Global Change and Trophic Interaction Diversity: Complex Local and Regional Processes

The structure and functioning of ecosystems across the globe are rapidly changing due to several components of global environmental change (GEC). My dissertation aims to illustrate how regional and local aspects of GEC impact diverse assemblages of species and species interactions. All organisms are embedded in complex networks of species interactions, and future efforts to predict and mitigate the impacts of GEC on ecological communities will be facilitated by such studies that incorporate a suite of species and species interactions. This study advances our understanding of how GEC will impact ecological communities by investigating two questions about GEC: 1) How will shifts in global climate cycles (e.g., El Nino Southern Oscillation), as a consequence of global warming, impact a diverse assemblage of butterflies that exist across a heterogeneous landscape? 2) What are the consequences of woody plant encroachment on complex, specialized interactions between plants, insect herbivores, and natural enemies (e.g., insect parasitoids)? Furthermore, I helped develop a tool to identify characteristics of ecological communities that are essential for promoting the diversity of trophic interactions. While the loss of species diversity is well recognized, interactions among species are vanishing at an astonishing rate, yet we know little about factors that determine the diversity of interactions within a community. Using data from a long-term butterfly monitoring dataset, I was able to demonstrate the utility of large-scale climate indices (e.g., ENSO) for modeling biotic/abiotic relationships for migratory butterfly species. Next, I used encroaching juniper woodlands in the Intermountain West to uncover that population age structure of dominant tress, such as juniper, can affect plant-insect dynamics and have implications for future control efforts in the expanding woodlands. Additionally, reductions of understory plant diversity, as a consequence of juniper expansion, resulted in significantly lower parasitism rates and parasitoid species diversity. Finally, simulated food webs revealed that species diversity and, to a lesser degree, consumer diet breadth, promote the diversity of trophic interactions. As ecosystems across the globe experience changes and the loss of species diversity continues, these findings offer insight into how GEC will impact species and species interactions

Global Change and trophic interaction diversity: complex local and regional processes

The structure and functioning of ecosystems across the globe are rapidly changing due to several components of global environmental change (GEC). My dissertation aims to illustrate how regional and local aspects of GEC impact diverse assemblages of species and species interactions. All organisms are embedded in complex networks of species interactions, and future efforts to predict and mitigate the impacts of GEC on ecological communities will be facilitated by such studies that incorporate a suite of species and species interactions. This study advances our understanding of how GEC will impact ecological communities by investigating two questions about GEC: 1) How will shifts in global climate cycles (e.g., El Nino Southern Oscillation), as a consequence of global warming, impact a diverse assemblage of butterflies that exist across a heterogeneous landscape? 2) What are the consequences of woody plant encroachment on complex, specialized interactions between plants, insect herbivores, and natural enemies (e.g., insect parasitoids)? Furthermore, I helped develop a tool to identify characteristics of ecological communities that are essential for promoting the diversity of trophic interactions. While the loss of species diversity is well recognized, interactions among species are vanishing at an astonishing rate, yet we know little about factors that determine the diversity of interactions within a community. Using data from a long-term butterfly monitoring dataset, I was able to demonstrate the utility of large-scale climate indices (e.g., ENSO) for modeling biotic/abiotic relationships for migratory butterfly species. Next, I used encroaching juniper woodlands in the Intermountain West to uncover that population age structure of dominant tress, such as juniper, can affect plant-insect dynamics and have implications for future control efforts in the expanding woodlands. Additionally, reductions of understory plant diversity, as a consequence of juniper expansion, resulted in significantly lower parasitism rates and parasitoid species diversity. Finally, simulated food webs revealed that species diversity and, to a lesser degree, consumer diet breadth, promote the diversity of trophic interactions. As ecosystems across the globe experience changes and the loss of species diversity continues, these findings offer insight into how GEC will impact species and species interactions

Synchronous population dynamics in California butterflies explained by climatic forcing

A long-standing challenge for population biology has been to understand why some species are characterized by populations that fluctuate in size independently, while populations of other species fluctuate synchronously across space. The effects of climatic variation and dispersal have been invoked to explain synchronous population dynamics, however an understanding of the relative influence of these drivers in natural populations is lacking. Here we compare support for dispersal-versus climate-driven models of interspecific variation in synchrony using 27 years of observations of 65 butterfly species at 10 sites spanning 2750m of elevation in Northern California. The degree of spatial synchrony exhibited by each butterfly species was used as a response in a unique approach that allowed us to investigate whether interspecific variation in response to climate or dispersal propensity was most predictive of interspecific variation in synchrony. We report that variation in sensitivity to climate explained 50% of interspecific variation in synchrony, whereas variation in dispersal propensity explained 23%. Sensitivity to the El Nino Southern Oscillation, a primary driver of regional climate, was the best predictor of synchrony. Combining sensitivity to climate and dispersal propensity into a single model did not greatly increase model performance, confirming the primacy of climatic sensitivity for driving spatial synchrony in butterflies. Finally, we uncovered a relationship between spatial synchrony and population decline that is consistent with theory, but small in magnitude, which suggests that the degree to which populations fluctuate in synchrony is of limited use for understanding the ongoing decline of the Northern California butterfly fauna

Synchronous population dynamics in California butterflies explained by climatic forcing.

A long-standing challenge for population biology has been to understand why some species are characterized by populations that fluctuate in size independently, while populations of other species fluctuate synchronously across space. The effects of climatic variation and dispersal have been invoked to explain synchronous population dynamics, however an understanding of the relative influence of these drivers in natural populations is lacking. Here we compare support for dispersal- versus climate-driven models of interspecific variation in synchrony using 27 years of observations of 65 butterfly species at 10 sites spanning 2750 m of elevation in Northern California. The degree of spatial synchrony exhibited by each butterfly species was used as a response in a unique approach that allowed us to investigate whether interspecific variation in response to climate or dispersal propensity was most predictive of interspecific variation in synchrony. We report that variation in sensitivity to climate explained 50% of interspecific variation in synchrony, whereas variation in dispersal propensity explained 23%. Sensitivity to the El Niño Southern Oscillation, a primary driver of regional climate, was the best predictor of synchrony. Combining sensitivity to climate and dispersal propensity into a single model did not greatly increase model performance, confirming the primacy of climatic sensitivity for driving spatial synchrony in butterflies. Finally, we uncovered a relationship between spatial synchrony and population decline that is consistent with theory, but small in magnitude, which suggests that the degree to which populations fluctuate in synchrony is of limited use for understanding the ongoing decline of the Northern California butterfly fauna

Global weather and local butterflies: variable responses to a large-scale climate pattern along an elevational gradient

Understanding the spatial and temporal scales at which environmental variation affects populations of plants and animals is an important goal for modern population biology, especially in the context of shifting climatic conditions. The El Nino Southern Oscillation (ENSO) generates climatic extremes of interannual variation, and has been shown to have significant effects on the diversity and abundance of a variety of terrestrial taxa. However, studies that have investigated the influence of such large-scale climate phenomena have often been limited in spatial and taxonomic scope. We used 23 years (1988-2010) of a long-term butterfly monitoring data set to explore associations between variation in population abundance of 28 butterfly species and variation in ENSO-derived sea surface temperature anomalies (SSTA) across 10 sites that encompass an elevational range of 2750 m in the Sierra Nevada mountain range of California. Our analysis detected a positive, regional effect of increased SSTA on butterfly abundance (wetter and warmer years predict more butterfly observations), yet the influence of SSTA on butterfly abundances varied along the elevational gradient, and also differed greatly among the 28 species. Migratory species had the strongest relationships with ENSO-derived SSTA, suggesting that large-scale climate indices are particularly valuable for understanding biotic-abiotic relationships of the most mobile species. In general, however, the ecological effects of large-scale climatic factors are context dependent between sites and species. Our results illustrate the power of long-term data sets for revealing pervasive yet subtle climatic effects, but also caution against expectations derived from exemplar species or single locations in the study of biotic-abiotic interactions

Synchronous population dynamics in California butterflies explained by climatic forcing.

A long-standing challenge for population biology has been to understand why some species are characterized by populations that fluctuate in size independently, while populations of other species fluctuate synchronously across space. The effects of climatic variation and dispersal have been invoked to explain synchronous population dynamics, however an understanding of the relative influence of these drivers in natural populations is lacking. Here we compare support for dispersal- versus climate-driven models of interspecific variation in synchrony using 27 years of observations of 65 butterfly species at 10 sites spanning 2750 m of elevation in Northern California. The degree of spatial synchrony exhibited by each butterfly species was used as a response in a unique approach that allowed us to investigate whether interspecific variation in response to climate or dispersal propensity was most predictive of interspecific variation in synchrony. We report that variation in sensitivity to climate explained 50% of interspecific variation in synchrony, whereas variation in dispersal propensity explained 23%. Sensitivity to the El Niño Southern Oscillation, a primary driver of regional climate, was the best predictor of synchrony. Combining sensitivity to climate and dispersal propensity into a single model did not greatly increase model performance, confirming the primacy of climatic sensitivity for driving spatial synchrony in butterflies. Finally, we uncovered a relationship between spatial synchrony and population decline that is consistent with theory, but small in magnitude, which suggests that the degree to which populations fluctuate in synchrony is of limited use for understanding the ongoing decline of the Northern California butterfly fauna

Experimental warming influences species abundances in a Drosophila host community through direct effects on species performance rather than altered competition and parasitism.

Global warming is expected to have direct effects on species through their sensitivity to temperature, and also via their biotic interactions, with cascading indirect effects on species, communities, and entire ecosystems. To predict the community-level consequences of global climate change we need to understand the relative roles of both the direct and indirect effects of warming. We used a laboratory experiment to investigate how warming affects a tropical community of three species of Drosophila hosts interacting with two species of parasitoids over a single generation. Our experimental design allowed us to distinguish between the direct effects of temperature on host species performance, and indirect effects through altered biotic interactions (competition among hosts and parasitism by parasitoid wasps). Although experimental warming significantly decreased parasitism for all host-parasitoid pairs, the effects of parasitism and competition on host abundances and host frequencies did not vary across temperatures. Instead, effects on host relative abundances were species-specific, with one host species dominating the community at warmer temperatures, irrespective of parasitism and competition treatments. Our results show that temperature shaped a Drosophila host community directly through differences in species' thermal performance, and not via its influences on biotic interactions