Transcript File

44
Ecological and Evolutionary
Consequences of Species
Interactions
Chapter 44 Ecological and Evolutionary Consequences
of Species Interactions
Key Concepts
• 44.1 Interactions between Species May Be
Positive, Negative, or Neutral
• 44.2 Interspecific Interactions Affect
Population Dynamics and Species
Distributions
• 44.3 Interactions Affect Individual Fitness and
Can Result in Evolution
• 44.4 Introduced Species Alter Interspecific
Interactions
The Number of Species of Life on Earth
No one knows the exact number
About 1.4 million species have been identified
and named
Insects and plants make up moth of these species
Number will increase
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.
Figure 44.1 Types of Interspecific Interactions (Part 1)
Concept 44.1 Interactions between Species May Be
Positive, Negative, or Neutral
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)
Concept 44.1 Interactions between Species May Be
Positive, Negative, or Neutral
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)
Concept 44.1 Interactions between Species May Be
Positive, Negative, or Neutral
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
Figure 44.1 Types of Interspecific Interactions (Part 4)
Concept 44.1 Interactions between Species May Be
Positive, Negative, or Neutral
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.
Concept 44.1 Interactions between Species May Be
Positive, Negative, or Neutral
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.
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.
optimal foraging
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
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 (Part 1)
Figure 44.3 Interspecific Competition Affects Population Growth (Part 2)
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
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.
The Competitive Exclusion Principles
2 species that have exactly the same
requirements cannot coexist in exactly the
same habitat
Ex) introduction of the gray squirrel into Great
Britain
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
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.
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
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.
Figure 44.4 Interspecific Competition Can Restrict Distributions
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species 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.
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
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.
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
Resource partitioning—different ways of using
a resource.
Example: Paramecium caudatum can coexist
with P. bursaria.
P. bursaria can feed on bacteria in the lowoxygen 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
Concept 44.2 Interspecific Interactions Affect Population
Dynamics and Species Distributions
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.
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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.
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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.
Figure 44.7 Finch Morphology Evolves in Response to Competition with Carpenter Bees (Part 1)
Figure 44.7 Finch Morphology Evolves in Response to Competition with Carpenter Bees (Part 2)
Figure 44.7 Finch Morphology Evolves in Response to Competition with Carpenter Bees (Part 3)
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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.
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Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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 nonnutritive or poisonous.
Figure 44.8 Defense Mechanisms and “Arms Races” (Part 1)
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
Strategies of consumers:
• Greater speed, size, or strength
• Keen senses
• Armor-piercing or crushing tools
• Means of detoxifying poisons
Figure 44.8 Defense Mechanisms and “Arms Races” (Part 2)
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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.
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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.
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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 nutrientpoor soils, but can be a liability in nutrient-rich
soils, where the cost of feeding the
mycorrhizae outweighs their value in nutrient
uptake.
Concept 44.3 Interactions Affect Individual Fitness
and Can Result in Evolution
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.
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.
Concept 44.4 Introduced Species Alter Interspecific Interactions
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.
Deliberate introductions (e.g., Europeans
brought many plants and animals to their new
homes). Species are still being transported—
ornamental plants, exotic pets, etc.
Concept 44.4 Introduced Species Alter Interspecific Interactions
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
Concept 44.4 Introduced Species Alter Interspecific Interactions
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.
Concept 44.4 Introduced Species Alter Interspecific Interactions
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.
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.
Answer to Opening Question
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.
Figure 44.11 A Fungal Garden
Answer to Opening Question
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.