Maj-Britt Pontoppidan:
Modelling the impact of roads on regional population of Moor frogs (Rana arvalis)

Date: 15-01-2013    Supervisor: Gösta Nachman

Over the last decade a growing amount of literature has documented the severe impacts of transport infrastructure on biodiversity, population persistence and gene flow, and there is an increasing awareness of the importance of finding agreement between nature conservation and land use. To ensure ecologically sustainable road planning conservation measures must be taken into consideration already in the earliest phases of road development. This requires adequate tools for assessment, prevention and mitigation of the impacts of infrastructure. For this reason the Danish Road Directorate decided to finance a PhD project with the objective of developing a management tool that could be used to substantiate that the conservation status of annex IV species would be unaffected by the a given road project. The purpose of the project was to provide a standardized and scientifically well founded basis for decisions concerning road lay-out and mitigation measures. As model species was chosen the Moor frog (Rana arvalis). Populations of Moor frogs are assumed to follow a pattern of metapopulation dynamics, with colonisation, extinction and recolonisation of suitable habitat patches. Thus, road constructions must be expected to have implication on both local and regional persistence; the former due to habitat destruction, the latter because of disrupted dispersal between subpopulations due to barrier effects.

The result of the project was the development of the model presented in this thesis. The model, called SAIA (Spatial Amphibian Impact Assessment), considers a landscape mosaic of breeding habitat, summer habitat and uninhabitable land. As input I use a GIS-map of the landscape with information on land cover. In addition, data on observed frog populations in the survey area are needed. The dispersal of juvenile frogs is simulated by means of individual- based modelling, while a population-based model is used for simulating long-term population dynamics. In combination, the two types of models generate output on landscape connectivity and population viability. To assess road impacts two scenarios have to be constructed and analysed. The first scenario should be a map of the area as it is before the planned road construction (scenario 0). This analysis measures the ecological performance of the original landscape and is a reference against which other scenarios are to be compared. The second map (scenario 1) should show the landscape as it is expected to be after the road constructions. In combination, the analyses of scenario 0 and scenario 1 make it possible to assess the effect of road construction on connectivity and population persistence. The analyses also constitute the basis for planning of mitigation measures. Analyses and comparisons of several alternative road projects can identify the least harmful solution. The effect of mitigation measures, such as new breeding ponds and tunnels, can be evaluated by incorporating them in the maps, thereby enhancing the utility of the model as a management tool in Environmental Impact Assessments.

The thesis consists of a synopsis and three manuscripts for scientific journals. An appendix contains examples of the result files SAIA produces. In the synopsis, I give an overview of the background theory and the conceptual model development.

In the first manuscript I introduce an alternative patch concept, the complementary habitat patch, and use a simple model to explore how intra-patch heterogeneity affects immigration and emigration probabilities. I find that the realised connectivity depends on internal structure of both the target and the source patch as well as on how habitat quality is affected by patch structure. Although fragmentation is generally thought to have negative effects on connectivity, the results suggest that, depending on patch structure and habitat quality, positive effects on connectivity may occur.

The second manuscript uses a light-version of SAIA and explores how changes in road mortality and road avoidance behaviour affect local and regional connectivity in a population of Moor frogs. The results indicate that road mortality has a strong negative effect on regional connectivity, but only a small effect on local connectivity. Regional connectivity is positively affected by road avoidance and the effect becomes more pronounced as road mortality decreases. Road avoidance also has a positive effect on local connectivity. When road avoidance is total and the road functions as a 100% barrier regional connectivity is close to zero, while local connectivity exhibit very elevated values. The results suggest that roads may affect not only regional or metapopulation dynamics but also have a direct effect on local population dynamics.

The third manuscript describes the full SAIA model. By means of a case study I demonstrate how SAIA can be used for assessment of road impact and evaluation of which management measures would be best to mitigate the effect of landscape fragmentation caused by road constructions.