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. Secondary 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 successions, including successions dominated by insects or other arthropods, have contributed 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 succession 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 comparing 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 comparison 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 confines 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 specialists 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

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

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 n n

* on herbaceous plants; ** on woody plants

Isi 03

occurs early during succession (first decade), when species richness and population 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 coexistence. 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 initially 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 abundant 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 lower densities of bark- and wood-feeding insects. These trends reflected the decomposition of woody residues remaining during early stages and the accumulation 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) suggested 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 symbiotic 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 fragmented 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 progresses. These insects and associated bacteria and fungi complete the decomposition 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 defining 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)

Succession Arthropods Stage Decay
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.

Succession Species
FIG. 10.4

Succession of arthropods on rabbit carrion during winter in Egypt. From Tantawi et al. (1996) with permission from the Entomological Society of America.

and on predators, especially ants (Tullis and Goff 1987, Wells and Greenberg 1994). This distinct sequence of insect community types, as modified by local environmental factors, has been applied by forensic entomologists to determine time since death.

Detritus-based communities develop in bromeliad and heliconia leaf pools (phytotelmata), as well as in low-order stream systems. Richardson and Hull (2000) and Richardson et al. (2000b) observed distinct sequences of arrival of dipteran filter feeders and gatherers during phytotelmata development in Puerto Rico. The earliest colonizer, of barely opened Heliconia bracts, was a small unidentified ceratopogonid, followed by an unidentified psychodid, cf. Pericoma. Subsequently, phytotelmata were colonized by two syrphids, Quichuana sp. and Copestylum sp. Older bracts with accumulated detritus and low oxygen concentration supported mosquitoes, Culex antillummagnorum, and finally tipulids, Limonia sp., in the oldest bracts.

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