Abstract
Seed dispersal is a fundamental process in the lifecycle of all flowering plants. Many plant species have evolved specialist associations with biotic vectors to facilitate dispersal. Such specialised interactions mean that these associations are potentially highly sensitive to disruption, e.g. from invasive species. However, despite this threat we still understand remarkably little about how such perturbations affect the dynamics and efficiency of the seed-dispersal process. In this study we quantify the impacts of an invasive ant across three key phases of the seed dispersal process: seed removal, distribution and placement, in order to determine the stages of seed dispersal most vulnerable to disruption by invaders. Using the Argentine ant (Linepithema humile) as a model, we show that invaded sites exhibited a significant decrease in seed dispersal services across all three phases of the dispersal process, relative to non-invaded sites. Seeds dispersed in invaded sites were: (a) less likely to be transported; (b) potentially distributed over a smaller spatial area, and (c) less likely to be placed at soil depths favourable for germination and establishment compared to those dispersed in non-invaded sites. These results reveal that ant-mediated seed dispersal services are significantly reduced by an invasive species at multiple stages in the dispersal process. Reductions in the efficacy of seed dispersal, combined with shifts in the ecological and geographical patterns of dispersal, may lead to cascading impacts on plant species composition and community structure. This study shows how an invasive ant can affect seed dispersal at several stages in the dispersal process.
Highlights
Seed dispersal is a vital process in the life cycle of all flowering plants (O’Dowd and Hay 1980; Howe and Smallwood 1982; Hanzawa et al 1988)
Using the Argentine ant (Linepithema humile) as a model, we show that invaded sites exhibited a significant decrease in seed dispersal services across all three phases of the dispersal process, relative to non-invaded sites
The fastest rates of removal in invaded sites were for the plant species Cirsium vulgare (52% [± SEM 7.9%] of seeds removed after 6 h) and Euphorbia serrata (65% [± SEM 6.2%] of seeds removed after 6 h); these removal rates were still significantly lower than seeds of the same plant species (C. vulgare 90% [± SEM 4.7%] and E. serrata 92% [± SEM 2.1%] dispersed in non-invaded sites (Fig. 2b)
Summary
Seed dispersal is a vital process in the life cycle of all flowering plants (O’Dowd and Hay 1980; Howe and Smallwood 1982; Hanzawa et al 1988). Invasions by nonnative species threaten the efficacy of biotic seeddispersers, with potentially serious knock-on effects to the natural plant community structure (Ricklefs and Renner 1994; Webb and Peart 2001; Christian 2001; Ozinga et al 2009; Ruxton and Schaefer 2012) Despite this threat, we often lack comprehensive data on how specific biological invasions alter important ecosystem processes like seed dispersal. We often lack comprehensive data on how specific biological invasions alter important ecosystem processes like seed dispersal This is because assessments are often based on studies that focus on only one aspect of the seed dispersal process (e.g. seed choice), without consideration of the entire process (e.g. whether seeds end up in favourable germination sites). This latter example can be used as a proxy for understanding the ultimate impact on plant community composition
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