Species loss from fragmentation occurs through two phases. Imminent (or immediate) extinctions are ‘cookie cutter’ losses, where all individuals in a population are physically removed during initial clearance. Biotic relaxation proceeds over time, as smaller, less viable populations are lost – this is the extinction debt incurred from habitat loss. Once fragmentation occurs, conservation interest tends to turn toward the viability of remaining populations and managing biotic relaxation. Theory and empirical evidence favour protection of larger habitat patches, because they tend to support larger populations with lower extinction rates. How effective this strategy is at conserving total species number depends on how well nested species are by area. However, meta-studies have shown high nestedness by area to be uncommon in many biotic groups. Smaller remnants in a fragmented landscape are most at risk of complete loss (attrition) through intentional clearance or environmental stochasticity (e.g., drought, fire). Systematic removal of smaller patches could result in imminent extinctions if some species’ populations were confined to these smaller patches. I tested this hypothesis using 57 published datasets on species incidence in vegetative habitat fragments by preferentially removing the smallest patches to a maximum of 20% of total area. I compared these losses with that predicted by the random placement endemics-area model.
Results/Conclusions
Attrition of the smallest remnants representing less than 20% of total area removed on average 9.0 ± 3.8 % (mean ± SE) of total species, although 25% of metacommunities lost no species at all. Species loss exceeded random-placement model predictions at the 5% level in 40.3% of metacommunities. Species loss was highest for invertebrates (13.5%) and lowest for mammals (3.5%). Thus, a scenario involving systematic loss of small remnants will remove species from the metacommunity perhaps 75% of the time, even if all larger patches remain unaffected. Many global-change scenarios present just such an elevated risk of attrition for small remnants. These ongoing imminent extinctions could also accelerate biotic relaxation by further reducing populations. The importance of smaller patches could be partly due to their comprising higher habitat quality and availability. For example, selective clearance of the most desirable and productive areas would likely leave only small remnants of this high-quality habitat. If such patches harbour a higher biodiversity potential, their conservation value is disproportionate to their size. The results support calls for consideration of habitat quality and availability in conservation prioritization and decision-making.