Abstract
Ecosystem engineers are increasingly recognized for their potential in facilitating habitat restoration efforts. An example of ecosystem engineering in aquatic habitats is bioturbation, the disruption of sediment at the water-sediment interface by animal activity. Among the varying effects they have on aquatic communities, bioturbating animals can facilitate zooplankton recruitment by digging up buried, resting eggs and returning them to the sediment surface, where they have a higher probability of hatching. Such facilitation has been demonstrated in studies involving lake and permanent-pond ecosystems, but the effects of bioturbation in temporary ponds, such as California vernal pools, have largely been overlooked. Vernal pools are home to a strong bioturbator, the endemic notostracan Lepidurus packardi. I hypothesized that bioturbation by L. packardi facilitates the hatching of buried, resting eggs by returning them to the sediment surface. I tested this hypothesis by removing L. packardi from mesocosms filled with natural vernal pool soil and comparing the resulting crustacean communities to those in unmanipulated mesocosms. I predicted that mesocosms with fewer L. packardi would have fewer crustacean individuals and/or taxa in the active community, because fewer buried eggs would be returned to the sediment surface. I directly tested L. packardi’s digging abilities by conducting complementary microcosm experiments where I buried propagules (resting eggs and plant seeds) at different depths and added freely roaming or caged L. packardi. These experiments also allowed me to determine whether L. packardi can influence the hatching of resting eggs through kairomones (chemical signals). I found no support for my hypothesis. In the mesocosm experiment, four taxa were actually more abundant, not less, in mesocosms with fewer L. packardi. This indicates that L. packardi was suppressing these taxa in the Control mesocosms, most likely through predation. In the microcosm experiments, I found that L. packardi did not translocate propagules buried ≥ 0.5 cm deep, and that it also consumed eggs (but not seeds) lying on the sediment surface. I further found no evidence for kairomones. Results from the microcosm experiments additionally suggest that i) egg translocation was not masked by egg predation; and ii) propagule translocation simply did not occur. I conclude that bioturbation by L. packardi does not facilitate crustacean recruitment in California vernal pools, and that this taxon influences other crustacean taxa primarily through predation on both resting and active stages.