September 2018
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Looking for easier (and cheaper) forest monitoring

The success of any initiative to conserve biodiversity relies on the availability of objective and timely information on its distribution, which is extremely complex and expensive. Counting the number of species in a forest can be overwhelming if one considers several groups of organisms (e.g. insects, spiders, birds, lichens, fungi), also because some of these groups are very difficult to identify. Such a sampling requires hundreds of hours to a team of well-trained field biologists.


If the variation in biological diversity would be congruent across groups of organisms, a single group could be used to draw conclusions on the others. Although the use of indicators is a well-established praxis, it is not yet clear how well these indicators work in European temperate forests. One of the open questions relates to the effect of spatial scale. In our new article “Congruence across taxa and spatial scales: Are we asking too much of species data?, just published in Global Ecology and Biogeography, we tried to find an answer.

We tried to understand whether the biodiversity of one group of species, correlates equally to the biodiversity of other groups, when changing either the grain (the size of the sampling unit) or the extent (the geographical area interested by the sampling).

We assembled biodiversity data for six groups of species (=taxa, singular=taxon): vascular plants, bryophytes, birds, epiphytic lichens, deadwood dependent beetles and wood-inhabiting fungi, collected across 354 plots in 23 forest sites situated in France, Italy and Hungary.


Changing the extent from site to continent

To understand the effect of the extent, we compared cross-taxon correlations when considering plots inside each forest (site extent), and across all forests in our dataset (continental extent). If cross-taxon congruence was not affected by this change, then one would expect the correlation coefficients between the same pair of taxa (e.g. lichens vs plants, or beetles vs birds) to remain constant.


This was not really the case for the number of species (=species richness). Not only the cross-taxon correlations were on average low (graph on the left), but when changing the extent (red vs. blue) some of these became stronger, other weaker, while some switched from being positive to being negative. When we consider the actual identities of the species (=species composition) correlation between pairs of taxa were always positive and they increased in strength with the extent of the analysis (graph on the right).

Changing the grain from plot to site

Results were slightly different when changing the grain. Moving from a finer to a coarser sampling unit (i.e. when considering the whole forest as a unit, instead of each individual plot inside the forest), correlations became on average much stronger, and in this case the results did not change much when considering species richness or composition.


In short, we found that cross-taxon congruence is strongly scale dependent. By increasing the spatial scale (both for extent and grain) broad scale factors (e.g., climate, biogeography) become more decisive than conditions directly or indirectly related to forest structure (e.g., gaps in the tree canopy, occurrence of deadwood). This may change the sign and the degree of congruence between different taxonomic groups. Therefore, when collecting biodiversity data we recommend sampling different species groups, since their diversity pattern are often not congruent. If this is not possible, then one should carefully choose the most promising indicators, and be aware that these only give a partial information.

Generally, the spatial extent has a greater influence on congruence between taxa than spatial grain, and species richness is more sensitive to shifts in spatial scale than species composition.

When considering fine-scale (plot grain, site extent) patterns of species composition we found consistent cross-taxon associations in many sites, especially between plants and other taxa. This is not surprising, because plants are the major structural and functional component of forest ecosystems and therefore influence forest structure and nutrient cycling. Sampling plant species composition in scattered plots across different sites may therefore be effective at summarizing the whole community composition.

Full reference

Burrascano*, S., R. B. De Andrade*, Y. Paillet, P. Ódor, G. Antonini, C. Bouget, T. Campagnaro, F. Gosselin, P. Janssen, A. Persiani, J. Nascimbene, F. M. Sabatini, T. Sitzia, and C. Blasi. 2018. Congruence across taxa and spatial scales: Are we asking too much of species data? Global Ecology and Biogeography 27: 980-990

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