Mendonca, E. & Lamelas-López, L.
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Gibbaranea occidentalis is an orb-weaver spider species occurring in eight islands of the Azorean archipelago (Azores, Portugal). It has a relatively large Extent of Occurrence (EOO = 42,175 km²) and relatively small Area of Occupancy (AOO = 404-1,940 km²). This species occurs mainly in Azorean native forest, but also in shrubland, exotic forest and other disturbed habitats, although these contain marginal subpopulations mostly dominated by juvenile stages. It is abundant in the canopies of endemic trees but can also be found on the forest floor and on exotic trees. It favours humid and sheltered microhabitats being a nocturnal species. Based upon the fragmentation of subpopulations and inferred decline in EOO, AOO, decline in the quality and structure of habitat, this species is assessed as Near Threatened (NT).
Gibbaranea occidentalis is an orb-weaver spider species occurring on eight islands of the Azorean archipelago (only absent in Corvo) (Azores, Portugal) (Borges et al. 2010). Within these eight islands it is known from 18 Natural Forest Reserves: Caldeiras Funda e Rasa and Morro Alto e Pico da Sé (Natural Park of Flores); Caldeira do Faial and Cabeço do Fogo (Natural Park of Faial); Mistério da Prainha, Caveiro and Caiado Pico (Natural Park of Pico); Pico Pinheiro and Topo (Natural Park of S. Jorge); Biscoito da Ferraria, Pico Galhardo, Caldeira Guilherme Moniz, Caldeira Sta. Bárbara e Mistérios Negros and Terra Brava Natural Park of Terceira); Atalhada, Graminhais and Pico da Vara (Natural Park of S. Miguel) and Pico Alto (Natural Park of S. Maria). The estimated Extent of Occurrence (EOO) is 42,175 km2 and the estimated Area of Occupancy (AOO) is between 404-1,940 km2.
This species can be considered one of the most abundant endemic Azorean spider species, with the highest concentration found in native forest. However, in exotic fragments individuals that are found are mostly juveniles, which means that these areas may be occupied by sink subpopulations.
Current Population Trend: Decreasing
The species occurs mainly in native forests and builds its orb weaver web in the canopies of endemic trees, but can also build them in the shrubs Vaccinium cylindraceum and Myrsine africana. It is also frequently found in man-made habitats, namely exotic forests and marginal fields, but with a predominance of juvenile life-stages in these marginal habitats, which may indicate that subpopulations in marginal habitats are sink subpopulations (see Borges et al. 2008). The species is active during the night and based on long-term data with SLAM traps (Borges et al. 2017) it is observed to occur in all seasons, but with adults being dominant in summer. The species may compete with the exotic species Metellina merianae (Scopoli) (Araneae, Tetragnathidae) that builds similar webs in the same habitats mostly at low to medium elevations.
In the past, the species has probably strongly declined due to changes in habitat size and quality (Triantis et al. 2010). However, the species seems to have survived in the remaining native forests of the Azores, as well as in some human modified habitats (still with marginal sink subpopulations, see Borges et al. 2008). The main current threats are Cryptomeria japonica wood and pulp plantation management, orchard management and the spread of invasive species namely Hedychium gardnerianum and Pittosporum undulatum on most islands, and Clethra arborea in S. Miguel, which are changing the structure of the native forest with impacts on web construction. At lower and medium elevations the species may compete with the exotic species Metellina merianae (Araneae, Tetragnathidae) which builds similar webs in the same micro-habitats. Based on Ferreira et al. (2016) the habitat will further decline as a consequence of climate change (increasing number of droughts, and habitat shifting and
The species is not protected by regional law, although its habitat is in regionally protected areas (Natural Parks of Faial, Flores, Pico, S. Jorge, Graciosa, Terceira, S. Miguel and S. Maria). Degraded habitats in some islands, degraded due to invasive plant species, should be restored and a strategy needs to be developed to address the current threat by invasive species in all islands and the future threat by climate change. A habitat management plan is needed and is anticipated to be developed during the coming years. 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 obtain adequate information on population size, distribution and trends. It is also necessary to develop a monitoring plan for the wider invertebrate community in its habitat in order to contribute to a potential future species recovery plan. Monitoring every ten years using the BALA protocol will inform about habitat quality (see e.g. Gaspar et al. 2011).