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Volume 2012 (2012), Article ID 796783, 8 pages
Effects of Additional Foods to Predators on Nutrient-Consumer-Predator Food Chain Model
Department of Mathematics, Daharpur A. P. K. B Vidyabhaban, Paschim Medinipur, West Bengal, India
Received 8 February 2012; Accepted 18 March 2012
Academic Editors: J. R. C. Piqueira and I. Rogozin
Copyright © 2012 Banshidhar Sahoo. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
We have proposed a nutrient-consumer-predator model with additional food to predator, at variable nutrient enrichment levels. The boundedness property and the conditions for local stability of boundary and interior equilibrium points of the system are derived. Bifurcation analysis is done with respect to quality and quantity of additional food and consumer’s death rate for the model. The system has stable as well as unstable dynamics depending on supply of additional food to predator. This model shows that supply of additional food plays an important role in the biological controllability of the system.
The interactions between living and nonliving organisms have significant role in ecological modelling. In every ecosystem, there always have been material fluxes from the outside to the system as well as from the system to outside. In a hypothetical steady state, these nutrient inputs and outputs balance. Many mathematical models have included these interactions. Effects of nutrient enrichment on a food chain model have been investigated, both empirically as well as theoretically by many scientists [1–3]. These nutrients enrichment may reduce species diversity and ecosystem functioning . Also, many researchers [5–7] have shown that nutrient enrichment can lead to a complex dynamics as well as extinctions of species. In the late 1970s, Pimm and Lawton  simulated a large number of food webs including omnivorous links, as nonlinear interactions. They discovered that these additional interactions in general stable internal equilibria to become statistically rare.
The role of additional food as a tool in biological control programs has become a topic of great attention for many scientists due to its ecofriendly nature. In recent years, many biologists, experimentalists, and theoreticians have concentrated on investigating the effects of providing additional food to predators in a predator-prey system [9–14]. Srinivasu et al.  have studied qualitative behavior of a predator-prey system in the presence of additional food to the predators, and they concluded that handling times for the available foods to the predator play a key role in determining the state of the ecosystem. In the controllability studies by Srinivasu et al. , it is observed that, for properly chosen quality and quantity of the additional food, the asymptotic state of a solution of the system can either be an equilibrium or a limit cycle. Sahoo and Poria  discussed the dynamic behaviour for seasonal effects on additional food in a predator-prey model. Very recently, Sahoo  discussed that existence of species in a system depends on interaction functions and supply of the quality of additional food. The decline of large predators at the top of the food chain has disrupted ecosystems all over the planet, according to a review of recent findings conducted by an international team of scientists and published in Science (Estes et al. ). They concluded that the loss of apex consumers may be the most pervasive influence on the natural world. Therefore, analysis of strategies related to effects of additional foods to predators is important in real world.
In this paper, we propose a model of nutrient-consumer-predator interaction (Figure 1) with additional food (characterized by predator’s handling time) to predator, at variable nutrient enrichment levels. We have derived the existence and local stability conditions of boundary and interior equilibrium points of the system. We have analyzed the behaviour of the proposed model through numerical simulations depending on some identified vital ecological parameters. We have done bifurcation analysis of our model with respect to quality and quantity of additional food and consumer’s death rate, respectively, and finally conclusion is given.
2. Model Formulation
We formulate a nutrient-consumer-predator model as
Here, stands for the amount of nutrients present within the ecosystem, and and denote the number of the consumer species and predator, respectively. Here, is time. Let be the constant rate of nutrient supply in the system; the constants and are conversion rates of nutrients supply to consumer; the constants and are conversion rates of consumer to predator for species and , respectively; and are constant death rates for species and respectively. The terms and are the nutrient regeneration rate from dead consumer and predator population. The constant is the half-saturation constant for .
If and are constants representing handling time of the predator per consumer item and ability of the predator to detect the consumer, then we have and , representing the maximum predation rate and half-saturation values of the predator , to be and , respectively. If represents the efficiency with which the food consumed by the predator gets converted into predator biomass, then , the maximum growth rate of the predator, is given by .
Now, we modify the model (1) by introducing “additional food” to predator population. We make the following assumptions:(a)predator is provided with additional food of constant biomass which is distributed uniformly in the habitat;(b)the number of encounters per predator with the additional food is proportional to the density of the additional food;(c)the proportionality constant characterizes the ability of the predator to identify the additional food.
Now, the modified model takes the following form:
If represents the handling time of the predator per unit quantity of additional food, and represents the ability for the predator to detect the additional food, then we have and . The term represents effectual additional food level. The system has to be analyzed with the following conditions: , , and .
To reduce the number of parameters and to determine which combinations of parameters control the behavior of the system, we nondimensionalize the system (2) with , , , and and obtain the following system of equations: where , , , , , , , , , and . The system (3) has to be analyzed with the following initial conditions: , , and .
The nutrient uptake rate per unit biomass of consumer per unit time is . Nutrient involved in the system also undergos loss due to leaching at a rate . Consumer growth rate per unit time is . The terms and are the nutrient regeneration rate from dead consumer and predator population. It is assumed that input of external nutrient supply is dependent on the amount of nutrient present in the system.
Here, represents the “quality” of the additional food (ratio between predator’s handling time towards additional food and consumer item), and represents the “quantity” of the additional food for predator. The parameters , are the parameters which characterize the additional food. We do not make any distinction regarding the additional food like complementary, essential, or alternative. Here, we only assume that the predators are capable of reproducing by consuming the available food sources. Next, we shall analyze the dynamics of the model (3) theoretically and numerically.
3. Theoretical Study
In this section, positivity and boundedness for the system (3) are established. Since the state variables , , and represent populations, positivity insures that they never become negative and population always survives. The boundedness may be interpreted as a natural restriction to growth as a consequence of limited resources.
3.1. Positive Invariance
The system (3) can be put into the matrix form with , where . is given by where and .
It can be seen that whenever such that, then (for ). Now any solution of with , say , is such that for all (Nagumo ).
Theorem 1. All the solutions of the system (3) which start in are uniformly bounded.
Proof. Let be any solution of the system (3) with positive initial conditions.
Let us consider that Using (3), we have Therefore, Since and , we get the following expression: where .
Hence, where .
Applying the theory of differential inequality, we obtain For , we have .
Hence, all the solutions of (3) that initiate in are confined in the region This proves the theorem.
3.3. Existence and Local Stability of Boundary Equilibrium Points
The system (3) always has two boundary equilibrium points. is the trivial equilibrium point. The axial equilibrium point is . The third boundary equilibrium point is the predator-free equilibrium point, where and .
The predator-free equilibrium point exists if and .
The Jacobian matrix of the system (3) at any arbitrary point is given by
Theorem 2. The trivial equilibrium point is always unstable. The axial equilibrium point is unstable if and . The predator-free equilibrium point is locally stable if , , and .
Proof. The Jacobian matrix at is given by
which has one positive eigenvalue and two negativeeigen values and , giving a point at the origin with nonempty stable manifolds and an unstable manifold. So, is always unstable.
The Jacobian matrix at is given by
From the Jacobian matrix , it is observed that it has one negative eigenvalue () and two positive eigenvalues if and and again has nonempty stable and unstable manifolds. Hence, the axial equilibrium point is unstable if and .
The Jacobian matrix at is given by The characteristic roots of the Jacobian matrix are and roots of the equation The predator-free equilibrium point is stable if and . Hence, the theorem is proved.
3.4. Existence and Local Stability of Interior Equilibrium Point
The interior equilibrium point of the system (3) is given by , where , , and is the positive root of the equation where , , and
The interior equilibrium point exists if
Theorem 3. The interior equilibrium point for the system (3) is locally asymptotically stable if the following conditions hold: , , and , where
Proof. The Jacobian matrix of the system (3) at the interior equilibrium point is
where , , , , , , , , . The characteristic equation of the Jacobian matrix is given by
Using the Routh-Hurwitz criteria , we observe that the system (3) is stable around the positive equilibrium point if the conditions , , and hold.
4. Numerical Study
For numerical simulation, we choose , , , , , , , , , , and which remains the same for all numerical simulations. The remaining two parameters (quality of additional food) and (quantity of additional food) are varied to obtain different types of behaviours of the system.
4.1. Bifurcation Analysis with respect to the Quality of Additional Food
We have done bifurcation analysis of the system (3) with respect to quality of additional food within the range taking as fixed. From Figure 2, we observe that the system shows chaotic behaviour without any additional food. If we increase availability of the quality of additional food after , the system shows periodic oscillations. The system again enters into chaotic region within , and after that, the system shows regular behaviour and finally settles down to steady state for . However, from these bifurcation diagrams, we observe that increase of quality of additional food up to a certain level reduces the prevalence of chaos, and the system enters into periodic region. Even, beyond a certain concentration level of food supply, the system will enter into a stable state. It shows that the consumer population has extinction risk for low quality of additional food. Figure 3 is the bifurcation diagram of the system with respect to quality of additional food taking death rate , instead of , . From Figure 3, we observe that the chaotic dynamics vanishes, and it shows oscillatory behaviour of the system (3) with suitable supply of additional food depending upon the death rate of the consumer.
4.2. Bifurcation Analysis with respect to the Quantity of Additional Food
We have done bifurcation analysis with respect to quantity of additional food within the range taking . Figure 4 represents the bifurcation diagram of consumer and predator with respect to for fixed . From Figure 4, we observe that in the absence of quantity of additional food , that is, at , the system shows chaotic behaviour. After , the system shows period 3, period 2, and limit cycle oscillation. With the increase of quantity of additional food after certain level, system goes to steady state. If we take , , the chaos totally disappears from the system and shows periodic behaviour which is shown in Figure 5. From these diagrams, we conclude that the consumer population has extinction risk for small quantity of additional food, but it has stable behaviour for high quanity of additional food. The supply of additional food to predator decreases the predation pressure on consumer species, and as, a result, the consumer species survives.
4.3. Bifurcation Analysis with respect to Death Rate of Consumer
Figure 6 is the bifurcation diagram of the system (3) with respect to consumer’s death rate . The bifurcation diagram shows that some consumer species have high extinction risk in the system. Another consumers species survives due to presence of additional food to predator. On the other hand, predator populations have no extinction risk. They always survive in the system, but the growth rate of predator population decreases with the increase of death rate of consumer.
Figure 7 is the diagram of stable and unstable dynamics of the system (3) with respect to quality of additional food for , , , , , , , , , , , and . The smooth lines indicate the stable dynamics, while the dashed lines indicate the unstable dynamics. From Figure 7, we can conclude that the system will have stable dynamic behaviour for proper choice of additional food.
In this paper, we have proposed a model of nutrient-consumer-predator interaction with additional food to predator. Here, we have derived boundedness criteria of our system. We have studied the existence and local stability conditions of boundary and interior equilibrium points of the system. We have done bifurcation analysis of our model with respect to quality of additional food , quantity of additional food , and death rate of consumer species, respectively. We observe that increasing quality and quantity of additional food supply, the system’s chaos can be controlled.
Through the theoretical study and bifurcation analysis, we conclude that nutrient-consumer-predator system in the presence of additional food exhibits very rich dynamics. From the bifurcation diagrams, we observe that by varying quality and quantity of additional food we can control chaos of a food chain. Notice that the system becomes regular for some range of values of the death rate of consumer and regeneration rate of nutrient due to consumer’s death. Therefore, the system’s dynamics is sensitive to the death rate of consumer. An important observation the Figure 7 is that the system has stable and unstable dynamics with respect to quality of additional food. Therefore, the stability of a system highly depends on proper supply of additional food.
We observe that consumer species has extinction risk for low quality and small quantity of additional food, but consumer can survive only when we supply high quality and large quantity of additional food. This happened as predator is taking additional food, and the predation pressure on consumer is decreasing, and thus, consumer can servive and have a stable dynamic behaviour. Therefore for biological conservation, additional food may be very useful for servival of consumer species in an ecosystem. So, we conclude that the proper choice of additional food to predator makes a food chain model more realistic, ecofriendly, and nonchaotic.
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