Extinction debt: Singapore forest fragments are arks of ginger diversity but they are all small, threatened populations (Niissalo et al., 2017)

From Ted Webb, “PhD student Matti Niissalo suggests that many of the gingers of Singapore are under immediate threat of extinction or have an unpaid extinction debt; but that the prolonged period before extinction provides a window of opportunity for conservation action.”

Niissalo, M. A., J. Leong-Škorničková, G. S. Khew & E. L. Webb, 2017. Very small relict populations suggest high extinction debt of gingers in primary forest fragments of a tropical city. American Journal of Botany, 104(1): 182-189 [journal link]

Tropical plant communities in fragmented forests are likely to experience an extinction debt, i.e., the habitat cannot support as many species as are present due to reduced habitat size and connectivity. There are few estimates of the number of species that represent extinction debt, and the number of extinctions over time has rarely been recorded. We recorded population sizes to assess threats and extinctions in gingers (sensu Zingiberales) in fragmented rainforest in Singapore, ca. 200 yr after fragmentation began.

We surveyed extant diversity and population sizes of gingers and used the results to estimate species survival. We critically assessed historic specimens to estimate initial extinctions and extinctions realized in present habitats.

We recorded 23 species, including five species previously presumed nationally extinct and four species omitted from the national checklist. The revised extinction rate is much lower than previously reported (12 vs. 37%). Most gingers have very small populations or miniscule ranges, implying that extinction debt has not been paid off.

Ginger diversity remains high, but the number of species at immediate risk of extinction outnumber recorded extinctions. Although tropical forest fragments remain arks of plant diversity for a long time, extinction debt may be prevalent in all plant groups in Singapore. Slow relaxation of extinction debt should be explicitly identified as a conservation challenge and opportunity. For conserving plant diversity in tropical fragments, relaxation must be reversed through restoration of degraded landscapes and, where feasible, targeted ex situ conservation and planting.

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