Experiments were created by destroying or creating precise amounts of habitat across replicate landscapes, allowing tests of fragmentation effects independent of habitat loss.
Tests were conducted within fragments that varied experimentally in area or edge, within fragments that were experimentally isolated or connected, or within experimental fragments compared to the same area within continuous habitat. In each experiment, different fragmentation treatments with replication were established, starting from continuous, nonfragmented landscapes and controlling for background environmental variation either by experimental design (blocking) or by measurement of covariates for use in subsequent analyses.
Distances to the edge of experimental fragments range to 500 m, encompassing edge distances found in more than half of forests worldwide ( Fig. The largest fragments across these experiments match the size of fragments commonly created by anthropogenic activities ( Figs. 2 and Supplementary Materials) and were designed to manipulate specific components of fragmentation-habitat size, isolation, and connectivity-while controlling for confounding factors such as the amount of habitat lost across a landscape ( Fig. The long-term fragmentation experiments we analyze here comprise the entire set of ongoing terrestrial long-term experiments. However, together with these correlative observations, experimental studies reveal that fragmentation has multiple simultaneous effects that are interwoven in complex ways and that operate over potentially long time scales ( 9). Observational studies of the effects of fragmentation have often magnified the controversy because inference from nonmanipulative studies is limited to correlation and because they have individually often considered only single aspects of fragmentation (for example, edge, isolation, and area) ( 8). Central to the controversy has been a lingering uncertainty about the role of decreased fragment size and increased isolation relative to the widespread and pervasive effects of habitat loss in explaining declines in biodiversity and the degradation of ecosystems ( 7). Early hypotheses based on the biogeography of oceanic islands ( 6) provided a theoretical framework to understand fragmentation’s effect on extinction in terrestrial landscapes composed of “islands” of natural habitat scattered across a “sea” of human-transformed habitat. Ecologists agree that habitat destruction is detrimental to the maintenance of biodiversity, but they disagree-often strongly-on the extent to which fragmentation itself is to blame ( 4, 5).