Ecology - Mod 11 (Ch 12): Predation and Herbivory

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

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 outright kill it

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

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

when predators are smaller than prey, can compensate using packs, schools, persistence

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 (synchronized mass fruit/seed production at irregular intervals)

only woodpecker to build nests in live pines

sticky resin of live pines traps insects for woodpeckers to eat

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)

compensation to reduce effects (compensation = increased growth upon removal of tissues)

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

structural defenses (e.g. armor plates on armadillo, armadillo girdled lizard, spines on catfish, hedgehog, porcupine)

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

behavioral defenses (e.g. running, early detection, seeking refuge, sacrificing body parts to either escape grip or use as a distraction, appearing larger to intimidate, expelling blood/urine/vomit/feces, feigning death, crypsis, 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

noxious species evolve to resemble each other

e.g. monarch, viceroy, and queen butterfly

e.g. British bumblebees

harmless species resemble noxious species

e.g. milk snake vs coral snake

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

a harmless species acts like a noxious species

e.g. speckled rattlesnake, spotted leafnose snake, black racer (shakes tail in leaves)

presence of predators (fish or dragonfly larva) affecting prey (tadpole)

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

see figure 12.10 in text

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)

newts have toxins in skin, but some snakes are resistant

resistant snakes may be immobilized for hours

toxicity and resistance vary between populations

populations with snakes more resistant to toxin move slower

a series of reciprocal evolutionary adaptations in 2 species

e.g. Heliconius and passion flowers

Heliconius lays eggs on passionflowers, caterpillars feed on sleeves

flower produces a toxin, 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

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

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

may be caused by predator-prey interactions

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

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

predators eat prey, reduce prey numbers

predators go hungry, predator numbers drop

remaining prey survive better, prey numbers build

predator populations build as prey increases

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

modification of exponential growth formula

without predation, prey population will grow exponentially

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)

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

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 (r/a = # of predators causing the prey population to stay constant)

predators: dP/dt = faNP - mP = 0 when N = m/fa (m/fa = # of prey causing the predator population to stay constant)

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

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

i.e. predators never get full