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Land-use changes and climate change are considered current key threats to biodiversity. Therefore, even under the assumption that current networks of protected areas adequately conserve natural habitats, climate change and associated land-use changes are anticipated to alter a range of landscapes' habitats in the near future. As a consequence, neighboring sites (i.e. falling within the dispersal distance of a given species) that currently protect a sufficient area of suitable habitat to support viable populations of species might no longer contribute to overall network connectivity if climate change eliminates habitat from one or both sites, or landscape resistance between them inhibits movements. Likewise, new sites may facilitate connectivity in the future if land-use changes enable expanding into, or through, such favorable habitats.

To assess the efficiency of conservation networks from a macroscopic, network point of view, a novel comprehensive and complementary methodology is proposed, taking into consideration both global changes and species traits. The proposed methodology aims to investigate the impacts of global changes at a European level, focusing on the connectivity of the Natura 2000 conservation network, for different forest types and different groups of species.

The methodology developed in the present study involved two basic steps. First, a dynamic vegetation model was used to simulate the potential current and future geographical ranges of three major forest types (Mediterranean, Boreal and Temperate) under scenarios of both land-use and climate change. Next, a landscape graph-based approach was applied by considering species' dispersal ability, habitat preference and minimum area requirements to assess the potential connectivity under the current and predicted land cover maps of existing protected areas (see Figure). The area of interest covers Europe with the protected areas of the European conservation network representing the units at which connectivity is assessed.

Network structure of the current (2001) and future (2030) modeled distribution of a hypothetical group of species that inhabit temperate forests and have a maximum dispersal distance of 18km and a minimum area requirement of 50ha. All patches of temperate forests recognized in the figure are located within the protected sites of the Natura 2000 network.

Three main conclusions were drawn. First, despite the often acknowledged benefits of long-distance dispersal for connectivity, the applied methodology clearly demonstrated that there is a trade-off between dispersal and area requirement, with long-distance dispersers facing a more serious risk with networks consisting of fewer sites, and having a larger proportion of isolated sites. Second, it become apparent the current spatial configuration of the European conservation network might absorb future changes, maintaining or even enhancing macroscopic properties of the network for some forest types, while it might prove insufficient for others. Third, interestingly, connectivity was maintained at levels similar to those estimated prior to simulated changes and even improved for some forest types. The latter conclusion indicates that current conservation policies, aiming to maximize the protection of vulnerable habitat types such as Boreal forests, already seem to contribute to maintaining connectivity, at least at the studied scale and resolution.

This comprehensive methodology, although it is subjected to various limitations (e.g. use of hypothetical species, assumption that intervening sites are unsuitable, lack of consideration of viable population sizes, assumption that the landscape between interconnected nodes is homogeneous in terms of resistance or facilitation of dispersal), offered us the opportunity to raise some first insights on connectivity issues at a European scale. At the present, the Natura 2000 network covers more than 17% of the terrestrial territory of the European Union and represents the most extensive network of conservation sites on the globe. Under the prism of the global environmental changes, it is critical that efficiency of such networks needs to be maintained or even enlarged. Towards this direction, the increase of the connectivity between protected sites is highly acknowledged. It is therefore critical that we focus our actions on conserving and restoring natural habitats within PAs by further reducing human related threats to biodiversity. In addition, it is critical to increase connectivity through the establishment of green corridors, and the selection and inclusion in the conservation network of key-sites that serve as stepping stones located in the habitat outside the Natura 2000 sites.

Publications: (NOT PUBLISHED YET)

Papanikolaou, A.D., Kallimanis A.S., Leshten V., Pe'er G., Henle, K. Panis J.D., Mazaris A.D. Connectivity of the European conservation network: A graph-theory based analysis considering dispersal capacities and minimum area requirements of species. Under review