The colonization-competition tradeoff has been proposed as a mechanism that drives ecological succession in communities. However, as communities gain diversity, additional factors (habitat heterogeneity, predation) may alter the colonization-competition tradeoff as species are required to allocate resources to respond to these new factors. A relatively simple model system for investigating these tradeoffs is the metazoan community in aquatic containers. In the US Midwest this community consists of 10-12 filter-feeding larval Diptera and a generalist larval predator (Toxorhynchites rutilus). These species colonize new containers in a predictable sequence indicative of ecological succession. To determine the relationships among colonization, competition, and predation resistance for these species, and how that three-way trade-off may affect succession, we conducted three studies. (1) We established ten water-filled containers in an oak/hickory forest in St. Louis, MO, and recorded colonization times of species. (2) We assessed and ranked competitive ability of seven species in the laboratory by determining RIndex, the detritus amount at which each species was estimated to attain equilibrium finite rate of increase. (3) We assessed predator vulnerability of early colonizers by recording colonization patterns of T. rutilus in response to field manipulations of colonization by different prey species.
The relationship between RIndex and colonization times was quadratic (quadratic regression coefficient significant: p=0.0230), with competitive ability of species first increasing, and then decreasing as a function of colonization time. The decrease in competitive ability coincided with mean colonization time of predator T. rutilus. This suggests that an additional tradeoff occurs in mid-late colonizing species, with good competitors more vulnerable to predation. Removal of the most competitive (and presumably most predator-vulnerable) species from field containers did not significantly alter T. rutilus colonization time (survival analysis, p=0.8404), or T. rutilus abundance over time (repeated-measures MANOVA, p=0.5854). This suggests that early colonizing species are no more predator-resistant than later colonizing, more competitive species. Therefore, predation resistance may not be simply an inverse function of competitive ability, but rather trades off with both colonization and competitive abilities. We will also discuss preliminary data on predator resistance to T. rutilus, and the relationships among predator resistance, colonization times, and RIndex of species.