Hochkirch, A. & Gerlach, J.
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The Desertas Islands (Madeira, Portugal) are the sole home of one of the largest and rarest wolf spider species worldwide, Hogna ingens (Blackwall, 1857) (Araneae, Lycosidae). Despite its size, it inhabits a single valley in the north of the Deserta Grande Island, Vale da Castanheira, currently invaded by the non-native grass Phalaris aquatica. This invasive species competes with the native flora and was subject to several erradication experiments, namely through fire and chemicals. The current distribution of H. ingens covers 23 ha, a recent reduction from its original 83 ha, corresponding to the entire valley. A total of 4,447 and 4,086 adults were estimated to live in the valley during 2011 and 2012, respectively. We found a significant negative impact of P. aquatica cover on the presence and abundance of H. ingens and that chemical treatment specifically directed towards the plant may be the only way to effectively recover the spider’s habitat. We suggest as future measures: (1) regular monitoring of the spider’ population; (2) possibly extend chemical treatments to other areas (subject to tests on side-effects to other native species); and (3) ex situ conservation with future reintroduction of adult specimens. Based upon the small geographic range of the species with only one location and continuing decline of its habitat and area of occupancy, it is assessed as Critically Endangered.
Hogna ingens is known to historically occur only in a valley at the northern end of Deserta Grande Island (Desertas, Madeira, Portugal). The Vale da Castanheira is approximately 2.8 km long and its width varies between 180 m to 400 m. The estimated area is 83 ha. The elevation of the valley ranges from 150 up to 350 m asl.
The estimated population size of H. ingens was 4,447 adult specimens in 2011 and 4,086 in 2012.
The Vale da Castanheira, single site of occurrence of the species, was originally occupied by a mix of native grasses and bare rock/soil. Most of it is now occupied by Phalaris aquatica, an invasive grass that recently expanded (expansion detected in 2005). This invasive plant covers the entire surface of the soil and crevices, preventing the accessibility to shelters that are usually occupied by the spider as well as other endemic fauna.
In the absence of native terrestrial mammals, this spider with a 40 mm body length is a top predator in its small habitat, and although its major prey consists of other invertebrates, such as the staphylinid beetle Ocypus olens (Müller, 1764) or the invasive millipede Ommatoiulus moreleti (Lucas, 1860), adults have even been seen predating on juveniles of the lizard Lacerta dugesii mauli Mertens, 1938. The latter, along with birds and mice, should nevertheless be the major predators of H. ingens, mostly during its juvenile stage. It is in this stage that the spider is most vulnerable to predators, because adding to its smaller size, it tends to disperse in order to find new shelters, thus maximizing the likelihood of encounters with potential predators (including conspecific adults). As spiders grow and find proper shelters (mostly below rocks but also in soil crevices), their dispersal propensity gradually decreases. It takes about two years for a spider to reach maturity.
The Vale da Castanheira is currently mostly covered by a non-native invasive grass, Phalaris aquatica L.. The settlement of this grass was hidden for some years, due to the presence of rabbits (Oryctolagus cuniculus L.), who grazed and stalled the proliferation of the grass. Given that rabbits were eradicated from the Vale da Castanheira in 1996, P. aquatica lost its main consumer and now proliferates. This grass appears to not only displace the native herbs, but also many of the native animals, as one of the most common arthropods is now the invader diplopod Ommatoiulus moreleti (Lucas, 1860). Also, P. aquatica covers the surface of the soil and rocks, making the microhabitats below the rocks harder to access for arthropods, which take shelter there during daytime.
We suggest as future conservation measures:
(1) Regular monitoring of the trends of the distribution and population size of the species. This will provide more robust data for ecological and conservation studies. Although this is already done on a voluntary basis, it is not regular or exhaustive.
(2) Taking into account possible side-effects on other species (an aspect which needs to be studied further), to extend the preliminary chemical treatments to the areas currently affected by the presence of P. aquatica. This could prove to be essential to remove the invasive grass from Vale da Castanheira, and may prove to also be beneficial to other endemic and native invertebrates.
(3) Implement ex situ conservation measures for the species for future reintroduction of adults into the Vale da Castanheira. This could be easily and inexpensively done by the Madeira Natural Park authorities with eventual collaboration from zoos. This would ensure that several juvenile specimens would reach maturity and could be used to colonize previously unoccupied areas. The ex situ animals also constitute an insurance against the extinction of the species in its natural habitat. This measure would further be a way to showcase a remarkable endemic species to the visitors of Deserta Grande, thus connecting the public with a nearly unknown value of the Madeira natural patrimony.