Environmental Engineering Reference
In-Depth Information
natural habitat, conventional near or far from natural habitat), however, ex-
perienced greatly reduced diversity and abundance of native bees, resulting
in some cases in failure of the native bees alone to pollinate crops.This im-
plies that farmers need to consider managing farmland for both natural pol-
linator habitat as well as crop lands.
Pest Control
Herbivorous insect species are a leading cause of crop plant damage and ac-
cordingly annual loss of crop production. Suppression of such insect pests
by their natural enemies has been identified as a potentially important
ecosystem service of biodiversity (Cardinale et al. 2003).
A case in point involves one of the most abundant pests of alfalfa crops,
the pea aphid, (
Acyrthosiphon pisum
).Three of the most important natural
enemies of pea aphids are the ladybird beetle (
Harmonia axyridis
), the damsel
bug (
Nabis sp.
), and the parasitic wasp (
Aphidius ervi
). Cardinale et al. (2003)
conducted an experiment in alfalfa fields in Wisconsin, United States, to
compare the effectiveness of the three natural enemies individually (low di-
versity) and in combination (high diversity) in controlling aphid abundance.
They discovered that the abundance of the pea aphid was suppressed more
under high diversity than low diversity conditions. Aphid reduction trans-
lated into higher alfalfa production.
Moreover, the degree to which pea aphids were suppressed in the high
diversity condition was greater than one would expect simply from sum-
ming the impact of each enemy species alone.This nonadditive effect arose
in part because different predator species were functionally complementary
in their effects on different age classes of the aphid, and hence set up a pre-
dation gauntlet.The parasitic wasp deposits eggs into the body of young
aphids.The eggs hatch and the wasp larvae consume the internal tissues of
the young aphids. The ladybird beetle and the damsel bug actively hunt
older aphids. Cardinale and colleagues, however, caution against ascribing
universality of this finding for all natural enemy species. In some cases, nat-
ural enemies may overlap in ecological roles (functionally redundant) and
may even attack each other as well as their shared prey (called an intraguild
effect) in which case the effectiveness of the diverse natural enemy assem-
blage is compromised.The underlying message is that natural enemy di-
versity is effective provided one is strategic about predator species selection.
Predator species chosen for a particular biological control problem should
be ones that complement each other in their effects on a pest species. But,
