Review: Dethier, McDonald, Strathmann (2003) Colonization and connectivity of habitat patches for coastal marine species distant from source populations. Conservation Biology, 17(4):1024-1035.

Feature Paper: DOWNLOAD * Dethier, McDonald, Strathmann (2003) Colonization and connectivity of habitat patches for coastal marine species distant from source populations. Conservation Biology, 17(4):1024-1035.

Author Abstract: The exchange of propagules or mobile adults between isolated habitat patches is of critical importance for some types of preserves, especially for species that cannot propagate locally. In the marine realm, the role of planktonic dispersal in maintaining viable local populations can be tested by examining life-history traits of species that colonize (or do not colonize) isolated habitat patches. We compared the abundances of benthic species on an exposed rocky jetty surrounded by dissimilar habitats on the coast of Washington (U.S.A.) with those of species at distant bedrock sites within potential source areas. Despite its isolation, the jetty lacked only a small proportion of the potential algal species; these absences could result from the 40- to 100-km distances to larger source areas or from subtle habitat differences at the jetty. Coralline algae are expected to be poor dispersers, both because propagules are short-lived and because adults are unlikely to float. These algae were absent on the study jetty, although they occur on other isolated jetties on this coast. Short-term transplant experiments indicated that corallines can survive locally once they colonize. Few animals were absent; one was a chiton that settles and feeds on coralline algae. Animals with obligate dispersal of offspring were abundant on the jetty despite their inability to propagate locally and despite dilution of larvae dispersing in the plankton from distant sources. Conversely, some animal species with no planktonic phase were also present; thus, organisms with a wide range of life-history traits can persist at this distant and small patch of suitable habitat. Isolation along this shoreline did not eliminate either poor dispersers or obligate dispersers.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: Today we'll look at an important topic in conservation biology… the connectivity of organisms within an aquatic region. The authors point out that connectivity is more well-known for organisms with short dispersal capacities compared to organisms with long dispersal ranges. 

As the authors point out, "the extreme variation in dispersal of marine plants and animals presents a problem for the design of systems of marine preserves. How well can any single combination of size and spacing of protected areas serve all the target species?"

The whole purpose of the paper is summarized by the authors below: 

"For terrestrial organisms, conservation practices have been informed by island biogeography, metapopulation models, and landscape analyses (Hanski & Gilpin 1997). In the marine realm, however, application of terrestrial models is problematic because most benthic organisms live in two landscapes at different stages in their life histories, one the sea bed and the other the overlying water; the water moves constantly and variably (Sammarco & Heron 1994; McEdward 1995). This connectivity via water ( and planktonic propagules ) constitutes a proposed advantage of marine reserves—their potential for extensive dispersal out into surrounding sites (e.g., Allison et al. 1998; Gerber et al. 2002)—but the connectivity over long distances is subject to debate (e.g., Roberts 1997; Cowen et al. 2000)."

To address their question of how to address connectivity of patchy or isolated marine ecosystems, the authors look at a number of case studies of isolated islands with depauperate faunas and floras and then expand their findings further.

The authors then conducted marine littoral (seashore and nearshore submerged and intertidal habitats) zones around an area of coast in the northwest Pacific coast of the Americas. The authors counted organisms within quadrates. To address whether certain groups of organisms were missing because of habitat or distribution (i.e., dispersal barriers) the authors did transplantation experiments.

The authors paid particular attention to obligate dispersers. A few final observations of the authors were:

"Algae, most or all of which are poor dispersers, were more depauperate even though a rare colonization event should have resulted in good local self-recruitment."

"Overall, our results are encouraging for the design of broadly effective systems of marine reserves. Our results indicate that such varied marine organisms can persist at an isolated site by quite different means. For species with a long pelagic larval period, connectivity and persistence can result from larvae that are dispersed far from the parental area if source populations are large. For species with little or no transport of propagules, persistence can result from much rarer transport between reserves because of a higher capacity for local recruitment."

"Moreover, the absence of some expected species, particularly algae, on isolated jetties suggests that spacing reserves at 50 km could reduce connectivity too much for their arrival or persistence, although distance from sources is not yet demonstrated to be the direct cause of these absences."