Abstract
Setting relevant temporal baselines is critical to understanding biodiversity change and the full impact of various pressures on biodiversity. Current knowledge of biodiversity change in European boreal waterbird communities is based on monitoring and other data from the last 35 years. However, the impact of the presumed main drivers of changes in these communities, i.e., eutrophication and alien predators, started decades before this. We used data of 35 breeding waterbird communities, representing both oligotrophic and eutrophic lakes, in southern Finland from 1951–1970 and 1996–2015 to study changes in biodiversity against a baseline from a period when the presumed main drivers were not yet fully effective. We found that species richness increased from 1951–1970 to 1996–2015 at oligotrophic lakes but not at eutrophic lakes; total abundance in turn increased at the former lake type but decreased at the latter. Breeding numbers of many historically abundant species declined at the eutrophic lakes to such a degree that the increases of other species were not sufficient to compensate for the declines. Population increases prevailed at the oligotrophic lakes and the slight declines of some previously less abundant species, were compensated for. The species level results revealed that local abundances of different species likely are affected by different drivers, suggesting that we need an autecological approach in the conservation management of boreal waterbird communities. Increased predation risk rather than eutrophication appeared to be the main biodiversity stressor in the waterbird communities studied.
Highlights
There is a wide consensus that biodiversity at the global scale is in decline, and we have a good understanding of what the main global drivers impacting biodiversity are (Sage, 2020)
The lakes that were a priori classified oligotrophic are situated at higher elevations (m from sea level; mean ± SD; 109.9 ± 18.4, n = 16) than the lakes that were classified eutrophic (t = 4.386, df = 34, p < 0.001). This difference gives further support to the relevancy of our lake classification in terms of trophic status, as Lindholm et al (2019) found in the nearby study area that lakes high in the landscape are surrounded by conif erous forest and peatlands and have low macrophyte species richness, while lakes low in the landscape are surrounded by arable lands and human settlements and generally have high macrophyte species richness
We considered the following metrics of biodiversity in local communities: species richness, total abundance, species-specific abundances and species proportional abundances
Summary
There is a wide consensus that biodiversity at the global scale is in decline, and we have a good understanding of what the main global drivers impacting biodiversity are (Sage, 2020). Setting the baselines for biodiversity change appropriately is important for several reasons. We need to set the baseline appropriately with respect to the action of the presumed main drivers of change to assess their relative importance and understand the full impact of various pressures on biodiversity (Collins et al, 2020). Due to the shifting baseline syndrome (Pauly, 1995), our expectations of the state of the natural environment may alter. The syndrome arises if each generation of researchers accepts as a baseline the condition of the natural environment that occurred at the beginning of their careers and uses it to evaluate changes; this is likely to result in gradual acceptance of increasingly degraded envi ronmental conditions as a baseline (Pauly, 1995; Soga and Gaston, 2018). Historical data of species’ population sizes are important for judging changes in biodiversity, because abundance itself is among the most important essential biodiversity variables (Schmeller et al, 2018), and data on population abundances provide the most useful basis for
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