The Effect of Ddt on the Fauna of a Central African Stream by H
Total Page：16
File Type：pdf, Size：1020Kb
[Reprinted from the 'Annals of Tropical Medicine & Parasitology,' Vol. 56. No. 1. April, 1962] THE EFFECT OF DDT ON THE FAUNA OF A CENTRAL AFRICAN STREAM BY H. B. N. HYNES AND T. R. WILLIAMS (From the Department of Zoology, University of Liverpool, and the Department of Parasitology and Entomology, Liverpool School of Tropical Medicine) (Received for publication December 22nd, 1961) In recent years it has become common practice in many parts of the world to use DDT for the control of the larvae of stream-dwelling insects, particularly Simu/ium. The effects of insecticides on stream communities have been reviewed by Hoffmann (1959), and several authors have expressed disquiet at the possible effects of their widespread use in streams and the dangers of upsetting the ecological balance of the community (Hynes, 1960 and references therein). This effect of insecticides is well known in agriculture, where, for instance, Pimentel (1961) has recently shown that they may actually increase the numbers of a pest by destroying its natural enemies. Similarly, Hinton (1955), in a review of the effect of insecticides on the balance of natural populations, goes so far as to write ' We create our own pests but not from choice : they are one consequence of our interference with nature, with our attempts to control it.' There are, indeed, already indications that attempts to control Simu/ium with insecticides have had the reverse effect : Davies (1950) reports a very large increase in the numbers of these insects in the years following application of DDT to a stream in Ontario. It was against this background of possible danger that one of us appealed for caution in the use of DDT against Simu/ium in Africa (Hynes, 1960). Caution seems particularly called for in a region where so very little is known about the biology of the rapid streams in which Simu/ium breeds. Ecological studies of South African streams have recently been started (Harrison, 1958; Harrison and Elsworth, 1958; Oliff, 196oa, 196ob) ; but in Central Africa, apart from notes in several scattered faunistic and taxonomic works, little ecological investigation of small streams and rivers has been carried out. The only exceptions are the work of van Someren (1952) on a trout stream in Kenya, which is, however, primarily concerned with the trout themselves, and that of Marlier (1954a), who gives a general description of the fauna to be found in different types of streams in the eastern Congo. Other ecological observations have been limited to certain groups or species (Marlier, 1954b; Hynes, 1953), or to fishes (e.g., van Someren, 1952; Marlier, 1953). Against the background of our ignorance of African stream communities, we should like to quote from the agricultural world : Because communities have a structure and a physiology they should be studied as a whole. This approach has been neglected in most insect control studies' (Pimentel, 1961). Similarly, Stern et al. (1959), discussing problems arising from the widespread and often indiscriminate use of insecticides, state : Few studies have included basic investigations on the effects the chemicals might have on other components of the 78 79 ecosystems to which the pests belong', and later, 'because fundamental knowledge is lacking, the investigator may be unaware of the intricate nature of the biotic complex with which he is dealing, and of the destructive potential that many chemicals in use today have on the environment of the pests.' During a visit to Uganda in December, 1960, and January, 1961, we were therefore very pleased to have the opportunity of studying a stream which had been dosed with DDT. We are well aware of the limitations of our work, confined as it was to a short period, but, in that we attempted to study the entire animal community, we feel that our data may be of value in assessing the dangers of the use of insecticides in African streams. Corbet's (1958) description of the effect of DDT upon the fauna of the Victoria Nile is the only other published study of this kind from Central Africa, and is much more limited in its scope. The River Manafwa The river Manafwa rises at a height of 13,900 feet on Mount Elgon in eastern Uganda, and flows in a south-westerly direction, eventually to join Lake Kioga. For the first nine miles it flows through dense and almost impenetrable forest, from which it emerges at about 6,000 feet. It then flows for five miles through cultivated land, on which plantains and coffee are the main crops, to the township of Bulucheke at 4,400 feet, and our study followed it for a further 71 miles, through similar country, to the mouth of its tributary, the river Sala, at 3,950 feet. From the forest edge, where our study began, to the mouth of the Sala, the main stream receives several tributaries of various sizes, and these steadily increase its size but do not much alter its general appearance. At the forest edge the river is about four yards wide, flowing swiftly over stones among large rounded boulders, which presumably it can move during spates. At 5,200 feet, the highest point on the motorable track up the valley, it is slightly wider, and the largest stones are about head-sized. It continues in this condition, although widening steadily to about seven yards at Bulucheke and to 10-12 yards at the mouth of the Sala. In a few places virgin rock projects up from the stream bed, which is composed of stones and gravel, but everywhere the current is swift, even at a time of low water. There are no deposits of very fine gravel, sand or silt, but we found leaves, sticks and other vegetable debris lodged amongst the stones. At the time of our visit the water was shallow (although as deep as two feet in places), but it was clear that during rainy periods, which are seasonally rather ill-defined on Mount Elgon, the water rises at least 2-3 feet. Everywhere the stones were smooth and clean and were very little coated with algae. The only visible vegetation was a sparse coat of a creeping liverwort, which occurred on some of the larger stones at stations from 4,600 to 4,200 feet. This condition appeared to be general in streams on the mountain, although in some streams at higher altitudes we found a little of the alga Lemanea, and some moss at very much higher altitudes. Fishes are scarce in the stream and are reported not to occur much above Bulucheke, up to which point the siluriid Amphilius jacksoni (Boulenger) ascends, and also a species of barbel. We ourselves collected A. jacksoni only at 4,200 feet, and we found a single small Clarias at 4,150 feet. As our collecting methods, particularly for crabs, were such as to allow us readily to catch and observe fishes, it is clear that they are an unimportant element of the fauna. 8o The Dosage with DDT On December 2nd, 1960, in the course of testing a new preparation of DDT, Mr. M. A. Prentice, of the Department of Medical Entomology, Kampala, had applied insecticide to the stream st Bulucheke at a point just below our collecting-station at 4,400 feet. This was done in such a way as to maintain a concentration of one part per million of DDT in the river water for 30 minutes, which was completely effective in eliminating Simulium neavei from crabs for a considerable distance downstream. We should, however, County boundary Road Track Forest 0 Collecting station • Dosing point MAP of the section of the river Manafwa on the slopes of Mount Elgon in which the investigations were carried out. emphasize here that this single treatment was designed as a test of a new insecticide prepara- tion, and not as a control measure for S. neavei. For the latter, repeated treatments during several weeks would have been carried out. On the same day, the river Sume, a tributary which joins the main stream at 4,300 feet, was treated at a high altitude with a soluble block containing DDT ; but, as this had little effect on the fauna of the tributary, our study was largely confined to samples taken from the river Manafwa alone. Data from the Sume are, however, given in the Appendix. 8 METHODS In order to study the fauna, a large general collection was made at each of eight stations distributed from the forest edge to the mouth of the river Sala. These were made on December 31st, 1960, and January znd, 1961, i.e., 29-31 days after the application of the DDT. The collections were made by kicking up the substratum and overturning stones upstream of a fine-meshed hand-net (6o meshes per inch), which was periodically emptied into a dish. The entire collection was then preserved unsorted in formalin. This method of collecting is to some extent selective, failing to take ancylid limpets and other closely adherent animals (Macan, 1958). At each station, therefore, a special search was made for such animals as Burnupia and Afropsephenoides, and their presence or absence was noted. Apart from these, however, the net samples can be expected to contain a fair representation of the fauna and to be comparable between stations. At the same time, extensive collections were made with a coarser-meshed net for crabs (Potamon spp.) and fishes, and any invertebrates taken in this way were noted if they had not been seen in the fine-mesh samples. In the laboratory in England, the fine-meshed samples were washed free of formalin, and the animals and light debris were floated off in a saturated solution of calcium chloride, by a technique modified from that of Beak (1938) and described by Hynes (1961).