Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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Metric
Dealing with Environmental Change
Levels of Persistence and Stability in Lotic Systems
Examples of Persistence and Stability in Lotic Systems
Levels of Persistence and Stability in Lentic Systems
Examples of Persistence and Stability in Lentic Systems
Persistence and Stability Summary
Persistence and Stability of Local Associations
Persistence, Stability, and Control of Fish Assemblages
THE FIRST TWO CHAPTERS in Part 2 examined how fish species and assemblages are affected by broadscale landscape features, how various models relate assemblages to the environmental variables, how fish assemblages are formed, and the role that movement plays over different life-history stages in allowing fishes to access new habitats and to move among habitats so that their fitness is maximized. This chapter focuses primarily on the temporal and spatial dynamics of fish assemblages, or how fish populations and assemblages cope with relatively short-term physical and biotic challenges.
Understanding the type, frequency, and magnitude of variability in fish assemblages is important for several reasons (e.g., Grossman et al. 1990; Matthews 1998). First, assessing the impact of anthropogenic environmental changes requires knowing the background level of natural variation in assemblages. Second, the degree to which assemblages are resistant to changes over space and time is related to the strengths of control mechanisms operative within the assemblage. Although assemblages generally are structured and not random collections of species from a regional species pool (Chapter 5), once established, they may be acted upon by external or internal processes. With some exceptions (Strong 1983), assemblages showing high variation in species composition and abundances may primarily be governed by external, stochastic (i.e., random) processes such as floods, droughts, or other major events. These events can control such processes as species persistence, colonizations, or even extinctions. In communities with strong stochastic influences, the importance of biotic interactions (i.e., competition or predation) in affecting community structure is considered to be lessened because of the frequent changes in species composition. In contrast, assemblages that show little variation may be controlled primarily by deterministic processes, such that the characteristics of the environment result in a particular suite of species (e.g., the landscape filters described in Chapter 4). In assemblages that show little variation in species composition, the possibility of well-developed biotic interactions is considered to be greater (Grossman et al. 1982; Lepori and Malmqvist 2009). Importantly, processes controlling communities should not be viewed in an either-or situation. Stochastic and deterministic processes can act hierarchically (i.e., stochastic processes influence the species on which deterministic processes act). The relative importance of stochastic versus deterministic controls varies with disturbance levels, although not necessarily monotonically (Lepori and Malmqvist 2009).
RESPONSES TO ENVIRONMENTAL PERTURBATIONS
Types of Perturbations
Natural perturbations have shaped the evolution of fish populations and, in the case of severe events, have resulted in the local extirpation of populations or the total extinction of species. For instance, large-scale Cenozoic climatic changes resulted in the extinction of numerous western North American fishes at the end of the Miocene and also the early Pleistocene (G. R. Smith 1981). Natural perturbations include droughts, floods, fires within the watershed, climatic changes, and biotic changes (such as the addition or loss of a predator). Human-induced changes might include chemical spills or piscicide applications; changes in land use, such as mining, agriculture, or timber harvesting resulting in flooding, increased water temperature, nutrient or herbicide runoff, and erosion; major barriers to fish movement as a result of dams or water diversions; stream channelization; and the introduction of nonindigenous species.
One way to view both natural and human-caused disturbances is by their extent. Events that persist longer than the life spans of the species in an assemblage and impact large spatial areas are referred to as press disturbances, in contrast to pulse disturbances, which 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