Madagascar, Hawaii, and Easter Island only about 1000 years ago (Martin and
Steadman 1999).
Although there is considerable debate on the effects of these human migrations,
one school of thought, discussed in Martin and Klein (1984), MacPhee (1999),
and Worthy and Holdaway (2002), holds that the arrival of people resulted in the
extinction of large mammals either directly through hunting or indirectly through
habitat change. Thus, in North and South America mammoths and giant ground sloths
disappeared, in New Zealand the large ratites (moas) were hunted to extinction,
in Madagascar both giant ratites (elephant birds, Aepyornis) and giant lemurs
(Megaladapsis) vanished, and in Polynesia a variety of birds such as the giant flight-
less galliform (Sylviornis neocaledoniae), twice the size of a turkey, became extinct
with the arrival of people (Martin and Steadman 1999). Another school of thought
holds that rapid climate change caused their extinction (Guthrie 1990). For example,
the giant Irish elk (Megaloceros giganteus) is thought to have died out at the end of
the ice age coincident with change in climate.
Knowledge of past events allows us to answer such questions as why Africa has
a wide diversity of large mammals whereas North America and Europe do not
(Owen-Smith 1999). When we ask questions concerning the distribution of a
species, for example why the white-tailed deer (Odocoileus virginianus) is found in
South America or why the nine-banded armadillo (Dasypus novemcinctus) is found
in Texas, we need to know not only about their individual adaptations of habitat
selection, diet, and behavior, but also about their historical distributions due to the
movement of continents and the effects of the ice ages.
A new evolutionary force now affects animal communities: intensive agriculture
and industrialization. This is a post-Pleistocene development which has altered many
habitats through pollution and large-scale clearing for agriculture and industry
(Morrison et al. 1992).
The abiotic environment includes the sets of conditions that determine where an
animal can live and reproduce. Conditionsare those factors such as temperature
and rainfall that affect an animal but which are not themselves influenced by the
population. Because environments are not constant, animals are adapted to a range
of conditions, and usually the less constant the conditions the wider the range (Stevens
1989). The limits of adaptation are called the tolerance limitsfor the animal, and
we need to specify whether we mean the limits for reproduction or for occupation.
The latter are usually broader. Section 7.4 discusses the conditions, and the adapta-
tions to these by individuals, that determine the distribution of a population and the
position of the range boundary.
Individuals differ from each other genetically as well as physically and behaviorally
in sexually reproducing species. As an example of genetic differences leading to dif-
fering behavior, females of 13-lined ground squirrels (Spermophilus tridecemlineatus)
mate with several males at the start of the breeding season (Schwagmeyer and
Wootner 1985). What advantage is this to the males? Do they all stand a chance
of producing some offspring, does the first male to mate contribute to all or most
of the conceptions, or does the last male to do so score most of the conceptions?
It turns out that the first male to mate contributes to 75% of the conceptions
(Foltz and Schwagmeyer 1989) and so being first is clearly an advantage. There is
ANIMALS AS INDIVIDUALS 27
3.6 The abiotic environment
3.7 Genetic characteristics of individuals
