Critical Issues

Resolution of the debate concerning potential regulatory roles of insects in natural ecosystems may not be possible, given the need for large-scale manipulation of insect populations and long-term, multidisciplinary comparison of ecosystem processes necessary to test the hypothesis. However, more data are needed on long-term consequences of insect activities in relatively natural ecosystems, including effects of population changes on mass balances of energy and nutrient fluxes, because these may mitigate or exacerbate effects of acid rain, carbon flux, and other processes affecting global change. Our perspective on the role of insects determines our management approaches. Whether we view insects as disturbances that destabilize ecosystems or as regulators that contribute to stability determines not only our approach to managing insects in natural or engineered ecosystems but also our approaches to managing our ecosystem resources and responding to global changes.

Clearly, exotic species freed from both bottom-up and top-down regulation function in the same way as pollutants or exotic disturbances (i.e., with little ecosystem control over their effects), at least initially. By contrast, population size and effects of native species are regulated by a variety of bottom-up, top-down, and lateral factors. Adaptations of native species to disturbances shape responses to natural or anthropogenic alteration of vegetation and landscape structure, with effects that often are contrary to management goals but perhaps conducive to ecological balances. If native insects function as regulators that contribute to ecosystem stability, then traditional management approaches that emphasize suppression may interfere with this natural feedback mechanism and maintain anthropogenic imbalances, at least in some ecosystems. In any case, insect outbreaks usually are responses to high density or stress of host plants, or both, making outbreaks a form of feedback that stabilizes ecosystem conditions, rather than a pest problem. Long-term solutions, therefore, require remedies for the departure from stability, rather than simply suppression of outbreaks.

Predicting and alleviating effects of anthropogenic changes requires understanding of insect roles and how these roles affect ecosystem responses to anthropogenic changes. Anthropogenic changes will continue to trigger insect outbreaks, whether as destructive events or regulatory responses. Land use, in particular, affects patch structure and interactions among demes, greatly altering the spatial and temporal patterns of insect abundances. Ruderal plant species, valued for crop production but also adapted for rapid colonization of new habitats, are increasingly likely to dominate fragmented landscapes. The rapid growth and poor competitive ability of these species in crowded ecosystems make them targets for their associated insects. Such ecosystems will require constant human intervention. Protection or restoration of natural ecosystems will require attention to interactions necessary to maintain key species, including pollinators, seed dispersers, and decomposers.

Accomplishment of this primary goal requires broadening of research approaches to address the breadth of insect effects on ecosystem structure and function. This, in turn, requires changes in research approaches and integration of population and ecosystem models. Testing of ecosystem-level hypotheses involves different approaches than does testing of population- and communitylevel hypotheses. At least three considerations are particularly important.

First, experimental design requires attention to statistical independence of samples. Whereas individuals within populations can serve as replicates for population and community properties, data must be pooled at the site (ecosystem) level for comparison of ecosystem variables. Ecosystem studies often have provided inconclusive data because a single site representing each of several ecosystem types or experimental treatments (e.g., Fig. 16.2 B-1 and B-2) provides

Design type


A-1 Completely randomized

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A-2 Randomized block

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A-3 Systematic

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B-1 Simple segregation

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B-2 Clumped segregation

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B-3 Isolative segregation

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I Chamber 1 I | Chamber 2 I

B-4 Randomized, but with inter-dependent replicates

I Chamber 1 I | Chamber 2 I

B-4 Randomized, but with inter-dependent replicates

B-5 No replication

Three representations (A-1-A-3) of acceptable experimental designs with interspersed, independent replicates of two treatments (shaded vs. unshaded boxes) and five representations (B-1-B-5) of experimental designs in which the principle of interspersed, independent replicates can be violated. From Hurlbert (1984) with permission from the Ecological Society of America. Please see extended permission list pg 573.

no error degrees of freedom for statistical analysis. Multiple samples collected within each site are not statistically independent (Hurlbert 1984). Furthermore, treatment effects are subject to confounding effects of geographic gradients between treatment plots. Therefore, experimental designs must incorporate multiple, geographically interspersed, replicate sites representing each ecosystem type or treatment (Fig. 16.2 A-1-A-3). A larger number of replicate sites provides a greater range of inference than do multiple samples within sites (that must be pooled for statistical analysis), requiring a tradeoff in sampling effort within sites and between sites.

Second, research to evaluate insect responses to, or effects on, ecosystem conditions should address a greater range of ecosystem variables than has been common in past studies of insect ecology. Insects respond to multiple factors simultaneously, not just one or a few factors subject to experimental manipulation, and their responses reflect tradeoffs that might not be reflected in studies that control only one or a few of these factors. A greater breadth of parameters can be addressed through multidisciplinary research, with experts on different aspects of ecosystems contributing to a common goal (Fig. 16.3). Involvement of insect ecologists in established multidisciplinary projects, such as the International Long Term Ecological Research (ILTER) sites in many countries, can facilitate integration of insect ecology and ecosystem ecology. Specifically, insect ecologists can contribute to such programs by clarifying how particular species respond to, and shape, ecosystem conditions, including vegetation structure, soil properties, biogeochemical cycling processes, etc., as described in Chapters 12-14; how insects affect the balance of nutrient fluxes within and between

| Interdisciplinary research on insect effects on log decomposition at the H. J. Andrews Experimental Forest Long Term Ecological Research Site in western Oregon, United States. A: Logs tented to exclude wood-boring insects during the first year of decomposition. B: Logs inoculated with different initial heterotroph communities (bark vs. wood-borer, mold vs. decay fungi; ribbon color indicates inoculation treatment; plastic shelters reduced wood moisture relative to unsheltered logs). Data loggers at each replicate site measured ambient temperature and relative humidity and vertical and horizontal temperature and moisture profiles in logs. Sticky screens were used to measure insect colonization, emergence traps were used to measure insect emigration, PVC (polyvinyl chloride) chambers were used to measure CO2 flux, and funnels under logs were used to measure water and nutrient flux out of logs. Scheduled destructive sampling of logs provided data on changes in wood density, excavation by insects, and nutrient content.

ecosystems (e.g., from aquatic to terrestrial ecosystems or across landscapes as populations move or expand, as described in Chapter 7); and how species diversity within guilds or functional groups affects the reliability of community organization and processes (Chapter 15).

Third, spatial and temporal scales of research and perspectives must be broadened. Most ecosystem studies address processes at relatively small spatial and temporal scales. However, population dynamics and capacity to influence ecosystem and global properties span landscape and watershed scales, at least. Feedbacks often may be delayed or operate over long time periods, especially in ecosystems with substantial buffering capacity, requiring long-term institutional and financial commitments for adequate study. Linkage of population and ecosystem variables using remote sensing and GIS (geographic information system) techniques will become an increasingly important aspect of insect ecology. Nevertheless, ecosystems with large biomass or high complexity require simplified field mesocosms or modeling approaches to test some hypotheses.

The complexity of ecosystem interactions and information linkages has limited incorporation of detail, such as population dynamics, in ecosystem models. Modeling methodology for ecosystem description and prediction is necessarily simplified, relative to that for population models. However, population models have largely ignored feedbacks between population and ecosystem processes. Hierarchical structure in ecosystem models facilitates integration of more detailed insect population (and other) submodels, and their linkages and feedbacks with other levels, as data become available (see Fig. 11.15).

Several ecosystem components should be given special attention. Subterranean and forest canopy subsystems represent two ecological frontiers. Logistical difficulties in gaining nondestructive or nonintrusive access to these two subsystems have limited data available for insect effects on canopy-atmosphere and canopy-rhizosphere-soil interactions that control climate and energy and matter fluxes. Improved canopy access methods, such as construction cranes (Fig. 16.4) for ecological use (Schowalter and Ganio 1998, D. Shaw 1998, 2004), and rhizotron technology (Sackville Hamilton et al. 1991, Sword 1998) offer opportunities for scientific advances in the structure and function of these subsystems.

Finally, principles of insect ecology must be applied to improved management of insect populations and ecosystem resources. Ecosystem engineering can make crop systems more or less conducive to insect population irruptions. Alternative cropping systems include protection of soil systems to enhance energy and matter availability and polyculture cropping and landscape patterns of crop patches and remnant native vegetation (see Fig. 16.1) to restrict herbivore dispersal among hosts or patches (Coleman et al. 1992, Kogan 1998, Lowrance et al. 1984, Rickson and Rickson 1998, Risch 1980,1981). These cropping systems also enhance conditions for predators that control potentially irruptive insect species. Promotion of interactions that tend to stabilize populations of irruptive species is more effective in the long term than is reliance on pesticides or genetically engineered crops. Examples include provision or retention of hedgerows, ant-attracting plants, or

Example About Expiremental Plants

| Canopy cranes are a new tool for experimental access to forest canopies. For example, the gondola of the Wind River Canopy Crane (75-m tall tower, 84-m long jib) can access 700,000 m3 of 60-m tall canopy, as well as the canopy-atmosphere interface, over a 2.3-ha area in a 500-year-old Pseudotsuga/Tsuga forest in southwestern Washington, United States. Photo by J. F. Franklin, from D. Shaw (2004). Please see extended permission list pg 573.

| Canopy cranes are a new tool for experimental access to forest canopies. For example, the gondola of the Wind River Canopy Crane (75-m tall tower, 84-m long jib) can access 700,000 m3 of 60-m tall canopy, as well as the canopy-atmosphere interface, over a 2.3-ha area in a 500-year-old Pseudotsuga/Tsuga forest in southwestern Washington, United States. Photo by J. F. Franklin, from D. Shaw (2004). Please see extended permission list pg 573.

other refuges within agricultural landscapes that maintain predator populations (Kruess and Tscharntke 1994, Rickson and Rickson 1998). Furthermore, insect effects on ecosystems, including agroecosystems, are complex. Net effects of outbreaks on multiple parameters should be considered in deciding whether to suppress outbreaks. Given that outbreaks often reflect simplification of ecosystem conditions and function to restore complexity and, perhaps, stability, control of native species in natural ecosystems may be counterproductive. Letting outbreaks run their course could serve management purposes under some conditions.

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