The ecological roles of amphibians and subsidies in ponds
Freshwater ponds are often underappreciated landscape features, despite the fact they comprise a major portion of all freshwater ecosystems. Ponds support diverse food webs and species not found in other habitat types. Ponds ecosystems do not exist in isolation, and what happens in and around them influences their food webs and connections to nearby terrestrial ecosystems. Habitat alterations near ponds have strong potential to reduce animal diversity. In freshwater systems, loss of sensitive species, such as amphibians, is occurring rapidly and with large ecological consequences. Furthermore, although many amphibians are imperiled, there is a lack of data regarding their precise role in ecosystems. My research tries to fill this gap by adding crucial data to expand our knowledge of the ecological structure of pond food webs and the role of pond-breeding amphibians. I used a combination of experiments, field observations, and statistical tools to address three main objectives: 1) what is the relative importance of top consumers vs. resources in aquatic food webs, 2) quantifying pond ecosystem response to variation in leaf inputs (i.e., such as would happen with land-use change), and 3) understanding how food webs and energy sources shift in ponds with varying leaf litter inputs. I used experimental ponds to establish a model food webs consisting of larval spotted salamanders (Ambystoma maculatum) and Southern leopard frog tadpoles (Rana sphenocephala). Experimental ponds standardize pond features such as age, shape, and depth while retaining microbial and plankton communities similar to natural ponds. They have been used with high success in answering important basic and applied ecological questions. Primary production can be controlled through bottom-up (e.g., resources) or topdown (e.g., predators) constraints. My experiment manipulated resources by altering light (low/high) and nutrient (low/high) availability; omnivores with the presence/absence of tadpoles; and predators with the presence/absence of spotted salamander larvae in an experiment conducted over 14 weeks. I observed that both bottom-up and top-down effects were important in predicting lower trophic level biomass. In particular, tadpoles were highly influential in the food web. We also found large temporal differences in food-web dynamics. These results underscore the need for more information into how ecosystem functioning could be altered by land use and amphibian species losses. One central problem of amphibian recovery efforts is that we still do not really know why some ponds are better for amphibian growth and survival than others. The amount of tree cover over and around ponds can vary drastically, thus I hypothesized that these differences may impact ponds through leaf litter. I completed a second large experiment testing how ponds respond to gradients of leaf litter subsidies. I manipulated leaf litter inputs across seven levels (0.25, 0.5, 0.75, 1, 1.5, 2, 3 kg) in a replicated design using the model food web from the first experiment. I measured ecosystem responses such as dissolved oxygen, nutrients, algae, and amphibian responses. Data from this experiment indicate that nutrient concentrations in increase with increasing leaf litter, but that frogs and salamanders respond differently to leaf litter than field observations would predict. Additionally, I observed a large proportion of the ecosystem responses were nonlinear with increasing leaf litter. These data will be useful in developing models to inform the recovery of threatened amphibian species. To round out my exploration of pond food webs, I conducted an experiment to understanding shifts in pond food web structure as a function of leaf litter input (0.5, 1, 1.5, 2.5, 4 kg) the relative strength of brown (detrital) vs. green (algal) energy pathways in ponds. I used the same experimental pond set-up as in previous years, but also included a top predator in the form of larval dragonflies. This experiment took advantage of the relatively simple food webs found in ponds to better understand food-web dynamics. I used two stable isotopes to trace algal and leaf litter through the food web, contributing greatly to understanding of energy pathways in ponds. I found evidence that consumers shift their energy source and trophic level with increasing leaf litter. This research will identify how leaf litter affects pond food webs and their sources of energy. All together, my research takes a novel approach integrating common scientific tools to a bring together theory and conservation. The most important findings of my dissertation include: 1) amphibians can have strong and lasting effects on pond ecosystems, 2) leaf litter switches ponds from green to brown energy, and 3) consumer trophic position and energy sources can shift as a function of leaf litter, often in nonintuitive ways.
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