Within closely related species, those that have large local populations also tend to have a high level of occupancy across a landscape, and species with small local populations tend to occur in fewer sites and have a lower level of occupancy. Although positive abundance-occupancy relationships have been observed across systems and species, there remains little consensus of the mechanisms responsible for producing this relationship. We analyzed a 19-year long-term dataset of crustacean zooplankton and rotifer abundance and occupancy across 28 lakes in the Adirondack State Park in New York, USA, to determine if planktonic invertebrates exhibit a positive abundance-occupancy relationship. We also examined the role of species traits such as body size as well as their environmental niche width and niche position in explaining the residual variation in abundance-occupancy relationships. We hypothesized that zooplankton would exhibit a positive abundance-occupancy relationship, and we expected that smaller species would have larger populations and occur across more lakes due to potential differences in dispersal capabilities among different size classes. Additionally, we expected species with higher local abundance and higher occupancy to have a wider niche width than rarer species.
Results/Conclusions
Positive abundance-occupancy relationships were found for all zooplankton in the dataset. The slope of the abundance-occupancy relationship was not significantly different among rotifers, copepods, and cladocerans. However, abundance-occupancy relationships of rotifers and copepods had a significantly higher intercept than cladocerans. Therefore, across Adirondack lakes, rotifers and copepods occurred at a higher level of abundance for a given level of occupancy. It is likely that Adirondack lakes are sufficiently calcium limited, with a lower pH and lower acid neutralizing capacity to prevent large populations of cladocerans from becoming established. We found that traits such as environmental niche width, environmental niche position, family, taxonomic group, length, and biomass did not explain a significant amount of variation in abundance. However, both niche position and niche width explained a significant amount of variation in occurrence (p<0.0001). Species that occurred across more sites tended to occur in non-marginal habitats (p<0.0001), and species with a wide environmental niche had a higher level of occurrence (p=0.0013). Although biological traits did not explain residual variation in abundance-occupancy relationships, a species environmental niche and their tolerance of Adirondack lake conditions may contribute to explaining why some species are common and locally abundant, while others are rare.