by Peter Erridge
The longest running detailed study of ecological predator/prey interaction food webs in the world, Peter Erridge updates us on this 40 year project.
In the 1970s Andrew Hildrew, a natural sciences graduate, interested in ‘food webs’ set out to investigate Broadstone Stream on Ashdown Forest. This runs from Broadstone Farm under the B2110 to the River Medway east of Forest Row. Many biological surveys are snapshots of species or habitats and not systematically followed up. Surveys of this watercourse have continued for over 40 years and recognised internationally as the longest running biological survey of a stream. The work has since been mirrored in streams around the world.
A food web is the links of feeding among living things in a habitat. Generally large predators eat smaller prey but they may have preferences, called links. When larger feeders are very selective less favoured species flourish and vice versa. This is one of the main factors determining the numbers and species populating a habitat. Environmental factors and human activity will moderate this.
The chain starts with aquatic vegetation decay providing nutrients for algae, food for crawling insects and places to hide from predators. Other food sources originate from streamside – leaves from bushes and trees, insects, eggs laid by insects or other animals etc.
A wide range of species of animals without backbones, called invertebrates, can be found in streams. These range in size from 1-50mm or more. They feed on living and dead plant matter and on other smaller species of invertebrates. An example of life in a stream is that damselfly eggs hatch into nymphs; (newly hatched insects not resembling the adult). Nymphs feed on other life forms but themselves are food for other maturing insects. Invertebrates found in the stream include Mayfly, Smooth newt larvae plus two major predators, the Caddis fly, voracious feeders, and the Alderfly. These invertebrates, are in turn, taken by higher forms such as frogs and fish.
Detailed research over four decades carried out by the originator and subsequently his students, has shown how the range of species and numbers vary over time. The smallest species in the food web are found predominately in the upper reaches of the stream, which are less accessible to their predators. The food naturally flows from the upper reaches down the watercourse to the main river. The population of the stream will vary with the rate of water flow.
Long periods of heavy rain have the effect of washing species further downstream but Broadstone Stream has many low flow areas, small pools called refugia, which help to retain the community of invertebrates. Any decrease in a predator group allows their prey species to proliferate. In 2002 modification of stream conditions allowed brown trout from the main river system to come into the tributary resulting in a substantial decrease in number of caddis flies, thus allowing their prey to increase in number.
Certain factors such as temperature, oxygen level and acidity or the introduction of toxic substances, affect the fauna and flora of streams. As acidity increases biodiversity decreases. Frogs can survive in more acidic conditions than one of their prey, mayfly larvae. Fewer adult mayflies means fewer eggs and reduced numbers of mayflies the following year. Acid rain in the 1980s affected Ashdown Forest increasing acidity of the Broadstone stream. Golden ringed dragonflies preferred the more acidic conditions for breeding. Reduction in atmospheric sulphur and other toxic emissions resulted in decreased acidity then to a decline in the numbers of these dragonflies. By 1988 invertebrate species in one section of the stream had reduced to 28, yet by 2002 had risen to 128.
Improving the environment can only be successful with better understanding of each habitat and the balance of nature. Even minor changes can have disastrous effects. This research showed the range of responses by individual species, the ecosystem processes, and the biodynamics of the whole stream and has since been applied to water systems worldwide. What was intended as a ‘weekend’ study has become a life’s work and stimulated worldwide interest in a small corner of Sussex.