# Dynamical Models of Interactions between Herds Forage and Water Resources in Sahelian Region

Authors: Jean Jules Tewa; Alassane Bah; Suares Clovis Oukouomi Noutchie
Journal: Abstract and Applied Analysis
 (2014)

---

## Abstract

Optimal foraging is one of the capital topics nowadays in Sahelian region. The vast majority of feed consumed by ruminants in Sahelian region is still formed by natural pastures. Pastoral constraints are the high variability of available forage and drinking water in space and especially in time (highly seasonal, interannual variability) and the scarcity of water resources. The mobility is the main functional and opportunistic adaptation to these constraints. Our goal in this paper is to formalize two dynamical models for interactions between a herd of domesticate animals, forage resources, and water resources inside a given Sahelian area, in order to confirm, explain, and predict by mathematical models some observations about pastoralism in Sahelian region. These models in some contexts can be similar to predator-prey models as forage and water resources can be considered as preys and herd’s animals as predators. These models exhibit very rich dynamics, since it predicts abrupt changes in consumer behaviour and disponibility of forage or water resources. The dynamics exhibits a possible coexistence between herd, resources, and water with alternative peaks in their trajectories.


---

## Body

## 1. Introduction

It is well known that pastoralism is the name given to the subsistence practice in which people care for and domesticate animals, usually ungulates such as camels, cattle, reindeer, sheep, and goats [1].Pastoralism will continue for the near future in poor nations [2–4], especially in Africa, because it is generally an efficient, low energy requiring subsistence base for semiarid regions. During the 20th century, however, most national governments tried to force pastoralists to stop their migrations and to reduce the size of their herds in order to prevent overgrazing. These efforts at controlling them were consistently resisted by pastoralists. They usually saw large herds as symbols of wealth and as security against unpredictable climates and periodic epidemics among their animals [5].Livestock in the Sahel is now basically a usurper such as land use regarding the vegetation that grows there naturally. The pastoral farming systems are those in which more than 90% of the dry matter consumed by livestock comes from grazing. The researchers focus in this paper on these pastoral-farming systems. The transition from one system to another is periodic and depends on climate and economic context [6]. Part of pastoral population in the rural population varies greatly from country to country. Livestock numbers are not known with precision, because large part of statistics does not differentiate production systems. Based on national statistics, thus including agricultural areas, small ruminants are the most numerous, before cattle. Animal density can only be determined on well-defined regions, for which there are the number of both animals and areas used by livestock. The data always differ by authors.The pastoral system is moving towards a systems approach mixed crop-livestock farming. This trend goes in North Africa with the grazing of steppe. In the Sahel, the environmental consequences are less obvious [2, 7, 8]. The vast majority of feed consumed by ruminants in Sahelian region is still formed by natural pastures. Savannas, steppes, and training fallow provide the basis of livestock feed, even in sedentary farming or being intensified. It is important to evaluate or to monitor their progress [1, 6].There have been many problems about pastoral livelihood vulnerability. The dry lands cannot support sustained and reliable agriculture because of low and variable rainfall and high temperatures. Pastoralism, however, is extremely well suited to this type of environment. Pastoralists make optimum use of the dry lands by practicing a mobile and extensive livestock-keeping system. They move according to where and when fodder becomes available and use different herd management strategies such as herd splitting, herd diversification, and herd maximization to ensure that they spread the risk of livestock loss from droughts, diseases, and theft. All the while, they make maximum use of the available vegetation without degrading the environment [1].Different explanations have been advanced for the increasing vulnerability of pastoralists. Population growth in pastoral areas has created pressure on land. Climate change has increased the frequency of droughts, floods, and livestock diseases. However, these natural factors only exacerbate the effects of a harsh policy and legal environment that is focused on “modernizing” and settling pastoralists. The increased vulnerability of pastoral livelihoods to shocks and other drivers of change are in many ways a function of the cumulative effect of these policies [1].Long-term sustainable pastoral development requires a good knowledge of the dynamics of multiple factors underlining pastoralism and here research has a crucial role to play. Alongside support to improve primary and secondary education is the need to strengthen institutions of higher learning and research in pastoral areas of Africa [5]. Such institutions require support to conduct research on a range of political, social, and natural science subjects and at levels, which range from local adaptation to regional integration and global trends. The links between research and policy also need to be strengthened, so that policy responds to the dynamics of pastoral livelihoods in an efficient manner [5]. This paper is our contribution to this research in pastoral areas of Africa.


## 2. Model Construction

In Africa, the Sahel has a wet season (June to October) and a dry season (November to May) which are very distinct, and livestock is greatly influenced by the amount of grass and shrubs. As we said previously, more than 90% of dry matter consumed by livestock comes from grazing. Some problems as overgrazing or scarcity of water are frequently observed. These problems force some pastoralists to migrate with their herd. According to these facts, the intrinsic production of forage resources can be a negative number, even the intrinsic disponibility of water resources. The intrinsic productionr of forage resources can be written as r=r0-r1, where r0=r01p+r02 as given in [8], p denotes the precipitation, r02 is the growth rate of the grasses and others without precipitation, and r1 is the disappearing rate due to many reasons (competition, respiration, and human harvesting) different from herd’s nutrition. The particularity of this model, which is the difference with classic predator-prey models in the literature, is the fact that the intrinsic production rate can be negative (r<0). This situation is frequently observed in Sub-Saharan Africa, particularly in Sahelian region when precipitations are scarce. Intensification of this situation can conduct pastoralists to migrate with their herds: this is the transhumance. We also know [9] that tropical livestock unit (TLU) is a basic criterion for a head of cattle weighing 250 kg. The daily volume of consumption of dry matter per TLU amounted to 6.5 kg. However, since this criterion varies, there is obviously the case with differences depending on the country, even within each country. The test described here is also currently used officially by the Ministry of Agriculture and Livestock in Niger [9].ParameterK denotes the maximal number of livestock in this paper. When considering the question of the number of livestock that can be raised on a given surface, it is first important to know the amount of forage needed for livestock. To calculate the volume needed to feed livestock, measured by the volume of solids, the amount of assimilation of dry materials by livestock is generally estimated to be approximately 1.4% to 3.0% of the weight of livestock. In the Sahelian region, by experience, the following values are generally used on the basis of tropical livestock unit, in order to compute the daily volume of required dry matter per head [9].One cattle: 5.2 kg [6.5 kg (TLU) × 0.8 (index of the considered space)]. Two goats and sheep: 1.0 kg [6.5 kg (TLU) × 0.15 (index of the considered space)]. We can calculate the number of heads that can be raised with the formula below [9]. Rearing capacity = [(volume of feed supply of natural grassland × utilization rate) divided by (daily volume required per capita solids)] × number of days of reliance on natural grassland.Let us give some hypotheses concerning the models in this paper.(H1)
The herds, forage, and water resources considered in this paper are in the same area.(H2)
When the intrinsic production is nonnegative, if feed resources are not consumed, they change their amount to the maximal production capacity and therefore storage. In this case the feed resources grow logistically. When the intrinsic production is negative, the feed resources will disappear at long time, and this situation conducts pastoralists to migrate with their herds: this corresponds to the transhumance situation.(H3)
If the herd is helpless, its number of animals or its tropical livestock unit (TLU) decreases and may disappear if nothing is done.(H4)
Interactions between the herd and forage resources are following the functional response of Michaelis-Menten or Holling function response of type II.(H5)
Consumption of resources has an instant effect on the reduction of forage resources and increased biomass of the herd, in proportion to their consumption.In the study of interactions between herds, forage, and water resources, it is crucial to determine which specific form of functional response, describing the amount of resources consumed by an animal in the herd per unit time, is ecologically plausible and provides a solid basis for theoretical development. As in [11–13], density dependence of the resources will be the starting point, which gives a functional response. In the simplest case, such a function is a linear function of the forage resources density (R), which is integrated into the classic Lotka-Volterra model. The linear functional response is a limited case and can only be seen over a short period. But one can use the Michaelis-Menten or Holling response function of type II defined by ϕ(R)=(Bω0R/(1+Bω1R)), where R denotes the forage resources density, ω0 and ω1 denote, respectively, the time taken by a herd’s animal to search and consume forage resources, and B is the herd consumption rate per unit of time. With the Holling function response of type II, it is well known that the diminution of forage resources due to the herd consumption increases and the forage density decreases and becomes constant at the end. In the model formulated here, γ is the removal rate from the herd (death, off take…); parameter e denotes the conversion rate of the forage resources consumed by the herd’s animals into their biomass. When there is no herd, the dynamics of forage resources can be governed by the logistic equation, but the intrinsic production can be negative, such that (0,0) is a stable equilibrium. Setting H(t) as the tropical livestock unit of the herd at day t, the function g(R,H)=Bω0R(t)H(t)/(1+Bω1R(t)) can also be written as g(R,H)=qR(t)H(t)/(1+aR(t)), where q=Bω0 denotes the utilization rate of forage resources and a=Bω1 denotes the satiety rate of herd’s animals. The following differential equations model interactions between forage resources and animals of the herd:
(1)dR(t)dt=(r01p+r02-r1)(1-RK)R-qRH1+aR,dH(t)dt=eqRH1+aR-γH.According to a study conducted by the Japan Green Resources Corporation (JGRC) [9], in a region where the average annual precipitation is 500–600 mm, the volume of dry matter production of grass per hectare in natural grassland was in tons 1.54 in 1997, 1.6 in 1998, and 1.8 in 1999 (according to research conducted on mowing in October each year). Among wild herbs growing in this region are the grasses that cattle prefer, but they also appreciate the taste of some legumes.Until now, in many cases (excluding reservoir dams) small artificial wells and natural ponds were used to supply livestock with drinking water. In all these cases, the storage capacity of water is low, and we find ourselves in a situation of chronic lack of water during the dry season. In addition, the storage capacity of water ponds in some places decreases by the reduction of vegetation in the water accumulation area and the influx of land in the ponds. There is sometimes also conflict when cattle in two villages must share the same drinking water source [9]. It is well known that it is not easy to get water in Sahelian region. For sustainable use of water sources that are ponds, it is also necessary to take a measure of water conservation preventing the influx sand around ponds. For example, one study of the Japan Green Resources Corporation showed that establishment of lines of stones can retain the sand and promote the restoration of wild vegetation. The earth is accumulating inside line of stones (the side from which comes the influx); the storage capacity of water becomes higher and ability to grow fodder crops and planting fodder trees becomes larger. This allows obtaining fodder and shade for livestock, and it becomes possible to revive the natural clam that was impoverished around the pond, as a grazing area including a watering place for the cattle [9].The pool of Eda (which is straddled between the villages of Magou and Eda in Niger) had long been used as a point of watering the cattle moving to the Burkina Faso, but due to the decrease in its subsequent storage capacity of water to the influx of sand in the pool, it could not provide the volume of water required to move livestock and livestock of the two villages. But thanks to the digging of the pond and the establishment of lines of stones (some with citizen participation), the revegetation of weeds is increasing, and livestock numbers even greater than in the past are not gathering [10].The variableH(t) is the tropical livestock unit of the herd at time t, R(t) is the forage resources density (kilograms of dry matter per hectare) at time t, W(t) denotes the water resources at time t, m is the removal rate from the herd, e1 is the conversing rate of forage resources consumed into herd biomass, and e2 is the conversing rate of water resources consumed into herd biomass; as previously, r1=r011p+r021-r11 is the intrinsic production of forage resources and r1=r012p+r022-r12 the intrinsic disponibility of water; p is the precipitation rate in the area; K1 is the maximal capacity of the considered area to support forage resources; K2 is the maximal capacity of the area to contain large quantity of water; q1 and q2 are, respectively, the utilization rates of forage and water resources.Following [14, 15], we set qi=(ω0i/ω1i) and ai=(1/Bω1i). The dynamical model for interactions between herds, forage resources, and water is given by the following system of differential equations:
(2)dRdt=(r011p+r021-r11)(1-RK1)R-q1RHa1+R,dWdt=(r012p+r022-r12)(1-WK2)W-q2WHa2+W,dHdt=-mH+e1q1RHa1+R+e2q2WHa2+W.We therefore have three trophic levels: one predator and two preys. The intrinsic production ratesr1 and r2 defined, as previously, can be negative values. In the absence of forage and water resources (r1<0 and r2<0), the herd is doomed if nothing is done. In this case, pastoralists will migrate with their herds (transhumance). Its workforce decreases and goes to extinction if nothing is done. But, in the absence of the herd, forage resources are stored and accumulate until the limit capacity when r1>0; forage resources can disappear when r1<0. The water resources can accumulate to exceed the carrying capacity, but it will very quickly go back to limit capacity when r2>0; water resources can disappear when r2<0.Let us give now a mathematical analysis of System (1) and interpretations of the results in terms of pastoralism, using the data [10].


## 3. Mathematical Results and Interpretations in Sahelian Context

### 3.1. Results of System (1)

Lemma 1.
The nonnegative orthantIR+2 is positively invariant by the trajectories of System (1), and the set D={(R,H)∈IR+2/R≤K} is a compact forward and absorbing set for System (1).Remark 2.
This theorem confirms the fact that forage resources quantity and the number of animals in the herd are always nonnegative numbers, since their trajectories are always inIR+2. Moreover, the forage resources quantity is bounded. This is ecologically plausible since temperatures are usually high and the stockage capacity is limited.Lemma 3.
The sign of the intrinsic production r has an incidence on the dynamics of forage resources and then on the dynamics of the herd’s animals.(1)
Ifr>0, then the forage resources converge to the maximal number of livestock K for a long time.(2)
Ifr<0, then the forage resources will disappear for a long time if nothing is done. This situation conducts to the transhumance of pastoralists with their herds.Lemma 4.
Much equilibrium exists for System (1).(1)
EquilibriumE0=(0,0) and equilibrium E1=(K,0) exist without any condition.(2)
Whenr>0, E2=(R*,H*)=(γ/(eq-aγ);(r/q)(1+aR*)(1-(R*/K))) is an ecologically acceptable equilibrium if eq>aγ and the threshold R1=(eqK/γ(1+aK)) is such that R1≥1. Therefore, with these conditions satisfied, there are three equilibria for System (1).(3)
Whenr<0, E2=(R*,H*)=((γ/(eq-aγ));(r/q)(1+aR*)(1-(R*/K))) is an ecologically acceptable equilibrium if eq>aγ and the threshold R1=(eqK/γ(1+aK)) is such that R1<1. Therefore, with these conditions satisfied, there are three equilibria for System (1).Theorem 5.
The following properties hold concerning System (1).(1)
The equilibriumE0 is a saddle-node when r>0 and a stable node when r<0.(2)
Ifr>0, E1 is a globally asymptotically stable node when R1<1 and a saddle-node with stability for forage resources and instability for herds when R1>1. If r<0, E1 is a saddle-node when R1<1 and a locally unstable node when R1>1. E1 is a globally asymptotically stable node when R1=1. The centre manifold in this case is given by Wc={x=h(y):h(0)=K,h′(0)=a1}, where h(y)=K+a1y+a2y2+O(y3), a1=-(qK/r(1+aK)), a2=-(a12(2ra+((r+γ)/K))+qa1/r(1+aK)).(3)
The equilibriumE2 is not ecologically acceptable when R1<1. If 1<R1≤1+(eq/aγ(1+aK)), then E2 is a globally asymptotically stable focus and when R1>1+(eq/aγ(1+aK)), E2 is an unstable focus and there exists a limit cycle for System (1). This phenomenon corresponds to supercritical Hopf bifurcation [16].Remark 6.
The stability of equilibriumE0=(0,0) means that there will be no more forage resources and no animal of the herd in the considered area for a long moment. There can be many explanations to this situation. Droughts can cause animals mortalities through starvation, emergency slaughtering, and sales, or definitive herds migrations (transhumance), which can create severe drops in the herd sizes. The Sahelian region is particularly affected by such climate shocks [17]. Droughts can also cause forages’ disappearance, and then the equilibrium E0 is stable. When this removal concerns only animals of the herd, depending on some climatic changes, the forage resources growth towards the maximal quantity needed for livestock and the equilibrium E1=(K,0) is stable. Concerning the coexisting equilibrium E2=(R*,H*), there is a Hopf bifurcation. The pastoral interpretation of Hopf bifurcation is that animals in the herd will coexist with the forage resources, exhibiting oscillatory balance behavior. We have a peak for the herd trajectory, followed by a peak for forage trajectory.Theorem 7.
Stability ofE1 and E2(1)
If1<R1≤1+(eq/aγ(1+aK)), then E2 is a globally asymptotically stable focus when r>0 and an unstable focus when r<0.(2)
IfR1>1+(eq/aγ(1+aK)), then E2 is an unstable focus when r>0 and a globally asymptotically stable focus when r<0.(3)
IfR1=1+(eq/aγ(1+aK)), then E2 is a center point, with neutral stability.(4)
IfR1<1, then E1 is a globally asymptotically stable node when r>0.(5)
IfR1>1, then E1 is an unstable saddle-node when r>0 and equilibrium E2 exists.(6)
IfR1=1, then E1 is a globally asymptotically stable node when r>0 and just a locally asymptotically stable node when r<0.Remark 8 (Hopf bifurcation).
There is a limit cycle whenR1 passes through the value 1+(eq/aγ(1+aK)). This phenomenon is known as Hopf bifurcation. Since the limit cycle is stable, it is a supercritical Hopf bifurcation. The pastoral interpretation of Hopf bifurcation as we said is that animals in the herd will coexist with the forage resources, exhibiting oscillatory balance behavior.
This can be explained by the resilience of vegetation, which means the ability of the ecosystem to withstand unusual stress and recover spontaneously once they have disappeared. Thus, an investigation in [18] showed that cereal production in the Sahel went from a deficit of one million tons in 1987 to surplus 1 million tons in 1988.


### 3.2. Results of System (2)

Lemma 9.
The nonnegative orthantIR+2 is positively invariant by the trajectories of System (2), and the set D2={(R,W,H)∈IR+3/R≤K1,W≤K2} is a compact forward and absorbing set for System (2).Remark 10.
Any trajectory with initial condition in the nonnegative orthantIR+3 is trapped and will stay inside. The nonnegative orthant IR+3 is then positively invariant and System (2) is mathematically well posed. We can then say that System (2) is well posed, since forage resources R(t), water resources W(t), and the tropical livestock unit of the herd H(t) are always nonnegative quantities.Theorem 11.
Equilibria of System  (2)
The equilibriaE10=(0,0,0), E11=(K1,0,0), E12=(0,K2,0), and E13=(K1,K2,0) of System (2) exist without any condition, and
(3)E2=(R*,W*,H*)=(R*,ma1a2+(ma2-e1q1a2)R*(e2q2a1-ma1)+(e1q1+e2q2-m)R*,r1q1(1-R*K1))
is an ecologically acceptable equilibrium if R* is a nonnegative value which satisfies equation
(4)c0(R*)4+c1(R*)3+c2(R*)2+c3R*+c4=0,
with R*>K1, W*>K2, and e1q1<m<e2q2, and the coefficients ci are given in Appendix A.Proposition 12 (descartes criterion).
The number of positive real roots of the polynomial equation is less than or equal to the number of changes in coefficient of ordered polynomial coefficients, and these two numbers have the same parity.Proposition 13 (descartes rule of signs-I).
The number of positive roots of the polynomial equation with real coefficients does not exceed the number of sign changes in its coefficients. A zero coefficient is not counted as a sign change.Proposition 14 (descartes rule of signs-II).
The number of positive roots of the polynomial equation with real coefficients does not exceed the number of sign changes in its coefficients and differs from it by a multiple of two. A zero coefficient is not counted as a sign change.
Using Descartes criterion and Descartes’ rule of signs, the number of positive real roots of the polynomial equation given by (3) depends on the number of sign changes in its coefficients(c0,c1,c2,c3,c4).Theorem 15.
Stability of equilibria(1)
The equilibriumE10=(0,0,0) is a locally asymptotically stable node if r1<0 and r2<0. E10 is a saddle-node if r1<0 and r2>0 or r1>0 and r2<0.(2)
The equilibriumE11=(K1,0,0) is a locally asymptotically stable node if r1>0, r2<0, and e1(q1K1/(a1+K1))<m.(3)
The equilibriumE12=(0,K2,0) is a locally asymptotically stable node if r1<0, r2>0, and e2(q2K2/(a2+K2))<m.(4)
The equilibriumE13=(K1,K2,0) is a locally asymptotically stable node if r1>0, r2>0, and e1(q1K1/(a1+K1))+e2(q2K2/(a2+K2))<m. E13 is a saddle-node if one or two of the three conditions are not satisfied and an unstable node if the three conditions are not satisfied.(5)
The coexisting equilibriumE2=(R*,W*,H*) of System (2), given as previously by
(5)E2=(R*,W*,H*)=(R*,ma1a2+(ma2-e1q1a2)R*(e2q2a1-ma1)+(e1q1+e2q2-m)R*,r1q1(1-R*K1))when it exists, can be locally asymptotically stable if the Routh-Hurwitz conditions
(6)H1=v1>0,H2=v1v2-v3>0,H3=v3>0
are satisfied, where v1, v2, and v3 are given as in the proof in Appendix B. Moreover, the system exhibits periodic oscillations with alternative peaks of forage, water, and the herd trajectories.Remark 16.
It is difficult for cattle to adequately live in Sahelian region when forage resources or water resources have dried up completely. When at least one of two completely lacks resources, pastoralists can be forced to migrate with livestock in order to avoid disaster. In addition, feed resources and water resources are often somewhat related since the fresh leaves have water content of about 80%. Transhumance can therefore take place when one of the two resources has failed to herd and in this case, pastoralists migrate while the second resource is available in the area. The stability ofE10, E11, E12, and E13 characterizes these situations. In usual predator-prey models, it is virtually impossible to have stability for E10, E11, and E12, becausestability of E10, for example,means thatthe entire population will disappear, which is not the main objective when coupling preys and predators. However, this is easily explained in pastoralism as it corresponds to transhumance.


## 3.1. Results of System (1)

Lemma 1.
The nonnegative orthantIR+2 is positively invariant by the trajectories of System (1), and the set D={(R,H)∈IR+2/R≤K} is a compact forward and absorbing set for System (1).Remark 2.
This theorem confirms the fact that forage resources quantity and the number of animals in the herd are always nonnegative numbers, since their trajectories are always inIR+2. Moreover, the forage resources quantity is bounded. This is ecologically plausible since temperatures are usually high and the stockage capacity is limited.Lemma 3.
The sign of the intrinsic production r has an incidence on the dynamics of forage resources and then on the dynamics of the herd’s animals.(1)
Ifr>0, then the forage resources converge to the maximal number of livestock K for a long time.(2)
Ifr<0, then the forage resources will disappear for a long time if nothing is done. This situation conducts to the transhumance of pastoralists with their herds.Lemma 4.
Much equilibrium exists for System (1).(1)
EquilibriumE0=(0,0) and equilibrium E1=(K,0) exist without any condition.(2)
Whenr>0, E2=(R*,H*)=(γ/(eq-aγ);(r/q)(1+aR*)(1-(R*/K))) is an ecologically acceptable equilibrium if eq>aγ and the threshold R1=(eqK/γ(1+aK)) is such that R1≥1. Therefore, with these conditions satisfied, there are three equilibria for System (1).(3)
Whenr<0, E2=(R*,H*)=((γ/(eq-aγ));(r/q)(1+aR*)(1-(R*/K))) is an ecologically acceptable equilibrium if eq>aγ and the threshold R1=(eqK/γ(1+aK)) is such that R1<1. Therefore, with these conditions satisfied, there are three equilibria for System (1).Theorem 5.
The following properties hold concerning System (1).(1)
The equilibriumE0 is a saddle-node when r>0 and a stable node when r<0.(2)
Ifr>0, E1 is a globally asymptotically stable node when R1<1 and a saddle-node with stability for forage resources and instability for herds when R1>1. If r<0, E1 is a saddle-node when R1<1 and a locally unstable node when R1>1. E1 is a globally asymptotically stable node when R1=1. The centre manifold in this case is given by Wc={x=h(y):h(0)=K,h′(0)=a1}, where h(y)=K+a1y+a2y2+O(y3), a1=-(qK/r(1+aK)), a2=-(a12(2ra+((r+γ)/K))+qa1/r(1+aK)).(3)
The equilibriumE2 is not ecologically acceptable when R1<1. If 1<R1≤1+(eq/aγ(1+aK)), then E2 is a globally asymptotically stable focus and when R1>1+(eq/aγ(1+aK)), E2 is an unstable focus and there exists a limit cycle for System (1). This phenomenon corresponds to supercritical Hopf bifurcation [16].Remark 6.
The stability of equilibriumE0=(0,0) means that there will be no more forage resources and no animal of the herd in the considered area for a long moment. There can be many explanations to this situation. Droughts can cause animals mortalities through starvation, emergency slaughtering, and sales, or definitive herds migrations (transhumance), which can create severe drops in the herd sizes. The Sahelian region is particularly affected by such climate shocks [17]. Droughts can also cause forages’ disappearance, and then the equilibrium E0 is stable. When this removal concerns only animals of the herd, depending on some climatic changes, the forage resources growth towards the maximal quantity needed for livestock and the equilibrium E1=(K,0) is stable. Concerning the coexisting equilibrium E2=(R*,H*), there is a Hopf bifurcation. The pastoral interpretation of Hopf bifurcation is that animals in the herd will coexist with the forage resources, exhibiting oscillatory balance behavior. We have a peak for the herd trajectory, followed by a peak for forage trajectory.Theorem 7.
Stability ofE1 and E2(1)
If1<R1≤1+(eq/aγ(1+aK)), then E2 is a globally asymptotically stable focus when r>0 and an unstable focus when r<0.(2)
IfR1>1+(eq/aγ(1+aK)), then E2 is an unstable focus when r>0 and a globally asymptotically stable focus when r<0.(3)
IfR1=1+(eq/aγ(1+aK)), then E2 is a center point, with neutral stability.(4)
IfR1<1, then E1 is a globally asymptotically stable node when r>0.(5)
IfR1>1, then E1 is an unstable saddle-node when r>0 and equilibrium E2 exists.(6)
IfR1=1, then E1 is a globally asymptotically stable node when r>0 and just a locally asymptotically stable node when r<0.Remark 8 (Hopf bifurcation).
There is a limit cycle whenR1 passes through the value 1+(eq/aγ(1+aK)). This phenomenon is known as Hopf bifurcation. Since the limit cycle is stable, it is a supercritical Hopf bifurcation. The pastoral interpretation of Hopf bifurcation as we said is that animals in the herd will coexist with the forage resources, exhibiting oscillatory balance behavior.
This can be explained by the resilience of vegetation, which means the ability of the ecosystem to withstand unusual stress and recover spontaneously once they have disappeared. Thus, an investigation in [18] showed that cereal production in the Sahel went from a deficit of one million tons in 1987 to surplus 1 million tons in 1988.


## 3.2. Results of System (2)

Lemma 9.
The nonnegative orthantIR+2 is positively invariant by the trajectories of System (2), and the set D2={(R,W,H)∈IR+3/R≤K1,W≤K2} is a compact forward and absorbing set for System (2).Remark 10.
Any trajectory with initial condition in the nonnegative orthantIR+3 is trapped and will stay inside. The nonnegative orthant IR+3 is then positively invariant and System (2) is mathematically well posed. We can then say that System (2) is well posed, since forage resources R(t), water resources W(t), and the tropical livestock unit of the herd H(t) are always nonnegative quantities.Theorem 11.
Equilibria of System  (2)
The equilibriaE10=(0,0,0), E11=(K1,0,0), E12=(0,K2,0), and E13=(K1,K2,0) of System (2) exist without any condition, and
(3)E2=(R*,W*,H*)=(R*,ma1a2+(ma2-e1q1a2)R*(e2q2a1-ma1)+(e1q1+e2q2-m)R*,r1q1(1-R*K1))
is an ecologically acceptable equilibrium if R* is a nonnegative value which satisfies equation
(4)c0(R*)4+c1(R*)3+c2(R*)2+c3R*+c4=0,
with R*>K1, W*>K2, and e1q1<m<e2q2, and the coefficients ci are given in Appendix A.Proposition 12 (descartes criterion).
The number of positive real roots of the polynomial equation is less than or equal to the number of changes in coefficient of ordered polynomial coefficients, and these two numbers have the same parity.Proposition 13 (descartes rule of signs-I).
The number of positive roots of the polynomial equation with real coefficients does not exceed the number of sign changes in its coefficients. A zero coefficient is not counted as a sign change.Proposition 14 (descartes rule of signs-II).
The number of positive roots of the polynomial equation with real coefficients does not exceed the number of sign changes in its coefficients and differs from it by a multiple of two. A zero coefficient is not counted as a sign change.
Using Descartes criterion and Descartes’ rule of signs, the number of positive real roots of the polynomial equation given by (3) depends on the number of sign changes in its coefficients(c0,c1,c2,c3,c4).Theorem 15.
Stability of equilibria(1)
The equilibriumE10=(0,0,0) is a locally asymptotically stable node if r1<0 and r2<0. E10 is a saddle-node if r1<0 and r2>0 or r1>0 and r2<0.(2)
The equilibriumE11=(K1,0,0) is a locally asymptotically stable node if r1>0, r2<0, and e1(q1K1/(a1+K1))<m.(3)
The equilibriumE12=(0,K2,0) is a locally asymptotically stable node if r1<0, r2>0, and e2(q2K2/(a2+K2))<m.(4)
The equilibriumE13=(K1,K2,0) is a locally asymptotically stable node if r1>0, r2>0, and e1(q1K1/(a1+K1))+e2(q2K2/(a2+K2))<m. E13 is a saddle-node if one or two of the three conditions are not satisfied and an unstable node if the three conditions are not satisfied.(5)
The coexisting equilibriumE2=(R*,W*,H*) of System (2), given as previously by
(5)E2=(R*,W*,H*)=(R*,ma1a2+(ma2-e1q1a2)R*(e2q2a1-ma1)+(e1q1+e2q2-m)R*,r1q1(1-R*K1))when it exists, can be locally asymptotically stable if the Routh-Hurwitz conditions
(6)H1=v1>0,H2=v1v2-v3>0,H3=v3>0
are satisfied, where v1, v2, and v3 are given as in the proof in Appendix B. Moreover, the system exhibits periodic oscillations with alternative peaks of forage, water, and the herd trajectories.Remark 16.
It is difficult for cattle to adequately live in Sahelian region when forage resources or water resources have dried up completely. When at least one of two completely lacks resources, pastoralists can be forced to migrate with livestock in order to avoid disaster. In addition, feed resources and water resources are often somewhat related since the fresh leaves have water content of about 80%. Transhumance can therefore take place when one of the two resources has failed to herd and in this case, pastoralists migrate while the second resource is available in the area. The stability ofE10, E11, E12, and E13 characterizes these situations. In usual predator-prey models, it is virtually impossible to have stability for E10, E11, and E12, becausestability of E10, for example,means thatthe entire population will disappear, which is not the main objective when coupling preys and predators. However, this is easily explained in pastoralism as it corresponds to transhumance.


## 4. Numerical Simulations and Interpretations

The parameters values come from [9, 10] and references therein, and we use the same method as in [9] to compute some parameters values. Let us recall these formulas, in order to have the livestock capacity in natural grassland.(1)
Assuming that the production volume of fresh grasses is 8000 kg/ha, the estimated water contained in fresh grasses is around 80% (this means 20% of dry matter).(2)
The dry matter (DM) obtained for the livestock is 5.2 kg/day for cattle and 1 kg/day for goats and sheep.The production volume of the forage resources with 40% as utilization rate can then be computed:8000 kg/ha × 0.2 × 0.4 = 640 kgDM/ha.The number of animals which is possible in pasture is then given by640 kgDM/ha: (5.2 kgDM × 365  days) = 0.34 head/ha for cattle,640 kgDM/ha: (1.0 kgDM × 365  days) = 1.75 head/ha for goats and sheep.We also have in Tables1 and 2 the parameters values obtained in [9, 10], which correspond to the situation where the intrinsic production r=r01p+r02-r1 is negative. This means that, at this moment, the removal quantity of forage resources, which is due to competition or which is used to feed the livestock, is greater than the intrinsic production. This situation can force pastoralists to leave the area and migrate elsewhere in order to feed their herds.Table 1
Parameter values for transhumance in System (1).
Parameters
Description
Estimated value/range
Reference
r
01
Intrinsic production coefficient
10–20 kgDM/ha
[9]
r
02
Intrinsic production without precipitations
5–10 kgDM/ha
[9]
p
Precipitations
500–600 mm/yr
[9]
r
1
Intrinsic disappearance of forage resources
90–120 kgDM/ha
[9]
e
Forage biomass conversion rate
0.015 per day
[10]
a
Herd’s satiety rate
0.01/ha
[10]
K
Maximal capacity of forage production
500–600 kgDM/ha
[10]
q
Utilization rate of forage resources
0.4 per day
[9]
γ
Removal rate in the herd
1.0 per day
[10]Table 2
Parameter values for transhumance in System (2).
Parameters
Description
Estimated value/range
Reference
r
01
1
Intrinsic production of forage
10–20 kgDM/ha
[9]
r
01
2
Intrinsic production of water
700–800 kgDM/ha
[9]
r
02
1
Forage production without precipitations
5–10 kgDM/ha
[9]
r
02
2
Water production without precipitations
100–200 mm/yr
[9]
p
Precipitations
500–600 mm/yr
[9]
r
1
1
Intrinsic disappearance of forage resources
90–120 kgDM/ha
[9]
r
1
2
Intrinsic disappearance of water resources
150–250 mm/yr
[9]
e
1
Forage biomass conversion rate
0.015 per day
[10]
e
2
Water biomass conversion rate
0.0154 per day
[10]
a
1
Herd’s satiety rate from forage
0.03/ha
[10]
a
2
Herd’s satiety rate from water
0.02/ha
[10]
K
1
Maximal capacity of forage production
7000–9000 kgDM/ha
[10]
K
2
Maximal capacity of water production
900 mm/yr
[10]
q
1
Forage utilization rate
0.4 per day
[9]
q
2
Water utilization rate
0.6 per day
[9]
m
Herd’s removal rate
1 per day
[10]Figure1 illustrates global stability of equilibrium (0,0). Figure 2 illustrates coexistence and local stability of equilibria (0,0) and (K,0). Figure 3 illustrates the global asymptotic stability of equilibrium (K,0). Figures 4 and 5 illustrate the global asymptotic stability of equilibrium E2=(R*,H*). Figures 6, 7, and 8 illustrate existence of periodic solutions and then Hopf bifurcation with a stable limit cycle.Figure 1
Phase portrait of the herd’s animals and forage resources when r < 0. This illustrates the global stability of equilibriumE0=(0,0) for System (1). This is a new situation, which happens during migration of pastoralists with their herds, calling transhumance; the pastoralists leave the area because of scarcity of forage resources. The parameters in this case are given in Table 1.Figure 2
Phase portrait of System (1). Illustration of the local asymptotic stability of equilibrium E0=(0,0) and equilibrium E1=(K,0). These situations happen during transhumance; the pastoralists leave the area because of scarcity of forage resources or some local conditions. The rest of parameters in this case are given in Table 1, with r01=200 kgDM/ha, r02=55 kgDM/ha, r1=80 kgDM/ha, and K=30 kgDM/ha such that r>0.Figure 3
Chronological evolution and phase portrait of the herd individuals and forage resources of System (1) when r01=200 kgDM/ha, r02=55 kgDM/ha, r1=80 kgDM/ha, K=30–600 kgDM/ha, e=0.05/day, a=0.013/ha, q=0.4/day, and γ=0.07/day. Global stability of equilibrium E1=(K,0) for System (1). This illustrates the case when pastoralists decide to migrate for reasons different from unavailability of forage resources.Figure 4
Chronological evolution of the herd’s animals and forage resources of System (1) when r=500 kgDM/ha, e=0.05/day, a=0.013/ha, K=10–40 kgDM/ha, q=0.4/day, and γ=0.07/day. Stability of the coexisting equilibrium E2=(R*,H*).Figure 5
Phase portrait of the herd’s animals and forage resources of System (1) when r=500 kgDM/ha, e=0.05/day, a=0.013/ha, K=8–30 kgDM/ha, q=0.4/day, and γ=0.07/day. Illustration of the stability of the unique coexisting equilibrium E2=(R*,H*) for four initial conditions.Figure 6
Phase portrait of the herd’s animals and forage resources of System (1) when r=500 kgDM/ha, e=0.05/day, a=0.013/ha, K=8–30 kgDM/ha, q=0.4/day, and γ=0.07/day. Illustration of the stability of the unique coexisting equilibrium E2=(R*,H*) for four initial conditions.Figure 7
Chronological evolution of the herd’s animals and forage resources of System (1) when r=650 kgDM/ha, e=0.05/day, a=0.013/ha, K=800 kgDM/ha, q=0.4/day, and γ=1.0/day. This illustrates the global stability of the unique coexisting equilibrium E2=(R*,H*). This means that the herd’s animals and forage resources have periodic curves since there is a stable limit cycle. The two populations can exist together.Figure 8
Phase portrait for illustration of a stable limit cycle around coexisting equilibrium of System (1) for E2=(R*,H*) for r=700 kgDM/ha, e=0.05/day, a=0.013/ha, K=960 kgDM/ha, q=0.4/day, and γ=1.0/day. This means that there is a supercritical Hopf bifurcation.Figure9 illustrates the global asymptotic stability of equilibrium (0,0,0). Figure 10 illustrates the local stability of equilibria E10, E11, and E12. Figure 11 illustrates existence of periodic solutions with alternative peaks of trajectories. Figure 12 illustrates the global stability of equilibrium E13=(K1,K2,0).Figure 9
Phase portrait corresponding to the disappearing of forage resources and water resources and removal of the pastoralists with their herds for System (2). We can observe here the global asymptotic stability of E10=(0,0,0) when r1=-0.4<0 and r2=-60<0. The rest of parameters are in Tables 1 and 2.Figure 10
Phase portrait of System (2) showing the coexistence of E10, E11, E12. Equilibria E11 and E12 correspond to the removal of the pastoralists with their herd and persistence of forage or water resources. We can observe here the local asymptotic stability of E11=(K1,0,0), E12=(0,K2,0), and E10=(0,0,0) when r1=400 kgDM/ha and p=3000 mm. The rest of parameters are in Tables 1 and 2.Figure 11
Illustration of periodicity of the trajectories at equilibriumE2=(R*,W*,H*) for System (2). Alternative peaks are observed between the trajectories of the flock, feed resources, and water resources when r1=800 kgDM/ha, r2=700 kgDM/ha, p=600mm, K1=1000 kgDM/ha, and K2=900 kgDM/ha. The rest of parameters are in Tables 1 and 2.Figure 12
(a) Chronological evolution of herd’s animals, forage, and water resources of System (2). We observe the global stability of equilibrium E13=(K1,K2,0) when r1=200 kgDM/ha, r2=300 kgDM/ha, p=600mm, K1=800 kgDM/ha, and K2=900 kgDM/ha. (b) Phase portrait of System (2) showing the local stability of equilibria E11 and E12. The rest of parameters are in Tables 1 and 2.


## 5. Conclusion

A mathematical model for pastoralism has been considered in order to explain and predict some situations in Sahelian region. This is one of the first deterministic models of pastoralism analyzed with interpretations. The results obtained in this paper have been simulated with the data in [9, 10] and confirm many observations throughout pastoralism’s literature concerning interactions of forage resources and herds. At the end of this first part, we can say that since forage resources can be also considered sometimes as water resources, it will be better for us to add in the previous equations of System (1) another equation in order to take into account the water resources.The statistics show the importance of livestock to the national economy in tropical arid countries. Two opposing trends develop: the continued contribution of livestock in national economy and the growing risk of deterioration of pastoral resource savings.Sustainable pastoral resource management, equitable and secured access to pastoral resources, and peace and security which are always the aims of framework policy are also the guidelines of our contribution to pastoralism by mathematical models. Thus, it is necessary that these issues be addressed through a comprehensive pastoral policy, which confers full political, social, economic, and environmental benefits to the pastoral communities. The deterministic models proposed and analyzed in this paper are firstly to participate to these efforts. Although considerable efforts with some positive results are being made throughout Africa, a great deal more still needs to be done. This calls for both commitments from individual countries and cooperation at the regional and continental levels. We think that mathematical models can considerably contribute to increasing comprehension of pastoralism and therefore increase the quality of life of pastoralists. The models considered in this paper exhibit periodic oscillations when some conditions are satisfied. The comprehension of conditions to have periodic oscillations and then coexistence of herds, forage, and water resources can help pastoralists to better understand their environment. With this coexistence, there are alternative peaks of livestock tropical unit, forage resources, and water resources. This means that when these conditions are satisfied, pastoralists can live in some areas without transhumance, with disponibility of forage and water resources.



---

*Source: 138179-2014-07-22.xml*