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SCALETOOL IntroductionDriversBiodiversityPolicies and managementConnectivity and protected areas
Differences between structural and functional connectivity How to assess connectivity - methods and tools From movement to dispersal to connectivity Connectivity and Natura 2000 Key messages for the connectivity of protected areas
 

Key messages for the connectivity of protected areas

In conservation policy-making, connectivity is important when considering the spatial arrangement of protected areas of habitat, and whether species of interest can successfully disperse between such patches. It is commonly assumed that greater connectivity is beneficial for conservation, but benefits and risks should be weighed together for specific objectives. Some general rules and key messages are tentatively offered.

Connectivity and scale

Ecological connectivity is a property of networks of habitat patches. It is naturally considered at the scale of a metapopulation: a group of local populations of a species that are spatially separated, reducing but not abolishing dispersal of individuals among them. Connectivity may be said to determine the level of such movements, which typically constitute dispersal, but may also include pollination and other foraging movements. A "metapopulation scale" is therefore ecologically defined: it implies a spatial extent many times larger than a single population patch, within which patches are separated by less than some maximum dispersal distance. The actual geographical scale will therefore depend on the species of concern and the distribution pattern of their habitat. To facilitate the analysis of connectivity in regard to species traits and habitat patterns, the SCALES project has collated a database of dispersal distances for a range of species (see also Götzenberger et al. 2011).

Metapopulations must span such distances, and their extent can be increased by increasing ecological connectivity. This may mean creating corridors or intervening "stepping stones" of habitat, or abolishing barriers, so as to enable individuals of certain species to move between the existing patches more regularly and reliably. Other means of increasing connectivity would be to move patches closer together, or to create additional main patches; these options have additional implications which are not considered here. The connectivity guidelines below, therefore, are relevant for a wide range of spatial scales and policy options, especially land-use planning and proposals for green infrastructure.

Protected areas and habitat patches

The concept of protected areas belongs to the legal side of conservation science. Here we consider a protected area as the legal embodiment of a habitat patch. A habitat patch, in turn, is an ecological concept referring to some contiguous region in which all the environmental (niche) requirements of one or more species tend to be satisfied, at least transiently. Many habitat patches exist that are not legally protected; conversely, there are protected areas comprising habitat that is degraded such that it poorly fulfils the purpose of its designation. This overview primarily considers ecological principles for habitat networks: how different amounts and kinds of ecological connectivity among habitat patches relate to a range of conservation objectives. The table below lists both benefits and risks of connectivity for each objective, and then tentatively articulates some general rules. These will suggest some messages and guidelines for policy makers who seek to enact legal frameworks concerned with the geometry of protected areas.

Conservation objective Benefits of connectivity Risks of connectivity General rules
Genetic diversity
  • Reduces inbreeding depression
  • Increases selection opportunities for strongly beneficial alleles
  • Reduces diversification through neutral evolution (drift)
  • Reduces likelihood of novel alleles establishing for local adaptation (outbreeding depression).
  • Low rates of dispersal can still have important effects on genetic processes (Ray & Excoffier 2010), so connectivity can be defined using lower migration thresholds than for other processes.
Population viability
  • May integrate vulnerable populations, increasing overall viability
  • Can create metapopulations out of independent populations, reducing stochastic extinction risk.
  • Enables mobile individuals to escape from local disturbances
  • Facilitates range shifts in response to climate change
  • May reduce viability of metapopulations by collapsing them into simple populations, increasing stochastic extinction risk.
  • May facilitate spread of endemic or epidemic diseases
  • Benefits of connectivity are generally assumed to outweigh risks (McCarthy et al. 2005).
  • Number of connections should be proportional to size of patches (Frank 1998).
  • Connections that traverse broad-scale temperature gradients are important in the context of climate change.
  • Connections themselves may constitute viable habitat and so effectively increase patch sizes (see below).
Community diversity
  • Benefits for population viability (above), plus:
  • Facilitates arrival of new species, potentially increasing local (?) diversity.
  • Risks for population viability (above), plus:
  • May facilitate invasions by competitive or predatory species, decreasing overall (?) diversity and threatening specialist species
  • Long-range connectivity may decrease among-site (?) diversity.
  • For habitat-specialists, connections should be of comparable quality to core habitat.
  • Edge (ecotone) habitats are crucial for many species
  • Solutions should consider the needs of taxa most prone to habitat fragmentation
Ecosystem functions and services
  • For any keystone species, see benefits for population viability (above).
  • Resilience may increase with local diversity (see benefits for community diversity, above)
  • For any keystone species, see risks for population viability (above).
  • Resilience may increase with among-site diversity (see risk of long-range connectivity, above)
  • Should consider disservices (pest species/communities) as well as services.
  • Connections can also serve as a distribution network for services that are experienced locally to the habitat (e.g. pollination, biocontrol, aesthetic services).
Overall
  • Incorporating community diversity with population viability, moderate connectivity has many benefits.
  • Valuing neutral genetic diversity, or certain ecosystem functions, may entail more risks from connectivity.
  • Often intermediate levels of connectivity perform best, but the optimum level will depend on the mobility of the species of concern, the amount of area required to maintain viable populations, and the aspect of conservation concerned.

Key messages

  • Ecological connectivity is taxon- and scale- specific. Connections should always be discussed with respect to a specified class of organisms with characteristic dispersal behaviour. In particular:
    • "connectivity" for small mammals that disperse on the ground will require contiguous habitat corridors, whereas other taxa can be "connected" by habitat islands.
    • "connectivity" for plant species that normally disperse on scales of metres will require much more closely-spaced habitat islands than taxa such as birds and butterflies that can disperse over kilometres.
  • While connectivity is relatively easily increased, it is also easily lost by attrition of the small habitat patches or corridors that constitute it, or by the creation of new barriers such as transport links.
  • It is difficult to specify general rules for connectivity, but in general terms, an intermediate degree of certain types of connectivity will provide the best compromise among different conservation objectives. The nature of this solution will depend upon the relative weights given to the various objectives, which is a political and ethical question.


References

Frank, K. (1998). Optimizing a network of patchy habitats: from model results to rules of thumb for landscape management. Pp. 59-72 in Munro, N.W.P. & J.H.M. Willison (eds.): Linking Protected Areas with Working Landscapes. Conserving Biodiversity Science and Management of Protected Areas Association (Wolfville/Canada)

Götzenberger L., Zobel M., Tamme R., Pärtel M., Lange R., Pe'er G., Trochet A., Stevens V. & Schmeller D.S. (2011). Review manuscript on dispersal traits and distances, including the underlying trait database (preliminary version). Project SCALES.

McCarthy M.A., Thompson C.J. & Possingham H.P. (2005). Theory for designing nature reserves for single species. American Naturalist, 165, 250-257.

Ray N. & Excoffier L. (2010). A first step towards inferring levels of long-distance dispersal during past expansions. Mol. Ecol. Resour., 10, 902-914.

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