As a result of forest fragmentation, tropical edge species have increased in abundance. Yet little is known about the response of edge species to global warming. We investigated effects of elevated CO2 and temperature on the vine Merremia umbellata (Convulvulacea) and larvae of a specialized, herbivorous beetle Acromis sparsa (Chrysomelidae). Larvae were reared on hostplants at 2-levels of CO2 (normal and double) and 2-levels of temperature (ambient and ca. 4°C above ambient).
Results/Conclusions
Doubling CO2 (2xCO2) more than doubled plant biomass, whereas temperature had no effect. Accordingly leaves expanded faster in 2xCO2, but leaves grown at warmer temperatures and normal CO2 had the greatest percentage of C. Leaf nitrogen was unaffected by the treatments. Larval growth and survivorship respond to warmer temperatures directly, but larvae also respond indirectly to the effects of warmer temperatures and 2xCO2 on their hostplant. Proportion of larvae pupating was disproportionately low in the 2xCO2 and high temperature treatment. Hostplants in high temperature and 2xCO2 had the greatest C:N ratio and the low availability of nitrogen probably increased larval mortality. However pupae were smaller in the elevated temperature, overall, with 2xCO2 interacting with temperature to increase pupal mass in elevated temperatures but decrease pupal mass in ambient temperatures. This rank order differs from effects of temperature and CO2 on larval mortality, but high temperature and 2xCO2 treatment may have generated the smallest larvae, and they simply didn’t pupate (and hence were not weighed). Days to pupation did not differ among treatments, but larvae reared at elevated temperatures required more time as pupae to eclose to adult. In addition, fewer pupae survived at high temperatures, but pupal survivorship was slightly better in 2xCO2. Because pupae were placed in a common area within a day of pupation, effects of temperature and CO2 on pupae are residual effects on the larvae. Rank order of treatments with highest to lowest proportions of pupae surviving to adult follows that of treatments with highest to lowest pupal masses suggesting that those treatments in which the larvae grew to make heavier pupae were more likely to eclose to adult. Elevating atmospheric CO2 and temperature together caused declines in hostplant quality and larval survivorship that were not observed when only one environmental feature was altered. This interaction will be deeply consequential to the forest edge, impacting multi-trophic systems like Merremia and its specialized herbivore that depends on its host for defensive compounds.