Wildlife Gardening – Population Changes
N.B. If your new to David’s monthly column then please be sure to read the ‘setting-the-scene’ opening piece first.
In recent years egrets have, quite naturally, moved into the UK. Presumably their required niche became available due to climate change or possibly due to an advantageous genetic change in their population. The original founder organisms bred and the species has now spread to occupy a broad swathe of Southern England, including my own area.
Plants will move too. For example tongue orchids seem to have colonised parts of the West Country from wind-blown seed arriving from across The Channel.
Once an organism has colonised a new land or, as in red kites or sea eagles been reintroduced, then their numbers can possibly increase. Then they will spread to fill their available niches until numbers cease growing and the population will stabilise. However, as the total wild food supply within the UK will not have changed some other competing or prey species must have lost out and their numbers will have declined.
Perhaps you have noticed new species in your own backyard over recent years. Possibly you will have lost some too.
Here, at Forest Edge, we have lost all our starlings and have only occasional sightings of house sparrows. Both were once common. Buzzards and kites are now frequently seen, but that was not the case some ten years ago. Our short-tailed voles and shrews have also declined.
Much scientific study on animal population numbers has been carried out by the University of Oxford in its woodlands at Wytham Woods, near Oxford. Here they’ve studied voles, stoats, tawny owls and great tits amongst many others. Generally many of these organisms have, over an extended period and in a fairly constant environment, maintained near stable populations. Naturally levels will change year on year, and environmental fluctuations are partly responsible for that, but occasionally a population will crash due to disease or competition.
Some organisms (most?) are very directly affected by human activity. For example otter numbers dived during the mid C20 due to habitat loss / change, pesticide poisoning and the effects of hunting / water keepers. With a more benign approach to the animal’s ecology the population has now become comparatively greater.
One organism whose population is easy to study is the holly-leaf miner. This insect’s larval stage spends its days eating away at the tissues inside a holly leaf. Seek out a local holly bush and randomly collect fifty leaves. Now count the number of larval trails (often they combine to form a ‘blister’) that you can spot – this will give an indication of the population size which can be compared from year to year.
But it is even more fun than that! If the larva survives to adulthood you should be able to see a small exit hole. If the larva is not visible by shining a bright light beneath, then it will have died due to disease or if the trail ends as a ripped hole, it has been eaten by a bird.
There is more detailed information on:
Certainly it is worth a few minutes to explore a local holly bush or tree to see the signs of this often over-looked animal.
Population size is regulated by many aspects of the environment, with food availability being especially important. One other controlling influence is the density of the organism. In my own garden we have populations of small mammals – short and long-tailed voles, woodmice etc and also rabbits – and stoats, owls and kestrels that feed off them. As the numbers of the prey species increases so we see more of the predators, to such an extent that our patio long-tailed vole and wood mouse populations have been killed off completely during late 2008. High density of prey will attract predators and also makes disease transmission easier. It is not just more prey are killed, it is a higher percentage that are killed with higher population levels.
With a wildlife garden one has the opportunity to watch population changes that may be much less visible in a conventional garden. Certainly we take great interest in the relative numbers of small mammals with time.
Snails make good study species as well. Their populations can be compared in different environments and even their evolution. There is free study material on the OU web site, for example: http://openlearn.open.ac.uk/course/view.php?id=1646
Next month – The Approach of Spring in the Garden.
David is an ex-lecturer in Environmental / Biological Sciences.
He has written for most of the UK gardening magazines, including the RHS.
Forest Edge’s garden has been widely covered in magazines and has been featured in a BBC’s Gardeners’ World special.
He also maintains a blog at https://nwhwildlife.org/