Forest invertebrate communities and atmospheric change

Predicting the responses of invertebrate species, and the communities they form, to global change is one of the great challenges facing modern ecology. Invertebrates play vitally important roles in forests, underpinning fundamental ecosystem processes like nutrient cycling and pollination. Changes in the composition of our atmosphere, associated with increased levels of carbon dioxide (CO2) and ozone (O3), have the potential to affect the abundance, diversity and structure of invertebrate communities and the ecosystems they support. This chapter reviews the findings from the body of work looking at the responses of invertebrates to changes inCO2 and O3 concentrations with a special focus on the results from Free-Air Enrichment studies. The most consistent finding across the studies we review is the idiosyncratic nature of the responses of invertebrate species to the elevation of CO2 and/or O3. This finding can be explained to some extent by bottom-up and top-down processes. These include the species- and genotype-specific responses of host plant chemistry and differences in the abilities of individual insect species to physiologically and behaviourally overcome changes in resource quality. Although evidence is clearly mixed, certain general conclusions can be made regarding the influence of CO2 and/or O3 on invertebrates. Forest invertebrate herbivores tend to respond negatively to elevated concentrations of CO2. This response is likely due to diminished food-plant quality. Conversely, predators and parasitoids may benefit under enriched-CO2 conditions as prey susceptibility increases. Elevated O3 concentrations generally have opposing effects: herbivores show a tendency to consume more and develop faster while higher trophic levels experience declines in performance. Therefore, simultaneous elevation of both gases, such as is found in reality, may moderate the effects of either gas in isolation. There also appears to be some capacity for invertebrate communities to rebound over time, as evidenced by long-term studies. From the few community-level studies available, the current conclusion is that the structure of invertebrate communities will not be strongly disrupted by increases in CO2 and O3. This suggests that the ecosystem processes underpinned by these communities may be maintained under future atmospheres in these systems, though more work is needed. Looking forward, we emphasize the critical need for long-term studies of invertebrate responses at the population and community-level within natural systems. Such studies will be particularly important in tropical regions where no such information currently exists. Studies incorporating multiple climatic and atmospheric factors will also be of great value, such as those looking at the combined effects of atmospheric change and alterations in water availability. These studies will allow us to better predict the effects of future climates on these fundamental ecological systems.