INTECOL 2013 Poster - Trophic cascades in a changing climate
Atmospheric concentrations of CO2 are predicted to increase significantly during this century. Increased availability of CO2 may increase photosynthesis by 11 – 39% potentially stimulating plant biomass by 10-20%. The response of plants to elevated CO2 is, however, likely to be species- and genotype-specific. Moreover, projected gains in plant productivity in a CO2 enriched world seldom account for trophic interactions, which may be indirectly affected by changes to the plant. While it has been known for sometime that the carbon:nitrogen (C:N) ratio increases in plant tissue in response to CO2 enrichment, most herbivory studies only consider the effects on primary consumers. This is despite the potential for cascading effects on higher trophic levels. For example, high C:N ratio in plant tissues results in a concomitant increase in the tissue of aphids; which also led to compensatory predation by ladybirds facing nutritionally poorer prey items. The same study, however, also found lower aphid densities corresponded with the high C:N in plant tissue, possibly impacting on the foraging behaviour of predators when trying to compensate for reduced prey quality.
Despite the growing number of studies investigating the biotic impact of elevated CO2, there remains a gap in our understanding of species interactions across trophic levels. Here we present a series of controlled-environment experiments to explore the consequences of atmospheric CO2 enrichment on organism performance and trophic interactions. We used a simple, tri-trophic chain comprising of the European red raspberry, Rubus idaeus, the large raspberry aphid, Amphorophora idaei, and the predatory ladybird Harmonia axyridis. Like most crop species, there are multiple cultivars of European red raspberry that vary in their resistance to aphid herbivory. We used three cultivars representing a gradient of resistance to the large raspberry aphid. As predicted, raspberry plants respond to CO2 enrichment by a cultivar specific increase in biomass. Aphid abundance did not change on susceptible and partially resistant cultivars in elevated CO2, despite the positive response by the plants. CO2 enrichment did, however, result in a significant increase in aphid abundance and a 65% increase in aphid colonisation on a cultivar resistant to aphid herbivory in ambient conditions. When a higher trophic level was present, this significant effect of CO2 on aphid abundance was removed. These findings will be discussed in the wider context of other experiments we have performed to create a picture of how this multi-trophic system may respond to CO2 enrichment.