The biological and physical mechanisms that establish and maintain species boundaries in the ocean are controversial. Contributing factors are offspring of species with planktonic larvae being physically transported outside their natal range, and adults thriving when transplanted into regions immediately beyond their natural distributions. It is unclear, however, why long-distance dispersal of a benthic organism's larva should persist on evolutionary timescales. There is more larval dispersal from natal habitat than would seem propitious. Furthermore, long larval duration is known to increase reproductive output for species persistence, makes population retention of favorable alleles less likely, and reduces the genetic diversity of the population.
The Co-PIs have shown that maintenance of range boundaries for a species are governed by a function analogous to that derived for allelic frequency/genetic clines in the coastal ocean. As with other recent advances in biodiversity theory, this work suggests a convergence between conditions that maintain the distribution of alleles within species and those that maintain the distribution of species themselves. This confluence of theory provides substantial opportunity for development of inter- and intra-species competition in an advective environment. It potentially would unify genetic and population-level theory, and create a holistic view of life in advective environs.
It is clear from preliminary work that a synthesis would depend critically on tradeoffs between dispersal mode and successful reproductive output. The theory would be developed both by pushing its analytical envelope, and by drawing upon extensive, existing databases to quantitatively constrain reproductive and dispersal tradeoffs. For example, although tradeoffs between larval quantity versus quality (i.e., many "energetically cheap" larvae versus few "highly provisioned" individuals) have long been the subject of qualitative models, they have not been quantitatively defined for life history characteristics of different benthic marine taxa. Combining analytical developments and observed life-history tradeoffs would provide 1) evolutionarily stable states for a range of dispersal strategies, 2) mechanisms that define species boundaries as a function of physical (e.g., temperature and alongshore variation in currents) and biological (like larval mortality) parameters and 3) quantitative origins of dispersal behaviors that would locally retain larvae, and result in relationships between inter- and intra-species fitness.
Such findings would predict species boundary locations and the presence/absence of various dispersal strategies as a function of local circulation, environmental conditions and their gradients. Predictions would be tested against data on species ranges gathered as part of an extensive literature and database search.
Broader impacts: This research would allow a better mechanistic understanding species' ranges that occur due to changes in the Earth's climate. For example, this study will test the hypothesis that warming favors species with longer larval planktonic duration. Therefore, high-latitude areas now dominated by species with direct development would shift to a mixture of planktonic dispersers and direct developers as the climate warms. The research would allow managers to understand how disruption to habitat can alter species ranges by changing alongshore sources and transport of planktonic larvae. A quantitative theory of species range will also help managers understand what sets the ultimate limits of recently introduced exotic species, allowing improvement of management strategies. This proposal includes the following education components. Two graduate students will be trained in cutting-edge techniques in the fields of quantitative phylogeography and biogeography. In addition, two undergraduate students each year will assist with all aspects of the project and will present their work at a national meeting. The students will be mentored to write REU proposals to NSF. Undergraduates will be recruited with the help of the Louis Stokes Alliance for Minority Participation, of which UGA is a flagship member. Undergraduates will also be recruited from the Research and Discovery Program at UNH, from colleges with limited opportunities for undergraduate research.
Boundaries in the ocean and the disadvantages of having a long larval life On land, it is easy to imagine that some species cannot reach some places. A mountain, a desert, a river—depending on the organism, these or other landscape features can act as barriers to movement. But what stops organisms in the ocean? Although ocean waters appear to connect all marine habitats, different regions have water masses with different properties, and the associated variation in temperature, salinity or nutrients may affect species distribution. Traditionally, temperature has been viewed as the most important factor to explain distribution of species in the ocean, but recent studies suggest that currents also play an important role (Fig 1). Ocean currents can transport spores, eggs and larvae of marine species. Many marine invertebrates have a larval stage in their life cycle, which can last from days to months, during which they drift or swim in the ocean. It is generally accepted that greater time spent in the water is associated with greater dispersal distance. But more time in the water also means more time to be negatively affected by bad currents. Depending on their direction and pattern of circulation, currents can carry the tiny larvae far from suitable habitat, and thus act as a boundary between ocean areas. If currents are important to explain boundaries in the coastal ocean, we would expect more boundaries for those species with larvae that stay in the water for a longer period, and the distribution of species boundaries in the ocean to be related to the pattern of circulation. To explore these ideas and understand better what determines species boundaries in the coastal ocean, we analyzed the distributions of ~1800 marine invertebrates (crustaceans, mollusks, annelids, echinoderms and cnidarians) along the eastern coast of North America. We gathered information on depth distribution of the species—that is, whether they inhabit shallow or deep waters—and we classified species by "short" or "long" larval duration, depending on the time that larvae spend in the water to complete development (Fig 2). Did all species have the same chance of being affected by a boundary? No. Species with different characteristics showed different locations of boundaries. Some locations along the coast had more boundaries for shallow species; some locations were more important barriers for deep species. When comparing similar locations, we found more boundaries blocking movement from south to north than from north to south. Looking closer at the northern boundaries, we found, as expected, more boundaries in species with long larval duration (Fig 3). Overall, our results suggest that the pattern of circulation can affect the location of range boundaries, particularly in species with long larval duration. The consistency of these current patterns and their strong effect on larvae can explain why species boundaries of so many different species are concentrated in narrow portions of the coast. Also these currents can affect the evolution of larval dispersal strategies because some hydrodynamic environments will make some strategies unviable. Furthermore, the constraints set by currents will affect how climate change influences species distributions, which may depend not only on changes in ocean temperature, but also in ocean circulation. Broader impacts: Eight publications will result from this project. Also, we have used this work to shape an Honors evolution class on the interaction of climate change, biogeography, and evolutionary dynamics with a publically-accessible web resource: (http://pisaster.genetics.uga.edu/groups/evolution3000/ ). We posted a popular synopsis of our work on species distribution ranges on the Ecography Blog: (www.ecography.org/blog). Also, a presentation summarizing our range work is posted on the Faculty of 1000 Posters: http://f1000.com/posters/browse/summary/1097501 We trained 2 postdoctoral researchers, 1 PhD student, and one technician who is now pursuing a PhD in marine ecology. We anticipate several additional publications in high quality journals over the next 1-2 years that will continue to advance our understanding of how linkages between ocean physics and biology set species distribution patterns and how these might change with climate conditions.
