Spatial and Temporal Dependent Shifts in Grassland Invasibility

Abstract

A biological community can become invaded by non-resident species when the resources necessary for establishment and growth coincide with invader propagules. There are many ecological factors that affect below- and aboveground resource availability in a community and this research integrates both spatially and temporally varying components. In a managed grassland, we measured the interactive effects of disturbance patch size (DPS), grazing intensity (GI), and soil resource availability (SRA) on the invasion success of smooth pigweed Amaranthus hybridus (L.) and velvetleaf Abutilon theophrasti (L.). Importantly, we considered two life history stages of each focal invader (i.e., emerged and established seedlings) because invasion success may depend on when resource shifts coincide with particular life history stages. Our empirical data generally support the following predictions: 1) thresholds in ISP exist across DPS, the magnitude of which increases with higher SRA levels, 2) the life history stage of the invader strongly affects ISP in smaller sized patches containing high SRA, 3) regardless of invader size, grazing effects are large in small DPSs in high SRA area 4), in the absence of grazing, SRA within a given DPS can have opposing effects on ISP. Integrating disturbance type and intensity across other ecological dimensions is important in elucidating the mechanisms of community invasibility as well as offering sound recommendations to managers of native grassland communities.