Increased fishing pressure, and the subsequent collapse of many fisheries, "have forced marine conservationists and environmental managers to re-evaluate traditional methods of resource management. In the last decade, marine reserves have become increasingly popular as an alternative to traditional management options." So notes a study in the journal Ecology Letters.

A 2002 review of 112 independent empirical measurements of 80 different reserves concluded that "average values of all biological measures were strikingly higher inside marine reserves compared to reference sites (either the same site before the reserve was created or equivalent sites outside the reserve). Relative to reference sites, population densities were 91% higher, biomass was 192% higher, and average organism size and diversity were 20-30% higher in reserves." Furthermore, the review found that these values were "independent of reserve size, indicating that even small reserves can produce high values."

The few existing studies that have measured temporal responses of biological communities in indiviual marine reserves provide, says the Ecology Letters review "no consistant pattern." Examples exist in which biological measures "increased within reserves through time, showed little change over time, had values that initially climbed but then fell back to original levels, and decreased over time."The authors of the Ecology Letters study propose that, because most of these reviews focus on particular species or groups of species, their varying results likely "stem from the diverse life histories, trophic position, or degree of harvest © of the organisms studied in each case. For example, as predator densities increase due to protection, prey populations may decrease in a classic trophic cascade."

The Ecology Letters review notes that "there have been few attempts to develop theory on the biological response to reserve protection," and "efforts to evaluate overall reserve performance are relatively nascent". "However, its own findings were that "the establishment of marine reserves appears to result in significant increases in average levels of density, biomass, and likely diversity within 1-3 [years], and these values persist through time. Furthermore, these results are independent of reserve size. These are very encouraging results for those facing societal and management expectations that marine reserves provide rapid and persistent biological responses".

However, the authors caution that the "analyzed mean responses, and results for a particular species will certainly depend on their life-histories. For example, after massive reserve closures in the George's Bank area cod stocks have been slower to respond to protection, whereas scallop populations quickly grew to enormous size.

"Second, rapidity of response of a species to reserve protection will also depend to some extent on the degree of exploitation of that species. Heavily targeted species are more likely to respond quickly to the implementation of reserves, assuming recruitment occurs at high enough levels, because the main factor limiting the population size and demography of the target species (fishing) is suddenly removed."

Notwithstanding such caveats, the paper's authors conclude that their results "indicate that some of the anticipated functions of marine reserves (e.g. species conservation, increased production) should be attained relatively quickly and persist through time."

Source: Halpern, Benjamin S., and Robert R.Warner. 2002. Marine reserves have rapid and lasting effects. Ecology Letters 5: 361-366