Seed survival of Australian Acacia in the Western Cape of South Africa in the presence of biological control agents and given environmental variation.

ABSTRACT

Studies of invasive Australian Acacia have shown that many seeds are still produced and accumulate in soil stored seed banks regardless of the presence of seed-targeting biological control agents. This is despite claims of biological control success, although there is generally a lack of data on the seed production of invasive Australian Acacia before and after the release of the respective agents. We aimed to quantify seed production and seed survival of invasive Australian Acacia currently under biological control. The seed production and survival (proportion of aborted, predated and surviving seeds) of A. longifolia, A. pycnantha and A. saligna were each studied at four to five sites in the Western Cape of South Africa. The relationships between seed production and stand characteristics were determined and the relative effects of seed predation and abortion on seed survival were established. The investigated invasive Australian Acacia produced many seeds that survived the pre-dispersal stage despite long-term presence of released biological control agents. It was shown that seed crop size is the only significant factor influencing seed survival of the studied Australian Acacia species. Furthermore, the seeds surviving per tree and per square meter were related to tree size. No quantitative evidence was found to suggest that seed-reducing biological control agents are having an impact on the population dynamics of their Australian Acacia hosts. This study illustrates the importance of studying the seed ecology of invasive plants before biological control agents are selected and released.