Understanding the relative importance environmental and anthropogenic variables in driving native and invasive plant and community structure and cover helps predict future plant community assemblages in ecosystems that are highly vulnerable to biological invasion. Using a stratified random sampling approach, we carried out a landscape-scale survey of 330 vegetation plots on San Clemente Island (SCI), California, USA, an island with a Mediterranean-type climate and numerous endemic and endangered plant species. We recorded percent cover of native and invasive plants and also measured climatic and topographic variables (precipitation, elevation, slope, aspect, topographic wetness index, and distance to streams), soil abiotic and biotic variables (moisture, pH, carbon, nitrogen, soil microbial biomass), and human influence (distance to roads). Using nonmetric multidimensional scaling (NMDS) and regression analyses, we quantified the relative influence of these factors on plant community composition and percent cover of native and invasive plants. We also evaluated relationships between invasive plants and overall species richness and native plant cover.
Results/Conclusions
NMDS results revealed that the changes in community composition were driven by a combination of abundance of invasive species, elevation, and distance from nearest road. Plant communities dominated by invasive species had lower soil organic matter content (r=-0.27, p=0.009) and lower total nitrogen (r=-0.26, p=0.01). Regression results showed that native plant cover increased with increasing distance from paved roads (a geographic gradient that represent increased human disturbance frequency and intensity on SCI), from southwest to northeast (a geographic/climatic gradient that represents increasing moisture), and with increasing soil pH. In contrast, invasive plant cover decreased with increasing distance from paved roads and increased from southwest to northeast. There was also a significant inverse association (p<0.001) between invasive plant cover and native plant cover. Our results suggest complex and interplaying effects of climate, topography, soil, and human factors on native and invasive plant communities on an oceanic island. Furthermore, an apparent negative effect of invasive plants on soil organic matter, nitrogen, and native plant cover suggests the potential for future community reassembly as invasive species become increasingly dominant. Our results illustrate the importance of simultaneously considering climatic, abiotic and human factors for future research and management of native and invasive plants.