A metapopulation can go extinct even when suitable habitat remains

Human actions (such as land development) often convert large tracts of habitat into sets of spatially isolated habitat fragments (see Figure 9.7). Such habitat fragmentation can cause a species to have a metapopulation structure where it did not have one before.

The idea that all populations in a metapopulation might go extinct while suitable habitat remains was developed further in studies on the northern spotted owl (FIGURE 9.16). The northern spotted owl (Strix occidentalis caurina) is found in the Pacific Northwest region of North America. It lives in old-growth forest, where nesting pairs establish large territories that range in size from 12 to 30 km² (territories are larger in poor-quality habitat). Lande (1988) modified Levins's model to include a description of how owls might search for vacant “patches,” which were interpreted as sites suitable for individual territories. Lande estimated that the entire metapopulation would collapse if the area covered by old-growth forest were reduced by logging to less than 20% of the total area of a large region. This result had a powerful impact: it illustrated how a species might go extinct if its habitat dropped below a critical threshold (in this case, 20% suitable habitat), and it contributed to the 1990 listing of the northern spotted owl as a threatened species in the United States. The importance of conserving old-growth forest has been highlighted by the effects of a recent invader, the barred owl (Strix varia): the arrival of barred owls can cause spotted owls to become locally extinct within forest patches, but such extinctions are less likely in old-growth forests that cover a large area (Dugger et al. 2011).

FIGURE 9.16 The Northern Spotted Owl The northern spotted owl (Strix Occidentalis Caurina) thrives in old-growth forests of the Pacific Northwest; such forests include those that have never been cut, or have not been cut for 200 years or more. View larger image