Since their invasion, Asian clams have become the most widespread non-native, freshwater bivalve in North America. Asian clam occurrences decrease with increasing latitude, establishing primarily in areas less than 40°north, however, a warming climate may facilitate a northern range expansion and further establishment. The objective of this research was to determine if benthic community responses to Asian clam invasions differ along a latitudinal gradient. Specifically, we quantified how Asian clam population densities change with latitude and temperature, and documented differences in native macroinvertebrate community responses to Asian clam invasions from temperate to tropical climates. Sites in temperate regions included rivers in southeastern Michigan (43°north) and southern Ohio (39°north), and subtropical and tropical locations included southeastern Georgia (31°north) and Puerto Rico (18°north). At each region, 8 invaded sites with varied Asian clam densities were assessed. We quantified Asian clam populations in all river substrates with a ½m2 quadrat, in triplicate locations at each site. Clams were removed from the quadrat area and counted. Macroinvertebrates were collected, targeting riffle zones in accordance with the Environmental Protection Agency’s rapid bioassessment protocol, and a suite of diversity and tolerance metrics were conducted. Samples were preserved and family-level classifications are ongoing.
Results/Conclusions
With increased reported Asian clam occurrences in southern regions, we predicted higher population densities in southern sampling sites, however, this was not consistent with our findings. We found dense Asian clam populations in northern, temperate Michigan where no previous sightings were reported. When comparing population densities in temperate Michigan to tropical Puerto Rican sites (Ohio and Georgia are in processing), we found no difference in clam density (Welch’s t-test, p=0.64), however, macroinvertebrate community responses differed for all metrics (Shannon diversity, Simpson’s dominance, evenness, %EPT composite index, Hilsenhoff Biotic Index, Benthic Index of Biotic Integrity; ANCOVA, p<0.05). Macroinvertebrate community diversity and tolerance in temperate river sites showed a positive linear relationship with Asian clam densities, while a negative relationship was noted in tropical sites. Though there was no difference in Asian clam density between temperate and tropical rivers, macroinvertebrate community responses differed significantly as Asian clam densities increased. In northern temperate climates, where Asian clam invasions were previously unreported, recent surface temperatures increases may have facilitated new, or expanded existing Asian clam populations. New populations can introduce habitat heterogeneity in river substrates that can be utilized by the benthic community, however, these benefits may not be evident in long-term tropical invasions.