Effects of overpopulation are unpleasant. In nature when a population of organisms grows beyond the limits of available resources two things tend to happen — birth rate declines and the death rate increases.
The theory that comes from population ecology is that when resources [food, water, space] are in short supply organisms allocate energy to their own survival often taking it away from attempts to reproduce. When it is crowded resources are harder to find and defend. This makes survival more costly than normal — it takes more time and effort to find and keep hold of what is needed to survive. At the extreme individuals starve, become more susceptible to disease, or fight resulting in more deaths than usual.
Fewer births and more deaths lowers the population growth rate — and eventually population size — easing the overpopulation problem.
The reverse also is also true.
Where resources are plentiful organisms tend to reproduce earlier and more frequently, and are more likely to survive. This combination sees population growth rates increase.
Ecologists call this response of birth and death rates to available resources and numbers density-dependence and have spent many a research project trying to demonstrate its presence.
There is enough evidence from a wide range of animal, plant and microbial species to suggest that the theory holds and that the effects of overpopulation are real.
Overpopulation describes the case where this density-dependence has not dampened population growth fast enough or where there has been a sudden collapse in a key resource or spike in an environmental condition [drought, flood, storm].
Resources and conditions no longer sustain the population.
At best reproduction slows or stops — at worst mature individuals die — in between there is considerable unpleasantness.
This excerpt from a new book, Missing Something by Mark Dangerfield describes the elephant problem in northern Botswana....
A newly prosperous country of a little over 600,000 km2 in southern Africa landlocked by South Africa, Namibia, Zimbabwe and Zambia, Botswana is home to at least 100,000 African elephants (Loxodonta africana) and possibly as many as 130,000. The actual number is not known. Although elephants are big and should be easy enough to count, they are widely spread and move constantly, and even from the air tree canopies can easily hide them. They tend to mill around when disturbed so counts must be as simultaneous as possible over wide areas and is a significant challenge.
The majority of Botswana’s elephants live in the north of the country in the land bordered by the edge of the Okavango Delta in the west and the international border with Zimbabwe in the east. Much of this area is National Park, Game Reserve or forestland. There are only a few villages and the small town of Kasane on the Chobe River in the north.
If we assume the lower population estimate of 100,000 elephants then there is roughly one elephant per square kilometre in this region. One square kilometre (100 ha) is a sizeable plot of land. Most suburban plots in Sydney are 450 square metres so it would be possible to fit over 2,000 house lots in 100 ha. And even though an adult African elephant cow at 2,500 kg needs to eat 300 kg of plant material each day, in most habitats she can readily find it in 100 ha. The conventional wisdom from those who have worked in reserves with elephants is that one elephant per square kilometre is about right. Fewer is fine, but any more and the system usually starts to change.
The reason is that elephants also need water and in the savannas of southern Africa, surface water is not evenly spaced, especially during the six to eight months of the dry season. Natural pans that hold rainwater dry up and in this region of northern Botswana only the Chobe River, Linyanti swamps and Okavango Delta provide a natural year-round supply. In the dry season elephants congregate within foraging distance of these water sources, behaviour that dramatically increases elephant density. It provides the spectacular vista of the Chobe floodplain with a herd of a thousand or more elephants – the picture of legend and of African adventure – but such numbers put enormous pressure on the vegetation.
Now, invasive shrubs cover large sections of the Chobe River that had mature riparian forest fringe a few decades before. Elephants have ring barked or pushed over the trees and literally opened up the canopy. When this happens the soil water balance changes and the carbon pools and fluxes do the same. It is a cascade to a very different type of system on an altered water balance that slows down primary production. Only when the deep-rooted canopy trees are recruited will the moisture retention in the upper soil layers improve. This recruitment is unlikely without good rains and fewer hungry elephants.
Wildlife managers stalled the problem of habitat change by creating water points away from the natural sources. Diesel pumps sucked up groundwater into pans that would then hold water through the dry season. It worked to keep some of the elephants away from the riparian fringes, but it also kept them near good food supplies. Naturally they did well and continued to reproduce. So a costly and high maintenance intervention made the problem worse.
Some of the effects of overpopulation for elephants in order of severity are
Many similar effects of overpopulation are present with human populations
However, our technology and economic prowess buffers us from these effects. We can readily exist alongside them and our ability to mobilise new resources keeps raising the bar on how many people can be supported.
Many would argue that human ingenuity is such that we will never be overpopulated. There will always be ways found to feed, clothe and house people no matter what world population growth generates.
Currently we do grow enough food, have just about enough water and build infrastructure sufficient to meet the needs of 7 billion souls.
There are some distribution and sharing out problems that results in about 1 billion people living with hunger but we have mostly kept pace with our extraordinary growth.
Just that the effects of overpopulation can be sudden. If resource supply drops when there are many mouths to feed there is a riot and quickly a famine. Many people die. Many more are displaced.
Birth rate decline and the death rate increase are inescapable.
The solution is to be aware of the issue.
Accept that there are limits to resources and that there are consequences of living close to them. This will ensure attention is given to food security through innovations and investment.
Think carefully about free trade, food distribution and adaptation to help buffer against inevitable effects of climate change and economic shocks.
Perhaps even look for alternatives to the economic growth model of development because elephant or human, the effects of overpopulation are nasty.
elephant numbers in northern Botswana have increased steadily
Jul 26, 15 07:19 AM
Global environmental issues fall into three broad categories based on the extent of their effects. Thinking this way helps us to know when the issue is real.
Jul 26, 15 06:59 AM
The GFC hit hard and fast, reverberating through economies everywhere. There are a number of ways that this impact fuelled environmental issues.
Jul 26, 15 05:47 AM
Fracking is drilling and injection of liquid to fracture deep rock layers and release natural gas. It is similar to drilling for oil but far more controversial — find out why.