are of short duration and are generally point source or brief hydrologic events (Bender et al. 1984; Detenbeck et al. 1992). Based on Detenbeck et al. (1992), press disturbances would include impacts of channelization, large-scale habitat alterations, timber harvesting, mining, and changes in nutrient input; pulse disturbances would include floods, chemical spills, droughts, nonchemical removal of biota, and localized construction activity.
Determining what amount of environmental variation actually represents a disturbance or perturbation (since the terms generally are used interchangeably although there are exceptions; e.g., Pickett and White 1985) to aquatic organisms is also challenging—especially for terrestrial, hominid biologists! Natural variations in physical conditions, even some viewed as “a disturbance,” are generally beneficial in the longterm to the well-being of aquatic systems. This would include changes in stream flow (including flow into lakes, ponds, and reservoirs), turbidity, temperature, ice cover, or insolation. For instance, without periodic high, scouring flows in streams, streambed complexity (Mount 1995) and complexity of riverine food webs (Wootton et al. 1996; Power et al. 2008) can be greatly reduced, resulting in population declines or loss of fish species. Likewise, the annual or semiannual turnover in many lakes results in redistribution of nutrients to surface waters and oxygenation of bottom waters (Wetzel 2001).
The recognition of the value of periodic disturbance in ecological communities in the 1970s and 1980s led to models of how periodic disturbance fostered increased species diversity. This corresponded with the recognition that most communities probably did not exist at some sort of steady state or equilibrium (Levin and Paine 1974; Sousa 1984). The intermediate disturbance hypothesis (Levin and Paine 1974; Connell 1978) predicts that the greatest species richness would occur at some intermediate level (intensity and/or frequency) of disturbance. The logic is basically that intermediate levels of disturbances provide sufficient time for species to colonize affected patches of habitat yet keep the habitat from being dominated by only a few species (Connell 1978; Sousa 1984). In a similar way, the dynamic equilibrium model (Huston 1979) predicts that diversity of communities is the outcome of two processes—the rate of population growth of competing species, balanced against the frequency of population reductions, caused by various types of disturbances. In contrast to disturbance functioning by mediating competitive interactions between species, the role of intermediate disturbance in a study of stream macroinvertebrates was due to the removal of more sensitive species, so that invertebrate communities converged to a core group of species moderately resistant to disturbance (Lepori and Malmqvist 2009).
What constitutes a disturbance also changes over ecological and evolutionary time and among taxa. Viewed in the evolutionary context of species and assemblages, a force that once was a major disturbance might be less so today given the strong selection for populations to withstand environmental 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,
