Species interactions in ephemeral patch systems: spatial, temporal, and spatio-temporal influences

Citation

Duthie AB (2013) Species interactions in ephemeral patch systems: spatial, temporal, and spatio-temporal influences. Doctor of Philosophy. Iowa State University.

Abstract
This thesis can be accessed at: http://lib.dr.iastate.edu/etd/13001/ This dissertation demonstrates the importance of spatial, temporal, and spatio-temporal mechanisms in influencing species interactions within ephemeral patch systems. This work extends prior theory using individual-based modeling to show that the autocorrelation of habitat can have a strong influence on plant-pollinator communities. Pollinators in plant-pollinator communities must regularly disperse to receptive plants, and in model systems often feed on ephemerally available seeds. Because the probability of a seed being eaten by a pollinator will be directly affected by the number of pollinators that visit a plant flower, the frequency at which seed-eating pollinators visit flowers has a major impact on the costs and benefits associated with the mutualism. High pollinator visitation increases per capita pollination, but also the number of seeds consumed. The number of pollinators visiting a plant is expected to increase with increasing plant density. Where plant habitat is especially dense, plant density will be high. If pollinator dispersal is limited, plants in these regions of high density will support high densities of pollinators that are able to move efficiently between plants. As a result of frequent pollinator visitation, plants with many nearby conspecific neighbors will produce disproportionately more pollinators than seeds. Because pollinators compete with non-pollinating exploiters of mutualisms, plants in these locations will also have lowered rates of exploitation. This is the case for the plant-pollinator interaction between the Sonoran Desert rock fig (Ficus petiolaris) and its wasp pollinator (Pegoscapus), wherein fig trees with a higher number of conspecific neighboring trees are visited by more foundress pollinators, produce fewer seeds, and are less likely to be exploited by non-pollinators. Because the costs, benefits, and stability of mutualisms with seed-eating pollinators are all strongly influenced by pollinator and exploiter birth rates, and because figs in particular are often a keystone resource for local frugivores, the work in this dissertation is of broad interest to ecology, evolution, and conservation. This dissertation also extends prior theory on competitors in ephemeral patch systems through the introduction of lottery models, which are used demonstrate how a temporal storage effect facilitates competitor coexistence when competitors develop within patches in overlapping generations. Conceptually unique to this work, the mechanism generating the temporal environmental variance on which coexistence relies is space itself, in the form of varying minimum between patch dispersal distance. When the minimum dispersal distance required to move from ephemeral patch to ephemeral patch changes over time, the superiority of competitors using patch resources can also change over time if they invest differently in fecundity versus dispersal ability. Given a trade-off in fecundity and dispersal ability, coexistence is predicted over a wide range of biologically realistic parameters for non-pollinating competitor fig wasps. An empirical prediction of this modeling work is that the fecundities of competitors in fig wasp communities will be negatively correlated with wasp dispersal abilities. Egg load estimates and two independent metrics of wasp dispersal ability show that non-pollinating competitors associated withF. petiolarishave negatively correlated fecundities and dispersal abilities. These results demonstrate that wasp population storage and a fecundity-dispersal ability trade-off are critical mechanisms for maintaining fig wasp diversity in at least one fig wasp community, and likely many others. And they more broadly demonstrate the importance of a fluctuating environment on species interactions and coexistence in ephemeral patch communities.