Biology Reference
In-Depth Information
Table 6.2
Dilution advantage from grouping. Wild horses in the Camargue,
southern France, were kept in a group of three or in a group of 36.
Although the larger group attracted more biting tabanid flies, individual
horses suffered fewer attacks in the larger group (Duncan & Vigne, 1979).
Mean number of biting flies
Number of horses
per group
per horse
Small group (3)
30
10
Large group (36)
108
3
Synchrony in time: predator
swamping
Dilution may also be achieved by
synchrony in time, which swamps the
capacity of predators to capture prey. The
synchronous emergence of mayflies
provides a dramatic example of this effect.
In North America, the emergence of
Dolania americana
occurs over a two
week period in late May to early June,
when the larvae transform into winged
adults at the water surface, just before
sunrise each day. The adults immediately
mate, lay eggs and then die all within an
hour or so. They are preyed upon at the
water surface by beetles, and after
emergence by dragonflies, bats and birds.
Individual mayfly are safest from predation
during days when more adults emerge
(Fig. 6.3). It could be argued that this
synchronous emergence has primarily
evolved to enhance mating success.
However, parthenogenetic mayflies have
similar synchrony in emergence, which
suggests that reducing predation through predator swamping is a major selective pressure
favouring synchrony (Sweeney & Vannote, 1982).
100
Jun 2
Jun 6
80
Jun 9
J
un 3
60
Jun 8
Jun 4
40
Jun 5
20
0
0
50
100
150
200
250
Numbers of Dolania per day
Fig. 6.3
Synchronous emergence
swamps predators. The percentage of
adult female mayflies
Dolania americana
preyed upon by aquatic and aerial
predators combined, during seven days
in June. Individual mayfly are safest on
days where more females emerge. From
Sweeney and Vannote (1982).
Synchrony in time
can also dilute
predator attacks
per individual
prey
Selfish herds
Just as a mayfly in the middle of the emergence period is safer than one at either end, so
individuals in the middle of a group may enjoy greater security than those at the edge.
W.D. Hamilton (1971) suggested that individuals should approach others to reduce
