Extinction Dynamics Under Extreme Conservation Threat: The Flora of St Helena

Phil Lambdon,Quentin Cronk

doi:10.3389/fevo.2020.00041

Abstract

The flora of the island of St Helena provides an amplified system for the study of extinction by reason of the island’s high endemism, small size, vulnerable biota, length of time of severe disturbance (since 1502) and severity of threats. Endemic plants have been eliminated from 96.5% of St Helena by habitat loss. There are eight recorded extinctions in the vascular flora since 1771 giving an extinction rate of 581 extinctions per million species per year (E/MSY). This is considerably higher than background extinction rates, variously estimated at 1 or 0.1 E/MSY. We have no information for plant extinctions prior to 1771 but applying the same extinction rate to the period 1502 to 1771 suggests that there may be around ten unrecorded historical extinctions. We use census data and population decline estimates to project likely extinction forward in time. The projected overall extinction rate for the next 200 years is somewhat higher at 625 E/MSY. However, our data predict an extinction crunch in the next 50 years with 4 species out of the remaining 48 likely to become extinct during this period. It is interesting that during a period when the native plant areas dropped to 3.5% of the original, the extinction rate appears to have remained shallowly linear with under 30% of the endemic flora becoming extinct.

Highlights

Extinction Dynamics Under Population DeclineHabitat loss leading to species extinction has been called the signature problem of our times (He and Hubbell, 2011), and recent studies have indicated the extent of the problem (Gray, 2019; Humphreys et al, 2019; Tollefson, 2019)
Species extinction is of particular importance as there is widely perceived to be a species extinction crisis (Ceballos et al, 2015; Díaz et al, 2019), with anthropogenic extinction running much higher that the natural extinction rate
The decrease of habitat due to these factors is expected to cause extinction, as there are generally more species on large areas of land than on small ones. This led to early attempts to predict extinction using the species–area relationships (SARs) that had been successfully established in the ecological literature to predict the numbers of species in plots or landscape features of differing areas (McGuinness, 1984)

Summary

Introduction

Extinction Dynamics Under Population DeclineHabitat loss leading to species extinction has been called the signature problem of our times (He and Hubbell, 2011), and recent studies have indicated the extent of the problem (Gray, 2019; Humphreys et al, 2019; Tollefson, 2019). A large proportion of terrestrial land area has been converted to what Janzen (2001) called “agroscape” (i.e., the agricultural landscape of ranches, plantations, and crop fields, along with associated infrastructure of roads, buildings, and drainage or irrigation ditches) In this agroscape, relict wildland trees often exist as non-viable populations or single non-reproducing individuals that Janzen has called the “living dead” (Janzen, 2001). The decrease of habitat due to these factors is expected to cause extinction, as there are generally more species on large areas of land than on small ones. It was subsequently realized that ecological SARs greatly overestimate the observed extinction and the naive use of SARs to predict extinction was abandoned (He and Hubbell, 2013)

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Conclusion