Bird predation and landscape context shape arthropod communities on broccoli

Birds increase crop yields via consumption of pests in some contexts but
disrupt pest control via intraguild predation in others. Landscape
complexity acts as an inconsistent mediator, sometimes increasing,
decreasing, or not impacting pest control. Here, we examined how landscape
context and seasonal variation mediate the impact of birds on arthropod
pests and natural enemies, leaf damage, and yields of broccoli (Brassica
oleracea) on highly diversified farms that spanned the USA West Coast. Our
study had two complementary components: a bird exclusion experiment and
molecular diet analysis of 357 fecal samples collected from the most
commonly captured bird species that also foraged in Brassica fields
(American Goldfinch, Spinus tristis; American Robin, Turdus migratorius;
Savannah Sparrow, Passerculus sandwichensis; Song Sparrow, Melospiza
melodia; and White-crowned Sparrow, Zonotrichia leucophrys). Bird access
yielded higher, rather than lower, numbers of pest aphids and increased
their parasitism, while no other arthropods examined were consistently
impacted. Independent of bird presence, percent natural cover in the
landscape sometimes increased and sometimes decreased densities of
arthropods in the mid-growth period, with diminishing impacts in the
late-growth period. Herbivore feeding damage to broccoli leaves decreased
with increasing amounts of natural land cover and in the late-growth
period. Molecular diet analysis revealed that Brassica pests and predatory
arthropods were relatively uncommon prey for birds. Landscape context did
not alter the prey items found in bird diets. Altogether, our
bird-exclusion experiment and molecular diet analysis suggested that birds
have relatively modest impacts on the arthropods associated with broccoli
plantings. More broadly, the limited support in our study for net natural
pest control services suggests that financial incentives may be required
to encourage the adoption of bird friendly farming practices in certain
cropping systems.