Effects of climate change and introduced species on Tropical island stream structure and function
Climate change and introduced species are among the top five threats to freshwater systems face. Tropical regions are considered to be especially sensitive to the effects of climate change, while island systems are more susceptible to species introductions. Climate-driven changes in rainfall are predicted to decrease streamflow and increase flash flooding in many tropical streams. In addition, guppies (Poecilia reticulata), an invasive fish, have been introduced to many tropical freshwater ecosystems, either intentionally for mosquito population control, or accidentally because of the aquarium trade. I examined the effects of climate-driven change in rainfall and introduced guppies on stream structure and function in Trinidad and Hawaii.
In Trinidad, I used a time series to examine how nutrient recycling of guppies changes in the first 6 years after introduction to a new habitat. I found that when guppy populations establish, they show considerable variation in excretion rates through time resulting from changes in guppy density within the first two years post-introduction, and changes in individual excretion in subsequent years. I was able to demonstrate that the effects of an introduced species are not static through time. This research was in collaboration with Dawn Phillips (University of the West Indies), Andrés López-Sepulcre (Washington University, S.t Louis), and Piatã Marques (Rio de Janeiro State University).
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In Hawaii, I utilized a rainfall gradient that mimics forecasted changes in rainfall and resulting changes in streamflow to examine the effects of climate change on invertebrates and their food resources. I learned that the drying of streams across the gradient was associated with a decline in resource quality and invertebrate biomass. Invertebrate taxa switched to species with faster turnover rates. In addition, invertebrate excretion declined at the drier end of our rainfall gradient. Under the current climate scenario, invertebrate community excretion supplies up to 70% of the nutrient demand, which I predict to decline ten-fold with projected climate changes in flow.
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Freshwater systems often face multiple human impacts and I wanted to understand how climate-driven changes in flow might alter the impact of invasive guppies. Using the same rainfall gradient in Hawaii, this time including several guppy-invaded streams, I found that the two stressors had synergistic effects on invertebrate biomass and nutrient recycling. I was able to show that climate change enhances ecosystem effects of introduced guppies. This research was done in collaboration with Rich MacKenzie (US Forest Service), and Piatã Marques (Rio de Janeiro State University).
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Collecting atyid shrimp, gobies, and insects to measure their excretion rates
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Implications of an amphibian decline for the
energy flow through a Neotropical food web
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Amphibian populations in the Neotropics are declining dramatically, mainly due to the chytrid fungus infection. While we have some understanding of the causes of these declines, it is unclear how these losses will influence stream ecosystem structure and function. I created a quantitative food web in a Panamanian headwater stream, using trophic basis of production, to identify major food resources and measure the energy flow from the resources to the invertebrate consumers. Overall, consumption of allochthonous materials was greater than autochthonous and omnivory was prevalent across the majority of taxa. My results provided much needed quantitative information on the basic structure and function of neotropical headwater streams. Based on my results, I predicted that amphibian declines will decrease the relative importance of energy flow from detritus to consumers and increase the significance of autochthonous energy flows via increases in algal resources and compensatory responses by grazing invertebrates.
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The fate of genetically modified corn in
Midwestern headwater streams
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In the Midwestern US, corn represents a significant allochthonous input into agricultural streams and 35 % of planted corn has been genetically modified to express Bacillus thuringiensis (Bt) δ–endotoxins. I deployed litter bags in several streams to quantified the rate of Bt leaching from corn leaves. Bt δ-endotoxin concentration in leaves was measured using an Enzyme-Linked ImmunoSorbent Assay. Within 1 hr, 61 % of the Bt-toxin leached from the leaves and after 70 days, <18% of the endotoxin remaining in corn. To examine the potential influence of the Bt-corn leachate, I measured microbial respiration on sediment cores in streams with and without Bt-corn planted adjacently. These results suggested that leached Bt δ-endotoxin has the potential to reduce microbial respiration in streams.
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The beauty of a ditch
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Taking sediment cores to measure microbial respiration
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Laying out litter bags with corn for leaching experiment
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The role of the wolf spider and fungal pathogens
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Installing grasshopper cages
I investigated how grasshopper densities are regulated by both food and predation. Spider predation on grasshoppers is considered compensatory in many systems, meaning that over time mortality due to predation balances with the mortality caused by other factors (e.g. competition). Therefore, the time at which predators are introduced to a population can impact the survival rate of the prey. I wanted to test this theory by introducing wolf spiders to enclosures containing grasshoppers at two densities during three different time intervals (day 1, 5, and 10). I hypothesized that grasshoppers, which are exposed to the predatory spiders the longest, display the strongest compensatory mortality response.
Instead, a lethal fungal pathogen spread across the grasshopper cages, resulting in 55% population mortality and a lack of the expected compensatory response. However, I noticed that grasshopper populations, which were exposed to predators for a longer time, had lower fungal pathogen casualties. This resulted in higher grasshopper survival rates in cages with predatory spiders. We concluded that predators can mediate effects of fungal pathogens on their prey. These results have lead to my first co-authorship as an undergraduate and an early lesson on how research can be unpredictable and collecting ancillary data is essential.
Instead, a lethal fungal pathogen spread across the grasshopper cages, resulting in 55% population mortality and a lack of the expected compensatory response. However, I noticed that grasshopper populations, which were exposed to predators for a longer time, had lower fungal pathogen casualties. This resulted in higher grasshopper survival rates in cages with predatory spiders. We concluded that predators can mediate effects of fungal pathogens on their prey. These results have lead to my first co-authorship as an undergraduate and an early lesson on how research can be unpredictable and collecting ancillary data is essential.
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