Assessment Rationale:

Acorigone zebraneus is a single-island endemic money spider species restricted to São Jorge island in Azores, Portugal (Borges et al. 2010). It is a rare species, with a restricted Extent of Occurrence (EOO) (4-20 km2) and Area of Occupancy (AOO) (4-20 km²), but these values are suspected to be at the upper end of these estimates. There is a continuing decline in the EOO, AOO, extent and quality of habitat as wellas the number of mature individuals as a result of the invasions of non-native plants and trampling of soil by dairy cows. The species occurs in a single patch of native forest at Topo Nature Reserve. In the past, the species has probably strongly declined due to changes in habitat size. The only patch of native vegetation where it occurs in under severe threat due to invasive plants and dairy-cattle disturbance. Therefore, we suggest as future measures of conservation: (1) a long-term monitoring plan of the species; (2) control of invasive species, and (3) restrict the access of cattle. The species is assessed as Critically Endangered (CR).

Geographic Range:

Acorigone zebraneus is a single-island endemic money spider species restricted to São Jorge island in Azores, Portugal (Borges et al. 2010), occurring in the Natural Forest Reserve of Topo (Natural Park of S. Jorge). The estimated Extent of Occurrence (EOO) is 4-20 km2 and the estimated Area of Occupancy (AOO) is 4-20 km2.

Population:

This is a very rare species, only known from a single sustainable subpopulation with very few individuals found during BALA project intensive sampling (Borges et al. 2016). A continuing decline in the number of mature individuals is inferred from monitoring schemes and from the ongoing habitat degradation due to invasions of alien plants (namely Hedychium gardnerianum) and soil erosion and degradation due to soil trampling by dairy cows.

Current Population Trend: Decreasing

Habitat and Ecology

The species is restricted to a hyper-humid native forest and builds the sheet weaver web in the ground between holes and the trunks of Juniperus brevifolia and Ilex perado subsp. azorica. The ground is also covered with mosses (Sphagnum spp.) and ferns (Borges and Wunderlich 2008). The species occurs in the same habitat of another single-island endemic Linyphid spider, Savigniorrhipis topographicus (see Crespo et al. 2013), which is also restricted to the same native forest fragment (Topo, S. Jorge). The species is active during the night.

Systems: Terrestrial

Major Threat(s):

In the past, the species has probably strongly declined due to a reduction in habitat size and quality (Triantis et al. 2010), and the species seems to have survived only at a single site at high elevation native forest of S. Jorge island. The main current threats are: the spread of an invasive plant species, Hedychium gardnerianum, which is changing the structure of the forest and the cover of bryophytes and ferns with impacts on web construction, and soil erosion due to trampling by dairy cattle (cattle are entering the native areas in S. Jorge island without control by the Conservation Managers). Based on Ferreira et al. (2016) the habitat will further decline as a consequence of climate change (increasing the number of droughts, and habitat shifting and alteration).

Conservation Actions

The species is not protected by regional law. Its habitat is in a regionally protected area (Natural Park of S. Jorge). However, the Topo Natural Forest Reserve has a low level of protection (currently classified in category V of the IUCN) and is managed mainly for landscape conservation and recreation, one of the lowest protection levels. As also indicated by Crespo et al. (2013), we strongly recommend a future change to category I, a wilderness area managed mainly for wilderness protection, so that its natural features can be properly safeguarded. Degraded areas, degraded due to invasive plant species and trampling by dairy cattle, should be restored and a strategy needs to be developed to address the current threat by invasive species and cattle and the future threat from climate change. Formal education and awareness are needed to allow future investments in restored habitats invaded by invasive plants; while further research is needed into its ecology and life history in order to find additional specimens in other areas of native forest of S. Jorge and to obtain adequate information on population size, distribution and trends. An area-based management plan is also necessary for the most disturbed sites including invertebrate monitoring to contribute to a potential species recovery plan. Monitoring every ten years using the BALA protocol will inform about habitat quality (see e.g. Gaspar et al. 2011).