Bottom-up and top-down pressures mediate competition between two generalist insects

Legacy effects in temporally separated tadpole species are not mediated by invasive Western Mosquitofish (Gambusia affinis)

Temporally separated species are often thought to have limited competition over a shared resource. However, early arriving species may consume a limited resource such that later-arriving species have access to fewer resources and thus experience competitive effects, even if they are temporally separated (i.e., they experience legacy effects from the early species). The presence of a predator might affect potential legacy effects by influencing the behavior or survivorship of the early species. Using a mesocosm experiment, I examined whether the presence of nonnative Western Mosquitofish (Gambusia affinis) mediated legacy effects in the interaction of two temporally separated species of tadpoles, early arriving American Toads (Anaxyrus americanus) and late-arriving Bullfrogs (Rana catesbeiana). Anaxyrus americanus tadpoles reduced R. catesbeiana tadpole growth despite all A. americanus tadpoles metamorphosing 8 days before the introduction of R. catesbeiana tadpoles into the mesocosms (i.e., legacy effects). Gambusia affinis had limited effects on A. americanus (1 day delay in metamorphosis but no effect on survivorship or size at metamorphosis) and positive effects on R. catesbeiana (increased growth). There were no significant interactions between the A. americanus tadpole density and G. affinis treatments. In conclusion, I found evidence of significant legacy effects of A. americanus tadpoles on R. catesbeiana tadpoles, but no evidence that G. affinis mediated the legacy effects.

Bottom-up and top-down pressures mediate competition between two generalist insects

The effects of competition can have far-reaching consequences for individuals, populations, and communities and therefore we should strive toward a deeper understanding of competitive interactions. In some cases, dietary generalists may be predicted to experience weak competition effects because of their ability to use a wide range of host plants. However, competition between insects frequently occurs indirectly, which can hinder insects' abilities to avoid competitive interactions. Therefore, competition may be as strong among dietary generalists as among dietary specialists. Yet competition between insects that are dietary generalists is infrequently studied. We tested for evidence of competitive interactions between two common, temporally separated, generalist insects: the western tent caterpillar (Malacosoma californicum), which feeds early in the season, and the fall webworm (Hyphantria cunea), which feeds later in the season. Both species frequently use a common host plant species (chokecherry) as a preferred host at our field sites. We tested the relative strength of bottom-up effects resulting from competitive interactions between these two generalists with laboratory-rearing trials at the relevant time of year for each insect. We recorded three common fitness measures (development time, pupal mass, and survival) for caterpillars reared on chokecherry with no damage from either of our focal species, with tent caterpillar damage, and with fall webworm damage. To test the strength of top-down pressures on fall webworm larval fitness and any potential interactions with bottom-up effects, we reared larvae in the field either exposed to or protected from predators on host plants that either did or did not have tent caterpillars feeding on them earlier in the season. We found evidence of bottom-up fitness effects on tent caterpillars and top-down and bottom-up fitness effects on fall webworms confirming that tent caterpillars and fall webworms compete indirectly. Tent caterpillars had lower pupal mass when reared on leaves from shrubs damaged by fall webworms. Fall webworms had lower pupal mass and longer development time when reared on leaves from shrubs damaged by tent caterpillars. In field trials, fall webworms reared on shrubs damaged by tent caterpillars had a lower survival and pupal mass. We show evidence of indirect competition in temporally separated generalists through leaf quality (bottom-up effects) and natural enemies (top-down effects).