Quantifying invasion resistance: the use of recruitment functions to control for propagule pressure

Abstract

Abstract. Invasive species distributions tend to be biased towards some habitats compared to others due to the combined effects of habitat-specific resistance to invasion and non-uniform propagule pressure. These two factors may also interact, with habitat resistance varying as a function of propagule supply rate. Recruitment experiments, in which the number of individuals recruiting into a population is measured under different propagule supply rates, can help us understand these interactions and quantify habitat resistance to invasion while controlling for variation in propagule supply rate. Here, we constructed recruitment functions for the invasive herb Hieracium lepidulum by sowing seeds at five different densities into six different habitat types in New Zealand's Southern Alps repeated over two successive years, and monitored seedling recruitment and survival over a four year period. We fitted recruitment functions that allowed us to estimate the total number of safe sites available for plants to occupy, which we used as a measure of invasion resistance, and tested several hypotheses concerning how invasion resistance differed among habitats and over time. We found significant differences in levels of H. lepidulum recruitment among habitats, which did not match the species' current distribution in the landscape. Local biotic and abiotic characteristics helped explain some of the between-habitat variation, with vascular plant species richness, vascular plant cover, and light availability, all positively correlated with the number of safe sites for recruitment. Resistance also varied over time however, with cohorts sown in successive years showing different levels of recruitment in some habitats but not others. These results show that recruitment functions can be used to quantify habitat resistance to invasion and to identify potential mechanisms of invasion resistance.

Testing weed risk assessment paradigms: Intraspecific differences in performance and naturalisation risk outweigh interspecific differences in alienBrassica

“…Many suitable microsites may remain uncolonised unless seed is very widely dispersed, highlighting that the spatial extent of propagule dispersal is an important component of propagule pressure (Lockwood, Cassey, & Blackburn, ). Indeed, our results support the view that propagule pressure (both the number of sites at which seed is introduced and the number of seeds reaching those sites) is a key determinant of invasion outcomes (Miller et al., ). In the light of this, weed risk assessment (WRA) protocols which do not account for introduction effort may fail to accurately assess invasion risk (Hulme, ).…”

Meta‐analysis suggests biotic resistance in freshwater environments is driven by consumption rather than competition

“…This conflicts with the idea that competition may be most relaxed amongst marine consumers due to the prevalence of omnivory in marine communities (Thompson et al 2007). Competitive biotic resistance may also be moderated by native species density (Smith et al 2004, Alofs andFowler 2013) and invasive propagule pressure (Von Holle and Simberloff 2005, Miller et al 2014; however, we focused on effects measured at ambient density where possible (see Methods). Frequently, these experiments measured the effect of interspecific competition (between native and invasive species) in contrast to intraspecific competition (between individual invaders), while keeping the density of individuals in the two treatments constant.…”

How propagule size and environmental suitability jointly determine establishment success: a test using dung beetle introductions

“…Importantly, it is the interaction between these two factors that is central to understanding how and why introduced species establish (D'Antonio et al 2001;Rouget and Richardson 2003;Leung and Mandrak 2007;Miller et al 2014). This is because the relationship between founding population size and establishment probability can vary as a function of environmental suitability: at locations where suitability is high few founding individuals may be required for a population to establish, while more individuals are required for success at sites of low suitability.…”

“…Although this interaction has been identified and modelled using species occurrence data (Rouget and Richardson 2003;Leung and Mandrak 2007;Eschtruth and Battles 2011) no studies have clearly demonstrated its importance for population establishment in the field. Studies of plant populations have shown that the form of the relationship between the number of seeds added to plots (initial population size) and the number of seedlings that recruit varies depending on environmental conditions (Thomsen et al 2006;Miller et al 2014) but these studies have not assessed population establishment. Greenhouse and laboratory studies of insect introductions have shown strong effects of both founding population size and environmental suitability on establishment success, but no evidence of a significant interaction between the two (Hufbauer et al 2013;Szűcs et al 2014).…”