Dispersal, migration and management - Ecological Applications: Toward a Sustainable World - page 104

Fig. 4.16 (a) Mean
number of pollen grains
deposited by native
bees per 10-minute
observation period in
relation to the
proportion of upland
habitat within 2.4 km of
the watermelon fi eld.
Circles denote organic
farms and triangles
conventional farms. (b)
Estimated total pollen
deposition per day by
native bees. Each
watermelon plant
requires 500-1000
pollen grains to set
marketable fruit. (After
Kremen et al., 2004.)

(a)

7

6

5

4

3

2

1

0

0 0.2 0.4 0.6 0.8

(b) Proportional area of upland

100,000

10,000

1000

100

10 0

0.2 0.4 0.6 0.8

Proportional area of upland

these natural habitats. None of the environmental variables was related to honeybee
pollination - remember that many of these bees would have been from hives
imported to the farms for this purpose.
Kremen and her colleagues converted their records of pollen deposited per
10-minute observation period to the total per day. This allowed them to determine
the proportion of surrounding land that must consist of upland native habitat in
order to yield the 500-1000 pollen grains required per melon plant (Figure 4.16b).
About 40% of natural habitat within 2.4 km of a fi eld is suffi cient to provide for
melon pollination needs. Their results provide a strong economic argument for
conserving these natural habitats. For farms that are far from natural habitat, active
restoration with native plants in hedgerows and ditches and around fi elds, barns
and roads, might allow a target of about 10% native habitat to be achieved. Accord-
ing to Figure 4.16b this would equate to 20-40% of watermelon pollination needs
being met by native bees.
Pollination is just one of the many economically important 'ecosystem services'
provided by native plants and animals. (Others include climate regulation, waste
treatment, provision of health-enhancing products, and even spiritual refreshment
- they will be discussed in detail in Chapter 9.) To achieve the economic benefi t of
native bees, it turns out that attention must be paid to the mobility of the species
in relation to the mosaic of their habitat patches in the landscape. Once again, dis-
persal traits help us understand how to manage our natural resources.