Testing for Microgeographic Effects on the Strength of Interspecific Competition

Local adaptation to an environment can vary across very fine scales - as little as a few meters in some species, kilometers in others. This divergence at microgeographic scales has been linked to dispersal ability and could be responsible for geographic variation in the strength of species interactions. For example, the spread of maladaptive traits across short distances could lead to inferior performance and local extinctions across the landscape. We utilized a model study system (headwater streams of New Hampshire) with known differences in dispersal, gene flow, and intraspecific competition to test for microgeographic variation in interspecific competition. We conducted a common garden experiment and measured survival and growth of larval Spring Salamanders from different stream reaches (fish and fishless) in response to the presence of conspecific salamanders or heterospecific Brook Trout fingerlings. We predicted that Spring Salamanders from reaches with fish would have higher competitive performance with fish than naïve salamanders from reaches without fish. No significant differences were detected in salamander survival. Overall salamander growth was negative but was not affected by reach, heterospecific competitor, or the interaction between those two factors. Based on our results, microgeographic variation does not appear to be important in determining the strength of interactions between larval Spring Salamanders and Brook Trout. Salamander dispersal between our collection reaches is possible, and high gene flow of maladaptive traits could be responsible for the overall negative growth patterns. Our research highlights the importance and complexity of testing species interactions in model systems with known microgeographic variation.