INTRODUCTION
Humans have altered wetland ecosystems in numerous ways across the globe, especially through drainage for agricultural land (Davidson, 2014; Gibbs, 2000; Hu et al., 2017; Kingsford et al., 2016). In boreal forest areas, peatlands and other riparian forest habitats have been ditched to increase wood production (Kuusisto et al., 1998). In addition to direct wetland destruction, climate warming (McMenamin et al.,2008) and over-exploitation of wetland creating beavers (Halley et al. 2021) have had an indirect effect on the amount and distribution of wetlands. Furthermore, wetland quality is threatened for instance due to eutrophication and alien species introductions (Fox et al., 2019; Ramsar Convention Secretariat 2010; Ma et al., 2010; Guillemain et al.,2013; Nummi et al., 2019a). All in all, human-induced environmental wetland change has therefore impacted aquatic animals at multiple levels of organization from individuals to landscapes (Sievers et al., 2018).
Habitat characteristics are important determinants of breeding densities and production of ducks on boreal wetlands (Holopainen et al.,2015). Many boreal lakes lack the habitat structure and sufficient food resources to support breeding ducks, making them unsuitable for brood rearing (Sjöberg et al., 2000). Indeed, duckling mortality at lakes with limited food resources, in particular invertebrates, is high (Gunnarsson et al., 2004; Nummi & Hahtola, 2008). In spring, however, patterns of snowmelt create annual variation in the nature and extent of shallow flooded lakeshores affecting littoral ecosystem productivity (Larmola et al., 2004). Seasonal floods and ponds that commonly dry during the summer offer important food-rich habitats for ducks (Holopainen et al., 2014), despite their highly variable occurrence. In addition, habitat engineering by beavers (Castor spp.) modifies oligotrophic, sharp-edged boreal lakes into productive shallow wetlands with ambiguous shorelines. Both beaver ponds and seasonal ponds typically have varying shorelines and possibly no fish or low fish densities (Nummi & Hahtola, 2008).
Habitat selection of breeding ducks is not straight-forward, but will possibly lead to trade-off situations, both between and within different stages of the breeding season. For example, experimental data by Gunnarsson and Elmberg (2008) suggests a trade-off between wetland use and nest survival in forested versus agricultural landscapes in the mallard (Anas platyrhynchos ); wild waterfowl, including mallard, seemed to prefer agricultural landscape, while facing higher nest predation risk there. While predation risk largely determines nest site use and nesting success, food resources and habitat structure are the key characteristics affecting habitat use by duck pairs and broods as well as subsequent breeding success in boreal lakes (Holopainen et al., 2015). At wetlands, complex habitat structure and luxuriant vegetation are linked, as the abundance of emergent vegetation typically increases from nutrient-poor oligotrophic to nutrient-rich eutrophic lakes (Kauppinen & Väisänen, 1993; Holopainen & Lehikoinen, 2021).
Changes in important boreal environmental characteristics may already have affected breeding ducks negatively. Finnish national duck pair surveys show declining trends for several species, but those breeding in eutrophic lakes have declined more than in oligotrophic lakes (Lehikoinen et al., 2016; Pöysä et al., 2013). In addition to detrimental effects of over-growth of eutrophic waters, disproportionally increased predator pressure is one of the suspected reasons for the differences in population trajectories between habitats and also between species within habitats (see Pöysä et al., 2019; Pöysä & Linkola 2021), potentially impacting flyway-level trends in population size and structure (e.g. Brides et al., 2017). Indeed, artificial duck nest experiments with wildlife cameras in northern Europe have shown that nest predation rate is high especially around wetlands and in agricultural landscape, where also alien mammal species now occur (Holopainen et al., 2020a). As a result, duck species nesting on eutrophic lakes and especially those preferring shorelines as nesting places have, in all likelihood, experienced a major increase in predator diversity and abundance, which may have contributed to their declining population trends (Holopainen et al., 2021; Pöysä & Linkola, 2021).
In this article the complex habitat-based associations with ducks breeding success will be analysed. We will assess whether and how habitat selection and brood production by boreal breeding ducks result from a trade-off between nest predation risk and food availability. Specifically, wildlife cameras were used at artificial nests (mimicking dabbling duck nests) to measure nest predation risk at both the local habitat (shoreline nests versus forest nests) and landscape (proportion of agricultural land versus forest in the landscape) scales. Next, habitat selection of both breeding pairs and broods were assessed emphasising the role of landscape and food availability (invertebrate abundance). In addition, brood production at the landscape scale was measured. The hypothesis is that while eutrophic wetlands in agricultural landscapes typically produce more invertebrates, they will also have a higher nest predation rate and thus lower brood production. Furthermore, contrary to permanent lakes, it is expected that flooded ponds offer the most food-rich brood habitats but also safe nesting places.
METHODS