UC Santa Cruz

Species interactions are core to our understanding of communities and ecosystems. Much of the theoretical work that forms the basis of our understanding of species interactions is based on the assumption of constant environmental conditions and the achievement of equilibrium. However, real ecosystems are often characterized by changing conditions and nonequilibrium dynamics. Fluctuations in environmental conditions may affect species interactions in a number of ways: by excluding one partner, by changing the dominance relations of interactors, the magnitude, or even the direction of interaction strength between pairs of species. In this dissertation I studied the effects of fluctuating environmental conditions on predation and competition of threespine stickleback (Gasterosteus aculeatus) in bar-built estuaries along the central coast of California. Due to seasonal variation in rainfall, bar-built estuaries are only intermittently open to the ocean and vary seasonally in important abiotic conditions such as salinity and dissolved oxygen. In Chapter 1 I surveyed threespine stickleback in estuaries of various sizes. I found that larger estuaries supported perennial streamflow, breeding habitat for predatory salmonids and sculpins, and therefore stickleback had higher frequencies of alleles coding for a complete set of armor plates. In Chapter 2 I tested directly for selection on Ectodysplasin-A (Eda), the gene controlling armor plates, by comparing the genotypes of breeding adult stickleback to juveniles recovered from the stomachs of predatory salmonids. I found no evidence of selection over a single season, indicating that the genetic differences in Chapter 1 built up slowly. In Chapter 3 I used empirical dynamic modeling to show that fluctuations in the abundance of endangered tidewater goby (Eucyclogobius newberryi) and threespine stickleback represented independent responses to changes in environmental conditions rather than context-dependent competition. Specifically, stickleback were negatively impacted by breaching, and the peak abundance of both species was offset by several months, potentially indicating that reproductive allochrony alleviates competition. In both the case of predation and competition, long-term effects were evident but short-term effects were not. This pattern suggests that species interactions can and do play an important role in fluctuating environments but that those interactions may be difficult to detect in the short-term or under all conditions.