(sensu, Diamond 1975) could be applied to the speciesrich Ozark stream fish assemblages of eastern North America. The study focused on six small streams occupied by a suite of 13 largely insectivorous species in the minnow, silverside, and topminnow families. All of the species had overlapping ranges even though each species did not occur in all of the streams. One of the hypotheses tested was that pairs of the common/abundant species were associated randomly among the six small streams. The alternative hypothesis, that species occurred only in certain combinations, would support the idea of assembly rules. The outcome of this statistical experiment showed that the occurrence of species pairs based on the random assortment of species into watersheds (keeping both the number of watersheds in which a species was common/abundant and the number of species per watershed equal to observed values) was not different from the observed values. From this, Matthews (1982) concluded that no other process, other than random assortment, was required to explain the pattern of mutual abundance of the small insectivorous fishes in watersheds.
Although this simple statistical experiment is appealing, a follow-up paper (Biehl and Matthews 1984) used more sophisticated statistical approaches to reexamine the results of Matthews (1982). Relative to the previous conclusion, they found that both the way the simulations were computed as well as the way the observed and expected values were compared (chi-square test) may have affected the outcome of the study (i.e., reducing the confidence in the conclusions). Subsequent studies have built on Matthews’s work; not surprisingly, the following two examples are from biologists who did their doctoral studies under the direction of W. J. Matthews.
Winston (1995) tested predictions coming from Diamond’s assembly rules with assemblages of phoxinin cyprinids (i.e., all minnow genera present in the study area except the leuciscin, Notemigonus) inhabiting streams of the Red River drainage in Texas, Oklahoma, and Arkansas. He found that species pairs that were most similar morphologically, and thus most likely to experience greater competition, co-occurred significantly less often than expected by chance. A potential problem with this outcome is that the absence of species’ co-occurrences could be due to historical as well as ecological reasons. For instance, closely related species could have arisen through allopatric speciation and thus have different spatial distributions by virtue of being adapted to slightly different environments (Winston 1995).
In a second test, Winston (1995) was able to rule out the historical explanation by showing that species pairs that were evolutionarily most closely related (based on a cladistic analysis) did not show co-occurrence patterns that differed from random patterns. Consequently, he argued that interspecific competition was the “most likely mechanism causing the pattern.” These results support a prediction of Diamond’s assembly rules—namely that certain species combinations do not occur in nature. However, the actual mechanism(s) responsible for the nonrandom pattern of species occurrence in this study are unknown.
In a study of benthic fishes living in small headwater streams of northeastern Oklahoma, Taylor (1996) also showed that species occurrence patterns can be affected by other resident species. Taylor compared the actual species area relationship with one predicted by a null model and showed that actual species richness relative to the area of habitat declined much more steeply than expected for smaller habitats. This suggested that some factor in addition to area was influencing the number of species. By following this regional analysis with manipulative field experiments, Taylor (1996) was able to show that asymmetrical interference competition between Banded Sculpin (Cottus carolinae) and Orangethroat Darter (Etheostoma spectabile) was responsible in part for the difference between observed and expected species densities (see also Chapter 11).
Thus there is evidence that freshwater fish assemblages can be structured, as well as evidence that competition between species pairs is one of the mechanisms causing such structuring (see also Chapter 11). However, in general, studies addressing factors governing the formation of fish assemblages have been surprisingly rare and have been restricted to only one general region, the Great Plains of North America.
A Return to Historical Effects
The concept of community assembly rules, as it has developed, focuses primarily on current ecological interactions. As I have already emphasized in earlier chapters (see Chapter 2; Figure 2.1), contemporary species interactions must be viewed in the context of history. For instance, Gorman (1992) analyzed assemblage formation in several Ozark upland streams and argued that general patterns of habitat use and ecological interactions were “ancient and not the result of an ongoing process.” Consequently, the occurrence of fish species on a macrogeographic level (i.e., drainage) was best predicted by historical, biogeographic factors. In contrast, ecological processes best predicted abundance patterns in local assemblages. Thus community assembly was largely determined by historical factors, whereas contemporary ecological processes affected patterns of species’ abundances but not occurrences. The contrast with the studies by Winston (1995) and Taylor (1996) is perhaps one of scale—at what spatial level do contemporary interactions control not only abundances but also presence or absence of species?
Species Characteristics and Assemblage Formation
Are species characteristics such as body size, geographic range, trophic position, and population size related to their ability to colonize habitats? In a long-term study of fishes in the Cimarron River in Oklahoma, Gotelli and Taylor (1999b) found that colonization potential of native fish species at 10 study sites was, not surprisingly, correlated with species’ abundance, but that geographic range size and body size were not. The species pool considered by Gotelli and Taylor (1999b) was based on the known native fish fauna of the Cimarron River from the Oklahoma study sites, so the concern of adaptation to the regional environment was not an issue.
Other studies have examined the colonization potential of species that are not native to the general region of the study systems. Information on these situations can provide general insight into factors important in the addition of species to assemblages (see also Chapter 15).
THE MATCH BETWEEN SPECIES TOLERANCES AND THE PHYSICAL HABITAT Moyle and Light (1996a, b) developed a set of empirically based predictions regarding colonization success in fishes. Although the predictions were largely developed to understand invasions of nonnative species, most are also applicable to colonization potential of native species with the caveat that they were developed for western (California) fish assemblages. A principal conclusion of Moyle and Light’s work in California streams and estuaries was that “the most important factor determining the success of an invading fish species is the match between the invader and the hydrologic regime.”
Colonization in Some Systems but Not Others
In general, most studies support the view that all aquatic systems are potentially invasible, although systems that have been highly altered tend to be more prone to invasions (Moyle and Light 1996a). Numerous authors have suggested that species-rich assemblages, such as those found in the southeastern United States, are more resistant to invasion by nonnative taxa when compared with species-poor systems, although there are exceptions to this (e.g., Ross 1991; Lodge 1993). However, given the condition that where there are species-rich systems, and that these systems have gained species richness both by in situ speciation and colonization, it stands to reason that species with appropriate physiological tolerances and ecological characteristics can enter and become established.
Colonization by Some Taxa and Not Others: The Role of Trophic Position
Resource use, and especially trophic position, are thought to be important in predicting the invasion success of nonnative fishes. In California streams, successful invaders of generally unmodified aquatic systems tend to be at one extreme or the other of the trophic pyramid—either top predators or detritivores/omnivores. In both cases, these are trophic levels that tend not to be food limited (Moyle and Light 1996b). A more recent study incorporating
