Protected areas (PAs) are the main instrument for biodiversity conservation, which has triggered the development of numerous indicators and assessments on their coverage, performance and efficiency. The connectivity of the PA networks at a global scale has however been much less explored; previous studies have either focused on particular regions of the world or have only considered some types of PAs.

There is indeed a growing recognition that PAs cannot be conceived and managed as “islands” isolated from other PAs and from the rest of the landscape context. It is projected that climate change will make some PAs inhospitable for many of the species they cur-rently harbor, requiring species to move to new locations matching their environmental requirements, typically at higher latitudes or altitudes. In the absence of connectivity in the PA systems, individual PAs may turn into cli-matic traps under warming, hampering their ability to meet theirlong-term conservation goals.

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The project presents Protected Connected (ProtConn), an indicator of the connectivity of PA systems that improves the detail and comprehensiveness of previous related assessments. ProtConn is based on graph theory (network analysis) and accounts forboth the land area that can be reached within PAs and that reach-able through the connections between different PAs.

Methods

They downloaded the public version of the World Databaseon Protected Areas (WDPA) for June 2016 as a file geodata base from Protected Planet. WDPA is managed by the World Conservation Monitoring Centre (WCMC) of the United Nations Environment Programme (UNEP) in collaboration with the International Union for Conservation of Nature (IUCN). They considered all PA types, including PAs with not reported or not assigned IUCN category in the WDPA. The connectivity analyses were performed for each of the 827 terrestrial ecoregions of the world delineated as a biogeographic regionalization of the global terrestrial biodiversity that can serve to support the development of large scale conservation strategies. The ecoregions are large units of land, each containing a distinct composition of natural communities which share similar environmental conditions, with boundaries that approximate the original extent of these natural communities prior to major land use change. The Terrestrial Ecoregions ofthe World data set (TEOW) was used for the spatial analyses.

They selected all PAs within each ecoregion, calculated the centroid of these PAs, and projected the PA layer to an azimuthal equidistant projection centered in the coordinates of that centroid for subsequent accurate distance calculation (including PA buffering) in each ecoregion. They considered four median dispersal distances (d) of 1, 10, 30 and 100 km, which covered values up to the median dispersal abilities of the large majority of terrestrial species.

The probability of direct dispersal (p_ij) between two PAs i and j was calculated through a negative exponential function of the distance separating the PAs, in which p_ij= 0.5 for those PAs separated by a distance equal to the species median dispersal distance.

Two related graph based metrics underlie the PA connectivity indicators presented in this study: the Probability of Connectivity (PC) and the Equivalent Connected Area (ECA). PC and ECA measure the reachable habitat resources in a landscape or region, accounting for both theresources that can be reached within the habitat patches (intrapatch connectivity) and those made available by (reachable through) the connections with other habitat patches (interpatch connectivity).

For calculating PC and ECA, scientists represented the PA systemsin each ecoregion as a weighted probabilistic graph (network). In this graph, nodes (patches) corresponded to PAs, weighted by a certain attribute as describedbelow. Links represented the possibility for movement between nodes (PAs) and were weighted by the probabilityof direct dispersal between them (pij). The Equivalent Connected Area (ECA) is defined as the size (area) that a single PA should have to provide the same amount (area) of reachable protected land as the network of PAs in an ecoregion.

They defined and calculated a set of indicators to assess the connectivity of PA systems (Table 1). These connectivity indicators arebased on PC and ECA and have the Protected Connected land (ProtConn) as the main indicator.

Source: Saura et al. (2016)

Note that while all indicators provide useful and complementaryinformation, the meaningful connectivity indicator is ProtConn,and not RelConn. The Protected Connected (ProtConn) land indicator can be partitioned into three fractions (Table 1) that are expressed as apercentage of the total ProtConn value, hence summing up to 100:

ProtConn [Prot] + ProtConn [Unprot] + ProtConn [Trans] = 100

The indicator values were aggregated at the global, realm and biome level by calculating the average of the ecoregion level indicator values weighted by the area of each ecoregion.

Results

In average, Protected Connected lands (ProtConn) only covered9.3% of the area of the world’s terrestrial ecoregions for a mediandispersal distance d = 10 km. Protected Connected land was, for all considered d, noticeably smaller than the global PA coverage of 14.7%. Further partitioning the ProtConn indicator showed that species could reach most of the protected connected land by moving only through PAs within the ecoregion (ProtConn[Prot]).

Source: Saura et al. (2016)

Conclusions

Ideally, the coverage of a protected area network should be accompanied by comparable levels of Protected Connected land. The results have shown, however, that the spatial arrangement of PAs is only partially successful in ensuring the connectivity of protected lands. The PA networks only seem to achieve intermediate levels of connectivity for most of the terrestrial species, as given by an average relative connectivity of 63% for a reference median dispersal distance d = 10 km.

Source: Saura et al. (2016)

Connectivity indicator relies on the assumption that PAs are effectively conserved and managed in order to ensure sufficient connectivity levels that allow the successful movementof species through protected lands. There is however, in many cases, a gap between the formal protection of an area and the actual implementation of appropriate conservation and management measures in that area. Assessment of progress towards management effectiveness is therefore of crucial importance to ensure that PAs are able to play their full role as functional connectivity providers in a wider network of protected sites.

ProtConn is the result of the combination, through network analysis, of the sizes, coverage and spatial arrangement of PAs with the species dispersal distances considered. Despite the underlying complexity andmultiple fractions involved, the indicator is, easy to communicate to end users and to compareagainst national and global targets for PA coverage and connectivity.The application of the indicator to the terrestrial realm has evidenced highly uneven levels of PA connectivity across the world’s ecoregions, and the need for targeted actions to improve the con-nectivity of PA systems.

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Source: Santiago Saura, Lucy Bastin, Luca Battistella, Andrea Mandrici, Grégoire Dubois Protected areas in the world’s ecoregions: How well connected are they? European Commission, Joint Research Centre (JRC).

Main photo credit: Isaac Sanz