110 ANIMAL ECOLOGY
The Sword-billed Hummingbird,
a native of South America, has a long
bill, which enables it to suck up nectar
from the long flowers of Passiflora
mixta, a species of passionflower. As
it feeds, it spreads the plant’s pollen.Animals may be capable of killing
much smaller prey, but it is simply
not worth the effort. Wolves hunt
medium-sized or large mammals
such as elk. If those mammals
disappear from their environment,
they find it hard to catch sufficient
numbers of smaller animals such
as mice to sustain them; the
energy they use finding small prey
is greater than the energy they
gain by consuming them.
Plants cannot run away or fight
back, so different considerations
apply to herbivores when it comes
to food size. There is a maximum
size of seed that a given finch,
for example, can fit in its bill, so
larger finches have an advantage
over smaller species. Similarly,
individual species of hummingbirds
can drink nectar only from flowers
up to a certain size, depending
on the length of their bill.Ecological niches
An animal or plant’s niche is its
ecological role or way of life. For
American zoologist Joseph Grinnell,working in the early decades of the
20th century, an organism’s niche
was defined as its habitat. He
studied birds called thrashers in
California and observed how they
fed, nested, and hid from predators
in the dense undergrowth of the
chaparral shrubland. However, a
niche is more complex than simply
the place where an organism lives.
Oxpeckers and buffalo share exactly
the same habitat—open grassland –
but their requirements for survival
are very different: the buffalo graze
on the grasses, while oxpeckers
derive their food from the ticks they
peck from the buffalos’ hide.
Charles Elton explored the
concept of ecological niches in
more depth. For him, food was
the primary factor in defining an
animal’s niche. What it ate and
what it was eaten by were crucial.
Depending on the habitat, a
particular niche could be filled by
a different animal. Elton cited the
example of a niche that was filledSnowshoe hare and
lynx population cyclesIn Canada’s boreal forests,
the favored prey of lynx are
snowshoe hares. Charles Elton
examined the relationship
between the populations
of these two species, using
data covering the period
1845–1925. When hares are
numerous, lynx hunt little
else. After their population
reaches its peak density,
the hares struggle to find
enough plant food. Some
starve, while others are
weakened and are more
easily caught by predators,
including lynx, which feed
very well for a time. When
hare numbers continue to
fall, this affects the lynx.
They are forced to hunt
less nutritious prey, such
as mice and grouse.
As they struggle to find
enough to eat, lynx produce
smaller litters or even stop
breeding altogether. Some
starve to death. A decline in
the lynx population sets
in one or two years after the
hare population has bottomed
out, a cycle that repeats every
eight to eleven years.A Canadian lynx captures a
snowshoe hare, its preferred prey.
When hares are plentiful, a lynx
will eat two every three days.Observation of species in the
wild convinces me that the
existence and persistence of
species is vitally bound up
with environment.
Joseph GrinnellUS_106-113_Animal_Ecology.indd 110 12/11/2018 17:34