An application of the Lotka -Volterra model to competition between two lotic grazers.

Abstract

To date the importance of competition in structuring stream communities is unclear. Compelling evidence that competition might affect benthic communities in several Michigan trout streams comes from recent studies that monitored population shifts in periphyton-grazing taxa, in response to a microsporidian infection of the caddisfly <italic>Glossosoma nigrior</italic>, a dominant grazer in many cold-water streams. The most dramatic response to the parasitic-induced collapse of <italic>Glossosoma</italic> was that of <italic>Goera stylata </italic>, a caddisfly previously unreported from these streams. I conducted series of laboratory experiments to (1) measure changes in competition between these taxa during their larval development and (2) parameterize the Lotka-Volterra model and to use it to understand the mechanism at work. Experiments used a target-competitor design to measure the relative effect of competition on the foraging time of <italic>Glossosoma</italic> and <italic> Goera</italic> larvae, as measured from time-lapse video recordings. For each of three instar pairings examined (4<super>th</super>- and two 5<super>th </super>-instar <italic>Glossosoma</italic> vs. 3<super>rd</super>-, 2<super> nd</super>-, and 5<super>th</super>-instar Goera, respectively) competitive equivalence (alpha) was estimated from the rate at which per capita foraging time diminished with increased treatment densities of conspecifics and competitors. On a per capita basis, competition was symmetric only between 5<super>th</super>-instar <italic>Glossosoma</italic> and 2<super>nd</super>-instar <italic> Goera</italic>, whereas it strongly favored <italic>Goera</italic> in the other two pairings. The explanation for <italic>Glossosoma</italic>'s competitive dominance was understood only after applying the Lotka-Volterra model and considering the importance of each species' relative carrying capacity (K). <italic> Glossosoma</italic>'s mean maximum abundance was &ge;11x that of <italic> Goera</italic>'s in three Michigan streams. <italic>Glossosoma</italic>'s finer mouthpart structure might allow it to crop periphyton below what is available to <italic>Goera</italic> and thereby contribute to <italic>Glossosoma </italic>'s larger K. The strong relationship between instar-specific alpha and instar biomass ratios allowed me to predict alphas for all possible instar pairings and then to consider how developmental phenology and life history strategies of these taxa might influence their competitive outcome. This application of the Lotka-Volterra model identified competitive bottlenecks for <italic>Goera </italic>, both when <italic>Glossosoma</italic> exhibits distinct cohorts and when there is broad overlap in its cohort structure, and it provided a framework for considering how <italic>Glossosoma</italic> maintains its competitive dominance over other grazer taxa.