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Pastures new: assisted migration of Ascension Island spurge as a climate mitigation strategy

by | Apr 25, 2022 | Endemic flora

Globally, many species face a growing problem as climate change intensifies. Species’ current ranges are often becoming hotter and drier. Suitable environmental niches are now generally moving upslope or away from the equator, towards cooler and wetter conditions. Species must be able to disperse into newly-suitable habitats in order to survive, yet they may not be able to migrate quickly enough nor overcome barriers such as habitat fragmentation. The CRACAB project aims to tackle this issue for the critically endangered Ascension spurge (Euphorbia origanoides) by translocating plants across barriers in one of the world’s first trials of assisted migration.

Newly-translocated plants at the high altitude restoration site. Plants from the nursery are supported by shade fencing and irrigation to overcome transplant shock.

Arid environment

Endemic to Ascension Island, Euphorbia origanoides, currently exists in fragmented populations across some of the driest, most barren areas of the island close to sea level. It is thought to have been excluded from wetter lowland areas by the introduction of herbivorous mammals, invasive plant pests, and competition from invasive plant species. Meanwhile, drought is hypothesized to be a major cause of recent population declines and low seedling recruitment. Soil probes, deployed as part of the CRACAB project, confirm that the spurge already faces a challenging moisture regime for much of the year in its arid lowland habitats (see Figure 1), with moisture content even in deep soil layers regularly less than 7%. Climate change may thus make survival increasingly difficult in its current range. Furthermore, the species is unlikely to be able to migrate naturally to wetter areas: low fecundity, drought, and the density of invasive competitors, pests and grazers across the island pose significant barriers to dispersal.

Figure 1. Soil moisture profiles of the assisted migration sites 2021-2022. Surface moisture is variable at all sites, while higher altitude sites have more deep moisture than lower sites. The CRACAB project will determine a precise moisture threshold for the Ascension spurge’s survival.

Assisted migration

As part of a long-term trial, the CRACAB project has established three experimental restoration sites along an altitudinal gradient. The lowest site, at sea level, is near the driest and hottest extreme of the spurge’s current range. An intermediate site at around 250 m altitude is marginally wetter than the current range, while the third site at 500 m elevation is significantly damper. These sites have been cleared of invasive vegetation, fenced to exclude grazing animals, and have rain catchers and water storage tanks to passively support the first generation of translocated plants with additional moisture. Thirty six nursery-grown spurges have now been translocated to the high-altitude site and the majority appear to have recovered strongly from the inevitable transplant shock to show new growth. The first batch of plants for the mid-altitude are currently being hardened-off to the equatorial sun before planting. The success of the trial will be measured by how well translocated plants establish a viable seed bank at new sites, followed by the germination and survival rate of seeds set.