Impact of disturbance on arthropod community structure: Nutrient enrichment, fire and the invasive Chinese tallow tree (Sapium sebiferum)
Hartley, Maria Kate
Doctor of Philosophy
Invasive species, fire suppression and nutrient deposition, as distinct disturbance factors, have altered Texas prairie communities. Arthropod diversity, abundance, and community composition may be modified by these factors either directly or by changes in plant community structure. Because arthropods provide essential ecosystem services and represent a large proportion of terrestrial biodiversity, understanding the responses of arthropod communities to disturbance is the central theme of this doctoral thesis. Land managers commonly utilize fire to control woody species encroachment into prairies and anthropogenic nutrient deposition increases plant productivity. The responses of arthropods to fire and fertilizer manipulations were examined. Fire maintained arthropod diversity by limiting woody plant cover. Path analysis indicated fertilization decreased arthropod diversity by promoting woody plant invasion. Direct negative effects of fire on the arthropod community were small compared to the indirect, positive effects. Arthropod abundance and species richness on the invasive species Chinese Tallow Tree (Sapium sebiferum) were compared to arthropods on three native tree species: Silver Maple (Acer saccharinum), Sycamore (Platanus occidentalis), and Sweetgum ( Liquidambar styraciflua). Although arthropod community composition differed on Sapium, species richness and abundance were similar to levels on native trees. Sapium experienced less chewing, fungal, and mining damage and supported relatively abundant predators and detritivores whilst there were relatively fewer herbivores. These observations are in accord with the Enemies Release Hypothesis that predicts low herbivore loads may contribute to Sapium's invasive success. When Sapium was compared to native plant dominated habitats, predators and detritivores were again relatively abundant while total arthropod diversity was comparatively low. This implies that Sapium has not acquired an insect fauna comparable to native plants in Texas. Insect assemblages on Sapium were examined along a geographical gradient, representing time since introduction. Sapium trees in Florida and Georgia (present longer) had higher insect loads compared to those in Louisiana and Texas (more recently introduced), but species richness did not differ. Herbivore and omnivore abundance and species richness were higher where Sapium has been present longer. This suggests that time since introduction is an important factor constraining herbivore diversity and abundance. Accumulation of herbivores may limit Sapium populations in the future.