Domestic bees alter the structure and functioning of networks of interaction between plants and pollinators.
The domestic bee (Apis mellifera, Apidae) is a species native to Eurasia and Africa, but has been introduced almost everywhere in the world for honey production and pollination of certain crops. The domestication of the honeybee possibly began about 7000-10000 years ago, and was a widespread practice during Egyptian, Greek and Roman civilizations. However, in recent decades there has been a considerable boom in this agricultural activity globally, increasing the number of hives in the natural environment. In fact, the origin of the great majority of domestic bees that we see today in nature is due to beekeeping.
The domestic bee is considered a super-generalist pollinator (visits many plant species): it competes with other native pollinators for floral resources (nectar and pollen) and displaces them. In addition, given its foraging pattern, it increases the transfer of pollen both within the individual plant itself and between species, thus affecting the production of fruits and seeds in wild plants. Given that beekeeping considerably increases the density of domestic bees on a local scale, and these in turn can travel several kilometers, it is proposed that this agricultural activity could trigger a series of consequences on natural systems by altering ecosystem services as important as pollination.
A group of researchers from the Estación Biológica de Doñana (EBD-CSIC) and the Instituto de Productos Naturales y Agrobiología (IPNA-CSIC) has carried out a comparative study in the Teide National Park (Tenerife) taking advantage of the introduction of up to 2700 beehives that takes place each spring within the Park. To this end, they established two time scenarios marked by the absence (phase-pre) and presence (phase-apis) of hives in the same location to subsequently analyse a series of ecological parameters that define the structure and functioning of the plant-pollinator interaction network. The work has been published in the journal Scientific Reports.
Beehives in the Teide National Park (Tenerife, Canary Islands) (Photo: A. Valido).
Once the hives are installed, "a significant decrease in the number of pollinator species and the frequency of visits they make to the flowers is detected, practically overnight". That is, several species of bees, flies, butterflies, birds and lizards that are detected frequently during the censuses in the pre-phase, practically disappear from the system in the phase-apis. This reduction in pollinator biodiversity in turn results in "a large majority of plant species now receiving a much smaller list of pollinator species". The introduction of domestic bees would therefore generate a "crisis of pollinators inside the Teide National Park".
The study also analyzes how this reduction in pollinator biodiversity restructures the network of mutualist interactions. The introduction of domestic bees "reduces the interactions of native generalist pollinators and dismantles the functional units (modules) of the network dominated by these native pollinators". As a consequence, "the mutualist network is reorganized into a larger number of modules (groups of species that interact more with each other than with species from other groups), and is now more vulnerable to, for example, climate change resulting from anthropogenic action".
But that's not all. The results show that "the massive presence of domestic bees does not really compensate for the loss of native pollinators". Many of the functional processes that occur within ecosystems are directly related to interactions between species. Therefore, it would be expected that the aforementioned changes have some functional implications in the plant-pollinator network. In this respect, "it was detected that those plant species most visited by domestic bees during the phase-apis produced fruits with a lower number of seeds when compared to what happened during the phase-pre". In addition, in another experiment carried out exclusively with the Teide broom (Spartocytisus supranubius, Fabaceae), it was observed that "brooms that were closer to the hives produced fewer seeds per fruit, and these in turn were smaller when compared with those brooms farther away from the hives". Therefore, it is confirmed that "manifest changes in the structure of the mutualist networks have immediate functional implications for the system".
The results of this work warn about the vulnerability of natural ecosystems to the massive increase of domestic bees due to the direct effect of beekeeping. "Their high densities not only negatively impact other native pollinator species, but also have multiple cascading effects on ecosystem functioning, sometimes as unobvious - but far-reaching - as shown in this study".
For more information on the topic: 'Honeybees disrupt the structure and functionality of plant-pollinator networks' (Alfredo Valido, Mª Candelaria Rodríguez, Pedro Jordano, 2019, Scientific Reports 9: 4711).
By Alfredo Valido.
