Agricultural intensification with seasonal fallow land promotes high bee diversity in Afrotropical drylands

Abstract

The exponential increase in the human population in tandem with increased food demand has caused agriculture to be the global‐dominant form of land use. Afrotropical drylands are currently facing the loss of natural savannah habitats and agricultural intensification with largely unknown consequences for bees. Here we investigate the effects of agricultural intensification on bee assemblages in the Afrotropical drylands of northern Tanzania. We disentangled the direct effects of agricultural intensification and temperature on bee richness from indirect effects mediated by changes in floral resources. We collected data from 24 study sites representing three levels of management intensity (natural savannah, moderate intensive and highly intensive agriculture) spanning an extensive gradient of mean annual temperature (MAT) in northern Tanzania. We used ordinary linear models and path analysis to test the effects of agricultural intensity and MAT on bee species richness, bee species composition and body‐size variation of bee communities. We found that bee species richness increased with agricultural intensity and with increasing temperature. The effects of agricultural intensity and temperature on bee species richness were mediated by the positive effects of agriculture and temperature on the richness of floral resources used by bees. During the off‐growing season, agricultural land was characterized by an extensive period of fallow land holding a very high density of flowering plants with unique bee species composition. The increase in bee diversity in agricultural habitats paralleled an increasing variation of bee body sizes with agricultural intensification that, however, diminished in environments with higher temperatures. Synthesis and applications. Our study reveals that bee assemblages in Afrotropical drylands benefit from agricultural intensification in the way it is currently practiced. However, further land‐use intensification, including year‐round irrigated crop monocultures and excessive use of agrochemicals, is likely to exert a negative impact on bee diversity and pollination services, as reported in temperate regions. Moreover, several bee species were restricted to natural savannah habitats. To conserve bee communities and guarantee pollination services in the region, a mixture of savannah and agriculture, with long periods of fallow land should be maintained.

Local and landscape scale woodland cover and diversification of agroecological practices shape butterfly communities in tropical smallholder landscapes

“…In temperate systems, set‐aside habitats such as flower strips show promising benefits for butterfly abundance and richness, mediated by an increase in flowering plant species richness (Boetzl et al, 2021; Wix et al, 2019). Given the financial and temporal constraints on smallholder farmers in our study region, we do not necessarily suggest set‐aside habitats in our context, although they promote bee diversity in East Africa (Lasway et al, 2022). Our results, however, reveal a large benefit in butterfly abundance for a relatively small increase in flowering plant species richness.…”

“…Though flowering plant species richness on our transects was generally low, our results indicate similar effects in agroecosystems in our study region. We assume that a higher number of flowering crop species ensure more continuous resources through time (Lasway et al, 2022). In temperate systems, set‐aside habitats such as flower strips show promising benefits for butterfly abundance and richness, mediated by an increase in flowering plant species richness (Boetzl et al, 2021; Wix et al, 2019).…”

A dataset of occurrence of wild bees and their interaction with foraging plants along a livestock grazing gradient of northern Tanzania

“…Two standardized sampling methods, UV-reflecting colored pan traps and random walks with hand nets were employed in the study area to sample bees [ 4 , 5 ]. Four pan trap clusters (each with white, yellow, and blue UV-reflecting colors) were passively used to collect bees [6] .…”

“…A drop of odorless liquid soap per approximately 1 L was used to break the surface tension of water so that bees landing on the pan trap were more likely to be captured. Pan traps were left in the field for 48 hours to collect bees [ 1 , 4 ]. Bees collected were temporarily preserved in 70% ethanol before being pinned and identified to species and morpho species levels [ 1 , 3 , 4 ].…”

“…Pan traps were left in the field for 48 hours to collect bees [ 1 , 4 ]. Bees collected were temporarily preserved in 70% ethanol before being pinned and identified to species and morpho species levels [ 1 , 3 , 4 ].…”