Near-Complete Extinction of Native Small Mammal Fauna 25 Years After Forest Fragmentation

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Science  27 Sep 2013:
Vol. 341, Issue 6153, pp. 1508-1510
DOI: 10.1126/science.1240495

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Futile Forest Fragments

Most of the planet's terrestrial biodiversity is found in tropical forests, but much of this critical habitat now persists as fragmented patches surrounded by agriculture. Smaller forest patches sustain fewer species than larger patches or contiguous forest. However, the numbers of species that will disappear from a forest fragment—and the rate of species loss—remain poorly understood. Gibson et al. (p. 1508) surveyed islands in a reservoir in Thailand to measure the rate of loss of small mammals from small forest fragments. Collapse of the entire native community (up to 12 species) from 16 forest fragments was observed after 25 years of isolation. Thus, small forest fragments hold little value for mammalian biodiversity, and conservation efforts should instead focus on the preservation of large forest expanses.


Tropical forests continue to be felled and fragmented around the world. A key question is how rapidly species disappear from forest fragments and how quickly humans must restore forest connectivity to minimize extinctions. We surveyed small mammals on forest islands in Chiew Larn Reservoir in Thailand 5 to 7 and 25 to 26 years after isolation and observed the near-total loss of native small mammals within 5 years from <10-hectare (ha) fragments and within 25 years from 10- to 56-ha fragments. Based on our results, we developed an island biogeographic model and estimated mean extinction half-life (50% of resident species disappearing) to be 13.9 years. These catastrophic extinctions were probably partly driven by an invasive rat species; such biotic invasions are becoming increasingly common in human-modified landscapes. Our results are thus particularly relevant to other fragmented forest landscapes and suggest that small fragments are potentially even more vulnerable to biodiversity loss than previously thought.

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