Hundreds of papers talk about the importance of connectivity in marine conservation planning. Almost all of these papers treat MPA as islands in a sea of nothingness. Some express disappointment that it is not more explicitly considered in planning Marine Protected Area (MPA) networks.
The lack of explicit consideration of connectivity in MPA planning maybe because most MPA are not established as part of networks, or because practitioners do not consider it something that they need to prioritise.
This topic arises every year in our MPA course and has long surprised me about why people think it is something we need to worry about. As a field ecologist, it is obvious to me that the same habitat and species occur inside and outside MPA. Therefore, they are not islands.
The question of which species “connectivity” needs to be considered often arises. Species have greatly different dispersal potential. Most marine species have larvae which disperse to varying extents in the plankton. Besides, most marine species are mobile as adults (most crustaceans, fish, molluscs and worms). Usually, the species considered for connectivity studies are those targetted by fisheries based on the reasonable assumption that their populations will be larger inside MPA. The fact that their population is sufficient to support a fishery outside the MPA, however over-fished, means that they are not islands for fished species either.
In this recent paper, David Connor and I explain why we think conservation planning does not need to worry about connectivity. The important issue is to have a network that encompasses all species. This is achieved by it being “representative” of biodiversity by including replicated examples of all habitats in a biogeographic region (Costello and Connor 2019). A simple way to do this is to place MPA in areas with high numbers of rare and/or endemic species. This approach will automatically capture the common species.
Our article was prompted by a thorough critical review of the literature on marine connectivity by Manel et al. (2019a). They showed that almost half of marine connectivity studies underestimated animal dispersal because the animals went outside the extent of their study area. Their review indicated, using genetic methods which indicate population-level connectivity best, that marine species populations extend over 1,000 km distances, not the tens of kilometres we and others had imagined in previous studies. Manel et al. (2019b) responded to our paper and agreed with us that MPA should not be treated as islands.
Of course, connectivity is important in ecology and evolution, but I think connectivity it is not something we need to worry about in designing MPA networks because MPA (1) are not islands for either marine habitats of species; (2) marine species disperse tens to thousands of km.
Costello MJ, Connor DW. 2019. Connectivity is generally not important for marine reserve planning. Trends in Ecology and Evolution, 2542, online. DOI https://doi.org/10.1016/j.tree.2019.04.015
Manel S., et al. 2019. Long-distance benefits of marine reserves: myth or reality? Trends Ecol. Evol. 34, 342–354.
Manel S., et al. 2019. Long-distance marine connectivity: poorly understood but potentially important. Trends Ecol. Evol. 34 (8), 688–689. DOI:https://doi.org/10.1016/j.tree.2019.05.011