Carolin Mayer

The Abundance and Pollen Foraging Behaviour of Bumble Bees in Relation to Population Size of Whortleberry (Vaccinium uliginosum)

Habitat fragmentation can have severe effects on plant pollinator interactions, for example changing the foraging behaviour of pollinators. To date, the impact of plant population size on pollen collection by pollinators has not yet been investigated. From 2008 to 2010, we monitored nine bumble bee species (Bombus campestris, Bombus hortorum s.l., Bombus hypnorum, Bombus lapidarius, Bombus pascuorum, Bombus pratorum, Bombus soroensis, Bombus terrestris s.l., Bombus vestalis s.l.) on Vaccinium uliginosum (Ericaceae) in up to nine populations in Belgium ranging in size from 80 m2 to over 3.1 ha. Bumble bee abundance declined with decreasing plant population size, and especially the proportion of individuals of large bumble bee species diminished in smaller populations. The most remarkable and novel observation was that bumble bees seemed to switch foraging behaviour according to population size: while they collected both pollen and nectar in large populations, they largely neglected pollen collection in small populations. This pattern was due to large bumble bee species, which seem thus to be more likely to suffer from pollen shortages in smaller habitat fragments. Comparing pollen loads of bumble bees we found that fidelity to V. uliginosum pollen did not depend on plant population size but rather on the extent shrub cover and/or openness of the site. Bumble bees collected pollen only from three plant species (V. uliginosum, Sorbus aucuparia and Cytisus scoparius). We also did not discover any pollination limitation of V. uliginosum in small populations. We conclude that habitat fragmentation might not immediately threaten the pollination of V. uliginosum, nevertheless, it provides important nectar and pollen resources for bumble bees and declining populations of this plant could have negative effects for its pollinators. The finding that large bumble bee species abandon pollen collection when plant populations become small is of interest when considering plant and bumble bee conservation.

The importance of monkey beetle (Scarabaeidae: Hopliini) pollination for Aizoaceae and Asteraceae in grazed and ungrazed areas at Paulshoek, Succulent Karoo, South Africa

The relative importance of monkey beetles (Hopliini, Scarabeidae) as pollinators of Asteraceae and Aizoaceae in the Succulent Karoo as well as the influence of livestock grazing on their abundance and diversity was investigated. Hopliine beetles proved to be the, or among the, most abundant flower visitors of 12 investigated plant species. However, during single flower observations at three Aizoaceae species, bees (Apoidea), bee flies (Bombyliidae) and pollen wasps (Masaridae) were the most frequent flower visitors. However, monkey beetles carried the highest Asteraceae and Aizoaceae pollen loads, and are therefore considered to play a vital role in the pollination of these two families. Abundance, species richness and diversity of Hopliini did not appear to be heavily affected by livestock grazing. Annual variation in the composition of monkey beetle populations was more dramatic. Still, some species showed higher abundances on heavily grazed rangeland while others only occurred under low grazing pressure. It is presumed that changes in the composition of the vegetation, especially the observed decrease of perennial plants in favour of annuals and geophytes (Todd and Hoffman 1999) could in turn affect the composition of monkey beetle assemblages.

Pollination services under different grazing intensities

A study was conducted to investigate whether a lack of pollinators, pollination and therefore, a shortage of fruit set of perennial plants is more likely to occur in overgrazed land than farmland without grazing pressure, as this could have considerable effects on the reproduction of certain plant species. The study was conducted at two adjacent BIOTA biodiversity observatories (www.biota-africa.org) in Namaqualand, South Africa. Individuals of certain plant species of the family of Aizoaceae were measured and flower and fruit production recorded. Malaise traps and transect walks were used to document insect diversity. Preliminary results indicate that not all plant species show reduced flower production but that fruit set is indeed compromised on overgrazed land. Insect diversity and abundance is partially reduced under heavy grazing pressure, especially in locations where the vegetation cover is low and dominated by unpalatable plant species.RésuméUne étude a été conduite afin de déterminer si un manque de pollinisateurs, de pollinisation et par conséquent une diminution du nombre de fruits sur les plantes pérennes a plus de chance d’être observée dans une région avec une pression de pâturage élevée que dans une région agricole à faible pâturage, car cela peut avoir une incidence considérable sur la reproduction de certaines espèces végétales. L’étude a été menée dans deux biotopes contigües (BIOTA biodiversity observatories, https://doi.org/www.biota-africa.org) dans le Namaqualand en Afrique du Sud. Des individus de certaines espèces de plante de la famille des Aizoaceae ont été mesurés et la production de fleurs et de fruits dénombrée. La diversité des insectes a été estimee à l’aide de différentes méthodes (piège Malaise et pièges d’interception). Les résultats préliminaires montrent que la diminution du nombre de fleurs n’affectent pas toutes les plantes mais que la fructification est en fait compromise dans la région fortement pâturée. La diversité et l’abondance des insectes est partiellement réduite lorsqu’il y a une forte pression de pâturage, en particulier dans les localites où le couvert végétal est faible et dominé par des espèces de plantes au goût désagréable.

Synchrony of pollinators and plants in the winter rainfall area of South Africa—observations from a drought year

The arid winter rainfall area of South Africa is a centre of bee diversity and endemism. Host plant synchrony is hypothesized as one possible mechanism for an elevated rate of speciation in desert bees, where rainfall is considered to initiate for the emergence especially of oligolectic (pollen specialist) bees. In Namaqualand in 2003, a drought year, the absence of early precipitation postponed the main flowering season by five weeks to early/mid September. Despite the lack of rain and flowers bee activity was already very high in August as recorded with colour trapping. Five highly specialised species of Rediviva (Melittidae) as well as many other bee species had emerged before the flowering season had started. The majority of bees were probably not nesting as none of the collected individuals carried pollen. Therefore failure in reproduction can be assumed. These observations show that bees in Namaqualand are not synchronised with their host plants and that rainfall might not be an abiotic cue for their emergence which contradicts current hypotheses. Usually, precipitation as well as flowering season in Namaqualand is fairly predictable. Thus it seems that evolutionary processes did not lead to adaptation of (oligolectic) bees to their host plants as one would expect in a xeric environment.

Does Grazing Influence Bee Diversity

In Namaqualand, the north-western part of the Succulent Karoo of South Africa, a study was conducted to investigate the influence of livestock grazing on the abundance and diversity of bees (superfamily Apoidea). Bees were collected on adjacent rangeland sites which are characterized by a significant fence line contrast, one site showing effects of heavy grazing. Application of different sampling methods (Malaise and colour plate trapping) reveal different results, indicating that methodological influences are significant. Colour traps, in particular, may provide poor estimates of bee abundance due to their apparent sensitivity to competition from surrounding flowers for insect attraction.