Forum Paper |
Corresponding author: Jan Olof Helldin ( j-o.helldin@slu.se ) Academic editor: Cristian-Remus Papp
© 2022 Jan Olof Helldin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Helldin JO (2022) Are several small wildlife crossing structures better than a single large? Arguments from the perspective of large wildlife conservation. In: Santos S, Grilo C, Shilling F, Bhardwaj M, Papp CR (Eds) Linear Infrastructure Networks with Ecological Solutions. Nature Conservation 47: 197-213. https://doi.org/10.3897/natureconservation.47.67979
|
Crossing structures for large wildlife are increasingly being constructed at major roads and railways in many countries and current guidelines for wildlife mitigation at linear infrastructures tend to advocate for large crossing structures sited at major movement corridors for the target species. The concept of movement corridors has, however, been challenged and pinching animal movements into bottlenecks entails risks. In this paper, I address the SLOSS dilemma of road ecology, i.e. the discussion whether a Single Large Or Several Small crossing structures along a linear barrier would produce the most benefit for wildlife, using the case of crossing structures for large wildlife in Sweden. I point out risks, ecological as well as practical, with investing in one large crossing structure and list a number of situations where it may be more beneficial to distribute the conservation efforts in the landscape by constructing several smaller crossing structures; for example, when the ecological knowledge is insufficient, when animal interactions are expected to be significant, when the landscape changes over time or when future human development cannot be controlled. I argue that such situations are often what infrastructure planning faces and that the default strategy, therefore, should be to distribute, rather than to concentrate passage opportunities along major transport infrastructures. I suggest that distributing passage opportunities over several smaller crossing structures would convey a risk diversification and that this strategy could facilitate the planning of wildlife mitigation. What to choose would however depend on, inter alia, landscape composition and ecology and on relationships amongst target species. A single large structure should be selected where it is likely that it can serve a large proportion of target animals and where the long-term functionality of the crossing structure can be guaranteed. New research is needed to support trade-offs between size and number of crossing structures. Cost-effectiveness analyses of wildlife crossing structures are currently rare and need to be further explored. Camera trapping and video surveillance of crossing structures provide opportunities to analyse details concerning, for example, any individual biases according to sex, age, status and grouping and any antagonism between species and individuals. Wildlife ecology research needs to better address questions posed by road and railway planning regarding the importance of specific movement routes and movement distances.
Mitigation planning, Sweden, SLOSS, wildlife crossing structure
One of the most significant ecological impacts of roads and railways are their barrier effects for terrestrial wildlife (
Monitoring of over- and underpasses for large wildlife has provided frequent proof that they are used by a variety of species (
Despite having recognised both the problem with barrier effects and its potential solution, in infrastructure planning practice, many transport agencies still seem to consider crossing structures for wildlife to entail external or unexpected costs. Accordingly, such constructions have to be argued for on a case-by-case basis and often end up being rather few. In response, environmental planners tend to advocate for as large wildlife crossing structures as possible and put much effort into finding the ideal locations for those crucial constructions. This situation is reflected not least in current European guidelines for mitigation of barrier effects at transport infrastructures; many of these have their focus on methods to identify major wildlife corridors and state ideal rather than optimal dimensions of crossing structures (
While crossing structures may be necessary measures to safeguard the connectivity for wildlife across large linear infrastructures, they inevitably create bottlenecks for animal movements, irrespective of location and size. Funnelling animals from larger areas into movement bottlenecks may have a number of ecological disadvantages, for example, increased predation (
The size is one of the most cost-driving factors for crossing structures and, in the infrastructure planning reality, the cost-effectiveness of measures has to be considered. Wildlife crossing structures, from culverts to viaducts and green bridges, may range in investment cost by orders of magnitude (
Examples of differently sized crossing structures in Sweden used by large wildlife, with rough estimates of investment costs. The precise costs depend on a number of site-specific factors, and values given are intended to serve as indications. Images by courtesy of Trafikverket and PEAB.
The question of size vs. number of wildlife crossing structures is analogous with that of the so-called SLOSS dilemma in conservation, i.e. the question whether a Single Large Or Several Small protected areas would be more effective for species conservation (
The issue of SLOSS wildlife crossing structures has previously been addressed by
In this paper, I develop the SLOSS dilemma of road ecology using the case of crossing structures for large wildlife in Sweden. Based on ecological and pragmatical arguments, I list a number of situations where it may be more beneficial to distribute the conservation efforts in the landscape by constructing several small crossing structures rather than one or a few large. I argue that the situations described for Sweden are not unique, but may apply to other taxa and geographical regions. I conclude by suggesting how the SLOSS discussion could provide information for planning of wildlife mitigation at linear infrastructures and by proposing some directions for future research in the field.
Populations of many large mammals are currently relatively strong in Sweden and species such as moose (Alces alces), deer (red deer Cervus elaphus, fallow deer Dama dama, roe deer Capreolus capreolus), wild boar (Sus scrofa) and large carnivores (wolf Canis lupus, bear Ursus arctos, lynx Lynx lynx) range over large parts of the country (
Within the managed boreal forest, ungulates may prefer certain stand types, for example, clear-cuts, young or dense forest stands and linear landscape elements, such as riparian areas and edge zones (
Animal movements may also change due to sudden human influences in the surrounding landscape, such as new housing, mining or industry and increased outdoor recreation adjacent to crossing structures (
Extensive site-specific empirical data on wildlife movements are in short supply, in Sweden as in other countries (
Some Scandinavian mammals are territorial, amongst these being roe deer and large carnivores (
In addition, game and prey species, such as ungulates, may adapt their spatial distribution, habitat choice and activity patterns to the risk of being hunted or predated (
This Swedish case of large wildlife ecology describes a number of situations that – each individually and all taken together – suggest that distributing conservation efforts on several small crossing structures may perform better than a single large crossing, namely:
• In relatively intact or homogenous landscapes, where animal movements are dispersed.
• Where animal movement routes are expected to gradually change over time due to landscape changes.
• Where future human development cannot be controlled and natural habitats surrounding crossing structures may suddenly deteriorate.
• Where animal movement habits simply are not known.
• When wildlife mitigation needs to target multiple species with different habitat choices and no ideal site can be appointed.
• When target species are territorial or competitors and there is a risk that some individuals or species monopolise the area in and around the crossing structure.
• When target species are sensitive to hunting, poaching or predation and enemies (human or natural predators) may ambush at sites where movements of prey are pinched.
The Swedish Transport Administration (STA), the responsible manager for the public road and railway network in Sweden, currently works along a strategy for landscape connectivity for large wildlife that partly take a SLOSS approach. According to the national ecological standards (
The standards describe a range of larger to smaller crossing structures as suitable for moose and roe deer (
Due to the lack of an explicit SLOSS approach in the planning for large wildlife mitigation, opportunities for better ecological function and more cost-effective mitigation measures may still be missed. For example, regional differences in data availability, plasticity in animal movements or target species for mitigation would imply different output depending on the region. In northern Sweden, investing in few large crossing structures at major migration routes may be warranted. Thorough ecological data should be collected and compiled to identify the ideal sites for these crossing structures and considerable efforts should be made to secure their long-term effectiveness through adapted management of the surrounding landscape. In more southern parts of the country, however, sufficient overall permeability of infrastructures may be achieved by several smaller crossing structures, including non-wildlife bridges which tend to be plentiful along most major roads and railways.
Though based on the specific case of Swedish large wildlife, I believe that many of the situations described above are what infrastructure planning often faces. Site-specific knowledge of animal movement patterns tends to be sparse (
Following this line of argument and with support from the results from the modelling approach adopted by
A planning strategy aiming at several smaller crossing structures rather than a single large structure could facilitate the planning of wildlife mitigation in a few ways. It may not be necessary to put as much effort into finding the best siting or design of each crossing structure, which may save both time and costs at early planning stages. Instead crossing structures may have a standard design and be spaced out on pre-defined intervals along the infrastructure or where the ground conditions (topography and soil) are ideal from a technical perspective. Non-wildlife bridges or culverts used by wildlife may also be included in the wildlife mitigation plan. While the goal of wildlife mitigation plans should not be to save money, but to minimise wildlife-traffic conflicts, the SLOSS issue will open the question of how to get the most out of available investments or how to reach conservation goals with a minimum of cost and it may, therefore, help the matter by redirecting the focus in planning from costs to savings.
Trade-offs between size and number of crossing structures in wildlife mitigation schemes may require that road ecology research take a somewhat different angle than that currently prevailing. Research and monitoring of over- and underpasses during the last decades have provided a basic understanding of how well different type of structures correspond to the demands of different species or taxa (
A strategy to construct several small crossing structures should entail an increased demand for research on how to make also narrower crossing structures more functional for wildlife, for example, by adapting vegetation and limiting human disturbance. However, squeezing down the size of crossing structures would also mean approaching a lower limit for functionality and, in the light of this, a much better understanding of the ecology of narrow crossing structures is needed.
I suggest a stronger emphasis in monitoring of crossing structures, not only on how different species use them differentially (such as described by, for example,
To this, we need better knowledge of what happens between animals at crossing structures, for example, predation risk (real and perceived), interference competition, territoriality, dominance and other antagonistic types of behaviour that can expel some target animals from the sites. The well-developed methods, using camera traps and video surveillance of crossing structures, provide opportunities for studying both animal categories and types of behaviour to a larger extent than is currently done.
Finally, I call for more efforts in wildlife ecology research to develop the knowledge of animal movements, to specifically address the questions posed by road and railway planning, of movement routes (importance of certain routes, their stability over time and reliable methods to map them) and potential movement distances along fences to find safe passages (
This paper is based on conference presentations held at ICOET (Raleigh, NC, USA, 20–24 September 2015), ACLIE (Skukuza, South Africa, 10–15 March 2019) and IENE (Online, 12–14 January 2021). I thank Manisha Bhardwaj, Lars Nilsson, Heinrich Reck and academic editor Cristian Remus Papp for helpful comments on earlier drafts of the paper. The writing was financed by the Swedish Transport Administration, through the research project TRIEKOL (https://triekol.se/).