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Herbivory (General) -- Summary
In our Editorial of 15 April 2000 -- Biodiversity and CO2 -- we suggested that "the elevated levels of primary production that elevated levels of CO2 induce in earth's plants provide the basis for greater populations of herbivores." That this is likely to be the case is further suggested by the review of plant-animal interactions in 51 terrestrial ecosystems conducted by McNaughton et al. (1989), who determined that the biomass of terrestrial herbivores is a strongly increasing function of aboveground primary production. It is also supported by the review of Cyr and Pace (1993), who found that the herbivore biomass of aquatic habitats likewise increases in response to increases in marine and freshwater vegetative productivity.

Clearly, however, not all herbivores will fare equally well in a CO2-enriched world of the future; and there could well be some losers among the many winners. Hence, this field of science is beginning to garner ever greater attention. In fact, fully 26 atmospheric CO2 enrichment studies on aphids alone had been conducted at the time of the review of Holopainen (2002), who found their performance to be increased in six studies, decreased in six others, and unaffected in the remaining 14 studies, for an overall nil response. This result is encouraging, however, for it suggests that outbreaks of this important insect pest will not be promoted in a CO2-enriched and, therefore, greener world of the future.

Likewise, in an earlier review that dealt with numerous types of herbivorous insects, Whittaker (1999) had found a slight increase in the activity of phloem-feeding aphids in CO2-enriched air; but either no change or actual reductions in the abundance of chewing insects, which suggests that we can probably expect little net change, or possibly even a slight decrease, in the fraction of agricultural crop production that is typically destroyed by herbivorous insects as the air's CO2 content continues to rise.

In an original research study, Sanders et al. (2004) studied the effects of atmospheric CO2 enrichment on the plant and arthropod communities of the understory of a closed-canopy sweetgum plantation -- which reduces the light available to the understory by between 70 and 95% during the growing season -- in a FACE experiment where the air's CO2 content was increased by approximately 50%. In doing so, they found there were large adjustments in the relative productivities of the five dominant species that account for more than 90% of the biomass and annual production of the understory vegetation. Overall, however, the total understory productivities of the two CO2 treatments were not significantly different from each other. Also, Sanders et al. determined that "C:N ratios for four of the five dominant plant taxa did not differ between ambient and elevated CO2," and that "there were no overall treatment or species x treatment effects" with respect to this parameter. In addition, they found "no effect of elevated CO2 on herbivory," and that "even for the one species that showed an effect of CO2 on C:N ratio, herbivores did not compensate by foraging more." Neither did total arthropod abundance differ between ambient and elevated CO2 plots, nor did abundances of detritivores, omnivores or parasitoids. Hence, the researchers concluded that "idiosyncratic, species-specific responses to elevated CO2 may buffer one another," as "the abundances of some species increase while others decrease."

More recently, Stiling and Cornelissen (2007) described the results of various meta-analyses they employed to determine "the effects of elevated CO2 on both plants (n = 59 studies) and herbivores (n = 75 studies)," where ambient CO2 concentrations ranged between 350 and 420 ppm and elevated concentrations ranged between 550 and 1000 ppm. These analyses revealed that "elevated CO2 significantly decreased herbivore abundance (-21.6%), increased relative consumption rates (+16.5%), development time (+3.87%) and total consumption (+9.2%), and significantly decreased relative growth rate (-8.3%), conversion efficiency (-19.9%) and pupal weight (-5.03%)," while "host plants growing under enriched CO2 environments exhibited significantly larger biomass (+38.4%), increased C/N ratio (+26.57%), and decreased nitrogen concentration (-16.4%), as well as increased concentrations of tannins (+29.9%)." Consequently, with plant biomass increasing and herbivorous pest abundance decreasing (by +38.4% and -21.6%, respectively, in response to an approximate doubling of the atmosphere's CO2 concentration), it would appear from their analyses that in the eternal struggle to produce the food that sustains all of humanity, man's crops will fare ever better as the air's CO2 content continues its upward climb, and that there will be a concomitant expansion of the vegetative food base that sustains all of the biosphere.

With respect to past and possible future global warming, Andrew and Hughes (2007) write that "individualistic responses of species to current and future changes, especially differential migration rates, will result in the progressive decoupling of present day ecological interactions, together with the formation of new relationships potentially leading to profound changes in the structure and composition of present day communities," which changes, we would add, are almost universally claimed by climate alarmists to be detrimental to the species involved and, ultimately, bad for the entire biosphere. Hence, they "investigated how the relationship of herbivorous insects and their host plants may change under a warmer climate" by "transplanting a host plant species to locations subject to mean annual temperatures 1.2C higher than at the species' current warmest boundary and 5.5C higher than at its coolest edge," after which they "compared the structure and composition of the herbivorous insect community that colonized the transplants (i) to that of the host plant species within its natural range and (ii) to a congeneric plant species that grew naturally at the transplant latitude." In addition, they "investigated whether the herbivore community and rates of herbivory were affected by the latitudinal origin of the transplants."

As a result of these several efforts, the two Australian researchers found that "rates of herbivory did not significantly differ between the transplants and plants at sites within the natural range," and that "there were no significant differences in herbivore species richness or overall rates of herbivory on the transplants originating from different latitudes." Thus, they concluded that "if this result holds for other plant-herbivore systems, we might expect that under a warmer climate, broad patterns in insect community structure and rates of herbivory may remain similar to that at present, even though species composition may change substantially." Or, as they rephrase it, "if these results can be generalized to other plant hosts, we might predict that as climate zones shift poleward and mobile organisms like flying insects respond by migrating to stay within their current climatic envelope, plants will be colonized by new herbivore species within similar guilds to those currently supported," and that "changes in the composition, but not necessarily the structure, of these new communities may, therefore, result."

Consequently, in a warmer world of the future, the biosphere may well be significantly different in terms of who does what to whom; but all of its major functions are likely to be preserved with little alteration, which is pretty much what will likely be the case in an atmospherically CO2-enriched world of the future.

References
Andrew, N.R. and Hughes, L. 2007. Potential host colonization by insect herbivores in a warmer climate: a transplant experiment. Global Change Biology 13: 1539-1549.

Cyr, H. and Pace, M.L. 1993. Magnitude and patterns of herbivory in aquatic and terrestrial ecosystems. Nature 361: 148-150.

Holopainen, J.K. 2002. Aphid response to elevated ozone and CO2. Entomologia Experimentalis et Applicata 104: 137-142.

McNaughton, S.J., Oesterheld, M., Frank, D.A. and Williams, K.J. 1989. Ecosystem-level patterns of primary productivity and herbivory in terrestrial habitats. Nature 341: 142-144.

Sanders, N.J., Belote, R.T. and Weltzin, J.F. 2004. Multitriphic effects of elevated atmospheric CO2 on understory plant and arthropod communities. Environmental Entomology 33: 1609-1616.

Stiling, P. and Cornelissen, T. 2007. How does elevated carbon dioxide (CO2) affect plant-herbivore interactions? A field experiment and meta-analysis of CO2-mediated changes on plant chemistry and herbivore performance. Global Change Biology 13: 1-20.

Whittaker, J.B. 1999. Impacts and responses at population level of herbivorous insects to elevated CO2. European Journal of Entomology 96: 149-156.

Last updated 28 January 2009