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Trente ans de lutte contre l’onchocercose en Afrique de l’Ouest. Traitements larvicides et protection de l’environnement

 | 
Laurent Yaméogo
, 
Christian Levêque
, 
Jean-Marc Hougard

Références des articles présentés sur le cédérom / Papers references presented on CD-ROM

Longitudinal zonation of lotic insects in the Bandama River System (Ivory Coast)

François Marie Gibon et Bernhard Statzner

Résumé

The longitudinal zonation of three lotic insect groups, viz. Hydropsychidae (Trichoptera), Philopotamidae (Trichoptera), and the Simulium damnosum complex (Diptera) in the Bandama River System (West Africa) is anomalous when compared to the patterns known from other geographical areas: In general from the highest to the lowest riffles downstream there appears to be no clear species replacement. Instead, there is a steady increase of species numbers due to the occurrence of additional species. These distributions are related to the gradient in the length of the period without flow from the north (temporary streams) to the south (permanent running waters), the consequent lack of well-defined sources and the relatively gentle slopes of the channels.

Texte intégral

Introduction

1Recently Lévêque et al. (1983) evaluated the longitudinal zonation of the macrofauna in the Bandama basin, a river System extending about 1000 km from north to south through the Ivory Coast (West Africa). They distinguished three zones: (1) headwaters and small affluents, (2) a considerable, long mid-reach, which was relatively uniform and (3) a relatively short lower reach (estuary). With regard to invertebrates the mid-reaches in the main courses of the Bandama basin were mainly characterized by the caddisfly family Hydropsychidae, particularly by Cheumatopsyche falcifera and C. digitata at periods of low discharge (see Fig. 15 and 16 in Lévêque et al., 1983).

2This is an odd zonation pattern compared to those found in other geographical areas (Illies & Botosaneanu, 1963; Hawkes, 1975; Botosaneanu, 1979). A recent study of the taxonomy and the distribution of the Hydropsychidae in the Ivory Coast (Statzner, 1984; Statzner & Gibon, 1984) allows a more detailed definition of the above zonation. Results on Hydropsychidae and two other lotic insect groups so far available, the caddisflies Philopotamidae (Gibon, 1984) and the blackflies of the Simulium damnosum complex (Vajime & Quillévéré, 1978; Quillévéré, 1979 and person. commun.) give evidence, that the long mid-reaches exhibit no uniform faunistic stream zone.

Methods and the area studied

3The papers mentioned in the introduction, as far as they deal with the Ivory Coast, contain detailed information on the methods used and the area studied. Thus only the essentials will be briefly reviewed here.

4The distribution patterns of the:

  1. Simulium damnosum complex are exclusively based on benthic samples from 35 localities,
  2. Hydropsychidae are mainly based on benthic samples, for some localities additional information from light trap samples was available. Only a few places were surveyed by light trap sampling. The number of sampling stations in the Bandama basin exceeded 50,
  3. Philopotamidae are exclusively based on light trap samples, since there are still unsolved problems in the larval taxonomy. 31 localities were considered in the Bandama basin.

58 monitoring points in the Bandama System were sampled for Trichoptera at monthly intervals over more than one year, and therefore more information was available for them than for the other stations. The main courses of the Bandama basin were more intensely studied than the smaller affluents. The differences in the sample techniques, discussed in detail by Statzner (1984), lead us to treat the results as follows: even if a species was missing at a certain locality it was considered to be ‘present’ there, if we found it north and south of this locality in the appropriate affluent at some time. Thereby we exclude the north-south-north-shifts of species (Quillévéré, 1979; Elouard, 1983), related to the climatic annual cycle, from our considerations, and deal with the ‘maximal distribution’ reported in any of the above groups. It should be noted that the uppermost distribution limits of the Trichoptera species are less precise (Statzner, 1984), since (1) it was not always possible to sample the northern parts of the streams due to cessation of the flow and (2) no samples were available from the upper Bandama and upper N’Zi from the period before regular insecticide treatment of these stream reaches were introduced through the Onchocerciasis Control Programme. The latter may have affected the presence of Macronematinae in those regions. No data exists from the period before the construction of the Kossou reservoir. We assume that the species composition in the reach now dammed was similar to that we found upstream of the lake. Taabo reservoir was build after the entomological sampling programme in the area.

6The most obvious longitudinal gradient in the abiotic factors of the Bandama system is to be found in the discharge pattern. On average the northernmost streams in the basin are running waters for only just over half the year. In the Southern direction the period without flow becomes shorter and shorter, until the stream is a permanent one. Due to large variations in the annual climate the periods without flow as well as the borderline between temporary and permanent stream are not fixed. In stream reaches in which flow ceases water usually remains in pools during the dry season.

7Thus, in contrast to nearly all streams so far studied with regard to the longitudinal zonation of the macrozoobenthos, the main channel and all its large affluents do not start with a well-defined source. Therefore, for most of the year, all streams originate in a lower reach where the slope is gentle.

8Compared to the key factor ‘discharge pattern’ all other abiotic factors so far considered as important in stream zonation studies exhibit relatively weak longitudinal gradients in the Bandama system (Lévêque et al., 1983).

Results and discussion

9The longitudinal distribution of the three groups under study (coded in Table 1) gives evidence that the ‘zone à Hydropsychidae’ sensu Lévêque et al. (1983) is not homogeneous in the Bandama River system (Fig. 1). In smaller streams situated west (Niouniourou) or east (Agneby) of the Bandama these three lotic insect groups extend further south than in the Bandama itself, where no rapids occur in the lower reach.

10In the main courses of the Bandama basin a steady increase in the species number was generally observed from north to south and almost no loss of northern species on the way south downstream to the lowest riffles of the Bandama. If a species disappears on the way south in the ‘zone à Hydropsychidae’ it is certainly a rare one, e.g., Aethaloptera sp. (2), Polvmorphanisus sp. I (3) and II (4), or Protomacronema pubescens (K). The latter was found further south in other stream Systems. Apparently major changes in the Trichoptera distribution take place at locations sampled at monthly intervals over a longer period (monitoring points!), and isolated occurrences were usually reported from these localities. The distribution patterns of these rare or isolated species are rather uncertain. The smaller affluents of the Bandama basin contain a fauna similar to that of the headwaters of the main rivers of the system. The example included in Figure 1 is a combination of data from various tributaries studied at various distances from the main courses and therefore represents an ecological and not a purely geographical classification. A typical species of small tributaries is Macrostemum inscriptum.

11The general pattern of species distribution from ‘sources’ downstream to the lowest rapids of the Bandama is not a clear species replacement, as is usually found in benthic macroinvertebrates (see reviews referred to in the introduction), but a steady increase of species numbers on the way downstream due to the occurrence of additional species. We assume this to be the typical zonation pattern of lotic insects in streams running from north to south over a considerable distance, having no well-defined source, and a gentle slope in this part of Africa.

Fig. 1. Schematic presentation of the longitudinal distribution reported for three lotic insect groups in the Bandama River system before the construction of reservoirs as well as their occurrence in smaller streams situated west (Niouniourou) or east (Agneby) of the Bandama. Presence of species in small tributaries of the Bandama System consisting of samples from several streams. The streams generally flow from north to south. starting in the sudanese savanna. crossing the guinean savanna, and then entering the forest area. The length of the Bandama is approximately 1000 km. Codation of species in Table I. Arrows: monitoring stations (higher sampling frequency); dot behind the species code: isolated occurrence.

Table 1. Code for Hydropsychidae. Philopotamidae (Chimarra spp.). and Simuliidae (Simulium spp.) used in Fig. 1. Letters: adults and associated larvae; numbers: unassigned larvae; graphic symbols: species found only in small stream west or east of the Bandama System.

A

Cheumatopsyche digitata

B

C. copiosa

C

C. falcifera

D

C. albomaculata

E

C. gibbsi

F

C. akana

G

C. sexfasciata

1

C. sp VIII

C. sp XI

H

Amphipsyche senegalensis

I

A. herneri

J

Protomacronema barnardi

K

P. pubescens

L

Macrostemum alienum

M

M. inscriptum

M. pulcherrimum

N

Aethaloptera dispar

2

A. sp

O

Polymorphanisus angustipennis

P

P. hargreavesi

Q

P. elisabethae

3

P. sp I

4

P. sp II

Leptonema sp II

R

Chimarra sassandrae

S

C. petri

T

C. intextu

U

C. prodhoni

*

C. triangularis occidentalis

V

Simulium damnosum ss.

w

S. sirbanum

X

S. soubrense

Y

S. sanctipauli

O

S. vahense

12Specific differences in the ability of Hydropsychidae to survive periods without flow as larvae have been reported elsewhere (Statzner, 1982). In addition, whether adults are capable of surviving droughts or recolonizing a stream reach after the resumption of flow by immigration from other localities will mainly determine, how far a species extends to the north. In the southernmost rapids of the Bandama basin several species occur which are generally found in streams of the forest area. In this region the guinean savanna is extending further south than alsewhere in the country and the species distribution indicate a zone of faunistic transition.

13Received 29 March 1984; in revised form 4 June 1984; accepted 4 June 1984

Acknowledgement

14The linguistic advice of Mrs. Liz Mole (Weinheim) is gratefully acknowledged.

Bibliographie

References

Botosaneanu. L., 1979. Quinze années de recherches sur la zonation des cours d’eau: 1963-1978. Bijdr. Dierk. 49 : 103-134.

Elouard, J. M., 1983. Impact d’un insecticide organophosphoré (le téméphos) sur les entomocénoses associées aux stades préimaginaux du complexe Simulium damnosum Theobald (Diptera: Simuliidae). Thesis Univ. Paris-Sud. Orsay. 576 pp.

Gibon, F. M., 1984. Recherches sur les Trichoptères d’Afrique de l’Ouest. 3. Philopotamidae de Côte d’Ivoire. Rev. Hydrobiol. trop. 20:

Hawkes. H. A., 1975. River zonation and classification. In B. A. Whitton (ed.), River ecology. Blackwell Scientific Publications, Oxford: 312-374.

Illies, J. & L. Botosaneanu. 1963. Problèmes et méthodes de la classification et de la zonation écologique des eaux courantes, considérées surtout du point de vue faunistique. Mitt. int. Ver. Limnol. 12 : I 57.

Lévêque. C., C. Dejoux & A. Iltis. 1983. Limnologie du fleuve Bandama. Côte d’Ivoire. Hydrobiologia 100 : 113-141.

Quillévéré. D., 1979. Contribution à l’étude des caractéristiques taxonomiques, bioécologiques et vectrices des membres du complexe Simulium damnosum présents en Côte d’Ivoire. Trav. Docum. O.R.S.T.O.M. 109, Paris. 304 pp.

Statzner. B., 1982. Population dynamics of Hydropsychidae (Insecta; Trichoptera) in the N’Zi River (Ivory Coast), a temporary stream partly treated with the insecticide Chlorphoxim. Rev. Hydrobiol. trop. 15: 157-176.

Statzner. B.. 1984. Keys to adult and immature Hydropsychinae in the Ivory Coast (West Africa) with notes on their taxonomy and distribution. (Insecta: Trichoptera). Spixiana 7: 23-50.

Statzner. B. & F. M. Gibon. 1984. Keys to adult and immature Macronematinae (Insecta: Trichoptera) from the Ivory Coast (West Africa) with notes on their taxonomy and distribution. Rev. Hydrobiol. trop. 20: Vajime, C. & D. Quillévéré, 1978. The distribution of the Simulium damnosum complex in West Africa with particular reference to Onchocerciasis Control Programme area. Tropenmed. Parasitol. 29: 473-482.

Table des illustrations

Légende Fig. 1. Schematic presentation of the longitudinal distribution reported for three lotic insect groups in the Bandama River system before the construction of reservoirs as well as their occurrence in smaller streams situated west (Niouniourou) or east (Agneby) of the Bandama. Presence of species in small tributaries of the Bandama System consisting of samples from several streams. The streams generally flow from north to south. starting in the sudanese savanna. crossing the guinean savanna, and then entering the forest area. The length of the Bandama is approximately 1000 km. Codation of species in Table I. Arrows: monitoring stations (higher sampling frequency); dot behind the species code: isolated occurrence.
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Auteurs

O.R.S.T.O.M., Hydrobiologie, 24 rue Bayard, F-75008 Paris, France

Zoologisches Institut I, Universität (T. H.), Postfach 6380, D-7500 Karlsruhe. F.R.G.

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