Earthworm abundance and availability does not influence the reproductive decisions of black-tailed godwits in an agricultural grassland

Maintaining the biodiversity of agricultural ecosystems has become a
global imperative. Across Europe, species that occupy agricultural
grasslands, such as Black-tailed Godwits (Limosa limosa limosa), have
undergone steep population declines. In this context, there is a
significant need to both determine the root causes of these declines and
identify actions that will promote biodiversity while supporting the
livelihoods of farmers. Food availability, and specifically earthworm
abundance (Lumbricidae), during the pre-breeding period has often been
suggested as a potential driver of godwit population declines. Previous
studies have recommended increasing the application of nitrogen to
agricultural grasslands to enhance earthworm populations and aid
agricultural production. Here we test whether food availability during the
pre-breeding period affects when and where godwits breed. Using
large-scale surveys of food availability, a long-term mark-recapture
study, focal observations of foraging female godwits, and tracking devices
that monitored godwit movements, we found little evidence of a
relationship between earthworm abundance and the timing of godwit
reproductive efforts or the density of breeding godwits. Furthermore, we
found that the soils of intensively managed agricultural grasslands may
frequently be too dry for godwits to forage for those earthworms that are
present. The increased application of nitrogen to agricultural grasslands
will therefore likely have no positive effects on godwit populations.
Instead, management efforts should focus on increasing the botanical
diversity of agricultural grasslands, facilitating conditions that prevent
hardening soils, and reducing the populations of generalist predators.