Abstract
During the last few centuries oceanic island biodiversity has been drastically modified by human-mediated activities. These changes have led to the increased homogenization of island biota and to a high number of extinctions lending support to the recognition of oceanic islands as major threatspots worldwide. Here, we investigate the impact of habitat changes on the spider and ground beetle assemblages of the native forests of Madeira (Madeira archipelago) and Terceira (Azores archipelago) and evaluate its effects on the relative contribution of rare endemics and introduced species to island biodiversity patterns. We found that the native laurel forest of Madeira supported higher species richness of spiders and ground beetles compared with Terceira, including a much larger proportion of indigenous species, particularly endemics. In Terceira, introduced species are well-represented in both terrestrial arthropod taxa and seem to thrive in native forests as shown by the analysis of species abundance distributions (SAD) and occupancy frequency distributions (OFD). Low abundance range-restricted species in Terceira are mostly introduced species dispersing from neighbouring man-made habitats while in Madeira a large number of true rare endemic species can still be found in the native laurel forest. Further, our comparative analysis shows striking differences in species richness and composition that are due to the geographical and geological particularities of the two islands, but also seem to reflect the differences in the severity of human-mediated impacts between them. The high proportion of introduced species, the virtual absence of rare native species and the finding that the SADs and OFDs of introduced species match the pattern of native species in Terceira suggest the role of man as an important driver of species diversity in oceanic islands and add evidence for an extensive and severe human-induced species loss in the native forests of Terceira.
Highlights
Relative to their area, islands make a disproportionately large contribution to global biodiversity but have long been severely impacted by human intervention leading several authors to consider that the present biodiversity crisis is acute in island ecosystems [1,2,3,4]
These two oceanic islands belong to different volcanic archipelagos (Madeira and Azores) that lie in the North Atlantic: Madeira is located between latitudes 32 ̊-33 ̊ N and longitudes 16 ̊-17 ̊ W, while the Azores range between 37 ̊-40 ̊ N and 25 ̊-31 ̊ W
Species richness was considerably different between the two islands for both spiders (S = 21 in Terceira, S = 40 in Madeira) and ground beetles (S = 7 in Terceira, S = 34 in Madeira) and sampling completeness was similar for Madeira samples, but higher for spiders than ground beetles in the Azores (Tables 2 and 3)
Summary
Islands make a disproportionately large contribution to global biodiversity but have long been severely impacted by human intervention leading several authors to consider that the present biodiversity crisis is acute in island ecosystems [1,2,3,4]. The number of documented extinctions of terrestrial arthropods is small, even though this speciesrich group of invertebrates has undoubtedly been the most severely affected by human driven extinctions over the last few centuries [7, 13,14,15]. The characterization of rarity and conservation status of species is hampered by the poor information on species distributions and abundances over time, and sensitivity to ecosystem disturbance [17]. The assessment of terrestrial arthropod rarity has mostly depended on the analysis of recent data on species abundances and distributions [18, 19]
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