Ayres Abstracts

Ayres, M. P. 1993. Global change, plant defense, and herbivory. Pages 75-94 in P. M. Kareiva, J. G. Kingsolver and R. B. Huey, editors. Biotic interactions and global change. Sinauer Associates, Sunderland, MA.  pdf
Climate change could impact plant-herbivore interactions in many ways. The challenge is to identify mechanisms that will most directly transduce climate change into ecological impacts. This is expedited by physiological models of plant allocation and herbivore nutrition, but the most appropriate theoretical framework seems to vary depending upon the herbivore guild and the specific environmental change (Larsson 1989). I find it useful to distinguish between herbivores of mature plant tissue, herbivores of developing plant tissue, and wood-boring beetles.

Changes in atmospheric CO2, soil nutrients, cloud cover, water availability, and temperature all affect the composition and palatibility of mature plant tissue (Table 6). Increased CO2, reductions in soil nitrogen, and sometimes increased temperature, tend to reduce tissue nitrogen. Reductions in soil nitrogen, reductions in cloud cover, and sometimes increases in temperature and drought stress, tend to increase concentrations of plant secondary metabolites. However, we cannot generalize that global change will be detrimental to herbivores of mature plant tissue. Such predictions require that we forecast changes in (1) photosynthetically active radiation, (2) plant water balance, and (3) soil nutrient availability. Yet changes in each will presumably vary in direction and magnitude from region to region. Both temperature and CO2 are likely to increase on a global scale (Schneider, this volume), but the consequences for plant-herbivore interactions of doubling CO2 could easily be reversed by changes in one or more other factors (the effects of CO2 on host plant quality may be the least among five aspects of global change, Table 6). We expect nonlinear effects of temperature and water potential on plant defense, thus meaningful predictions require that we know the initial state of the plant, the direction and magnitude of the environmental change, and the precise form of the plant physiological responses. This knowledge is lacking for virtually all plants. Moreover, we need a better understanding of the way that various environmental perturbations interact. Can the effect of simultaneously increasing temperature and reducing precipitation be predicted from the additive effects of each?

Wood-boring beetles deserve special recognition because of their extraordinary economic and ecological impact, and because their outbreaks have been so often linked to climatic anomalies. Lorio et al. (1990) are accumulating support for a model of tree defense that relies on water balance to predict the timing of an ontogenetic switch from poorly defended earlywood to well defended latewood.

Numerous herbivores, including perhaps half of the economically important outbreak insects, specialize on developing plant tissue that is of high quality but only ephemerally available. For these herbivores, a phenological mismatch of a few days, perhaps caused by differential effects of temperature on plants and herbivores, can halve fecundity (Ayres and MacLean, 1987a; Keese and Wood, 1991), and thus result in huge impacts on plant/herbivore communities. Plants can reduce damage through slight phenological displacement from the herbivore (Eidt and Little, 1970; Aide, 1988; Crawley and Akhteruzzaman, 1988; Townsend, 1989; Tuomi et al., 1989). The availability of high quality food (or, from the plants perspective, the period of vulnerability) is strongly temperature sensitive. Warmer temperatures will reduce the temporal availability of food, but will also increase the rate of consumption and development in poikilothermic herbivores. Unless the temperature sensitivity of plant development and insect development are identical, changes in temperature will favor either the plant or the insect (Figure 5). Virtually nothing is known about the relative temperature sensitivity of insects and their host plants, but greenhouse experiments in Finnish Lapland indicate that an increase of 1C can potentially triple population growth of the herbivore.

Physiological models of plant allocation and herbivore nutrition already allow general predictions regarding the effects of climate change. Carbon/nutrient balance provides a parsimonious explanation for many phenotypic responses to alterations of CO2, light, and nutrient availability. Its failures suggest the role of natural selection in adapting plant responses to the ecological challenges of particular environments (optimal allocation hypothesis). Growth/differentiation balance explains the general importance of immature plant tissue for herbivore nutrition, and provides a robust framework for evaluating the effects of water balance on plant resistance. I anticipate the largest effects on plant-herbivore interactions to accompany changes in temperature and water availability. These are the same areas where our theoretical and empirical understanding seems weakest. Nonlinear responses to temperature and moisture imply that changes in climatic variability (even without a change in the mean) can alter plant-herbivore interactions. Assessing the ecological and economic risks that accompany global change demands experiment-based research that builds on existing physiological models to predict responses of populations, communities, and ecosystems. Progress will be expedited through the selection of experimental systems that involve naturally interacting plants and herbivores.

Herbivore/ Resource acquisition/ Allocation/ Phenotypic response/ Nutrition/ Energetics/ Environmental variables/ Physiological ecology/ Plant defense

 

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