Table of Contents
Introduction
Fire dependent ecosystems are ecosystems that have evolved with fire as an essential contributor to habitat vitality and renewal. Many South African vegetation types depend on fire and majority of plant species in fire dependent ecosystems need fire to germinate, establish, or to reproduce therefore suppressing fire in these ecosystems does not only help the plant species but also helps the animals that depend on those plant species.
About seventy percent of the ecosystems covering South Africa are fire dependent, which particularly include the fynbos, grassland and savanna. These fire dependent ecosystems need to burn in order to maintain their ecological integrity, whereas fire sensitive ecosystems are ecosystems that did not evolve with fire and they do not depend on it for their reproduction and to complete their life cycle. In South Africa about thirty percent of the ecosystem are fire sensitive.
Discussion
A fire regime refers to a long term nature of fire in an ecosystem and the effects of fire that characterize such an ecosystem. They usually defined according to frequency, severity, season, duration depth of burn and type of fire (Matthew et al 2014). Fires are a natural feature of many of South Africa’s ecosystems, and they occur regularly in the dry season in fynbos shrub lands, grasslands and savannas across the country. Plant species that occur naturally in fire-prone ecosystems are adapted to survive fires, either by re-sprouting after fire, or by germinating from seeds that survive the fire. They are not only adapted to survive fires but they also often require fires to complete their life cycles, or to remove competition from other plants. The vegetation is not only fire-adapted, it is often fire-dependent. In savannas, the balance between grasses and trees is maintained by fire, and it has been clearly demonstrated that exclusion of fires for long periods leads to the elimination of grass and dominance by trees and shrubs.
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The invasions by alien vegetation in South Africa have become a very big challenge which limits the management of indigenous biodiversity and indigenous ecosystems. The Department of Environmental Affairs defines invasive alien vegetation as plant species that are exotic, non-indigenous or non-native to an ecosystem, they are brought to South Africa from other countries both intentionally and unintentionally.
They are capable of causing human, environmental or economic harm. Alien invasive vegetation spread in silence across the ecosystems and they are capable of directly affecting native plants by becoming donors of limiting resources and they can also affect native plants and change ecosystems indirectly by altering soil stability, promoting erosion, reducing agricultural productivity and threatening globally significant biodiversity. (Kraaij. T et al 2018)
According to (Matthew, L et al 2014) the success of invasive plants results from their ability to outcompete local plant species and alter the environment to suit themselves, because the evaporation rate of many species of invasive alien vegetation especially shrubs and trees is higher than the evaporation rate of indigenous plant species therefore they use more water than the native vegetation. River flow and groundwater reserves become reduced due to the increased rates of evaporation therefore the dilution capacity becomes reduced as well, resulting in higher concentrations of nutrients and pollutants which will increase salinity and alter buffering capacity of the ecosystems.
The existence of alien vegetation in an ecosystem changes the fuels that are characteristic to the native vegetation. These alien plants grow rapidly and allow the vegetation to burn far more frequently than it did before the invasion of alien plant species, allowing fires to spread more quickly and to burn larger areas. In fynbos the invasive alien plants are dominated by trees and shrubs, which do not increase fire frequency, but they increase the intensity of fire and promote erosion after fires. (Van Wilgen 2012)
Plant invasions often involve the establishment of new life forms which may have intrinsic fuel properties that differ from those of native species, they may alter fire regimes through changes to the flammable biomes for example the pines and Australian acacias in fynbos they increase fuel load which therefore lead to higher fire intensity. Fuel attributes of invasives such as moisture content and fuel size and arrangement also affect fire behaviour. The moisture content of some invasive vegetation may be higher than that of endemic plant species (Matthew et al 2014)
Some invasive plants cause the change in soil chemistry and structure resulting in soils being unsuitable for indigenous plant species that release chemicals which inhibit indigenous plant growth and may result in severe erosion due to the lack of native vegetation cover in the ecosystems. An increase of invasive alien plant species is also a concern because they increase fuel loads and high fuel loads exacerbate the fire intensity and heat of fires, making it more difficult to control and suppress.
The invasive alien vegetation do not affect either the weather or the frequency and distribution of ignitions, but they change the structure of the vegetation which lead to changing both the amount and the type of fuel available to support fires (Chamier, J 2012). As a result, alien plants can introduce fires into areas where fires did not occur previously, and by increasing the amount of plant material in fire-prone ecosystems, they can make fires more intense. For example, the invasion of fynbos shrub lands by alien vegetation such as pines can rapidly increase the above-ground plant mass, increasing the amount of fuel available to burn, and make fires more intense and more difficult to be managed or controlled.
In un-invaded fire dependent ecosystems, soils are not negatively affected no matter how intense the fire is, but in invaded ecosystems the higher fire intensity leads to the development of water-repellent layers in the soil which then lead to severe erosion during the following rainy season, degrading catchment areas and leading to flood damage. Infrastructures that are situated next to invaded ecosystems are at higher risk of destruction during wildfires because of the increases in fire intensity associated with higher fuel loads Chamier j 2012.
According to Kraaij 2018 areas such as the arid Karoo which hardly ever burn because there is too little fuel, invasion by an alien vegetation could change the vegetation sufficiently to allow fires to burn, and if they do burn very few of the Karoo vegetation would be able to survive and this would ultimately affect the core natural resources that support the Karoo economy.
Invasive alien trees and shrubs constitute a major threat to the fynbos ecosystems because they are fire adapted and their ability to produce large numbers of seeds facilitates their proliferation and spread after fires. Managers are unable to deal effectively with very large numbers of seedlings that germinate after fire and that becomes a major problem to the successful control of these species (van Wilgen 2012). Invasive alien trees and shrubs increase biomass and add to fuel loads, leading to increased fire intensity and erosion (van Wilgen 2012).
Conclusion
Fires are both inevitable and necessary in many of our ecosystems, but fires and invasive alien plants interact in many and complex ways that can substantially increase the difficulty of managing fires. Invasive alien plants make the fire control problem worse, and they aggravate the effects of what would otherwise be an ecologically beneficial process. Because invasive alien plants are spread by fires, fire also increases their impacts on water resources, rangeland productivity, and biodiversity in fire-prone fynbos, grasslands and savannas. It is then important to increase attempts to remove invasive alien plants from fire prone ecosystems, as well as to prevent their introduction and spread in areas that are currently not invaded.
The problem of alien invasive plant species is huge and complex, it affects everyone either directly or indirectly therefore volunteers are needed to contribute in alien hacks in the local communities to fight the spread of alien species.
References
- Chamier, J., Schachtschneider, K., Le Maite, D.C., Ashton, P.J. and van Wilgen, B.W. 2012. Impacts of invasive alien plants on water quality, with particular emphasis on South Africa. Vol.38
- Kraaij, T., Stephan, C, and Singles, E. 2018. Invasive alien plants in South Africa pose huge risks, but they can be stoped.
- Matthew, L. B., Carla, M. D., David, M.R., James, B.G., Jon, E.K., Joseph. M.D., Richard, J.H., Mike, P., and David, P. 2014. Effects of Invasive Alien Plants on Fire Regimes.
- Moretti, M., De Caceres, M., Pradella, C., Obrist, M., Wermelinger, B., Legedre, P., and Duelli, P. Fire-induced taxanomic and functional changes in saproxylic beetle communities in fire sensitive regions. Ecography. 2010, vol. 33(4), pp. 760-771
- Van Wilgen, B.W., Forsyth, G.G, and Prins., P. 2012. The management of- adapted ecosystems in an urban setting: the case of Table Mountain National Park, South Africa. Ecology and Society 17(1): 8
