The Volterra Principle

I’ve been cleaning up my hard disk, deleting old files and so on and I found this piece I’d written some years ago for an environmental education display on local plants, so thought I’d reproduce it here. It’s one of the reasons why I find ecology such a fascinating science.

 

The Volterra Principle: why you shouldn’t use pesticides in your garden

Do you use pesticides in your garden? Judging by the number of insecticide sprays available in garden centres and supermarkets, most people do.

The book Silent Spring by Rachel Carson appeared first in 1962 and has been credited with being the catalyst that started the environment movement throughout the world. It documents the wholesale use of insecticides and pesticides which was common at that time and points out the long-term effects on wildlife and humans. If you haven’t read it, there’s probably a copy in your local library. It should certainly put you off using pesticides for ever.

The following is a very interesting excerpt from The Machinery of Nature by American ecologist Paul Ehrlich:

“If predator and prey populations are more or less in balance with one another and if an environmental change raises the death rates in both predator and prey populations, there will be a disproportionate decline in the number of predators.

This result is known as the Volterra Principle. It predicts what will happen, all else being equal, after an application of a broad-spectrum pesticide (one that kills many species of insects) to a farmer’s field in which herbivorous insects are being eaten by predatory insects. The surviving prey, whose death rate in the aftermath of the pesticide is now lower due to the absence of predators, will suffer less than the surviving predators, which cannot find sufficient prey to increase their birthrate quickly. The population of prey (the pests) will thus recover from the pesticide much more rapidly than the predators and build up a larger population than was previously present. Thus an even-handed assult on a predator-prey system will tend to promote the prey and suppress the predators—in this case making the pest problem worse.”

In simple terms, what does this mean?

Suppose you have a rose bush which is being attacked by aphids. More than likely, a predatory insect such as a ladybird or a lacewing, will be feeding on the aphids at the same time. Because of the way predator-prey systems work, there will always be more prey than predators, i.e. more aphids than ladybirds or lacewings. (By the same token, there will always be more rose bush than aphids.) A bit of thought will see why this is the case. If the system was ‘top-heavy’ i.e. hundreds of ladybirds, dozens of aphids and very little rose bush, then each predator would quickly eat all the prey and then starve to death. Balanced systems are ‘bottom heavy’ and work the other way—they wouldn’t be balanced if they didn’t.

Suppose on your rose bush, prey and predators exist in the ratio 100:1 i.e. 100 aphids to 1 ladybird. There might be 1000 aphids, in which case there would be 10 ladybirds. Along you come with your ‘Bugoff’ spray and douse the whole bush with insecticide. Suppose that 90% of all insects, both ladybirds and aphids, are killed. That means 100 aphids will survive but only 1 ladybird. The 100 aphids will quickly build up their numbers again but the single ladybird wont.

So, an even-handed attack on pests in your garden can do more harm than good. The very thing we shouldn’t do is upset the natural balances; they have evolved over countless millions of years and they must be maintained for the health and survival of all species. The cost of allowing the natural balances to survive is very small. We must simply accept that a small amount of damage to our garden plants is an inevitable consequence of the natural world we live in, and not rush for the spray every time we see a hole in a leaf.

Another thing to consider is that some of the insects which survive the poison may well have an inbuilt resistance to it and this will be bred into successive generations. Eventually the poison may have very little effect.

By growing local plants in our gardens, we restore the naturally-balanced ecosystems that used to exist in our local areas. There are very few problems with pests in such a garden. It is well worth putting up with a few chewed leaves for the bonus of having a diversity of birds, butterflies, insects, lizards and frogs in the garden.