An ecosystem is the sum total of all the living things and all of the non living things in a particular area and (potentially more importantly) all of the interactions that link them together. This is a big definition; it is pretty much all the stuff alive or dead in an area doing all the things that they do, all the time!
Ecosystems are big and complicated things, they have so many components that we have yet to count them all. To give you an example, there are a million insect species described globally, we estimate there may well be anywhere between 6 and 12 million species in total. Even at our best guess we have only discovered 1/6 of the total number of insects in the world. Now we have to do the same for plants, worms, crustaceans, fish and bacteria etc (On top of this insects are actually a pretty well studied group, all those Victorian butterfly collectors did get up to something).
So ecosystems are big, they are also complex. There are many interactions between each species in the system. Let us look at a simple example of a food web:
This food web has only 7 types of organisms in it. We have a root organism that represents a decomposer, (something like an earthworm, but more likely a bacteria species) who is happy to munch on the remains of all of the other animals and plants in our simple system. Then we have a and b, our producers. These are most likely plants and are not eating our root rather they take in material (Nitrogen) from the soil made by our hard working earthworm. D is our herbivore and feeds only on plants (specifically just b), most likely a caterpillar. C, e and f are all predators; as they feed on other animals, c and f also happen to be cannibals, quite happy to eat one of their own if they get the chance. We only have 7 types of creatures here, but already we have 18 different interactions.
This is just a simple food web; we haven’t included the interactions with the non living environment and the interactions both between species and within them that are not related to feeding (all the growth, reproduction, contest and territoriality). Even in our simple 7 species diagram without all of these extras thrown in it is very difficult to predict the change in one species given a change of any other one thing in the ecosystem. Indeed throw some equations in to try and explain these diagrams and even senior maths professors start to develop headaches.
So we have established that ecosystems are big and complex. Indeed they are so big and so complex that we haven’t even come close to describing them in terms of their individual bits (not that it stops us, we are scientists after all). So how do we tell if they have gone wrong? How do we know if they are damaged if we don’t know what is in them and how they work?
The key is that we don’t need to know everything to tell if something isn’t working well, we just need a few critical bits of information. Just like how playing a little bit of sport makes you informed enough to know if a professional player is good, great or should never ever earn that much money, we can look for key signs in ecosystems that show they are either healthy or on the verge of collapse.
Ecosystem integrity deals with this. It turns out there are a few critical signs that show if the overall health of an ecosystem is in dire straits:
1) Build up of waste material; this means that our root organisms from our diagram have been overloaded or wiped out. Healthy ecosystems don’t waste very much; everything is broken down and re used fairly rapidly. Our use of heavy plastics, dumping of industrial waste and release of radioactive material all lead to situations where natural ecosystems can’t handle the strain.
2) Loss of keystone species; if we come back to our diagram we can see that c has 6 links, there are 18 in total so c represents 1/3 of our total links. C is a very well connected guy (for a nasty cannibal), removing C will cause all sorts of damage and disruption to the food web. We have observed that the loss of single species can alter whole systems, the removal of the sea otter from California seas for their pelts led to the explosion of sea urchin numbers (a tasty treat for a hungry otter), these ate the young kelp and prevented the kelp forests from re growing, the kelp was key habitat for just about everything else in the ecosystem and so fish numbers crashed.
3) Large incidences of disease; this means that there are either alien disease species that are entering the ecosystem or that the environment is degraded to the point where many species are having difficulty getting enough food to maintain a healthy immune system. In nature disease is nearly universally present but it is not prevalent. Wild cattle do get diseases, indeed weak or old individuals often die from disease, but it is very rare for healthy natural populations to develop epidemics. If you look at human examples, it is common for a new disease to emerge and harm a few people (e.g. SARS death toll only 630) but only after World War 1 where there were millions of humans living without good food shelter or water did influenza kill 50 million people. It is the condition of the environment that determines the effect of a disease.
4) The migration of whole species into or out of a region; great migrations are common for certain species, but the removal of entire groups occurs only in the cases of natural disaster such as prolonged drought or flooding. Clear felling trees or burning forest are manmade activities that inspire similar results.
5) The reduction of diversity and the establishment of monoculture. When ecosystems begin to fail, there are often one or two hardy opportunistic plants or animals that can exploit the changing environment, they either feed on the dead or happen to be able to cope with new temperatures or conditions. In the early stages of ecosystem degradation these guys often become dominant and form monocultures. We have learnt to replace this process with intensive agriculture; we remove or destroy all other competitors, leaving only one plant growing in an ecosystem. These systems are then very unstable, prone to disease and collapse just as is described above.
I object to many modern waste management, fishing, agricultural, industrial and technological practices not on moral principle (“Industry is wrong man”) but on specific evidence that particular types of these practices lead to the collapse of ecosystem integrity. Dredging the bottom of the sea with a trawler beam is stupid, as it breaks the very system that produces what you catch. You wouldn’t break into your bank with a bulldozer every time you wanted to make a withdrawal, you would be harming your investment, it simply doesn’t make any sense.