The potentially important role that larval dispersal may play in determining gene flow, distributions, and population structure of marine invertebrates remains unclear despite many hundreds of descriptive comparisons of pelagic duration and population genetic structure. This lack of clarity suggests many factors may influence population genetic structure and their interactions may be complex. Difficulties studying these factors include (under normal circumstances) distinguishing local from exogenous recruitment and therefore the true distribution of dispersal distances. For example, experiments that normally could be undertaken to explore this issue are very small scale relative to the distances that many marine taxa may disperse.
In August 2011, a large-scale natural removal experiment was initiated along a 100 km stretch of the central California coast. The PIs propose to use this rare opportunity to clarify the effects of dispersal and species interactions on marine population genetic variation and community structure. They propose to study three species that suffered very high rates of mortality: an ecosystem engineer (Strongylocentrotus purpuratus, ~100% mortality), a keystone species (Pisaster ochraceus, ~10-70% mortality), and one of its competitors (Leptasterias sp., ~100% mortality). Their objectives during this first year following the natural large-scale die-off are to [1] quantify the abundance and distribution of the target species at sites across the impacted range and reference sites to the south and north, [2] develop and use genetic markers to identify the sources and dispersal distances of new recruits of P. ochraceus, Leptasterias sp., and S. purpuratus that recolonize the impacted range, and [3] describe changes in abundance of these three species and their prey and competitors at sites throughout the impacted range.
In Fall 2011 and during 2012, we documented the extent of a widespread and severe mass mortality of rocky shore invertebrates along the north-central California coast in the northeast Pacific Ocean, following a harmful algal bloom (which occurred in August 2011). Formerly abundant intertidal populations of the ecologically important purple sea urchin (Strongylocentrotus purpuratus) and the predatory six-armed sea star (Leptasterias sp.) were extirpated from approximately 100 km of coastline; some purple sea urchins survived at depth subtidally. Other invertebrates, including the ochre sea star (Pisaster ochraceus) and gumboot chiton (Cryptochiton stelleri), also experienced elevated mortality. The severity and spatial scale of effects suggest the potential for long-term population, community, and ecosystem consequences. Continued surveys in 2013 and 2014 showed that purple sea urchins are recolonizing the coastline from which they were extirpated, albeit spatially patchily and mostly in low numbers; the six-armed sea star has yet to return to most sites. (The ochre seastar subsequently experienced a second mass mortality in 2014 attributed to seastar wasting disease.) Genetic analyses are underway to discover the diversity and geographic origins of the new colonists. Understanding the cause and extent of, as well as recovery from, large scale disturbances may provide insight into marine rocky intertidal population dynamics, community dynamics, and potential short- and long-term consequences of future environmental change.
