1 research outputs found
Environmental correlates of wild bee assemblages in pastoral landscapes in Ireland (Hymenoptera, Apidae)
Declines in wild bees have been reported internationally and attributed to
habitat loss associated with agriculture. Recent research has led to a better
understanding of the responses of bumblebees, especially within mixed,
arable or fruit-growing agricultural landscapes. However gaps remain in
scientific understanding of the responses of bees, particularly of solitary bees,
to agricultural intensification in pastoral landscapes. In general, studies of
bee declines have focussed on measures such as total abundance, species
richness and diversity indices and there is a need for studies of changes in
assemblages’ species composition in order to identify resilient and vulnerable
species.
The aim of this study is to examine the responses of solitary bees and
bumblebees to grassland intensification in lowland, pastoral landscapes. The
study identifies factors that influence wild bee diversity, abundance and
assemblage composition. The relative importance of landscape composition;
agricultural management (considered at the field, farm and landscape level);
habitat structure and quality and more immutable environmental conditions
such as latitude and longitude, altitude and edaphic factors on bee responses
are studied. Based on differences in foraging distances and social behaviour,
it is likely that solitary bees and bumblebees respond to anthropogenic
change occurring at different scales. This hypothesis is tested.
A field survey of bees across fifty agricultural sites, together with a survey of
the environmental conditions at these sites was undertaken. Correlations
were investigated using Mantel’s tests and Mantel correlograms, Procrustean
rotations and indirect gradient analyses with ordinations. The relative
importance of environmental variables was evaluated using a combination of
methods. The variance in species composition of bee assemblages was
decomposed between environmental predictors using distance-based
Redundancy Analysis (RDA). Bayesian and Information theoretic methods
were used to evaluate the relative importance of predictors of bee diversity
and abundance.
The abundance and species richness of bees in pastoral landscapes in Ireland
are impacted by intensifying grassland management, with solitary bees
showing a response to intensification at the field scale and bumblebees to
intensification at the landscape scale. A shift in bee assemblage composition
from assemblages dominated by diverse solitary bees to assemblages
dominated by a few common bumblebees and a small number of solitary bee
species was observed as field management became more intense. This
gradient was also associated with a calcicole-calcifuge vegetation gradient
and it is not possible to distinguish between the effects of the two factors on
bee assemblage. Solitary bees showed greater sensitivity to site management
than bumblebees, possibly due to previous local extinctions of more sensitive
bumblebee species from the majority of farm sites. A model of extinction order
from wild bee assemblages in the face of intensifying grassland management
is proposed.
There is high natural variability of bee assemblage composition, associated
with solitary bees. Biogeographical influences were more influential in
shaping assemblage composition than agricultural management. Spatial
effects upon bees were significant for distances up to approximately 10km.
Associations with vegetation contributed to this spatial pattern. Autogenic
factors were also influential in spatial patterning of solitary bees.
The mechanisms by which intensification of grassland management impacts
on bees remain to be identified. To date, agri-environmental schemes have
focused on the conservation of hedgerows. The conservation of bees in
pastoral landscapes requires initiatives focused on grassland management at
both the field and landscape scale. This will aid the conservation of common
bee species and the maintenance of pollination services across regions. Rarer
bees require conservation initiatives targeted at the locations where they
survive. This study has suggested characteristics of such locations which may
aid in their identification. The proposed ‘order of extinction’ model requires
validation. It has potential to be used in quantifying the level of impact on
bee assemblages and in monitoring the effects of environmental degradation
and restoration on wild bees