Ecological barriers mediate spatiotemporal shifts of bird communities at a continental scale.

Species' range shifts and local extinctions caused by climate change lead to community composition changes. At large spatial scales, ecological barriers, such as biome boundaries, coastlines, and elevation, can influence a community's ability to shift in response to climate change. Yet, ecological barriers are rarely considered in climate change studies, potentially hindering predictions of biodiversity shifts. We used data from two consecutive European breeding bird atlases to calculate the geographic distance and direction between communities in the 1980s and their compositional best match in the 2010s and modeled their response to barriers. The ecological barriers affected both the distance and direction of bird community composition shifts, with coastlines and elevation having the strongest influence. Our results underscore the relevance of combining ecological barriers and community shift projections for identifying the forces hindering community adjustments under global change. Notably, due to (macro)ecological barriers, communities are not able to track their climatic niches, which may lead to drastic changes, and potential losses, in community compositions in the future.

Mechanisms of density dependence in ducks: importance of space and per capita food.

The growth rate of populations usually varies over time, often in a density-dependent manner. Despite the large amount of literature on density dependence, relatively little is known of the mechanisms underlying the density-dependent processes affecting populations, especially per capita natality. We performed a 20-year study on the density dependence of brood production in two duck species differing in the stability of habitat use. Our study was conducted in a boreal watershed in southern Finland. We predicted that a diving duck common goldeneye Bucephala clangula, with more stable habitat use, would show stronger density dependence than a dabbling duck common teal Anas crecca. We investigated reproductive output in relation to the duck pair density per se as well as in relation to per capita food availability. As predicted, the reproductive output of the goldeneye showed a more density-dependent pattern than that of the teal. The number of goldeneye broods per pair decreased when the pair density increased. This was not the case with the teal. However, when the breeding success was measured by taking into account per capita food availability, both species showed density dependence. Our results imply that the occurrence of density dependent processes may vary even in sympatric ducks breeding in the same, relatively stable landscape. Our analysis also emphasizes that it is important to take into account per capita resource availability when studying the density dependence of breeding success. Both findings have important implications for the management and conservation of species.

Balancing between predation risk and food by boreal breeding ducks.

Wetlands belong to the globally most threatened habitats, and organisms depending on them are of conservation concern. Wetland destruction and quality loss may affect negatively also boreal breeding ducks in which habitat selection often needs balancing between important determinants of habitat suitability. In Finland duck population trajectories are habitat-specific, while the reasons behind are poorly understood. In this research, we studied the balance of nest predation risk and invertebrate food abundance in boreal breeding ducks in Finland at 45 lakes and ponds in 2017 and 2018. We surveyed duck pairs and broods from these and 18 additional water bodies. We evaluated nest predation by monitoring artificial nests with camera traps over a 7-day exposure period and sampled invertebrates from water bodies using emergence and activity traps. Camera trap results indicate that predation risk was higher in the water bodies surrounded by agricultural land than in forestland. Ponds (seasonal, beaver, and man-made) had lower nest predation risk, and they were also more invertebrate-rich than permanent lakes. In addition, artificial nests further away from water bodies had higher survival than shoreline nests. Habitat use of duck pairs was not associated with invertebrate food, but duck broods preferred habitats rich in food. High nest predation pressure in shorelines of especially agricultural landscapes may contribute to the declining population trends of ducks in Finland. Controlling predators could be an important conservation action to improve duck breeding success. This research underlines the benefits of the availability of different water body types for breeding ducks. There is an urgent need to pay attention to protecting seasonal ponds, while the lack of flooded waters may be mitigated by favouring beavers or creating man-made ponds.

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.

Role of forest ditching and agriculture on water quality: Connecting the long-term physico-chemical subsurface state of lakes with landscape and habitat structure information.

Increasing anthropogenic pressures have affected the status of surface freshwater ecosystems. Eutrophication, water browning, acidification, and several other processes may be channelled through the food web. In this study, we evaluate the role of hydrology impacting anthropogenic pressures, flows from urban, farmland and ditched forest areas, and how they explain the physico-chemical quality of lakes and ponds in the boreal biome of Finland. We study the long-term effect around 445 waterfowl survey sites that had physico-chemical measurements (total phosphorus, total nitrogen, pH, water clarity and colour) produced by Finnish environmental authorities done in years 1986-2020. Furthermore, we investigate whether a long-term national-level citizen science study focusing on rather robust visible habitat structures measured by the volunteers can reveal physico-chemical water quality using data from >270 lakes where the waterfowl habitat survey and physico-chemical measurements could be spatio-temporally matched. Farmland occurrence around the lakes was positively associated with pH, colour and nutrient concentrations but negatively associated with water clarity. Furthermore, ditch length was positively associated with nitrogen concentration and water colour, while being negatively associated with pH and water clarity. Overall, the studied lakes showed a negative trend in nutrients and clarity but a positive trend in pH and colour. As expected, nutrient concentration increased and clarity decreased along the gradient from oligotrophic to eutrophic lake habitat classifications, which suggests that the citizen science classification seem to reflect the subsurface physico-chemical status of the lakes. We conclude that farming and forest ditching practices in particular seem to associate with the state of the study lakes and that the ecological impacts of intensified turbidity and brownification in wetland ecosystems should be studied further in the future. Sustainable improvement of water quality rests upon scientific understanding of biogeochemical processes in lake ecosystems and the primary sources of the nutrient and sediment loading.

Invertebrates are declining in boreal aquatic habitat: The effect of brownification?

Surface water browning affects boreal lakes in the Northern Hemisphere. This process is expected to increase with global warming. Boreal lakes are the most numerous lakes on Earth. These ecosystems are particularly sensitive to disturbances due to their low biodiversity compared to other aquatic environments. The recent darkening of surface water is expected to hinder key ecosystem processes, particularly through lower primary productivity and loss of biodiversity. However, studies based on long-term data collections have rarely been conducted on the ecological consequences of water browning on aquatic food webs, especially concerning its impacts on invertebrate communities. For the first time, our analysis based on two decades of data collection in Finnish lakes highlighted a relation between water browning and a decline in aquatic macroinvertebrate abundances. Aquatic invertebrates are the main food resource for many secondary predators such as fish and waterbirds, hence such effect on their populations may have major consequences for boreal ecosystem functioning.

Changes in species richness and composition of boreal waterbird communities: a comparison between two time periods 25 years apart.

Global measures of biodiversity indicate consistent decline, but trends reported for local communities are more varied. Therefore, we need better understanding of mechanisms that drive changes in diversity of local communities and of differences in temporal trends between components of local diversity, such as species richness and species turnover rate. Freshwater ecosystems are vulnerable to multiple stressors, and severe impacts on their biodiversity have been documented. We studied species richness and composition of local boreal waterbird communities in 1990/1991 and 2016 at 58 lakes distributed over six regions in Finland and Sweden. The study lakes represented not only local trophic gradients but also a latitudinal gradient in the boreal biome. While species richness tended to be lower in 2016 than in 1990/1991, species turnover was relatively high. Within foraging guilds, local species richness of diving ducks and surface feeding waterbirds decreased, whereas that of large herbivores increased. The number of species gained in local communities was higher in lakes with rich vegetation than in lakes with sparse vegetation. Conservation of boreal freshwater ecosystems would benefit from recognizing that large-scale environmental changes can affect local diversity via processes operating at finer scales.