Influence of elevated CO₂ on interspecific interactions at higher trophic levels

Abstract

We report the results of a study investigating the influence of elevated CO2 on species interactions across three trophic levels: a plant (Brassica oleracea), two aphid herbivores (the generalist Myzus persicae and the specialist Brevicoryne brassicae), and two natural enemies (the coccinellid Hippodamia convergens (ladybird) and the parasitoid wasp Diaeretiella rapae). Brassica oleracea plants reared under elevated CO2 conditions (650 ppmv vs. 350 ppmv) were larger and had decreased water and nitrogen content. Brevicoryne brassicae reared on plants grown in elevated CO2 were larger and accumulated more fat, while there was no change in M. persicae traits. Fecundity of individual aphids appeared to be increased when reared on plants grown in elevated CO2. However, these differences were generally lost when aphids were reared in colonies, suggesting that such changes in plant quality will have subtle effects on aphid intraspecific interactions. Nevertheless, CO2 treatment did influence aphid distribution on plants, with significantly fewer M. persicae found on the shoots, and B. brassicae was only found on senescing leaves, when colonies were reared on plants grown in elevated CO2. We reared B. brassicae and M. persicae in competition on plants grown at both the CO2 concentration treatments. We found a significantly lower ratio of M. persicae: B. brassicae on plants grown under elevated CO2 conditions, strongly suggesting that increasing CO2 concentrations can alter the outcome of competition among insect herbivores. This was also reflected in the distribution of the aphids on the plants. While the CO2 treatment did not influence where B. brassicae were found, fewer M. persicae were present on senescing leaves under elevated CO2 conditions. Changes in plant quality resulting from the CO2 treatments did not appear to alter aphid quality as prey species, as the number consumed by the ladybird H. convergens, and the number parasitised by the parasitoid wasp D. rapae, did not change. To our knowledge, this study provides the first empirical evidence that changes in host plant quality mediated by increasing levels of CO2 can alter the outcome of interspecific competition among insect herbivores.

Climate Change and Tritrophic Interactions: Will Modifications to Greenhouse Gas Emissions Increase the Vulnerability of Herbivorous Insects to Natural Enemies?

“…Concerning the searching behavior of predators, Vuorinen et al (2004) showed that, unlike plants grown under ambient CO 2 , the spined soldier bug Podisus maculiventris (Say) does not discriminate herbivore-damaged plants from intact plants under elevated CO 2 , regardless of host plant cultivar. Other results tend to show similar consumption rates under both ambient and elevated CO 2 conditions for Hippodamia convergens Guérin-Méneville (Stacey and Fellowes 2002), regardless of the aphid species being predated on. Two other studies conducted in the early 2000s investigated the impact of natural enemies on the population dynamics of tree aphid species.…”

“…There were several noticeable differences in the methods used by each study. For example, the difference in the CO 2 concentration between the ambient and elevated treatments was lower in the study by Stacey and Fellowes (2002) compared with Klaiber et al (2013). Also, a lower modification in CO 2 concentration could lead to a lower impact on parasitoid efficiency.…”

“…For instance, found no difference in the parasitism rate of the parasitoid Aphidius matricariae Haliday between ambient and elevated CO 2 treatments. Furthermore, an increase in atmospheric CO 2 concentration did not influence the preference and efficiency of the aphid parasitoid Diaretiella rapae (McIntosh) (Stacey and Fellowes 2002). In contrast, Klaiber et al (2013) showed a decrease in the efficiency of D. rapae under elevated CO 2 concentrations compared with ambient CO 2 concentrations, with $50% reduction in parasitized hosts number and short-lived emerging adults.…”

“…Experimental conditions may have also influenced the results. For instance, Stacey and Fellowes (2002) and Klaiber et al (2013) studied the same host-parasitoid species complex, but obtained different results. The former study found that increased CO 2 did not affect the performance of D. rapae, whereas the latter study found a reduction in efficiency.…”

Crop and pasture response to climate change

“…Recent research has highlighted the key role of competition between C 3 crop and C 4 weed species under different climate and CO 2 concentrations (27). CO 2 -temperature interactions are recognized as a key factor determining plant damage from pests in future decades; CO 2 /precipitation interactions will be likewise important (58,59). Most studies continue to investigate pest damage as a separate function of either CO 2 (60-63) or climate, mostly temperature (64)(65)(66).…”