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The Effects of Disturbance and Succession on Wildlife Habitat And This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. 11 The Effects of Disturbance K EVIN S. McKELVEY and Succession on Wildlife Habitat and Animal Communities his chapter discusses the study of disturbance and al l affect both the postdisturbance wildlife community Tsuccession as they relate to wildlife. As such, the and, more importantly, the trajectory of the postdistur­ discussion is confined to those disturbance processes bance community. Even for well-studied species, co­ that change the physical attributes of habitat, leading herent understandings of their relationships to distur­ to a postdisturbance trajectory. However, even with bance across time and space are therefore often vague. this narrowing of the scope of disturbances discussed, Commonly, we look at successional changes in habi­ there remain formidable obstacles prior to any co­ tat quality by using spatial samples of different ages as if herent discussion of disturbance. The first, and most they were a temporal series, which implies spatiotem­ fundamental, is definitional: what constitutes distur­ poral constancy in successional dynamics. This assump­ bance and succession, and what is habitat? The con­ tion has served wildlife research well, but in the face of cepts of disturbance, habitat, and succession are highly directional climate change and the nearly continuous scale-dependent; disturbances at one scale become addition of exotic species, this approach is becoming part of continuous processes at a larger scale, and ideas increasingly untenable. We need to embrace the idea associated with succession require assumptions of con­ that postdisturbance succession is increasingly unlikely stancy, which become highly problematic as spatiotem­ to produce communities similar to those that the dis­ poral scales increase. Literature on the effects of distur­ turbance altered: short-term successional patterns are bance and succession on wildlife, however, focuses on a likely to be influenced by the large and dynamic pool of narrow range of spatial scales, primarily occurs within exotic plants and animals and longer-term succession a narrow temporal window immediately following dis­ by directional climate change. turbance, and seldom includes interactions between Together, these observations indicate a need for areas within the disturbed patch and the landscape studies of disturbance and succession at larger spatia­ that surrounds it. While these largely descriptive stud­ temporal scales. It is, however, important to assess ies undoubtedly have great local value, more general the feasibility of scaling up studies in time and space. information about the relationships between organ­ Clearly, studies that expand both the spatial and tem­ isms and environments shaped by disturbance and poral dimensions of data collection quadratically in­ succession is remarkably limited. Multiple small-scale crease costs; longer time frames do not fit into current descriptive studies of the immediate postdisturbance competitive funding structures and contain a variety environment do not appear to coherently aggregate of negatives such as the potential loss of data. Broad­ into larger understandings of the effects of disturbance scale targeted monitoring, however, can provide a and succession on wildlife. Context is important: the framework allowing acquisition of data at these scales conditions at the time of the disturbance, in adjacent and can be designed to produce both immediate and undisturbed patches, and within the broader landscape longer-term results. 144 RESEARCH AND CONSERVATION and other habitat features). Temporal habitat patterns Disturbance, Succession, and Questions are as critical for conservation as are spatial patterns, of Scale but much harder to study. In many cases, the events Bormann and Likens (1979) defined disturbance as dis­ that structure landscapes and define species ranges are ruption of the pattern of the ecosystem, principally by rare- often singular. For example, the genetic popula­ external physical forces. This idea, however, assumes tion structures of many species are strongly associated that ecosystems function as idealized Newtonian sys­ with glacial vicariance that occurred during the Pleis­ tems, in stasis until external energy is applied. But few tocene (see Shafer et al. 2010 for a review). To study ecosystems exist in equilibrium (Sousa 1984 ). Further, these rare events, we primarily look to the past to gain Rykiel (1985) noted that whpt±rer disturbance is viewed insight into their frequency, size, and postevent succes­ as changing the state of an ecosystem or being part of sional and evolutionary trajectories. Further, for prac­ that state is entirely a function of scale. At one scale, a tical reasons, we frequently use habitats of different tree-fall is a state-altering disturbance; at a larger scale, ages as surrogates for the passage of time, making the it is part of a continuous process that creates and main· tacit assumption that if we were to project one area tains the state of an old-forest ecosystem. The same forward (or backward), it would be similar enough to a thing is true for larger disturbances; at one scale, fire in sunogate area that we can infer its future or past state western US forests represents a significant disturbance by studying that surrogate. I refer to this approach as to ecological function, radically altering wildlife habi­ "trading space for time." tats. At a larger scale, fire is a part of the ecosystem­ The study of temporal patterns of habitat over time, many ecosystems are dependent on fire to mruntai.n and specifically the validity of using spatial surrogates to the presence, patterns, and juxtapositions of plant and infer temporal patterns, is tightly linked to the concept animal species (Habeck and Mutch 1973· Covington of succession. Here I use this term in a neo-Clemensian I and Moore 1994; Nowacki and Abrams 2008). fashion (Clements 1916; Daubenmire 1952): suc- Similar-scale dependencies are associated with cession assumes a pattern of orderly and predictable the effects of disturbance on wildlife habitat. Habitat changes in species presence and abundance that occur is often thought of with a particular scale in mind: over time after disturbance, leading to a fairly stable perhaps a forest stand or home-range area evaluated terminal state, or climax. As Gleason (1927) noted, the across a year or the lifespan of an organism. If succes­ idea of succession is very appealing: if understood, it sional thinking is applied, habitat may be defined as allows us to predict the future and to see into the past existing within a specific sere (e.g., a species may be without needing any data other than what we collect considered to be associated with early seral or late sue· in the present. However, successional concepts cannot cessional forests). However, the habitat requirements be accepted naively. The validity of this concept, and its for population persistence are, like disturbance, com­ resulting popularity (or lack thereof), is directly related plex and span many scales in both space and time. In to the complexity of the system studied and the scale space, wildlife habitat spans spatial domains measured at which the system is evaluated. Simple systems have in meters (e.g., specific resources for denning), to ki­ fewer succession pathways and fewer species; hence, lometers (e.g., sufficient resources to support a local the vegetative trajectories are more predictable. For viable population), to hundreds of kilometers (e.g., a example, in the western forests of the United States, mosaic of resources and connectivity sufficient to sup­ tree communities are often simple and contain large port long-term metapopulation [Levins 1969, 1970] areas of intact natural vegetative communities. In persistence and abundance). Additionally, at all spatial these systems, succession-based classification systems sca1es, juxtaposition and spatial patterns of habitats (e.g., habitat types; Pfister and Arno 1980) and con­ are important (e.g., Iverson ~tal. 1987). Temporally, cepts (e.g., potential natural vegetation [PNV]; Kuchler the definition of habitat is even more complex. Again, 1964) are popular. In highly modified landscapes con­ scales vary from almost instantaneous (e.g., the timing cepts, however, concepts like PNV become abstract as of ice breakup in the Arctic) to millennia! (e.g. , the pro­ none of the vegetation within a study area may exist cesses of erosion and deposition that create soils, caves, in its putative potential condition (Zerbe 1998). For E F FECTS O F DISTURBANCE AND SU C CESSI O N O N WILD LIF E H ABITAT 145 these reasons, looked at objectively, succession is often process, we discuss and label this process as part of a a problematic concept, but one that has proved useful separate body of literature. Similarly, we are prone to in many communities and without which our ability view what we do as being more of a disturbance than to study systems and build predictive models would be what other organisms do. For example, beavers (Cas­ severely limited. However, regardless of its historical tor canadensis) instigate a variety of disturbances (tree validity, there is good reason to doubt its relevance as a felling, house and dam construction), oftentimes re­ tool to predict future habitat conditions; the likelihood moving all accessible trees from the area adjacent to of rapid directional climate change
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