Thursday, March 15, 2007

The Influence of Invasive Alien Species on Native Plant Reproduction

The impacts of invasive alien species on the structure and composition of native vegetation and the abiotic environment are well documented but the underlying mechanisms have not received much attention. Thus there is little documentation on the effects of invasive alien species on native plant reproduction (Traveset & Richardson 2006).

Traveset & Richardson (2006) indicate potential ways in which native pollinator-plant interactions could be disrupted but there is not as yet confirmation for all of the given scenarios. Of the scenarios given, both the effects of foreign pollen on and in the stigma and hybridization could logically also apply to wind pollinated species. In the Western Cape Province of South Africa, the invasive Acacia spp. are adapted for wind pollination though they do also attract insects.

As with pollination, Traveset & Richardson (2006) indicate potential ways in which native disperser-plant interactions could be disrupted. Two of the scenarios have been recorded in South Africa. In Natal, Solanum mauritianum is favoured above native plant species by the rameron pigeon (Columba arquatrix) (Traveset & Richardson 2006). In the Fynbos Biome, the native myrmecochory is disrupted where the Argentine ant (Linepithema humile) has invaded. In this case, the invasive alien species displaces the native ant species without fulfilling their dispersal role (Witt & Giliomee 2004, Traveset & Richardson 2006). Argentine ants tend to feed on elaiosomes in situ rather than transporting the seeds to their nests (Witt & Giliomee 2004).

The effects of invasive alien species are visible in the seedbank. Prolonged invasion of Sand Plain Fynbos by Acacia saligna leads to loss of species from the seed-bank and changes in the species composition of the seed-bank. Collection of seed from species likely to be lost from the seed-bank and soil disturbance (to expose more deeply buried seed) are possible recommendations for the restoration of Sand Plain Fynbos (Holmes 2002).

Understanding of the impacts and mechanisms of impact of invasive alien species on native flora is necessary for effective conservation, management and restoration (though complete restoration is unlikely to be possible) (Traveset & Richardson 2006).

Holmes, P.M. (2002) Depth distribution and composition of seed-banks in alien-invaded and uninvaded Fynbos vegetation. Austral Ecology 27: 110—120.

Traveset, A. & Richardson, D.M. (2006) Biological invasions as disruptors of plant reproductive mutualisms. Trends in Ecology and Evolution 21: 208—216.

Witt, A.B.R. & Giliomee, J.H. (2004) The impact of an invasive ant, Linepithema humile (Mayr), on the dispersal of Phylica pubescens Aiton seeds in South Africa. African Entomology 12: 179—185.

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