Community Dynamics

Differential herbivory among plants and plant species in an ecosystem affects both the distribution of individuals of a particular plant species and the opportunities for growth of plant species resistant to or tolerant of herbivory.The intensity of herbivory determines its effects on plant communities. Low to moderate intensities that prevail most of the time generally ensure a slow turnover of plant parts or individual plants. High intensities during outbreaks or as a result of management can dramatically reduce the abundance of preferred species and rapidly alter vegetation structure and composition. However, D. Inouye (1982) and Paige and Whitham (1987) demonstrated that herbivory can increase seed production.

Overgrazing by domestic livestock has initiated desertification of arid grasslands (by reducing vegetation cover, causing soil desiccation) in many parts of the globe (e.g., Schlesinger et al. 1990). Herbivory by exotic insect species (but rarely native species) is capable of eliminating plant species that are unable to compensate (McClure 1991). Patterns of herbivory often explain observed geographic or habitat distributions of plant species (Crawley 1983,1989, Huntly 1991, Louda et al. 1990a, Schowalter and Lowman 1999). Herbivory has a variety of positive and negative effects on plant growth and fitness, even for a particular plant species (D. Inouye 1982; see earlier in this chapter). Herbivory can prevent successful establishment or continued growth, especially during the vulnerable seedling stage (D. Clark and Clark 1985, P. Hulme 1994, Wisdom et al. 1989). Louda et al. (1990a) reported that patterns of herbivory on two species of gold-enbushes, Happlopappus spp., explained the significant difference between expected and observed distributions of these species across an environmental gradient from maritime to interior ecosystems in southern California (Fig. 12.8). Louda and Rodman (1996) found that chronic herbivory by insects was concentrated on bittercress, Cardamine cordifolia, growing in sunny habitats and largely explained the observed restriction of this plant species to shaded habitats. Schowalter et al. (1981a) suggested that differential mortality among pine species (as a result of southern pine beetle, Dendroctonus frontalis) in the southern United States largely explained the historic patterns of species distributions over topographic gradients.

Herbivory on dominant plant species can promote persistence of associated plant species. Sousa et al. (2003) found that predation by a scolytid beetle, Coccotrypes rhizophorae, on seedlings of the mangrove, Rhizophora mangle, prevented establishment of R. mangle in lightning-generated gaps and permitted a shade-intolerant species, Laguncularia racemosa, to co-dominate the mangrove community on the Caribbean coast of Panama. McEvoy et al. (1991) documented changes in plant community structure resulting from herbivore-induced mortality to the exotic ragwort, Senecio jacobeae, in western Oregon. Ragwort standing crop declined from >700gm-2 (representing 90% of total standing crop of

Coccotrypes Rhizophorae
A I II III IV

B Maritime C Interior

Zones of San Diego county

| Herbivore effects on plant species distribution. A: Gradients in observed frequencies of two goldenbushes, Happlopappus venetus (yellow) and H. squarrosus (orange), from maritime to interior montane sites in San Diego County, California. B and C: Observed frequency accounting for herbivore effects (solid lines) compared to potential distribution in the absence of herbivory (dashed line) based on several measures of performance of control plants when insects were excluded. From Louda et al. (1990a). Please see extended permission list pg 572.

vegetation) to 0.25 g m-2 over a 2-year period following release of the ragwort flea beetle, Longitarsus jacobaeae. Grasses responded rapidly to declining ragwort abundance, followed by forbs, resulting in relatively constant vegetation standing crop over the 8 years of measurement.

Herbivory often facilitates successional transitions (see Chapter 10). Selective herbivory among plant species suppresses those on which herbivory is focused and provides space and other resources to others, resulting in altered plant community composition (e.g., Davidson 1993, McEvoy et al. 1991, Schowalter 1981, Schowalter et al. 1986). V.K. Brown and Gange (1989),V.K. Brown et al. (1988), and Gibson et al. (1990) reported that chemically reduced above-ground her-bivory resulted in lower plant species richness after 2 years, whereas V.K. Brown and Gange (1989) found that reduced below-ground herbivory resulted in higher plant species richness, largely reflecting differential intensities of herbivory among various grass and forb species. V. Anderson and Briske (1995) simulated herbivory by livestock in a transplant garden containing mid-seral and late-seral grass species to test alternative hypotheses that (1) mid-seral species have greater tolerance to herbivory or (2) herbivory is focused on late-seral species to explain species replacement in intensively grazed grasslands in the southern United States. They found that late-seral species had greater competitive ability and equivalent or higher tolerance to herbivory, indicating that selective herbivory on the late-successional species is the primary mechanism for reversal of succession (i.e., return to dominance by mid-seral species under intense grazing pressure). Conversely, Bach (1990), Coley (1980,1982,1983), Coley and Aide (1991), and Lowman and Box (1983) reported that intensities of herbivory by insects were higher in earlier successional stages than in later successional stages. Schowalter et al. (1981a) suggested that southern pine beetle is instrumental in advancing succession in the absence of fire by selectively killing early succes-sional pines, thereby favoring their replacement by later successional hardwoods (see Fig. 10.5).

Davidson (1993) compiled data indicating that herbivores may retard or reverse succession during early seres but advance succession during later seres. She suggested that herbivory is concentrated on the relatively less defended, but grazing tolerant, mid-successional grasses, forbs, and pioneer trees (see Bach 1990). Increased herbivory at early stages of community development tends to retard succession, whereas increased herbivory at later stages advances succession. Environmental conditions may affect this trend. For example, succession from pioneer pine forest to late successional fir forest in western North America can be retarded or advanced, depending primarily on moisture availability and condition of the dominant vegetation. Under conditions of adequate moisture (riparian corridors and high elevations), mountain pine beetle advances succession by facilitating the replacement of host pines by the more shade-tolerant, fire-intolerant, understory firs. However, limited moisture and short fire return intervals at lower elevations favor pine dominance. In the absence of fire during drought periods, herbivory by several defoliators and bark beetles is concentrated on the understory firs, truncating (or reversing) succession. Fire fueled by fir mortality also leads to eventual regeneration of pine forest. Similarly, each plant species that became dominant during succession following Hurricane Hugo in Puerto Rico induced elevated herbivory that facilitated its demise and replacement (Torres 1992). The direction of succession then depends on which plant species are present and their responses to environmental conditions.

Changes in plant condition, community composition, and structure affect habitat and food for other animals and microorganisms. Changes in nutritional quality or abundance of particular foliage, fruit, or seed resources affect abundances of animals that use those resources. Animals that require or prefer nesting cavities in dead trees may be promoted by tree mortality resulting from herbivore outbreaks.

Grazing on above-ground plant parts can affect litter and rhizosphere processes in a variety of ways (Bardgett et al. 1998). Reduced foliar quality resulting from induced defenses or replacement of palatable by less palatable plant species can reduce the quality of detrital material (Fig. 12.9). Seastedt et al. (1988)

Community Foliar Physiological effects herbivory S V. effects y^^'iis^s i i f*)

Community Foliar Physiological effects herbivory S V. effects

Coccotrypes Rhizophorae
From Bardgett et al. (1998) with permission from Elsevier Science.

reported that simulation of herbivore effects on throughfall (precipitation enriched with nutrients while passing over foliage) affected litter arthropod communities. Schowalter and Sabin (1991) found that three taxa of litter arthropods were significantly more abundant under experimentally defoliated (<20% foliage eaten) Douglas-fir saplings, compared to nondefoliated saplings. Reynolds et al. (2003) experimentally evaluated effects of herbivore-derived litter components on litter invertebrates. They found that addition of herbivore feces increased abundances of Collembola and fungal- and bacterial-feeding nematodes; addition of throughfall increased abundances of fungal- and bacterial-feeding nema-todes; litterfall exclusion reduced abundances of oribatid and prostigmatid mites. Altered carbon storage in roots (Filip et al. 1995, Holland et al. 1996) affects resources available for below-ground food webs (Fig. 12.10). Bardgett et al. (1997, 1998) reported that microbial biomass, nematode abundance, and soil respiration rates were consistently reduced by removal of sheep grazing (Fig. 12.11). Gehring and Whitham (1991,1995) documented significantly reduced mycorrhizal activity on roots of pinon pines subject to defoliation by insects compared to nonde-foliated pines.

Insect herbivores or their products constitute highly nutritious resources for insectivores and other organisms. Caterpillars concentrate essential nutrients several orders of magnitude over concentrations in foliage tissues (e.g., Schowalter and Crossley 1983). Abundances of insectivorous birds and mammals often increase in patches experiencing insect herbivore outbreaks (Barbosa and

Soil Respiration

FIG. 12.10

Carbon allocation as a function of intensity of herbivory (measured as shoot biomass remaining) in A: shoots, B: roots, C: soluble root exudates, D: respiration from roots and soil, E: rhizosphere soil, and F: bulk soil. Data were normalized for differences in CO2 uptake; 1 kBq = 1000 disintegrations sec-1. Shoot biomass was inversely related to leaf area removed by herbivores. Regression lines are shown where significant at P < 0.05. Open circles represent ungrazed plants, and solid circles represent grazed plants. From Holland et al. (1996) with permission from SpringerVerlag. Please see extended permission list pg 572.

MICROBIAL BIOMASS

500 400 300 200 100 -0

500 400 300 200 100 -0

Spring Summer Autumn Winter

Llyn Llydaw brown earth

Spring Summer Autumn Winter

Llyn Llydaw podzol

500 400 300 200 100 -0

Spring Summer Autumn Winter

Cwm idwal brown earth

Spring Summer Autumn Winter

Cwm idwal podzol

500 400 300 200 100 -0

NEMATODES (m-2 x 104)

500 400 300 200 100 -0

500 400 300 200 100 -0

Spring Summer Autumn Winter

Llyn Llydaw brown earth

Spring Summer Autumn Winter

Llyn Llydaw podzol

500 400 300 200 100 -0

Spring Summer Autumn Winter

Cwm idwal brown earth

Spring Summer Autumn Winter

Cwm idwal podzol

Wagner 1989). Arthropod tissues also represent concentrations of nutrients for decomposers (Schowalter and Crossley 1983, Seastedt and Tate 1981).

A variety of organisms use honeydew accumulation from aphids, scales, and other Homoptera. Ants, honey bees, Apis mellifera, hummingbirds, and other animals forage on the carbohydrate-rich honeydew (N. Edwards 1982). Stadler and Müller (1996) and Stadler et al. (1998) reported that the presence of honey-dew significantly increased the growth of a variety of epiphytic bacteria, yeasts, and filamentous fungi on the surface of conifer needles, potentially affecting pho-tosynthetic efficiency of underlying foliage.

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