-scale diversity was offset by increasing compositional similarity across the landscape. Competitive and trophic interactions may have diminished the expected benefit of connectivity for diversity. Finally, disturbance and isolation did not always decrease local diversity as expected, suggesting a strong role for context-dependent dispersal behaviour and in situ recovery processes. Our results indicate that rescue effects mediated by habitat corridors can improve recovery of local community diversity following disturbance, but they may not be effective at maximizing regional diversity. Predator diversity may be more likely to benefit from rescue effects than prey diversity at the regional scale.

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Habitat destruction and fragmentation are dominant disturbances in tropical landscapes, but consequences of these changes for invertebrates and ecosystem functioning are poorly described and explained. In northwestern Costa Rica I used pitfall sampling and litter bag experiments to investigate consequences of two land-use changes (forest conversion to orange groves and forest fragmentation) for litter invertebrates and decomposition. I infer effects of forest conversion and fragmentation based on comparisons of intact forest with orange groves and forest fragments, respectively. Invertebrate diversity differed among habitat types. Invertebrate family richness and evenness in orange groves were 24% and 56% lower, respectively, relative to intact forest. Beta diversity (dissimilarity in invertebrate composition) among orange groves was high, likely due to variation in microclimate with grove age and/or management regime. Forest patch diversity was similar to that of intact forest, and composition was marginally more dissimilar between forest patches than between intact forest sites. Consistency in local richness between intact and fragmented forest was largely attributed to a suite of disturbance-adapted taxa detected exclusively in forest patches. Approximately 11% of the families that were naturally common in intact forest were rare or range-limited in forest fragments. These results emphasize the need for large forest reserves to prevent considerable losses of intact forest fauna. Losses of intact forest invertebrates in both orange groves and forest patches were explained by habitat modification (increases in litter temperature) and were more likely for families that are naturally rarer. Forest conversion and fragmentation had divergent effects on litter decomposition. During the wet season, decomposition was 9% faster in orange groves relative to forest. This pattern was explained by higher temperatures and lower litter cover in orange groves; I discuss both indirect and direct microenvironment mechanisms. In contrast, dry season decomposition rates were 7% slower in forest fragments than those in intact forest. Fragmentation effects on decomposition were explained by the action of shredder and/or saprophagous macroinvertebrates, which enhanced decomposition rates in intact forest but not in forest patches. The seasonal aspect of these results emphasizes the importance of accounting for intra-annual variation when assessing disturbance effects in natural systems.

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