Conclusions

Much remains to be learned about responses by pest insect populations in agro-ecosystems to anticipated climate shifts (Cammell and Knight, 1992; Lawton, 1995; Lindroth, 1996b; Cannon 1998; Davis et al., 1998; Harrington et al., 1999). Insect pest populations can build up to large numbers rapidly and unexpectedly in response to environmental changes, including weather (Lansberg and Smith, 1992; Cappuccino and Price, 1995; Dempster and McLean, 1998), often resulting in significant plant damage (Hewitt and Onsager, 1983). Potential impacts on insect pest populations will reflect the combined, integrated responses to direct effects from altered physical environments that influence physiological responses, coupled to indirect consequences of variable food quality resulting from elevated CO2 and associated drought stress (Lansberg and Smith, 1992). Geographic range shifts of arthropod pests and their natural enemies in response to changing temperatures or plant communities are likely. Temperature and CO2 levels are only part of the story (Lawton, 1995), however, and species interactions that are very difficult to predict in deterministic environments, let alone changing ones, may be most important. Combined direct and indirect effects from species interactions will alter key demographic responses by insect populations as well as interactions among populations within and between trophic levels.

The effects of elevated CO2 on insect herbivores can be summarized according to a standard model described in Figure 13.4; this model ignores effects of species interactions within and between trophic levels. In a nutshell, elevated temperature resulting from increases in greenhouse gases will have direct effects on insect performance, which in turn affects population processes and species interactions. CO2 will most likely have its greatest direct impact on food quality; food quality will be reduced for most insect herbivores. In many cases, insects can compensate for lowered N levels by eating more leaf or root material, but often not completely. Tissue loss in turn affects crop productivity, although plants also exhibit significant tolerance to losses from herbivores and

Temperature CO2 ^ Precipitation

Foliar

Plant

{

Quality

Productivity

Plant Communities

Insect Herbivores

* "■"*

Decreased Stress Increased Stress

I Precipitation CO,

Temperature^ Herbivory |

Foliar Quality

Decreased Stress Increased Stress

Temperature^ Herbivory |

Low C:N

High C:N

High C:N

Low C:N

Figure 13.4 General model of effects of climate change on insect herbivores. Multiple interactions that are anticipated to operate within boxes, including a variety of indirect ones, are not represented, but are discussed in detail in the text. (A) Temperature has two main paths in its impact on insect herbivores, through direct effects on insects by affecting metabolic rates, microhabitat selection and time budgets, and indirect effects on food quality. (B) Food nutritional quality (e.g., C:N, or concentrations of secondary chemicals) responses to abiotic conditions including temperature in association with precipitation and CO2. Such changes can greatly influence insect herbivore population dynamics.

pathogens, so the impact of consumption must be translated into actual effects on food-producing capacity.

Important but often unstated assumptions underlie scenarios proposed to deal with the impact of climate changes on agricultural pests:

1. The crops remain where they are and will suffer the effects of an altered physical environment.

2. Climate change is gradual.

3. Species assemblages will track conditions as a community unit.

4. Ecological interactions are deterministic, mostly linear and continuous (without thresholds) that could lead to alternate states with small changes in environmental conditions.

We expect that each assumption will not hold under many situations, and relaxing any of these assumptions makes it difficult to predict responses by agricultural pests as suggested by niche-based scenarios of Section 13.2.

Agronomists and agricultural economists expect that farmers and ranchers will adapt to new climatic conditions by growing the most appropriate crops for local conditions and cropping patterns will track changes in climate. Corn may be grown farther north than is now the case, such that the resulting environmental conditions in these new regions will be much the same as they are now. Agricultural pests will likely follow after some time lag. If so, the resulting impacts from pests on crops may not change, or they may be more affected by changes caused by final species in the assemblage in new regions rather than because of changes in prior species or community interactions due to climate.

Table 13.1 indicates possible changing conditions that may affect insect population abundance, and hence effects on crop productivity. In some cases, responses are reasonably deterministic and anticipate outcomes to changing climates. Many other situations are not so predictable with available information and understanding of interactions among species, and additional empirical research will be required to fill in the necessary gaps to support reasonable predictions.

Table 13.1 Factors Affecting the Likelihood that Insect Pest Populations Will Increase or Decrease in Response to Changing Environmental Conditions

Increased risk from agricultural pests following climate change is likely if the following conditions are met:

I. Species continue to inhabit a given location under changed global conditions and key participants (e.g., herbivores and host plants) continue to co-occur. Shifts in a species range with climate change may also cause new problems if a previously absent pest can now live in an area after climate changes that already has its host plant, or its previously successful natural enemy disappears.

II. Resources affecting population size (e.g., carrying capacity) are not reduced by:

a. Climatic adversity disrupting insect populations or meta population dynamics, causing local extinction (Hanski, 1998).

b. Disruption of phenological timing between herbivores and critical stages of host plants (Cammell and Knight, 1992; Cannon, 1998; Harrington et al., 1999).

c. New physical conditions slow rates of herbivore growth, development or reproduction because they have moved from more to less optimal states (Logan et al., 1976; Wermelinger and Seifert, 1999) (Figures 13.1 and 13.2).

III. Increased probability of outbreaks are expected if:

a. Pest was previously enemy limited, and density-dependence feedback from enemies decreases in impact (Hassell et al., 1993; Gutierrez et al., 1994; Cornell and Hawkins, 1995).

b. Pest was previously food limited, and density-dependent feedback from host plant decreases (Auerbach et al., 1995; van der Meijden et al., 1998).

c. Prior limitation by physical conditions change to be more favorable for pest population growth, without compensating for increases in density dependence from food or enemies (den Boer, 1998; Logan et al., 2003).

d. Pest outbreaks are exacerbated by positive feedbacks from the host that occurs because of feeding under new environmental conditions (Mattson, 1980; Mattson and Haack, 1987; Joern, 1992; White, 1993).

e. Pests increase the number of generations in a normal growing season as the window of suitable conditions is expanded (univoltine to bivoltine, or bivoltine to trivoltine, etc.).

Note: References are representative, not exhaustive.

Source: Based on Lansberg, J., and M.S. Smith. 1992. Aust. J. Botany, 40:565-577.

Some important issues are not covered in this chapter. Pollination is critical for many agricultural situations, resulting from both diffuse and specialized interactions between plant floral phenology, presence of pollinators, and the need for genetic outcrossing to support seed production. Mutualis-tic interactions such as pollination differ somewhat from those described above, but the underlying approach to their study will be similar. Also, evolutionary responses to changing conditions have been ignored. Predictions basically assume that species will retain attributes characteristic of prior environmental conditions. Adaptive stasis is highly unlikely as natural selection will certainly lead to changes in the underlying adaptive ability to function in new environments. For example, phonological decoupling between previously interacting species such as insect herbivores and parasitoids may initially lead to pest outbreaks. After a relatively small number of generations, these species (or others) may reestablish the control of natural enemies on the pest as the natural enemy populations evolve to track prey. Anticipated evolutionary responses can become very complex (Malcom, 1993; Thomas et al., 2001), and are not considered further here, but the capability for evolutionary responses affecting species interactions must not be forgotten. Finally, the ability to predict responses by agricultural pests to climate or other types of environmental change will be very different depending on whether the changes are gradual, or whether they are sudden. Current species interactions are much more likely to survive intact under gradual changes.

The basic theme of this chapter concerns the difficulty in predicting responses by agricultural pest insects to climate change, especially those promoted by elevated CO2. There is no longer any doubt that important responses by insect herbivores to climate change will occur. The biggest problem lies in our ability to predict the outcome of these changes with regard to food security. Clearly, more research on this important topic is needed.

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