Abstract
Simple SummaryWe studied the diversity of arthropods in native forests along a 1000 m elevation gradient on Terceira Island, Azores (Portugal). These forests form an isolated and threatened habitat with unique endemic species. We analysed the change in alpha and beta diversity of arthropod species with elevation and if the diversity of endemic, native non-endemic and introduced species responds differently to elevation. Resident arthropods were sampled using SLAM (Sea, Land and Air Malaise) traps between 2014 and 2018. Spiders (Araneae), beetles (Coleoptera), true bugs (Hemiptera) and barklice (Psocoptera), as well as endemic, native and introduced species, were analysed separately. Total species richness decreases with elevation for all species, Coleoptera and Psocoptera, and particularly so for introduced species, but peaks at mid-high elevation for Araneae and endemic species. These patterns are probably driven by unfavourable climatic conditions at higher elevations while being influenced by human disturbance at lower elevations. Total species diversity along the whole elevation gradient is shaped by this decreasing richness as well as the replacement of species at different elevations.We present an analysis of arthropod diversity patterns in native forest communities along the small elevation gradient (0–1021 m a.s.l.) of Terceira island, Azores (Portugal). We analysed (1) how the alpha diversity of Azorean arthropods responds to increasing elevation and (2) differs between endemic, native non-endemic and introduced (alien) species, and (3) the contributions of species replacement and richness difference to beta diversity. Arthropods were sampled using SLAM traps between 2014 and 2018. We analysed species richness indicators, the Hill series and beta diversity partitioning (species replacement and species richness differences). Selected orders (Araneae, Coleoptera, Hemiptera and Psocoptera) and endemic, native non-endemic and introduced species were analysed separately. Total species richness shows a monotonic decrease with elevation for all species and Coleoptera and Psocoptera, but peaks at mid-high elevation for Araneae and endemic species. Introduced species richness decreases strongly with elevation especially. These patterns are most likely driven by climatic factors but also influenced by human disturbance. Beta diversity is, for most groups, the main component of total (gamma) diversity along the gradient but shows no relation with elevation. It results from a combined effect of richness decrease with elevation and species replacement in groups with many narrow-ranged species.
Highlights
Biodiversity is a complex concept that is most often assessed by analysing taxonomic species richness at different spatial scales [1,2]
We addressed three main questions: (1) How does the alpha diversity of Azorean arthropods respond to increasing elevation; (2) how do alpha diversity patterns across the elevation gradient differ between endemic, native non-endemic and introduced species; (3) what are the contributions of species replacement and species richness differences to beta diversity partitioning along the elevation gradient? Previous studies on the impact of land-use changes on the Azorean arthropod fauna have shown that endemic and native non-endemic species tend to be restricted to native vegetation [50,51,52,53,54,55], while introduced species favour non-native and disturbed vegetations [51,52]
A total of 134 species were recorded at the five transect sites; see Supporting Information, Table S1 for a list of all species and their recorded altitudinal ranges. This included all the orders identified to species level, which represented almost 40% of the specimens collected in summer, but only about 25% of the specimens collected over the whole year and 9% of the specimens collected at 930 m a.s.l
Summary
Biodiversity is a complex concept that is most often assessed by analysing taxonomic species richness at different spatial scales [1,2]. Lande [4] suggested an additive partition proposing that gamma = alpha + beta. This meant that beta diversity could be measured in terms of species numbers in the same way as alpha and gamma, instead of being the unitless measure of turnover resulting from multiplicative partition. This framework addresses the effects of spatial scale, which is an essential factor in diversity studies [5,6,7]. Beta diversity can be partitioned into turnover and nestedness-resultant dissimilarity/richness difference components [9]
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