Anthropogenic bottom-up and top-down impacts on boreal breeding waterbirds.

Wetland habitats are changing under multiple anthropogenic pressures. Nutrient leakage and pollution modify physico-chemical state of wetlands and affect the ecosystem through bottom-up processes, while alien predators affect the ecosystems in a top-down manner. Boreal wetlands are important breeding areas for several waterbird species, the abundances of which potentially reflect both bottom-up and top-down ecosystem processes. Here, we use long-term national monitoring data gathered from c. 130 waterbird breeding sites in Finland from the 1980s to the 2020s. We hypothesised that the physico-chemical state of the waters and increasing alien predator abundance both play a role in steering the waterbird population trends. We set out to test this hypothesis by relating population changes of 17 waterbird species to changes in water chemistry and to regional alien predator indices while allowing species-specific effects to vary with foraging niche (dabblers, invertivore divers, piscivorous divers, herbivores), nesting site, female mass and habitat (oligotrophic, eutrophic). We found niche and nesting site-specific, habitat-dependent changes in waterbird numbers. While the associations with higher phosphorus levels and browning water were in overall positive at the oligotrophic lakes, the numbers of invertivore and piscivore diving ducks were most strongly negatively associated with higher phosphorus levels and browning water at the eutrophic lakes. Furthermore, increased pH levels benefitted piscivores. Invertivore diving duck species nesting on the wetlands had declined most on sites with high alien predator indices. Large herbivorous species and species preferring oligotrophic lakes seem to be successful. We conclude that the large-scale breeding waterbird decline in Finland is closely connected to both bottom-up and top-down processes, where negative associations are emphasised especially at eutrophic lakes. Niche-, nest site- and habitat-specific management actions are required to conserve declining waterbird populations. Managing wetlands on catchments level together with alien predator control may provide important approaches to future wetland management.

Burning of logged sites to protect beetles in managed boreal forests.

Natural disturbance-based management and conservation strategies are needed to protect forest biodiversity. Boreal forests of northern Europe are typically clearcut and otherwise intensively managed for timber production. As a result, natural disturbances such as forest fires have became rare and the volume of dead wood has decreased. These changes have had a profound negative effect on species that depend on dead wood (saproxylic). Therefore, it is important to determine whether modifications of forest management methods can enhance the survival of these species. In our study area in southern Finland, we determined whether burning of logged sites and leaving trees (i.e., retention trees) on the sites benefited saproxylic, rare, and red-listed beetle species and how long the burned sites remained suitable habitat for these species. We surveyed the beetle fauna at 40 sites logged 1-16 years previously, 20 of which were burned after logging. The abundance and species richness of saproxylic beetles were positively affected by burning, but the effect depended on the retention of trees in the otherwise clearcut stands. The difference between burned and unburned sites increased with the number of retention trees, and the effect of burning was not significant when there were fewer than approximately 15 retention trees/ha. Most important, the species groups that were unlikely to persist in ordinarily managed forests (rare saproxylic and red-listed beetles), benefited strongly from burning and tree retention. The species richness of saproxylic beetles decreased with time since logging at both burned and at unburned sites. We conclude that burning of logged sites and leaving an adequate number of retention trees may be useful in the conservation of disturbance-adapted species and can be used to improve the environmental quality of the matrix surrounding protected areas. Unfortunately, sites remained high-quality habitat for only a short time; thus, a continuum of burned areas must be ensured.