Ideas to save the world. Ecology: the intermediate disturbance hypothesis

Human dominated environments are chronically over-disturbed and so low in biological diversity. Ecological theory tells us that if we reduce disturbance, diversity will return.

Human dominated ecosystems such as farmland are frequently disturbed by activities like herbicide spraying. Frequently disturbed ecosystems tend to be low in biodiversity according to ecological theory.

The intermediate disturbance hypothesis (IDH) is a concept I have a mild obsession with. When I see the old man that lives near me cutting his lawn every week I think IDH. When I see farmers spraying their fields with herbicide I think IDH. When I see someone cover their garden with plastic grass or wooden decking I think IDH. When I see a new road or housing estate being built I think IDH. The IDH is to ecology what natural selection is to evolutionary biology; what atomic structure is to chemistry; what relativity is to physics. It is a simple but very deep concept that you can carry around in your head to help you decide whether an ecosystem (the sum of all living and nonliving things in any given area) is being managed in a responsible way that promotes biodiversity. It seems to me an idea with the potential to create a whole world of citizen ecosystem managers working to maximise biological diversity wherever they go. But no one except professional ecologists and university students seem to know about it. So here goes.

Stated simply the IDH proposes that biological diversity is increased by intermediate levels of disturbance to the living things in an area. Very low or very high levels of disturbance lead to low levels of biological diversity. Here ‘disturbance’ usually means the death or significant curtailment of some or all of the living things in an area. The basic premise of the IDH, that ecosystems must be disturbed to maximise their biological diversity, may surprise some people. Many amateur naturalists I see on social networks advise that we should simply let land alone to maximise its diversity and in some respects they are correct. Already-diverse, well-established natural ecosystems in places that humans rarely tread like pristine rainforest or the patches of fairly natural land remaining in most countries already receive a range of natural disturbances such as storms, drought, frost, and disease and are probably best left alone or managed by humans very carefully on a case to case basis. However, in human dominated environments like towns and cities and agricultural areas the IDH seems to me a very applicable and very useful idea to help us manage and maximise biological diversity.

To understand the IDH a bit better let us go back to my old friend and his lawn. He disturbs the ecosystem that is his lawn very frequently by mowing each week. If you look closely at the lawn you will see that it is woefully lacking in diversity and consists mainly of grass and moss. Now if the old feller was to chill a bit and maybe switch on the telly or read a book each time he had the urge to cut his grass and reduce mowing to, say, once every couple of months his lawn would still contain grass and moss but we would see other plants like dandelions, daisies and plantain in there as well: it would be more diverse. Now let us imagine that the old chap suffers some kind of life crisis and no longer gives a damn about his lawn and never mows it. Now we would find that dominant species like clover would creep into the lawn and swamp all the other plants in there. The diversity of his lawn would go down again because he is no longer disturbing it at all. This is how the IDH is thought to work. Intermediate levels of disturbance occasionally wipe out the dominant species that tend to overcome the many weaker species that can contribute to biodiversity. The IDH is best and most commonly applied to plant communities but this doesn’t lessen its importance: if you can create a diverse plant community you are almost guaranteed to create a diverse community of creatures such as insects and birds that feed on those plants. Plant community diversity is key to overall ecosystem diversity.

So taking all that we now know let us consider some environments that we are familiar with and consider how frequently they are disturbed and how this is affecting biological diversity in them. I have drawn a picture to help us with this (see Fig. 1). Most human dominated environments (towns, cities, gardens, parks, roadsides, agricultural areas) as well as receiving the storm, drought, frost, and disease disturbances that natural environments experience, receive an additional raft of human induced disturbances. Human dominated environments can be thought of as catastrophically and intentionally over-disturbed by humans to artificially maintain them in a constant artificial state of low diversity. We mow our lawns too much. We weed our gardens too much. Council workers mow park lawns and roadsides too often and weed plant borders too often. Trees are frequently removed from urban woodlands and roadside. Greenbelt is permanently disturbed by building new housing estates over it. Farmers disturb the natural plant communities in and around their fields with constant herbicide application and tilling. They over-disturb grass meadows by putting too many animals too frequently in them to graze. All this contributes to a living environment and countryside for most of us that is low diversity, bland and of minimal value for wildlife. But now that we are familiar with the IDH the solution is simple. We should not stop disturbing/maintaining human dominated environments altogether but we should reduce the frequency at which we disturb them. We must stop over tending our gardens. Be like me and basically leave your garden alone and cut it back once in autumn: seriously disturb it once per year. We must insist that local councils reduce the levels of disturbance that urban parks, roadsides, and semi-natural urban areas are subjected to. Less mowing and less weeding is required and the rate at which mature trees and shrubs are removed and cut back should be reduced. Farmers must be forced one way or another to reduce the quantity and frequency of herbicide spraying and tilling and they must reduce the intensity of animal grazing or we are destined to a countryside that is basically a low diversity industrial production line. It might at first seem an insurmountable task to increase biological diversity across the nations we live in, but it isn’t. It’s easy. All that you and me and the council workers and farmers have to do is less; sit on our asses more and disturb the land around us less so that the land gets closer to this sweet spot of intermediate disturbance where diversity flourishes.

Figure 1. The intermediate disturbance hypothesis in natural and human dominated environments. Human dominated environments are chronically over-disturbed and so low diversity. As a society we need to climb up the humped line in the plot by reducing disturbance to human dominated environments. If we do this biodiversity will increase.

There will be cynics among you who couldn’t care less about biological diversity and can’t see its importance but this is a deeply misguided point of view. Personally I am not especially interested in economic arguments for biological diversity but if you wish to pursue this avenue consider the discovery that the innocuous Madagascan flowering plant the rosy periwinkle can be used to develop effective anti-cancer drugs, enhancing the life of many and creating a billion dollar industry. Lose diversity and the next cancer drug, the next great discovery, the next big area of economic growth, will be lost too. More interesting to me is the interaction between environmental stress and biodiversity and how this will play out during the ever-growing threat that is global warming. Along with the IDH, another principle to emerge from the science of ecology is that diverse ecosystems tend to be more stable and withstand stress better: they don’t die away when you stress them with something like a rise in temperature or erratic weather; they withstand it; they ‘fight’ it if you like. Mainly for the reasons outlined in this article the UK is placed 189th out of 218 countries for the management of its biodiversity. It concerns me that in the face of the serious global warming that seems increasingly inevitable the remaining natural resources of the UK and other developed nations that have been stripped of their biological diversity will simply wilt and die.

And what of the IDH and society? Can we not all think of one or two dominant individuals in our social network whose removal would allow those remaining to flourish and diversify? And what of wider society; of state and corporation? The ordurous duo that tell us we cannot build home and work land because, well, the land does not belong to us. It belongs to those with capital. Those in-love bedmates that systematically created the crisis of climate and biodiversity and who, laughably, we now look to for solutions. Is it not time those two received a little disturbance to liberate those toiling under the heavy yolk? Surely it is and when it is done and all we have to depend upon is each other, only then will we see the world as it can be. Varied, abundant, even, stable. Only then will we know what beauty truly is.