Habitat variation and trophic plasticity in widespread, common zooplankton
Widespread geographic distributions have been observed in many taxa, yet even among closely related species range size can vary by orders of magnitude. Range size may be positively correlated with niche breadth, suggesting species with wide geographic ranges exploit a wide range of resources, whereas narrowly distributed species may exploit a narrower range of resources. It was our goal to compare the lake environments and trophic positions of populations of common and rare freshwater zooplankton to determine how niche breadth changes with range size. We sampled 13 lakes on a large geographic scale in the Adirondack State Park, New York where the common copepod Leptodiaptomus minutus and the rare copepod Aglaodiaptomus leptopus occur. We measured environmental characteristics of each lake: size, productivity, seston quality and dissolved organic carbon. We then measured the δ15N and δ13C isotopic signatures of each copepod species to compare their trophic position and trophic niche.
Characteristics of lakes containing A. leptopus and L. minutus were not statistically different, although A. leptopus was found in shallower lakes on average. The mean trophic position of L. minutus and A. leptopus was 2.4 and 2.6, respectively, however there was no statistical difference. Trophic positions of copepods among lakes ranged from 1.6-2.8 for L. minutus and 2.3-3.1 for A. leptopus, suggesting that on a species level, the common L. minutus feeds at a wider range of trophic positions than the rare A. leptopus. A. leptopus had a more depleted mean δ13C signature than L. minutus, suggesting this species feeds on resources derived from the pelagic zones of the lakes, or feeds on grazers that interact with methanogens at the lake-sediment interface. Even though the lake environments of common and rare species were not different, common species feed at a wider range of trophic positions and thus may exhibit greater dietary plasticity than rare species, which could be a potential strategy or a result of maintaining a large range size.