Succession generally progresses toward the community type characteristic of the biome within which it occurs (e.g., toward deciduous forest within the deciduous forest biome or toward chaparral within the chaparral biome; e.g.,Whittaker 1953, 1970). However, succession can progress along various alternative pathways and reach alternative endpoints (such as stands dominated by beech, Fagus, maple, Acer, or hemlock, Tsuga, within the eastern deciduous forest in North America), depending on a variety of local abiotic and biotic factors. Substrate conditions represent an abiotic factor that selects a distinct subset of the regional species pool determined by climate. Distinct initial communities reflecting disturbance conditions, or unique conditions of local or regional populations, can affect the success of subsequent colonists. These initial conditions, and subsequent changes, guide succession into alternative pathways leading to distinct self-perpetuating endpoints (Egler 1954, Whittaker 1953). Herbivory and granivory can guide succession along alternative pathways (Blatt et al. 2001, Davidson 1993).
Substrate conditions affect the ability of organisms to settle, become established, and derive necessary resources. Some substrates restrict species representation (e.g., serpentine soils, gypsum dunes, and lava flows). Relatively few species can tolerate such unique substrate conditions or the exposure resulting from limited vegetative cover. In fact, distinct subspecies often characterize the communities on these and the surrounding substrates. Contrasting communities characterize cobbled or sandy sections of streams because of different exposure to water flow and filtration of plant or detrital resources. Finally, sites with a high water table support communities that are distinct from the surrounding communities (e.g., marsh or swamp communities embedded within grassland or forested landscapes).
Successional pathways are affected by the composition of initial colonists and survivors from the previous community. The initial colonists of a site represent regional species pools, and their composition can vary depending on proximity to population sources. A site is more likely to be colonized by abundant species than by rare species. Rapidly growing and expanding populations are more likely to colonize even marginally suitable sites than are declining populations. For example, trees dying during a period of minimal bark beetle abundance would undergo a delay in initiation of heterotrophic succession, dominated by a different assemblage of insect species associated with different microorganisms (e.g., Schowalter et al. 1992). Wood initially colonized by decay fungi, such as inoculated by wood-boring beetles, wasps, and termites, decays more rapidly, thereby affecting subsequent colonization, than does wood initially colonized by mold fungi, such as inoculated by bark and ambrosia beetles (Kaarik 1974, Schowalter et al. 1992).
Many individuals survive disturbance, depending on their tolerance to (or protection from) disturbance, and affect subsequent succession (Egler 1954). Disturbance scale also affects the rate of colonization. Succession initiated primarily by ruderal colonists will differ from succession initiated by a combination of ruderal colonists and surviving individuals and propagules (e.g., seed banks). Such legacies from the previous community contribute to the early appearance and advanced development of later successional species. These may preclude establishment of some ruderal species that would lead along a different succes-sional pathway. Large-scale disturbances promote ruderal species that can colonize a large area rapidly, whereas small-scale disturbances may expose too little area for shade-intolerant ruderal species and be colonized instead by later suc-cessional species expanding from the edge (Brokaw 1985, Denslow 1985, Shure and Phillips 1991). Fastie (1995) identified distance from each study site to the nearest seed source of Sitka spruce, Picea sitchensis, at the time of deglaciation as the major factor explaining among-site variance in spruce recruitment at Glacier Bay, Alaska.
The sequence of disturbances during succession determines the composition of successive species assemblages. For example, fire followed by drought would filter the community through a fire-tolerance sieve then a drought-tolerance sieve, whereas flooding followed by fire would produce a different sequence of communities. Harding et al. (1998) and Schowalter et al. (2003) demonstrated that arthropod communities in stream and forest litter, respectively, showed responses to experimental disturbances that reflected distinct community structures among blocks with different disturbance histories. Disturbance also can truncate community development. Grasslands and pine forests often dominate sites with climatic conditions that could support mesic forest, but succession is arrested by topographic or seasonal factors that increase the incidence of lightning-ignited fires and preclude persistence of mesic trees.
Longer-term environmental changes (including anthropogenic suppression of disturbances) also affect the direction of community development. Ironically, fire suppression to "protect" natural communities often results in successional replacement of fire-dominated communities, such as pine forests and grasslands. The replacing communities may be more vulnerable to different disturbances. For example, fire suppression in the intermountain region of western North America has caused a shift in community structure from relatively open, pine/larch woodland maintained by frequent ground fires to closed-canopy pine/fir forest that has become increasingly vulnerable to drought and stand replacing crown fires (Agee 1993, Schowalter and Lowman 1999,Wickman 1992).
The importance of animal activity to successional transitions has not been recognized widely, despite obvious effects of many herbivores on plant species composition (e.g., Louda et al. 1990a, Maloney and Rizzo 2002, Torres 1992; see Chapter 12). Vegetation changes caused by animal activity often have been attributed to plant senescence. Animals affect succession in a variety of ways (Davidson 1993, MacMahon 1981, Schowalter and Lowman 1999, Willig and McGinley 1999), and Blatt et al. (2001) showed that incorporation of herbivory into an old-field successional model helped to explain the multiple successional pathways that could be observed. Herbivorous species can delay colonization by host species (Tyler 1995, D. Wood and Andersen 1990) and can suppress or kill host species and facilitate their replacement by nonhosts over areas as large as 106 ha during outbreaks (Schowalter and Lowman 1999). Bullock (1991) reported that the scale of disturbance can affect animal activity, thereby influencing colonization and succession. Generally, herbivory and granivory during early seres halts or advances succession (V. K. Brown 1984, Schowalter 1981, Torres 1992), whereas herbivory during later seres halts or reverses succession (Davidson 1993, Schowalter and Lowman 1999). Similarly, Tullis and Goff (1987) and Wells and Greenberg (1994) reported that predaceous ants affected colonization and activity of carrion feeders and affected succession of the carrion community.
Granivores tend to feed on the largest seeds available, which most often represent later successional plant species, and thereby inhibit succession (Davidson 1993). Herbivores and granivores can interact competitively to affect local patterns of plant species survival and succession. For example, Ostfeld et al. (1997) reported that voles dominated interior portions of old fields, fed preferentially on hardwood seedlings over white pine, Pinus strobes, seedlings, and competitively displaced mice, which fed preferentially on white pine seeds over hardwood seeds near the forest edge. This interaction favored growth of hardwood seedlings in the ecotone and favored growth of white pine seedlings in the old field interior.
Animals that construct burrows or mounds or that wallow or compact soils can kill all vegetation in small (several m2) patches or provide suitable germination habitat and other resources for ruderal plant species (D. Andersen and MacMahon 1985, MacMahon 1981; see also Chapter 14), thereby reversing succession. Several studies have demonstrated that ant and termite nests create unique habitats, usually with elevated nutrient concentrations, that support distinct vegetation when the colony is active and facilitated succession following colony abandonment (e.g., Brenner and Silva 1995, Garrettson et al. 1998, Guo 1998, King 1977a, b, Lesica and Kannowski 1998, Mahaney et al. 1999). Jonkman (1978) reported that the collapse of leaf-cutter ant, Atta vollenweideri, nests following colony abandonment provided small pools of water that facilitated plant colonization and accelerated development of woodlands in South American grasslands.
Predators also can affect succession. Hodkinson et al. (2001) observed that spiders often are the earliest colonizers of glacial moraine or other newly exposed habitats. Spider webs trap living and dead prey and other organic debris. In systems with low organic matter, nutrient availability, and microbial decomposer activity, spider digestion of prey may accelerate nutrient incorporation into the developing ecosystem. Spider webs are composed of structural proteins and may redistribute nutrients over the surface. In addition, webs physically stabilize the surface and increase surface moisture through condensation from the atmosphere. These effects of spiders may facilitate development of cyanobacterial crusts and early successional vegetation.
Relatively few studies have evaluated community development experimentally. Patterns of arthropod colonization of new habitats represent a relatively short-term succession amenable to analysis. D. Strong et al. (1984) considered the unwitting movement of plants around the world by humans to represent a natural experiment for testing hypotheses about development of phytophage assemblages on a new resource. They noted that relatively few arthropod colonists on exotic plants were associated with the plant in its native habitat. Most arthropods associated with exotic plants are new recruits derived from the native fauna of the new habitat. Most of the insects that colonize introduced plants are general-ists that feed on a wide range of hosts, often unrelated to the introduced plant species, and most are external folivores and sap-suckers (Kogan 1981, D. Strong et al. 1984). Miners and gall-formers represent higher proportions of the associated fauna in the region of plant origin, likely because of the higher degree of specialization required for feeding internally. For example, endophages represented 10-30% of the phytophages associated with two species of thistles in native European communities but represented only 1-5% of phytophages associated with these thistles in southern California where they were introduced (D. Strong et al. 1984).These results indicate that generalists are better colonists than are specialists, but adaptation over ecological time increases exploitation efficiency (Kogan 1981, D. Strong et al. 1984).
In one of the most ambitious studies of community development, Simberloff and Wilson (Simberloff 1969, Simberloff and Wilson 1969, E. Wilson and Simberloff 1969) defaunated (using methyl bromide fumigation) six small mangrove islands formed by Rhizophora mangle in Florida Bay and monitored the reestablishment of the arthropod community during the following year. Sim-berloff and Wilson (1969) reported that by 250 days after defaunation, all but the most distant island had species richness and composition similar to those of untreated islands, but densities were lower on treated islands. Initial colonists included both strong and weak fliers, but weak fliers, especially psocopterans, showed the most rapid population growth. Ants, which dominated the mangrove fauna, were among the later colonists but showed the highest consistency in colonization among islands. Simberloff and Wilson (1969) found that colonization rates for ant species were related to island size and distance from population sources. The ability of an ant species to colonize increasingly smaller islands was similar to its ability to colonize increasingly distant islands. Species richness initially increased, declined gradually as densities and interactions increased, then reached a dynamic equilibrium with species colonization balancing extinction (see also E.Wilson 1969). Calculated species turnover rates were >0.67 species per day (Simberloff and Wilson 1969), consistent with the model of MacArthur and Wilson (1967).
These studies explain why early successional stages are dominated by r-selected species with wide tolerances (generalists) and rapid reproductive rates, whereas later stages are dominated by K-selected species with narrower tolerances for co-existence with more specialized species (see Chapter 5). The first arthropods to appear on newly exposed or denuded sites (also glaciated sites) usually are generalized detritivores and predators that exploit residual or exogenous dead organic material and dying colonists unable to survive. These arthropods feed on less toxic material than do herbivores or on material in which the defensive compounds have decayed. Herbivores appear only as their host plants appear, and their associated predators similarly appear as their prey appear.
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