Seminar Series 2003

Ecology of Mutualism: Biotic and Abiotic Factors Influence Goby and Shrimp Populations

Andrew Thompson, University of California, Santa Barbara

Although it is slowly becoming accepted that mutualism plays an important role in the function of ecological systems, the science of ecology has traditionally downplayed the role of positive interactions. One reason that mutualism has been discounted is that early theoretical models predicted that mutualistic populations were exceptionally unstable and were vulnerable to unchecked population growth or rapid extinction. To bolster the understanding of forces affecting mutualist ecology, I developed a mathematical model of mutualist population dynamics. Model results indicate that accounting for habitat limitation and intraspecific competition for mutualists prevents population explosion, and that immigration buffers mutualistic populations from extinction. Further, a main finding of the model was that immigration can alleviate negative effects of predators on mutualist abundance. To determine whether impacts of habitat, competition, predation and immigration were in accord with theoretical predictions, I then conducted lab and field experiments that examined the population ecology of fierce shrimp gobies (Ctenogobiops feroculus) and snapping shrimp (Alpheus djeddensis) in Moorea, French Polynesia.
Mutualistic gobies and shrimp are common circumtropically and interact as follows: shrimp construct burrows in which both gobies and shrimp reside, and gobies warn shrimp of predators through touch-based signals. Experiments delving into the nature of benefits demonstrated that goby mortality is significantly reduced when paired with a shrimp, whereas shrimp growth is five times greater when paired with a goby. Surveys of 686 3x3m quadrats throughout the north shore of Moorea indicated that populations sizes were ultimately constrained by habitat appropriate for burrow construction by shrimp (i.e. a mixture of app. 70% sand and 30% rubble). Next, adding marked gobies to plots in the field showed that gobies compete intraspecifically for shrimp, and that large gobies were competitively dominant over smaller individuals. Surveys of locations with appropriate habitat, but variable predator densities, evinced a negative correlation between predator and large goby densities, a positive correlation between the proportion of small gobies residing with large shrimp and predator density, but no correlation between goby and predator densities. A field experiment then demonstrated that predators readily consumed large gobies, but because recently settled gobies rapidly paired with orphaned large shrimp when there was a change in goby size structure but no overall change in density. Hence, as predicted by the model, habitat limitation and intraspecific competition limit population sizes, whereas immigration mitigates predator impacts.