The Rise and Fall of
It seems an obvious point, but population levels of plants and animals
dont stay the same from one year to the next, whether humans are interfering or not.
Populations will constantly fluctuate in response to a variety of other factors such as
the weather, the food supply and predators, parasites and diseases.
In some years population numbers may take a nose-dive, only to bounce
back again in future years. In boom years, more of the offspring produced will survive,
while in bust years, most of them will die. Predators reliant on prey populations for
food, will follow similar cycles to their prey populations.
There is currently some talk of attempting to control sparrowhawk
numbers in order to protect songbird populations. In practice, this is totally
unnecessary. The sparrowhawk populations are naturally limited by songbird numbers. They
do not eat anything else. If sparrowhawk numbers are high, then it is only because there
are enough songbirds there to feed on and more besides.
The key factor which needs to remain stable in order for populations to
continue to perform their intricate dance of variation, is the availability of suitable
habitat. If the habitat is there for songbirds, they will continue to thrive, no matter
how many sparrowhawks there are.
However, where predators are not tied to specific prey populations,
control may be necessary. Crows and magpies, which prey on the eggs and youngsters of
songbirds, are relevant examples of this. In this case, their numbers can remain high even
when songbird numbers are low, as they have many alternative food sources.
Life in Nature is very uncertain. A host of competitors, predators,
parasites, diseases and natural disasters lie in wait for the weak, inexperienced or
merely randomly unfortunate. In practice, very few individuals of any species will ever
make it to maturity
To compensate for this, Nature has devised an extremely efficient
over-production system. Most plants and animals produce enormous numbers of offspring in
their lifetimes, very few of which will actually survive. This is just as well, or the
earth would be drowned in surplus organisms! However, provided as little as one individual
survives to replace its parent, the numbers will be maintained.
Those annoying aphids attacking our garden plants in the summer, as we
all know, reproduce extremely rapidly and successfully (without the benefit of males). If
all the progeny of a single female aphid were to survive to maturity, within one year, the
world would be blanketed in a layer of aphids 14 km thick! This gives us some idea of the
war of attrition which goes on against aphids. The number which must be killed or eaten is
simply staggering. Yet each year there are more than enough to come back and do the same
thing all over again.
A single Oak tree in its life span of around 300 years, will produce
tens, possibly even hundreds of thousands of acorns. It is clearly impossible for all of
these to grow into mature oaks - the space is simply not there, even without competition
with other species. All that is needed to maintain the oak population at a stable level,
is for just one of those many thousands of acorns to grow to maturity.
Populations can be exceedingly resilient. Humans have been attempting
to eradicate pest species such as rats for centuries. So far our efforts have been to no
avail. A single pair of rats in their lifetime can be responsible for producing 15,000
youngsters. No matter how many we manage to exterminate, as long as the food supply and
the habitat remains, there is a never ending supply of youngsters to take the place of
those who fall. It is this enormous overproduction which guarantees the overall robustness
of populations, provided there is suitable habitat available.
Nature is the ultimate recycler. None of those which fail to survive go
unused. They will either provide food for something else, or will rot, turning into
nutrients to be returned to the soil for reuse by plants and bacteria. One organisms
disaster is anothers benefit.
Long does a Bird Live?