Poulet, Alain

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EVOLUTION OF THE RODENT POPULATION OF A DRY BUSH SAVANNA 
IN THE SENEGALESE SAHEL FROM 1969 TO 1977 
ALAIN R. POULET 
Laboratoire de.Zoologie Appliquee, O.R.S.T.O.M., BP 1386, Dakar, Senegal 
bl 
I 
ABSTRACT 
An eight-year study of changes in rodent populations in a Sah- ' areas include the ability to rapidly attain high population levels 
after an initial reinvasion. Habitat preference and resource uti- 
lization within the dry bush savanna are critical factors to SUC- 
cessful increase in population levels and their maintenance. 
el savanna habitat in northern Senegal is presented. Character- 
istics of population dynamics for the principal species that are 
most important in dowing repopulation of dry bush savanna 
INTRODUCTION 
The evolution of small rodent populations was 
followed in a Sahel savanna in northem Senegal 
from 1969 to 1977. Trappings were effected at more 
or Iess regular intervals (every one to three months) 
L. for the following purposes: to determine the com- 
' ,- 'position of the population and its eventual temporal 
I variations; and to determine the most important 
'4 characteristics of population dynamics for the prin- 
cipal species, including periodicity and magnitude 
of reproduction, and maximal and minimal densities 
during the yearly cycle. 
The first part of this study, included in the Inter- 
national Biological Program from 1969 to 1972, oc- 
curred during an extreme draught which peaked in 
1972. Rainfall since that time, although still below - 30-year averages, has increased throughout the 
z Sahel. BioIogical phenomena have thus undergone 
a renewal, illustrated by the more or less peneral- 
ized outbreaks of insects and especially rodents in 
1975-1976. 
The purpose of the present report is to describe 
this latter phenomenon in a natural environment 
and to establish a qualitative and quantitative com- 
parison with observations made during the period 
of drought. These observations were localized to 
the Sahe! savanna of Fete-Ole, in nort'hern Senegal 
(northern Ferlo). 
Average yearly rainfall of about 300 mm supports 
a dry bush savanna, established on old dune reliefs 
whose topography governs the distribution of veg- 
etation, which includes large grassy areas with oc- 
casional trees on the dunes and dense bushy un- 
dergrowth in interdune depressions. 
The rainy season lasts only three or  four months, 
between July and October. The rest of the year is 
dry, becoming chilly from November to March and 
hot from April to June. Rainfall from 1969 to 1976 
113 
is shown in Table 1. Biological-activity is concen- 
trated in the rainy season and-the first part of the 
dry season. The majority o€ the animals reproduce 
during this period. Animal's with an annual cycle,' 
such as rodents, generally reach their abundance 
maximum between October and January. Popula- 
tion densities then decrease, reaching their mini- 
mum just before or at the onset of the rains of the 
following season. The characteristics of a given in- 
nua1 cycle are thus functions of both the minimum 
density of the preceding year and the peculiarities 
of the present reproduction (length and fecundity), 
all of which determine the growth level of the pop- 
ulation and thus the maximum annual number of 
individuals in the population. 
The rodent population of Fete-Ole was studied 
from 1969 to 1972 and the population dynamics of 
the most abundant species, Taterillirs pygargus ,  
was established (Poulet, 1972n, 19726). Other stud- 
ies concerning the effects of the drought were per- 
formed in i972 and 1973 (Poulet, 1971). The spec- 
tacular change in the rodent population was studied 
- -  - 
Table I.--Anniial Rainlull in Fere-Ole from 1969 IO 1976 ( in 
mm). 
Amount of 
Year rainfall 
1969 32 1 
1970 209 
1971 202 
1972 33 
1973 209 
1974 316 
1975 311 
1976 343 
114 
. .  .. 
NO. 6 BULLETIN CARNEGIE MUSEUM OF NATURAL HISTORY 
extensively from 1975 to 1977 by samplings per- 
formed during the dry season in order to obtain in- 
formation conceming the fauna and demography of 
the region. The detailed analyses of these data are 
forthcoming (Poulet and Poupon, 1978; Poulet, in 
preparation). . .  
DISCUSSION 
Rodents of Fete-Ole 
Gerbil1idae.-The most common rodents on the 
Fete-Ole Sahel are Taterillrts. T. pygargus is found 
in ail the habitats, but this species is the only one 
which is capable of permanently occupying the 
dunes, because it is particularly adapted to the 
sandy habitat. T. gracilis, which is much more rare, 
- inhabits only the bushy depressions. The two spe- 
cies coexist in these depressions, but T. pygnrgits 
consistently is the dominant species. This situation 
explains why T.  gracilis appears to have virtually 
disappeared during the drought, a condition that is 
much more unfavorable to it than to T. pygargus. 
Desmodillisclts braiceri is a small subdesert ro- 
dent, which is -very difficult -to study because it does 
not enter the traps. Its presence is ,known by i,ts 
occurrence in pellets of owls and, although difficult 
to trap, it may be stalked at night and caught by 
hand. Its abundance' apparently vanes inversely 
with that of all the other rodents; i t  is rare during 
periods of abundant rainfall and becomes more nu- 
merous during droughts. It seems to have dissap- 
peared since the outbreak of the other rodents in 
1975- 1976. 
A Tarera with 52 chromosomes appeared in 1976 
af the latitude of Fete-Ole but its occurrence must 
be considered as exceptional. Its appearance seems 
to be exclusively related to a temporary extension 
of the area of distribution of the species toward the 
north following the outbreak of 1975-1976. 
Muridae . -Arvican this and Masr om ys are usu- 
ally not found in the Fete-Ole Sahel; Arvicanrhis 
niloticris normally inhabits the campsites of nomads 
and iMastomys erythroleitcus is occasionally found 
in certain permanent settlements of the Sahel. Ar- 
viconthis invaded the Fete-Ole savanna in the rainy 
season of 1975. Family groups inhabited all the 
depressions with bushy covering and often adopted 
a diurnal arboreal behavior. Mastomys eyvthrolen- 
CNS also inhabited the depressions but was less 
abundant than Awicanthis. The samples taken re- 
vealed only several individuals per depression. 
Pullitlation of Taterillus 
Following the rains of 1974, thus two years after 
the great drought, the rodent density had risen to 
' . ' 
~ 
a level comparable to that of 1969-1971. There are 
no precise data for this period but the several Ta- 
rerillus captures made indicated a considerable re- 
production beginning in the months o€ October. The 
-.possibility of a rapid growth of rodent populations 
was indicated by the abundance of herbaceous veg- 
etation as well as acridian multiplication. Ropula- 
tion samplings were undertaken in January-Febru- . 
ary, April, and July of 1975 and then in February, 
April, and June 1976 (Table 2). 
A maximum of population density was reached 
in April 1975, with approximately 40 Taterillus per 
ha. The reproduction season, at least 6 months 
long, resulted in the appearance o€ two successive 
generations, thus considerably increasing the pop- 
ulation density. The period of population decrease 
resulting from the arrest of reproduction at the end 
of the dry season was very brief; from July 1975 
onward, reproduction recommenced at already high 
densities, approximately 30 individuals per ha with 
an average of eight young per litter. 
Considering an overall mortality rate of 15 to 20% 
per month and two successive litters of eight and 
six young with a starting density of 30 per ha, we 
may calculate an average maximum density of 180 
(135-250) at the end of September. The demograph- 
ic analysis of samplings of February 1976, show that 
reproduction during the rainy season of 1976 was 
indeed very short and that no new individuals ap- 
peared beginning with early October 1975. The pe- 
nod of population decrease in 1975-1976 was thus 
very long (October 1975-August 1976) and the pop- 
ulation density decreased from 70 per ha in Feb- 
ruary to 30 in July, representing an overall mortality 
rate of 25% per month. 
The 1976-1977 reproduction season began rather 
late, toward the end of September. The annual min- 
imum density was reached at this time, with 20 in- 
dividuals per ha, a level which is still considerable. 
The pullulation period could be considered as vir- 
tually terminated, because the following cycle 
showed neither as great a difference between min- 
imum and maximum densities nor as rapid a vari- 
ation of numbers of individuals. 
The pullulation thus occurred in two stages-1) 
a progressive but important rise in the population 
i 
i 
I? 
4 
... 1 ... 
... 
1978 PO ULET-SENEG ALES E RODENT POPULATIONS 115 
Table 2.-Esrittrarion of densities and biomass of Taterillus PY- 
gargus on rhe dunes of Fete-Ole during rhe rodent outbreak of 
1975-1976. Average weight of an individual is 36 g. 
considered that Toterillits pygargus is very suitably 
adapted to the dry bush environment of the Sene- 
galese Sahel. This does not, however, mean that 
the outbreak of Tarerillus was without repercus- 
sions. Seed productivity in the savanna and the de- 
1 
' Density h 
Y Date ualslha) (dha) tails of the granivorous diet of rodents are poorly 
i\ (individ- Biomass 
January-February 1975 29 1,033 
April 1975 40 1,440 
July 1975 28 1,008 
February 1976 86 3,096 
April 1976 54 1.944 
June 1976 28 1,008 
level, due to a very long reproductive season, fol- 
lowed by a period of population decrease which was 
too short to bring the density to a low level (1974- 
1975); and 2 )  a following reproductive season char- 
acterized by a highly elevated fertility which af- 
fected the already elevated population level. The 
outbreak ensued and the population was quadru- 
pled from the first litter .( 1973-1976). 
.The abrupt population increase during the second 
phase led to an overpopulation whose first and fore- 
most effect was to rapidly block reproduction. Be- 
cause the vegetation did not yet have the time to be 
degraded, the block had to be at the behavioral 
level; the promiscuity of .individuals was reduced, 
thus Ìnhibiting all subsequent gestations. 
In spite of the abundance of Taterillus, mortality 
during the entire period of population decrease re- 
mained limited to a low level of 20% per month. 
There were no changes in the size of each age group 
noted betkeen the beginning and the end of the dry 
season; there was thus no state of famine. 
The Tarerillus pygnrgus-Sahel savanna relation- 
ships appear to be especially well in equilibrium. 
Just as the great drought.of 1972 had no irreversible 
consequences €or the Taferillus population, the rap- 
id reproduction of 1975-1976 did no! lead to any 
considerable degradation of the habitats. I t  may be 
' 
- 
known. I t  is not possible to precisely estimate veg- 
etation changes, which, although not spectacular, 
could nevertheless be important. Furthermore, the 
competitive relationships among consumers were 
probably changed. Thus, the exceptional abun- 
dance of rodents could furnish an explanation for 
the disappearance of all reproduction among certain 
gran i v o r o u s birds , w h ich p re v i o us 1 y reproduced 
quite satisfactorily at the end of the dry season (the 
case of certain species of turtle-doves: G .  Morel, 
personal communication}. 
Invasion by Arvicanthjs nilot,icus 
The invasion of the Sahel savanna by A. niloticris 
at the end of the 1975 dry seasdn resulted from mas- 
sive migrations induced by an overpopulation at the 
moment of the maximum population density at the 
first phase of the pullulation in cultivated areas and * 
in  the villages where iln~ica/7this lives permanently. 
The intense reproduction of the newcomers. in- 
habiting the bushy depressions, explains the spec- 
tacular increase in the number of An~icmzfhis be- 
tween October and December.. 1975. The dietary 
requirements of a population exceeding 100 individ- 
uals per ha of habitat and a resulting biomass great- 
er than 1 1  kg, are such that a state of famine could 
not he long in developing. The consequences of this 
chain of events were a significant reduction in phys- 
ical parameters of the individuals of, a given age 
(size and especially weight), a lifespan not exceed- 
ing 9 months due to an intense mortality and a very 
clear degradation of the woody stratum. Mortality 
was amplified by predation resulting from abnormal 
concentrations of diurnal Palearctic birds of prey. 
Table 3 shows the population densities in the Sah- 
. 
. 
~ 
. 
. 
' 
Table 3.-€stimarion of densities and biomass of rodenî popirlariotrs of "buslzy depressions" in 1976. Taterillus sp. represents T. 
pygargus and T. gracilis in a 60:40 ratio. Values are given in ha of effective biorope. rhar is,  ha of bushy depression. 
Taterillus sp. Arvicanrhis niloricris 
Density/ Average B iomassl Density/ Average B ¡omass/ 
Date ha wt (9) ha (g) ha wt (€9 ha (g) 
February 1976 66 38 2,508 100 112 11,200 
June 1976 15 38 570 17 ' 84 1,428 
April 1976 32 38 1,216 73 98 7,154 
I 
116 
Table 4.-Maxima and minima of population densities, in nunlber of Taterillus pygargus per hu of Sahel savanna, for  ench anni 
cycle from 1969 IO 1977. The average value of popdation density is calcrilated from the maxitnrrrn assuming a conslant decrease 
20% per month until the minimum. The average weight of a T. pygargus is estimated as 36 p und its daily seed comsiiniptioti al 3 
BULLETIN CARNEGIE MUSEUhl OF NATURÁL HISTORY 
x = very low density. 
i 
Annual maximum Annual minimum 
Average .Biomass 
Date Date Density Date Density density (9) Seed 
~-  
1969-1970 
1970-1 971 
1971-1972 
1977-1973 
1973-1974 
1974- I975 
1975-1976 
1956-1977 
~ 
April 1970 
October 1970 
November 1971 - 
- 
April 1975 
September 1975 
December 1976 
9 
7.8 
0.7 - 
- 
40 
180 
45 
August 1970 
August 1971 
? - 
- 
July 1976 
September 1976 
July 1977 
4. I 
0.6 
x 
- 
- 
30 
20 
10 - 
6 
3.2 
0.3 
? 
29.5 
83 
23.4 
X 
216 
115 
I I  
- 
- 
1,062 
2,992 
842 
6,480 
3,456 
324 - 
- 
3 1.860 
80,773 
25.272 
. .  
el depressions during the dry season of February to 
June 1976 for T.  pygargrcs and T. gracilis (in a 60:40 
ratio) and for Arvicanthis niluticus. The invasion 
was massive but was reduced very rapidly. In real- 
ity it affected only the "pond" biotope (bushy un- 
10% of the Fete-Ole surface. It may be concluded 
that the permanence of the A. niloticics population 
in the dry bush savanna is questionable, which is 
fortunate for the natural environment as it can tol- 
erate the presence of these murids for only a shorr 
dergrowth in depressions), which represents only time ivithour irreversible damage. I 
The frequent drougl,ts in the Sahe 
CONCLUSIONS 
do not enable 
permanent populations of murids to be maintained 
in the dry bush savanna. The particular circum- 
stances of an exceptional demographic outbreak are 
required for the murids to invade this zone. These 
new populations are nonetheless not capable of 
maintaining themselves, because of the unaccepta- 
ble disequilibrium they impart to the habitats they 
invade. The invasion of Arvicnnthis niioticus and 
Mnstomys er)?throleucrts in the dry bush Sahel sa- 
vanna represents the limits of their ecological po- 
tentialities; they must be considered not 'as normal 
dwellers but rather as invarders with deleterious 
actions. 
Taterillits, especially T. pygargus, are the char- 
acteristic animals of the Sahel savanna. They may 
undergo enormous variations in abundance without 
producisg irreversible disequilibrium in the habi- 
tats. Table 4 summarizes data gathered between 
1969 and 1977 concerning minimum and maximum 
population densities, biomass, and seed consump- 
tion. 
Biomass was increased by a factor of 300 be- 
tween l..e drought of 1977- anL the maximum pop- 
ulation outbreak of 1976. These are extreme cases,l 
related to climatic extremes of the Sahel. True Sah- 
el species are those capable of undergoing large 
variations in population numbers in the absence of 
irreversible repercussions, neither to the species i t -  
self nor to the sheltering environment. This is in: 
deed the case for Taterillus bur not for Arvicaizthis. 
Seed productivity of 1 ha of Fete-Ole Sahel has 
been estimated at 29 kg (Bille, 1977). Availabilities 
from previous years bring the reserve to 40 to 60 
kg. These figures are largely in excess of the needs 
of Taterillus during a drought, but it must be sup- 
posed that availabilities of the environment in 1976 
were at least double, because the quantity of seeds 
required to support a Tateril[us population estimat- 
ed at 80 per ha is about 80 kg. This represents an 
indirect confirmation of the opinion of Bille, ac- 
cording to which, after the dry period, there is "a 
compensatory phenomenon enabling the very rapid 
biological rise of the ecosystem, both for the trees 
and grasses." 
' 
. 
1 
i 
1978 POULET-SENEGALESE RODENT POPULATIONS 
LITERATURE CITED 
AKONY hiouS. 1972. Recherches ecologiques sur une  savane sah- 
elienne du FerIo septentrional, Senegal. Terre et  Vie, Nu- 
mero Special, P. B. I.-Orstom-Sahel, 26:340472. 
BILLE, J. C. 1977. Etude de la production primaire nette d'un 
ecosysteme sahelien. Travaux et documents de  I'ORSTOM, 
POULET, A. R. 1972~. Recherches ecologiques sur une savane 
sahelienne d u  FerIo septentrional. Senegal: les Mammiferes. 
Terre et Vie, 26:440-472. 
-. 1973.  Caracteristiques spatiales de TuferiNits pygnrzrrs 
dans le sahel senegalais. Mammalia, 36579406. 
-. 1974. Recherches ecologiques sur une savane sahelienne 
65: 1-82. 
I17 
du Ferlo septentrional, Senegal: quelques effets de la seche- 
resse sur le peuplement mammalien. Terre et Vie, 18: 114- 
130. - I 1978. La pullulation des rongeurs dans le milieu naturel 
de Fete-Ole en 1975-1976. In preparation. 
POULET, A. R., AND H. POUPON. 1978. L'invasion d'Arvicanrliis 
ni/oricus (Rodentia, Muridae) dans le milieu naturel sahelien 
en 1975-1976 et ses consequences pour la strate ligneuse. 
Terre et Vie, 32:161-193. 
. 
This paper was cottiiniotiicuted by Berriard Hubert. 


Evolution of the rodent population of a dry bush savanna in the Senegalese Sahel from 1969 to 1977

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