Search

Bumblebees inhabit spatially heterogeneous landscapes that are likely characterized by population sources and sinks. To date, most studies of bumblebee habitat requirements have examined static relationships between worker abundance and habitat characteristics. However, if habitat types are linked by dispersal, source‐sink dynamics could alter overall population sensitivity to habitat loss, changing conclusions from static approaches. Here, we used empirically derived spatial matrix models to study source‐sink population dynamics of bumblebees in heterogeneous environments. We used these models to assess population sensitivity to habitat loss and to examine the population dynamics that could occur under rapid habitat change. Loss of natural habitat reduced long‐term population growth rates, and more natural habitat was required to sustain bumblebee species with long‐distance dispersal than species with short‐distance dispersal. The long‐term relative density of colonies in each habitat type depended on landscape structure and bumblebee dispersal ability. Under most conditions, high‐quality habitat contained higher densities of colonies than low‐quality habitats. However, low‐quality habitat could contain higher densities than high‐quality habitats if populations were in long‐term decline. Rapid loss or restoration of natural habitat produced transient population dynamics that differed from long‐term dynamics. After large landscape perturbations that affected population structure, transient dynamics persisted for 4–8 years and were longest for species with short dispersal in highly aggregated landscapes. While transient dynamics were short in duration, they caused large effects on long‐term population density. Synthesis and applications. When habitats are linked by dispersal or populations have recently experienced perturbations, patterns of abundance on the landscape may not reflect variation in habitat quality. Spatially structured matrix population models are practical tools than can be used to account for nonequilibrium and source‐sink dynamics. For bumblebees, population sensitivity to habitat loss and the duration of transient dynamics depend on dispersal ability and landscape configuration. Demographic approaches such as ours can help to disentangle population patterns from processes and will therefore be valuable for guiding conservation and management.

Summary 1 Concern about a global decline in wild pollinators has increased interest in how pollinators are affected by human land use, and how this, in turn, affects crop pollination. 2 We measured wild bee visitation to four summer vegetable crops, and investigated associations between flower visitation rates and land-use intensity at local and landscape scales. We studied 29 farms in New Jersey and Pennsylvania, USA. Over 2 years we recorded >7400 bee visits to crop flowers and identified 54 species of wild bees visiting crops. 3 Wild bees were the dominant flower visitors at three of the four crops studied; domesticated honeybees, Apis mellifera L., provided the remainder of visits. 4 Ordination of the two best studied crops showed that the wild bee species visiting tomato, Solanum lycopersicum L., were distinct from those visiting watermelon, Citrullus lanatus (Thunb.) Matsum. & Nakai. 5 Crop visitation by wild bees was not associated with organic farming, nor with natural habitat cover at either local or landscape scale. 6 Synthesis and applications. The extent of crop visitation by wild bees observed in this study is among the highest recorded. In contrast to previous studies of crop visitation by wild bees, we did not find negative effects of conventional farming or natural habitat loss. In our study system, organic and conventional farms differ little in field size, crop diversity and weedy flower diversity, unlike some systems where organic farms have smaller fields with greater crop and weed diversity. Such variables may be more important than organic vs. conventional farming practices. Second, small patches of natural habitat are dispersed throughout our entire study system, and this habitat heterogeneity may support high bee abundance even in landscapes with a low proportion of natural habitat overall. Our findings suggest that agri-environment schemes and similar programmes that work through local habitat restoration should target farms in intensively agricultural (homogeneous) landscapes to gain maximum conservation benefits.

Summary 1. Global declines in honeybees have led to concerns about negative impacts on food production because of low levels of pollination. This is exemplified in California where the demand for honeybees Apis mellifera to pollinate almond Prunus dulcis is increasing, but problems with honeybee health suggest it may not be sustainable to rely solely on the pollination service of a single species. 2. We investigated the effect of the quantity of surrounding natural habitat, organic management and strips of semi-natural vegetation on flower visitation frequency of wild and managed pollinators and fruit set in 23 California almond orchards (15 conventional, 8 organic). Five conventional and four organic orchards were surrounded by a low percentage ( 30%). A further five conventional orchards with a low percentage of surrounding natural habitat had an adjacent strip of semi-natural vegetation and were included in the study to represent a realistic option for orchard management in intensive agricultural landscapes. 3. Wild bee species visited almond flowers but only in orchards with adjacent semi-natural habitat or vegetation strips. Organic management increased the flower visitation frequencies of hover flies and wild bees. The presence of a strip of semi-natural vegetation in orchards with a low percentage of surrounding natural habitat increased the number of species and the flower visitation frequency by wild pollinators but only at orchard edges and not to the degree seen when natural habitat was abundant. 4. Wild bee species richness and flower visitation frequency, but not honeybee frequency, were related to fruit set. Fruit set increased with increasing percentage of natural habitat surrounding the orchards. Organic farming or the presence of a vegetation strip did not increase fruit set. 5. Synthesis and applications. The restoration of high quality habitat strips along the edges of crop fields in highly intensified agricultural landscapes should be encouraged and monitored to conserve pollinators and to determine whether benefits for agriculture can be realized. Although honeybees are the main and most important pollinating insects for many plants, wild pollinators may be necessary to ensure high fruit set. Organic farming alone will not sustain wild pollination services for almond in California.