Similarity between agricultural and natural land covers shapes how biodiversity responds to agricultural expansion at landscape scales

The impact of agriculture on biodiversity depends on the extent and types
of agriculture and the degree to which agricultural land contrasts with
the natural ecosystem. Most research on the latter comes from studies on
the influence of different agricultural types within a single ecosystem
with far less study on how the natural ecosystem context shapes the
response of biodiversity to agricultural production. We used citizen
science data from agricultural areas in Canada’s Eastern Hardwood-Boreal
(forest ecosystem, n=108 landscapes) and Prairie Pothole (prairie
ecosystem, n=99) regions to examine how ecosystem context shapes the
response of avian species diversity, functional diversity and abundance to
the amount of arable crop and pastoral agriculture at landscape scales.
Avian surveys were conducted along 8km transects of Breeding Bird Survey
routes with land cover assembled within a 20km2 landscape around each
transect. The amount of agriculture at which species diversity peaked
differed between the forest (15%) and prairie (51%) ecosystems, indicating
that fewer species tolerated the expansion of agriculture in the former.
In both ecosystems, functional diversity initially increased with
agriculture and peaked at higher amounts (forest: 42%, prairie: 77 %) than
species diversity suggesting that functional redundancy was lost first as
agriculture increased. Species turnover with increasing agriculture was
primarily among functional groups in forest where a shift from a low to a
high agriculture landscape led to a decline in the percent of the
community represented by Neotropical migrants, insectivores, upper foliage
gleaners and bark foragers, and an increase in the percent of the
community represented by short-distance migrants, granivores, omnivores
and ground gleaners. There were few distinct shifts in the percent of the
community represented by different functional groups in the prairie
ecosystem. Total abundance was the least sensitive measure examined in
both ecosystems and indicated that species losses with agriculture are
likely followed by numerical compensation from agriculture tolerant
species. Our results highlight the importance of ecosystem context for
understanding how biodiversity is affected by agricultural production with
declines in diversity occurring at lower agricultural extents in
ecosystems with lower similarity between natural and agricultural land
covers. These findings allow for more specific conservation
recommendations including managing for species intolerant to agriculture
in prairie ecosystems and limiting the expansion of high contrast
agriculture and the loss of semi-natural habitat, such as hedge rows, in
historically forested ecosystems.