🦉Intro to Ecology Unit 7 Review

Predation, herbivory, and parasitism all follow the same basic pattern: one organism benefits at the expense of another. But they differ in important ways.

Predation kills and consumes prey organisms. The interaction is typically quick and lethal.
Herbivory involves consuming plant tissue (leaves, seeds, roots). It often damages the plant without killing it outright.
Parasitism is a longer-term relationship where the parasite lives on or inside a host, drawing nutrients over time. The host is harmed but usually not killed immediately.

These interactions differ in their population-level effects. Predation tends to cause more immediate, dramatic shifts in prey numbers, while herbivory and parasitism often exert slower, chronic pressure. All three can trigger trophic cascades, where effects ripple across multiple levels of a food web.

Each interaction also drives coevolution: consumers and consumed organisms develop adaptations and counter-adaptations over time. Predators get faster, so prey get faster too. Plants evolve chemical defenses, so herbivores evolve detoxification enzymes. Parasites evolve ways to evade host immune systems, and hosts evolve stronger immune responses.

Ecological consequences

Trophic cascades look different depending on the interaction involved:

Predation cascade: Removing a top predator allows herbivore populations to explode, which reduces plant biomass. The classic example is the reintroduction of wolves to Yellowstone, which reduced elk browsing and allowed streamside vegetation to recover.
Herbivory cascade: Overgrazing strips vegetation, which increases soil erosion and reduces water retention in the landscape.
Parasitism cascade: Parasites can alter host behavior in ways that reshape ecosystems. The parasite Toxoplasma gondii, for instance, makes infected rats less fearful of cats, increasing the rats' chance of being eaten and completing the parasite's life cycle.

Keystone species effects show up across all three interaction types. Predators can maintain prey diversity by preventing any single prey species from dominating through competitive exclusion. Herbivores shape plant communities (elephants in African savannas knock down trees, maintaining open grassland). Parasites influence host population sizes and can shift competitive balances between host species.

Indirect effects add another layer of complexity:

Apparent competition occurs when two prey species share a common predator. If one prey species becomes abundant, it boosts predator numbers, which then increases predation pressure on the other prey species, even though the two prey don't compete directly.
Trait-mediated indirect interactions happen when a predator's presence changes prey behavior (like foraging less), which then affects other species the prey interacts with.

Predation's impact on prey

Population regulation and dynamics

Predation acts as density-dependent mortality: as prey populations grow denser, predators find and kill more of them. This prevents prey from overexploiting their own resources and helps maintain population stability.

Ecologists describe how predation rate changes with prey density using three functional response types:

Type I (linear): The predation rate increases proportionally with prey density, with no upper limit until the predator is satiated. Filter feeders like baleen whales approximate this pattern.
Type II (decelerating): The predation rate rises quickly at low prey density but levels off as the predator spends more time handling each prey item. This is the most common response for individual predators.
Type III (sigmoidal): At low prey density, predation rate is low because predators focus on alternative prey. As prey density increases, predators "switch" to that prey type, and the rate climbs steeply before leveling off.

Predators also show numerical responses, meaning their own population size changes in response to prey abundance. When prey are plentiful, predator populations grow; when prey decline, predators decline too. This feedback loop produces the cyclical population dynamics seen in the famous lynx-hare cycles in boreal forests, where populations of snowshoe hares and Canada lynx rise and fall roughly every 10 years.

Keystone predation occurs when a predator maintains species diversity by keeping dominant competitors in check. Robert Paine's experiments with the sea star Pisaster ochraceus in Pacific intertidal communities showed that removing this single predator allowed mussels to monopolize space, reducing overall species diversity.

Behavioral and community effects

Predation doesn't just kill prey; it changes how surviving prey behave. These non-lethal effects can be just as ecologically significant:

Habitat use shifts: Prey avoid areas where predators are active, even if those areas have good food resources.
Altered foraging: Prey spend less time feeding or restrict themselves to safer locations, which can reduce their growth and reproduction.
Reproductive changes: In high-predation environments, some prey species mature earlier or invest more in reproduction at the cost of longevity.

Mesopredator release happens when top predators are removed from a system. Mid-level predators (mesopredators) then increase in abundance and can devastate smaller prey populations. For example, the extirpation of wolves from parts of North America allowed coyote populations to expand, increasing predation pressure on smaller mammals and ground-nesting birds.

Apparent competition between prey species can also restructure communities. Arctic hares and lemmings don't compete for the same resources, but they share predators like arctic foxes. When lemming populations crash, foxes switch to hunting hares more heavily, linking the two prey populations indirectly.

Predator-prey adaptations

Morphological and physiological adaptations

Predators evolve traits that make them more effective hunters:

Sensory capabilities like echolocation in bats, which allows hunting in complete darkness
Speed and agility, such as the cheetah's flexible spine enabling bursts up to 110 km/h
Camouflage for ambush hunting, like the cuttlefish's ability to change color and texture in milliseconds
Specialized structures like venomous fangs in snakes or the powerful talons of raptors

Prey evolve a corresponding toolkit of defenses:

Cryptic coloration (camouflage), such as leaf-mimicking katydids that are nearly invisible on foliage
Aposematism, where bright warning colors advertise toxicity. Poison dart frogs use vivid reds, blues, and yellows to signal danger.
Chemical defenses like the tetrodotoxin in rough-skinned newt skin, which is lethal to most predators
Physical defenses including porcupine quills, turtle shells, and armadillo armor

These adaptations fuel a coevolutionary arms race. Mollusks evolve thicker shells, and in response, predators like mantis shrimp evolve appendages that can strike with the force of a bullet to crack them open.

Behavioral strategies

Predator behavioral adaptations include:

Optimal foraging, where predators maximize energy gain relative to effort. Bees, for example, use central place foraging, choosing flowers that give the best nectar return relative to travel distance from the hive.
Cooperative hunting in wolf packs and lion prides, which allows taking down prey much larger than any individual could handle.
Search image formation, where predators learn to spot common prey types more efficiently, temporarily overlooking rarer forms.

Prey behavioral adaptations include:

Vigilance and alarm systems. Meerkats post sentinels that scan for predators while the group forages, and many bird species have distinct alarm calls for aerial versus ground predators.
Group living reduces individual predation risk. Fish schools and bird flocks confuse predators through coordinated movement (the "confusion effect").
Mimicry comes in two main forms:
- Batesian mimicry: A harmless species evolves to resemble a dangerous one. The viceroy butterfly mimics the toxic monarch's wing pattern, gaining protection without producing toxins itself.
- Müllerian mimicry: Multiple genuinely toxic or unpalatable species converge on similar warning signals, reinforcing predator avoidance. Various Heliconius butterfly species share similar bold wing patterns.

Frequency-dependent selection helps maintain variation in prey appearance. Predators develop search images for the most common prey morph, which gives rare morphs a survival advantage. Over time, this keeps multiple forms in the population. Grove snails, for instance, maintain a range of shell colors and banding patterns partly through this mechanism.

Herbivory's effects on plants

Plant defenses and responses

Plants can't run from herbivores, so they've evolved an impressive array of defenses.

Physical defenses create barriers to feeding:

Thorns, spines, and prickles (roses, acacias, cacti)
Trichomes, the fine hairs on leaf surfaces that make it difficult for insects to feed or lay eggs
Tough, leathery leaf tissue (like holly) that resists chewing

Chemical defenses make plant tissue toxic or unpalatable:

Secondary metabolites like alkaloids, tannins, and terpenoids don't contribute to basic plant metabolism but deter herbivores. Nicotine in tobacco plants, for example, is a potent neurotoxin to insects.

Induced defenses are activated only after herbivore damage occurs, saving the plant the cost of maintaining defenses constantly. When a tomato plant is attacked by caterpillars, it releases signaling molecules (like jasmonic acid) that trigger increased production of defensive compounds throughout the plant, even in parts that haven't been damaged yet.

Some plants show compensatory growth after being grazed. In certain cases, grazed grasses actually outperform ungrazed ones because grazing removes older, less photosynthetically efficient tissue and stimulates new growth. This phenomenon is called overcompensation.

Ecosystem-level impacts

Herbivory influences ecosystems well beyond individual plants.

Nutrient cycling is affected because herbivores process plant material and return nutrients to the soil through waste. African savanna "grazing lawns," where large herbivores repeatedly crop grasses short, show faster nutrient turnover than ungrazed areas.

Plant community composition shifts when herbivores feed selectively. Deer that preferentially browse certain understory species can transform forest composition over time, favoring plants they avoid and eliminating palatable species.

Some herbivores function as ecosystem engineers, physically modifying habitats. Elephants topple trees, maintaining open grassland in savannas. Beavers build dams that create entirely new wetland ecosystems. These habitat modifications affect dozens of other species.

The impact of herbivory depends on both its intensity and the environmental context. Plants in nutrient-rich environments can often tolerate more herbivory because they have the resources to regrow lost tissue. In nutrient-poor environments, the same level of herbivory may cause lasting damage.

🦉Intro to Ecology Unit 7 Review