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Population genetic studies on zooplankton: patterns of genetic variation in organisms inhabiting insular habitats as a means of comparing the merits of different nature conservation strategies
Journal Contribution - Journal Article
Zooplankton organisms are rarely the target of nature conservation efforts, probably because they are small and not well known to the public. Yet, information on zooplankton organisms may be very useful for nature conservation. First, some rare species have very specific habitat requirements. Secondly, the species composition and size distribution of the zooplankton community may reveal a wealth of information on habitat characteristics, including on biotic factors such as fish predation pressure which is often difficult to assess directly. In addition, zooplankton organisms play a key role in the trophic cascade, and knowledge on their ecology is therefore essential to the restoration of water quality in lakes through biomanipulation (CARPENTER & KITCHELL 1993). Another important factor is that many zooplankton species, and especially members of the genus Daphnia, are well-suited for population genetic studies, because they typically occur in large numbers and have a short generation time. In addition, quantitative genetic studies are encouraged because several Daphnia species are relatively easy to rear in the laboratory and one can work with clones (DE MEESTER 1996b). Moreover, as zooplankton typically inhabits insular habitats such as ponds and lakes, studies on the genetic variation in these organisms may reveal patterns that are of general relevance for nature conservation strategies related to the design of nature reserves (which are often islands in a cultural landscape) and the consequences of habitat fragmentation (which is a process of insulating populations) (see SHAFER 1990, LOESCHCKE et al. 1994, SPELLERBERG 1996). We have started a survey study on the genetic diversity within and the genetic differentiation among populations of several cladoceran taxa, including several Daphnia species (D. magna, D. pulex and the introduced species D. ambigua) and some in our region less common species (Sida crystallina, Polyphemus pediculus and Megafenestra aurita). From each (sub)population studied, 40-60 individuals are analyzed for genetic variation at four to nine polymorphic loci by cellulose acetate electrophoresis. As all taxa studied are cyclical parthenogenetic, the genetic diversity of populations can be assessed by calculating clonal diversity. Our preliminary results on D. magna, Sida crystalline and Megafenestra aurita suggest that ( 1) there is a weak but significant relationship between habitat area and clonal diversity. For both the littoral species Sida and Megafenestra, the relationship is better when the surface area of the littoral zone rather than the surface area of the whole water body is taken into consideration. (2) There is a strong inverse relationship between genetic differentiation among populations and the average clonal diversity (VANOVERBEKE & DE MEESTER in press). This relationship reflects a drift phenomenon, and blurs the relationship between genetic and geographic distance. (3) Quantitative genetic data on phototactic behaviour illustrate that Daphnia magna populations can show a pronounced local adaptation (DE MEESTER 1996a). These data suggest that one should be prudent in linking different isolated habitats in an effort to create larger habitats, because this may lead to outbreeding depression and a loss of genetic diversity on a regional scale. We are currently studying genetic diversity in a Daphnia metapopulation to assess the effect of corridors among habitats with respect to local and regional genetic diversity. In addition, we study the erosion of genetic diversity during the course of the growing season of moderately large Daphnia populations to learn how clonal selection and drift phenomena lead to the observed relationship between clonal diversity and habitat size.
Journal: Biologisch Jaarboek (Dodonaea)
Number of pages: 2