This question is a source of debate, with many arguing that many parks lack the management capacity necessary to effectively protect biodiversity. Two recent studies support the consensus that protected areas are key in reducing adverse land-cover change.
In the first, our colleagues at the University of Oxford co-authored a recent study in Biological Conservation, in which a systematic review approach was applied to investigate the evidence from peer reviewed and grey literature on the effectiveness of PAs to protect habitat cover and species populations (Geldmann et al., 2013). They analysed 76 studies that evaluated impacts on habitat cover, and 42 studies on species populations. Their analyses showed (a) there is good evidence that PAs have conserved forest habitat (b) there is inconclusive evidence that PAs have been effective at maintaining species populations and (c) the effects of management on conservation is rarely examined in the literature. Overall, the authors conclude that the available evidence suggests that PAs deliver positive outcomes, but data is lacking and we have a weak understanding of the effects of protection efforts and how PA management successes/failures deliver conservation outcomes.
In the second study, satellite imagery was used to measure rates of land-cover change over a 20 year period in both protected and unprotected Important Bird Areas (IBA's) across sub-saharan Africa (Beresford et al., 2013). They found that the overall rate of forest loss was about double that of all natural land-cover loss, but that rates of loss of natural land-cover were 58% lower inside protected IBAs than unprotected IBAs. They conclude that protection is effective at reducing land-cover change at sites of conservation importance.
Geldmann J, Barnes M, Coad L, Craigie ID, Hockings M, Burgess ND, 2013. Effectiveness of terrestrial protected areas in reducing habitat loss and population declines. Biological Conservation 161:230-238.
Beresford AE, Eshiamwata GW, Donald PF,Balmford A, Bertzky B, Brink AB, Fishpool LDC, Mayaux P, Phalan B, Simonetti D,Buchanan GM, 2013. Protection Reduces Loss of Natural Land-Cover at Sites ofConservation Importance across Africa. PLoSONE 8:e65370.