illustrates some general features of habitats that have universal signifi- cance in bee conservation.
From Table 4.1 we can illustrate some important principles and practices for bee habitat conservation. First, it is apparent that bee activi- ty and reproduction are optimized in open, sunny habitats with an abun- dance and diversity of food plants, in contrast to those in flower-poor, shaded woodlands. Warm, sunny, relatively bare patches of ground are preferred nesting sites, in part because warm conditions speed the devel- opment time of immatures and promote flight activity of adults. This is especially important for annual nests of solitary bees and bumble bees and helps explain why soil-nesting bees often dig nests in patches of soil with a southern aspect that maximizes exposure to the sun (Potts and Willmer, 1997). Second, as the diversity of nectar- and pollen-bearing plant species increases, the diversity of bee species in a habitat also increases (Banaszak, 1983). Unfortunately, modern agriculture tends to promote the exact opposite type of habitat – large monocultures, fre- quently of cereals or other nectar-poor plants. If the monoculture hap- pens to be a good bee forage, then there may be many bees but the number of bee species will tend to be low. Honey bees are the most like- ly bee visitors in these cases since they excel at exploiting rich food resources (Schaffer et al., 1983). The reverse of this situation is natural habitats which may have a smaller density of plants but whose diversity of plant species will support a more diverse bee fauna.
The increase in pine forest monocultures in North America does not bode well for bee populations except in those areas with good nectar-producing understorey and margin plants such as bramble, gallberry, and palmetto. Trees such as red maple, sourwood, and tulip poplar are good pollen or nectar sources, but even in these types of forests the bees are more likely to nest at the forest margins that have sun and a variety of nesting sites and flowering plants.
At this point we can generalize that for ground-nesting solitary bees or bumble bees, land conservation efforts should focus on sunny, open undisturbed meadows, field margins, sun-drenched undisturbed patches of bare soil, roadsides, ditch banks, and woodland edges. In any year, a farm can have large areas of such idle land, and using it for bee sanctu- ary costs next to nothing and involves mostly a willingness on the part of the land owner to leave it undisturbed for the long term. By ‘undis- turbed’ we mean no draining, herbicides, ploughing, or compacting with heavy machinery. Maintaining such undisturbed areas will increase the abundance of bee nesting sites and diversity of flowering plant species on farms. The presence of such bee sanctuaries in the agricultural land- scape of western Poland is one reason Banaszak (1992) gives for the sur- prising constancy of bee diversity there over 40 years. Another reason was plantings of agronomic crops that are very rich bee forages, namely lucerne, clover, oilseed rape, and sunflower. These rich forages can par- tially compensate for lost diversity of native plants.
28 Chapter 4
The importance of habitat permanence
The richness of plant and bee species in undisturbed fallowed fields increases with time (Gathmann et al., 1994). This means that one should plan bee sanctuaries for the long term. As the years go by a land manager can expect increasing numbers of plant and bee species in these undisturbed sanctuaries. But one catastrophic event, such as ploughing, can undo years of progress.
Mowing management
The most effective bee sanctuaries are mid-successional plant commu- nities, having an abundance of herbaceous perennials and few or no invading trees (Dramstad and Fry, 1995). Biannual mowing is advis- able to keep a sanctuary from succeeding into shaded woodlands or scrub lands. It is best to mow in winter when it is less likely to destroy active bumble bee colonies. A light mower is preferable to a heavy tractor-mounted implement that may crush nests of overwinter- ing soil-nesting bees.
Livestock pasture management
Pastures used for hay or animal grazing are generally inhospitable to bees, but there are ways to manage them in a manner consistent with a bee conservation programme (Osborne et al., 1991). One way to do this is to make pastures more-or-less permanent. The older the pas- ture, the more likely it is to have suitable bee nest sites and numerous plant species. Temporary pastures, such as those grown in crop rota- tion, have very low plant diversity even though the cover crop may be a rich resource for one season. Overgrazing is inadvisable because it promotes invasion of fast-growing grasses that crowd out nectar-yield- ing herbaceous plants. Herbicides similarly can reduce the number of pasture plant species. It is preferable for land managers to delay cutting pasture for hay until after a certain period of bloom. Cutting forage plants before they bloom essentially renders the pasture nutri- tionally useless to bees.
The importance of nesting materials
An ideal bee habitat must also provide nesting materials (mud, leaves, etc.) appropriate to the needs of a particular bee species. For example, a shortage of mud could be a limiting factor with orchard mason bees
Bee Conservation 29
(Osmia spp.), a group that uses mud in nest construction. Bumble bees need grassy thatch or abandoned rodent burrows in which they build their nests. If a local bee sanctuary meets the more important general standards discussed above, appropriate nesting materials will probably follow suit, but the matter should not be disregarded. One commercial blueberry grower in south Georgia, USA set out shelled maize under sheets of plywood around his orchard in an attempt to increase the number of rodent nests in the area which, in turn, would provide future nest sites for pollinating bumble bees.