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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
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.
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 Competition
Figure 44.1 Types of Interspecific Interactions Commensalism
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.
Figure 44.1 Types of Interspecific Interactions Mutualism
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
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 44.2 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
resources decreases as population density increases.
Interspecific interactions (between members of different
species) also modify per capita growth rates. They can
lead to extinction.
Interspecific competition —effect of the other species
would be subtracted (prey) or added (predator) in the
per capita growth rate, depending on the interaction.
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 Can Lead to Extinction
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. Why?
P. bursaria 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.
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.
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.
A species has a growth advantage when it is at a low
density and its competitor is at a high density. Huh?
• This rarity advantage prevents the species from
decreasing to zero. Result is coexistence.
• Example prey can become harder to find as they
become rare. Hide in best locals, have more
resources per capita.
Figure 44.5 Resource Partitioning Can Result in Intraspecific Competition Being Greater than
Interspecific Competition
When 2 species of birds differ in resource
use, individuals will have a greater affect
on resource availability to the same species
than to the other species
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.
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
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 (Part 1)
Homework Answer the questions that follow
Figure 44.6 Resource Paritioning Allows Competitors to Coexist (Part 2)
Figure 44.6 Resource Paritioning Allows Competitors to Coexist (Part 3)
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 2)
Concept 44.3 Mechanisms for Survival, Fooling, Foiling, Fighting
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)
E. Balteatus is a
harmless hoverfly. It
gains protection by
mimicking V. vulgaris a
stinging wasp which
predators avoid
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
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 44.4 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 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.
Concept 44.4 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. Remember the chpt. 43 case study?
Concept 44.4 Introduced Species for Pest Control
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.