Regulation of Net Primary Productivity by Insects

During the 1960s, a number of studies, including Crossley and Howden (1961), Crossley and Witkamp (1964), C. Edwards and Heath (1963), and Zlotin and Khodashova (1980), indicated that arthropods potentially control energy and nutrient fluxes in ecosystems. Clearly, phytophages could affect, without regulating, ecosystem properties. However, phytophages respond to changes in vegetation density or physiological condition in ways that provide both positive and negative feedback, depending on the direction of deviation in primary production from nominal levels (Figs. 12.5 and 15.6).

Mattson and Addy (1975) introduced the hypothesis that phytophagous insects regulate primary production, based on observations that low intensities of herbivory on healthy plants often stimulate primary production, but high intensities of herbivory on stressed or dense plants suppress primary production.

Net Primary Production
| Stimulation of primary production at net primary productivity (NPP) < K and suppression of primary production at NPP > K by phytophages (see Fig. 12.5) could stabilize primary production. From Schowalter (2000) with permission from CABI.

Furthermore, productivity by surviving plants often is greater following herbivore outbreaks (see Chapter 12). Schowalter (1981) proposed that phytophage outbreaks, triggered by host stress and density as resources become limiting, function to advance succession from communities with high demands for resources to communities with lower demands for resources. Davidson (1993) and Schowalter and Lowman (1999) refined this hypothesis by noting that herbivores and granivores can advance, retard, or reverse succession, depending on environmental conditions. Belovsky and Slade (2000) demonstrated that grasshoppers can accelerate nitrogen cycling and increase primary productivity, especially by plants that are better competitors when nitrogen is more available, at intermediate levels of herbivory. At low levels of herbivory, grasshoppers had too little influence on nitrogen cycling to affect primary production. At high levels, grasshoppers depressed plant growth and survival more than could be offset by increased nitrogen cycling and plant productivity.

Despite the obvious influence of animals on key ecosystem processes, their regulatory role has remained controversial and largely untested. Herbivorous insects possess the characteristics of cybernetic regulators (i.e., low maintenance cost and rapidly amplified effects, sensitivity to deviation in ecosystem parameters, and capacity to dramatically alter primary production through positive and negative feedback) and appear, in many cases, to stabilize NPP. For example, inconsequential biomass of phytophagous insects, even at outbreak densities, is capable of removing virtually all foliage from host plants and altering plant species composition (see Chapter 12). Virtually undetectable biomass of termites accounts for substantial decomposition, soil redistribution, and gas fluxes that could affect global climate (see Chapter 14).The following model for insect effect on ecosystem stability focuses on herbivores, but detritivores, pollinators, and seed dispersers also are capable of modifying ecosystem conditions in ways that might promote stability (e.g., decomposer enhancement of nutrient availability, plant growth, and herbivory [Holdo and McDowell 2004] provides feedback on herbivore effects on litter quality and availability [S. Chapman et al. 2003]).

Primary production often peaks at low to moderate intensities of pruning and thinning (see Fig. 12.5), supporting the grazing optimization hypothesis (Belovsky and Slade 2000, S. Williamson et al. 1989). Herbivores apparently stimulate primary production at low levels of herbivory, when host density is low or condition good, and reduce primary production at high levels, when host density is high or condition is poor (Fig. 15.6), potentially stabilizing primary production at intermediate levels. Furthermore, primary production often is higher following herbivore outbreaks than during the preoutbreak period (e.g., Alfaro and Shepherd 1991, Romme et al. 1986), suggesting alleviation of stressful conditions that could lead to instability. By stabilizing primary production, herbivores also stabilize internal climate and soil conditions, biogeochemical fluxes, etc., that affect survival and reproduction of associated organisms. Romme et al. (1986) reported that mountain pine beetle, Dendroctonus ponderosae, outbreaks appeared to increase variation (destabilization) of some ecosystem properties. However, these outbreaks represent a response to an anthropogenic deviation in primary production (i.e., increased tree density resulting from fire suppression).

No data are available to indicate whether long-term variation in ecosystem parameters is reduced by such outbreaks. However, annual wood production following mountain pine beetle outbreaks equaled or exceeded preoutbreak levels within 10 years, suggesting relatively rapid recovery of primary production (Romme et al. 1986).

Outbreaks of phytophagous insects are most likely to occur under two interrelated conditions, both of which represent responses to departure from nominal ecosystem conditions, often resulting from anthropogenic alteration (Schowalter 1985, Schowalter and Lowman 1999). First, adverse environmental conditions, such as inadequate water or nutrient availability, changing climate, and atmospheric pollution, cause changes in plant physiological conditions that increase suitability for phytophages. High intensities of herbivory under these conditions generally reduce biomass and improve water or nutrient balance or, in extreme cases, reduce biomass of the most stressed plants, regardless of their abundance, and promote replacement by better adapted plants (e.g., Ritchie et al. 1998, Schowalter and Lowman 1999). Second, high densities of particular plant species, as a result of artificial planting or of inhibitive successional stages, enhance host availability for associated phytophages. High intensities of herbivory represent a major mechanism for reversing site dominance by such plant species, facilitating their replacement and increasing diversity.

If communities evolve to minimize environmental variation, then herbivore interactions with disturbances are particularly important. Although outbreaks of herbivores traditionally have been viewed as disturbances (together with events such as fire, storm damage, and drought), their response to host density or stress often appear to reduce the severity of abiotic disturbances. Herbivore outbreaks commonly co-occur with drought conditions (Mattson and Haack 1987, T. White 1969,1976,1984), suggesting that plant moisture stress may be a particularly important trigger for feedback responses that reduce transpiration and improve water balance (W. Webb 1978). Fuel accumulation, as a result of herbivore-induced fluxes of material from living to dead biomass, often predisposes ecosystems to fire in arid environments. Whether such predisposition is stabilizing or destabilizing depends on the degree to which outbreaks modify the severity and temporal or spatial scale of such disturbances. Schowalter (1985) and Schowalter et al. (1981a) suggested that herbivore-induced disturbances might occur more regularly with respect to host generation times or stages of ecosystem development, as a result of specific plant-herbivore interactions, and thereby facilitate rapid adaptation to disturbance or postdis-turbance conditions. Although such induction of disturbance would seem to increase variation in the short term, accelerated adaptation would contribute to stability over longer time periods. Furthermore, increased likelihood of disturbance during particular seres should maintain that sere on the landscape, contributing to stability over larger spatial scales. The following example demonstrates the potential stabilization of ecosystem properties over the large spatial scales of western North America.

Conifer forests dominate much of the montane and high latitude region of western North America. The large, contiguous, lower elevation zone is characterized by relatively arid conditions and frequent droughts that historically maintained a sparse woodland dominated by drought- and fire-tolerant (but shade-intolerant) pine trees and a ground cover of grasses and shrubs, with little understory (Fig. 15.7). Low-intensity ground fires occurred frequently, at intervals of 15-25 years, and covered large areas (Agee 1993), minimizing drought-intolerant vegetation and litter accumulation. The relatively isolated higher elevation and riparian zones were more mesic and supported shade-tolerant (but fire- and drought-intolerant) fir and spruce forests. Fire was less frequent (every 150-1000 years) but more catastrophic at higher elevation as a result of the greater tree densities and understory development that facilitated fire access to tree canopies (Agee 1993, Veblen et al. 1994).

As a result of fire suppression during the past century, much of the lower elevation zone has undergone succession from pine forest to later successional fir

Fire Suppression Effects Human Body

| The relatively arid interior forest region of North America was characterized by open-canopied forests dominated by drought- and fire-tolerant pines, and by sparse understories, prior to fire suppression beginning in the late 1800s (A). Fire suppression has transformed forests into dense, multistoried ecosystems stressed by competition for water and nutrients (B). From Goyer et al. (1998) with permission from the Society of American Foresters.

| The relatively arid interior forest region of North America was characterized by open-canopied forests dominated by drought- and fire-tolerant pines, and by sparse understories, prior to fire suppression beginning in the late 1800s (A). Fire suppression has transformed forests into dense, multistoried ecosystems stressed by competition for water and nutrients (B). From Goyer et al. (1998) with permission from the Society of American Foresters.

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forest (see Fig. 15.7), a conspicuous deviation from historic conditions. Outbreaks of a variety of folivore and bark beetle species have become more frequent in these altered forests. During mesic periods and in more mesic locations (e.g., riparian corridors and higher elevations) the mountain pine beetle has advanced succession by facilitating the replacement of competitively stressed pines by more competitive firs. However, during inevitable drought periods, such as occurred during the 1980s, moisture limitation increases the vulnerability of these firs to several folivores and bark beetles specific to fir species (Fig. 15.8). Insect-induced mortality of the firs reversed succession by favoring the remaining drought- and fire-tolerant pines. Tree mortality can increase the severity and scale of catastrophic fires, which historically were rare in these forests, unless litter decomposition reduces fuel accumulation before fire occurs. However, this altered fire regimen likely will be mitigated in ecological time by eventual reestablishment of the pine sere following catastrophic fire. A similar situation has been inferred from insect demography in pine-hardwood forests of the southern United States (see Fig. 10.5). Van Langevelde et al. (2003) also suggested a cycle of alternating vegetation states maintained by interaction of fire and herbivores in African savanna.

Savanna Insects

| Phytophage modification of succession in central Sierran mixed conifer ecosystems during 1998. Understory white fir (Abies concolor), the late successional dominant, is increasingly stressed by competition for water in this arid forest type. An outbreak of the Douglas-fir tussock moth, Orgyia pseudotsugata, has completely defoliated the white fir (brown trees), restoring the ecosystem to the more stable condition dominated by earlier successional, drought- and fire-tolerant sequoias and pines (green, foliated, trees). Photo by J. H. Jones.

| Phytophage modification of succession in central Sierran mixed conifer ecosystems during 1998. Understory white fir (Abies concolor), the late successional dominant, is increasingly stressed by competition for water in this arid forest type. An outbreak of the Douglas-fir tussock moth, Orgyia pseudotsugata, has completely defoliated the white fir (brown trees), restoring the ecosystem to the more stable condition dominated by earlier successional, drought- and fire-tolerant sequoias and pines (green, foliated, trees). Photo by J. H. Jones.

To what extent do insects contribute to stability and "health" of various ecosystems? Until recently, insect outbreaks and disturbances have been viewed as destructive forces. The increased productivity of ecosystems in the absence of fire and insect outbreaks supported a view that resource production could be freed from limitations imposed by these regulators. However, fire now is recognized as an important tool for restoring sustainable (stable) ecosystem conditions and characteristic communities. Accumulating evidence also suggests that outbreaks of native insects represent feedback that maintains ecosystem production within sustainable ranges. Regulation of primary production by phytophagous insects could stabilize other ecosystem variables as well. Clearly, experimental studies should address the long-term effects of phytophagous insects on variability of ecosystem parameters. Our management of ecosystem resources, and in particular our approach to managing phytophagous insects, requires that we understand the extent to which phytophages contribute to ecosystem stability.

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