Background & Aims: Due to the universal distance decay in ecological similarity, the shape of the sampling unit should affect species richness counts, with more species being found in elongated vs. compact units. Similarly, where a sampling unit consists of several incontiguous plots within a larger spatial extent (e.g. rarefaction curves), it should contain more species than a contiguous unit of the same area (see simulation of Dengler & Oldeland 2010). Although the differences of elongated vs. compact (shape) and incontiguous vs. contiguous (arrangement) are theoretically clear, ecologists rarely acknowledge them when comparing biodiversity data originating from different sampling schemes, and from the few existing studies the effect sizes are hard to assess (Dengler 2008). With our study we thus aimed at quantifying the relative differences in species richness counts resulting from varying shapes and arrangements of sampling units at different, small spatial grain sizes. Methods: We used monitoring plots of the BiodivERsA project SIGNAL in semi-natural grasslands of six Eurasian countries (France, Germany, Italy, Hungary, Bulgaria, Turkey). In each study site we established six blocks of 2.80 m x 0.40 m, subdivided into 448 microquadrats of 25 cm² and recorded the vascular plant species composition in each of these. Then we calculated species richness values for different sampling unit sizes (4, 16, 64 microquadrats) for different shapes (1:1; 1:4, 1:16) and arrangements (contiguous vs. incontiguous, drawn from different extents). We tested for differences by means of linear mixed models. Main Results & Interpretations: Both shape and arrangement had highly significant effects on richness values of sampling units, and these responses were consistent across the six countries and the sampling unit sizes. Generally, the differences between squares and 1:4 rectangles were negligible while 1:16 plots clearly showed increased richness. This indicates that compact and slightly elongated plots of the same size can be combined without problems in the same study, while serious distortions are to be expected only for extremely long and thin plots. In contrast, the contiguous and the various incontiguous arrangements showed significant differences in species richness, with richness increasing strongly with the spatial extent from which the subplots were drawn. This shows that rarefaction curves are highly idiosyncratic and should not be compared between studies because their spatial extent is hardly ever the same (see also Chiarucci et al. 2009).

Effects of plot shape and arrangement on species richness counts in grasslands

CAMPETELLA, Giandiego;CANULLO, Roberto;
2014-01-01

Abstract

Background & Aims: Due to the universal distance decay in ecological similarity, the shape of the sampling unit should affect species richness counts, with more species being found in elongated vs. compact units. Similarly, where a sampling unit consists of several incontiguous plots within a larger spatial extent (e.g. rarefaction curves), it should contain more species than a contiguous unit of the same area (see simulation of Dengler & Oldeland 2010). Although the differences of elongated vs. compact (shape) and incontiguous vs. contiguous (arrangement) are theoretically clear, ecologists rarely acknowledge them when comparing biodiversity data originating from different sampling schemes, and from the few existing studies the effect sizes are hard to assess (Dengler 2008). With our study we thus aimed at quantifying the relative differences in species richness counts resulting from varying shapes and arrangements of sampling units at different, small spatial grain sizes. Methods: We used monitoring plots of the BiodivERsA project SIGNAL in semi-natural grasslands of six Eurasian countries (France, Germany, Italy, Hungary, Bulgaria, Turkey). In each study site we established six blocks of 2.80 m x 0.40 m, subdivided into 448 microquadrats of 25 cm² and recorded the vascular plant species composition in each of these. Then we calculated species richness values for different sampling unit sizes (4, 16, 64 microquadrats) for different shapes (1:1; 1:4, 1:16) and arrangements (contiguous vs. incontiguous, drawn from different extents). We tested for differences by means of linear mixed models. Main Results & Interpretations: Both shape and arrangement had highly significant effects on richness values of sampling units, and these responses were consistent across the six countries and the sampling unit sizes. Generally, the differences between squares and 1:4 rectangles were negligible while 1:16 plots clearly showed increased richness. This indicates that compact and slightly elongated plots of the same size can be combined without problems in the same study, while serious distortions are to be expected only for extremely long and thin plots. In contrast, the contiguous and the various incontiguous arrangements showed significant differences in species richness, with richness increasing strongly with the spatial extent from which the subplots were drawn. This shows that rarefaction curves are highly idiosyncratic and should not be compared between studies because their spatial extent is hardly ever the same (see also Chiarucci et al. 2009).
2014
9780958476652
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11581/369982
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