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