AMER. ZOOL., 22:361-369 (1982)
On the Size of African Riverine Fish Faunas1
DANIEL A. LIVINGSTONE
Department of Zoology, Duke University,
Durham, North Carolina 27706
MARY ROWLAND
Department of Fishery and Wildlife Biology, Colorado State University,
Fort Collins, Colorado 80523
PATRICIA E. BAILEY
Department of Zoology, Duke University,
Durham, North Carolina 27706
SYNOPSIS. An apparent anomaly exists in the size of riverine fish faunas of the Nile and
Zaire; the Nile while considerably longer, appears depauperate when compared with the
Zaire. This paradox is explained when discharge, not length, is used as a measure of river
size. Indeed, the number of freshwater fish species in African rivers is more closely related
to discharge than to length or catchment area. Discharge is directly proportional to terrestrial productivity of a river basin, which in turn affects total biomass offishand number
of species. Changes in the size of rivers during the geologic past affected their capacity
to sustain diverse fish faunas. Rivers flowing through especially arid lands during the latePleistocene were reduced in discharge, and concomitantly, fish faunas. Immigration of
fish from refuge rivers during the Holocene partially restored these diminished faunas.
We propose that fish are more mobile than they seem, and that the distinctiveness of
riverine fish faunas may be maintained by competitive pressure from established residents,
rather than by limited dispersal abilities of fish. Theories of the distribution of fish in
Africa are considered, and we suggest that discharge as affected by climatic stability is
largely responsible for the size of African riverine fish faunas.
length was a reasonable measure of size of
a
river, but it seems to us possible that
Dr. Humphrey Greenwood has drawn
some
other measure, such as the area of
our attention (Greenwood, 1976 and personal communication) to an apparent catchment used by Welcomme (1979),
anomaly in the sizes of the riverine fish might give a more realistic appraisal of the
faunas of the Nile and Zaire (formerly size of a river against which to compare
Congo) Rivers. Not taking into account the the size of its fish fauna.
Most comparisons of the size of biotas
large flocks of endemic fishes restricted to
headwater lakes, the Nile, second longest have used surface area as the measure of
river of the world, has a fauna of 115 habitat size, but area is easier to specify for
species, only 26 of which are endemic to terrestrial systems or for lakes than it is for
that river; the Zaire, although appreciably rivers. One might expect that the total surshorter, contains 669 species, no less than face area of the river and all its tributaries
would be comparable with the areas of is548 of which are endemic (Poll, 1973).
lands
used in the classic studies of Preston
We have gathered data on the number
of fishes inhabiting various African rivers (1960, 1962a, b) and MacArthur and Wilas well as various measures of size of the son (1967). Estimation of that parameter
same rivers. Greenwood implied that for even a small African river would be
very time consuming and would require
larger scale maps than exist for much of
the continent.
1
From the Symposium on Alternative Hypotheses in
Only three measures of size are available
Biogeography presented at the Annual Meeting of the
a large enough number African rivers
for
American Society of Zoologists and the Society of Systematic Zoology, 27-30 December 1980, at Seattle, to permit meaningful comparisons with
their faunas. Those measures are the greatWashington.
INTRODUCTION
361
362
LIVINGSTONE ETAL.
TABLE 1. Effects of river size on number of fish species (N) present in African rivers, analyzed by stepwise multiple
regression analysis.3
Linear model6
Independent variable
Discharge (D)
Length (L)
Area (A)
Overall R2, F
Contribution
toR»
Partial F
0.9398
0.0074
0.0025
0.9497
374.35C
3.24
1.09
138.40c
Log-log model
Order entered
in equation
Contribution
to R1
Partial F
Order entered
in equation
1
2
3
0.7208
0.0222
0.0002
0.7432
61.96C
1.99
0.02
21.23 C
2
3
I
a
Data are from Table 2.
The multiple linear regression equation is N = 19.61532 +0.01636(D) + 0.02051(L) - 0.00003(A). The
equation for the log-log model is In N = 0.24383 + O.30403(ln D) + 0.29393(ln L) - 0.01675(ln A).
c
Significant at 0.001 level; other values are not significant.
b
est length from mouth to tributary, used by
Greenwood, the total surface area of catchment, used by Welcomme (1979), and the
mean annual discharge. All three of these
measures of size are highly correlated with
each other. The Nile is an aberrant river,
extremely long for its discharge because it
rises in the humid tropics and then flows
for an enormous distance through desert
from which it gains little or no water.
FISH FAUNA AND SIZE OF
MODERN RIVERS
We have made a statistical analysis of the
relation between number of fishes, length,
area, and mean annual discharge of the 26
African rivers for which we could find all
four data. We used SPSS (Statistical Package for the Social Sciences, Nie etal., 1975)
to compute simple and stepwise multiple
correlation coefficients between the number of fish species and the various measures of river size, with and without logarithmic transformation of the variables
(Table 1).
There is a very high correlation between
untransformed number of species and discharge. A small amount of additional variance is explained by length and an even
smaller amount by area. Together, these
three variables account for 95% of the
variance in the number of fish species in
rivers. (Table 2).
Perusal of plots of the untransformed
data, however, is less than reassuring. The
high correlation between number of fish
species and discharge is due largely to the
Zaire River, which is much larger and has
many more species than its nearest rival.
The plot consists very largely of the Zaire
River at one end and most of the others in
a cluster at the other, with only the Zambesi lying at an intermediate position. We
do not have enough such intermediate
rivers to define the shape of the curve relating untransformed discharge and size of
fish fauna.
We lend more ecological credence to the
relation displayed by a plot of the logarithmically-transformed variables (Fig. 1). After transformation the correlation coefficients are not quite so high, but the data
points spread out enough to define the
shape of the curve relating size of fish fauna to size of river. Once again, the highest
correlation is with discharge, r2 between
log of fish species and log of discharge
being 0.74. Multiple correlation analysis
shows that log of length and log of discharge explain an additional small part of
the variance, but that neither addition is
significant.
On the log-log plot of species against
discharge (Fig. 1) the Nile and the Zaire
are not unusual. The number of species in
each falls well within the range of variation
of the rivers for which we have data—in
fact, close to the line of best fit that might
be drawn through the data—and Greenwood's paradox disappears.
It may seem extraordinary that the
number of species of fish in a river should
bear the same relation to discharge as do
the numbers of species in insular biotas to
the areas of the islands on which they live.
The dimensions of these two measures of
size are very different—cubic meters per
second and square meters. It is, of course.
SIZE OF AFRICAN RIVERINE FISH FAUNAS
363
possible that the total surface area of
rivers, for which we have no data, is highly
correlated with the discharge, and the linear log-log relation may result from that
correlation. We believe, however, that the
similarity has another root.
It is unlikely that fishes in a river system
are commonly severely restrained by mere
lack of space in which to live, for they can
tolerate extreme crowding in the isolated
pools of rivers at low stage and zero discharge. More likely, the biomass that a river can support depends on the primary
production available to sustain its trophic
pyramid, and the number of species depends in some Prestonesque fashion on
-I 0 1 2 3 4 5 6 7 8 9 10 n 12 13
the total fish biomass. Although some of
NATURAL LOG OF DISCHARGE
the primary production comes from phy- FIG. 1. Log species number versus discharge volume
toplankton in river water, more usually it for African rivers.
comes from rooted aquatics. For many
rivers, the productive base is primarily leaf
litter imported from the terrestrial part of the total rainfall (Gois, 1972). We have,
the catchment. It is likely, further, that then, two equations:
even the area of an island is of ecological
Discharge = Area X Precipitation X 0.1
significance in the species-area relationProductivity
= Area X Precipitation X k
ship primarily as an estimator of the
amount of solar energy available to sup- Evidently the total terrestrial productivity
port photosynthesis by plants, and hence of a river basin is directly proportional to
of the amount of energy available to the discharge from the basin, just as the
higher levels of the food chain.
total productivity of an island is directly
Lieth (1973) has shown that two climatic proportional to the area of the island. Toparameters explain most of the site-to-site tal production of a river basin is certainly
variance in primary production of terres- proportional to the land area of the basin
trial systems. They are mean annual tem- also, but the correlation between producperature and mean annual precipitation. tion and simple area is not so high as that
Temperature does not vary much among corrected for rainfall. This is presumably
most of the African rivers we are consid- why the correlation between fish fauna
ering, but mean annual precipitation on and discharge is higher than it is between
the catchment area varies quite signifi- fish fauna and area of the catchment.
cantly.
Many of the archipelagos that have been
Although Lieth's data were fitted best by used in the study of species-area relations
a more complex relation, over the range on land are relatively uniform with respect
of precipitation of interest here, produc- to rainfall, but it would be interesting to
tion — precipitation X k. The total pro- reinvestigate some that do present a range
duction in the catchment of a river is ap- of precipitation values. We predict that the
proximately production = area X pre- correlation between area and biotic diversity could be improved by taking rainfall
cipitation X k.
Discharge of a river is similarly a func- into account.
Regardless of the underlying basis of the
tion of the area and the rainfall per unit
area multiplied by a factor depending on relation shown in Figure 1, it is clear from
the part of the rainfall that runs off as riv- that figure, and also from corresponding
er discharge. For the rivers of Angola, at figures showing species with respect to rivleast, that part is not far from one tenth of er length and catchment area, that the Nile
364
LIVINGSTONE ETAL.
does not have a depauperate fauna. Rather, it has a number of species that general
experience of African rivers would lead
one to expect in a river of its size.
EFFECTS OF LATE-QUATERNARY
CLIMATIC CHANGE
To this point we have considered only
the size of modern rivers and taken no account of how changes in that size during
the geologic past may have affected their
capacity to sustain large numbers of fish
species. It is well established that there
have been significant changes in the humidity of the African climate during the
past twenty thousand years (Gasse, 1975,
1980; Livingstone, 1975, 1976, 1980;
Grove and Street, 1979). In particular,
Kendall (1969) has shown that for some
thousands of years prior to 12,500 years
ago no water flowed down the White Nile
from Lake Victoria, and Harvey (1976) has
shown that no water flowed to the White
Nile from Lake Mobutu Sese Seko (formerly Albert) during the same period of
time. All the evidence from the Ethiopian
plateau, and the valley of the Nile itself
(Gasse, 1975, 1980; Williams, 1980) indicates that discharge through the Blue Nile
and Atbara must have been low at the
same time. At present most of the annual
discharge of the lower Nile comes from
Ethiopia, but the supply of water from that
region is highly seasonal. It is the ten percent of total discharge from the almost
non-seasonal White Nile that maintains
flow in the lower river during low stage.
From these data it is not possible to estimate the annual discharge of the Nile during the arid late-Pleistocene, but it is clear
that the length and catchment area of the
river were both seriously reduced by loss
of the White Nile, and it is difficult to avoid
the conclusion that the lower river was a
vestige of its modern self, probably reduced to a series of isolated pools during
the dry season.
In the light of this history the 115 fishes
of the modern Nile seem surprisingly
many. One might expect a much smaller
fish fauna, perhaps only the 26 endemics,
to have survived the dry late-Pleistocene.
It is conceivable that all African rivers
have been affected in a similar way by the
climatic changes of the late-Pleistocene,
and that the relation of Figure 1 represents the conditions of that time. This possibility cannot be ruled out on strictly geological evidence, although it does seem
geologically unlikely, but the large differences in percentage of endemism among
the fishes of the various river basins make
it seem very unlikely.
If the late-Pleistocene fish fauna of the
Nile were actually much smaller than the
modern one, the question immediately
arises of where the extra species have come
from that inhabit the modern river. The
Zaire has a large fauna and shares a very
long common divide with the Nile, and
there are at least 22 species of fishes common to the two rivers. Seventy-five Nilotic
species are shared with the Niger River in
West Africa. Many of the non-endemic
Nilotic fishes actually range as far west as
the Volta and the Senegal Rivers (Greenwood, 1976).
This pattern of faunal similarities is surprising, and has sometimes been regarded
as very old, a remnant of a hypothetical
uniform fish fauna that was widespread in
Africa before partitioning of the continent
by the post-Oligocene tectonic activity that
produced the Rift. We agree with Beadle
(1974) that it reflects much more recent
faunal exchange, and in particular, that it
dates from the early Holocene wet period.
The early Holocene climate of the Soudan
was much moister than it is now, the level
of standing water in the Chad basin was
much higher, and there were drainage
connections between rivers that are now
quite separate, such as the Niger and influents of Lake Chad, which were connected through the Benue.
Conditions for fish migration seem to
have been much more favorable along the
southern edge of the Sahara than they
were between the Nile and the Zaire. The
incidence of river capture was probably
greater in the flat terrain of the Soudan,
where the greatest local relief is commonly
formed by sand dunes, than it was over the
largely rock-controlled Nile-Zaire watershed.
Fishes are commonly regarded as ani-
365
SIZE OF AFRICAN RIVERINE FISH FAUNAS
TABLE 2. Characteristics of African nvers used in multiple regression analysis.
River
1.
2.
3.
4.
5.
6.
7.
8.
Nile
Zaire
Niger
Zambesi
Orange
Benue
Volta
Cunene
9.
10.
11.
12.
Komoe
Tana
Bandama
Cavally
Number
offish
species
Length
(km)
Drainage
area (km1)
Discharge
(in'/sec)
49
945
3,349,000
3,457,000
1,125,000
1,300,000
677,000
64,000
390,000
83,000
2,640
39,000
6,100
7,070
122
6,650
4,700
4,200
2,580
2,100
1,400
1,270
71
1,160
72,000
250
115
669
130
129
15
128
345
382
1,175
215
25
710
800
151
54
1,050
800
38
700
97,000
22,400
13. Sassandra
14. Rufiji
57
650
25
640
75,000
170,000
3,431
15. Wami
25
480
46.000
93
16. Pangani
29
352
29,000
70
4
64
17
61
365
32
260*
1,680
16,190
9,500
29
20
190*
170*
130*
17
335
13
298
9
163
8,700
5,000
4,000
26,000
4,222
1,585
14
125
3,200
17. Sigi
18. Konkoure
19. Bia
20.
21.
22.
23.
24.
25.
Agneby
Boubo
Me
Tugela
Umkomaas
Umtamvuna
26. Umvoti
13
384
480
Authority
Fish
Greenwood, 1976
Poll, 1973
Greenwood, 1976
Bell-Cross, 1965-66
Jubb, 1967
Stauch, 1966
Roberts, 1967
Jubb, 1967
Daget and Iltis, 1965
Copley, 1958
Daget and Iltis, 1965
Daget and Iltis, 1965
Daget and Iltis, 1965
Bailey, 1969;
Copley, 1958
Bailey, 1969;
Copley, 1958
Bailey, 1969;
Copley, 1958
Bailey, 1969
Daget, 1962
Daget and Iltis, 1965
River
Showers, 1973
Showers, 1973
Welcomme, 1972
Welcomme, 1972
Showers, 1973
Van der Leeden, 1975
Welcomme, 1972
Showers, 1973;
Van der Leeden, 1975
Welcomme, 1972
Showers, 1973
Welcomme, 1972
Van der Leeden, 1975;
Welcomme, 1972
Van der Leeden, 1975
Welcomme, 1972;
Tanganyika Atlas
Tanganyika Atlas
Tanganyika Atlas
10
Tanganyika Atlas
Van der Leeden, 1975
Welcomme, 1972; Ivory
Coast Atlas
Daget and Iltis, 1965
Daget and Iltis, 1965
Daget and Iltis, 1965
South Africa Atlas
Shand,1971
Shand, 1971
25
Natal Water Bd.
353
71
50
32
32
17
40
Daget and Iltis, 1965
Daget and Iltis, 1965
Daget and Iltis, 1965
* Measured from 1:2,000,000 Atlas of Cote d'lvoire.
mals of very limited capacity for dispersion. We suspect that they have generally
unappreciated powers of dispersal, and
that competitive pressure from well-established local residents keeps fish faunal interchange to the low levels that are commonly observed. Only when fish are
presented with a largely unexploited habitat into which to expand, as they were in
Canada after retreat of the Laurentian ice,
or in northern equatorial Africa after reestablishment of moist conditions in the
early Holocene, does their true potential
for dispersion become apparent.
River capture may be a particularly effective mechanism for transferring fish
from one catchment to another not only
because it involves a large number of immigrants, but also because those immigrants are transferred together with a
366
LIVINGSTONE ETAL.
piece of the biotope in which they have
been living. On that small patch of native
turf they should have a much better
chance of surviving challenge by the established residents of their new river than
they would if they trickled in by waterspout or the talons of a careless fish-eagle.
A modern incident also suggests that
fish are good travellers but poor immigrants. The faunas of the Upper and Middle reaches of the Zambesi River are separated by the formidable barrier of
Victoria Falls. Until a few years ago many
species of Upper Zambesi fishes appeared
to be excluded from the Middle Zambesi
by inability to survive passage down the
Falls. Since the construction of Kariba
Dam on the Middle Zambesi, however, it
is reported that 13 species of Upper Zambesi fishes have established themselves in
Kariba Lake (Balon, 1974).
Occam's razor may suggest that no nonhuman agency should be invoked to explain fish range extension to a water where
fisheries biologists operate. If, however,
these new immigrants have actually
reached the Middle Zambesi by natural
means, their success in surviving there
now, and not earlier, must certainly be due
to dam-creation of the tranquil slow-flowing conditions that have long been characteristic of the gentler Upper Zambesi.
These suggestions of considerable dispersal ability among fishes come from
places that have undergone particularly
drastic environmental change. Less drastic
changes must have been very common
during the Quaternary, and must have
had significant ichthyogeographic effects.
As the area of stream-shading rain forest
expanded and contracted, the discharge
increased and decreased, or the concentration of dissolved salts in the river water
changed in response to Quaternary climatic change. Conditions in one river must
often have changed so much that they became more suitable for potential immigrant species of a neighboring river than
any of its native fishes.
If fish ranges do commonly shift in such
dynamic fashion, one might hope to identify range extensions in the fossil record.
Positive indications of extension are not
easy to find, however, because fish fossils
are commonly identifiable only to genus,
and the fish fossil record for Africa is
either rather incomplete or rather incompletely studied. To establish a range extension beyond reasonable doubt would require an inordinate amount of fossil
evidence. First entry of a species into a river can never be demonstrated, but only
suggested by the accumulation of cases in
which the fish is not found in deposits below some level above which it is generally
present. If some species are extending
their ranges to new waters, however, some
of the resident species must go extinct locally to make way for them. Such extinction can be established by discovery of a
single fossil of the species in a basin where
it cannot be found by modern ichthyologists.
The mochokid catfish genus Synodontis
provides a possible case. At present no
species of Synodontis is known from Lake
Idi Amin Dada (formerly Lake Edward) or
its tributaries. Yet the genus is known as
fossils from deposits in the beds laid down
in the early Holocene (Greenwood, 1959;
Greenwood and Howes, 1975). The same
basin lacks modern species of Lates, the
genus of the Nile perch, or mputa, yet
there is fossil evidence for the presence of
these fishes near the end of the Pleistocene. Such recent extinctions puzzle ichthyologists, and attempts have been made
to explain them in terms of catastrophic
deterioration of the environment. If fishes
are mobile and dynamic, moving with relative freedom from river to river, they
should occasionally establish themselves in
new rivers where they are better adapted
than the local residents. Extinction of natives is to be expected whenever the new
immigrant increases the faunal size of a
river beyond the level that its size and biotopic diversity will support. The Lake Idi
Amin Dada cases are readily apparent because they involve genera no longer living
in the lake. Extinction by competition from
a new immigrant is more likely to involve
species of the same genus, most likely
closely related species of the same genus.
Because of the difficulty of distinguishing
African fish species by their bones, most
SIZE OF AFRICAN RIVERINE FISH FAUNAS
cases of such dynamic extinction would
pass unnoticed.
367
nary, we would expect that the fish faunas
of pre-Miocene Africa were probably more
different from each other than those of
DISCUSSION
today, an expectation supported by such
African fish distribution is often inter- scraps of fossil information as are available
preted in the light of two ideas, neither of (Roberts, 1975).
which will seem strange to participants in
We would not expect the number of
the vicariance debate.
fishes in the rivers of peripheral parts of
According to the first idea, the fish fau- Africa to increase much solely by the pasna of Africa was much more uniform be- sage of time. However, we would expect
fore rifting began in the early Miocene. the faunas of these rivers to become even
This uniform fauna was fragmented when more individually distinctive with the pasthe developing rift disrupted previous sage of time than they are now, and to be
drainages; its daughter parts evolved the enriched by the spread of slowly-migrating
distinctive riverine faunas of modern Af- lineages that have not been able to reach
rica. The relatively uniform fauna of the these rivers of peripheral regions from the
Soudan is sometimes regarded as a rem- lowland refuge rivers during the 12,500
years since the beginning of the Holocene
nant of the old pan-African fauna.
According to the second idea, which is wet phase.
more favored by students of Southern AfAlthough most of recolonization of perican fishes (Gaigher and Potts, 1973), the ripheral rivers after the end of such a dry
Zaire River is the source from which the episode must have come from the lowland
fishes of less ichthyologically favored rivers refuges, a certain amount must have come
have come, and the modern faunas of from the peripheral rivers themselves,
these other rivers have developed as the each serving as a source of immigrants
result of a gradient of fish diversity away from among the reduced number of
from the Zairean source, modified by local species that persisted in it through the dry
speciation of the various stocks of fresh- period. Fishes of the peripheral rivers
water fishes as they have migrated away would have much to gain by such interfrom the Zaire River.
change between rivers. They would essenWe suggest a third way of viewing the tially be following the shell-hole strategy of
phenomena of fish distribution in Africa. infantry under heavy bombardment in the
The total number of fish in the fauna of First World War.
a river is controlled very largely by disIn such bombardment, every square mecharge. The barriers to movement of fish- ter of ground was subjected to shellfire,
es from one river to another are not ab- and an occupying force that merely held
solute, and depend upon competitive its place would be destroyed to the last
ecological pressure as well as the mechan- man. By continually spreading out, howics of dispersal. Climatic change alters the ever, and occupying each new shell-hole as
discharge, and hence the species-support- it was formed, a considerable part of the
ing capacity, of rivers and probably affects occupying force would come through even
rivers in the drier peripheral parts of Af- the heaviest bombardment alive.
rica more commonly than the well-watered
Shellfire is a more nearly random prolowland tropical Zairean and Guinean cess than Quaternary climatic catastrophe,
core. When favorable conditions return to which would affect many rivers in the
streams that have suffered partial or com- same climatic zone in the same way at the
plete extirpation of their fishes, conditions same time. Even a few rivers that largely
are excellent for immigration from the ref- escaped climatic catastrophe would prouge rivers, and a relatively uniform fauna vide a valuable refuge, and permit the surof low endemicity is developed in the re- vival of a much richer fauna than would
populated area.
persist because of the shellhole strategy
To the extent that pre-Miocene time was alone. The large fauna of the Zaire, and
climatically more stable than the Quater- especially its high level of endemicity, sug-
368
LIVINGSTONE ETAL.
gests the Zaire River basin as an area that the final typescript. Mrs. J. R. Bailey drew
was relatively unscathed by climatic catas- Figure 1.
trophe.
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SUMMARY
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2. The relation between faunal size and
discharge is:
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