Verena Keller

A resampling-based method for effort correction in abundance trend analyses from opportunistic biological records

Capsule Resampling data from biological records databases yielded abundance trend estimates better corrected for increasing observation effort. Aims To correct population trend estimates for the effects of annually changing observation effort in analyses of opportunistic data. Methods We developed a resampling-based abundance index for analysis of population trends based on opportunistic citizen-science observations. To correct for the huge recent increase in observation effort every year, we resampled (with replacement) a species-specific constant number of records from the data and computed our index. To validate our standardized index, we used counts from the national waterbird census as a benchmark. Results Over 22 winters (1991–2012), trend estimates based on resampled indices were substantially more similar to waterbird census-based trend estimates than were raw index-based trends. Raw index trends were off by 648% (se 72%) and typically overestimated trends, while trends computed from standardized indices were off by only 125–131% (se 12–13%) and over – and underestimated trends about equally frequently. Hence, our method of effort correction reduced the bias in trend estimates by a factor 5. Conclusion Our resampling method may be useful for improving trend analyses from collections of opportunistic biological records, as they become increasingly available, especially via the internet.

Evidence for an additive effect of hunting mortality in an alpine black grouse Lyrurus tetrix population

In the Canton of Ticino in southern Switzerland data from the monitoring programme for black grouse from 1981 to 2016 (population density, lek size, sex ratio in chicks and adults) were analysed together with information on bag statistics and hunting regulations to evaluate if mortality from hunting had an additive effect. In the study population the proportion of cocks at the beginning of the study period was only 23%. As hunting regulations for black grouse were tightened in the late 1970s the observed proportion of males showed an increase in particular during the first years but remained much lower than what would be expected from the sex ratio among chicks assuming equal survival between the sexes. The observed low proportion of adult males indicates a lower survival rate and as a consequence a smaller than natural lek size. High reproductive success two years before had a positive effect on lek size. The correlation coefficient between hunting bag and population size index increased with increasing hunting pressure and showed a decline over the years. Bag size in the period with a high hunting pressure (1981–1999, many hunting days) was driven by population density whereas in the second period (2000–2016, few hunting days) it was driven by hunting regulations. Our analysis showed that hunting affects population structure and presents indirect evidence that mortality due to hunting is additive. The study also shows that hunting management has to be continuously adapted to changes in the size and structure of the population as well as changes in habitat conditions. It is therefore essential to continue long-term monitoring of population size and of demographic parameters.