Patterns of canopy cover and gall wasp diversity in Quercus spp. (Fagaceae) along fragmented landscape in Mexico

"Padrões de cobertura de dossel e diversidade de vespas galhadoras em Quercus spp (Fagaceae) ao longo de uma paisagem fragmentada no México"

 

Pablo Cuevas-Reyes

Universidad Michoacana de San Nicolás de Hidalgo, Ciudad Universitaria,

Morelia, Michoacán, México

 

The life cycle of galling insects must be synchronized with the phenology of host plants because they require undifferentiated tissue to initiate gall induction. However, forest fragmentation by human activities modifies the microenvironment conditions of remaining forest patches and isolated trees that may alter the phenological patterns of individual trees. The timing and availability of resources (i.e. leaves) are a key features that determine the patterns and preferences of galling insects in a fragmented landscape. Cynipids or gall wasps (Hymenoptera: Cynipidae: Cynipinae) are closely associated with oak trees (Fagaceae: Quercus). Each wasp species is specific to a particular host oak species or species group. The cynipid life cycle progresses through two generations, a sexual generation in the spring or early summer and an asexual generation during the summer and autumn of the same year. We conducted a three-year study to test if the species richness of oak gall wasps associated with oaks as host-plants decreased in function of the degree of disturbance of remnant fragments assuming a modification in leaf phenology and canopy cover patterns of some oak species in fragments of different sizes and levels of disturbance. Fifteen permanent forest fragments were classified in three categories according to their size. We established permanent sampling sites on the fragment edge and one in the fragment interior at a distance of more than 100 m from the forest border. Twenty-five isolated oak trees were also selected. For each individual tree, the canopy cover of leaves, diameter at breast height (DBH), tree height, abundance and species richness of gall wasps were recorded every month during three years. The galling insects were identified on the basis of gall morphology and host plant species. Our results show that oak canopy cover, DBH and tree height were significantly higher in isolated oaks and small fragments, influencing the spatial and temporal variation of gall wasps abundance and species richness in most fragmented habitats. Isolated oaks could be considered “super-hosts” because they hosted approximately 80 oak gall wasp species in a fragmented landscape of oak forest in Mexico.