Size and Spacing
The size, shape and spacing of individual MPAs in an MPA network greatly influence the degree to which conditions in the wider environment affect the features of the MPA, and exchange of individuals between the MPA and adjacent environment. The three components of size, spacing, and shape should be considered in design of the MPA network to facilitate and promote connectivity between and within the MPA network.
Models of the impacts of MPAs on population dynamics suggest that multiple MPAs can have three kinds of effects on ecosystems:1
- Additive effects — for example, the increase in population size from the entire network is simply the sum of the increases that would be observed from each reserve in isolation.
- Declining effects if the marginal value of extra protection declines as more areas are protected. For example, the number of species protected within a reserve is a function of its placement, habitat heterogeneity and the area. Doubling the area in reserves will not necessarily double the number of species protected.
- Multiplicative effect on ecosystems if the reserve system, as a whole, has greater beneﬁts than the additive sum of the benefits of individual reserves.
Achieving these multiplicative effects should be the goal when designing reserve networks.
Deciding how many, how large, and how far apart MPAs should be is a challenge. Ultimately, size, shape and spacing design of an MPA network will vary with the goals and objectives of the MPA, as well as the social and economic environment in which it is located. However, there are some general design guidelines on size, shape and spacing of each MPA that will help to ensure maximum benefits to individual MPAs within a larger MPA network.2