2.5 Landscape-level Impacts and Cumulative Effects
2.5.2 Addressing Cumulative Effects on Wetlands
2.5.2.1 Catchment and wetland land-cover and its relation to wetland
Bedford and Preston (1988) believe that the first step in conducting a cumulative impact analysis is to establish appropriate boundaries for the analysis and to include all anthropogenic disturbances that fall within them. As such, the consideration of land-use and land-cover is imperative in cumulative effects analysis. The ‘Landscape Principle’ as described by Sheldon et al. (2005) is a simple principle that states that “the size, shape, and spatial relationships of land-cover types influence the dynamics of populations, communities, and ecosystems” (p2-4).
This is reiterated by Wu et al. (2003), who emphasize the importance of incorporating land use and land-cover change, and their influence on ecosystem services. The necessity for including this aspect in landscape-level studies involving wetlands is explained by Kotze (1999), who describes the occurrence of wetlands as “patches in an intervening landscape matrix, with exchanges of material, information and energy in both directions between wetland and matrix” (p134). These exchanges imply that matrix and wetland yield influence over each other, and that changes to this matrix in the form of land-cover change for example, would influence the functioning of the wetland influenced by that matrix (Kotze, 1999). The
ability of a wetland to perform a particular ecosystem service is therefore likely to be hindered or enhanced by the land-cover present even beyond the boundaries of the wetland.
This concept has been labelled the “Landscape Principle” by Sheldon et al. (2005), who describe this ecological principle as one that underlies a proper understanding of how wetlands function and how best they may be managed in order to protect their ability to perform ecosystem services. The Landscape Principle dictates that the landscape is comprised of a “spatial array of habitats and ecosystems” in the form of various land-cover types, whose size, shape, and spatial relationships all yield great influence over prevalent ecological processes. This is akin to the ‘matrix’ described by Kotze (1999), and is referred to by Sheldon et al. (2005) as a “landscape template” (p2-4), to which all ecological processes respond.
The importance of accounting for off-site impacts and their effect on ecosystem services provision such as water quality enhancement is expressed by Kotze (1999), in which he highlights the fact that given that South Africa is considered a ‘dry’ country (Section 2.2.8), much of the water supplied to wetlands is from the surrounding catchment, thereby making off-site impacts to water quality and quantity particularly consequential.
It should be noted however, that not all of the ecosystem services provided by a particular wetland may be altered with a change in the wetland’s context, and that the provision of ecosystem services may alter to different degrees or in different directions (Kotze, 1999). An example offered by Kotze (1999) is that while a wetland’s ability to support biodiversity may be compromised due to increased human activity in that wetland’s catchment, such as irrigated agriculture for example, the same implementation of irrigated agriculture may increase the value of the wetland for providing water quality enhancement. This is because the change from natural to irrigated agriculture will impair the quality of the water entering the wetland, thereby affording the wetland greater opportunity to enhance it. However it is pointed out by Kotze (1999) that the benefits that are yielded by such a wetland are also dependent on how effectively that wetland may be able to assimilate incoming pollutants.
2.5.2.2 The spatial configuration of wetlands and its relation to wetland function
It has been established that analysing landscape level impacts involves the consideration of a variety of factors, such as the effect of wetlands at different spatial scales, the spatial configuration of wetlands within the landscape (Kotze, 1999), as well as the influence of catchment land-cover types on the wetlands.
These aspects are of importance because a drainage network links all of the wetlands within a catchment, and the groundwater interconnectivity within catchments has strong consequences: pollution of one body of water, such as a lake, often causes groundwater to become polluted and inadvertently contaminates a seemingly unconnected stream; excessive groundwater removal from boreholes may drastically drop the water-table to dry up surface wetlands; and commonly, substantial water removal from upstream of a river drastically affects downstream morphology, with the silting and shallowing of estuaries greatly disrupting the natural environmental processes necessary to keep the habitat optimal (Davies and Day, 1998). Similarly, upstream land-cover may affect downstream processes. Thus, upstream impacts potentially impact wetlands downstream. Furthermore, it is likely that the location of wetlands within a catchment influences the overall cumulative impact of factors such as wetland degradation within that catchment. Wu et al.(2003) point out that empirical studies suggest that the way landscape elements are configured often yields considerable influence over ecosystem processes. Despite this interconnectedness of the landscape and the influences that these interactions may yield on wetland functions, most research and management has focused on functions and controls of functions within the wetland itself, rather than on the entire landscape or watershed (Sheldonet al., 2005).
The importance of considering these aspects may be illustrated by considering a simple development project. At the site scale, a project and its effects on natural resources may be evaluated and local impacts determined. When considering the bigger picture however, many critical issues may have been overlooked, such as the impacts of the project on resources as a whole, the total impacts brought by all anthropogenic activities in the vicinity, or the secondary impacts which may arise as a result from the impacts of the project interacting with prevalent anthropogenic activities (Bedford and Preston, 1988). Thus, by extending the spatial and temporal boundaries of the analysis, a more thorough and accurate assessment is gained. To illustrate the issue at hand, consider Figure 4.
Figure 4. Example of a catchment with multiple wetlands
In the example illustrated by Figure 4, it can be seen that Catchment A and Catchment B (depicted by the dashed lines) share a portion of their boundary length, but catchments A and B are nested within Catchment C. The application of the Method for Assessing the Cumulative Impacts on Wetland Functions at the Catchment or Landscape Scale as described in Section 2.3 for toxicant removal, will result in hectare equivalents of toxicant removal function for each wetland, based on the land-cover present in their catchments, their impact scores, their respective sizes in hectares, and the wetland types. Strictly speaking however, these hectare equivalents actually describe the toxicant removals that take place in Wetlands A and B, but due to their position in the landscape, the water entering Wetland C will have now been influenced by Wetland A and Wetland B (the water entering wetland C is a product of water leaving wetlands A and B). Wetland C therefore has less ‘work’ to do in terms of performing the function of toxicant removal, as Wetland B and Wetland C have already treated the water passing through them. If Wetlands A and B were removed, however, Wetland C would be required to perform the full function of toxicant removal for its entire catchment. The ability of wetland C to remove toxins must therefore be considered given these factors.
Clearly, the dynamics associated with cumulative impacts differ greatly from conventional ones. These differences pose new challenges and therefore require an alternative approach to natural resource regulation and management. Bedford and Preston (1988, p567) beautifully illustrate the importance of considering cumulative effects in the following analogy:
“Imagine a Renaissance mosaic of a mother and child, composed of tiles of various shapes and colours. With age, the mosaic has begun to lose tiles, and we must decide which tiles to reinforce to best preserve its value. If conventional environmental assessment strategies were used, the tiles would be evaluated in terms of their individual intrinsic value. Those of highest intrinsic value would be selectively preserved. This strategy would not preserve the image of mother and child. Yet theimage is the feature making the mosaic more valuable than the sum of the values of its component tiles; the image itself is the resource of concern. If the image in the mosaic is to be preserved, the value of each tile must be determined by its importance in conveying the central image of the mosaic within the spatial boundaries of the mosaic as a whole”.
There is clearly a need to evaluate wetlands on a scale that incorporates their ability to improve water quality based on their connectivity to, and influence on, other surrounding wetlands. Their obvious interaction means that impacts that may seem insignificant when considered individually become major when considered collectively over time and space. By assessing wetlands in such a manner, decision-making may be greatly aided, so that wetlands with the greatest potential for water quality maintenance may be prioritised for rehabilitation.
2.5.3 Accounting for cumulative effects in wetland prioritisation
The analysis of cumulative effects clearly incorporates a number of concepts and influential factors, each of which requires consideration when prioritising wetlands for rehabilitation and conservation purposes given a landscape-level scenario.
Bearing in mind the concept of opportunity for providing an ecosystem service such as water quality enhancement by a wetland, a rule of thumb offered by Kotze (1999) with regard to the context of the wetland and where wetland conservation and rehabilitation should be directed in order to maintain overall catchment water quality, is that “efforts should be directed to those wetlands with human activities in their catchments” (p139), as these wetlands would be afforded the greatest opportunity for enhancing water quality. This would imply that particular land-cover classes prevalent in a wetland’s catchment would offer greater potential
than other land-cover classes for the opportunity for water quality enhancement by the wetland.
Kotze (1999) further suggests that “there should be representation across different wetland size classes” (p139), as limiting focus to just large wetlands for example, is likely to lead to the loss of small wetlands and therefore to higher levels of isolation; to under-representation of specific wetland types which do not occur as large areas; and to reduced effectiveness in water quality enhancement functionality.