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Habitat Fragmentation Effects on Birds in Grasslands and Wetlands: A Critique of Our Knowledge


Figure 1: Line graph showing Percent of Patches Occupied for small and large patch sizes. 
Line for Common species = 89% (small) to 100% (large). 
Line for Rare species = 5.6% (small) to 75% (large).
Figure 1. The incidence function describes the proportion of habitat patches in a size class that contain a given species. Increasing incidence functions are frequently interpreted as evidence of area sensitivity in habitat selection by a species, but the functions are confounded with the commonness of the species.

Figure 2: Four-part graphic (A,B,C,D) of species distribution within large and small patches.
A (top,left) = rare species in 3 large patches.
B (top, right) = common species in 3 large patches.
C (bottom, left) = rare species in 18 small patches.
D (bottom, right) = common species in 18 small patches.
Figure 2. Simulated distribution of points, representing birds, randomly distributed across some region of uniform habitat, to demonstrate why the incidence function cannot distinguish between area sensitivity and regional abundance. The left figures represent an uncommon species; the right figures represent a very common species. The squares represent habitat patches; A and B show three large patches, and C and D show 18 small patches. The rare species, shown as dots in A and C, is more likely to be sampled on the large (A) than on the small (C) patches compared to the more common species shown as dots on the large (B) and small (D) habitat patches. The incidence function for the very common species (upper line in Fig.1) shows little effect of patch size: the species occurred in 89% of the small patches and 100% of the large patches. In contrast, the incidence function for the rare species suggests a patch-size effect: the species occurred in only 5.6% of the small patches versus 75% of the large patches (Fig.1). But no patch-size sensitivity was involved in the simulations; the apparent sensitivity is an artifact of the species' rarity

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