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; Mzabalazo Z Ngwenya; Karen J Esler

doi:10.7717/peerj.6816

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

PeerJ. 2019 Apr 29:7:e6816. doi: 10.7717/peerj.6816. eCollection 2019.

Authors

Matthys Strydom^{1

2

3}, Ruan Veldtman^{1

4}, Mzabalazo Z Ngwenya^{5

6}, Karen J Esler^{1

2}

Affiliations

¹ Department of Conservation Ecology and Entomology, Stellenbosch University, Matieland, South Africa.
² Centre of Excellence for Invasion Biology, Stellenbosch University, Matieland, South Africa.
³ Academy for Environmental Leadership SA, Upington, South Africa.
⁴ South African National Biodiversity Institute, Kirstenbosch National Botanical Garden, Cape Town, South Africa.
⁵ Statistics in Ecology, Environment and Conservation (SEEC), Department of Statistical Sciences, University of Cape Town, Rondebosch, South Africa.
⁶ Biometry, Agricultural Research Council, Stellenbosch, South Africa.

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.

Keywords: Allometric relationships; Invasive plants; Population dynamics; Reproductive ecology; Reproductive output; Seed survival; Seed-reducing agents.

Grants and funding

This research was undertaken with financial support from the DST-NRF Centre of Excellence for Invasion Biology and Working for Water Programme through their collaborative research project on “Integrated Management of invasive alien species in South Africa”, and in part by the National Research foundation of South Africa (Grant Number 103841 to Prof. Karen Joan Esler). There was no additional external funding received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.