Tuesday, 1 October 2013

Small mammals rapidly become extinct in small forest fragments

Near-complete extinction of native small mammal fauna 25 years after forest fragmentation

September 2013. As 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.

Luke Gibson, a biologist at the National University of Singapore, & colleagues surveyed small mammals on forest islands in Chiew Larn Reservoir in Thailand from 5-7 and 25-26 years after isolation and observed the near-total loss of native small mammals within 5 years from islands smaller than 10-hectares and within 25 years from 10-56 hectare islands.

Based on these results, they 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. These 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.

We surveyed small mammals on forest islands in a reservoir five separate times after isolation to assess the rate of species loss from forest fragments. Reservoirs can form useful natural laboratories to estimate extinction rates from isolated forest patches . 

Chiew Larn Reservoir
Chiew Larn Reservoir in southern Thailand was formed in 1986-1987 when 165 km2 of forest was flooded, creating over 100 islands in the process (Fig. 1) (17). We selected 16 islands in the reservoir ranging from 0.3 to 56.3 ha in area and surveyed small mammal communities 5 to 7 years (for 12 of the 16 islands) and 25 to 26 years after isolation.

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