Der Artikel geht auf die entscheidende Rolle heterogener Schutzkorridore für den Erhalt der Artenvielfalt innerhalb südafrikanischer Holzmosaiken ein. Darin werden die Herausforderungen durch menschliche Aktivitäten und den Klimawandel sowie die Notwendigkeit einer strategischen Naturschutzplanung diskutiert. Die Forschung konzentriert sich auf die Konzeption und Verwaltung dieser Korridore und unterstreicht die Bedeutung von Breite, Orientierung und Heterogenität der Lebensräume. Die Studie betont auch die Bedeutung der Aufrechterhaltung qualitativ hochwertiger Lebensräume und effektiver Managementpraktiken wie Brand- und Weidemanagement, um die ökologische Widerstandsfähigkeit zu gewährleisten. Die Ergebnisse unterstreichen das Potenzial dieser Korridore, ein mit Schutzgebieten vergleichbares Niveau der Artenvielfalt zu fördern und die Reichweite der Schutzbemühungen auf Landschaften auszuweiten.
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Abstract
It is urgent now to place greater emphasis on harmonising conservation of indigenous biodiversity with food and fibre production. This is especially important in countries like South Africa which support high levels of irreplaceable biodiversity. The local timber industry has responded to this challenge by retaining large-scale networks of conservation corridors of historic ecosystems in the forestry landscape. The corridors consist mostly of grassland, with patches of indigenous forest, thickets, wetlands, ponds, and rivers. The motivation is to future proof compositional and functional biodiversity for ecological resilience in these production environments in a rapidly changing world. We synthesise here the substantial evidentiary research on the effectiveness of conservation corridors in plantation forestry-dominated landscapes in the Maputaland-Pondoland-Albany biodiversity hotspot. We focus on six emergent themes: 1. corridor dimensions, orientation, and connectivity, 2. heterogeneity at different spatial scales, 3. maintaining aquatic and terrestrial habitat quality in the conservation corridors, 4. biodiversity value of conservation corridors relative to protected areas, 5. the plantation matrix, and 6. assessment of corridor network performance. Results show the importance of prioritising large, high quality conservation corridors, especially those with a high number of natural features and variety of environmental conditions, both terrestrial and aquatic. Alien clearing, grazing control, and appropriate fire regimes should be prioritised in these corridors. Where possible we need to retain, restore, or replicate the natural ecological regimes. Overall, this conservation approach in commercial forestry landscapes helps to conserve indigenous biodiversity and ecosystem integrity, improves connectivity across afforested landscapes while also having sustainable timber production, thereby safeguarding the resilience of these working landscapes well into the future.
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Introduction
South Africa is one of the most biologically diverse countries in the world, and the only country with three of the world’s biodiversity hotspots (Myers et al. 2000). However, human densification and impact today pose ever-increasing threats to its biodiversity (Hoveka et al. 2020). Stressors include landscape transformation, habitat loss, habitat fragmentation, invasive alien organisms, and changed fire regimes. All these impacts are aggravated by ongoing anthropogenic climate change and extreme weather events.
Several protected areas are established throughout the country, including World Heritage sites, Biosphere Reserves, RAMSAR sites, Important Bird Sites, and Key Biodiversity Areas. However, protected areas collectively cover less than 10% of the country’s total area (www.statssa.gov.za/?p=14732), and the unprotected spatial gaps that remain means that various taxa, including threatened species, are inadequately supported (Hoveka et al. 2020).
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Much biodiversity conservation action takes place at the spatial scale of the landscape. Yet any landscape is dynamic, with its abiotic and biotic components changing in space and time, and interactions between organisms also continually changing (Hilty et al. 2006). Promoting ecological and evolutionary dynamics is the basis for ensuring resilient landscapes in the face of a suite of threats and emergent future pressures. The conservation challenge today is that human-induced stressors are largely novel impacts on biota, for which there has been no natural selection in the past. A further challenge is that there is inadequate policy to improve functional connectivity across South Africa (Dalziel and Evans 2023).
These conservation challenges can be addressed with strategic conservation planning. This approach is based on analysis of all known options available so that high value conservation areas in any one region are maintained to support as much indigenous biodiversity as possible. However, this planning approach must recognise that setting aside natural landscapes and habitats is both expensive and often challenging when the focal areas are under human ownership. This means that there must be a socio-ecological approach that melds sustainability with economic feasibility (Samways et al. 2010). Ways of harmonizing biodiversity conservation with historical levels of fragmentation, and current human demands on landscapes and the provision of ecosystem services need to be found (Niemandt and Greve 2016; Starke et al. 2021). The way forward is to think beyond nature protection versus useable land to a situation where working landscapes are managed in a biodiversity-friendly manner (Kremen and Merenlender 2018).
In these rapidly changing times, it is essential to maintain and improve functional connectivity across landscapes to conserve biodiversity. An important means of doing this is to implement large-scale networks of conservation corridors (Jongman 1995). The aim of these networks is to provide both ecological and evolutionary resilience by enabling organisms to ebb and flow across landscapes according to daily and seasonal changes in environmental conditions, El Niño Southern Oscillation events, and global climate change, especially across transformed landscapes (Hilty et al. 2006). Empirical research that validates the effectiveness of these networks is important to help promote the adoption and further development of this conservation approach.
We focus here on sustainable South African plantation forestry. The planted compartments cover about 1.2 million ha in South Africa and their coverage range between 45 and 90% on forestry estates. Sustainability for timber considers the larger spatial scale, where timber compartments are separated by corridors of remnant ecosystems in a natural or near-natural state, with a cumulative area of about 0.5 million ha in South Africa. The corridors are largely interconnected, not only within a plantation but also across plantations, making it a regional approach. Furthermore, in ideal cases, the networks of conservation corridors are also functionally connected to protected areas (Samways and Pryke 2016).
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The stimulus for developing and sustaining these corridor networks in South African plantation forestry-dominated landscapes is two-fold. First, it aligns with global and national conservation policies, as South Africa is signatory to the Convention of Biological Diversity and has developed an effective National Biodiversity Strategy and Action Plan, now in its 2nd edition (Government of South Africa 2015). Second, there is an economic motive in that export of timber depends on fulfilling the criteria of the Forest Stewardship Council which ensures sustainable timber and pulp production. The conservation corridor networks are maintained by forestry companies, partly in consultation with national and local conservation authorities, for overall socio-economic benefit, optimal production, and optimal natural ecosystem function and biodiversity conservation.
Research prior to 2016 and relating to networks of conservation corridors in plantation forestry-dominated landscapes has been summarised (Samways et al. 2010; Samways and Pryke 2016). This foundational work highlighted corridor design and management principles that promote biodiversity, especially of grassland and freshwater ecosystems. Since then, there has been a concerted research effort to refine these principles through more in-depth testing of current landscape ecological and conservation concepts, through the inclusion of additional ecosystems such as indigenous forest and soils, and through inclusion of a broader range of taxa. The aim here is to review the evidentiary research, especially post-2016, and to synthesise key findings related to the effectiveness and management of South Africa’s networks of conservation corridors associated with commercial forestry. We emphasise how new findings build on the previous research and highlight important gaps for future research.
Methods
Study region
Biodiversity research has been mostly concentrated in the Maputaland-Pondoland-Albany (MPA) biodiversity hotspot in the Province of KwaZulu-Natal (Fig. 1). Planted areas consist of fast-growing, exotic Eucalyptus, Pinus and Acacia species from Australia, North America, and the Mediterranean basin (Forestry South Africa 2020; Kirkman and Pott 2002). In the conservation corridors, the dominant vegetation type is natural grassland, but the corridors also contain biodiversity-rich indigenous forests, thickets, savanna, wetlands, and riparian vegetation (Fig. 2).
Fig. 1
Examples of large-scale networks of conservation corridors among plantations in the relatively high-elevation Midlands, and among plantations in the lowland Zululand region of the KwaZulu-Natal Province, South Africa. Dark green areas on the satellite images are plantations and lighter green areas are the conservation corridors
Fig. 2
Images of biodiversity and ecosystems in conservation corridors alongside satellite images of the corridors. Top row of images: Gilboa Estate in the KwaZulu-Natal Midlands, bottom row of Nyalazi Estate, part of the SiyaQhubeka forestry in Zululand. Arrows show where and in which direction photographs were taken in the corridors. Satellite images taken from Google Earth
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Literature review
We performed a review of published literature on networks of conservation corridors in South African plantation-forestry landscape mosaics. To compile a comprehensive list of articles for inclusion in this review, we searched titles, keywords, and abstracts of published research on the Web of Science online database. First, we identified articles relating to South African research using the search terms: ("cape province*" OR "western cape*" OR "eastern cape*" OR "kwazulu*" OR “mpumalanga*” OR "northern cape*" OR "north west province*" OR "gauteng*" OR "free state*" OR "free-state*" OR "limpopo*" OR "south africa*" OR "southern africa*"). To relate specifically to conservation corridors, yet accounting for commonly used synonyms, the search was refined using terms: ("ecological network*" OR "corridor*" OR "connect*" OR "habitat fragmentation" OR "production landscape*" OR "production mosaic*" OR "fragmented landscape*" OR "transformed landscape*" OR "landscape ecology*" OR "pondscape*" OR "heterogeneity" OR "modified landscape*") AND ("plantation*" OR "timber*" OR "forestry*" OR "afforest*" OR "alien vegetation" OR "pine*" or "eucalyp*").
The search yielded a total of 120 papers published between July 2016 and May 2024 (supplementary file S1). Of these, 41 papers were selected for inclusion in the review after screening the titles and abstracts. Searched papers that did not evaluate the effectiveness of conservation corridors in plantation mosaics, or fell outside the geographic scope of South Africa, were excluded. Papers that reviewed conservation corridors, at least to some degree, were included. An additional eight papers were included, obtained from the reference lists of included articles. We then synthesised findings into six major themes that emerged from the recent research: 1. corridor dimensions, orientation, and connectivity, 2. heterogeneity at different spatial scales, 3. maintaining aquatic and terrestrial habitat quality in conservation corridors, 4. biodiversity value of conservation corridors relative to protected areas, 5. the plantation matrix, and 6. assessment of corridor network performance.
Corridor dimensions, orientation, and connectivity
Design attributes such as corridor dimensions, orientation, shape, and structural connectivity influence a wide range of arthropods (Pryke and Samways 2015; van Schalkwyk et al. 2022). An important design principle is to ensure that the corridors are wide enough to incorporate the range of habitats needed by most species, which is especially important for species vulnerable to extinction in fragmented landscapes in other regions (Azhar et al. 2017; Begotti et al. 2018). The edge influence from the timber compartments extends for most arthropod species roughly 30 m into the corridor (Pryke and Samways 2012b), meaning that corridors narrower than 60 m do not adequately support corridor interior species (those that require fully natural conditions without interference from the adjacent timber trees) (van Schalkwyk et al. 2017). In fact, recent evidence suggests that terrestrial grassland specialist species may require corridor widths 400 m or more due to the interaction of both edges (van Schalkwyk et al. 2020). As with terrestrial fauna, dragonfly abundance and species richness are positively correlated with corridor width (Kietzka et al. 2021).
Furthermore, a few wide corridors as opposed to several narrow corridors can have higher conservation value and should be prioritized during landscape planning to maximize the amount of interior habitat (van Schalkwyk et al. 2020). This does not mean that narrow corridors have no value as they may still function as movement corridors but have less value for residency (van Schalkwyk et al. 2017). Certain species also prefer narrower corridors. For dragonflies, size of natural terrestrial areas surrounding their freshwater habitats and degree of isolation shape species assemblage composition, where some species have high affinity for small terrestrial patches, where others mostly occur in ponds within large patches (Deacon et al. 2023b).
The size of natural forest patches within corridors significantly influences ground arthropod species composition, leading to an interaction between forest patch size and interpatch distance. Large patches support similar assemblages regardless of interpatch distance. Overall, this means that it is essential to conserve large and close patches, while also maintaining a variety of forest patch sizes to encompass all ground arthropod diversity (Yekwayo et al. 2016a).
Furthermore, both edge orientation as well as corridor width are important for biodiversity along grassland corridor edges. While wide corridors enhance overall species richness, some grassland specialist butterfly species for example prefer sunnier edges (i.e., north facing in the southern hemisphere), while forest specialists prefer shadier edges (south facing edges), especially so for residents over transient species, and among habitat specialists more strongly than generalists. The point here is that corridor orientation and width should both be considered in corridor design, with wide corridors with various orientations benefitting different butterfly species sets (Fig. 3) (van Schalkwyk et al. 2022).
Fig. 3
Habitat heterogeneity is important to ensure ecological resilience across the whole conservation corridor network and relative to plantations. Heterogeneity includes a range of vegetation types, wide corridors to maintain resident arthropod assemblages, narrow corridors that enable movement among natural patches, conservation corridors at various spatial orientations, and presence of meso-scale landscape features to provide habitat for a wide range of arthropod species
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A high level of connectivity of conservation corridor networks may also provide resilience to climate change by connecting patches together and allowing individuals to move away from suboptimal conditions. Remote sensing and machine learning were used to predict corridor functional connectivity and the responses on grasshoppers as sensitive indicators of habitat quality. Wide corridors with different orientations across an elevation gradient supported various grasshopper assemblages and promoted species turnover. The most sensitive species occurred in north-west facing corridors at higher elevations, with wide corridors also functionally connecting large high-quality remnant patches (Theron et al. 2022c). Habitat size and connectivity can interact (Yekwayo et al. 2016a), with habitat size also interacting with habitat quality (see below).
Corridor design is one of the most fundamental aspects of landscape ecology, yet in this landscape there is little more that can be done to improve them without extensive replanning. Yet, remaining questions should be considered. The first is whether pinch points (locations where there are corridor constrictions) have an inhibitory effect on the functional connectivity of the conservation corridors. Secondly, the biodiversity value of cul-de-sac corridors (linear strips of natural vegetation that end abruptly and walled off by plantation trees) need to be explored. Addressing these questions would benefit conservation planning through identifying pathways of functional movement across the landscape (Pliscoff et al. 2020). Furthermore, a more detailed understanding of interactions between corridor size, connectivity, and habitat quality will help inform corridor prioritization and planning. It will be valuable to know whether e.g. high habitat quality or high connectivity compensates for small corridor size.
Key messages: Corridors should be as wide as feasibly possible. A few wide corridors should be prioritized over several narrow corridors, although narrower corridors should be retained to promote movement and species with an affinity for smaller habitats. For indigenous forest patches within corridors, it is essential to conserve large patches and patches in close proximity to each other, while also maintaining a variety of forest patch sizes. The whole corridor, which includes both the edges and the interior, must be considered in the planning process, with wide corridors of different orientations being optimal.
Heterogeneity at different spatial scales
Maintaining as much natural landscape heterogeneity as possible is part of the good design process for ensuring ecological resilience of conservation corridors (Pryke and Samways 2015). This means incorporating not only macro- and microtopography, but also high levels of habitat heterogeneity across the whole conservation corridor network (Pryke and Samways 2015) (Fig. 3). Significantly, arthropod diversity depends on a variety of structural and compositional diversity among the plant communities, making vegetation heterogeneity conservation an important first step (van Schalkwyk et al. 2021a). Biotic heterogeneity is largely driven by geomorphology and hydrodynamics. Variable macro- and micro-topography are strong determinants of vegetation communities, moderated by factors like degree of insolation, wind exposure, and fire events. In turn, the vegetation determines the degree and rate of soil formation which feeds back into spatio-temporal vegetation heterogeneity. The result is that while the corridors are largely grassland, with various levels of water saturation, they are also characterised by natural forest patches.
There is great variation among terrestrial arthropod assemblages in natural forests and grassland (Yekwayo et al. 2017; Eckert et al. 2019, 2022a). Even when there are subtle environmental differences between biotopes such as between dry and hydromorphic grasslands, their associated soil arthropod assemblages differ greatly, as do the relative proportions of feeding guilds of predators, herbivores, detritivores, and omnivores (Eckert et al. 2019) (Fig. 4). These differences emphasise the sensitivity of the soil fauna to fine-scale habitat heterogeneity and a diversity of vegetation types (Eckert et al. 2022b).
Fig. 4
In KwaZulu-Natal (KZN), different soil types are associated with different ecosystems, and have different sets of soil fauna. Particularly, there are major differences in soil fauna assemblages among dry and hydromorphic grasslands
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Soil arthropod assemblages in the different biotopes have different traits that are well-suited to the environmental conditions of each biotope, which allows diverse soil assemblages to persist across the natural-plantation mosaic (Eckert et al. 2023c). This suggests that an effective conservation solution would be to ensure a mix of vegetation types in corridors such as grasslands, wetlands, and indigenous forests to maximise landscape heterogeneity and its arthropod biodiversity (Eckert et al. 2023b). However, as certain species require large areas of contiguous habitat, such as the grassland specialist butterflies in the study region, there can be a trade-off between the effects of compositional heterogeneity and suitable habitat area (Heidrich et al. 2020; Fahrig et al. 2011). Therefore, an understanding of the level of landscape heterogeneity that maximizes biodiversity in these landscapes would help refine conservation planning. Furthermore, high arthropod species turnover among patches of the same vegetation type emphasises the importance of having biotopes represented across different landscapes and plantation estates (Eckert et al. 2023a) (Fig. 4).
There is also complementarity among different freshwater ecosystems. For example, wallows, ponds, and marshes support different dragonfly assemblages (Pryke et al. 2015) (Fig. 5). Furthermore, individual ponds associated with different land uses, whether associated with plantations, sugarcane fields, or natural areas are all highly varied in terms of their dragonfly assemblages. The conservation significance is that the focus should be at the level of the pondscape rather than on individual ponds (Briggs et al. 2019a), especially when the ponds are well managed and support well vegetated banks and macrophytes (Briggs et al. 2019b).
Fig. 5
A protected river at high elevation (top left) along with a low elevation perennial pond (top right), a small grassland pond (bottom left), and an artificial water body that is frequently visited by many megaherbivore species (bottom right)
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Variation within freshwater ecosystems is also important. Dragonfly diversity is influenced by river width, water turbidity, water depth, and presence of invasive plants. In turn, dragonfly assemblage composition is influenced by water body type, flow rate, and substrate type. There are also differences in responses between the two sub-orders, Zygoptera and Anisoptera. In the focal corridors which were large and well-managed, human-induced effects have less impact on dragonfly species richness and composition than do natural environmental variables, emphasising the importance of conserving natural heterogeneity. These results indicate that dragonfly diversity is well-conserved in the conservation corridors when the systems are well-managed by ensuring that they are low on pollution, that their banks are free of invasive alien trees, and protecting a wide enough natural riparian zone for the species to carry out all their life functions (Kietzka et al. 2015).
In turn, artificial ponds (dams) are a common feature of many South African landscapes and are present in conservation corridors (Fig. 5). For dragonflies, beetles and bugs, vegetation cover is the most important environmental variable for maintaining species richness and composition. Of great conservation interest is that artificial reservoirs are attractive ecosystems for many species. These artificial water bodies increase area of occupancy for three-quarters of the species, and provide alternative habitats to natural ponds, leading to improved ecological resilience across the whole pondscape (Deacon et al. 2018).
Ecotones should also be considered. These are the transition zones between two different adjacent ecosystems. In these corridors, ecotones between plantation trees or indigenous trees and adjacent grassland, especially where there are indigenous bushes, are another important component of landscape heterogeneity for conserving biodiversity (van der Mescht et al. 2021). Edges between grassland and indigenous forests are especially rich in species, as shown for bush crickets (van der Mescht et al. 2021) and butterflies (Gaigher et al. 2024) (Fig. 3). Forest-grassland edges support species-rich butterfly assemblages, consisting of a mix of forest specialists, grassland specialists, and habitat generalists. For all butterfly groups, forest edges also support a high prevalence of butterfly behaviours indicative of habitat use, such as territory defence and courtship behaviour. In addition, some species preferentially use forest edges in the hot and windy season, indicating the importance of shelters provided by forest edges. For conservation, indigenous forest edges are significant for maintaining butterfly diversity and may help buffer butterfly populations against global change as they offer more opportunities for species to find refuge (Gaigher et al. 2024).
Heterogeneity is more than only different ecosystem types. At the mesoscale, various other landscape features, such as rocky areas, wetlands, and gullies, are also important relative to open grasslands (Fig. 3). Gullies are occupied by nested assemblages of rocky areas and open grasslands, suggesting that they have lower conservation importance than the other two features, but nevertheless should still be targets for restoration. Overall, the findings suggest that a variety of landscape features that represent natural landscape variation can best conserve the butterfly assemblages (Deacon et al. 2023a).
Heterogeneity is becoming increasingly important for protecting insect populations against climate change by providing a wide range of environmental conditions, resources, and refuges (Harvey et al. 2022). Future work that explicitly tests for the contribution of conservation corridors to climate-resilience in afforested landscapes would help to further optimize this conservation approach and further highlight the benefits of conservation corridors in the long-term. Future considerations on internal aspects of plantations, such as elevational gradients, valleys, hilltops, and other meso-scale features that influence species persistence and habitat use under different environmental conditions, are essential to understand long-term resilience of these corridor networks to climate change.
Key messages: Maintain as much natural environmental variation in the corridors as possible. This includes conserving a variety of natural vegetation types and the ecotones between them, different types of waterbodies including artificial dams, meso-scale features in the landscape and within-habitat heterogeneity.
Maintaining aquatic and terrestrial habitat quality in conservation corridors
Freshwater systems have been given high priority in past efforts to maintain ecosystem integrity in the conservation corridor networks. Riparian zones in these areas have been kept free of invasive alien trees which are known to cause considerable damage to freshwater biota (van Wilgen et al. 1998). However, it is also known that freshwater invertebrates, even localised and rare species, can spring back remarkably well as soon as the alien trees are removed from riparian zones, so long as the species are still locally present and can re-colonise from a nearby area (Samways et al. 2011).
There are no known local extinctions of aquatic fauna in the conservation corridor networks. One reason for this, besides removal of alien trees, is that timber trees in high-quality networks are no longer planted on hydromorphic soils, where they would otherwise interfere with hydrological processes. In areas where timber trees had previously been planted on hydromorphic soils, the trees are selectively removed through a process known as ‘delineation’. Doing this allows recovery of natural hydrodynamics, hydromorphic soils, and associated vegetation, as well as the dependent soil arthropods (Eckert et al. 2019).
A wide range of alien invasive plants occur in the conservation corridors, and ongoing control and monitoring of these plants is important to prevent them from proliferating and so reducing habitat quality. Research in the study area has focused mainly on American bramble (Rubus cuneifolius), as it is the most insidious threat to biodiversity in the corridors at higher elevations (Fig. 6) and is both highly invasive and is difficult to control. Super-resolution imagery has been used to understand its invasion dynamics and extent of impact with a high degree of accuracy. Tree harvesting and continuous prescribed burning are the major drivers that increase its cover of the landscape, and bramble cover is highest one year following plantation tree harvesting (Theron et al. 2022a-c). Bramble cover is also high near streams (Theron et al. 2022a). To maintain high-quality habitat heterogeneity, sites must be burned at least every three years (Theron et al. 2022b), with wide corridors at high elevations being a priority for maintaining functional connectivity (Theron et al. 2022c) (Fig. 6).
Fig. 6
American bramble (Rubus cuneifolius) invasion reduces biodiversity levels in various habitat types, and at various spatial scales. Use of insect indicators showed that bramble can effectively be managed through regular clearing, prioritizing streams prone to invasion, rotational harvesting of plantation trees, and regular burning
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Management activities in the networks of conservation corridors require good information on the historical effects of fire on biodiversity prior to the effect of plantation trees. Fire in both protected areas and conservation corridors has a strong effect on grassland plant communities which are adapted through many fire events in the past (Joubert et al. 2016b). The aim has been to mirror natural fire regimes within the conservation corridors (burning every 2–5 years (SANBI 2014)), and to implement patchy mosaic prescribed burns, which to date have been effective for maintaining plant diversity (Joubert-van der Merwe et al. 2019) (Fig. 7).
Fig. 7
Conservation landscape design and management are both important to ensure ecological resilience. Design aspects include high connectivity, habitat heterogeneity, and conservation corridors of varying shapes, width, and orientations. Conservation corridors must be suitably managed in terms of grazing and fire regimes to ensure ecological resilience into the future
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In turn, local arthropods associated with the plants are naturally well-adapted to fire. Their populations recover when the plants do, usually with the onset of adequate rainfall. Importantly, different taxa show differential resilience to fire (Yekwayo et al. 2018). This means that it is essential to recognise that fire refuges, such as rocky areas, ravines, riparian zones, and deep soil retreats, are essential in providing source populations for recharging surrounding areas once the fire has passed. Mosaic burning also provides refuge for arthropods by maintaining unburned grassland patches among burned areas. Putting these findings into practice in the corridors allows better informed and strategic management, especially when ecological succession is traded against intermediate-level, biodiversity-poor bush encroachment (Gaigher et al. 2019).
Furthermore, management focuses especially on maintaining domestic grazing in ways that mimic the natural grazing regimes, which historically have shaped the biodiversity in the natural areas prior to plantations being established (Fig. 7). At lower elevations in particular, there is today still a rich indigenous megaherbivore fauna within some plantations (Fig. 8). Indigenous grazing activity by these animals is important for maintaining local biodiversity (Pryke et al. 2016). Moreover, megaherbivore diversity is essential for maintaining dung beetle diversity, especially so for elephant dung that supports a wide range of dung beetle species (Pryke et al. 2022). Domestic livestock such as cattle in the absence of indigenous megaherbivores can create conditions also suitable for example for certain sensitive grasshopper assemblages. However, as cattle grazing behaviour is not as diverse as that of a variety of megaherbivores (Fig. 8), they are not a perfect substitute for the local megafauna (Samways and Kreuzinger 2001). They do though help maintain grassland quality by supressing woody vegetation, so promoting high arthropod diversity (Joubert et al. 2017; Joubert-van der Merwe et al. 2019). Importantly, it is essential that livestock grazing is maintained at moderate levels, and when associated with good fire management, livestock grazing promotes a healthy grass sward and high grasshopper diversity (Joubert et al. 2016a, b).
Fig. 8
Elephants in a eucalypt plantation (top left), a cow in a conservation corridor (top right), fire that had burned a conservation corridor and moving into a pine plantation (bottom left) and the American bramble, which is the most aggressive invasive alien plant species threatening the function of conservation corridors in the Midlands region (bottom right)
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These management activities pertaining to alien plant control, fire and grazing management augment the design features (Joubert et al. 2016a; van Schalkwyk et al. 2021b). When these two facets (design and management) are both implemented appropriately, optimal conservation corridors are equivalent to protected areas in the biodiversity that they support (Samways et al. 2020) (Fig. 7).
While the principles for maintaining integrity of grassland and freshwater ecosystems are relatively well-developed, we know little about the response of indigenous forest biodiversity to different stressors. The naturally patchy indigenous forests in these systems support disproportionately high biodiversity despite their small size, yet are impacted by livestock grazing, alien invasive plants, overharvesting, ringbarking of trees for medicinal use, among other pressures (Von Maltitz et al. 2003). A major priority is to better understand the effects of their most important stressors, and also to determine how to effectively protect these forests against degradation.
Key messages: Clearing of alien invasive trees from riverbanks and removal of planted tree stands from hydromorphic soils is essential. The adoption of rotational harvesting is best, rather than simultaneously harvesting several plantation blocks which greatly increases bramble invasion. Bramble removal programmes should prioritise riparian areas, along with control of bramble one year after timber harvesting. Implementation of prescribed fire regimes that mimic the natural fire frequency and patchy spatial patterning is important. In terms of grazing management, conserving indigenous megaherbivores is optimal, although rotational domestic grazing at moderate levels also benefits grassland integrity.
Biodiversity value of conservation corridors relative to protected areas
Previous research showed that, for several arthropod groups and plants, large, well-managed conservation corridors can support equally high biodiversity compared to nearby protected areas (Joubert and Samways 2014; Pryke et al. 2015; Pryke and Samways 2012a). Subsequently, equivalence between high-quality conservation corridors and protected areas has also been shown for dragonflies in riparian systems, whose abundance, species richness, and Dragonfly Biotic Index scores do not differ between conservation corridor networks and protected area sites (Kietzka et al. 2021). Furthermore, butterflies, large mammalian herbivores and dung beetles in coastal protected areas are well-represented in conservation corridors in adjacent plantation estates (Gaigher et al. 2021; Pryke et al. 2022). These additional findings further highlight the high conservation value of the conservation corridor networks as a mitigation measure for plantation forestry. These findings also confirm that the conservation corridors effectively extend the size of protected areas and so improve their ecological resilience. This is because increased spatial options are available to all populations and species (Fig. 9). Viewed the other way round, it also means that the corridors are connected to high value conservation areas, allowing individuals to move into corridors from source populations in the protected areas (Pryke and Samways 2012a; Smith et al. 2019).
Fig. 9
A panoramic view of Goodhope plantation (left). A protected area is in the top left far distance, along with a variety of natural ecosystems, grassland, and forest patches, interspersed with timber stands in a varying topography. A river in a corridor with a riparian zone in Gilboa estate (right)
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However, there is rarely a perfect match in species assemblages between the two types of land uses owing to large-scale landscape heterogeneity. For example, earlier work showed that certain butterfly species only occur in the protected area and not in the corridor networks, while for other species, it is vice versa (Pryke and Samways 2003). By conserving biodiversity that is unrepresented in existing protected areas, conservation corridors contribute to regional representativeness, as is the case for dragonflies and damselflies (van Schalkwyk et al. 2023). There can also be subtle shifts in assemblages due to changes to the broader landscape from afforestation. For example, butterflies and mammalian herbivores with an affinity for large, open savannas are more prevalent in the protected areas and those associated with woody vegetation thrive in the conservation corridors (Gaigher et al. 2021; Pryke et al. 2022), again emphasising the complementarity between the areas.
To assess ecological resilience of conservation corridors, it is important to understand whether ecological functioning and species interactions are intact. Recent results indicate that flower-visitor networks are well-maintained in conservation corridors (Hansen et al. 2018), whereas there are shifts in network structure between protected areas and conservation corridors for dung beetle-mammal interactions (Pryke et al. 2022). In both cases, the assemblage structure of interacting partners differs between the two systems.
More research is needed on this topic to understand whether conservation corridors benefit from being directly connected or near protected areas, and whether corridors enhance regional-scale connectivity beyond plantation borders. In addition, further assessment of ecological functioning under different design and management scenarios would be valuable.
Key messages: A regional approach that combines large and appropriately managed conservation corridors in forestry estates along with nearby protected areas conserve a wide range of species at the larger spatial scale.
The plantation matrix
The extent to which species utilize timber plantations vary among different contexts and among species with different levels of habitat specialization (Brockerhoff et al. 2008; Wolstenholme and Pedley 2021). In the focal study region, mature plantation stands are highly depauperate in aboveground invertebrates, and when eco-acoustic assessment is used, they are devoid of both singing insects and frogs (Grant and Samways 2016). However, several mammal species, many of which are red listed as threatened, traverse plantation stands as well as using the corridors, including some predatory species such as honey badger (Kheswa et al. 2018), black-backed jackal (Sosibo et al. 2022) and serval during winter months (Ramesh et al. 2016). In contrast, some species remain in natural forest and avoid plantations (e.g. tree hyrax (Sosibo et al. 2022)) or are large ambush predators (e.g. leopard) that do not find plantations as suitable habitat (Ramesh et al. 2017). The giraffe only uses the corridors when mature plantation trees are present (Fig. 2), while other species prefer bushy ecotones (e.g., kudu, nyala, red duiker) (Pryke et al. 2022). Elephant are general in their use of the plantation mosaic and may at times do damage to young plantation trees (Samways et al. 2021).
However, among arthropod assemblages, there is remarkably little spillover of habitat specialist species of either the ground or foliage species from grassland or forest into the timber stands (Fig. 3). In contrast, the high-quality natural areas conserve many rare and habitat specialist species, whereas overall low quality natural areas and plantations maintain low levels of diversity (van Schalkwyk et al. 2019). This indicates the plantation stands cannot be considered of any significant value for historical biodiversity, emphasising that the setting aside corridors is clearly the way to harmonise production and for ground and aboveground biodiversity conservation (van der Mescht et al. 2023).
An interesting exception is the belowground arthropods. While natural vegetation has high species richness, functional diversity, and species turnover of soil arthropods, surprisingly, so do pine plantations. The high soil arthropod diversity in plantations is due to high environmental variation among plantation sites, providing diverse niche opportunities. A possible explanation for the high proportion of shared species between natural areas and plantations is through colonization from natural areas into plantations and/or source populations surviving harvesting and tree planting, with their populations growing during the long period of timber tree maturation. This implies that in certain regions, soils in the plantation blocks may have relatively high levels of ecological functioning, which is positive for long-term soil health in the plantation blocks. Nonetheless, plantations and natural areas support different assemblages, emphasising that remnant natural vegetation is essential for supporting historic soil biodiversity (Eckert et al. 2022a).
Harvesting of timber trees is a major and sudden impact on local soil communities, while leaving timber residues that must be disposed of, or its volume reduced in-field using one of three procedures: (1) on-site retention of unburned harvest residue, (2) residue removal off site, (3) burning of residue. While there is an initial decline in soil arthropods at harvest, recovery can be remarkably rapid. Furthermore, there are no differences in arthropod assemblages between the three slash management procedures after one year, with a 50% recovery of soil biological activity after only six months (Eckert et al. 2023a). However, these patterns in recovery have only been assessed in the inland regions which have deep, humic soils. Further testing on coastal soils, which are sandy and highly erodible, will be necessary to assess the relative impacts of slash management practices on soil biota under these different conditions.
Many small indigenous forests that were previously in a grassland matrix, are now surrounded by plantations. While natural forest patches surrounded by grassland have higher ground species diversity than those surrounded by pines, there is greater sharing of species between alien pine trees (Pinus spp.) and natural forests. As matrix type matters for maintaining diversity, again the conclusion is to conserve a wide variety of natural forest patch sizes with various matrix types (Yekwayo et al. 2016a, b).
The plantation matrix within a plantation forestry landscape represents a huge amount of space and there is a need to make this extensive landscape more inhabitable, or at least more permeable, by native fauna (Pliscoff et al. 2020). Intuitively, we expect forest species to prefer and utilise the plantation areas (Brockerhoff et al. 2008), but this is not always the case here with only a few species found deep in plantation blocks (Yekwayo et al. 2017), especially in the case of mature plantation blocks. However, plantation matrices might contribute to overall functional connectivity for open habitat specialists when the timber trees are young and for forest species when the planted trees are mature (van Schalkwyk et al. 2021a). Maintaining a mosaic of stand age-classes in plantation landscapes may therefore improve connectivity for a wider range of species at any given time (Lindenmayer and Hobbs 2004). How to minimise the impacts of this matrix and how to make conditions more suitable for biodiversity is imperative to improving the functional connectivity in these networks and reducing the footprint of the timber compartments.
Key messages: Conservation of remnant natural vegetation types is essential to conserve historical biodiversity, including rare species and habitat specialists, that do not occur within timber-planted areas.
Overall assessment of corridor network performance
As effective management responses are needed to achieve societal goals of sustainability, a new monitoring protocol (Management Check: MATCH) that comprehensively evaluates management outcomes at the operational level, was developed for the corridor networks. Using a Driver-Pressure-State-Impact-Response framework, pressures influencing ecosystem integrity inside conservation corridors and commercial compartments, and in wetland and stream buffers, can be monitored. The pressures identified are domestic livestock grazing, fire management, invasive alien plants, and potential soil erosion from timber-access roads and harvested timber compartments. When actual practice across these plantation mosaics is compared with Best Operational Practice and reworked into a Weighted Index of Compliance per operational unit, the differentials are poor management of livestock grazing, but management of invasive alien trees along rivers, of dirt roads, and of timber compartments, and management of wetland and stream buffers is very good. Fire management was difficult to measure at the spatial and temporal scales of operations. Overall, MATCH is a valuable tool that helps improve management of these corridor networks (Joubert-van der Merwe et al. 2020).
To evaluate corridor network performance in the long term, it is important to understand trends in biodiversity on forestry landholdings. The Dragonfly Biotic Index, which assesses freshwater ecosystem integrity based on dragonfly assemblages, has been valuable for monitoring freshwater biodiversity and to prioritize sites for conservation (Samways and Simaika 2016). Similar systems for assessing and monitoring terrestrial ecosystem integrity and biodiversity, both of grassland and indigenous forests, would be a valuable tool to assess changes to these ecosystems. However, to date an easy-to-use simple biodiversity index has been elusive due to the complexity of the natural areas and weak correlations between responses of different taxa (Cawood et al. 2024), leading rather to a focus on quality of habitats and ecosystems as umbrellas for ensuring conditions are met for maintaining indigenous biodiversity.
Currently, there are no biodiversity monitoring schemes in place, yet new technologies, especially machine learning, drones, small cameras, and audio devices, are expected to make monitoring of biodiversity easier than ever before. However, we should be clear as to what we are monitoring and ensure that this is done in a clear and repeatable way.
Key messages: Although protocols are in place for monitoring management outcomes and the integrity of freshwater ecosystems, the development of further tools to assess terrestrial ecosystem integrity and monitoring of biodiversity will be important to help evaluate corridor network performance over time.
Conclusions
Several principles and management recommendations have emerged from the research and would be applicable to the broader forestry industry in South Africa and elsewhere in the world. Recommendations from this work suggest that we should prioritise large, high quality conservation corridors, especially those with a high number of natural features and wide variety of environmental conditions, both terrestrial and aquatic. Alien clearing, grazing control, and appropriate fire regimes should be prioritised in these corridors. Where possible we need to retain, restore, or replicate the natural ecological regimes. For example, it is better in grasslands to have cattle where indigenous megaherbivores are minimal to simulate grazing, although recognizing that this is inferior to having indigenous grazers.
Overall, this work confirms that timber production landscapes can be managed and designed in a way that conserves high levels of historical biodiversity, extends the reach of established protected areas, and enables structural and functional connectivity across plantation estates. This is done without compromising timber production, and these landscapes are therefore truly multifunctional. This is a critically important approach to complement formal protected area networks and to help safeguard South Africa’s exceptional biodiversity.
Acknowledgements
The Mondi Ecological Network Programme is supported by Mondi Group (donor number 14493268), additional funding from GreenMatter, Foundational Biodiversity Information Programme, National Research Foundation (South Africa) (Grant numbers: RDYR181213403534, SFH150723130214, SFH170610239104) and Stellenbosch University.
Declarations
Competing interest
We declare that there are no competing interests. Neither Mondi Group, nor any other commercial enterprise, had any influence on the production of the original research papers or on this review.
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