Environmental Biology o f Fishes 28: 179-187,1990. © 1990 Kluwer Academic Publishers. Printed in the Netherlands.
The proportion of different eco-ethological sections of reproductive guilds of fishes in some African inland waters
Michael N. Bruton & Glenn S. Merron J.L.B. Smith Institute of Ichthyology, Private Bag 1015, Grahamstown 6140, South Africa
Received 16.10.1989 Accepted 5.12.1989
Key words: Lakes, Floodplains, Rivers, Perturbation, Resilience, Resistance, Guarders, Non-guarders, Bearers, Altricial, Precocial, Predictable, Unpredictable, Ecology
Synopsis
The continent of Africa has a wide variety of inland waters ranging from rift valley lakes to endorheic and coastal lakes, floodplains and rivers. This paper makes a preliminary comparison of the number of species in different eco-ethological sections of the reproductive guild categories of non-guarders, guarders and bearers in ancient African Great Lakes (Malawi, Victoria and Tanganyika), fluctuating endorheic lakes (Ngami, Chad and Chilwa), typical rivers (Orange-Vaal, Limpopo, Phongolo, Sabi-Lundi, Middle and Lower Zambezi, Kafue, Cunene, Okavango, Niger, Luongo, Lower Zaire) and wetlands (Okavango Delta and Kafue floodplain). The results indicate that the highest percentage of bearers and guarders is found in the ancient African Great Lakes, which are characterised by relatively predictable physico-chemical regimes, whereas a higher percentage of non-guarders is found in the rivers and wetlands, which have less predictable physico-chemical regimes. The management implications of this observation are discussed, and the useful ness of the species as a unit in ecology is assessed.
Introduction bird niches into ‘groups of species having very simi lar ecological roles within a community’. Root de Although the Linnean system adequately classifies fined a guild as a ‘group of species that exploit the species according to their phylogenetic relation same class of environmental resources in a similar ships, it does not (and was not intended to) indicate way’, and recognised that not only trophic but also the nature of the species ecological relationship to reproductive characteristics need to be taken into other species and to the environment. The species account when determining a guild. One commonly is nevertheless almost universally used as a unit of used trophic classification is the Functional Feed study in ecology. There have been many attempts ing Group (FFG) of Cummins (1979). This mor- to develop an ecological classification, most of pho-behavioural classification, which has mainly which have concentrated on groupings of animals been applied to stream invertebrates, is based on which feed or breed in similar ways. Feeding habits their adaptations for food acquisition and facil determine, to a large extent, a species relationship itates an evaluation of the changing invertebrate to other species, whereas breeding modes are con assemblages found along a stream on a functional cerned with maximising an individual’s contribu rather than on a taxonomic basis. tion to its species’ gene pool. The term ‘guild’ was Classifications of reproductive modes have usu applied by Root (1967) in an attempt to arrange ally been based on the extent of parental care given 180 to the young, and were initiated by Noble (1927) 1978,1981b, 1985, Bruton 1986, Balon et al. 1988, for Amphibia and Nice (1962) for birds. Fishes Compagno 1990). were treated in a similar way by Blumer (1979, Reproductive guilds may be represented as al 1980). Kryzhanovsky (e.g. in Kryzhanovsky et al. ternate states from generalised to specialised types 1953) independently developed other criteria, such (Balon 1975b, 1981a, c, 1989, Bruton 1989, Table 1, as, for example, characters of early ontogeny (tem Fig. 1). The most generalised section is the non- porary respiratory circulation), spawning styles guarders which scatter gametes on open substrata and spawning ground characteristics, for the classi (including open water) and exercise no parental fication of reproductive groups in fishes of some care. Non-guarders produce a large number of eggs regional faunas. The above system was expanded and have ova that are poorly endowed with nutri into the most useful modern classification of repro ents. They invest a relatively small amount of ener ductive styles in fishes by Balon (1975a, b, 1981a, b, gy in each of a large number of young. The more 1985). His reproductive guilds are separated into specialised guilds are those in the sections of guard three main eco-ethological sections (Table 1). This ers and bearers which spawn in specially prepared ecological classification (Balon 1975a) distinguish nests or carry the young internally or externally. es species that are similar in their use of the ecosys They exercise more extensive parental care, have a tem and which have convergent developmental low fecundity but large-yolked ova, and invest a traits, irrespective of phyletic origin (Balon 1981b). large amount of energy in each of a small number of This classification of reproductive guilds by Balon young (Balon 1975b, 1985). thus bears no resemblance to the Linnean classifi Superimposed on these genotypic states are phe cation with the result that some phylogenetically notypic states whereby a fish can alter its life-histo- ancestral fishes (e.g. mustelid sharks, coelacanth) ry style within a generation or generations in re are found in advanced guilds and vice-versa (Balon sponse to the environment (Balon 1979, 1981c, 1984, 1986, Noakes & Balon 1982, Bruton 1989, Fig. 2). Altricial fishes produce small, incompletely Table 1. Summary of the eco-ethological sections and ecological developed young and are generalists capable of groups of fishes in the reproductive guild classification of Balon surviving in unstable, uncrowded environments in (1975b, 1981a). which they are mainly subject to density-independ ent mortality. Precocial forms produce large, well- Non-guarders - Spawners on open substrata (egg scatterers): seven guilds of developed young and are specialists that are best pelagic, gravel, plant and sand spawners which produce buoyant able to survive in a stable, crowded environment in or adhesive eggs and do not hide or guard the young which the community is mainly subject to density- - Brood hiders: five guilds of beach, gravel, cave and other dependent mortality. The interaction between the specialised spawners which hide the young but do not guard genotypic and phenotypic states allows the fish to them respond successfully to environmental changes in Guarders the long and short term. The mechanism for chang - Substratum choosers: four guilds of pelagic, above water, rock ing from an altricial to precocial state is simply an and plant spawners whose young are tended by the parents - Nest spawners: eight guilds of froth, gravel, plant, sand and alteration in the relative timing of key develop other specialised spawners which make protective cavities in mental events, e.g., size at first exogenous feeding which they guard the young (Balon 1985, 1989, Flegler-Balon 1989). Bearers The stability of an ecosystem is a major determi - External bearers: five guilds of mouth, gill-chamber, pouch nant of the nature of the animal community which and other spawners which carry the young in a cavity opening to inhabits that ecosystem (Margalef 1969, May 1974, the exterior in order to protect them Orians 1975, Bruton 1989). Climax ecosystems - Internal bearers: four guilds of spawners which carry the which have reached a high level of complexity and young inside the body cavity where they may receive some nutrition from the parent biotic interdependence over a long period of co evolution, achieve stability through relative con- 181
genotypic alternate states
Plesiomorphic Non-guarder Guarder Bearer open substrate spawner substrate chooser external brood hider nest spawner internal no parental care some parental care intensive parental care high fecundity low fecundity indirect development direct development low cost, high number high cost, low number investment investment Fig. 1. Diagrammatic representation of the different sections of reproductive guilds (sensu Balon 1975b, 1981a) as genotypic alternate states, and some of their associated characters (from Bruton 1989). stancy, i.e. relative lack of change or relatively of traits to that mentioned above, and it is hypothe predictable cyclical regimes. Examples of these sized that they are typically inhabited by species kinds of ecosystems are tropical forests and ancient with reproductive styles that are suited for envi deep tropical lakes. These ecosystems are charac ronments that have predominantly density-inde terised by high species diversity, high species inter pendent mortality, i.e. non-guarding guilds that dependence, the common occurrence of mutual invest a small amount of energy in each of a large isms, many rare and sedentary species, few migra number of small young. tory species, high speciation and extinction rates, and high species saturation (May 1974, Bruton 1989). Species in these highly competitive, biotical- Study areas ly harsh environments are hypothesized to make a large parental investment in each of a few young The Great Lakes of East Africa include large, deep which are relatively large and competitive when rift valley systems (e.g. Tanganyika, Malawi, Tur- they come in contact with the external environ kana) and shallower lakes between the eastern and ment, i.e. to have advanced breeding styles with western rifts (e.g. Victoria). In contrast to the extensive parental care (Bruton 1989,1990). ephemeral nature, geologically speaking, of many Ecosystems that are primarily driven by physico large, glacial lakes (such as the Laurentian Great chemical events which originate from outside the Lakes in North America), the African Great Lakes system are characterised by lower levels of com are of considerable permanence and antiquity. plexity and ever-changing biotic relationships, and Lake Tanganyika, with an area of 34000 km2 and achieve stability by the continual, though often depth of almost 1500 m, is the seventh largest lake unpredictable, repetition of cycles, e.g., marine in the world and has been in existence for at least 5 upwelling systems, many wetlands, riverine flood- million, possibly 20 million, years (Coulter et al. plains and rivers subject to episodic floods. These 1986). Lake Malawi is the ninth largest lake in the ecosystems are characterised by an opposite suite world with an area of about 30 000 km2 and a depth 182 alternate phenotypic states Altricial s. Precocial young small, incompletely developed young large, well-developed L _L ■xii.A U- l_ l L. -L. _L indirect development direct development generalists specialists pioneer and residual communities climax community weaker competition strong competition unstable, uncrowded environment stable, crowded environment density-independent mortality density-dependent mortality E = embryo L = larva J = juvenile A = adult S = senescence Fig. 2. Diagrammatic representation of altricial and precocial alternative phenotypic states (sensu Balon 1979,1981c, 1989) and some of their associated characters (from Bruton 1986,1989). of about 750 m, and the basin is thought to have their huge water volumes buffer most changes, at formed during the Lower Pleistocene (Coulter et least compared with shallow water bodies. al. 1965). Lake Victoria has an area of 68800km2 The African Great Lakes harbour the world’s and is the second largest lake in the world (Fryer & most diverse freshwater fish faunas (Coulter et al. lies 1972). 1986). Lake Victoria has over 250 endemic species Although they are seasonally thermally stratified of cichlid fishes and Lake Edward about 50 endem and have annual cycles of cooling and mixing, the ic cichlids (Greenwood 1981, Coulter et al. 1986). African Great Lakes have relatively stable and pre Lake Malawi may have as many as 500 endemic dictable physico-chemical regimes (Tailing & Tail species of cichlids (Ribbink et al. 1983, Coulter et ing 1965, Fryer & lies 1972), at least compared with al. 1986) and about 42 other species of fish, of more peripheral African water bodies such as many which 26 are endemic. Lake Tanganyika has about rivers, wetlands and small endorheic lakes. The 279 species of fishes (including about 200 endemic retention times in the deep lakes are long, and in species), of which 165 are cichlids and 114 are non- Lake Tanganyika and Malawi there is a deep hypo- cichlids (G.W. Coulter personal communication). limnion of relict water above which a shallower Adequate information on the fishes of Lakes Mala mixolimnion circulates (Coulter et al. 1986). The wi, Victoria and Tanganyika was available for in pH in Lake Malawi fluctuates between 7.7 and 8.6, clusion in this analysis. while in Tanganyika the water is more alkaline, The endorheic lakes included in the analysis are with the pH varying between 8.6 and 9.2. Within a Lake Ngami in Botswana (Bruton & Jackson 1983, single lake conductivities vary within a fairly nar Merron & Bruton 1988), Lake Chad in Chad (Car- row range, although the salts responsible are not mouze et al. 1983) and Lake Chilwa in Malawi the same in all cases (Fryer & lies 1972). Although (Kalk et al. 1979). These lakes undergo wide fluctu lake level fluctuations and internal waves induce ations in physico-chemical properties and water some variability in these systems (e.g. Eccles 1962), level, sometimes drying-up during periods of pro- 183 100 * 1 9 * 9 0 - •18 8 0 - % bearers 7 0 - & guarders o (0 1 •17 5 0 - 40 - • 16 3 0 6 . 1 2 *.,3 14 \1 5 1.° 7 * .11 20 1 . 3*5 1 0 - 2 * % *4 9 Rivers Wetlands Endorheic Ancient lakes Great Lakes Fig. 3. The percentage of bearers and guarders in the fish faunas of various African rivers, wetlands, small endorheic lakes and ancient African Great Lakes, showing the higher proportion of these advanced reproductive guild sections in the relatively stable ancient lakes: 1 - Orange-Vaal River, 2 - Limpopo River, 3 - Phongolo River, 4 - Sabi-Lundi River, 5 - Middle and Lower Zambezi River, 6 - Kafue River, 7 - Cunene River, 8 - Okavango River, 9 - Niger River, 10 - Luongo River, a headwater of the Zaire system, 11 - Lower Zaire River, 12 - Okavango Delta, 13 - Kafue floodplain, 14 - Lake Ngami, 15 - Lake Chad, 16 - Lake Chilwa, 17 - Lake Tanganyika, 18 - Lake Victoria, 19 - Lake Malawi. References for the different water bodies are given in the text. longed drought. The rivers include the Orange- Methods Vaal (Jubb 1967, Bowmaker et al. 1978, Skelton 1986), Limpopo (Jubb 1967, Gaigher 1973, Bow We have used Balon’s (1975a, 1981a) classification maker et al. 1978), Phongolo (Bowmaker et al. of reproductive guilds to describe the life-history 1978, Bruton & Kok 1979, Heeg & Breen 1982), differences between fishes in the various study ar Sabi-Lundi (Jubb 1967, Bowmaker et al. 1978), eas. The lists of fish species, genera or families Middle and Lower Zambezi (Jubb 1967, Bell-Cross contained in the above mentioned references on 1972, Bowmaker et al. 1978, Jackson 1986), Kafue African lakes, rivers and wetlands were compared (Bowmaker et al. 1978), Cunene (Jubb 1967, Bow with datafiles in the J.L.B. Smith Institute of Ich maker et al. 1978), Okavango (Skelton et al. 1985, thyology to determine whether a given taxon is a Merron & Bruton 1988), Niger (Welcomme 1986), non-guarder, guarder or bearer. Luongo (Balon & Stewart 1983) and Lower Zaire (Roberts & Stewart 1976), all of which are subject to widely varying flows. The wetlands include the Results Okavango Delta in northern Botswana (Jubb & Gaigher 1971, Skelton et al. 1985, Merron & Bru A synthesis of information on fishes in unpredict- ton 1988) and the Kafue floodplain in Zambia ably perturbed inland waters (e.g. rivers, flood (Chapman et al. 1971, Lagler et al. 1971, Kapetsky plains and endorheic lakes) and predictably per 1974). These systems are seasonally inundated and turbed systems (e.g. ancient Great Lakes) confirms have widely-fluctuating physico-chemical regimes. that the fish faunas of the latter systems have a 184 higher proportion of species belonging to advanced Balon (1981a), the proportion of riverine or wet reproductive guild sections (i.e. bearers and guard- land fishes in this guild ranged between 5.4 and ers) (Fig. 3). Conversely, fish communities in the 20% (Table 2) with the majority of species belong unpredictably perturbed systems are dominated by ing to a variety of other guilds, including non non-guarding, open-substratum egg scatterers. guarding and guarding lithophils, non-guarding This result is partly a consequence of the large and guarding phytophils, xerophils, aphrophils, proportion of cichlid fishes in the African Great polyphils, guarding speleophils, etc. Thus, while Lakes, but the point is nevertheless valid that a species diversity may be higher in the more predict family of fishes which has a large proportion of able ecosystems, these species are typically assem guarders and bearers has been highly successful in bled into a relatively small number of reproductive these lakes. Cichlid species are also found in Afri guilds. can rivers and wetlands but their proportion of the total ichthyofauna in these systems is lower (e.g. Jubb 1967, Roberts & Stewart 1976, Bowmaker et Discussion al. 1978, Balon & Stewart 1983, Skelton et al. 1985, Merron & Bruton 1988). Evidence from African inland waters suggests that It was furthermore found that wetlands and riv fishes from non-guarding reproductive guilds are ers have a greater variety of different individual better suited to environments characterised by reproductive guilds among their fishes than the abiotic harshness, e.g. with wide and less predict ancient lakes. Whereas 92.3% of the fish species in able variations in water temperature, oxygen, pH, Lake Malawi and 38.7% in Lake Tanganyika be etc., such as in the rivers and floodplains. These long to the single guild C.1.3 (mouth brooders) of species play a ‘high number/low investment’ game which is best suited for environments with predom inantly density-independent mortality. In contrast, Table 2. The proportion of guarders and mouthbrooders in the species in guarding and bearing guilds play a ‘low total ichthyofauna of some African inland waters (from Bow maker et al. 1978, Balon & Stewart 1983, Ribbink et al. 1983, number, high investment’ game and are better suit Coulter et al. 1986, Skelton et al. 1985, G.W. Coulter personal ed to conditions of biotic harshness, e.g. with communication, and unpublished information of the authors). strong competition for food and space and predom inantly density-dependent mortality, as found in % of total ichthyofauna the relatively stable African Great Lakes. These Guarders Mouthbrooders findings are not surprising considering that rela tively predictable ecosystems are characterised by Rivers having a greater number of species that are more Orange-Vaal 6.7 6.7 closely packed and which make more efficient and Limpopo 4.0 6.0 total use of available environmental resources Phongolo 5.4 5.4 (May 1974, Bruton 1989,1990). Kafue 3.2 20.4 Cunene 4.3 20.0 Complex ecosystems, like the African Great Okavango 7.8 18.2 Lakes, with their high biodiversity and rich interac Luongo 8.2 14.3 tion structure, are now considered to be, in gener Wetlands al, dynamically fragile compared with simpler eco Okavango Delta 9.1 21.8 systems (May 1974,1975, Bruton 1990). Although Endorheic lakes the species in these ecosystems are able to thrive in Ngami 6.3 26.0 predictable environments, they are likely to be vul nerable to perturbations that are beyond the range Ancient African Great Lakes Tanganyika 20.4 38.7 with which the ecosystem co-evolved. It also seems Malawi 0.4 92.3 reasonable to assume that the complex and sensi tive relationships of a mature ecosystem, once dis 185 located by disturbance, will be reconstituted less during the sexual process or during more distant easily than the simpler relationships of less stable transfer events (Maynard Smith 1989). communities (Bruton 1990). The species flock of cichlids in Lake Victoria has already shown itself to be highly susceptible to the effects of major man- Acknowledgements induced perturbations, such as overfishing and the introduction of alien species (reviewed in Coulter We sincerely thank E.K. Balon, A.J. Ribbink and et al. 1986, Ribbink 1987, Bruton 1990), and it is A.K. Whitfield for their useful comments on the likely that the cichlid fishes of Lakes Tanganyika manuscript. Sheila Coutouvidis kindly prepared and Malawi will be even more vulnerable because the figures. of their high endemicity and specialised nature. 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