Abstract
Large‐scale biodiversity studies can be more informative if observed diversity in a study site is accompanied by dark diversity, the set of absent although ecologically suitable species. Dark diversity methodology is still being developed and a comparison of different approaches is needed. We used plant data at two different scales (European and seven large regions) and compared dark diversity estimates from two mathematical methods: species co‐occurrence (SCO) and species distribution modeling (SDM). We used plant distribution data from the Atlas Florae Europaeae (50 × 50 km grid cells) and seven different European regions (10 × 10 km grid cells). Dark diversity was estimated by SCO and SDM for both datasets. We examined the relationship between the dark diversity sizes (type II regression) and the overlap in species composition (overlap coefficient). We tested the overlap probability according to the hypergeometric distribution. We combined the estimates of the two methods to determine consensus dark diversity and composite dark diversity. We tested whether dark diversity and completeness of site diversity (log ratio of observed and dark diversity) are related to various natural and anthropogenic factors differently than simple observed diversity. Both methods provided similar dark diversity sizes and distribution patterns; dark diversity is greater in southern Europe. The regression line, however, deviated from a 1:1 relationship. The species composition overlap of two methods was about 75%, which is much greater than expected by chance. Both consensus and composite dark diversity estimates showed similar distribution patterns. Both dark diversity and completeness measures exhibit relationships to natural and anthropogenic factors different than those exhibited by observed richness. In summary, dark diversity revealed new biodiversity patterns which were not evident when only observed diversity was examined. A new perspective in dark diversity studies can incorporate a combination of methods.
Highlights
Analysis of large-scale species richness patterns is an important tool for ecology, biogeography and conservation biology (Gaston 2000)
We aim to determine (1) whether species co-occurrence (SCO) and species distribution modeling (SDM) methods give similar dark diversity estimates; (2) the extent and significance of overlap between species composition in these dark diversities estimates; (3) whether consensus and composite dark diversity form different distribution patterns; and (4) whether different estimates of dark diversity and completeness of site diversity are related to observed richness, and whether they differ in their relations to various natural and anthropogenic factors
At the European scale both methods were fairly similar at intermediate dark diversity values; at the regional scale, the difference between dark diversity estimates was greater at low values but roughly agreed at larger values
Summary
Analysis of large-scale species richness patterns is an important tool for ecology, biogeography and conservation biology (Gaston 2000). An important facet to the study of diversity patterns can be obtained by including the set of species that could potentially inhabit a study site but are currently absent – the dark diversity (P€artel et al 2011a; Ronk et al 2015), that is, the absent portion of the site-specific (i.e., both abiotic and biotic filtered) species pool (Cornell and Harrison 2014; Zobel 2016). Dark diversity cannot be measured directly from local plant inventories (as opposed to observed species richness), rather it is estimated indirectly (P€artel et al 2011a)
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