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

Matthys Strydom,Ruan Veldtman,Karen J Esler,Mzabalazo Z Ngwenya

doi:10.7717/peerj.6816

Matthys Strydom, Ruan Veldtman + Show 2 more

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https://doi.org/10.7717/peerj.6816

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Journal: PeerJ	Publication Date: Apr 29, 2019
Citations: 11	License type: CC BY 4.0

Affiliation: Stellenbosch University, University of Cape Town

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.

Highlights

Assessment of the seed bank dynamics of invasive alien plants provides a framework to determine when control measures would be most effective during population development to reach management goals (Richardson & Kluge, 2008)
This was confirmed by the seeds surviving per tree not being significantly related to seed predation or abortion in the fitted Generalised linear mixed models (GLMMs) (Tables 4 and 5)
Based on seed production and seed bank data the released biological control agents are exerting no significant impact on the population dynamics of their Australian Acacia hosts

Summary

Introduction

Assessment of the seed bank dynamics of invasive alien plants provides a framework to determine when control measures would be most effective during population development (e.g., at what stage are seed banks the smallest) to reach management goals (Richardson & Kluge, 2008). Invasive Australian Acacia often cover vast areas of natural and agricultural land (Bar Kutiel, Cohen & Shoshany, 2004; Henderson, 2007; Marchante, Freitas & Hoffmann, 2011b) and have significant impacts on natural capital (e.g., reduced water availability) (Le Maitre et al, 2011; Rascher et al, 2011; Cohen & Bar Kutiel, 2017). Their impact has led to chemical and mechanical clearing operations (Van Wilgen et al, 2012) and biological control programs (Impson et al, 2011; Marchante, Freitas & Hoffmann, 2011a). In South Africa many resources have been spent in an effort to control these plants (Van Wilgen et al, 2012)

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