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.

Ecological restoration of ecosystems degraded by invasive alien plants in South African Fynbos: Is spontaneous succession a viable strategy?

“…In lowland fynbos ecosystems, acacia species (especially Acacia saligna and A. cyclops) and myrtle (Leptospermum laevigatum) are the most problematic alien tree invaders, although pines and hakeas have invaded some areas (Wilson et al, 2014). Acacias are difficult to clear because they accumulate large banks of long-lived seeds in the soil, and some species (notably A. saligna) resprout vigorously after fires or if not cut sufficiently close to the soil surface (Strydom et al, 2019). They also grow more quickly than pines and hakeas because of their capacity to fix nitrogen, and may rapidly outcompete fynbos species and exert a legacy of soil-enrichment, rendering conditions less favourable for native species (Yelenik et al, 2004;Nsikani et al, 2017Nsikani et al, , 2018a.…”

“…In terms of life-history, acacias produce a legacy of long-lived seeds that accumulate in the soil seed bank (Strydom et al, 2019) whereas pines are serotinous and store seeds in the canopy. Pines can be eliminated by felling followed by one prescribed burn to kill seedlings, whereas for acacias a portion of the seed bank remains dormant after a fire.…”

“…Furthermore, several introduced seed-destroying Melanterius weevils have been introduced as back-up biological control agents on acacias whose impacts are argued to accrue over time and reduce the species' reproductive fitness (Impson & Hoffmann, 2019). Nevertheless, acacias in dense monospecific stands where seed agents are present still accumulate stand-replacing seed banks (Strydom et al, 2019). It will take time, possibly several fire cycles, before the legacy of large seed banks of acacias can be significantly reduced, and follow-up alien control will remain an essential long-term component of any restoration strategy for the foreseeable future.…”

Quantifying range structure to inform management in invaded landscapes

“…The population threshold (2,500 plants/ha) was chosen based on our knowledge of these species' traits and the effort required to treat them. This threshold could be amended based on the suite of species to be tar- Strydom et al, 2017). In contrast, due to their long-distance dispersal abilities, pines and hakeas tend to increase in range before increasing in local population size (Richardson et al, 1994; i.e.…”

Review: The invasion of Acacia nilotica in Baluran National Park, Indonesia, and potential future control strategies

“…The use of seed-destroying biological control agents could reduce the rate of establishment and spread of seedlings (Janzen 1971;Moody and Mack 1988), diminishing seed banks within soil, and reducing the numbers of seedlings recruited into the population (Impson and Hoffmann 2019). South Africa is currently using seed predators for the biological control of invasive Australian acacias (Impson et al 2009;Strydom et al 2019). Portugal is using a bud-galling wasp to control invasive acacia and is currently investigating the use of seed predators (Marchante et al 2011;Marchante et al 2017).…”