Contribution of rare and common species to subterranean species richness patterns

Abstract

Aim: Common species contribute more to species richness patterns (SRPs) than rare species in most studies. Our aim was to test this hypothesis using a novel model system, species living exclusively in subterranean habitats. They consist of mainly rare species (small ranges), only a few of them being common (large ranges), and challenge whether rare species are less important for the development of SRPs in this environment. We separately analyzed aquatic and terrestrial species.

Location: Western Balkans in southeastern Europe.

Methods: We assembled two datasets comprising 431 beetle and 145 amphipod species, representing the model groups of subterranean terrestrial and aquatic diversity, respectively. We assessed the importance of rare and common species using the stepwise reconstruction of SRPs and subsequent correlation analyses, corrected also for the cumulative information content of the subsets based on species prevalence. We applied generalized linear regression models to evaluate the importance of rare and common species in forming SRPs. Additionally, we analyzed the contribution of rare and common species in species-rich cells.

Results: Patterns of subterranean aquatic and terrestrial species richness overlapped only weakly, with aquatic species having larger ranges than terrestrial ones. Our analyses supported higher importance of common species for forming overall SRPs in both beetles and amphipods. However, in stepwise analysis corrected for information content, results were ambiguous. Common species presented a higher proportion of species than rare species in species-rich cells.

Main conclusion: We have shown that even in habitats with the domination of rare species, it is still common species that drive SRPs. This may be due to an even spatial distribution of rare species or spatial mismatch in hotspots of rare and common species. SRPs of aquatic and terrestrial subterranean organisms overlap very little, so the conservation approaches need to be habitat specific.

Keywords: Dinarides; amphipods; beetles; caves; conservation planning; endemism; range size.

Figure 1

Representatives of the two taxonomic groups used in this study: Left is Niphargus stenopus—subterranean amphipod (photograph T. Delić) and right is Spelaeodromus pluto—subterranean beetle (photograph: P. Bregović)

Figure 2

Distribution of localities (amphipods—blue triangles; beetles—black dots) used in analyses of subterranean species richness pattern in Western Balkans in southeastern Europe. Gray color denotes karstic areas (Lambert Conformal Conical Projection)

Figure 3

Species richness patterns of subterranean amphipods (left column) and beetles (right column) in Western Balkans, presented per 20 × 20 km grid cell size, for (a, b) all species, (c, d) rare species,and (e, f) common species. Class delimitation is defined according to the highest number of species per cell, as described in the text. Exception are rare species of amphipods, where categories are different due to small number of species per cell

Figure 4

Range size frequency distribution for amphipods (gray) and beetles (black). Rectangle inside the plot is enlarged part of the histogram until 300 km. Red line indicates the percentage of single‐site species (23% in amphipods, 31% in beetles)

Figure 5

The correlation coefficients Kendall τ _b calculated between the species richness of subset and overall dataset, plotted against the percentage of species included in particular subset (a, b), or against the percentage of cumulative information in each subset, that is, expected binomial variance of subset's richness patterns (c, d). The subsets were built by addition of species one‐by‐one, ascendingly (yellow) or descendingly (blue) with respect to their MLEs. Single‐site species (MLE = 0 km), were included in the subsets at random; the plots show only median values of Kendall τ _b of 10,000 repetitions. The shaded region represents the null model generated by 10,000 random one‐by‐one additions of species regardless their MLEs. Red line indicates the percentage of species at which correlation coefficient exceeded .5 (solid line—descending order, dashed line—ascending order). The empirical p‐value indicates the probability of each point occurring at a distance from the median of the null distribution. (a, c) amphipods, (b, d) beetles

Figure 6

Plots of univariate regression between overall species richness and the number of species in 1st quartile—rare species (a, b) or 4th quartile—common species (c, d) in cells of the studied area. The blue line represents regression line according to generalized linear model; gray area is 95% confidence interval. AICc is Akaike's information criterion corrected for small sample size. (a, c) amphipods, (b, d) beetles

Figure 7

Contribution of species of different range quartiles to species‐rich cells (SRCs) for amphipods (a) and beetles (b). The x‐axis shows total number of species per cell, while y‐axis shows a percentage of species of certain range quartile within each SRC. Colors denote: Orange—1st quartile (rare species), dark gray—2nd quartile, light gray—3rd quartile, and blue—4th quartile (common species). The numbers refer to exact percentage of species within each SRC. Note that on average (blue line) common species represent higher share of species richness in SRCs than rare species (orange line). Note also that this effect is more pronounced in beetles