and that the regional-local species richness relationship was nonexistent within the minnow or darter families. Overall, within all levels of basin (regional) richness, there was great variation in numbers of species in local assemblages. They attributed the lack of strong regional effects to local physical factors at sites within basins, or to within-basin zoogeographic chance in movement or distributions of species. In an analysis of midwestern stream fishes at 65 sites in 13 drainages from Nebraska and Iowa south to Texas, local factors had more effect on species richness than did the overall size of the regional species pool (Marsh-Matthews and Matthews 2000). However, in contrast to emergent assemblage properties (i.e., species richness), primary assemblage structure (i.e., the occurrence of particular species) was strongly influenced by broad geographic factors, primarily latitude, reflective of the fact that many species have restricted north-south distributions (Conner and Suttkus 1986; Cross et al. 1986).
Regional and historic filters, as emphasized by Tonn et al. (1990), clearly can have a major influence on local assemblages and in some cases, especially southeastern streams and northern lakes, the richness of local fish assemblages is strongly affected by regional diversity. Species composition also can be influenced by large-scale factors such as latitude or divisions between major river basins. However, not all assemblages show a relationship between regional and local diversity, as evidenced by harsh midwestern streams and speciose upland streams. In addition, the scale of the study influences the outcome—namely, how small is the local area and how broad is the regional area.
SUMMARY
Fish populations and assemblages are shaped broadly by landscape features and by the mosaic pattern of patches within a landscape. The scale on which fishes perceive patches varies among taxa and life-history stage. Many fish populations likely comprise linear or dendritic metapopulations, although rigorous tests of metapopulation structure of North American freshwater fishes are extremely uncommon and one of the most rigorous studies supports an island-mainland metapopulation model.
Statistical models relating fish species and assemblages to environmental factors are increasingly widespread and generally fall into two groups. A priori models attempt to predict assemblage characteristics and, less often, species occurrence from general environmental features. Three common a priori models are the habitat template, the river continuum, and the landscape filter, and all have some predictive successes. A posteriori models use multivariate statistical techniques to find suites of environmental variables (both physical and biotic) that best predict the occurrence or abundance of species, assemblages, or functional groups. Such models rely on the approximately simultaneous collection of fishes and potentially predictive variables. However, a new class of multivariate models, “niche models,” uses museum collections of fishes in concert with independently collected digitized environmental data sets suitable for GIS programs. Because the various models have different strengths and weaknesses, the use of several models is often appropriate, with the choice based on the research questions and the nature of the data.
Fish assemblages are a product of local conditions and regional factors, including the regional fish fauna. Studies of both lentic and lotic fish assemblages suggest that the relative influence of local versus regional factors varies, although there is support for the influence of regional factors in both, as well as the importance of local factors, including the presence or absence of predators and habitat complexity. The importance of local versus regional effects seems to be heightened in harsher systems. However, as with population models, the number of studies of local versus regional effects on fish assemblages is quite limited, especially in terms of representation of different geographical regions.
SUPPLEMENTAL READING
Gotelli, N. J., and A. M. Ellison. 2004. A primer of ecological statistics. Sinauer Associates. Sunderland, Massachusetts. A useful source for understanding ecological statistics.
Hanski, I., and M. E. Gilpin. 1991. Metapopulation dynamics: Brief history and conceptual domain. Biological Journal of the Linnean Society 42:3–16. Provides a background of the metapopulation concept.
Leopold, A. 1949. A sand county almanac. Oxford University Press, New York, New York. A “must read” for students interested in conservation.
Turner, M. G. 2005. Landscape ecology: What is the state of the science. Annual Reviews of Ecology and Systematics 36:319–44. A good source for current views of landscape ecology.
Wiley, E. O., K. M. McNyset, A. T. Peterson, C. R. Robins, and A. M. Stewart. 2003. Niche modeling and geographic range predictions in the marine environment using a machine-learning algorithm. Oceanography 16:120–27. Provides a good background and examples of niche modeling.
FIVE
The Formation and Maintenance of Populations and Assemblages
CONTENTS
Application to Fish Assemblages
A Return to Historical Effects
Species Characteristics and Assemblage Formation
Colonization in Some Systems but Not Others
Colonization by Some Taxa and Not Others: The Role of Trophic Position
Movement and Assemblage Formation
Movement Inferred from Colonization Studies
Movement Inferred from Tagging Studies
ECOLOGISTS STUDYING FISH populations and assemblages most often are dealing with their subjects already formed, at least in the sense of ecological time. In Part 1, the examples of the colonization of new habitats that were opened up following glacial retreat showed how species might be added to such systems. Obviously, opportunities to study natural assemblages in the early stages of their formation are rare. However, opportunities do occur when assemblages repopulate after being eliminated due to loss of habitat during droughts or because of intense flooding that can cause local or regional extirpation of fishes.
This chapter deals with issues of how species are added to assemblages, both from the standpoint of how preexisting species might alter the outcome and from the standpoint of how adept fishes are in moving about their environment (see figure in Part 2). In thinking about how fish assemblages might be formed or reconstituted, a key question is how species are added to an assemblage, and whether there are “rules,” other than having a suitable habitat, that govern the order and kind of species additions. Are
