Biogeographic patterns, species interactions, and mechanisms of coexistence in nearshore marine communities

Abstract

The maintenance of ecological communities through space and time is driven by complex processes which often operate simultaneously. Understanding the mechanisms that underlie patterns of species diversity and coexistence across scale remains a key focus for ecologists. Empirical experiments across large spatial and temporal scales are critical to rigorously test prevailing ecological theory and identify context dependencies. Such studies are necessary to determine regional variation in the effects of local-scale processes, which should be explicitly incorporated into the examination of macroecological patterns. The striking richness of species concentrated at low compared to higher latitudes—known as the latitudinal diversity gradient— is one of the most well-documented patterns of global biodiversity. Ecologists continue to test hypotheses that can explain this broad-scale pattern and, recently, the biotic interactions hypothesis (BIH) has received clear empirical support in both terrestrial and marine systems. Under the BIH, the relative strength of species interactions increases toward the tropics and serves to maintain the high diversity of species observed there. Despite accruing support for the BIH, interaction outcomes can be quite variable and little is known about the extent and the drivers of variability in these community-scale processes that shape regional patterns. In my dissertation, I explore how multiple processes (e.g. predation, competition, and disturbance) shape patterns of assembly and coexistence through empirical examination of subtidal marine hard-substrate communities across varying spatial and temporal scales. To achieve this, I conducted standardized experiments in nearshore habitats across four biogeographic regions spanning 47-degrees of latitude of the eastern Pacific Ocean and across ocean basins in the tropics (Pacific and Atlantic Panama). Among my research objectives I was able to: (1) explore variation in consumer effects on prey community structure across latitude and identify key attributes of predator communities that shape impacts on prey in the tropics, (2) examine mechanisms contributing to coastal and seasonal variation in consumer pressure that underlies stronger predation in tropical communities, and (3) uncover a strong nested structure in networks of competing species across latitude that contributes to enhanced tropical coexistence. My dissertation provides novel insights into the complex nature of consumer-prey interactions across latitude, with particular focus on tropical communities, which have been identified as important hotspots for predation based on theoretical predictions and prior experimental studies. Further, results reported here improve our understanding of underlying mechanisms and the context dependencies of interactions that contribute to coexistence in diverse ecological communities. These large-scale experiments on naturally assembled communities provide critical insights into the processes that contribute to biodiversity maintenance and ecosystem functioning in a changing world.