Foraging Behavior and Drift Mechanisms of a Stream Insect Grazer (Baetis Tricaudatus).

The foraging behavior of Baetis tricaudatus (Ephemeroptera: Baetidae) was studied in field and laboratory experiments to determine how food abundance and distribution affects movement patterns on several spatial scales. Nymphs fed continuously and were more active at night than during the day when food (periphyton) was patchily distributed. Substantial diel variation in interpatch movement rate accounted for observed diel activity rate patterns. Search behavior within and between patches was compared with movement patterns predicted by optimal foraging theory. Nymphs spent significantly more time in patches than predicted if movements had been r and omly directed. Within-patch movements were highly systematic, the optimal pattern since food was uniformly distributed within patches. Interpatch movements were affected by food abundance in the previous patch and resulted in frequent revisitation of high quality patches. Because previously grazed patches were revisited, interpatch search was suboptimal in experimental habitats. The interpatch search mechanism would be adaptive in natural habitats if patch types are contagiously distributed, as suggested by field data. Because of systematic search, food intake rate of individuals within patches was positively correlated with food abundance and constant until a patch was entirely grazed, after which it declined markedly. These relationships defined two rules necessary to maximize intake rate in habitats: (1) graze the entire surface of patches, and (2) depart patches when intake rate declines. Observed behavior was consistent with both rules. In habitats containing two patch types, the time spent in a low quality patch decreased with increasing intake rate in the habitat upon encountering the patch. The results were consistent with an intake rate criterion for determining patch choice. The foraging experiments were used to design experiments which tested a passive drift-entry hypothesis. Starved animals during the day and fed animals at night showed no significant differences in activity rate or positioning on substrates, but drift occurred largely at night. The results refute a passive explanation for drift entry and diel changes in entry probability, and suggest that drift entry was active. It was hypothesized that active entry occurred when an individual's food intake rate in a habitat dropped below some threshold level.Ph.D.EcologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159762/1/8402306.pd