Ivy Jansen

Importance of core and linear marsh elements for wetland arthropod diversity in an agricultural landscape

The importance of linear habitat elements connecting core habitat patches for biodiversity conservation is still poorly understood. We surveyed reed strips along drainage ditches and reed marshes in an agricultural landscape to assess how both the density of linear habitat elements and the area of core habitat affect diversity and community composition of spiders, ground beetles, and long‐legged flies. For each taxonomic group, species composition of both ‘all’ and ‘typical wetland’ species, but not species richness was different between ditches and marshes. Overall local species richness and richness of species of conservation interest were affected at a landscape scale both by the density of ditches and by the area of core wetland. Strength and direction of these effects differed among groups. An increase in the density of reed ditches positively affected the total species richness of spiders and ground beetles and the species richness of typical wetland ground beetles, but not for long‐legged flies and typical wetland spiders. The positive effects were explained by improved network functionality, rather than by increase in available habitat area at landscape level. The number of red list spiders and long‐legged flies increased only with increasing core wetland area, while no significant effects were found for the number of red list ground beetles. Our study revealed that preserving or increasing the density of habitat corridors (more reed ditches) can be beneficial for the species richness of particular predatory arthropods, including species of conservation concern (especially ground beetles). Other groups react indifferently or are only positively impacted by an increase of core wetland area.

Reply to the comment on 'Working with population totals in the presence of missing data comparing imputation methods in terms of bias and precision' by Bogaart et al.

Bogaart et al. (2017) indicate in their comment that Onkelinx et al. (2017b) misinterpreted some aspects of the Underhill index (UIndex), Species Trends Analysis Tool for birds (birdSTATs) and TRends and Indices for Monitoring data (TRIM), and, as a consequence, do not sufficiently acknowledge the quality of those methods. We agree that some operational choices and underlying assumptions were not fully clear to us. However, if the documentation was incomplete and/or if variants existed, we made choices and filled in gaps, always in favour of Underhill and TRIM, to test our approach as thoroughly as possible and to guarantee a balanced comparison. In fact, in our paper we acknowledge that under certain circumstances, Underhill and TRIM can work properly (see the results and discussion of our paper), but our main point is that multiple imputation covers a broader range of situations and assumptions, and hence it is more flexible and robust. In practice, we cannot always be sure that the assumptions of Underhill or TRIM are valid. In these circumstances, a method that proves to be more robust is preferable. For instance, Bogaart et al. (2017) mention in their fifth point that it is still an open question within the eco-statistical community whether a negative binomial or a quasi-Poisson distribution is more appropriate. However, with multiple imputation, you can make your own choice according to the context or theoretical insight. With an analytical approach, for a new model, the source code needs to be adapted. In addition, in our paper, we caution that an appropriate model must be carefully selected, and we demonstrate what happens with a less appropriate model. In the following, we reply in greater depth to the statements in Bogaart et al. (2017). Each number corresponds to their numbering.