Habitat fragmentation is commonly cited as an important driver of biodiversity loss, but the primary mechanism by which fragmentation impacts biodiversity is unclear. This confusion is partially driven by the fact that habitat fragmentation is an aggregate process by which habitat area often declines simultaneously with changes in spatial configuration. Reviews of past studies have suggested that these aggregate effects of habitat fragmentation on biodiversity are driven largely by losses in habitat area, not changes in spatial configuration, and increasing habitat patchiness alone more often has a positive effect than a negative one. Much of this prior work has focused on terrestrial systems, however, and marine systems differ fundamentally in the nature of animal/material movement, matrix habitat structure, and the scales at which fragmentation is typically observed. We reviewed the literature to create a database of marine studies that measured habitat fragmentation effects while controlling for habitat area on any ecological response variable. We evaluated: (1) whether fragmentation effects are typically positive, negative, or neutral, (2) whether habitat area or fragmentation had a stronger effect on the ecological response, and (3) how fragmentation effects varied by focal habitat, trophic level of response taxa, and scale of the study.
Results/Conclusions
Our literature review returned 82 papers that evaluated the effects of habitat fragmentation independent of changes in habitat area on ecological responses in marine systems. These papers employed both experimental and observational study designs and spanned a range of habitat types including coral reefs, kelp forests, mud bottom, oyster reefs, seagrass beds, and worm reefs. Our initial results indicate that within marine systems, independent effects of fragmentation are common (39% of contrasts reported a significant or important effect) and that fragmentation effects are just a likely to be positive (57% of significant effects) as negative (43%). For studies that compared the relative strength of independent fragmentation and area effects, fragmentation effects were considered more important in 2/3 of these studies compared to 1/3 of studies that found area effects to be stronger. These initial findings stand in contrast to those from previous work on habitat fragmentation dominated by terrestrial examples. It is possible that stronger matrix effects, differences in species dispersal abilities, or the smaller scale of landscape measurement underlie these sharp contrasts between habitat fragmentation effects in marine relative to terrestrial systems.