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.

Conservation Priorities in South Africa

Two of the problems facing conservation in South Africa are limited resources and land reform initiatives. These problems, among others, make it necessary to prioritise. Prioritisation initiatives may be based on species distribution data or aggregate categories such as land classes. A combination of the two approaches may be advisable (Reyers et al. 2006). Reyers et al. (2006) used a GIS package and imagery from the National Landcover database (mostly captured 1994—1995) to prioritise the vegetation types identified in Low & Rebelo (1996). The land-cover classes were split into natural, degraded and transformed categories. The degraded category represented areas with lower vegetation cover while the transformed category represented areas with nearly complete to complete change in the structure and species composition of the vegetation. Information on protected areas was also considered (Reyers et al. 2006).

In order of priority, the top ten vegetation types identified by Reyers et al. (2006) were West Coast Renosterveld (Vegetation Type No. 62, Low & Rebelo 1996), Sand Plain Fynbos (Vegetation Type No. 68, Low & Rebelo 1996), Dry clay highveld grassland (Vegetation Type No. 36, Low & Rebelo 1996), South and South-west Coast Renosterveld (Vegetation Type No. 63, Low & Rebelo 1996), Short Mistbelt Grassland (Vegetation Type No. 47, Low & Rebelo 1996), Coastal Bushveld-grassland (Vegetation Type No. 23, Low & Rebelo 1996), Moist Cold Highveld Grassland (Vegetation Type No. 40, Low & Rebelo 1996), Sour Lowveld Bushveld (Vegetation Type No. 21, Low & Rebelo 1996), Afro Mountain Grassland (Vegetation Type No. 45, Low & Rebelo 1996) and Coast-hinterland Bushveld (Vegetation Type No. 24, Low & Rebelo 1996).

Reyers et al. (2006) do not appear to have considered invasive alien species among their threats – probably because identifying areas invaded by alien species is not likely to be efficiently done from satellite imagery or aerial photography and the costs (both financial and time) of extensive fieldwork are prohibitive. It is possible that heavily invaded sites would be included in the transformed land-cover category but this is not explicitly stated. This is possibly a limitation of the method and suggests that the calculated percentages of affected area are conservative.

The imagery used by Reyers et al. (2006) came from 1994-1995. Development is progressing rapidly in South Africa, particularly around urban areas. This is visible on the Cape Flats, e.g. on both sides of the Stellenbosch arterial road near Kuilsriver. Of the top ten conservation priority vegetation types, the expansion of the Cape Town metropolitan area is threatening both Sand Plain Fynbos and West Coast Renosterveld – the two vegetation types with the highest conservation priority. It is likely that the percentages of affected area calculated by Reyers et al. (2006) were already outdated by the time the article was published.

It is critical that formal conservation areas be designated to protect the vegetation types threatened by the expansion of Cape Town.

Low, A.B. & Rebelo, A.G. (1996) Vegetation of South Africa, Lesotho and Swaziland, Department of Environmental Affairs and Tourism, Pretoria. 0-621-17316-9 ISBN

Reyers, B., Fairbanks, D.H.K., Van Jaarsveld, A.S. & Thompson, M. (2006) Priority areas for the conservation of South African vegetation: a coarse-filter approach. Diversity and Distributions 7: 79—95.

Penhill’s Natural Vegetation

Penhill is a small community across the railway line from the rest of Eersterivier, which lies roughly south-west and south of Penhill. To the south-east of Penhill is a quarry for slate. East of Penhill is an old field. The undeveloped hill top to the north-east (approximately 33o59’24.32”S; 18o44’31.71”E) is covered by Sand Plain Fynbos (Vegetation Type No. 68, Low & Rebelo 1996) with some West Coast Renosterveld (Vegetation Type No. 62, Low & Rebelo 1996)where the soils are granitic. To the north of Penhill are farmlands. Another remnant of Sand Plain Fynbos lies north-west to west of Penhill (approximately 33o59’23.94”S; 18o43’51.65”E). This essay concerns the Sand Plain Fynbos remnants.

The Sand Plain Fynbos contains the typical components of Fynbos: a restioid component, a proteoid component, a geophytic component and an ericoid component. In the Penhill remnants, the natural structure of the vegetation is open shrubland. Of the species listed by Low & Rebelo (1996) as characteristic of Sand Plain Fynbos, the following definitely occur in the Penhill remnants: Erica mammosa (Fig. 1), Phylica cephalantha and Phylica stipularis. Phylica cephalantha is one of the dominants in the Penhill remnants. The pictures were taken in October 2006 and show a few of the species occuring in the north-west to west Penhill remnant.

Figure 1: Erica mammosa

Figure 2: A Tritonia sp.

Figure 3: Leucospermum hypophyllocarpodendron

Sand Plain Fynbos occurs on acid sands which are low in nutrients (Low & Rebelo 1996). The presence of Mediterranean grasses indicates that the soils of the Penhill remnants have been enriched – possibly by agricultural runoff (see Piessens et al. 2006) and the action of the woody legume invaders.

The Penhill remnants of Sand Plain are isolated from other Sand Plain Fynbos by urban development and agricultural land and are threatened by both agriculture and urban development.

The natural vegetation of Penhill has not been managed for conservation. Eskom brushcuts parts of the north-western remnant because of its proximity to major power lines. Both remnants suffer from too frequent burning. The frequent fires have favoured alien species resulting in dense invasions in places. The main woody alien is Acacia saligna. Acacia cyclops and Eucalyptus sp. are also present.

In conservation terms, Penhill falls within the Peninsula District centre of endemism within the Sand Plain Fynbos. In 1996, none of the vegetation of this centre of endemism was formally conserved (Low & Rebelo 1996). (The University of the Western Cape has a remnant of this vegetation but while some of it is adjacent to the Cape Flats Nature Reserve and presently managed with the reserve, it is not formally part of the reserve and is threatened by development of the university.) The Penhill remnants are thus valuable and should be formally conserved.

Low, A.B. & Rebelo, A.G. (1996) Vegetation of South Africa, Lesotho and Swaziland, Department of Environmental Affairs and Tourism, Pretoria. 0-621-17316-9 ISBN

Piessens, K., Honnay, O., Devlaeminck, R. & Hermy, M. (2006) Biotic and abiotic edge effects in highly fragmented heathlands adjacent to cropland and forest. Agriculture, Ecosystems and Environment 114: 335—342.