Principles of Life
Hillis • Sadava • Heller • Price
Instructor’s Manual
Chapter 44: Ecological and Evolutionary Consequences of Species
Interactions
OVERVIEW
The various types of interactions between species are described in this chapter,
along with their impacts on individual fitness, population dynamics, and evolution.
The first section describes the types of interactions in terms of positive and negative
effects on the participants, including competition, predation, herbivory, and
mutualisms. Descriptions and examples of how interactions affect growth rates,
species distributions, and fitness and evolution form a large part of the chapter.
Mechanisms for species coexistence are also discussed. The last section describes
how invasive species change interactions among native species and can have
detrimental effects.
KEY CONCEPTS/CHAPTER OUTLINE
44.1 Interactions between Species May Be Positive, Negative, or
Neutral
• Interspecific interactions are classified by their effect on per capita growth rates
• Many interactions have both positive and negative aspects
Interactions between species can be beneficial or detrimental to either of the species.
These interactions affect population densities, species distributions, and ultimately
lead to evolutionary changes. They can be categorized as consumer–resource
interactions (predation, herbivory, parasitism), mutualism, commensalism, or
amensalism.
44.2 Interspecific Interactions Affect Population Dynamics and Species
Distributions
• Interspecific interactions can modify per capita growth rates
• Interspecific interactions can lead to extinction
• Interspecific interactions can affect the distributions of species
• Rarity advantage promotes species coexistence
Interspecific interactions affect per capita growth rates and hence the fitness of
individuals. Negative effects of other species would be subtracted from the growth
model, positive effects would be added. Population dynamics can change in the
presence of other species: presence of a competitor always reduces growth rate; if
competitors coexist, they will both have lower equilibrium population densities. In
some cases, competition causes one species to go extinct. Interspecific interactions
can also affect distribution of species. A species at low density may have a rarity
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advantage when a competitor population is at high density. Rarity advantage can
keep prey species from going extinct. Resource partitioning is one mechanism by
which competing species can coexist.
44.3 Interactions Affect Individual Fitness and Can Result in Evolution
• Intraspecific competition can increase carrying capacity
• Interspecific competition can lead to resource partitioning and coexistence
• Consumer–resource interactions can lead to an evolutionary arms race
• Mutualisms can involve exploitation and cheating
Intraspecific competition may result in certain traits being favored by natural
selection, and carrying capacity for that phenotype increases (more resources
available). In interspecific competition, phenotypes that gain the most from a
positive interaction, or suffer least from a negative interaction, will increase in
frequency in the population, and the population will evolve. Resource partitioning
can lead to speciation and coexistence of species. In consumer–resource interactions,
the opposing interests of the consumer and resource species lead to an “evolutionary
arms race” in which prey evolve new defenses (e.g., more speed, better camouflage,
mimicry) and predators evolve new offenses (e.g., more strength, keener senses,
ability to withstand toxins). In mutualisms, both species are acting in their own selfinterest. Benefits of mutualism can be dependent on environmental factors, and can
even involve cheating.
44.4 Introduced Species Alter Interspecific Interactions
• Introduced species can become invasive
• Introduced species alter ecological relationships of native species
Species introduced into a new region may become invasive and have negative
impacts on native species. Invasive species are spread by many mechanisms: some
are moved accidentally, some deliberately. Invasive species can alter the
relationships among native species. They may be better competitors for food, space,
pollinators, or other resources. Some invasives can cause extinctions (e.g., chestnut
blight). Species that are introduced to control pest species may have unintended
effects (e.g., by switching to another resource species).
LECTURE OUTLINE
Chapter 44 Opening Question
How could the intricate ecological relationship between leaf-cutter ants and fungi
have evolved?
Concept 44.1 Interactions between Species May Be Positive, Negative,
or Neutral
Interspecific interactions (between individuals of different species) affect
population densities, species distributions, and ultimately lead to evolutionary
changes.
The interactions can be beneficial or detrimental to either of the species.
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FIGURE 44.1 Types of Interspecific Interactions (Part 1)
Interspecific competition refers to –/– interactions
Members of two or more species use the same resource.
At any one time there is often one limiting resource in the shortest supply relative
to demand.
FIGURE 44.1 Types of Interspecific Interactions (Part 2)
Consumer–resource interactions—organisms get their nutrition by eating other
living organisms.
+/– interactions—the consumer benefits while the consumed organism loses
Includes predation, herbivory, and parasitism.
FIGURE 44.1 Types of Interspecific Interactions (Part 3)
Mutualism benefits both species: +/+ interaction
Examples:
• Leaf-cutter ants and the fungi they cultivate
• Plants and pollinating or seed-dispersing animals
• Humans and bifidobacteria in our guts
• Plants and mycorrhizal fungi
• Lichens
• Corals and dinoflagellates
(VIDEO 44.1 Animal–animal mutualisms: Rattle ants and the small oak blue
caterpillar)
FIGURE 44.1 Types of Interspecific Interactions (Part 4)
Commensalism—one species benefits while the other is unaffected (+/0
interaction).
• Brown-headed cowbird follows grazing cattle and bison, foraging on insects
flushed from the vegetation.
• Cattle convert plants into dung, which dung beetles can use.
Dung beetles disperse other dung-living organisms such as mites and nematodes,
which attach themselves to the bodies of the beetles.
Amensalism—one species is harmed while the other is unaffected (–/0 interactions).
Tend to be more accidental than other relationships.
Example: a herd of elephants that crush plants and insects while moving through a
forest.
Relationships between species do not always fit perfectly into these categories.
Fish that live with sea anemones escape predation by hiding in the anemone
tentacles.
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Effects of this on the anemones is unclear. Do the fish steal some of their prey? Do
they get nutrients from fish feces? It may depend on the availability of nutrients.
(APPLY THE CONCEPT Interactions between species may be positive, negative, or
neutral)
FIGURE 44.2 Interactions between Species Are Not Always Clear-Cut
Concept 44.2 Interspecific Interactions Affect Population Dynamics and
Species Distributions
Density-dependent population growth reflects intraspecific (within-species)
interactions among individuals in a population.
They are usually detrimental because per capita resource availability decreases as
population density increases.
Interspecific interactions also modify per capita growth rates:
Interspecific competition—effect of the other species would be subtracted in the
growth model.
Consumer–resource interactions—effect of the consumer is subtracted in the
equation for the resource species; the effect of the resource is added in the
equation for the consumer, since the consumer benefits.
Populations show different dynamics in the presence or absence of other species.
This was demonstrated in classic experiments with species of Paramecium.
FIGURE 44.3 Interspecific Competition Affects Population Growth
Conclusions from the Paramecium studies, and from mathematical models:
• Presence of a competitor always reduces population growth rate.
• When two species coexist, they have lower equilibrium population densities than
either would alone.
• In some cases, competition causes one species to go extinct.
Other types of interspecific interactions have similar consequences:
• Per capita growth rate of each species is modified by the presence of the other,
positively or negatively.
• Population densities are increased in positive interactions and decreased in
negative interactions.
• In interactions with negative effects, extinction of one or both species is possible.
Interspecific interactions can affect the distributions of species.
Competitive interactions can restrict the habitats in which species occur.
Two barnacle species compete for space on the rocky shorelines of the North
Atlantic, with no overlap between zones occupied.
A classic experiment removed each species and observed response of the other
species.
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FIGURE 44.4 Interspecific Competition Can Restrict Distributions
Two competitors can coexist when each species suppresses its own per capita
growth rate more than it suppresses the per capita growth rate of its competitor.
Intraspecific competition must be stronger than interspecific competition.
A species has a growth advantage when it is at a low density and its competitor is at
a high density.
This rarity advantage prevents the species from decreasing to zero. Result is
coexistence.
Resource partitioning—different ways of using a resource.
Example: Paramecium caudatum can coexist with P. bursaria.
P. bursaria can feed on bacteria in the low-oxygen sediment layer at the bottom of
culture flasks.
P. bursaria has symbiotic algae that provides it with oxygen from photosynthesis.
FIGURE 44.5 Resource Partitioning Can Result in Intraspecific Competition Being
Greater than Interspecific Competition
Prey species may gain a rarity advantage that prevents them from being driven
extinct by their predators.
• They may be harder to find and predators may switch to other prey species.
• They may invest in more defenses—low density means more resources per capita.
• Other limiting factors may prevent predators from becoming numerous enough to
eat all the prey
Concept 44.3 Interactions Affect Individual Fitness and Can Result in
Evolution
Species interactions can affect individual fitness.
Phenotypes that gain the most from a positive interaction or suffer least from a
negative interaction will increase in frequency in the population, and the
population will evolve.
Intraspecific competition—density-dependent growth models assume all individuals
in a population are equally affected by density.
But individuals vary, and some traits may increase the ability to obtain resources.
Natural selection will favor the trait and its frequency will increase in the population
(directional selection). More resources will be available for this phenotype,
increasing the carrying capacity.
(LINK Concept 15.4 The three patterns of selection—directional, stabilizing, and
disruptive)
Interspecific competition
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Variation in traits can affect sensitivity to interspecific competition.
Resource partitioning as an evolutionary response: Finch species in the Galápagos
Islands have varying beak sizes; beak sizes match sizes of available seeds.
FIGURE 44.6 Resource Paritioning Allows Competitors to Coexist
In one finch species on the Galápagos Islands, small individuals feed more on
nectar, larger individuals feed more on seeds.
On islands where carpenter bees are present and compete for nectar, the finches tend
to be larger and eat more seeds.
The finch resource use has diverged from their bee competitors on islands where
they coexist.
(APPLY THE CONCEPT Interactions affect individual fitness and can result in
evolution)
FIGURE 44.7 Finch Morphology Evolves in Response to Competition with
Carpenter Bees
Consumer–resource interactions
The opposing interests of the consumer and the resource species can lead to an
“evolutionary arms race,”—prey continually evolve better defenses and predators
continually evolve better offenses.
Strategies of resource species:
• Use speed, size, or weapons to thwart predators.
• Hide or use camouflage
• Mimic unpalatable species
• Sessile species have thick armor or are non-nutritive or poisonous.
(VIDEO 44.2 Development of mimicry in a swallowtail species)
FIGURE 44.8 Defense Mechanisms and “Arms Races” (Part 1)
Strategies of consumers:
• Greater speed, size, or strength
• Keen senses
• Armor-piercing or crushing tools
• Means of detoxifying poisons
(VIDEO 44.3 Scent mimicry in a spider)
FIGURE 44.8 Defense Mechanisms and “Arms Races” (Part 2)
Plants produce a variety of toxic chemicals against herbivores and pathogens.
Some of these chemicals we use as spices, etc.: black pepper, chili peppers, caffeine.
Herbivores evolve ways to deal with the chemicals.
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Heliconius butterflies store or detoxify the cyanide compounds of passionflower and
use them as defense against their own predators.
Some passionflower species have leaf structures that resemble butterfly eggs.
Females will not lay eggs on a plant that already has eggs.
FIGURE 44.9 Using Mimicry to Avoid Being Eaten
Mutualisms
Species benefit other species because acting in their own self-interest happens to
benefit others.
Most pollinators visit flowers to get food and happen to pollinate the flowers in the
process; flowers provide food (usually as little as possible) to lure the animals.
(LINK Concept 21.5 The coevolution of plants and pollinators)
(VIDEO 44.4 Plant–animal mutualisms: Pollen transfer by honeybees)
(VIDEO 44.5 Plant–animal mutualisms: Pollination of a night-blooming cactus by a
bat)
All mutualisms involve the exchange of resources and services.
The fitness effect of the mutualism can vary depending on environmental conditions.
Example: Mycorrhizae benefit plants in nutrient-poor soils, but can be a liability in
nutrient-rich soils, where the cost of feeding the mycorrhizae outweighs their value
in nutrient uptake.
Cheating in mutualisms:
Some flowers mimic the form and smell of female insects and are pollinated when
males attempt to copulate with them.
Some bees bite holes in the base of flowers and eat the nectar without pollinating the
flower.
(ANIMATED TUTORIAL 44.1 An Ant–Plant Mutualism)
Concept 44.4 Introduced Species Alter Interspecific Interactions
Species introduced into a region where their natural enemies are absent may reach
very high population densities.
They may become invasive—reproduce rapidly and spread widely, and have
negative impacts on native species.
(VIDEO 44.6 The exotic invasive plant kudzu, Pueraria lobata)
(VIDEO 44.7 Biocontrol: The use of flea beetles to control the exotic invasive plant
leafy spurge, Euphorbia esula)
Invasive species are spread in many ways:
Marine species have spread by being carried in ballast water on ships.
Terrestrial species are carried unknowingly by humans as we have moved around
the globe.
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Deliberate introductions (e.g., Europeans brought many plants and animals to their
new homes). Species are still being transported—ornamental plants, exotic pets,
etc.
Invasive species can harm native species in various ways:
Invasive flowering plants can alter relationships between native plants and their
pollinators.
Purple loosestrife was introduced to North America in the early 1800s and now
dominates wetlands. It competes with the native Lythrum alatum, which receives
fewer visits from pollinators and produces fewer seeds when purple loosestrife is
present.
FIGURE 44.10 An Invasive Species
Some invasive species cause extinction of native species.
Example: A sac fungus blight caused extinction of American chestnut trees.
Chestnuts have been replaced by oaks.
Chestnut trees produced consistent nut crops each year, but acorn production
varies greatly, contributing to yearly fluctuations in rodents, ticks, and Lyme
disease in the northeastern U.S.
Species introduced to control specific pests can alter interactions of native species.
A weevil was introduced to North America to control invasive musk thistle.
When abundance of the thistle declined, the weevil began eating seeds of native
thistle species.
The weevil has become a competitor of native insects that eat thistles.
(VIDEO 44.8 Unintended targets of biocontrol: The effect of the Eurasian weevil on
non-target, native thistle species)
Answer to Opening Question
In the mutualism between leaf-cutter ants and the fungus they cultivate, both species
gain nutrition from the interaction.
Ants also disperse the fungus and protect it from pathogens.
It may have started when ants began eating the fungi growing on refuse in their
nests. Ants that provided better growing conditions had more fungus to eat and
thus higher fitness.
Fungi that provided ants with more nutrients were more likely to be propagated by
ants.
The ants expanded their food base by feeding leaves to the fungi (ants can’t digest
the leaves).
The fungi then had access to food they would not be able to use if ants did not chop
it up for them.
Leaf-cutter ants and their fungi have been very successful:
They are major herbivores in the Neotropics, and have expanded into dry
environments that are normally hostile to fungi.
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FIGURE 44.11 A Fungal Garden
KEY TERMS
amensalism
commensalism
consumer–resource interactions
herbivory
interspecific competition
interspecific interactions
intraspecific competition
invasive
limiting resource
mutualism
parasitism
predation
predator
prey
resource partitioning
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