Separating sources of density‐dependent and density‐independent establishment limitation in invading species

Abstract

Summary Successful colonization by invasive species depends on both the ability to disperse seeds to a site and an ability to establish once seeds have arrived. While seed and establishment limitation are known to jointly influence colonization, decomposing establishment limitation into density‐dependent and density‐independent components has remained challenging. Here, we couple theoretical models of recruitment with a multispecies invasion experiment conducted within a natural gradient of soil moisture and productivity to assess how variation in establishment limitation shapes outcomes for invasion. Recruitment was affected by both density‐dependent and density‐independent sources of establishment limitation in three of four species. Soil moisture stress and productivity both increased density‐independent mortality in one species, whereas density‐dependent mortality increased in locations with favourable soil moisture. Synthesis. Successful establishment of invading species can be limited by both density‐dependent and density‐independent mechanisms. In particular, the strength of density‐independent limitation may depend on natural gradients in abiotic factors. The varying strengths of establishment limitation suggest that patterns of invasion are likely to be uneven both in space and in time. Understanding how intraspecific competitive constraints and density‐independent limitation vary with abiotic gradients can assist with predicting when invasions are likely to occur, information that can be harnessed in the development of better methods for control.

Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

“…; Larios & Suding ; Spotswood et al. ), but the importance of litter accumulation on the growth and expansion of these species has not been rigorously assessed.…”

“…The high silica content (up to 89% of ash for the whole plant) of medusahead (Swenson et al 1964) has been suggested to be responsible for its low palatability but also for the formation of very dense litter thatch with low decomposability (Nafus & Davies 2014), from which only medusahead seedlings can emerge (Davies & Svejcar 2008;Kyser et al 2014). Both the exotic introduced and the exotic invasive species produce large amounts of litter compared to the native plant community (James et al 2015;Larios & Suding 2015;Spotswood et al 2017), but the importance of litter accumulation on the growth and expansion of these species has not been rigorously assessed.…”

Fine‐scale variables associated with the presence of native forbs in natural temperate grassland

“…The relative importance of competitive biotic interactions identified in our results is consistent with predictions for productive grassland (Scherrer et al ). We would expect fewer negative effects of litter and dead biomass in unproductive areas (Morgan ; Williams et al ) where there is less biomass produced, less competition from dominant species (Price & Morgan ), and greater filtering by abiotic conditions relative to biotic interactions (the stress gradient hypothesis) (Spotswood et al ). We acknowledge that forb occurrence in the study site may have been slightly affected by grazing by kangaroos and rabbits, even though populations of both are managed, indirectly because these wild grazers consume grass biomass, and directly because forbs are a part of the normal diet of rabbits (Travers et al ).…”

Quantifying niche availability, niche overlap and competition for recruitment sites in plant populations without explicit knowledge of niche axes

“…Equation 1 describes a recruitment curve with an asymptote at b i n i , which is the total number of safe-sites per plot and hence the number of individuals that would recruit if seeds landed in all safe-sites (i.e. the plot was saturated with seeds) (Aicher et al, 2011;Duncan et al, 2009;Miller et al, 2014;Spotswood et al, 2016).…”

“…Species-specific requirements for germination and seedling establishment mean that even small differences among microsites can alter conditions to favour the recruitment of one species over another (Fowler & Antonovics, 1981;Harper et al, 1965). Because the number of microsites suitable for a given species will vary among habitats and along environmental gradients (Duncan, Diez, Sullivan, Wangen, & Miller, 2009;Miller et al, 2014;Spotswood, Mariotte, Farrer, Nichols, & Suding, 2016), locations with greater numbers of microsites suitable for a given species coincide with areas of greater fundamental niche availability. Measuring the number of suitable microsites at different locations thus allows us to quantify niche availability without knowing precisely what underpins recruitment outcomes.…”