Biology Reference
In-Depth Information
Exemplary
groups
Body
weight
(kg)
Habitat
Diet
Group size Reproductive
unit
Anti-
predator
behaviour
Table 2.3
The social
organization of
African ungulates
in relation to their
ecology (Jarman,
1974).
Grade I
Dik-dik
Duiker
3-60
Forest
Selective
browsing;
fruit, buds
1 or 2
Pair
Hide
Grade II
Reedbuck
Gerenuk
20-80
Brush,
riverine
grassland
Selective
browsing
or grazing
2-12
Male with
harem
Hide, flee
Grade III
Impala
Gazelle
Kob
20-250
Riverine
woodland,
dry
grassland
Graze or
browse
2-100
Males
territorial in
breeding
season
Flee, hide
in herd
Grade IV Wildebeest
Hartebeest
90-270
Grassland
Graze
Up to 150
(thousands
on
migration)
Defence of
females within
herd
Hide in
herd, flee
Grade V
Eland
Buffalo
300-
900
Grassland
Graze
unselectively
Up to 1000
Male
dominance
hierarchy in
herd
Mass
defence
against
predators
(b)
Quantification of ecological variables
The ecological variables have not been quantified. For weaver birds, for example, are
insects 'dispersed' and are seeds 'patchy'? How exactly will these differences influence
the economics of exploitation by individuals?
(c)
Cause and effect
Consider the observation that weaver birds with a diet of seeds go about in flocks. Our
explanation was that seed eating selects for flocking because this is the best way to find a
patchy food supply. However, we could equally well have suggested that predation selects
for flocking and, as a consequence, the birds are forced to select locally abundant food so
all the flock can get enough to eat. In this case a diet of seeds is a consequence, or effect, of
flocking, not a cause. Maybe predation also selects for flocking in the forest insectivores
but because their diet is incompatible with flocking they have to forage singly.
(d)
Alternative adaptive peaks or non-adaptive differences
It is tempting when comparing between species to assume that differences are always
adaptive but some differences may simply be alternative solutions to the same ecological
pressures. An ecologist from Mars who visited the Earth would observe that in the
United States people drive their cars on the right hand side of the road while in the
United Kingdom they drive on the left. He would then perhaps make lots of measurements
in an attempt to find ecological correlates to explain the adaptive significance of the
difference. In fact, driving on the right and driving on the left may just be equally good
alternatives for preventing accidents (Dawkins, 1980).
