Over population is where abundance of organisms are out of balance — more births than deaths allow numbers to grow and unless resources become limiting, this can keep going indefinitely.
Usually though there is a shortage of food, water, and/or space and organisms must compete with each other [or other species] for access to these needs.
This competition stressful and those that consistently lose the contests go without. Eventually stress and malnourishment cause some to loose condition and they must give up costly activity like reproduction to survive. In time there are fewer births and more deaths [especially among the old and young exposed to the elements] and population growth slows or even declines.
Fewer organisms means less pressure on resources and competition eases eventually enough to allow population to grow again. Abundance eventually settles into a balance with the available resources.
This sequence is what ecologists call population regulation.
The paradigm is that populations of organisms in nature are contained by competition and its consequences [density-dependent processes]. Abundance may spike on occasion but in time numbers reach a balance with resources.
Over abundance can happen with invasive species when they first arrive in a disturbed habitat and with pest species that are presented with excess of a resource they like — think locusts on a grain crop.
It also happens if the organisms can overcome resource shortages. One organism is unique in this regard
At 7 billion and counting it could be argued that Homo sapiens are already over abundant except that we are still feeding ourselves, finding water and building shelter. We are [more or less] readily meeting the basic needs of 7 billion and still have positive population growth — 216,000 a day.
In a little over 48 hours we add more people than there are elephants in Africa.
There is an ongoing debate in Australia about human population size. What should it be, and how fast should it be reached?
22 million seems a small population for continent-sized country — compare it with Nigeria that has 169 million with just 12% of the land area.
At just 3 people per km2, Australia is 233rd of 243 on the list of countries by population density —yet has the sixth largest land area.
However in this context averages can be a bit misleading for 90% of Australians live within a spit and a shove of the ocean and most of them on the eastern seaboard. These days an average Australian is actually an urban dweller with plenty of near neighbors.
No matter the natural human tendency to aggregate, the issue of over population is rarely one of density.
The debate in Australia is whether the current 22 million should grow to 27 or 35+ in the coming decades.
And this is a choice because unlike world population growth the net increase in Australian residents is determined by immigration — like many mature economies births and deaths in Australia have been in balance for some time.
Government encourages residents to reproduce with free public education and health care — they even resorted to cash payments nicknamed the baby bonus. No doubt this has worked but with an ageing demographic not enough to see positive population growth rates.
There should be a debate about a policy that encourages procreation when we already have 9,000 new people per hour added to the global population. Instead the debate is about two things
Both are something of choice because the volume and rate of immigration over the coming decades will determine population size.
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