Figure 1 Commercial forestry compartments do not support much biodiversity that is typical of the local environment.

Unlocking the door to species movement through production landscapes...
By Dr Liza Joubert-van der Merwe

This is the third of a series of articles sharing key findings of on-going research conducted by the Mondi Ecological Wetland Programme at Stellenbosch University.

I have recently had the rather unfortunate experience of finding myself locked out of my house. Even worse, I did it to myself. As I pulled the door shut and the slam lock clicked into place, I realized that my key was still on the inside. I could see the key. I gazed at it with longing eyes through the glass window panes, but I could not reach it. It took a rather expensive call to a locksmith to gain access to my home again. So often, this is also the case for biodiversity in production landscapes. There are impenetrable barriers between where a species is and where it needs to be. Do we hold the key to open the door for these species to access and utilize natural habitat on the 'other side’ of barriers in these landscapes?

It is necessary to clarify how we view production landscapes before launching into this article. We do so by focusing on forestry plantations, which are divided into commercial afforestation zones and non-commercial conservation zones, also called ecological networks (ENs).  ENs comprise between 5 and 50% of each forestry plantation, which adds up to about 1/3 of plantation holdings at national level. The commercial zones of forestry plantations do not support much biodiversity that is typical of the natural environment (Figure 1). This contrasts with ENs, which can contribute as much to conservation as formally protected areas (e.g. nature reserves) if properly designed and managed.

The following guidelines for design of ENs is underpinned by a simplification of reality i.e. that commercial zones are not overall suitable, and ENs are habitable to locally-occurring, indigenous species (from here onwards referred to as ‘species’). This binary classification of the landscape is less appropriate to, for example, livestock farming where many species can inhabit areas designated for commercial production. However, this simplified reality also addresses the worst case scenario i.e. where species are only found in designated conservation areas.

An EN should be seen as a sum of all the different parts, with each part performing a function vital to the sustainability of the production landscape (Figure 2). We consider various design variables, while acknowledging that there is not a single correct answer for design of ENs everywhere.

Figure 2 Production landscapes consist of commercial afforestation zones and non-commercial ecological networks (ENs). ENs consist of different parts, with each part playing a role in safeguarding biodiversity and sustainability of commercial enterprises.

Hard vs soft edges
The edges between natural grassland patches and forestry compartments are very abrupt to the human eye (Figure 3). This is also so for grassland butterflies, which actively avoid forestry compartment edges. Analyses of butterfly flight patterns literally show how they ‘bounce off’ these edges, and deflect back into the EN.

Characteristics of hard edges are: 1) straight boundaries, and sharp transitions in 2) vegetation structure (grassland vs. plantation trees) and 3) plant assemblage composition, which impact upon availability of quality habitat.

The scope to soften forestry compartment edges is limited due to intensive management of weeds, and mechanical harvesting inside compartments. However, wetland delineation has inadvertently led to softer edges, as the spatial distribution of hydromorphic soils is in a curvilinear line. Species use these soft edges for shelter from adverse weather conditions, and to escape heat.

Figure 3 It is typical for commercial forestry compartments to have hard edges characterized by straight boundaries, and a sharp transition in vegetation structure and plant assemblage composition.

Edge effect
The effect of forestry compartments on biodiversity does not stop at the compartment edge. It reaches a further ~ 32 m into the EN due to, among others, shading by plantation trees (Figure 4). Consequently, the EN can be divided into an edge zone < 32 m from the forestry compartment edge, and a core zone further into the EN. It is only in core zones that plant and invertebrate assemblages closely resemble that of adjacent protected areas (Pryke and Samways 2012). Therefore, core zones have a higher conservation value than edge zones.

Figure 4 Ecological Networks can be divided into edge zones which are < 32 m from the forestry compartment edge, and core zones further away. Possible reasons for different biotic assemblages in edge zones are shading by plantation trees, vehicle tracks alongside compartment edges, and management practices inside compartments, e.g. herbicide spraying and mechanical harvesting.

Patch shape
Patch shape influences the relative amounts of edge and core habitat. A perfectly round patch will have the greatest proportion of core habitat. Meanwhile, a patch with an irregular shape has a greater perimeter and, therefore, greater proportion of edge zone (Figure 5). Because core zones are worth more than edge zones, a circular node will be more valuable for conservation than a linear corridor of the same size. Judging from sensitive grasshopper assemblages in Zululand, the biodiversity value of core zones in round patches does not differ from core zones in patches with irregular shapes (Bazelet and Samways 2011a). If sufficiently large, they can both make a very valuable contribution to conservation.

Patch size
The Single Large or Several Small (SLOSS) debate is about whether it is better to have a single large patch, or a few small patches designated for conservation in production landscapes. It is true that each pristine patch of natural habitat in a production landscape has value for conservation, as they can function as stepping stones for mobile species. However, size does count. As total patch area increased from 2 to 20 ha, sensitive grasshopper assemblages became more similar to reference, open grassland sites (Bazelet and Samways 2011b). Therefore, at least for pristine grassland, it is better to invest in larger patches. Larger patches are also easier to manage (Figure 5).

Figure 5 Patch shape, patch size and corridor width influences EN ability to conserve biodiversity.

Corridor width
Although having a larger proportion of edge zone, a wide corridor can perform functions that a round node of the same size cannot. Corridors can connect distant patches in the landscape, and so allow movement of individuals among sub-populations of the same species (i.e. metapopulation dynamics). Movement of individuals (and their genes) maintain genetic diversity in the landscape, and so decrease vulnerability of sub-populations to genetic bottlenecks and inbreeding depression, which could cause local extinctions.

Metapopulation dynamics might involve a multi-generational road trip for short-lived invertebrates, with grandchildren and great-grandchildren picking up where grandparents and great-grandparents have started. This requires corridors to function as habitat i.e. they should provide all the necessary resources for resident species to complete their life cycles. Habitat is mainly restricted to core zones in corridors wider than 64 m. But is 64 m wide enough? And if not, why not?

Using butterflies in the KwaZulu-Natal Midlands, we determined that wide corridors (> 250 m) functioned as habitat, while narrower corridors (50-250 m) only facilitated movement from one habitat patch to another (Figure 5). The rate of butterfly movement was also greater in narrower corridors. This reflects the large amount of time spent basking, resting, foraging and mating, which slows down butterfly movement rates in wide corridors (Pryke and Samways 2001). The richness and composition of butterfly assemblages in wide corridors resembles that of reference, open grassland sites (> 750 m), which lend further support that ENs can act as mini-reserves for small biota, such as butterflies, within a plantation forestry context.

Why so wide? Corridors facilitate species movement, but they also function as conduits of certain disturbances, most notably of domestic cattle and invasive alien species (IAS). These disturbances tend to be concentrated in narrower corridors, which erodes habitat quality and inhibit their ability to sustain resident species for the duration of their life cycles. It is necessary to make corridors 250 m wide to dilute the effects of these disturbances, and cater to the needs of sensitive, specialist species.

Connectivity
Connectivity is a prerequisite for metapopulation dynamics i.e. the movement of individuals of a species among sub-populations in the landscape. Metapopulation dynamics are analogous to a computer network. A basic computer network consists of a sender, conduit (e.g. cable or wireless connection, such as Wi-Fi), and a receiver. However, without a signal, you cannot send an e-mail. Similarly, for metapopulation dynamics to be successful, there must be a signal (or individuals of a species) that moves from the ‘sender’ patch via a conduit (or corridor) to a ‘receiver’ patch.

We refer to ‘structural connectivity’ when we refer to the path that a species can take, but to ‘functional connectivity’ when referring to actual species movement. To measure functional connectivity, we frequently use species presence at different points along the sender-conduit-receiver route as an indication of successful movement through ENs. Structural connectivity can be measured in many different ways. 1) We can simply indicate whether sender and receiver patches are isolated, or connected with a corridor, we can 2) measure the distance from sender to receiver patches, or we can 3) calculate the relative proportion of suitable habitat patches within a certain radius (Figure 6).

Figure 6 Structural connectivity in the landscape influences species movement. There are different ways to measure structural connectivity: 1) indicate whether there is a corridor to connect patches, 2) measure distance from inside the forestry plantation to a source, habitat patch outside the plantation boundary, and 3) calculate the proportion of suitable habitat within a certain radius.

Overall, structural connectivity is very important for relatively immobile plants and invertebrates (Pryke and Samways 2003; Bullock and Samways 2005; Bazelet and Samways 2011a), but less so for more mobile birds that can fly over forestry compartments (Lipsey and Hockey 2010). Furthermore, actual species movement very frequently depends upon habitat quality within these structural connections. For example, bramble invasion or domestic cattle grazing and trampling can lead to a decline in habitat quality, and block movement of sensitive pollinator groups, which could lead to a breakdown in plant-pollinator interactions.

Management of these disturbances, and how it impacts upon biodiversity in ENs, will be discussed in the next article in SA Forestry.

References
Bazelet CS, Samways MJ (2011a) Relative importance of management vs. design for implementation of large-scale ecological networks. Landsc Ecol 26:341–353. doi: http://dx.doi.org/10.1007/s10980-010-9557-z
Bazelet CS, Samways MJ (2011b) Grasshopper assemblage response to conservation ecological networks in a timber plantation matrix. Agric Ecosyst Environ 144:124–129. doi: http://dx.doi.org/10.1016/j.agee.2011.07.008
Bullock WL, Samways MJ (2005) Conservation of flower-arthropod associations in remnant African grassland corridors in an afforested pine mosaic. Biodivers Conserv 14:3093–3103. doi: http://dx.doi.org/10.1007/s10531-004-0379-7
Lipsey MK, Hockey PAR (2010) Do ecological networks in South African commercial forests benefit grassland birds? A case study of a pine plantation in KwaZulu-Natal. Agric Ecosyst Environ 137:133–142. doi: 10.1016/j.agee.2010.01.013
Pryke JS, Samways MJ (2012) Conservation management of complex natural forest and plantation edge effects. Landsc Ecol 27:73–85. doi: http://dx.doi.org/10.1007/s10980-011-9668-1
Pryke SR, Samways MJ (2001) Width of grassland linkages for the conservation of butterflies in South African afforested areas. Biol Conserv 101:85–96. doi: http://dx.doi.org/10.1016/S0006-3207(01)00042-8
Pryke SR, Samways MJ (2003) Quality of remnant indigenous grassland linkages for adult butterflies (Lepidoptera) in an afforested African landscape. Biodivers Conserv 12:1985–2004. doi: http://dx.doi.org/10.1023/A:1024103527611

*First published in SA Forestry magazine, October 2016