Mod 11 (Ch 12) - Predation and Herbivory

updated for spring 2026, included on exam 3

predator

an organism that catches individuals (prey), kills, and consumes them → removes individuals from the prey population

some actively hunt, some sit and wait

herbivore (2 definitions)

definition 1: an organism that primarily eats plant material

definition 2: an organism that eats parts of living plants, including tissues or internal fluids, but does not kill the plant

parasite

an organism that consumes parts of a living prey organism (host) in/on which it lives

definition is broader than the definitions for prey and predators, because some parasites kill their hosts, some don’t

predator/prey size differences

predators usually larger than prey because large prey may be impossible, dangerous, or energy-costly to subdue and consume

predators can compensate using packs (e.g. wolves), schools (e.g. piranhas), and persistence (e.g. komodo dragons), if they’re smaller than prey

carnivorous plants

plants that “hunt”

obtain nutrients (some or most) from trapped insects, other arthropods, protozoans, occasionally small mammals and birds

usually occur in nutrient-poor soils or water (need extra nutrients, so exerting the energy to “hunt” is worth it)

e.g. Venus flytrap, pitcher plant, sundew, butterwort, bladderwort

adaptations to avoid herbivory

adaptations developed by plants to avoid getting eaten

structural defenses (e.g. spines, hairs, tough seed coats)

production of sticky gums and resins (special case: red-cockaded woodpeckers)

production of toxic compounds (e.g. tannins, interfere with digestion of all proteins; other compounds that affect specific metabolic pathways or physiological processes)

masting

examples of structural defenses to avoid herbivory

cactus spines

tough seed coats

spines and hairs, e.g. nettles

examples of production of sticky gums and resins to avoid herbivory

pine trees

tree sap (trees that produce amber)

example of production of toxic compounds to avoid herbivory

tannins (like the trees by Florida’s liver)

example of masting to avoid herbivory

oak trees

special case: red-cockaded woodpecker

red-cockaded woodpecker is the only woodpecker to build nests in live pines

sticky resin of live pines traps insects for woodpeckers to eat

masting

synchronized mass fruit/seed production at irregular intervals

may have evolved to prevent predator satiation (too many to eat them all, not reliable enough to build predator population numbers, not reliable for seed predators to specialize)

other possible reason it evolved: pollination efficiency (optimizes successful pollination and fertilization; especially important for wind-pollinated species)

adaptations to tolerate herbivory

compensation to reduce effects

clipping, in some plants (herbivory or clipping of apical buds → decreased self shading → increased growth → growth of previously dormant axillary buds)

compensation

increased growth upon removal of tissues

an adaptation to tolerate herbivory (doesn’t discourage herbivory, but makes up for it)

adaptations to avoid predation

structural defenses (e.g. armor plates, spines)

chemical defenses (e.g. poison, venom, foul smell)

behavioral defenses (e.g. running, early detection, seeking refuge, sacrificing body parts to either escape or distract, appearing larger to intimidate, expelling blood/urine/vomit/feces, feigning death, crypsis, aposematic coloration)

examples of animals with structural defenses to avoid predation

armadillos and armadillo girdled lizards have armored plates

porcupines, hedgehogs, catfish have spines

examples of animals with chemical defenses to avoid predation

skunks

frogs with parotid glands

fire ants

bees and wasps

venomous snakes

aposematic coloration

certain coloring or marking (usually bright) to warn potential predators that you are bad-tasting, toxic, or dangerous

predators avoid such animals innately or due to learned response after an unpleasant experience

some animals manufacture chemicals, others get them from other plants

types: Mullerian mimicry, Batesian mimicry

types of Batesian mimicry: morphological, behavioral

Mullerian mimicry

noxious species evolve to resemble each other

all are noxious/toxic

e.g. monarch, viceroy, and queen butterfly

e.g. British bumblebees

Batesian mimicry

harmless species resemble noxious species

one species is harmless, pretending to be noxious

types: morphological, behavioral

e.g. milk snake vs coral snake (B is a harmless coral snake, mimicking venomous A and C (milk snake and scarlet king snake)

morphological Batesian mimicry

something looks harmful or dangerous but isn’t

e.g. the tip of a leaf is red and looks like a thorn, but isn’t

behavioral Batesian mimicry

a harmless species acts like a noxious species

e.g. speckled rattlesnake, spotted leafnose snake, black racer (doesn’t have a rattle, but shakes its tail in leaves to sound like a rattle)

startle coloration

a type of Batesian mimicry?

e.g. swallowtail butterfly and its caterpillar, peacock moth, false-eyed frog

example: costs of avoiding predators

time spent avoiding predators can mean less time spent eating or finding mates

e.g. presence of predators (fish or dragonfly larva) affects prey (tadpole)

when tadpoles spent a lot of time avoiding predators, their growth rate decreased

see figure 12.10 in text for a second example with snails and crabs

adaptations to improve predation (hunting)

camouflage (e.g. spider blending in with flower, cheetah, octopus)

speed (e.g. cheetah)

weapons (e.g. fangs, claws)

keen senses (e.g. owl eyes, snake nose)

example: costs to adaptations improving predation

newts have toxins in skin, but some snakes are resistant

resistant snakes may eat newts and survive, but be immobilized for hours

toxicity and resistance vary between populations (more toxic newt populations correlate with more resistant snake populations)

populations with more-venom-resistant snakes move slower

coevolution

a series of reciprocal evolutionary adaptations in 2 species

e.g. Heliconius butterflies and passion flowers

Heliconius lays eggs on passionflowers, caterpillars feed on leaves

flower produces a toxin to discourage herbivory, caterpillars become resistant to toxin

flower produces sugar deposits that attract ants/wasps that prey on butterfly and mimic eggs so it won’t lay more eggs

oscillation

repetitive variation in magnitude around a central point or between two different states

typically occurs in populations that exceed carrying capacity and then fall well below K with relatively regular periodicity

e.g. lynxes, lemmings

may be caused by predator-prey interactions

predator-prey cycles

predator and prey populations often cycle at similar frequencies, with predators slightly lagging behind due to delayed density dependence

e.g. in Canada: small herbivores and their predators have cycles of 4 years, larger herbivores and their predators have cycles of 9-10 years

predators eat prey → prey numbers decrease

predators go hungry → predator numbers decrease

remaining prey survive better because of less getting hunted → prey numbers increase

more abundant prey/food get eaten → predator numbers increase

(delayed density dependence)

lab experiment of predators and prey dynamics

G.F. Gause

Paramecium and Didinium in an enclosure, with and without refuge

without refuge: predator devoured all prey, then went extinct itself

with refuge: some prey escaped predation, then prey population reexpanded after the predator went extinct

could maintain predator-prey cycles by periodically adding more predators (i.e. immigration)

see figures 12.15 and 12.16 in book

Lotka-Volterra model of prey population

dN/dt = rN - aNP

a modification of exponential growth formula

big idea: without predation, prey population will grow exponentially. More prey, more predators, and efficiency of predation limit a prey population’s growth.

variables:

dN/dt = rate of change in prey population

rN = exponential growth in absence of predators

aNP = amount that predation reduces the growth rate of the prey population (i.e. how many prey are removed from the population by predators)

N = # of prey (more prey = easier to catch)

r = prey’s per capita exponential growth rate

P = # of predators (more predators = more prey caught)

a = efficiency of predation (more efficient predators = more prey caught)

Lotka-Volterra model of predator population

dP/dt = faNP - mP

a modification of exponential growth formula

big idea: a predator population’s growth rate depends on how much they eat, how well they can convert food into new babies or increased survival, and how many predators due to non-food-related causes

variables:

dP/dt = rate of change in predator population

f = conversion of prey to predator population growth (efficiency of converting food (consumed prey) to population growth)

aNP = removal of prey by predators

mP = death (mortality) rate of predators regardless of prey availability

P = # of predators

N = # of prey

a = constant expressing efficiency of predation

m = constant related to death of predators

zero growth isoclines

conditions required for a population size to stay constant

(i.e. dN/dt = 0 or dP/dt = 0)

prey: dN/dt = rN - aNP = 0 when P = r/a (there is some number of predators where the prey population will stay constant)

predators: dP/dt = faNP - mP = 0 when N = m/fa (there is some number of prey where the predator population will stay constant)

if isoclines are combined and graphed (one population on each axis), the intersection point is the perfect combination of predator and prey numbers to have equilibrium (both populations are stable)

combined zero growth isoclines

if isoclines are combined and graphed (one population on each axis), the intersection point is the perfect combination of predator and prey numbers to have equilibrium (both populations are stable)

population sizes of both predator and prey are stable

only one combination

change in P → change in N

change in N → change in P

Lotka-Volterra predator-prey model oscillations

changes in P and N are represented by vectors, combined into a single vector within quadrants

the vector in each quadrant corresponds to a region on a population cycle graph

predator population cycles lag slightly behind prey population

further population sizes are from equilibrium → larger amplitude of cycles → less likely to return to stability

changes disturb equilibrium → may result in population cycle

problem with Lotka-Volterra model

the model assumes that there is a lack of satiation of predators

i.e. predators never get full

models that incorporate satiation

type I: Lotka-Volterra (no satiation)

type II: satiation (predators get full, catch fewer prey than expected)

type III: satiation + low predator response at low prey density, because it is difficult to find prey when prey density is low

some models can also incorporate predators’ ability to respond to satiation