The processes, mechanisms, and results of vegetation succession have important implications for the abundance, distribution, and structure of wildlife populations and animal communities. This statement is intuitive to the point of being obvious, yet few observational studies have explicitly demonstrated how succession can influence wildlife populations, presumably because of the long-term nature of succession. Even fewer experimental studies have been conducted because of logistical issues and constraints associated with time and money.
Newton's (1991, 1993) long-term research on the European spar-rowhawk (Accipiter nisus) is a rare example that demonstrates explicit links between changes in vegetation structure and distribution and demography of an avian species. The European sparrowhawk is a woodland raptor that eats mainly small birds and is distributed throughout Great Britain and much of Europe. For about 20 years, Newton banded and observed breeding sparrowhawks, particularly females, and monitored their nest sites, productivity, and movements. Newton's research has shown that sparrowhawk nests located in young woods with small densely growing trees had the highest occupancy rates and reproductive success, but, as woodlands matured and trees became larger and more widely spaced, both occupancy and success declined. Furthermore, removal experiments confirmed the presence of nonbreeding individuals in the population; these individuals attempted to nest when sites in younger woods became available, but would otherwise remain as non-breeders despite the presence of vacant sites in older forest stands. Thus, the quality of the habitat (in this case, young woods with small, densely growing trees) was important to breeding success. As succession progressed, the quality of the habitat for breeding sparrowhawks declined.
Although there may be few other examples of the effects of succession on wildlife distribution and demography as explicit as Newton's sparrowhawks, the dynamic forces that shape the composition and structure of plant communities obviously have important impacts on animal communities (Box 4.2). These forces and processes, if understood by wildlife managers, can be manipulated to the benefit of some wildlife populations. In addition, there are some additional key points that should be considered by those interested in managing wildlife populations.
Box 4.2 Early successional stages as wildlife habitat
In most parts of the temperate region of the world, a forest covering a broad region (i.e., a forest covering hundreds of km2, undisturbed by humans) would provide a wide array of cover types for a large number of species. Although many environmental variables, such as soil type, hydrology, and elevation, and many natural disturbances, such as fire or wind, are involved in determining the distribution and abundance of species, it is often the structure of the vegetation that ecologists tend to measure to gain some understanding of community ecology.
Historically, wildlife managers recognized that many animals, particularly some sought-after game species such as grouse, quail, and deer, were more abundant where edge habitat was prevalent. Edge habitat describes those areas or ecotones where two or more cover types meet, such as mature forest, second-growth forest, and fields. For many species, this is ideal habitat, and in fact the "intermediate disturbance hypothesis" predicts that diversity will be greatest where disturbance is intermediate. For example, a forest with a mixture of mature stands of trees, second-growth trees, regenerating clearcuts, and either anthropogenic or natural openings will have greater species diversity than either a uniform old-growth forest (minimum disturbance) or a large clearcut (maximum disturbance). Throughout the middle of the last century, wildlife managers often manipulated vegetation to maximize edge.
In recent decades, however, the loss of older forests - those woodlands characterized by large trees, dense canopy cover, large standing and downed dead wood (i.e., snags and logs) - and the concern for species dependent on those older forests - such as spotted owls, northern goshawks, and red tree voles - has led to public pressure to reduce human-caused disturbances such as logging. This has led to widespread concern for habitat fragmentation, especially in forested ecosystems (Harris 1990).
Even more recently, biologists have become concerned with the loss of early successional communities (Askins 2001). As disturbed areas such as old farm fields and young stands of trees such as aspen disappear from the landscape through both natural succession and lack of periodic disturbances such as fire, tree harvest, or mowing, these early successional communities are disappearing from the landscape, especially in eastern North America (Trani et al. 2001). Along with them go an assemblage of many early-successional stage or disturbance-dependent species (Hunter et al. 2001; Litvaitis 2001). Biologists are now recommending ways to perpetuate and maintain these early successional communities on the landscape (Thompson and DeGraaf 2001).
First, the term "habitat" is often loosely used by wildlife biologists. It has been defined as the sum total of all the environmental components used by a species for its life history. Thus, references to cover types such as pine habitat or oak habitat used by sparrowhawks, or the desert, montane, or forested habitat of bobcats (Felis rufus), are too vague and narrow, and do not conform to the habitat concept. Hall et al. (1997:175) offered the following definition:
We therefore define "habitat" as the resources and conditions present in an area that produce occupancy - including survival and reproduction - by a given organism. Habitat is organism-specific; it relates the presence of a species, population, or individual (animal or plant) to an area's physical and biological characteristics. Habitat implies more than vegetation or vegetation structure; it is the sum of the specific resources that are needed by organisms.
Thus, the concept of "habitat" is much more than just plant cover. For some species, habitat can be quite complex - e.g., habitat for migratory
species includes areas needed not only for breeding, but for migration and wintering as well (Figure 4.8).
Second, the concept of habitat "quality" is increasingly interesting to wildlife biologists. An evaluation of habitat quality is often based on the demographic performance of the species of interest; if individuals show optimum reproductive output and high survivorship, then that habitat is thought to be of high quality (DeStefano et al. 1995). High density of individuals does not necessarily mean that those individuals are in high-quality habitat or will show high breeding success (van Horne 1983; Vickery et al. 1992a). However, beyond generalizations such as high abundance and availability of food resources, well-distributed nest or den sites, or adequate safety from predators or inclement weather, biologists have not been very successful in determining the exact characteristics of a habitat that make it high quality (although see Vickery et al. 1992b). Nonetheless, the concept of habitat "quality" or "fitness" (DeStefano et al. 1995) is important to management and conservation and deserves further study.
Third, although a complete definition of the term habitat implies more than vegetation or vegetation structure, it is usually vegetation that we try to manipulate and manage for wildlife populations. Extant vegetation is a product of historical events, plant propagules, and ecological
interactions (although the relative roles of these forces are frequently unknown). For example, fire regime, intensity of livestock grazing, and silvi-cultural systems shape the resulting community - this is the "legacy" idea to which forest ecologists sometimes refer.
Fourth, stochastic events, such as wildfires, windstorms, or floods, can greatly alter the rate and path of succession and can change wildlife habitat. This realization is especially important for small populations, including species threatened with extinction, which are often the major targets of management efforts. Extinction is a deterministic process, often punctuated by a stochastic event, which frequently results from habitat loss or fragmentation (Figure 4.9).
Fifth, Newton's studies of site fidelity among sparrowhawks showed how tenacious individual birds can be in their "loyalty" to a home site, and how beneficial this tenacity can be in terms of reproductive output and longevity. Many wildlife species display an incredible capacity to return to or stay at a site even after alteration, and there can be a delayed response after even severe changes in vegetation structure. Examples include a pair of northern goshawks returning to a nest site that was mature forest the previous year, but is now on the edge of a clearcut, and sage grouse (Centrocercus urophasianus) returning to a lekking ground that has been paved over since their last spring ritual.
It is important to remember, from a wildlife management perspective, that the outcome of vegetation succession and the subsequent effects on local animal populations will vary in time and space; that is, vegetation change and consequent alterations in animal communities are spatially and temporally specific, and these processes take place in a fluctuating environment (Morrison et al. 1998). We are not suggesting that managers are destined to "reinvent the wheel" at each specific time and location; rather, managers must acknowledge and appreciate modern succession theory, establish clear and realistic goals, and incorporate site-specific characteristics such as land-use history, past management techniques (e.g., prescribed burning, timber harvest patterns, soil disturbance), and presence of nonnative species. Few management plans involve long-term monitoring of the vegetation and responses of targeted wildlife populations. Efforts to alter the successional patterns of local plant communities should not be attempted without a well-designed and realistic monitoring program for plant and animal populations (Morrison et al. 1998; Thompson et al. 1998).
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