PUBLICATIONS

Issue 35 - Autumn 2005

Thoughts on Wildlife Lakes and Ponds

At the heart of conservation is the relationship between flora and fauna. Common to both, be it explicitly or implicitly, is their dependence on water.

To the landscape designer reared on ornamental horticulture, the ecologic or wildlife lake or pond might seem at first sight unkempt, overgrown or even chaotic. To the naturalist it is none of these things, but rather a scene of natural order and a thing of beauty in its own right. This is a question of approach and of idiom, and it must be realized from the start that the ecologic and wildlife landscape is fundamentally different from the purely ornamental.

By way of summary, a wildlife or ecologic lake can be briefly described as a body of fresh water in which all planting zones offer the maximum diversity of plant species. Adequate shelter, protection and food are provided for the fauna that inhabit, temporarily or permanently, those zones. There are areas in which those vegetative zones merge without obvious breaks. There is open water as well as open patches of ground, and the depth of water facilitates dabbling and diving wildfowl.

Islands, with sheltered bays [together with the shoreline of the lake] provide adequate protection from prevailing winds. Tree planting in the hinterland is accompanied by dense under-planting and open breaks; and the hinterland or surrounding area acts, where necessary, as a buffer zone between the water and run off from agricultural chemicals or pollutants. Finally, the planting of native species of vegetation is the abiding rule - not the exception.

It is worth making the point that to regard a lake as a distinct ecosystem in its own right is an artificial convenience. Factors far beyond the vicinity of the lake basin are critical to its survival as a biosystem, such as sunlight plus many factors closer to home.

The point is that a lake is not a self-sufficient entity, but then neither is any other so called ecosystem. So some line of demarcation has to be drawn. For a lake or pond that line is certainly not along its margins, but must also include the hinterland surrounding the lake, for it is the combination of land and water that makes up the support system described as an aquatic wildlife ecosystem.

The diversity of life that any lake can support depends upon many factors, especially upon the acidity or alkalinity of the water. This in turn is usually the result of geological factors.

Extensive conifer plantations and the blight of acid rain, among other factors, can adversely raise the acidity of a watercourse, sometimes to intolerable levels. The range and density of animal species, as well as their size, is usually restricted in acid waters. Dragonflies and damselflies are among the few genera which show a preference for, albeit mildly, acidic water, although vertebrate species often show a wide range of tolerance. Pike [Esox lucius], for example, are found in waters ranging from about 4.9 to 10.7 on the pH scale; perch [Perca fluviatilis], have a similar range, from 4 to 9.2, whereas the common carp [Cyprinus carpio] requires a pH above 5.5 to thrive. [A value of 7 represents neutral on the pH scale; below that the water is regarded as acidic.]

The diversity of plant genera is restricted in acid waters too. The Quillworts [Isoetes] are an exception, being found in waters with a pH value as low as 1.8. Molluscs, crustaceans and all arthropods [animals with a hardened skin or cuticle] so significant in the food chain, all require hard or lime based waters to be plentiful, and the same is true for most microorganisms. It is in slightly alkaline or mildly 'hard' water that by far the greatest diversity as well as density of plant and animal species are to be found.

Seasonal and longer term fluctuations of pH levels may take place in a water course, and there is such a thing as regression to the mean. As an example, water drawn from a remote source [for instance, a municipal water supply] if it shows an extreme pH value will almost certainly regress toward the mean if allowed to remain in a well vegetated, still water lake. The bed of the lake, if it has a different geological base from the water source, will be a major factor in changing the pH value, but so will the vegetation, which will tend to 'pull' the newly introduced water toward the mean. This is a useful phenomenon, since it means that an artificial lake that has to be topped up from time to time with water having an extreme pH should not be adversely affected. [Of course, the volume of introduced water must be small in relation to the volume of the lake if a constant fluctuation of the pH value is to be avoided]. It is not generally regarded as good ecological practice to artificially manipulate the pH of a lake, other than what can be achieved by vegetation in relation to the geological environment. Generally, aquatic and wetland conservation is best directed toward conserving what is indigenous in any given location. Clearly, a lake designed to fit as naturally as possible into its immediate environment offers its flora and fauna the best chance of long term survival.

So much is written about the need to achieve an 'ecological balance' in the landscape that it is easy to overlook the fact that 'balance' in any ecological sense is no more than a metaphor and a misleading one at that. Balance suggests a self perpetuating system free of change. In fact nature is in a continual state of flux. Plant succession inevitably follows upon the availability of habitat.

To the casual observer the wild landscape might appear as static, but on a larger timescale in even the most mature landscape change is, inexorably, taking place. A marsh will once have been a lake: as deposits of leaf litter and detritus raise the level of the shallows, the banks encroach inwards. Emergent plants are joined by bog plants which in turn give way to alder and willow carr .

Scrubland will eventually result, with comparatively few plant species supporting minimal wildlife. Habitat creation is the function of the landscape designer, and the aim should be to create the maximum diversity of habitat with the minimum maintenance regimen.

The notion that an ecologic lake can be allowed to 'run wild' is quite wrong. While it is certainly true to say that the maintenance of a 'natural' landscape is less than that required for an ornamental one, if the policy of supporting the maximum amount of flora and fauna is being pursued, without a maintenance programme the optimum diversity of the site will certainly not be realised. Only by what may be described as an interventionist policy of management will a wildlife site provide the diversity and number of habitats that it is capable of, with a corresponding diversity and richness of both animal and plant species.

Peter Semple

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