Dispersal, migration and management - Ecological Applications: Toward a Sustainable World - page 103

4.5.4 Bee business -
pollination services
of native bees
depend on dispersal
distance

A large proportion of the world's crops depend on insect pollinators and bees play
a predominant role. Farmers often rely on the ancient relationship with domesti-
cated honeybees ( Apis mellifera ), many importing hives when their crops are in
bloom. However, a multitude of native, unmanaged bee species also provide pollina-
tion services for various crops, and these are likely to become even more important
as populations of domesticated and feral honeybees decline.
Kremen et al. (2004) studied the role played by native bees in 26 watermelon
( Citrullus lanatus ) fi elds on farms in northern California (Figure 4.15). The farms,
some of which were organic (meeting set standards for pesticide and fertilizer use)
and others conventional (greater use of pesticides and fertilizer), varied in the pro-
portion of native and other habitats found nearby. Satellite imagery was used to
determine the amount of native upland habitat (woodland and chaparral), riparian
woodland (beside streams) and highly modifi ed land classes (agriculture, grassland
dominated by non-native species, urban land) in the vicinity of each fi eld. They
gathered information on pesticide use at the same scale and also recorded the total
number of plant species on and around each watermelon fi eld.
Each watermelon plant requires 500-1000 pollen grains to be deposited by insect
vectors to produce marketable fruit. The 30 native bee species in the area deposit
from 4 to 197 grains each time they visit a fl ower, whereas honeybees deposit 21
grains per visit. Kremen's team recorded on a number of sunny days over a 2-year
period a total of 3349 visits by native bees to watermelon fl owers and 7023 by hon-
eybees. They estimated the number of pollen grains deposited at each visit.
In an analysis to determine the importance of all environmental variables (includ-
ing local habitats, pesticide use, organic vs conventional farms, and plant species
richness) only the proportion of upland native habitat within 1-2.5 km of the fi elds
was related to pollen deposition by native bees (Figure 4.16a). This accords with the
maximum foraging dispersal distances of about 2.2 km for native bees that nest in

Fig. 4.15 Map of
northern California
showing the locations
of farms and surround-
ing habitat cover.
Circles denote organic
and triangles conven-
tional watermelon
farms. Gray, upland
(woodland plus
chaparral) habitat;
black, riparian (stream-
side) habitat; white,
agricultural or other
human-dominated land
type. The town of Davis
and Lake Berryessa are
indicated, and the scale
is in kilometers. (After
Kremen et al., 2004.)

(a) (b)

a.

Lake
Berryessa

Davis