Chapter 11 for details on stable isotope analysis) tested the Moyle and Light hypothesis, focusing on the base of the food web (Gido and Franssen 2007). In fact, the trophic position of nonnative fishes tended to be lower than native fishes in four of the five study rivers, although the differences were small and not significant at the 0.05 level. Because their study included five streams located from New Mexico to Kansas, they were also able to compare the trophic position of four species in streams where they were native and in those where they were nonnative. In this case, three of the four species showed significantly lower trophic positions in assemblages where they were nonnative relative to where they were native. Thus this study is at least consistent with the hypothesis that low trophic level facilitates colonization.
The San Juan River, a major tributary of the lower Colorado River, has been changed by impoundments and flow modifications and has a fish fauna that includes both native and nonnative species (Gido and Propst 1999). The food web of small-bodied fishes from secondary channels and from the margin of the main channel includes three trophic levels, as determined from stable isotope analysis (δ15N ratios): a primarily detrital base, a primary consumer level dominated by midges (Chironomidae), and a secondary consumer level that includes all fishes (Gido et al. 2006). Fishes from the secondary channels showed higher food overlap than those in the main channel, indicating convergence on the same food source. Also, fishes found in swifter water in the main channel, principally Speckled Dace (Rhinichthys osculus) and Channel Catfish (Ictalurus punctatus), had slightly elevated trophic levels. Although all fishes were grouped as secondary consumers, the more common native fishes tended to feed at a higher trophic level (mean native = 3.18) than most of the nonnative fishes (mean nonnative = 3.03), a finding at least consistent with part of the Moyle and Light hypothesis (Figure 5.1).
The Moyle and Light hypothesis, predicting the invasion of nonnative fishes at the highest or lowest trophic levels, has also been tested for fishes in the Gila River system in southwest New Mexico (Pilger et al. 2010). This portion of the Gila River has received relatively low human impacts but has been subject to widespread introduction of nonnative fishes, which potentially put continued persistence of native fishes at risk. The study was based on both stable isotope ratios and gut analyses and showed that native and nonnative fishes fed at multiple trophic levels. Large-bodied native fishes, Adult Sonora Sucker (Catostomus insignis) and Desert Sucker (Catostomus clarkii), fed at low trophic levels as adults, but at a higher trophic level as juveniles. In contrast, large-bodied nonnative fishes, such as Rainbow (Oncorhynchus mykiss) and Brown (Salmo trutta) trout, Smallmouth Bass (Micropterus dolomieu), and Yellow Bullhead (Ameiurus natalis) fed at lower levels as juveniles and then at higher trophic levels as adults. Native small-bodied fishes such as Longfin Dace (Agosia chrysogaster), Spikedace (Meda fulgida), Speckled Dace, and Loach Minnow (Tiaroga cobitis), were primarily insectivorous; the Headwater Chub (Gila nigra) was the only native piscivorous fish but also consumed large amounts of algae. Overall, nonnative fishes foraged at higher trophic levels on fishes and large, predaceous aquatic invertebrates, compared to native fishes. The introduction of nonnative fishes has extended the maximum food chain length in the upper Gila River fish assemblages to higher trophic levels, although long-term survival of at least some of the native fishes is in question as a consequence (Pilger et al. 2010).
FIGURE 5.1. Trophic position, as determined from stable isotope analysis (δ15N ratios), for native (black bars) and nonnative (gray bars) fishes in the San Juan River. Based on Gido et al. (2006).
Colonization Models
Strange and Foin (2001) developed a model to predict the invasion success of six fish species (including native and nonindigenous) to cool-water (trout zone) streams, the Sagehen and Martis creeks, located on the eastern slope of the Sierra Nevada mountains near Lake Tahoe. The model incorporated life-history responses to biotic factors of competition and predation and the physical factor of stream discharge. Predictions of the model were tested against a long-term data set of relative abundance of fishes for Martis Creek from 1979 to 1994 and Sagehen Creek from 1952 to 1961. Model predictions generally captured major trends in species’ relative abundances for Martis Creek and showed excellent fit overall to data for Sagehen Creek, although some species showed better fit than others (Figure 5.2). Key points from this study of a six-species system are that invasion sequence and biotic interactions (competition and predation) were all important, but that the relative importance of these factors varied based on physical conditions. In particular, the sequence of floods and droughts had a major impact on the relative abundance and potential for invasion of each species. One of the lessons from studies such as this, which incorporate the impact of physical habitat variation, is that the formation of fish assemblages is largely nondeterministic—that is, the final assemblage cannot be totally predicted by knowing the species pool available for colonization. As stated by Strange and Foin (2001), “assembly from the same species pool is likely to result in multiple or alternate states as the physical regime alters rates of biotic processes.”
Another set of studies used the extensive database on successful and unsuccessful introductions of fishes in California to develop models predicting characteristics of successful colonizers (Marchetti et al. 2004a, b). In terms of becoming established, the best predictive model included physical characteristics of the invading species (body size and physiological tolerance), demographic characteristics (area of population), niche characteristics (trophic position), and behavioral characteristics (parental care of young). Of these, greater physiological tolerance and the degree of parental care seemed particularly important in aiding establishment.
FIGURE 5.2. Relative abundances of five of the six species from Sagehen Creek, California, based on model predictions and observed values over a 10-year period. Numerals indicate starting and ending years; the dashed line shows a 1:1 relationship. Based on data from Strange and Foin (2001).
MOVEMENT AND ASSEMBLAGE FORMATION
Freshwater fishes, with few exceptions, are naturally limited in their access to new habitats by suitable aquatic connectivity. Thus understanding how fish assemblages are formed also requires understanding the movement capability of potential colonizers found in regional species pools. To a certain extent, this tempers what one can learn about community assembly by examining nonnative taxa, given that such taxa often move more through human intervention than by natural processes (see also Chapter 15).
The terms “dispersal” and “migration” are often used to describe the movement of organisms outside of their home range. Briefly, dispersal has most often been used to refer to a general outward spreading of individuals away from each other, such as from a starting group. Consequently, it is a group definition because it is based on the movement of one individual relative to the movement of one or more other individuals (Begon et al. 1996). Dingle (1996) recommended the term “ranging” in place of dispersal. Ranging refers to individual behavior and is “the departure from the current habitat patch (emigration), the seeking of a new patch, and the occupation of the first available and suitable habitat patch discovered (immigration).” In older works, the definition of migration was constrained to include movement to and from an area or areas. For instance, Harden Jones (1968) defined migration as “a class of movement which impels migrants to return to the region from which they have migrated.” If an organism did not return, however far or periodic its travels, then the term migration was deemed inappropriate. A recent trend is to recognize migration as a persistent movement of individuals of a species from one habitat to another so that the location of each life-history stage is optimized in terms of needed resources (Begon et al. 1996; Dingle 1996). Migration usually involves distinct pre- and postmigratory behaviors and includes physiological
