6.1 Linkages
At the smaller spatial scale, the interface between plantation patches and rem- nant, indigenous grasslands has a fuzzy effect on insects at the edge (Samways and Moore, 1991). Alien pine trees, for example, can influence grasshopper species richness and abundance 30 m, and occasionally more, into the natural grassland matrix. Indeed, pines have a much harder edge than do natural for- ests. This was shown for both butterfly abundance and species richness (Pryke and Samways, 2001) (Fig. 6.5). Less than 10% of grassland species penetrated 10 m into the pine patch, whereas 45% of the species penetrated natural forest.
Furthermore, the effect of the pine trees on these butterflies extended into the grassland as it did for grasshoppers. The effect was significant up to 50 m from the pine edge, whereas it was negligible relative to the natural forest edge.
This supports the recommendations of Fry and Lonsdale (1991) and Kirby (1992), that softening edges is beneficial for most insect species.
Management at the landscape spatial scale using linkages also takes into account temporal factors. A linkage that acts as a movement corridor enables those mobile organisms to locate suitable habitats to complete their life histor- ies. In the case of South African butterflies, only the most mobile, generalist species entered the narrow corridors (less than 50 m wide), when they flew 13 times faster than they did in wide (greater than 250 m) linkages. The slow- est movements were recorded in the widest linkages, which was due to the butterflies using these linkages as habitats (stopping to nectar, drink, sun- bask and rest). Interestingly, butterflies also flew faster through linkages that
were highly disturbed, had few nectar flowers, a high density of alien plants, short grasses and high impact from cattle. However, linkage width did not significantly affect movement speeds of many migrant species. Likewise, some of the sedentary and local endemic species were unaffected by linkage width, generally flying less than 200 m in both intermediate, as well as wide linkages. These specialists rarely entered linkages less than 250 m wide, and when they did, they only spent a short time in them.
Preferably, linkages must also be habitats per se, enabling completion of life histories. In other words, linkages can only be considered successful in the long term if they are a network of habitats (ecological network) that are resist- ant and/or are resilient enough in the face of environmental fluctuations to permit and promote long-term survival of all the local, indigenous species.
What evidence is there that the ecological networks being established across South Africa have the potential for critical conservation? The first results, on adult butterflies, were surprisingly positive (Pryke and Samways, 2001, 2003). Linkages more than 250 m wide were actual habitats within the network. Butterflies pene- trated deep into the ecological network, far from the outside indigenous grass- land. Specialists were limited to the least disturbed sites, both inside and outside the network, and shared some common traits, such as being small, sedentary and having specific habitat requirements, such as grassy slopes, hilltops and tall grasses. The important point is that the high quality linkages, even deep within the network, supported the rare, specialized and endemic species.
Good quality grassland corridors were rich in species, although three common widespread species were not recorded within them. Two of these Fig. 6.5. An ecological network in South Africa designed to optimize agroforestry while at the same time maintaining indigenous biodiversity within a global hotspot.
Although there is loss of biodiversity at the spatial scale of the pine stand, there is maintenance of quality biodiversity at the larger spatial scale of the landscape.
at least seem to prefer very wide open spaces, larger even than the major corridors. This illustrates that large, open nodes are also required to supple- ment these corridors. This is the case, for example, for the Karkloof blue, Orachrysops ariadne, a Red Listed species, which has very specific habitat requirements, including particular slopes, a very special host plant and an ant mutualist, all within a specific type of grassland. Indeed, this species now benefits from a large set-aside node, which is carefully managed within the ecological networks (Lu and Samways, 2002).
Surprisingly, these ecological networks also supported three other spe- cies that were not present in the natural nodes in the immediate vicinity. One of these species, Alaena amazoula, is a localized national endemic, emphasiz- ing that these networks can have add-on effects and act as important natural reserves in their own right.
The most important influence on the corridors detracting from their effect iveness was disturbance, especially from domestic cattle. The effects of their disturbance could be measured by changes in plant compositional and structural diversity. Major disturbance had a highly impoverishing effect, particularly upon abundance. Preliminary results on caterpillars (indicators of residency of butterflies), grasshoppers (herbivore functional group) and flower-inhabiting arthropods (pollinators, flower-eaters, seed-eaters, preda- tors) (Bullock and Samways, 2005) are also suggesting that the width of link- age and its interiorness are not critical for ensuring movement throughout the network. Nevertheless, to retain species and functional biodiversity in the long term, it is essential to have wide corridors and nodes, as well as adjacent natural reserves (Fig. 6.6).
Flower–arthropod associations remained intact whatever the character of the corridor, so long as the flowers were present. The associations were only lost when the plants were lost, not when the plants were present, but under some disturbance pressure. As a result of these studies, it is now pos- sible to develop design guidelines for such ecological networks (Fig. 6.7).
6.2 Dealing with linkages as human conduits
Although these corridors, which amount to about a third of the whole local land surface, mitigate the effects of the pine afforestation, they are, in places, also conduits receiving intense human impact. This means that implemen- tation of these ecological networks must also consider human social and commercial activities, as well as the biology of the organisms. Another way of viewing this is that these corridors are not necessarily conservation end points, but rather, they are a conservation-enabling strategy. The wider the linkage, and the more natural they are, the more they become nodes, and thus linkages intergrade into nodes. Such large linkages and nodes can accommodate the high impact of a limited number of vehicles, which, although locally intensive, constitute only a small amount of the total area.
More difficult to address are thoroughfares for domestic cattle, the impact of which is greatest where closest to their enclosures. As the effect of many
cattle is impoverishing upon the local plant and insect diversity, this impact must be viewed in the same way as a pine patch, subject to the same triage, and considered as a deficit for biodiversity.
6.3 Riparian corridors
These corridors must also function as retainers of hydrological processes.
This means that many of the linkages have roads or are riparian corridors.
Studies on dragonflies (Kinvig and Samways, 2000) have indicated that these riparian zones are maintaining quality aquatic diversity, as measured by the presence of localized, endemic species. This however, presumes that inva- sive alien plants, which radically alter the structural diversity of the riparian
6. Reduction of contrast between disturbed area and adjacent natural area
7.Maintenance of the metapopulation trio of large patch (habitat) size, good patch quality and reduced patch isolation
1. Adjacent natural reserve
2.Maintenance of quality habitat heterogeneity 4. Outside reserve,
maintenance of as much undisturbed land as possible
3. Linkage of quality habitat
5. Simulation of natural disturbance (burning, grazing)
Fig. 6.6. This ecological network is adjacent to a natural reserve (1) and includes considerable habitat heterogeneity (2). Linkages (3) and associated nodes (4) are an insurance for all the subtle, unrecorded biotic interactions that take place and need to be conserved across the landscape. This craggy hilltop (3) is essential as a thermal refugium and hilltopping site for insects, as well as a special habitat for many plants and insects. Management involves activities, such as encouragement of grazing by indigenous game, as well as limited number of domestic livestock (5). Ideally, there should be reduction of contrast between the impoverishing pine stands and the natural grassland (6). This can only be achieved in these networks by having wide (>250 m) linkages (3), as well as nodes (4) and adjacent reserves (1). At the population level, the aim is to maintain the metapopulation trio of large patch (habitat) size, good patch quality and reduced patch isolation (7).
Fig. 6.7. A summary of the design elements of ecological networks. The success of these networks hinges fi rst on the fi rst two key premises of maintaining undisturbed land (nodes) (4) and instigating linkages (3). These in turn, link with adjacent reserve areas (1). Throughout the reserve areas, nodes and linkages, the aim is always to maintain quality heterogeneity (2). The linkages are subject to edge effects because contrast between afforested stands and grassland remnants is great (6). This edge effect is mitigated by wide corridors (>250 m) that have central areas away from the pines, which are natural habitats. Within the ecological network, natural disturbance is simulated (5). Running through all the above landscape approaches is the golden thread of the metapopulation trio of large patch (habitat) size, good patch quality and reduced patch isolation, which is a function of good quality, wide corridors (7).
2. Habitat heterogeneity
3. Corridors
6. Reduced contrast 1. Reserves
• Adjacent to ecological network acts as a coarse filter
• Nodes inside the network act as fine as well as coarse filters
• Maintained throughout the web by incorporating topographic, hydrological, edaphic and other features
7. Metapopulation trio
• Corridors >250 m wide become large patches
• These wide corridors are good habitat
• Wide corridors reduce the isolation of nodes
5. Simulated disturbance
• Burning (some conflict between requirements of forestry and needs of\
biodiversity)
• Megaherbivore grazing By indigenous game By domestic livestock
• Mitigated by wide corridors. The outer 50 m of corridors is ‘edge’
• The main feature of an ecological network 4. Undisturbed land
• One-third area left ‘undisturbed’
• But there is disturbance at edges and where corridors are human conduits
zone, are removed. Indeed, preliminary results indicate that removal of these aliens leads to a remarkably fast recovery of rare, endemic and other odonate species (Samways et al., 2005).
6.4 Emergent properties of ecological networks
The context and contrast of adjoining landscape elements (Wiens et al., 1993) can result in emergent properties. Little evidence is yet available, but the ecotones between plantation trees and natural grasslands do appear to be favoured habitats for some invertebrates, e.g. scorpions (Ingham and Samways, 1996). The boundaries are also significant in that they attract certain vertebrates, including the threatened oribi antelope (Ourebia ourebi), which shelters in the plantation yet grazes in the linkages (R. Pott, Pietermaritzburg, 2001, personal communication).