http://www.nceas.ucsb.edu/Consensus/Consensus_Statement.doc
Basically, lots of scientists sign a concensus saying MPAs rock.
Ecological effects within reserve boundaries:
1) Reserves result in long-lasting and often rapid increases in the abundance, diversity and productivity of marine organisms.
2) These changes are due to decreased mortality, decreased habitat destruction
and to indirect ecosystem effects.
3) Reserves reduce the probability of extinction for marine species resident within them.
4) Increased reserve size results in increased benefits, but even small reserves have positive effects.
5) Full protection (which usually requires adequate enforcement and public
involvement) is critical to achieve this full range of benefits. Marine protected areas do not provide the same benefits as marine reserves.
Ecological effects outside reserve boundaries:
1) In the few studies that have examined spillover effects, the size and abundance of exploited species increase in areas adjacent to reserves.
2) There is increasing evidence that reserves replenish populations regionally via larval export.
Ecological effects of reserve networks:
1) There is increasing evidence that a network of reserves buffers against the vagaries of environmental variability and provides significantly greater protection for marine communities than a single reserve.
2) An effective network needs to span large geographic distances and encompass a substantial area to protect against catastrophes and provide a stable platform for the long-term persistence of marine communities.