FIGURE 4.3. Various conceptual models of metapopulation structure. Shaded circles are occupied; open circles are unoccupied; arrows indicate directions of movement. Based on Harrison and Taylor (1997), Wiens (1997), Fagan (2002), and Farina (2006).

      Metapopulations

      Given that aquatic habitats can be viewed as temporal and spatial mosaics of varying suitability, it is not surprising that populations of a species are not distributed uniformly across the aquatic landscape but instead tend to be aggregated in areas offering the most suitable habitat components. This view of how populations are distributed in space has been formalized as the metapopulation concept—namely a metapopulation is “any assemblage of discrete local populations with migration among them” (Hanski and Gilpin 1997) (Figure 4.3). As such, a metapopulation is “a set of populations that are interdependent over ecological time” (Harrison et al. 1988). Each local population or deme is subject to forces of local selection, including extinction, emigration, and immigration of individuals from other local populations. The balance of these forces determines the fate of local populations, the extent of sites occupied, and the overall size and genetic diversity of the metapopulation (Hanski and Gilpin 1997; Policansky and Magnuson 1998). The landscape concepts of patches and corridors and the ability of organisms to move between them are thus central to the metapopulation concept. (Movement is treated in Chapter 5.)

      The term metapopulation (literally a population of populations) was first used by Richard Levins in 1970, although the mathematical description of a population of a single species comprising interconnected local populations appeared in 1969 (Hanski and Gilpin 1991). Levins (1970) described a landscape of occupied and vacant patches as a result of the colonization and extinction of local populations, with the overall collection of local populations comprising a metapopulation (Hanski and Gilpin 1991; Hanski and Simberloff 1997). This is often referred to now as the “classical” or “Levins style” metapopulation (Figure 4.3A) (Hanski and Simberloff 1997; Gotelli and Taylor 1999a). The concept has been readily, and often uncritically, applied to fish populations because the occurrence of habitat patches can result in spatial structuring of populations, such as linear or dendritic metapopulations in streams (Figure 4.3B, C) (Fagan 2002; Campbell Grant et al. 2007). However, without actual assessment of local colonizations and extinctions, rates of movement among local populations, and genetic structure, just because a species may comprise discrete local populations does not automatically indicate that it fits one of the metapopulation concepts (Hanski and Simberloff 1997; Gotelli and Taylor 1999a). For instance, a species existing as a number of discrete populations would, in the absence of movement, constitute a nonequilibrial population and would not be considered a metapopulation (Figure 4.3D).

      Gotelli and Taylor (1999a) provide one of the few studies on freshwater fishes to rigorously test classical/Levins-style metapopulation predictions. They used a large data set of 46 fish species, censused 2–3 times per year for 11 years at 10 sites in the Cimarron River, Oklahoma. In the Levins-style metapopulation (Figure 4.3A), the probability of colonization should increase as the proportion of occupied sites increases because there would be more occupied sites from which dispersal could originate. Similarly, the probability of extinction should decrease as the proportion of occupied sites increases because of the increased odds of rescue from another site (Gotelli and Taylor 1999a). Using the 11-year data set for each species, Gotelli and Taylor constructed a matrix showing the occurrence of each fish species at a census site in each year. The proportion of occupied sites was the number of sites occupied in a year, divided by the number of censused sites. The probability of extinction was the number of occupied sites in year (t) that were not occupied in year (t + 1), divided by the number of occupied sites in year (t). Finally, the probability of colonization was the number of vacant sites in year (t) that were occupied in year (t + 1), divided by the number of censused sites in year (t).

      In contrast to the prediction of a Levinsstyle metapopulation, the probability of extinction overall was not related to the proportion of occupied sites, although 5 of the 36 species in the analysis did show a significant negative relationship and thus fit the prediction. Similarly, the probability of local colonization was not related overall to the proportion of occupied sites; at the level of individual species, only one species showed a significant positive relationship. Rather than site occupancy, the position in the river system was a more important predictor of colonization and extinction, with the probability of extinction increasing in upstream areas and the probability of colonization increasing in downstream areas. Although the Cimarron River fishes did not fit the predictions of the Levins-style metapopulation model, they did fit predictions of an island-mainland metapopulation model where local extinctions are independent of each other and colonizations occur from outside of the smaller patches (Figure 4.3E). An island-mainland model is appropriate when there is wide variance in the size of the local populations or high variation in patch quality (Harrison and Taylor 1997).

      Stream fishes are often viewed as having linearly arranged metapopulations (Figure 4.3B), and in some instances this may be appropriate when species, such as the Bayou Darter in Mississippi, occur almost exclusively in main-channel habitats (Ross et al. 1992b). In many other instances, fishes occur in a broader range of stream sizes so that a dendritic model (Figure 4.3C) would better capture their population structure.