ALIEN PLANT IMPACT
What impact are invasive alien plants having on EKZNW protected areas?
Over 9000 species of alien plants can be found within South Africa. To date, 198 of these species have been legally defined as alien invader plants. These species are non-native, non-indigenous, foreign or exotic and have the ability to spread naturally (without the direct assistance of people) in natural or semi-natural habitats. Invader plant species produce a significant change in terms of composition, structure, or ecosystem processes. From a nature conservation perspective, the mere presence of an alien invader plant species is a threat.
However, not only do alien invader plant species pose an ecological threat, but potentially have dire social and economical ramifications as well. For instance, a far-reaching ecological, as well as economic and social, implication is the depletion of South Africa’s water resource that alien invader plant species incur. In KwaZulu-Natal, alien invader plants use approximately 576 million m3 of water per annum more than the natural vegetation they have invaded and replaced.
Invasive alien plants also have a significant impact on the ecological integrity of our natural systems, the productive potential of land, the intensity of fires, flooding, erosion, the health of estuaries, water quality and quantity and the livelihoods of all those who depend on the life-support systems that these invasive alien plants undermine. KZN is no exception, with alien infestation having a major impact on the ecology throughout the province.
A number of examples are discussed below. Although these pertain primarily to Zululand, the principles thereof may in the majority of cases be similarly applied to the other regions of the province.
One of the biggest threats within the Zululand region is Chromolaena odorata (Triffid Weed). It is known to reduce the grassland, savannah and forest vegetations to monotypic vegetation irrespective of the systems properties (Goodall & Zachariades, 2002). It is this biotic homogenization and the suppressing of the indigenous plant communities that results in endemic species loss and habitat destruction (MacDonald and Jarman, 1985; McKinney and Lockwood, 1999). C. odorata poses a threat to species diversity. C. odorata has invaded many of Zululand’s Protected Areas and consequently many of the research initiatives within Zululand and its surrounds have concentrated on C. odorata.
Crocodiles (Crocodylus niloticus) which occurs in several of the Zululand Protected Areas have a temperature dependent sex determination (Hutton, 1987; Leslie, 1997). In a study by Leslie and Spotila (2000) they investigated the impact of C. odorata on C. niloticus nest site selection, nesting success and resultant hatching sex ratios in the Mpate River breeding area in the Greater St Lucia Wetland Park, KwaZulu-Natal. They found that the alien plant species poses a very serious threat to a continued survival of the C. niloticus in the Lake St. Lucia ecosystem. Their data confirmed that C. odorata has a shading effect on nesting sites, and that this shade has reduced incubation temperatures. Nests in shaded sites would, therefore, be well below the pivotal temperature for C. niloticus at St Lucia and would therefore be expected to produce a female-biased sex ratio. Of the six nesting site investigated, three have been invaded by C. odorata. If C. odorata remains uncontrolled and continues to spread, a female-biased sex ratio in all nesting areas may result in the eventual extirpation of C. niloticus from Lake St Lucia ecosystems (Leslie and Spotila, 2000).
Similarly, turtles and some lizards also have temperature-dependant sex determination (Hutton, 1987; Bull and Charnov, 1989; Janzen and Puakstis, 1991; Wibbels, Bull et al., 1992; Leslie, 1997), thus their existence may also be threatened by invading alien plants species such as C. odorata.
According to Samways and Taylor (2004), many of the endemic dragonflies and damselflies (Insecta: Odonata) are threatened by invasive alien trees, in particular Acacia mearnsii (Black wattle) and Acacia longifolia (Long-leaved Wattle). These trees shade out the vegetal understorey and, as a result, grasses that are perching sites for these species, and bushes that are oviposition sites, are disappearing.
In the Zululand Region, the globally threatened Pseudagrion newtoni (Harlequin Sprite) and the nationally threatened Lestinogomphus angustus (Spined Fairytail) are examples of such species that are threatened.
A study on ‘Invasion biology of the alien invasive plant C. odorata in a South African savanna’ is currently being conducted in HiP. Preliminary analyses show that the aggressive invasive nature of this species can be attributed to its exceptionally high growth rate and its enormous seed production. It appears that overall herbivore densities within C. odorata infested areas are low, but it is uncertain whether this is related the presence of C. odorata or merely a characteristic of the area in which the experiment is set up. However the results do indicate that grazers are negatively affected by C. odorata, while browsers are affected positively, or it is possible that the affect may be neutral. The plants species diversity in infested areas is not affected to a great extent, but the abundance of indigenous plant species is significantly decreased by the presence of C. odorata. In areas that were cleared of C. odorata, the herbivore community has shown a shift from purely browsers to a mix of browsers and grazers. This coincides with a shift from C. odorata to grassland.
The study also showed that C. odorata invasion is lower in grassland areas than in areas supporting woodlands. Fire appears to be effective in hampering the invasion of C. odorata, especially in areas where fire has been used as a tool to reduce bush encroachment (Ms M. te Beest, pers comm.)
In a study conducted in HiP on the impact of C. odorata on invertebrate density and diversity, spiders were used as biological indicators. C. odorata was shown to have a negative impact on spider diversity and density. Another aspect of this study determined if the system is able to rehabilitate itself after clearing operations. The results indicated that the treatments that were cleared 3-5 years ago have a higher spider diversity index value as compared to treatments that were cleared 1-2 years ago. This suggests rehabilitation is taking place (Ms M. Mgobozi, pers comm.)
A further study within HiP is investigating the effects of C. odorata on the habitat use of L. pictus and on small mammal diversity. L. pictus is among the most endangered species in the world, and is extinct in 25 of the 39 countries which it occupied. Because L. pictus have large home ranges, habitat loss is clearly a problem as it reduces population size thus increasing the extinction risk. In addition small mammal monitoring can be used as a method of indicating whether the functioning of an ecosystem is healthy or unhealthy and may facilitate the management of nature reserves and future development of natural areas.
From the preliminary data, it appears as if L. pictus is neither avoiding nor preferring the areas invaded with C. odorata. However, anecdotal data shows that they sometimes use C. odorata to improve capture by chasing their prey into dense stands of the weed. L. pictus has also denned in areas of high C. odorata infestation. This may help protect the den from Panthera leo (Lion) and Crocuta crocuta (Spotted Hyena). However, more analysis needs to be done on the data to confirm these findings. The same study also found that areas that have been invaded for long periods have the lowest small mammal diversities (Ms L. Dumalisile, pers comm.) .
The invasion by C. odorata is clearly having a significant impact on habitat dynamics within in HiP and elsewhere, however, the long term consequences of these invasions still need to be established. Questions have also been raised on what impact C. odorata is having on flagship species, such as the endangered D. bicornis (Black Rhino). Consequently, the need for ongoing research is vitally important in furthering our knowledge and understanding of all invasive alien plant species and their associated impacts.