Source: Exam 1 Review Guide 2, Prof. Jensen
Difficulty: Intermediate | Prerequisites: Basic population ecology concepts (population growth, carrying capacity, competition).
Predation sits at the heart of species interactions in ecology. It covers any interaction where one organism consumes another, and that includes herbivory and parasitism, not just a lion taking down a gazelle. This topic builds on population ecology by asking: what actually controls population size? Understanding predation is essential because it connects directly to community structure, coevolution, and real-world wildlife management decisions.
Predation is any interaction where one organism eats another, and it can regulate prey populations from the top down. Whether predation actually shrinks a population depends on whether the killed prey would have died anyway (compensatory mortality) or would have survived and reproduced (additive mortality). Predators and prey coexist more easily when the environment offers refugia, habitat variety, and prey switching, and plants and herbivores are locked in an ongoing coevolutionary arms race.
Predation
An interaction in which one organism (the predator) consumes all or part of another organism (the prey). This includes herbivory and parasitism. In simple terms, any time one living thing eats another, that counts as predation in the broad ecological sense.
Top-down regulation
Predators control prey population size by consuming prey individuals. Think of it as the population being shaped from above in the food chain.
Bottom-up regulation
Prey (or primary producers) control predator populations by limiting the energy and nutrients available. Think of it as population control flowing upward from the base of the food chain.
Compensatory mortality
Predation that does not reduce population size because the individuals killed would have died from other causes anyway. In simple terms, the predator is removing individuals the population was going to lose regardless.
Additive mortality
Predation that does reduce population size because the individuals killed would otherwise have survived and contributed to population growth. Think of it as deaths that stack on top of what would have happened naturally.
Habitat heterogeneity
Variation in the physical structure and layout of the environment. More complex environments offer more niches and hiding spots, which helps predators and prey coexist.
Refugia
Places where prey can hide and where predators are least likely to invade. In simple terms, safe zones that keep prey from being completely wiped out.
Prey switching
When predators shift to eating more abundant prey species as a particular prey becomes scarce. This prevents any single prey species from being driven to extinction.
Constitutive defenses
Defenses that are always present in an organism, regardless of whether a threat is nearby. For example, thorns on a plant or a hard shell on a beetle.
Inducible defenses
Defenses that are produced only when a threat is detected. These are resource-efficient because the organism only pays the cost of defence when it is actually needed.
Red Queen hypothesis
The idea that species in an ecological interaction (e.g. predator and prey, or herbivore and plant) continually evolve in response to each other, but neither side gains a lasting advantage. Named after the Red Queen in Through the Looking-Glass, who says you must keep running just to stay in place.
Predation is broad: it includes carnivory, herbivory, and parasitism.
Prey have evolved defenses: speed, coloration, chemical deterrents, and mimicry.
Predators have counter-adaptations: sharper vision, better hearing, stronger jaws, claws, and specialised digestive enzymes.
Whether predation controls a population depends on the system. In some cases (dingoes and kangaroos), predators clearly regulate prey numbers. In others (muskrats and mink), they do not.
Top-down: predators limit prey populations from above in the trophic hierarchy.
Bottom-up: prey availability (or producer availability) limits predator populations from below.
The hare/lynx experiment is a classic case study. Data suggest hare populations are limited by both predation and food availability, not just one or the other. When predators are excluded and food is added, hare density increases far more than from either factor alone.
Compensatory mortality: the prey killed by predators would have died anyway (from disease, starvation, old age). Predation does not shrink the population.
Additive mortality: the prey killed would have survived and reproduced. Predation does shrink the population.
This distinction has real management implications. Salmon management strategies differ depending on whether predation mortality is assumed to be compensatory or additive.
If mortality is compensatory, harvesting or predation has little impact on population size. If additive, every individual removed matters.
In simple lab settings, predators often drive prey to extinction (no coexistence).
Three factors promote coexistence in nature:
Habitat heterogeneity: environmental complexity gives prey places to escape.
Refugia: specific safe zones where predators cannot easily reach prey.
Prey switching: predators shift to more abundant prey when one species becomes rare, relieving pressure on the declining species.
Plants limit herbivore populations through chemical and physical defenses.
Constitutive defenses are always present (e.g. thorns, toxic alkaloids). Cost: the plant pays the metabolic price continuously.
Inducible defenses are activated only when the plant detects herbivore damage. Advantage: resource-efficient, because the cost is paid only when needed.
Herbivores may evolve tolerance to plant defenses, or even exploit them (e.g. monarch butterflies sequestering milkweed toxins).
This drives selection pressure for even stronger plant defenses, creating an ongoing arms race.
Species in close ecological interactions evolve reciprocally: each adaptation in one species selects for a counter-adaptation in the other.
Neither side "wins" permanently. Both must keep evolving just to maintain their current fitness relative to the other.
This concept recurs throughout the BBIO 180 course in the context of host-parasite interactions, sexual selection, and more.
Wildlife management decisions (e.g. salmon harvest limits) depend on whether predation mortality is compensatory or additive.
Understanding top-down vs bottom-up control informs conservation strategies for endangered species and invasive species management.
Students often think predation always controls prey population size. It does not. In many systems, competition or resource limitation plays a bigger role.
Students confuse compensatory and additive mortality. Remember: compensatory means the individual was going to die anyway, so population size is unaffected. Additive means the death is a net loss to the population.
Students assume inducible defenses are better than constitutive defenses in every situation. They are not. Inducible defenses save resources but are useless if the organism cannot detect the threat quickly enough.
Students sometimes think the Red Queen hypothesis means one species eventually wins. The whole point is that neither side wins; both keep evolving without gaining a permanent advantage.
⚠️ Be ready to distinguish compensatory from additive mortality and explain why the distinction matters for wildlife management (salmon is the go-to example).
⚠️ Know the three mechanisms that promote predator-prey coexistence: habitat heterogeneity, refugia, and prey switching.
⚠️ Understand both top-down and bottom-up regulation, and be able to interpret experimental data (like the hare/lynx study) to determine which is operating.
⚠️ Be able to explain why inducible defenses are advantageous (resource efficiency) and distinguish them from constitutive defenses.
⚠️ The Red Queen hypothesis will reappear later in the course. Nail the concept now.
True or False: Predation always reduces prey population size. (False: it depends on whether mortality is compensatory or additive.)
Fill in the blank: A place where prey can hide from predators is called a ________. (Refugium / refugia.)
True or False: Inducible defenses are always present in an organism. (False: they are produced only when a threat is detected. That describes constitutive defenses.)
Fill in the blank: When predators switch to eating whatever prey is most abundant, this behaviour is called ________. (Prey switching.)
True or False: The Red Queen hypothesis predicts that one species will eventually evolve to dominate the other. (False: neither side gains a permanent advantage.)
Q: Why are inducible defenses advantageous compared to constitutive defenses?
A: Inducible defenses make efficient use of resources because they are produced only when needed. The organism avoids the metabolic cost of maintaining defenses when no threat is present.
Q: A predator kills 100 deer in a population. Explain how the effect on population size differs depending on whether this mortality is compensatory or additive.
A: If the mortality is compensatory, those 100 deer would have died from other causes (disease, starvation, old age), so the population size is unchanged. If the mortality is additive, those 100 deer would have survived and potentially reproduced, so the population declines by up to 100 individuals plus their potential offspring.
Q: In a simple laboratory experiment, a predator drives its prey to extinction. Name three features of natural environments that promote coexistence instead.
A: Habitat heterogeneity (environmental complexity), refugia (safe hiding places for prey), and prey switching (predators shift to more abundant alternative prey when one species becomes rare).
Q: Why is the phrase "coevolutionary arms race" an appropriate way to characterise the long-term effects of species interactions?
A: Adaptations that give one species a fitness advantage in an interaction are likely to be countered by adaptations in the other species that eliminate this advantage. Each side continually evolves in response to the other, with neither gaining a permanent upper hand.
Q: Looking at hare/lynx experimental data where hare density was highest when both predators were excluded and food was added, what conclusion can you draw?
A: Hare populations are limited by both predation and food availability. Neither factor alone fully explains population regulation; both top-down and bottom-up forces are at work.
This connects to community ecology because predation is one of the key species interactions (alongside competition and mutualism) that shapes community structure. The Red Queen hypothesis reappears in discussions of sexual selection, host-parasite dynamics, and the evolution of sex. Top-down and bottom-up regulation links directly to trophic levels and productivity, where energy flow through food webs determines how much biomass each level can support.
Predation, predator-prey interactions, top-down regulation, bottom-up regulation, compensatory mortality, additive mortality, habitat heterogeneity, refugia, prey switching, constitutive defenses, inducible defenses, plant defense, herbivory, parasitism, coevolutionary arms race, Red Queen hypothesis, coevolution, population regulation, wildlife management, salmon predation, dingoes and kangaroos, muskrats and mink, BBIO 180, Intro Biology, ecology exam review