Fitness related diet-mixing by intraspecific host-plant-switching of specialist insect herbivores

Generalist insect herbivores may profit by feeding on a mixture of plant species that differ in nutritional quality. Herbivore performance can also be affected by intraspecific host plant variation. However, it is unknown whether conspecific plant individuals differ sufficiently to promote diet-mixing behavior in specialist herbivores. We experimentally tested this "specialist diet-mixing hypothesis" for specialist caterpillars (Chrysopsyche imparilis, Lasiocampidae) in a West African savanna. The caterpillars switched regularly between host tree individuals (Combretum fragrans, Combretaceae). To examine whether switching benefited caterpillar performance via diet-mixing, the caterpillars were reared either on leaves from several plant individuals (mixed diet) or on leaves from a single plant. The strongest effect of diet-mixing was found for fecundity, with females reared on a mixed diet laying significantly more eggs than sisters receiving a single-plant diet. In addition, a mixed diet decreased variability in egg size and increased the growth of second-instar caterpillars. Supplementary food choice experiments were conducted to assess a potential influence of lowered host quality (induced by herbivory) on caterpillar behavior; no such effect was found. By linking intraspecific host-switching behavior and herbivore performance, this study provides new information on the relevance of intraspecific plant variation for herbivorous insects.     

Niche complementarity for nitrogen: an explanation for the biodiversity and ecosystem functioning relationship?

The relationship between plant diversity and productivity has largely been attributed to niche complementarity, assuming that plant species are complementary in their resource use. In this context, we conducted an 15N field study in three different grasslands, testing complementarity nitrogen (N) uptake patterns in terms of space, time, and chemical form as well as N strategies such as soil N use, symbiotic N fixation, or internal N recycling for different plant species. The relative contribution of different spatial, temporal, and chemical soil N pools to total soil N uptake of plants varied significantly among the investigated plant species, within and across functional groups. This suggests that plants occupy distinct niches with respect to their relative N uptake. However, when the absolute N uptake from the different soil N pools was analyzed, no spatial, temporal, or chemical variability was detected, but plants, and in particular functional groups, differed significantly with respect to their total soil N uptake irrespective of treatment. Consequently, our data suggest that absolute N exploitation on the ecosystem level is determined by species or functional group identity and thus by community composition rather than by complementary biodiversity effects. Across functional groups, total N uptake from the soil was negatively correlated with leaf N concentrations, suggesting that these functional groups follow different N use strategies to meet their N demands. While our findings give no evidence for a biodiversity effect on the quantitative exploitation of different soil N pools, there is evidence for different and complementary N strategies and thus a potentially beneficial effect of functional group diversity on ecosystem functioning.     

Species-specific responses of herbivores to within-plant and environmentally mediated between-plant variability in plant chemistry

Allocation of resources to growth and defense against herbivores crucially affects plant competitiveness and survival, resulting in a specific distribution of assimilates and defense compounds within plant individuals. Additionally, plants rarely experience stable environmental conditions, and adaptations to abiotic and biotic stresses may involve shifts in resistance to herbivores. We studied the allocation of phytochemicals in Brassica oleracea (Brussels sprouts) due to leaf age, drought stress and herbivore damage and assessed effects on two lepidopteran herbivores differing in diet breadth: the generalist Spodoptera littoralis and the specialist Pieris brassicae. Glucosinolates as secondary defense compounds and total nitrogen and carbon were quantified and linked to plant palatability, i.e., herbivore feeding preference. Herbivore responses were highly species-specific and partially related to changes in phytochemicals. Spodoptera littoralis preferred middle-aged leaves with intermediate levels of glucosinolates and nitrogen over young, glucosinolate and nitrogen rich leaves, as well as over old leaves, poor in glucosinolates and nitrogen. In contrast, P. brassicae preferred young leaves. Both species preferred severely drought-stressed plants to the well-watered control, although analyzed glucosinolate concentrations did not differ. Both S. littoralis and P. brassicae feeding induced an increase of indole glucosinolate levels, which may explain a reduced consumption of damaged plants detected for S. littoralis but not for P. brassicae. By revealing distinct, sometimes contrasting responses of two insect herbivores to within-plant and stress-mediated intraspecific variation in phytochemistry of B. oleracea, this study emphasizes the need to consider specific herbivore responses to understand and predict the interactions between herbivores and variable plants.