Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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change (Sousa 1984). In ecological time, as elaborated later in the chapter, life-history stages of a species differ in their abilities to respond to disturbances, just as species differ in their responses. Furthermore, the seasonal timing of disturbance can affect the level of impact on fish populations and, because of the wide variation in body size and mobility among fish species, disturbance must be viewed relative to the spatial and temporal dynamics of species (Pickett and White 1985).
Efforts to define disturbance have taken two main approaches. One approach defines a disturbance by its magnitude, whereas the other defines a disturbance by the population, species, or community responses to it or to its impact on the physical environment (Resh et al. 1988; Matthews 1998). In the former case, a sudden change in water temperature or stream flow that exceeded some arbitrary value, say ± two standard deviations, would be judged as a disturbance, whereas a change of less than ± two standard deviations would not. In the latter case, if there were no apparent biological or physical response to what would seem to be a disturbance, such as a major flood event, then the event would not be considered a disturbance. The latter approach has generally been preferred (e.g., Resh et al. 1988), and a useful working definition of a disturbance proposed by White and Pickett (1985) and used by other authors (e.g., Resh et al. 1988; Yount and Niemi 1990) is “any relatively discrete event in time that disrupts ecosystem, community, or population structure, and that changes resources, availability of substratum, or the physical environment.” As such, a disturbance is “the primary event, or cause, from which certain effects follow” (Yount and Niemi 1990).
The Metric
Responses to environmental change can basically be measured by the presence or absence of species, irrespective of the actual numbers or relative abundance of individuals. This qualitative measure is referred to as persistence, in contrast to stability, which is based on abundance measures (Connell and Sousa 1983). Quantitative measures include relative abundances, or actual numbers or densities of the component species. The choice of metric has a strong influence on the detection of change, or lack thereof, in fish populations (Rahel et al. 1984; Yant et al. 1984; Matthews et al. 1988; Grossman et al. 1990; Rahel 1990; Matthews 1998). For instance, presence-absence, ranks in abundance, relative abundance measures, and actual numbers of individuals form a transformation series of increasing sensitivity to change. That numbers of individuals of a given species show the greatest variation is not surprising, especially because most long-term studies of fish assemblages employ sampling techniques that are not designed to provide rigorous quantitative data on population sizes (Matthews 1998).
Spatial and Temporal Scales
In addition to the appropriate metric, the spatial and temporal scales over which a measurement is made also affect the outcome. To assess stability, the temporal scale must encompass at least one full turnover in the assemblage (Connell and Sousa 1983); if it does not, then what is really being measured is simply the impact of long-lived organisms on the local community. This point can have a major impact on apparent regional differences in responses of fish assemblages to environmental change. Some southwestern fish assemblages, such as in the San Juan River of the Colorado River drainage, consist primarily of species like Flannelmouth (Catostomus latipinnis), Bluehead (C. discobolus), and Razorback (Xyrauchen texanus) suckers; Roundtail Chub (Gila robusta); Speckled Dace (Rhinichthys osculus); and Colorado Pikeminnow (Ptychocheilus lucius) (Tyus et al. 1982; Propst and Gido 2004). With the exception of the short-lived (ca. 3 years) Speckled Dace, these San Juan River species commonly live more than 20 years, and in the case of Colorado Pikeminnow and Razorback Sucker, over 40 years (John 1964; McCarthy and Minckley 1987; Scoppettone 1988; Lanigan and Tyus 1989; Osmundson et al. 1997). In contrast, southeastern fish assemblages, such as in Black Creek of the Pascagoula River drainage, Mississippi (Baker and Ross 1981; Ross et al. 1987), are composed primarily of small minnows, topminnows, darters, and sunfishes, most of which have life spans of only 1–5 years (Ross 2001). A study of 4–5 years would essentially capture one complete assemblage turnover for the Black Creek fishes, whereas an equivalent study in the San Juan River would need to extend to 20 years or more to achieve the same result. In probably the majority of studies, the temporal scale is defined more by the duration of funding or graduate student tenure (both commonly on the order of 1–5 years) than by consideration of the life history of the fishes—with some notable exceptions
The spatial scale of a study also has a major impact on the ultimate outcome (Connell and Sousa 1983; Rahel 1990). If the spatial scale does not include the normal population bounds of the component species (see Chapter 5), then it is likely that any measure will record extensive changes in assemblage structure. In contrast, a large study area might include a number of subpopulations comprising various metapopulations of the component species (see Chapter 4), so that variation or loss of taxa in one area is damped out by their survival in another. Connell and Sousa (1983) suggest that the spatial scale should correspond to the least area that is necessary for the recruitment of adults through successful reproduction, survival, and growth of young. Recalling the types and extent of movement in Chapter 5, this guideline would result in widely differing spatial scales, depending on the species and region. However, again with a few notable exceptions, the spatial scale of most studies is somewhat arbitrary or driven by sampling logistics or cost. Thus not only the analytical scale, as illustrated previously, but also the temporal and spatial scales of the analysis have strong effects on the outcome and, not surprisingly, because of the interactions between metrics and scales, there are conflicting views on the nature of change in fish populations and assemblages.
Assessing Assemblage Change
The stability and persistence of assemblages should be investigated on multiple levels within the hierarchical framework (Rahel 1990). Separating actual changes in species presence or absence from artifacts of sampling also is a pervasive and significant problem (Magnuson et al. 1994). Preferably, the goal of current research should not be to determine whether local assemblages change or not—virtually all local assemblages undergo change as individuals are added or removed (due to natality and mortality, or movements). Instead, as the temporal extent of data increases (several to many samples over a period of years to decades), and as change is measured on multiple spatial scales, it becomes of more interest to ask how much a local fish assemblage, or distribution of fish species within a watershed, have changed during various intervals (Ross and Matthews, in press). Also, an important issue is whether changes largely are driven by major events such as extensive droughts or floods, with little change during times that lack apparent disturbance factors. In other words, are the changes that can be observed in fish assemblages over long periods of time related more to gradual changes, or to dramatic events that may (or may not) leave their mark for years or more?
Dealing with Environmental Change
Fish populations can deal with environmental or biotic stressors in three primary ways. First, populations might lack means of dealing with perturbations and be eliminated from a region altogether. Second, individuals in a population might show resistance by withstanding such challenges through morphological, physiological, or behavioral adaptations, such as refuge-seeking behavior that would overall increase their tolerance to environmental perturbations. Finally, populations might emigrate from, or perish in, the stressed habitat but recover following a perturbation by return immigration of the displaced individuals or colonization by individuals from other populations, such that pre- and postdisturbance assemblage structures are the same or similar. This approach to dealing with environmental change was termed adjustment stability by Connell and Sousa (1983) and resilience by Dodds et al. (2004). Connell and Sousa (1983) considered that this response included two components: amplitude and elasticity. Amplitude is a measure of how far a population or assemblage can be displaced from its predisturbance state and still return; elasticity, drawing further on the analogy with a rubber band, is a measure of how quickly populations or assemblages can return to a predisturbance condition.
Resistance
It is not surprising that fish assemblages occurring in geographic regions prone to extreme climatic conditions are generally persistent in the face of such environmental challenges,