Leaf quality and insect herbivory in model tropical plant communities after long-term exposure to elevated atmospheric CO2

Abstract

Results from laboratory feeding experiments have shown that elevated atmospheric carbon dioxide can affect interactions between plants and insect herbivores, primarily through changes in leaf nutritional quality occurring at elevated CO. Very few data are available on insect herbivory in plant communities where insects can choose among species and positions in the canopy in which to feed. Our objectives were to determine the extent to which CO-induced changes in plant communities and leaf nutritional quality may affect herbivory at the level of the entire canopy. We introduced equivalent populations of fourth instar Spodoptera eridania, a lepidopteran generalist, to complex model ecosystems containing seven species of moist tropical plants maintained under low mineral nutrient supply. Larvae were allowed to feed freely for 14 days, by which time they had reached the seventh instar. Prior to larval introductions, plant communities had been continuously exposed to either 340 μl CO l or to 610 μl CO l for 1.5 years. No major shifts in leaf nutritional quality [concentrations of N, total non-structural carbohydrates (TNC), sugar, and starch; ratios of: C/N, TNC/N, sugar/N, starch/N; leaf toughness] were observed between CO treatments for any of the species. Furthermore, no correlations were observed between these measures of leaf quality and leaf biomass consumption. Total leaf area and biomass of all plant communities were similar when caterpillars were introduced. However, leaf biomass of some species was slightly greater-and for other species slightly less (e.g. Cecropia peltata)-in communities exposed to elevated CO. Larvae showed the strongest preference for C. peltata leaves, the plant species that was least abundant in all communites, and fed relatively little on plants species which were more abundant. Thus, our results indicate that leaf tissue quality, as described by these parameters, is not necessarily affected by elevated CO under relatively low nutrient conditions. Hence, the potential importance of CO-induced shifts in leaf nutritional quality, as determinants of herbivory, may be overestimated for many plant communities growing on nutrient-poor sites if estimates are based on traditional laboratory feeding studies. Finally, slight shifts in the abundance of leaf tissue of various species occurring under elevated CO will probably not significantly affect herbivory by generalist insects. However, generalist insect herbivores appear to become more dependent on less-preferred plant species in cases where elevated CO results in reduced availability of leaves of a favoured plant species, and this greater dependency may eventually affect insect populations adversely.

Consumption rates and food preferences of slugs in a calcareous grassland under current and future CO2 conditions

“…This finding confirms the results of another study conducted in these plots that showed that in situ herbivore damage was not significantly different in plots subjected to ambient or elevated CO 2 (Ledergerber et al 1997). Of the few studies in which herbivores were given a choice, most have not shown an increase in consumption by herbivores on plant species grown at elevated CO 2 (Arnone et al 1995;Traw et al 1996;Ledergerber et al 1997;Díaz et al 1998). One experiment showed an increase in overall consumption due to greater consumption of the legume T. repens, but this was conducted on greenhouse plants grown on fertile substrate (Ledergerber et al 1998).…”

“…While other studies have shown that elevated CO 2 may change feeding patterns through increased consumption (compensatory feeding, reviewed in Lincoln et al 1993), this study shows that host preference may also be altered by rising CO 2 levels (hypothesis 3). This result seems to support the idea of alternative feeding, in which herbivores adjust to rising CO 2 levels by choosing plants that retain high resource concentrations at elevated CO 2 (Arnone et al 1995;Körner 1996). Compensatory and alternative feeding represent two strategies for dealing with the decreased foliar nitrogen density with which many plant species respond to enriched CO 2 atmospheres.…”

“…Partly because most studies on plant-herbivore interactions in response to elevated atmospheric CO 2 have used single-species feeding trials in which host switching was not possible, few data are available on possible changes in herbivore food preferences. In the few studies in which herbivores had a choice, preference shifts were not observed (Arnone et al 1995;Traw et al 1996;Ledergerber et al 1997Ledergerber et al , 1998Díaz et al 1998). Remarkably, only one of these studies (Ledergerber et al 1998) revealed an increase in total consumption as is typically seen in single-species feeding experiments trials.…”

Effects of long‐term atmospheric CO2 enrichment on the mineral concentration of Citrus aurantium leaves

“…Many authors have noted the potential significance of COg-induced changes in leaf nutrient quality for herbivore feeding and development (Fajer, Bowers & Bazzaz, 1989;Lincoln, Fajer & Johnson, 1993;Lindroth, Kinney & Platz, 1993;Arnone et al, 1995), for litter decomposition and for carbon sequestration in soils (Strain, 1985;Rastetter et al, 1992;Comins & McMurtrie, 1993;Schimel, 1995). Based on the results of experiments that ended before the sampling programme of this study, it has been widely assumed that these phenomena would be significantly affected by COg-induced alterations in leaf nutrient concentrations.…”

Elevated CO 2 lowers relative and absolute herbivore density across all species of a scrub-oak forest

“…However, relatively few studies on the effects of enhanced CO 2 on plant-herbivore interactions have been conducted to test the effects of elevated CO 2 on attack rates of herbivores by natural enemies, probably because of the difficulties inherent in maintaining all three trophic levels in laboratory conditions. More commonly, studies on the effects of elevated CO 2 on plant-herbivore interactions have focused on plant chemistry and rates of insect digestion and development, where it is relatively easy to feed insects on foliage grown in elevated CO 2 (Lincoln et al 1993;Arnone et al 1995;Bezemer and Jones 1998). However, in these laboratory conditions, plants are usually not nutrient limited, herbivores cannot choose where to feed and natural enemies are absent.…”