How Natural Enemies Shape Gall Wasp Diversity in Oak Cynipid Communities

Abstract

The action of natural enemies within a model tri-trophic system, such as oak cynipid communities, can provide valuable insight into the mechanisms underlying the variation in gall inducer diversity across geographic gradients. Two hypotheses were formulated: (1) The diversity and abundance patterns of cynipid gall wasps, their parasitoids and inquilines differ between high- and low-altitude habitats. (2) Parasitoids and inquilines are involved in mechanisms that influence the abundance patterns of gall wasps. In order to test these hypotheses, we established a study site at various altitudes in the northeastern region of Algeria, selecting three localities: Bouzizi, Ghora and Mechrouha. In each of these localities, two distinct habitats were identified: one located at a lower altitude and the other at a higher altitude. Bud and acorn galls on Quercus faginea were collected, and the insects that emerged from these galls were then identified. The study showed that gall wasps had markedly greater abundance, species richness and diversity in low-altitude habitats. In contrast, parasitoids showed significantly higher abundance, species richness and diversity in high-altitude habitats. Conversely, there is no significant difference in the abundance and diversity of inquiline species between high and low altitudes. The rates of parasitism and inquilinism escalated at elevated elevations but diminished at lower altitudes. A negative correlation was seen between the parasitism rate and the gall wasp population (r = 0.87, r² = 0.77, p < 0.001), as well as between the inquilinism rate and the gall wasp populations (r = 0.86, r² = 0.75, p < 0.001). The results presented in this study confirm the involvement of parasitoids and inquilines in the mechanisms that influence the abundance patterns of gall wasps through their varying pressures between high- and low-elevation habitats.