Species interactions

Species interactions

A predator is an organism that feeds directly upon another living organism, whether or not it kills the prey in doing so

Prey most successfully on slowest, weakest, least fit members of target population

Reduce competition and population size
Predation exerts a selective force

Coevolution – “Arms Race”

Natural selection promotes traits that help prey escape or deter predation

It also promotes traits that make predators more successful at capturing or eating prey

Types of predation


Predator-prey in the microscopic world

When the prey (Paramecium) are used up in the test tube, the predator (Didinium) also dies

Population dynamics

Herbivores can have dramatic effects on plant populations.

Major basis of bio-control

Prickly pear introduced into Australia in 1839

By 1925 was in 240,000 km2

Released moth from South America in 1925

Prey adaptation to avoid predation

Prey and predators are not evolutionarily passive

There exist a large suite of adaptation of prey that decreases the likelihood of being consumed

Such variety, and commoness of these traits is evidence of the strong selective pressure of predation on evolution

Prey Defenses




Warning coloration






Chemical defenses

Can be used in attack, or following consumption

Cane Toad and native animals

Naïve predators


Crypsis and catalepsis

Coloration and markings to blend in to the surroundings

Development of a frozen posture

Warning coloration

Aposematic coloration


Batesian mimicry - Harmless species evolve characteristics that mimic unpalatable or poisonous species

Mimicry - Batesian


Mimicry of a unpalatable species (MODEL) by a palatable one (MIMIC)

Mimic success is dependent upon mimic:model abundance

High mimic abundance can harm model


Mullerian Mimicry - Two unpalatable species evolve to look alike

Mimicry - Mullerian

Evolutionary convergence of unpalatable (or dangerous) species to look and act the same

Yellow-black striping in wasps and bees

There are often Batesian mimics of Mullerian mimicry complexes!

Many harmless yellow/black flies

Intimidation displays

Many organisms can “increase” size in response to danger

Give appearance of being larger – and thus bigger threat

Can also just startle predator – deters ambush predators


Synchronous production of many progeny by all individuals in a population

Saturates predators

High juvenile mortality, but less than if only a few offspring produced


Physical deterrent to predator

Can be induced

Phenotypic plasticity


Spikeless vs Spiked forms
Chemically induced

Behavior to avoid defense

Predator Responses

Predator adaptations include stealth, camouflage, and ways to avoid chemical repellents


Symbiosis - Intimate living together of members of two or more species

Commensalism - One member benefits while other is neither benefited nor harmed

Mutualism - Both members benefit

Parasitism - One member benefits at the expense of other



Difficult to distinguish

Can cross over into mutualism or parasitism or have characters of both


Both species benefit

Mutualisms can be obligate or facultative

Some are more intimate relationships than others

Yucca and yucca moth

An obligatory mutualism

Each species of yucca is pollinated by only one species of moth

Moth larvae can grow only in that one species of yucca


Acacia and ants

Ants get nectar and a place to live

Ants provide protection and reduction in competition

Cleaner fish

Seed dispersal


Association between plant and fungus

Can strongly influence growth and survival


Parasites drain nutrients from their hosts and live on or in their bodies

Defined by size and location on host

Ectoparasites vs. endoparasites

Microparasites vs. macroparasites

Natural selection favors parasites that do not kill their host too quickly

Have complex life cycles