Small groves: important oases for pollinators
Categorie(s): Agriculture, Ecology, Trees
Pollinators are nowadays an ‘hot topic’. Pollinating agricultural crops makes them important economical players, contributing in our food supply. And many pollinators like bees, butterflies and hoverflies are very visible and attractive for nature lovers. Many of these species are occurring less and less.
One of the most important causes of the decline of wild pollinators is the intensivying of agriculture since World War II. this caused an essential increase of productivity but a great decline of biodiversity Upscaling caused the elimination of small elements in the landscape like wooded ridges, free standing trees and shrubs. Increasing use of agrochemicals, like pesticides and fertilisers, caused a decrease of pollinators. Pesticides are directly toxic for the insects, but also cause decrease in plantdiversity, which means less food sources for insects. Also other changes in agricultural practices harmes the quality of the habitats: less use of red clover as green manure during the last decades caused an enormous decline of bumblebees.
Ecology of pollinators
Pollinators depend on two things: food and micro habitats for their reproduction. Practically all bees and hoverflies depend on pollen and nectar, so flowery habitats. Nectar contains much sugar, is a fast source of energy much-needed to fly. Pollen is rich in proteins, so essential for reproduction in supplying the material for prosducing eggs. Especially beesdepend very much on pollen. Because the larvae feed on pollen, the parents must build a supply for their offspring. Most hoverflies andmany species of bees are not very particular in their choice of food. But there are also species who depend on one or a few species for their pollens. The knautiabee (Andrena hattorfiana) for instance depends completely on field scabiuous (Knautia arvensis).
Knautiabee on field scabious (Knautia arvensis)
Which habitat is needed for reproduction depends on the species. Bees build nests in whicht they place a supply of pollen. Underground nesting bees dig holes and build a nest in the dirt; above ground species look for holes in dead wood, old walls or hollow stalks of plants to make their nests. Bumblebees nest, depending on the species, in old holes of mice, hollow trees, nesting boxes, etc.
Because important host plant are not always found on the same spot as the nesting sites, and bees collecting pollen have to fly to and fro between host plant and nest, it is important that both places are not too far apart. this makes bees extra vulnerable for loss of habitat. Hoverflies do not make a nest, the drop their eggs simply in a suitable habitat.Many species have larvae that eat aphids and live in the vegetation. Other species have larvae that develop in cow’s dung, mud, roots or even as parasite in the nests of bees or wasps.
Small forest fragments as habitat voor pollinators
In agricultural regions fitting habitatys for pollinators has largely disappeared or degraded. One of the types of habitats that still can be found relatively much in agricultural landscapes is forest. These forest fragmenst are mostly very small, but they can play an important role in natural control of pests, watereconome, prevention of erosion and other functions. I have been doing research about the role of small forestfragmenst for the preservation of a diverse community of pollinators.
In the fragmented Flemisch landscape small rests of forests play an important role for pollinators. © Yves Adams
The community of bees in the forest fragments consists mainly of underground nesting bees.. They live in greater numbers in the fragmentswith much naked soil and sloping spots on the southern side. We found more different species when there were hedges in the direct surrounding. In old parts. In old groves with a rich herb layer we found in spring, during the blossoming of the herb layer, more bees than in young forests. In old groves we also found more species of hoverflies, where there were more microhabitats than in young forests. In open sunny fringes of the forests we found more bees and hoverflies than in shadowy fringes
In old forest fragments spring flowers like wood anemone(Anemone nemorosa) offer food for the pollinators in spring.
In the forest fringes with many species of pollinators plants are more often visited by pollinators and more flowers were pollinated. If the pollinators of the forest fringe can also improve the pollination of the nearby agricultural vegetation, this would be an economical argument to protect these forest fragments for the benefit of the pollinators
What has to be done?
Owners and managers can make (small) forests more suitable for wild pollinators by purposeful mamangment
1.Create slowly slopint fringes of the grove with much light and create open naked spots for underground nesting bees
2.Preserve small forest fragments, especially old fragments with a rich herb layer. They will be a food source for bees andcontain habitats for forest hoverflies.
3. Preserve as much as possible dead wood as nesting place for pollinators nesting aboveground.
4. Preserve exiasitng hedges en rows of trees and plant more if possible to add to the community of wild pollinators .
Of course forests and groves are not the only or most important habitat for most pollinators. In degraded regions without species rich grasslands or berms forests ca serve as real ‘oases’ in ‘biodiversity deserts – and help to preserve a rich community of pollinators’
Source: Willem Proesmans: (2019) ‘Kleine bosfragmenten: oases voor bestuivers in een groene woestijn?’ in Bosrevue 78a