ECOLOGY

The assessment of major impacts of alien invading plants on ecological processes and biodiversity.

An understanding of the ecology of invasive alien plants, and of the impacts that they have on ecosystem function, is central to the development of an effective control programme. In the first instance, a thorough understanding of the life cycle of the target weed, its mechanisms and rates of spread, and its vulnerability to various forms of treatment are needed to design and improve control methods. In the second instance, the impacts of invasive alien plants on ecosystem features such as susceptibility to fire and erosion are critical areas that need to be understood. Much of an ecosystem’s ability to deliver goods and services for the benefit of humans is dependant on retaining the biodiversity that supports such goods and services. An understanding of the impacts of invasion on biodiversity, and therefore on ecosystem stability, is therefore necessary for prioritising control interventions.

The ecology research programme formed the smallest component of the research work in the year under review. The focus was mainly on the effects of fires in invaded and uninvaded areas on the physical properties of the soil, on the ability of soil-stored seed banks to regenerate, and on the best approaches to clearing given the risks of accidental fires after clearing operations had taken place. One project also focussed on the ecological effects of herbicide use in clearing programmes.

The research programme produced a number of interesting findings during the year under review. The study into the impacts of invading alien plants on catchment stability after fire, indicate that after the first rains subsequent to the Cape Peninsula fires of January 2000, no net change in soil surface level could be found in fynbos, whilst over 100m³/ha soil moved in alien infested areas. What was also interesting was the fact that nearly twice as much soil moved in cleared areas, which indicates that the stacking of cleared biomass needs to be revisited.

The question of whether or not to fell trees prior to burning, or whether to remove the felled biomass prior to burning formed the subject of another study. This study revealed that burning standing trees promotes fynbos recovery. However, because of the practical problems associated with seed spread following "burn standing" treatments, managers have little option but to apply the "fell and burn" treatment. The results of this study illustrate the dangers of this, and highlight the need for intervention before areas become densely invaded. They also highlight the need for effective biological control agents to reduce rates of spread of aggressively invasive species.

A study on the impacts of alien plant clearing on vegetation in mountain catchment of the Western Cape, showed that areas that have been invaded by dense alien stands for a relatively short period (1 to 2 fire cycles), recover fairly well following a fell and burn treatment, with very little rehabilitation effort required. This finding is significant, as fears are often expressed that ecosystems cleared of dense stands of aliens will not be able to recover.

It is intended to initiate new projects that will address priority areas of research in the ecological field in the coming year. In order to ensure that priority areas for research are clearly identified, an ecology research strategy will be completed by March 2002. In the meantime the research review panel has identified preliminary research questions of importance. These include the importance of understanding aspects relating to the rehabilitation of sites cleared of invasive alien plants. In addition, the research into the effects of fire on the Cape Peninsula has been expanded to look at vegetation re-growth after the fires on 50 additional plots.