Forecasting native and exotic plant species richness and interactions

Abstract

Plant species richness patterns tend to follow environmental (climatic and topographic) gradients. Species richness-environmental relationships are used to predict species richness in different locations which can benefit conservation efforts. For instance, predicted native species richness can be used to identify native richness ‘hotspots’, and predicted exotic (i.e. non-native) richness can be used to determine which areas are most at risk of invasion by the most exotic species. Context-specific factors also influence native and exotic richness, suggesting that other factors may be important for predicting native and exotic species richness patterns. For native plants, species richness is affected by biogeographic factors, such as landmass area and degree of isolation; and exotic richness is higher in locations that have higher levels of human impact. I tested the relative importance of different factors on plant richness by developing models that linked combinations of native and exotic C3 and C4 grass species richness to environmental and human impact factors at a large (100 × 100 km) spatial scale across Australia. I found that native and exotic species richness were (i) positively correlated in areas they co-occurred possibly because (ii) they had similar associations with environmental and human impact variables, implying (iii) predicted native species richness may provide a template for potential exotic species richness. For exotic C4 grasses, Northern regions of Australia had particularly high native richness but relatively low exotic richness, suggesting these regions are suitable for supporting much greater numbers of exotic C4 grass species. I tested criteria (i) and (ii) for a further 20 common plant families within Australia to determine whether native and exotic richness respond similarly to environmental gradients more generally, and to develop guidelines for using the native richness template in other locations. Use of the native richness template was supported by four additional plant families, again highlighting Northern Australia as a region that should be able to support many more exotic species. Families that did not meet criteria (i) or (ii) suggested that use of native richness template may be restricted when native species richness are influenced by non-environmental factors and when exotic species are strongly dispersal-limited. I tested the importance of environmental and biogeographic factors (land area and isolation) on species richness by comparing native and exotic C3 and C4 grass richness in Australia and New Zealand. I found that that richness patterns in Australia predicted native richness patterns in New Zealand, likely because native species have conserved tolerances to environmental gradients. For its environmental conditions, New Zealand supported fewer native species relative to Australia, consistent with island biogeographic theory which posits smaller and more isolated landmasses support fewer species because of increased dispersal barriers. New Zealand supported many exotic species, likely because human activities overcome dispersal barriers that have previously isolated plant species assemblages. Finally, I tested the effect of changes in a critical resource (water availability) on the competitive impacts of exotic species on a native community. Under drought conditions, the exotic grasses had higher survival and greater biomass than native species, and one exotic species still competitively suppressed the native community. These findings suggest that native species may not escape competitive effects of exotic species during resource-poor periods.

Date of Award	2022
Original language	English
Awarding Institution	University of Canberra
Supervisor	Richard Duncan (Supervisor) & Lizzie Wandrag (Supervisor)