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#predator population

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Predator Prey Model

In a system with alternate prey, predator population may:

Alternative prey can maintain or increase predator numbers even while the principal prey declines because the predator’s energy intake depends on the total accessible resource base. Generalists may switch toward the alternative, and reproduction or survival supported by that food can offset losses caused by scarcity of the main prey. In a multispecies growth equation, positive contributions from alternative prey can keep predator per-capita growth above mortality. Predator abundance is therefore neither necessarily constant nor unaffected by the main prey’s decline. This subsidy can have an important indirect effect: sustained predators may continue attacking the depleted focal prey, creating apparent competition between prey species and potentially preventing recovery. If predators switch strongly away from rare prey, however, alternative food can stabilize coexistence. Outcomes depend on prey profitability, encounter rates, switching behavior, and whether prey abundances covary. The central prediction is energetic rather than categorical—when alternative prey contribute enough food, predator carrying capacity and population growth need not track the main prey alone.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 14

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Predator Prey Model

In the absence of prey, predator population will:

Without prey or another usable food source, predator intake falls to zero while metabolic maintenance, mortality, and emigration continue. In the Lotka–Volterra equation dP/dt = baNP − mP, setting prey density N to zero gives dP/dt = −mP. Predator abundance then declines exponentially at rate m, assuming a closed population and no immigration. Stabilization would require replacement of deaths through feeding-supported reproduction or external subsidy, while increase is impossible under the model because prey conversion supplies the positive growth term. Oscillation also requires reciprocal feedback between both populations and cannot continue when one is absent. Real predators may persist temporarily on stored reserves, enter dormancy, migrate, scavenge, or switch to alternative prey, so “absence of prey” must mean absence of all effective food. Those mechanisms change the model assumptions rather than the basic energetic conclusion. The population-level decline arises from the balance of births and deaths, not because every individual dies immediately when prey disappear.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 14

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Predator Prey Model

The predator population decreases over time due to:

Predator abundance depends on energy and nutrients obtained from prey. When prey decline, predators encounter food less often, reducing intake, body condition, survival, and reproductive output. Starvation and emigration can cause an immediate decrease, while reduced recruitment creates a delayed response. This lag is why predator peaks often follow prey peaks in coupled population cycles. High prey abundance ordinarily supports predator growth rather than decline, and an increased predator reproductive rate would raise population size unless outweighed by mortality. Lack of competition may relieve a constraint but cannot compensate indefinitely for insufficient food. In the Lotka–Volterra model, predator change is dP/dt = baNP − mP = P(baN − m); below the prey threshold N = m/(ba), predator mortality m exceeds gains from consuming prey and predator numbers fall. Alternative prey, stored reserves, or immigration can weaken this relationship in nature, but the mechanistic expectation remains a predator decline when its effective resource base falls below replacement needs.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 14

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