Section 2 – Species Interactions
The Niche and Competition
 An organism’s niche is affected by both its tolerance
and competitive interactions
 Habitat: general place it lives
 Niche: an organism’s habitat, resource use, and
fundamental role in a community
 Habitat
 Food Source
 How/When it Reproduces
 Interactions with other Organisms
Tolerance
 Tolerance: the ability to survive and reproduce under
changing environmental conditions
 Specialists: very restricted tolerance ranges
 Ex. Panda Bears
 Generalists: wide tolerance ranges
 Ex. Rats
 BOTH may not be able to adapt when conditions
change.
Competition
 Multiple organisms seek the same limited resource.
 Food, Light, Water, Space
 Direct Competition: physically, for territory
 Indirect Competition: flowers, for pollinators
 Intraspecific Competition: competitive interactions
among members of the same species
 Interspecific Competition: competitive interactions
among members of two or more different species
Competitive Exclusion
 Competitive Exclusion: if one species is a very effective
competitor, it may exclude another species from
resource use entirely
 Mussel and Clam Populations in Lake St. Clair
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Lake St. Clair connects Lake Erie and Huron
20 native mussel species
By 1997, they were all gone
When Zebra Mussels attack, shellfish cannot open their
shells – they eventually suffocate or starve
Fundamental and Realized
Niche
 Usually, neither competing species fully excludes the
other.
 Instead, competing species tend to adjust to each
other, minimizing competition.
 Changing Behavior
 Using only a portion of the resources they are capable of
using
 Fundamental Niche: full niche of a species
 Realized Niche: a niche restricted by competition
Resource Partitioning
 Resource Partitioning: a process that allows different
species to share common resources
 If 2 bird species eat the same type of seeds
 1 specializes in eating larger seeds
 1 specializes in eating smaller seeds
 OR
 1 is active in the morning
 1 is active in the evening
Character Displacement
 Character Displacement: resource partitioning can
lead to the evolution of physical characteristics among
the competing species that reflect their specialized role
in the environment
 Darwin’s Finches – Galapagos Islands
 Birds specialize in eating small seeds = small bills
 Birds specialize in eating large seeds = large bills
Predation, Parasitism, and
Herbivory
 “0” = relationship in which there is no effect, or the
effect is neutral
 “-/-” = negative effect on both organisms
 Competition
 “+/-” = beneficial for one participant, but harmful to the
other
 Predation
 Parasitism
 Herbivory
Predation and Population Cycles
 Predation: the process by which individuals of one
species (the predators) hunt, capture, and feed on
individuals of another species (the prey)
 Population Cycles
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INCREASE in Prey Population
INCREASE in Predator Population
DECREASE in Prey Population
DECREASE in Predator Population
and Repeat
Predation and Evolution
 Natural Selection – evolution of better predators
 Predators skilled at capturing prey – more successful
 Prey – evolution of Defense Mechanisms
 Strong selective pressure – IMMEDIATE DEATH
Coevolution and Evolutionary
“Arms Race”
 Coevolution: the process by which two species evolve
in response to changes in each other
 Newt develops toxins that kills predators
 In response, predators evolve immunity to the toxins
 In response, newt evolve stronger toxins
 Arms Race: each species develops stronger and
stronger “weapons” in response to the other
 ALL SPECIES ADAPT TO BETTER SURVIVE AND
REPRODUCE IN THEIR ENVIRONMENT.
Parasitism
 Parasitism: a relationship between two organisms
from different species in which one organism (the
parasite) depends on the other (the host) for
nourishment or some other benefit
 Tapeworms
 Ticks
 Symbiosis: a long term and physically close
relationship between two organisms from different
species in which at least one organism benefits
 Commensalism
 Mutualism
 Parasitism
Herbivory
 Herbivory: the act of feeding on a plant
 Insects – most common type of herbivore
 Hervibory usually doesn’t kill the plant
 Affects growth and population
 Natural Selection has given plants defenses:
 Produce Toxic/Distasteful Chemicals
 Thorns, Spines or Irritating Hairs
 In response, herbivores evolve ways to overcome.
Mutualism
 Mutualism: (+/+) a relationship between two organisms
from different species in which both organisms benefit
 Plant Roots + Fungi = Mycorrhizae
 Plant provides energy, fungus assists in nutrient
absorption
 Pollination
 Pollinators enable sexual reproduction in plants
 Pollinators receive nectar as a reward
Commensalism
 Commensalism: (+/0) a relationship between two
organisms from different species in which one benefits
and the other is unaffected
 Example:
 Palo Verde trees in the Sonoran Desert
 Trees create shade and leaf litter that allow the soil to hold
moisture
 Soil becomes cooler with more moisture, making it easier for
young plants to germinate and grow
 Other desert plants grow beneath the “nurse” tree
Section 3
Section 4