PhD, University of California, San Diego
Determining the legacy effects of invasive species once they have been eradicated
The impacts of global changes on ecosystems are shaped by evolution of organismal traits, plasticity and turnover in community structure. Ecological and evolutionary history can influence community responses to environmental change, though time scales of experimental studies are typically too short for such shifts to occur. Thus, the contributions of phenotypic selection within and among species to community resilience are largely unknown. To test if history alters community responses to environmental change, we conducted a plankton community transplant experiment. Our experiment is a community-level analog to a common garden, designed to measure the impact of the environment and genotype on the phenotype of an organism. We established communities of aquatic organisms originating from four types of lakes in the California Sierra Nevada mountains: all factorial combinations of alpine and sub-alpine elevations, with and without introduced trout predators. We grew these four types of plankton communities at three elevations crossed with the presence of fish predators in the experimental pond mesocosms. Our goal was to test the hypothesis that evolutionary and ecological history of elevation and fish predation would affect the contemporary population, community and ecosystem level responses of plankton to these same perturbations.
Results/Conclusion We found that the response of communities to elevation and predators was contingent on the environment from which they originated. We expected that local adaptation would produce plankton with the highest fitness in their home environment, but this prediction was not supported. Instead, zooplankton that originated in lakes with fish obtained a higher total community biomass in the absence of fish than communities that were originally from fishless lakes. This pattern was largely driven by changes in Daphnia pulicaria biomass (increases in both abundance and body size) suggesting fish select for a faster intrinsic growth rate in Daphnia populations and this effect lasts for many generations. The history of fish predation played a greater role in contemporary community structure and response to environmental change than the history of elevation. Our experiment indicates that shifts in species composition and evolutionary history of populations along environmental gradients determine the aggregate response of communities to changes in predation and climate.
Abstract: Dramatic changes in environmental conditions or community composition may impose severe selective pressures on resident populations. These changes in the selective regime can lead to demographic bottlenecks or local extinction. The consequence of demographic contraction is often a reduction of standing genetic variation. Since the level of adaptive genetic variation in populations plays an important role in persistence and adaptive response, understanding genetic resilience and the time course for re-establishment of genetic diversity following demographic perturbations is a critical component of assessing the consequences of changing environments. The introduction of nonnative fish into historically fishless lakes is a particularly dramatic environmental change frequently contributing to demographic bottlenecks and local extinction of native populations. We examine the quantitative- and molecular-genetic recovery of two alpine populations of the zooplankton Daphnia melanica from the Sierra Nevada, California, USA. These populations were extirpated by introduced salmonids and subsequently re-established following the experimental removal of nonnative fish. We obtained data for nuclear and mitochondrial markers and conducted a common-garden experiment to assess the levels of molecular- and quantitative-genetic variation following experimental fish removal. Reestablished D. melanica populations attained levels of nuclear genetic diversity only slightly lower than surrounding fishless populations in the first year following fish removal and substantial mitochondrial and quantitative-genetic diversity within 8 years. This high level of genetic resilience was likely facilitated by multiple sources of genetic variation, including immigration from neighboring populations and hatching from a local reservoir of diapausing eggs. Our results highlight the genetic resilience of taxa with reservoirs of genetic variation in seed or egg banks.
Pub.: 27 Apr '10, Pinned: 07 Jun '17
Abstract: Ecological factors are known to cause evolutionary diversification. Recent work has shown that evolution in strongly interacting predator species has reciprocal impacts on ecosystems. These divergent impacts of predators may alter the selective landscape and cause the evolution of prey. Yet, this link between intraspecific variation and evolution is unexplored. We compared the life history of a species of zooplankton (Daphnia ambigua) from lakes in New England in which the dominant planktivorous predator, the alewife (Alosa pseudoharengus), differs in feeding traits and migratory behaviour. Anadromous alewife (seasonal migrants) exhibit larger gapes, gill-raker spacing and target larger prey than landlocked alewife (year-round freshwater resident). In 'anadromous' lakes, Daphnia are abundant in the spring but extirpated by alewife predation in summer. Daphnia are rare year-round in 'landlocked' lakes. We show that Daphnia from lakes with anadromous alewife grew faster, matured earlier but at the same size and produced more offspring than Daphnia from lakes with landlocked or no alewife across multiple temperature and resource treatments. Our results are inconsistent with a response to size-selective predation but are better explained as an adaptation to colder temperatures and shorter periods of development (countergradient variation) mediated by seasonal alewife predation.
Pub.: 29 Jan '11, Pinned: 07 Jun '17
Abstract: The effects of global and local environmental changes are transmitted through networks of interacting organisms to shape the structure of communities and the dynamics of ecosystems. We tested the impact of elevated temperature on the top-down and bottom-up forces structuring experimental freshwater pond food webs in western Canada over 16 months. Experimental warming was crossed with treatments manipulating the presence of planktivorous fish and eutrophication through enhanced nutrient supply. We found that higher temperatures produced top-heavy food webs with lower biomass of benthic and pelagic producers, equivalent biomass of zooplankton, zoobenthos and pelagic bacteria, and more pelagic viruses. Eutrophication increased the biomass of all organisms studied, while fish had cascading positive effects on periphyton, phytoplankton and bacteria, and reduced biomass of invertebrates. Surprisingly, virus biomass was reduced in the presence of fish, suggesting the possibility for complex mechanisms of top-down control of the lytic cycle. Warming reduced the effects of eutrophication on periphyton, and magnified the already strong effects of fish on phytoplankton and bacteria. Warming, fish and nutrients all increased whole-system rates of net production despite their distinct impacts on the distribution of biomass between producers and consumers, plankton and benthos, and microbes and macrobes. Our results indicate that warming exerts a host of indirect effects on aquatic food webs mediated through shifts in the magnitudes of top-down and bottom-up forcing.
Pub.: 26 Sep '12, Pinned: 07 Jun '17