Complementary Occupancy of Semi-permanent Wetlands by Tiger Salamander (Ambystoma tigrinum) Larvae and Fishes as Indicated by Zooplankton Size and Color: How Respiratory Physiology Explains Community Ecology

Hydrology and dissolved oxygen (DO) availability drive wetland occupancy by zooplantivorous predators in the Prairie Pothole Region of the northeastern Great Plains, as follows. Permanent wetlands favor fishes, while the shallow waters of seasonal wetlands favor invertebrates and the larvae of the amphibian spring-breeding guild, including Tiger Salamanders (Ambystoma tigrinum). Semi-permanent wetlands fluctuate between these two extremities, holding water during wet years but drying during droughts, which means the aquatic vertebrate occupancy of these wetlands typically switches between fishes (wet periods) and amphibian larvae (droughts). Further, the shallow water of wetlands promotes hypoxic/anoxic conditions, creating “summerkills” and “winterkills,” primarily of fishes, which obligately breathe DO. Spring-breeding amphibian larvae avoid summerkills by air gulping—using their lungs to access atmospheric oxygen—and avoid winterkills by metamorphosing. Zooplankton, in turn, have several responses to these wetland conditions and their predators. In reaction to visually mediated, size-selective fish predation, small and semi-transparent zooplankton are selected for, and they undertake daily vertical migrations; the literature on this ecological interaction is vast. In response to intense invertebrate predation pressure, zooplankton become large. In response to wetland hypoxia, zooplankton upregulate hemoglobin (i.e., create blood), which makes them conspicuously red. Nocturnally active Tiger Salamander larvae use senses other than vision (likely lateral line mechano- and electroreceptors) to detect prey, permitting zooplankton to be large and red. The presence or absence of fishes or amphibians, combined with zooplankton morphology (small/large, semi-transparent/red), therefore create hypotheses regarding recent wetland conditions. This variability, while characterizing semi-permanent wetlands, is much more dramatic and dynamic than has previously been reported for palustrine basins. As such, by this and other criteria, I offer a synthesis built upon an understanding of respiratory physiology to explain the ecological communities observed in prairie wetlands.