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The Rise and Fall of Populations

It seems an obvious point, but population levels of plants and animals don’t 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.

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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 organism’s disaster is another’s benefit.

 

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