The planning region for our study was centered on the Subtropical Thicket Biome (Low & Rebelo 1996); it covers 105,454 km2 and straddles the Western and Eastern Cape Provinces of South Africa (Fig. 1). It was subdivided into six primary water catchments and a coastal region (hereafter referred to as catchments) (Fig. 1), encompassing the eight biogeographic subdivisions of the thicket biome (Vlok et al. 2003). Sixteen percent of the planning region has been transformed to agriculture, urbanization, afforestation, and alien invasive plants, and 12% has been severely degraded by overgrazing, leaving 72% of the habitat intact. Eight percent of the planning region is highly threatened by development pressures (urbanization, agriculture, or afforestation) that are likely to affect biodiversity negatively over the next 20 years (Cowling et al. 2003b). Almost half the region faces minimal land-use pressures over this time period. Areas of particular concern are mainly along the coastal belt. The semiarid interior of the planning region faces low-impact land-use pressures.
Figure 1. The location of the Subtropical Thicket Biome and the Subtropical Thicket Ecosystem Planning (STEP) planning domain. Subtropical thicket vegetation is classified as “solid” or “mosaic” (see text). Major rivers are indicated.
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Subtropical thicket is composed of dense, spiny, and usually succulent thicket up to 3 m tall, which may occur in solid stands or as a mosaic of thicket clumps with other vegetation types (Vlok et al. 2003). Subtropical thicket has high plant species richness and endemism, most endemics being succulents and geophytes, and is associated with two globally recognized centers of succulent plant endemism: the Little Karoo Center of the Succulent Karoo in the west and Albany Center in the east (van Wyk & Smith 2001). The Subtropical Thicket Biome is contained in the southwestern sector of the Maputaland-Pondoland-Albany hotspot recognized by Conservation International (Steenkamp et al. 2005).
The fauna of the Subtropical Thicket Biome, although diverse, does not demonstrate the level of endemism of the flora. Mammal diversity is relatively high, with 48 species of large and medium-sized mammals. Unfortunately, many of these species have been extirpated, and all have undergone extensive reductions in their distribution. An important feature of the mammal fauna is the presence of two megaherbivores (African elephant [Loxodonta Africana] and black rhinoceros [Diceros bicornis]), which are recognized as keystone species in structuring subtropical thicket plant communities (Kerley et al. 2002). The avifauna is diverse, with 421 species of birds recorded within the planning region (no endemics). Ten “important bird areas” occur within the planning domain (Barnes 1998). The reptile fauna includes five tortoise species and relatively high endemism (13 species) among the lizards and snakes (Branch 1998). The amphibian fauna includes at least five endemic species (Passmore & Carruthers 1995). Although the invertebrate diversity and endemism is probably high, little is known about this group.
The STEP project was a 4-year initiative (July 2000–June 2004) funded by the Global Environment Facility. The overall aims of STEP were to conduct a conservation assessment to identify priority areas that would ensure that the long-term conservation of the subtropical thicket biota and that the assessment outcomes were implemented through the policies and practices of public and private-sector agencies responsible for land-use planning and management of natural resources in the region (Cowling et al. 2003b; Knight et al. 2003a; Pierce 2003).
The need for such a conservation assessment resulted from (1) the high diversity and endemism of the subtropical thicket biota; (2) an existing biased protected area system; (3) an escalation in land-use pressures that threaten biodiversity in this area; (4) diminishing capacity of institutions responsible for land management; (5) a general lack of awareness of the importance, economic and otherwise, of subtropical thicket biodiversity; (6) opportunities associated with a shift to biodiversity-based rural economies, especially game farming and ecotourism; (7) current conservation initiatives (e.g., Greater Addo Elephant National Park); and (8) rapidly unfolding opportunities to mainstream the outcomes of this assessment into land-use legislation and policy.
Cowling et al. (2003b) provide a detailed description of the conservation assessment, including biodiversity features, biodiversity targets, land-use opportunities, and constraints. Here we provide a brief summary of the assessment.
The STEP conservation assessment, undertaken at the 1:100,000 scale, used as biodiversity features 169 vegetation types (of which 112 are thicket types), three wetland types, and five spatial surrogates (hereafter components) of ecological and evolutionary processes (Table 1). Models were used to determine the potential distribution and community-adjusted abundance of 48 species of large and medium-sized mammals (Boshoff et al. 2001). Here we used habitat suitability for the African elephant, a focal species in the subtropical thicket biome (Kerley et al. 2002), to enhance corridor design. We used a simple spreadsheet model to estimate the potential elephant density based on forage availability within the mammal habitats, partitioned within the herbivorous guilds, and the metabolic requirements of the mammals (see Boshoff et al.  for more details). We rescaled elephant density from 0 to 100 to quantify habitat suitability for elephants.
Table 1. Biodiversity features considered in the STEP conservation assessment to ensure biodiversity representation and persistence.
|Habitat types||169 vegetation and 3 wetland types mapped at 1:100,000||10–26% of original (pretransformation) area||Desmet & Cowling 2004|
|Wildlife suitability||habitat suitability for focal species (elephant)||1000 individuals in planning region||Boshoff et al. 2001; Kerley et al. 2003|
|Spatially fixed processes||biome interfaces, riverine corridors, and sand movement corridors||100% of extant area||Rouget et al. 2003|
|Spatially flexible processes||upland-lowland and macroclimatic gradients||at least one in each biogeographic region|| |
We set conservation targets for all the biodiversity features we used in this study (Table 1). Vegetation-type targets, expressed as a percentage of the type's area before transformation, were set based on species-area data derived from phytosociological relevés (Desmet & Cowling 2004) and ranged from 10% to 26%. Targets for wetland and forest types were set at 100%, as required by South African legislation. Overall, vegetation-type targets are higher in the western part of the planning region and lower in the east, although for subtropical-thicket types, targets peak in the central parts. These target patterns reflect patterns of species rarity among vegetation types (Desmet & Cowling 2004): Local endemism is highest in the fynbos and succulent karoo vegetation in the west (Pressey et al. 2003) and lowest in grassland and savanna vegetation in the east, whereas in subtropical thicket, local endemism peaks in the central part of the planning region (Vlok et al. 2003).
C-Plan (New South Wales National Parks and Wildlife Services, Armidale, available from http://www.ozemail.com.au/~cplan), a conservation assessment software, was linked to ArcView (ESRI, Redlands, California) and used to calculate irreplaceability pattern (Ferrier et al. 2000) based on the biodiversity features and targets mentioned above. Irreplaceability measures the likelihood of selecting planning units for achieving representation targets. Irreplaceability values range from 0 (not needed) to 1 (irreplaceable, essential for achieving the set of targets) (Pressey et al. 1994). The units of selection for the assessment—the planning units—were based on cadastral data and included statutory protected areas.
Planning for Persistence
A key component of the STEP conservation assessment was planning for the persistence of biodiversity (Cowling et al. 1999; Rouget et al. 2003). Spatially fixed processes were mapped and included in the irreplaceability analysis (see below), whereas spatially flexible processes (i.e., gradients) were captured by designing corridors. The most extensive ecological and evolutionary processes in the Subtropical Thicket Biome are aligned along several major biological gradients. These are largely nested within distinct biogeographic regions associated with the major (north-south aligned) river drainage systems of the planning region (Gouritz, Gamtoos, Sundays, Fish, Buffalo, and Kei rivers) but also are aligned along east-west trending climatic gradients (e.g., along the Great Escarpment, a major topographic feature running east and west in the northeastern part of the planning region) and the coastal dune systems (Vlok et al. 2003). Our overall aim in designing corridors was to represent these biological gradients (north-south upland-lowland and east-west macroclimatic gradients) within each biogeographically distinct water catchment. Corridor design, therefore, focused primarily on ensuring biodiversity persistence (i.e., the long-term maintenance of ecological and evolutionary processes), on which the conservation assessment is founded.
We translated the persistence goal into four key functions that the corridors must fulfill (in priority order): (1) maintain ecological processes (gradients) in subtropical thicket vegetation to enable movement of biota over ecological and evolutionary time scales; (2) ensure habitat retention and connectivity; (3) maximize wildlife habitat suitability; and (4) represent biodiversity pattern (to integrate biodiversity persistence and representation).
We used cost-distance analysis in a geographic information system (GIS) to design corridors. Cost-distance functions in ArcInfo (version 7.2, ESRI, Redlands, California) provide a spatially explicit framework that incorporates these criteria for identifying the least costly (or the most efficient) route to connect a landscape. Corridors were derived in three stages: stage 1, primarily driven by biological process considerations, identified the core area of the corridor (referred to as “conservation paths”); stage 2 expanded the core area to improve representation of habitats and the persistence of processes; and stage 3 further expanded corridors into areas of high irreplaceability value for biodiversity pattern (see below). We named conservation corridors and paths after their associated river catchments.
Identification of Conservation Paths (Stage 1)
Conservation paths aim to capture the processes associated with upland-lowland and climatic gradients operating at a macroscale. Although these macro-scale gradients could occur in various parts of the region, the functionality of such gradients relies on several ecological and human factors. Based on the first three key functions mentioned above, we hypothesized that maximum functionality would be achieved when gradients, in decreasing order of importance, (1) run through subtropical thicket vegetation types, (2) are not in transformed habitats (urban areas excluded from the analysis), (3) run through habitats highly suitable for wildlife, (4) encompass other process components (i.e., riverine corridors, biome interfaces, sand movement corridors), (5) link protected areas, and (6) are not in areas likely to be transformed in future.
We developed criteria to quantify the functionality of these gradients. These relate to (1) the presence of subtropical thicket vegetation and its condition, (2) the occurrence of process components, (3) the degree of suitability of wildlife habitat (with suitability of elephant habitat as a surrogate), (4) the location of protected areas, and (5) future land-use pressures (Table 2). Criteria relating to 4 and 5 illustrate how we incorporated implementation issues into the location of the paths. Table 2 indicates the relative importance of each criterion and the respective cost incurred. Given the cost values assigned, criteria of higher rank override lower rank criteria (i.e., intact habitat was always more suitable than transformed or degraded habitat irrespective of wildlife habitat suitability). The relative cost of each criterion reflects the priority order of the key functions mentioned above.
Table 2. Criteria used to derive conservation paths in relation to the functionality of macroclimatic and upland-lowland gradients that the conservation paths aim to achieve.
|Criteria||Objective||Value||Change in cost|
|Thicket biome||favor thicket biome||subtropical thicket vegetation||0|
|Habitat transformation||avoid transformed areas||natural (untransformed)||0|
|Habitat degradation||avoid degraded areas (including invaded areas)||intact||0|
|Elephant suitabilitya (categorical)||favor suitable habitat for wildlife||high||0|
|not suitable||+900 |
|Process component||include spatially fixed processes||process||0|
|no process||+150 |
|Protected areas||link protected areas||statutory protected areasb||0|
|nonstatutory protected areas||+20 |
|outside protected areas||+80 |
|Land-use pressures||avoid areas likely to be transformed||0 (no threat)||0|
|1 (low threat)||+15 |
|2 (medium threat)||+30 |
|3 (high threat)||+45 |
|Elephant suitability (continuous)||favor suitable habitat for wildlife||from 0 (high) to 14 (low)||0–14|
We developed a cost surface (referred to as a map of landscape suitability) that reflects the options for achieving upland-lowland and macroclimatic gradients by combining all these criteria. The cost surface was first derived at a 25-m resolution by adding all criteria (with their respective cost) and was then aggregated to 1000 m based on the mean cost value (Fig. 2). Thus low-cost areas represent the nearly optimal location for such ecological process components (gradients). In our case, protected areas of pristine subtropical thicket, which were also highly suitable for wildlife, were considered the best areas to achieve these gradients, whereas highly transformed habitat that was not subtropical thicket was considered least suitable.
Figure 2. Landscape suitability surface showing the options for achieving upland-lowland and macroclimatic gradients based on the cost surface (see Table 2 for the list of criteria used). The shading relates to the landscape suitability (light shading means more suitable). Options for achieving the paths were greatest in the eastern part of the planning domain, where large tracts of untransformed subtropical thicket still occur.
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We constrained the 1-km-wide conservation paths within single primary water catchments by anchoring them to major river mouths and ending them at the northern margin of subtropical thicket. River mouths were selected because of key ecological processes associated with their estuaries and wetlands (Heydorn & Tinley 1980). Based on the landscape suitability surface, least-cost surface analysis identified the best option to link start and end points. Urban areas, including rural settlements, were excluded (i.e., the paths could not traverse urban areas). This procedure selected conservation paths with the highest landscape suitability for the considered criteria.
Expanding Conservation Paths toward Corridors (Stage 2)
The 1-km-wide conservation paths represent a nearly optimal location and the bare minimum extent for conserving processes along upland-lowland and macroclimatic gradients. We expanded these paths to (1) buffer the conservation path, (2) include fixed process surrogates, (3) achieve targets for vegetation types, (4) select areas highly suitable for wildlife (with the African elephant as a surrogate species), and (5) incorporate existing protected areas. The expansion was adjusted to avoid areas threatened by future land-use pressures. In doing this, we identified large conservation corridors of contiguous, extant habitat that achieved conservation targets for process and pattern and considered implementation opportunities and constraints.
We identified criteria—similar to those for the conservation paths—to expand these paths into functional corridors. A new cost surface was required to consider areas of high irreplaceability for pattern targets (biodiversity pattern was not used to identify the conservation paths). This second cost surface was controlled by the extent of untransformed thicket, irreplaceability values for achieving vegetation type targets, wildlife habitat suitability (based on areas suitable for elephant), distribution of protected areas, and future land-use pressures. Figure 3a illustrates how we assigned a cost value to each criterion. Although we put a high cost on habitat transformation and degradation to avoid transformed areas, preliminary analyses showed that maintaining large ratios of relative cost of each criterion had a greater influence on corridor design than small variations of the cost assigned to each criterion.
Figure 3. Criteria used for developing the cost surface which controls the expansion of conservation paths toward corridors in (a) intact subtropical thicket habitat and (b) areas of high pattern irreplaceability (Irr.). The relative cost of each criterion determines its importance and a maximum cost (Max. cost) was assigned to each. For example, intact subtropical thicket habitat that is highly suitable for wildlife habitat had the lowest cost (i.e., highest suitability for expanding the conservation paths), whereas transformed or degraded nonsubtropical thicket areas had the highest cost.
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Using least-cost analysis, we identified the extent to which the conservation paths could be expanded based on the second cost surface (Fig. 3a). The expansion was controlled by calculating the incremental cost extending at a right angle from the conservation paths; the expansion was stopped when the cost of corridors reached 0.25% of the total cost (overall cost of all the 1-km2 cells in the planning domain). This cutoff was arbitrarily set to control the maximum width of the conservation corridor. The actual width of the corridors varied according to the landscape suitability. The paths were most easily expanded in untransformed thicket vegetation of high irreplaceability. In hostile areas (i.e., transformed nonsubtropical thicket) the extent of the corridor was restricted to the 1-km-wide conservation path. We then adjusted the boundaries to planning units, with the exception of the Dune corridor, where the planning unit size was much larger than the width of available untransformed dune subtropical thicket vegetation, which is often confined to a very narrow strip immediately inland of the coast.
Expanding the Corridors into High Irreplaceability Areas (Stage 3)
Finally, we explored the extent to which corridors could be expanded to capture areas of high-irreplaceability for biodiversity. Irreplaceability values were recalculated in C-Plan for planning units, starting from the current configuration of corridors (from stage 2). We considered the contribution of corridors to targets for biodiversity features (Table 1), assuming that each of the corridors was afforded conservation management relatively consistent with that of protected areas. Spatially flexible processes were excluded because the corridors achieved them. The identified planning units were important for achieving remaining biodiversity targets. A new cost surface was required to update the irreplaceability pattern. This third cost surface was controlled by irreplaceability values for achieving biodiversity targets, extent of untransformed areas (we no longer differentiated between subtropical thicket and nonsubtropical thicket vegetation types), wildlife habitat suitability (with African elephant suitability as a surrogate), distribution of protected areas, and future land-use pressures. Figure 3b illustrates how we assigned cost to each criterion. The lowest cost was allocated to intact habitat of high irreplaceability.
As in stage 2, we identified the extent to which the conservation paths could be expanded based on this new cost surface (Fig. 3b). The expansion stopped when the cost of corridors reached 0.25% of the total cost of the planning domain. The boundaries of the corridors were then adjusted to planning units except for the Dune corridor, where the size of the planning units was much larger than an appropriately sized corridor (see above).
Assessing Corridor Effectiveness
To test the adequacy of our approach, we compared the corridors identified here with a simple corridor designed to follow the courses of the major rivers (Fig. 1) and the dune coast. Such corridors, albeit simply designed, would nonetheless ensure biodiversity persistence by capturing the major east-west and north-south gradients. A similar design was used in the conservation assessment for the Cape Floristic Region (Cowling et al. 2003a). Our simple corridor design consisted of river buffers. We buffered the major rivers with a fixed width, which was adjusted to match the same area as the STEP corridors (2,225,000 ha). We compared both sets of corridors in relation to the criteria mentioned above: extent of natural area, thicket representation, elephant suitability, achievement of pattern targets (vegetation types), achievement of process targets, avoidance of land-use pressures (i.e., implementation constraints), and linkages to protected areas (implementation opportunities).