Diane Srivastava

Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Abiotic and biotic factors creating variation among bromeliad communities (2017)

Many ecological communities show variation from place to place; understanding the causes of this variation is the goal of community ecology. Differences in community composition will be the result of both stochastic and deterministic processes. However, it is difficult to know to what degree deterministic processes will shape community composition. In this thesis I combined observational and experimental approaches to quantify deterministic processes within a particular ecological community -- they phytotelmata of bromeliad plants. In my thesis I describe three studies at different scales of organization: 1) do organisms of different size respond equally to changes in their environment 2) how do predators interact to influence prey survival 3) what mechanisms underly the response of similar species to the same environmental gradient, bromeliad size. In Chapter 1, I tested an hypothesis developed from previous observational data -- that smaller organisms respond less than larger ones to the same environmental gradient -- different bromeliad species that occur under different forest canopies. After removing variation caused by dispersal, I found that environmental variation explained little variation for bacteria, more for zooplankton and most of all for macroinvertebrates. In my second chapter, I examined ecological determinism on a smaller scale -- within a single trophic level (macroinvertebrate predators). I found that predators may interfere with each other, reducing predation rates and increasing prey survival. In Chapter 3, I examine macroinvertebrate responses to bromeliad volume. I use both null models and a field experiment to show that for at least one such pair, a difference in abiotic tolerances may be the plausible mechanism. Together these results illustrate when, and to what degree, bromeliad communities respond to deterministic factors. All three chapters first demonstrate a pattern, testing it against a suitable null distribution, before attempting to quantify possible mechanisms with a field experiment. This combination of observation and experiment is an approach which can contribute to our understanding of how ecological systems work.

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The influence of spatial processes at multiple scales on local community structure and function (2014)

Spatial patterns, at multiple scales, can influence the functioning of local communities, but studies of community interactions are often conducted at a local scale. To investigate how spatial complexity at local, regional, and geographic scales influences a local aquatic macroinvertebrate community, I designed experiments using the natural mesocosm of bromeliad phytotelmata, quantifying community structure (functional diversity and trophic structure) and function (decomposition) of an entire food web in Puerto Rico, Costa Rica, and Brazil. At the local scale I studied the effects of habitat patch size on trophic structure and decomposition rates at all three sites. The relationship between decomposition and bromeliad size varied by site. In Costa Rica, size-sensitive top predators drove strong top-down control of leaf decomposition in larger bromeliads causing a negative relationship between decomposition and bromeliad size. In Puerto Rico this relationship was positive, as the lack of a large top predator allowed reduced top-down control in larger bromeliads. In Brazil, climatic conditions potentially shifted decomposition processes to microbial control, and no relationship between bromeliad size and decomposition rate existed. At the regional scale, by enclosing varying numbers of bromeliads I determined how functional traits of species determined extinctions within the metacommunity. Small metacommunities became less diverse and local extinctions were more stochastic than in large ones. Large-bodied predators and invertebrates requiring resources in the terrestrial matrix were most sensitive to metacommunity size. At the geographic scale, I combined experimental replicates in each site and community-analogues in common sites to identify mechanisms of context-dependency of community structure and function relationships among study sites. Top-down control of detritivore communities by predators was influenced by geographic differences in species traits, whereas cascading effects of predators on decomposition were influenced by geographic variation in environmental conditions. With these studies I was able to identify relationships between community functions and spatial processes operating at a hierarchy of spatial scales. I identified how biogeographical shifts in climate and large-scale connectivity among study sites can influence traits in the species pool which can affect both metacommunity dynamics and sensitivity of species to local extinctions due to habitat patch size.

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Distributions and interactions of insect herbivores as influences on host plant density and performance (2012)

Biological control programmes to reduce the density of invasive weeds often introduce multiple species of insect herbivores before reductions of the weed population occur. The factors leading to successful control need to be identified to improve success rates and reduce the number of insect introductions. These factors may be traits of the biocontrol agent-plant system, or may be external influences such as the presence of other biocontrol agents or environmental conditions.A characteristic that varies among insect herbivores is their distribution among plants. I developed a simulation model that demonstrates how insects distributed in direct proportion to plant density cause the fastest rate of host plant population decline. This agrees with observations of a successful species, Larinus minutus, compared to an unsuccessful one, Urophora affinis, in the diffuse knapweed (Centaurea diffusa) biocontrol system.Using the successful biocontrol agent, L. minutus, I experimentally tested whether adult as well as larval feeding increased control of diffuse knapweed. My results suggest that agents with both adult and larval feeding can control plants in a wider range of environments.With a meta-analysis I tested if the presence of other biocontrol agents alters the relationship that a natural enemy has with the host-plant, and showed that the reduction in plant performance caused by one natural enemy is, on average, independent of the second. I also show that the occurrence of non-independent interactions between natural enemies can be predicted by a small range of enemy or plant attributes. (direct interactions and attack to reproductive plant parts). My experiment using the root herbivore, Cyphocleonus achates and the aboveground herbivore L. minutus showed that interactions between the two species can increase the reduction in seed produced by the plant, potentially enhancing biocontrol.My thesis demonstrates that attributes of the biocontrol agents themselves and their interactions with traits of other agents can alter plant performance or plant population decline. Research into attributes associated with successful, and unsuccessful, biocontrol will improve success rates.

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Notes on species coexistence, invasion and ecosystem function (2008)

Species coexistence theory attempts to explain and predict the mechanisms that promote speciesdiversity and the ecological consequences of this diversity. In this thesis I used observational andexperimental field studies to test the predictions of several coexistence theories, and developedspecific predictions in a theoretical study. The observational study was used to test generalpredictions made by two mechanisms, neutral interactions and spatial niche partitioning, usingbromeliad-dwelling mosquito larvae in Costa Rica. Results from this study were only consistentwith spatial niche partitioning, and showed how local, within-bromeliad interactions could scaleup to meso-scale (among-bromeliad) distributions. The experimental study, based in the borealforest understory, used the standard rank-abundance relationships of plant species to test bothcompetitive and facilitative coexistence mechanisms that differentiate between the effects ofdominant species and species diversity. In particular, removals of a consistent biomass thattargeted one dominant or many low-abundance species were used in conjunction with seedlingadditions to test the roles of different species, and species diversity, in limiting the establishmentof new species. High mortality of new seedlings in completely cleared areas indicated thatfacilitation was important. However, small-scale disturbances (7% of community biomassremoved) either had no effect on seedling survival, or increased survival, indicating competitiveeffects. These competitive effects were limited to a single dominant species, and wereinconsistent with current models of resource niche partitioning. The theoretical study usedcomputer simulations to investigate the effects of regional habitat heterogeneity on localdiversity in communities that differed in their connectivity (dispersal among patches) andneutrality (niche overlap among species). The model suggested that dispersal and niche overlaphave synergistic effects on local diversity by increasing the size of sink populations, and likewisedestabilizing coexistence regionally. However, they have opposite effects on resource-use,causing either positive or negative diversity to resource-use relationships. Together, the threestudies illustrate that different processes can scale up to general patterns of species distributions,but that these processes have very different implications for understanding and protecting speciesdiversity and the functions this diversity may provide.

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Trophic Effects on Nutrient Cycling (2008)

No abstract available.

Master's Student Supervision (2010 - 2018)
Non-linear ecosystems : intertwined consumptive and non-consumptive pathways following increase in predator habitat (2018)

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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Forest fragmentation changes macroinvertebrate community composition in neotropical treeholes (2016)

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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Direct and indirect effects of drought on community structure and ecosystem processes in an aquatic ecosystem (2014)

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 and distribution in Southwestern Canada (2013)

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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The effects of tropical forest fragmentation and land use on leaf litter decomposition and nutrient cycling (2013)

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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Interactive effects of dispersal rate and disturbance synchrony on microarthropod diversity at multiple spatial scales : rescue effects do not increase regional richness (2012)

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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Losses of rare forest invertebrates and divergent rates of litter decomposition under different land uses (2012)

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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