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Unwelcome Guests? The Seed Biology of Invasive Species

Acacia melanoxylon (Leguminosae) seeds, an Australian species invasive in Southern Africa (scale bar = 2mm)

This project is one of a series in the theme 'Climate and Reproductive Biology'.

The introduction of invasive plants is one of the major threats to plant biodiversity as well as impacting negatively on ecosystem function e.g. watershed hydrology, the yields of harvested food and medicines and scenic beauty. Thus, in the USA, the total economic impact of invasive species has been estimated at $137 billion per year. In addition, invasive species compete with native species, resulting in a loss of species richness and often mono-dominance by the invading species. This competitive advantage of invasive species over native species has received considerable attention with the majority of studies focussing on traits of adult plants, such as growth rates, survival, tolerance to herbivory and nutrient and water use efficiency.

However, there has been little effort to understand how seed biology contributes to the success of introduced species. Most effort in this area has been directed at control measures for adult plants and their soil seed. Whilst forming a large, persistent seed bank may be advantageous for invasive species, the presence of soil seed banks is irrelevant if the seeds do not germinate at the appropriate time for maximum seedling establishment. Consequently, an understanding of seed yield, persistence and the underlying seed germination biology should provide significant insights into the adaptive success of invasive species. In addition, seedling competitiveness and resilience to environmental stresses is positively related to seed and hence seedling size. This coupled with the tentative suggestion that seed size of invasive species is larger in their invasive than native ranges may potentially explain the competitive nature of invasive species.

To address this issue, we are conducting within-species comparisons of seed traits to test whether shifts in traits such as mass, resource allocation patterns and fecundity contribute to the invasiveness of species within their introduced range. In addition, we will conduct between species comparisons on con-generic / familial groups of species to test for differences in seed traits between co-occurring invasive, exotic species and native species. These complementary approaches should lead to an enhanced understanding of the processes underlying species invasion and lead to the development of predictive tools that may allow the identification of potentially invasive species.

We have observed that for 114 of the most invasive plant species, seed mass in the introduced, invasive range is generally greater than in the native range (see Annex material). In addition, for co-occurring native and invasive Asteraceae and Poaceae in California we have found that the seed mass of invasive species is, on average, greater. Since large seeds result in larger seedlings, which are usually more competitive and more tolerant of adverse conditions, shifts in seed mass may contribute to species’ invasions.

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