Null Models for Assessing Ecosystem Conservation Priorities: Threatened Birds as Titers of Threatened Ecosystems in South America

Abstract

We set ecosystem conservation priorities based on a comparison of observed levels of threat and protection versus expected levels derived from the use of null models. We used differences in the proportion of threatened and nonthreatened bird species among ecosystems to assess levels of threat and differences in the coverage of parks and protected areas relative to land area to assess levels of protection. We applied this approach to terrestrial biomes of continental South America and determined where recommendations for species and ecosystem conservation converge and diverge. We calculated the percentage of range in each biome for 132 bird species threatened by habitat destruction and 288 nonthreatened bird species. Three biomes (evergreen forest, broadleaf forest, and cold desert) were not included because few bird ranges fell primarily within them. Tropical humid forest and mountain biomes supported the greatest levels of avian species diversity. But only subtropical and temperate rainforests and tropical dry forests supported a higher proportion of threatened bird species than expected, making them ecosystem destruction hotspots. Tropical humid forests and tropical and temperate grasslands supported lesser proportions of threatened bird species than expected (i.e., coldspots), whereas mountain and warm deserts supported similar proportions of threatened and nonthreatened bird species (i.e., neutral). These patterns were corroborated by the apportionment of human population densities among biomes. Highest densities (> 25 per km2) occurred in subtropical and temperate rainforests and tropical dry forests, whereas tropical humid forests and warm deserts supported the lowest human densities (< 1 per km2). The largest proportions of protected areas were in tropical humid forest and mountain biomes, which is in accord with the distribution of avian diversity but not with our assessment of habitat destruction. Protected areas were especially underrepresented in tropical dry forests, making them the most threatened biome.