Plant-herbivore coevolution in a changing world

Human impacts on the environment, such as habitat destruction and fragmentation, climate change, invasive species, over-exploitation and pollution are causing a rapid loss of biodiversity. We only have limited knowledge on the impacts of biodiversity loss on ecological and evolutionary processes. Much of what we know of current diversity loss is on single species level and little is known about the impact on species interactions (Leimu et al. 2012).

Coevolution as proposed by Ehrlich and Raven (1964) in their classic paper was responsible for much of the fantastic diversity found in modern insects. Plants evolved chemical defenses to protect themselves from herbivore damage and consequently insects evolved adaptions to these chemicals. Coevolution is seen to be a dominant process that has shaped much of the biodiversity on the planet.

Human induced environmental changes such as habitat fragmentation reduce populations sizes. Climate change may reduce the size of optimal habitat available for populations therefore contributing to population reduction. Small populations can lead to inbreeding and reduced genetic variation. These factors are likely to influence plant-herbivore coevoluntionary processes by affecting a species ability to respond to selection pressure (Leimu et al 2012).

The ability of some plants to become weeds in a non-native habitat has been attributed to their release from their coevolved natural enemy allowing reallocation of resources from chemical defense into growth and reproduction (Zangerl and Berenbaum, 2005). The introduction of biocontrol agent, generally a species that has coevolved with the invasive species may control the pest. Zangerl and Berenbaum (2005) give an example how a plant weed become more toxic after its natural enemy was released as a biocontrol agent. In the absence of its natural enemy the toxicity of the plant reduced but when the coevolutionary relationship was re-introduced the plant again began producing toxic chemicals. As shown by this example there is a need to consider the coevolutionary history of interacting species when proposing biocontrol of an invasive species.

The group discussion queried how would it be possible to determine if plant-herbivore coevolution had been conserved or even if it is possible to conserve such relationships.

The paper discussed concludes with Leimu et al. (2012) determining that urgent study is needed to investigate the interactive effects of the major drivers of biodiversity loss. A criticism of this continuing wish for more study was brought up by a member of the discussion group. How much investigation is needed before conservation action is taken? Martin et al. (2012) document the demise of a small insectivorous bat endemic to Christmas Island. The decline of the bat was documented from 1986 and the species has been monitored intensively since 2004. Sadly the bat is now extinction because of a lack of action even though it was very obvious that this species was in trouble with only 20 individuals left.

Is it possible to conserve plant-herbivore coevolution? Does it really matter whether or not ALL coevolving relationships are conserved? New Zealand plant species evolved without browsing mammals, can they or are they already coevolving with these new herbivores?

References

Ehrlich, P. R., & Raven, P. H. (1964). Butterflies and Plants: A Study in Coevolution? Evolution, 18, 586-608.

Leimu, R., Muola, A., Laukkanen, L., Kalske, A., Prill, N., & Mutikainen, P. (2012). Plant-herbivore coevolution in a changing world. Entomologia Experimentalis et Applicata, 144, 3-13.

Martin, T. G., Nally, S., Burbidge, A. A., Arnall, S., Garnett, S. T., Hayward, M. W., et al. (2012).

Acting fast helps avoid extinction. Conservation Letters, 0, 1-7.

Zangerl, A. R., & Berenbaum, M. R. (2005). Increase in toxicity of an invasive weed after reassociation with its coevolved hervivore. Proceedings of the National Academy of Sciences of the United States of America, 102(43), 15529-15532.