10 Community Dynamics I. Short-Term Change in Community Structure II. Successional Change in Community Structure A. Patterns of Succession B. Factors Affecting Succession C. Models of Succession III. Paleoecology IV. Diversity versus Stability A. Components of Stability B. Stability of Community Variables V. Summary COMMUNITY STRUCTURE CHANGES THROUGH TIME AS SPECIES abundances change, altering the network of interactions. Short-term (e.g., sea- sonal or annual) changes in community structure represent responses to envi- ronmental changes that favor some species or affect interaction strength (see Chapter 8). Longer-term (e.g., successional) changes in community structure often reflect relatively predictable trends during community development on newly available or disturbed sites. Finally, changes in community structure over evolutionary time reflect responses to long-term trends in environmental conditions. Among the major environmental issues facing governments worldwide is the effect of anthropogenic activities (e.g., altered atmospheric or aquatic chemistry, land use, species redistribution) on the composition of natural communities and the ecosystem services they provide to humans. How might changes in commu- nity structure affect epidemiology of human diseases? How stable is community structure, and how sensitive are communities and ecosystems to changes in species composition? Our perception of communities as self-organizing entities or random assemblages has significant implications for our sensitivity to species loss and our approach to management of ecosystem resources. As with population dynamics, study of changes in community structure requires long periods of observation. Few studies have continued over sufficiently long time periods to evaluate many of the factors presumed to affect community 283 010-P088772.qxd 1/24/06 10:47 AM Page 283 structure. However, paleoecological evidence and studies of community recov- ery following disturbance have provided useful data. Research on factors affecting community structure over a range of temporal scales can enhance understanding of the degree of stability in community structure and anticipation of responses to environmental changes. I. SHORT-TERM CHANGE IN COMMUNITY STRUCTURE Community structure changes over relatively short time periods.Short-term vari- ation in community structure reflects interactions among species responding differently to fluctuating abiotic conditions and species interactions. Relatively few studies measured effects of seasonal or annual changes in arthropod com- munities over extended periods. Several studies represent annual to decadal dynamics in arthropod communities. Fluctuating weather conditions and disturbances can cause appreciable changes in arthropod community structure. Changes in precipitation pattern can elicit dif- ferential responses among arthropod species. Schowalter et al. (1999) found that particular arthropod species, as well as the entire arthropod community, associated with creosotebush, Larrea tridentata, in southern New Mexico showed distinct trends in abundance over an experimental gradient in precipitation volume.Abun- dances of several species increased with moisture availability, whereas abundances of others declined with moisture availability, and some species showed nonlinear or nonsignificant responses. Multivariate analysis indicated distinct community structures on plants subjected to different amounts of precipitation. Polis et al. (1997b, 1998) studied community changes on desert islands in the Gulf of California during a 5-year period (1990–1994), which included an El Niño event (1992–1993). Winter 1992 precipitation was 5 times the historic mean and increased plant cover 10–160-fold. Insect abundance doubled in 1992 and 1993, compared to 1991 levels, with a significant shift in dominance from detritivores supported by marine litter to herbivores supported by increased plant biomass. Spider densities doubled in 1992 in response to prey abundance, but declined in 1993, despite continued high plant and prey abundance, as a result of increased abundance of parasitoid wasps, partially supported by nectar and pollen resources. These changes were consistent among islands throughout the archi- pelago, indicating that general processes connecting productivity and consump- tion governed community dynamics in this system. Changes in precipitation pattern in western Oregon, United States, between 1986 and 1996 altered the relative abundances of dominant folivore and sap- sucker species in conifer canopies (Fig. 10.1). In particular, western spruce budworm, Choristoneura occidentalis; sawflies, Neodiprion abietis; and aphids, Cinara spp., were abundant during a drought period, 1987–1993, but virtually absent during wetter periods.A bud moth, Zeiraphera hesperiana, was the domi- nant folivore during wet years but disappeared during the drought period. Schowalter and Ganio (2003) described changes in arthropod community structure in tropical rainforest canopies in Puerto Rico from 1991 to 1999. Hurricane Hugo (1989) created 30–50-m diameter canopy gaps dominated by early successional shrubs, vines, and Cecropia schreberiana saplings. Several 284 10. COMMUNITY DYNAMICS 010-P088772.qxd 1/24/06 10:47 AM Page 284 species of scale insects and a phytophagous mirid bug, Itacoris sp., were signifi- cantly more abundant on foliage in canopy gaps, compared to nongaps, in 1991 and again following Hurricane Georges (1998), suggesting positive response to storm disturbance. Scale insect and folivore abundances were significantly more abundant during a record drought (1994–1995), compared to intervals between disturbances, providing further evidence of responses to disturbances. Factors that increase competition or predation can reduce population sizes of particular species.Some species may become locally extinct, whereas others show population irruptions. Changes in species abundances affect interactions with other species. Both the strength and direction of interaction can change greatly. Herbivores that have little effect on their hosts at low abundances can interact in a more predatory manner at high abundances. Reduced abundance of one member of a mutualism can jeopardize the persistence of the other. I. SHORT-TERM CHANGE IN COMMUNITY STRUCTURE 285 A. A. Z. Co. Co. A. Z. Co. Ch Ci. N. 0 3 6 10 30 60 100 300 1986 1992 1996 Year Abundance (no./kg plant material) Folivores Sap-suckers Pollen and seed feeders Predators Fungivores FIG. 10.1 Temporal change in arthropod abundances in old-growth Douglas fir canopies at the H. J. Andrews Experimental Forest in western Oregon; 1989 and 1996 were relatively wet years; 1992 was in the middle of an extended drought period (1987–1993). Z., Zeiraphera hesperiana; Ch., Choristoneura occidentalis; N., Neodiprion abietis; Ci., Cinara spp.; A., Adelges cooleyi; Co., Coccoidea (4 spp.). Note the log scale of abundance. Data from Schowalter (1989, 1995 and unpublished data). 010-P088772.qxd 1/24/06 10:47 AM Page 285 Changes in species composition and abundance alter species diversity, food web structure, and functional organization. Change in abundance of species at one trophic level can affect the diversity and abundance of species at lower trophic levels through trophic cascades. For example, reduced predator abun- dance usually increases herbivore abundance, thereby decreasing plant abun- dance (Carpenter and Kitchell 1987, 1988, Letourneau and Dyer 1998). II. SUCCESSIONAL CHANGE IN COMMUNITY STRUCTURE Relatively predictable changes in community structure occur over periods of decades to centuries as a result of succession on newly exposed or disturbed sites. New habitats become available for colonization as a result of tectonic activity, glacial movement, sea level change, and sediment deposition or erosion. Species colonizing newly exposed surfaces usually are small in stature, tolerant of exposure or able to exploit small shelters, and able to exploit nonorganic or exogenous resources. Disturbances to existing communities affect each species differently, depending on its particular tolerances to disturbance or postdistur- bance conditions (see Chapter 2). Often, legacies from the predisturbance com- munity (such as buried rhizomes, seed banks, woody litter, and animals surviving in protected stages or microsites) remain following disturbance and influence the trajectory of community recovery. The process of community development on disturbed or newly exposed sites is called ecological succession. The succession of populations and communities on disturbed or newly exposed sites has been a unifying concept in ecology since the time of Cowles (1911) and Clements (1916). These early ecologists viewed succession as analogous to the orderly development of an organism (ontogeny). Succession progressed through a predictable sequence of stages (seres), driven by biogenic processes, which culminated in a self-perpetuating community (the climax) determined by climatic conditions. Succession is exemplified by the sequential colonization and replacement of species: weedy annual to perennial grass to forb, to shrub, to shade-intolerant tree, and finally to shade-tolerant tree stages on abandoned cropland. Succession following fire or other disturbances shows a similar sequence of stages (Fig. 10.2). Although the succession of species and communities on newly exposed or dis- turbed sites is one of the best-documented phenomena in ecology, the nature of the community and mechanisms driving species replacement have been debated intensely from the beginning. Gleason (1917, 1926, 1927) argued that succession is not directed by autogenic processes but reflects population dynamics of indi- vidual species based on their adaptations to changing environmental conditions. Egler (1954) further argued that succession could proceed along many potential pathways, depending on initial conditions and initial species pools. E. Odum (1969) integrated the Clementsian model of succession with ecosystem processes by proposing that a number of ecosystem properties, including species diversity, primary productivity, biomass, and efficiency of energy and nutrient use, increase during succession. Drury and Nisbet (1973) viewed succession as a temporal gra- dient in community structure,similar to the spatial gradients discussed in Chapter 286 10. COMMUNITY DYNAMICS 010-P088772.qxd 1/24/06 10:47 AM Page 286 9, and argued that species physiological tolerances to environmental conditions were sufficient to explain species replacement. More recently, the importance of disturbances and heterotroph activity in determining successional processes and preventing ascension to the climatic climax has been recognized (e.g., Davidson 1993, MacMahon 1981, Ostfeld et al. 1997, Pickett and White 1985, Schowalter 1981, 1985, Willig and Walker 1999). The concept of succession as goal-oriented toward a climax has succumbed to various challenges, especially recognition that succession can progress along various pathways to nonclimatic climaxes under different environmental con- ditions (Whittaker 1953). Furthermore, the mechanism of species replacement is not necessarily facilitation by the replaced community (e.g., Botkin 1981, Connell and Slatyer 1977, H. Horn 1981, McIntosh 1981, Peet and Christensen 1980, Whittaker 1953, 1970). Nevertheless, debate continues over the integrity of the community, the importance of autogenic factors that influence the pro- cess, and the degree of convergence toward particular community composition (Bazzaz 1990, Peet and Christensen 1980, Glenn-Lewin et al. 1992, West et al. 1981). II. SUCCESSIONAL CHANGE IN COMMUNITY STRUCTURE 287 N Forest floor 0–1 2–5 6–25 26–50 Age (yr) Fire Newly burned stage Herb–tree seeding stage Shrub-tree sapling stage Dense hardwood stage (birch and aspen) Mature hardwood stage Mixed white spruce– hardwood stage Mature white spruce– moss stage 51–100 100–200 200–500+ Bedrock FIG. 10.2 Diagrammatic representation of upland white spruce forest succession in Alaska following fire. From van Cleve and Viereck (1981) with permission from Springer-Verlag. Please see extended permission list pg 571. 010-P088772.qxd 1/24/06 10:47 AM Page 287 A. Patterns of Succession Two types of succession can be recognized. Primary succession occurs on newly exposed substrates (e.g., lava flows, uplifted marine deposits, dunes, newly deposited beaches, etc.). Primary succession usually involves a long period of soil formation and colonization by species requiring little substrate modification. Sec- ondary succession occurs on sites where the previous community was disturbed and is influenced by remnant substrate and surviving individuals.Although most studies of succession have dealt with trends in vegetation, heterotrophic succes- sions, including successions dominated by insects or other arthropods, have con- tributed greatly to perspectives on the process. Insects and other arthropods dominate the development of freshwater communities and litter (especially woody litter and carrion) communities, and succession in these habitats occurs over shorter time scales than does succession involving longer-lived plant species. Succession varies in duration from weeks for communities with little biomass (e.g., carrion feeders) to centuries for communities with abundant biomass (e.g., forests). Shorter successions are amenable to study by individual researchers. However, forest or desert succession spans decades to centuries and has not been studied adequately throughout its duration (see Fig. 10.2). Rather, forest succes- sion usually has been studied by selecting plots of different age since disturbance or abandonment of management to represent various seres (i.e., the chronose- quence approach). Although this approach has proved convenient for compar- ing and contrasting various seres, it fails to account for effects of differences in initial conditions on subsequent species colonization and turnover processes (e.g., Egler 1954, Schowalter et al. 1992). Even Clements (1916) noted that com- parison of the successional stages is less informative than is evaluation of the factors controlling transitions between stages. However, this approach requires establishment of long-term plots protected from confounding activities and a commitment by research institutions to continue studies beyond the usual con- fines of individual careers. Characterization of succession is a major goal of the network of U.S. and International Long Term Ecological Research (LTER) Sites (e.g., Van Cleve and Martin 1991). Long-term and comparative studies will improve understanding of successional trajectories and their underlying mechanisms. A number of trends have been associated with vegetation succession. Gener- alists or r-strategists generally dominate early successional stages, whereas spe- cialists or K-strategists dominate later successional stages (Table 10.1, see Fig. 10.2) (Boyce 1984, V.K. Brown 1984, 1986, Brown and Hyman 1986, Brown and Southwood 1983, Grime 1977, Janzen 1977, D. Strong et al. 1984; see Chapter 5). Species richness usually increases during early-mid succession but reaches a plateau or declines during late succession (Peet and Christensen 1980,Whittaker 1970), a pattern similar to the spatial gradient in species richness across ecotones (Chapter 9). E. Wilson (1969), based in part on data from Simberloff and Wilson (1969), suggested that community organization progresses through four stages: nonin- teractive, interactive, assortative, and evolutionary. The noninteractive stage 288 10. COMMUNITY DYNAMICS 010-P088772.qxd 1/24/06 10:47 AM Page 288 II. SUCCESSIONAL CHANGE IN COMMUNITY STRUCTURE 289 TABLE 10.1 Life history strategies of insects from different successional stages. Updated from V. K. Brown (1984) by permission from V. K. Brown and the American Institute of Biological Sciences, © 1984 American Institute of Biological Sciences. Characteristic Successional Stage Source Ruderal Early Mid Late 0–1 yr 1–5 yr 7–11 yr 60+ yr Mobility (% fully winged species) 94 84 80 79 Heteroptera (V. K. Brown 1982) Generation Time (% species >1 generation/yr) 43 50 33 3 Exopterygote herbivores (V. K. Brown and Southwood 1983) 41 37 10 12 Heteroptera (V. K. Brown 1982) Size (mean body length, mm, ±SEM) 3.68 ± 0.57 3.59 ± 0.63 3.86 ± 0.63 4.14 ± 0.67 all insect species (V. K. Brown 1986) Reproductive potential (mean number of 70.0 ± 4.4* 50.2 ± 2.0 ** aphids (V. K. Brown and Llewellyn 1985) embryos ±SEM) Niche breadth (scale 1–5; 1 = highly specialized) 3.35 3.10 2.87 1.79 sap feeders (V. K. Brown and Southwood 1983) 1.60 1.29 1.33 3.05 weevils (V. K. Brown and Hyman 1986) * on herbaceous plants; ** on woody plants 010-P088772.qxd 1/24/06 10:47 AM Page 289 occurs early during succession (first decade), when species richness and popula- tion densities are too low to induce density-dependent competition, predation, or parasitism. As species number increases and densities increase, interaction strength increases and produces a temporary decline or equilibrium in species number, as some species are excluded by competition or predation. The assorta- tive stage occurs over long disturbance-free time periods as a result of species persistence in the community on the basis of efficient resource use and co- existence. Niche partitioning allows more species to colonize and persist. Finally, co-evolution over very long time periods increases the efficiency of interaction and permits further increase in species number. However, most communities are disturbed before reaching the assortative stage. The intermediate disturbance hypothesis predicts that species richness is maximized through intermediate levels of disturbance that maintain a combination of early and late successional species (Connell 1978, Sousa 1985). Arthropod communities also change during vegetative succession (see Table 10.1) (V. K. Brown 1984, Shelford 1907, Weygoldt 1969). E. Evans (1988) found that grasshopper assemblages showed predictable changes following fire in a grassland in Kansas, U.S.A. The relative abundance of grass-feeding species ini- tially increased following fire, reflecting increased grass growth,and subsequently declined, as the abundance of forbs increased. Schowalter (1994, 1995), Schowalter and Crossley (1988), and Schowalter and Ganio (2003) reported that sap-sucking insects (primarily Homoptera) and ants dominated early successional temperate and tropical forests, whereas folivores, predators, and detritivores dominated later successional forests.This trend likely reflects the abundance of young, succulent tissues with high translocation rates that favor sap-suckers and tending ants during early regrowth. V. K. Brown and Southwood (1983) reported a similar trend toward increased representation of predators, scavengers, and fungivores in later successional stages. They noted, in addition, that species richness of herbivorous insects and plants were highly correlated during the earliest successional stages but not later successional stages, whereas numbers of insects and host plants were highly correlated at later stages but not the earliest successional stages. Brown and Southwood (1983) suggested that early colonization by herbivorous insects depends on plant species composition but that population increases during later stages depend on the abundance of host plants (see also Chapters 6 and 7). Punttila et al. (1994) reported that the diversity of ant species declined during forest succession in Finland. Most ant species were found in early successional stages, but only the three species of shade-tolerant ants were common in old (>140-year-old) forests.They noted that forest fragmentation favored species that require open habitat by reducing the number of forest patches with sufficient interior habitat for more shade-tolerant species. Starzyk and Witkowski (1981) examined the relationship between bark- and wood-feeding insect communities and stages of oak-hornbeam forest succession. They found the highest species richness in older forest (>70 years old) with abun- dant dead wood and in recent clearcuts with freshly cut stumps. Densities of mining larvae also were highest in the older forest and intermediate in the recent clearcut. Intermediate stages of forest succession supported fewer species and 290 10. COMMUNITY DYNAMICS 010-P088772.qxd 1/24/06 10:47 AM Page 290 lower densities of bark- and wood-feeding insects. These trends reflected the decomposition of woody residues remaining during early stages and the accu- mulation of woody debris again during later stages. Torres (1992) reported that a sequence of Lepidoptera species reached outbreak levels on a corresponding sequence of early successional plant species during the first 6 months following Hurricane Hugo (1989) in Puerto Rico but disappeared after depleting their resources. Schowalter (unpublished data) observed this process repeated following Hurricane Georges (1998). Davidson (1993), Schowalter (1981), and Schowalter and Lowman (1999) sug- gested that insect outbreaks and other animal activity advance, retard, or reverse succession by affecting plant replacement by nonhost plants (see later in this chapter). Heterotrophic successions have been studied in decomposing wood, animal carcasses, and aquatic ecosystems. These processes can be divided into distinct stages characterized by relatively discrete heterotrophic communities. In general, succession in wood occurs over decadal time scales and is initiated by the penetration of the bark barrier by bark and ambrosia beetles (Scolytidae and Platypodidae) at, or shortly after, tree death (Ausmus 1977, Dowding 1984, Savely 1939, Swift 1977, Zhong and Schowalter 1989). These beetles inoculate galleries in fresh wood (decay class I, bark still intact) with a variety of symbi- otic microorganisms (e.g., Schowalter et al. 1992, Stephen et al. 1993; see Chapter 8) and provide access to interior substrates for a diverse assemblage of sapro- trophs and their predators. The bark and ambrosia beetles remain only for the first year but are instrumental in penetrating bark, separating bark from wood, and facilitating drying of subcortical tissues (initiating decay class II, bark frag- mented and falling off).These insects are followed by wood-boring beetles; wood wasps; and their associated saprophytic microorganisms, which usually dominate wood for 2–10 years (Chapter 8). Powderpost and other beetles, carpenter ants, Camponotus spp., or termites dominate the later stages of wood decomposition (decay classes III–IV, extensive tunneling and decay in sapwood and heartwood, loss of structural integrity),which may persist for 5–100 years,depending on wood conditions (especially moisture content) and proximity to population sources. Wood becomes increasingly soft and porous, and holds more water, as decay pro- gresses. These insects and associated bacteria and fungi complete the decompo- sition of wood and incorporation of recalcitrant humic materials into the forest floor (decay class V). Insect species composition follows characteristic successional patterns in decaying carrion (Figs. 10.3 and 10.4), with distinct assemblages of species defin- ing fresh, bloated, decay, dry, and remains stages (Payne 1965,Tantawi et al. 1996, Tullis and Goff 1987,Watson and Carlton 2003). For small animals,several carrion beetle species initiate the successional process by burying the carcass prior to oviposition. Distinct assemblages of insects characterize mammalian versus reptilian carcasses (Watson and Carlton 2003). For all animal carcasses, the fresh, bloated, and decay stages are dominated by various Diptera, especially calliphorids, whereas later stages are dominated by Coleoptera, especially dermestids. The duration of each stage depends on environmental conditions that affect the rate of decay (compare Figs. 10.3 and 10.4) (Tantawi et al. 1996) II. SUCCESSIONAL CHANGE IN COMMUNITY STRUCTURE 291 010-P088772.qxd 1/24/06 10:47 AM Page 291 292 10. COMMUNITY DYNAMICS FIG. 10.3 Succession of arthropods on rabbit carrion during summer in Egypt. From Tantawi et al. (1996) with permission from the Entomological Society of America. 010-P088772.qxd 1/24/06 10:47 AM Page 292 [...]... 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... 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 II SUCCESSIONAL CHANGE... 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... 1997, Hooper and Vitousek 1997, Schulze and Mooney 1993, Tilman et al 1997; see Chapter 15) An early 305 306 10 COMMUNITY DYNAMICS Intermediate and specialized 50 All Generalized 225 a 209 40 30 b 248 20 K/T meters 10 500 218 0 436 10 681 –20 –30 234 259 –40 205 252 –50 1495 –60 207 –70 404 –80 0 10 20 30 40 Percent of leaves damaged 50 0 5 10 15 20 25 # Damage types FIG 10. 9 (a) Frequency analyses (percentage)... herbivory similar to that produced by modern insects (Boucot 1990, Labandeira 1998, 2002, Scott and Taylor 1983) Boucot (1990) reported a unique example of an extant insect species associated with extant genera in an Upper Miocene deposit in Iceland The hickory aphid, Longistigma caryae, occurred in the same deposit with fossil leaves of Carya (or Juglans), Fagus, Platanus, and Acer This aphid species... 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 generalists that feed on a wide range of hosts, often unrelated to the introduced plant species, and most are... 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... 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... Poinar and Poinar 1999) and even the consequences of prehistoric changes in climate (Wilf and Labandeira 1999, Wilf et al 2001) or other disturbances (Labandeira et al 2002) Similar morphological features of fossil and extant organisms imply similar functions and associated behaviors (Boucot 1990, Poinar 1993, Scott and Taylor 1983), helping to explain fossil records as well as to understand long-term... 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, . 1993, Schowalter and Lowman 1999). Similarly, Tullis and Goff (1987) and Wells and Greenberg (1994) reported that predaceous ants affected colonization and activ- ity of carrion feeders and affected succession. particular tolerances to disturbance or postdistur- bance conditions (see Chapter 2). Often, legacies from the predisturbance com- munity (such as buried rhizomes, seed banks, woody litter, and animals. which included an El Niño event (1992–1993). Winter 1992 precipitation was 5 times the historic mean and increased plant cover 10 160-fold. Insect abundance doubled in 1992 and 1993, compared to 1991