How much survey effort is enough?

New paper by Jacqueline Loos, Jan Hanspach, Henrik von Wehrden, Monica Beldean, Cosmin I. Moga and Joern Fischer: Developing robust field survey protocols in landscape ecology: a case study on birds, plants and butterflies, Biodiversity and Conservation, DOI 10.1007/s10531-014-0786-3.

Alma Vii 14.7 (2)

In order to better understand biodiversity patterns along landscape gradients, powerful data is needed to detect relations to environmental parameters. However, given financial and logistics constraints, ecologists often face a trade-off between the number of sites they can survey and the necessity to repeat surveys in the same site (for example, to cover seasonal variation in species composition). With our recent publication in Biodiversity and Conservation, we present an assessment of the trade-offs between alternative survey strategies for plants, birds and butterflies in Southern Transylvania. This pilot study helped designing surveys on a larger spatial scale, which we conducted as a follow-up.

In this study, we applied different survey techniques, including a so-called “cartwheel approach” for plants, in which we randomly placed ten one square meter plots within a round-shaped one hectare site. In the same sites, we conducted ten-minute point counts for birds and we adapted 200 m Standard Pollard walks for butterflies.

We then reduced the total sampling size for each taxon and investigated whether species richness, species turnover and species composition changed. We correlated the pattern that we achieved from the “full survey effort” with results from randomly “reduced survey effort”.

1

Correlations between data from reduced survey effort (1 to 9 plots for plants; 1 to 3 repeats for birds and butterflies) and the maximum survey effort (10 plots for plants; 4 repeats for birds and butterflies). Reduced survey effort was simulated by randomly sub-setting the full data set 1,000 times for each level of data reduction.

We also conducted a power analysis, which allowed us to estimate the required number of survey sites to being able to detect landscape effects on species richness. Based on the patterns we observed in the correlations, we concluded that it is possible to reduce survey effort without losing the “bigger picture” of species richness distribution. Overall, this study showed us that in the highly heterogeneous farmland of Southern Transylvanian, at least three temporal replicates on at least 100 study sites were required to find landscape scale effects on diversity patterns of birds and butterflies, while for plants, seven one square meter plots in at least 100 sites showed sufficient power to detect trends.

2

Power analysis with simulated data. Minimum detectable effect (MDE) is plotted as a function of the number of survey sites. MDE was defined as the absolute change in species richness along the observed heterogeneity gradient in arable fields that could be etected in a linear model with given sample size.

We recommend other landscape ecologist to conduct pilot studies in order to test and adapt different sampling schemes before conducting their main study. By doing so, it is possible to identify the most efficient use of available project resources. With the help of our study, we detected that diversity patterns remained relatively stable within certain thresholds.

The full paper is available here: http://link.springer.com/article/10.1007/s10531-014-0786-3?sa_campaign=email/event/articleAuthor/onlineFirst

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