Download Interspecific Interactions

Interspecific Interactions
Reading: Freeman, Chapter 49
The Niche
 The niche is one of the most important concepts in
ecology. Paradoxically, it is also one of the hardest
to define (Ecology is still a young science).
 In essence, an organism’s niche is how it makes a
living: the environmental conditions it tolerates, the
important resources it needs to survive, and its ways
of obtaining those resources.
 In obtaining energy, nutrients, etc.. a populations of
one species frequently interact with populations of
other species.
Types of Interspecific Interactions
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Effect on
Species 1
Neutralism
Competition
Commensalism
Amensalism
Mutualism
Predation,
0
+
+
-
 Parasitism, Herbivory
Effect on
Species 2
0
0
0
+
+
Neutralism
 Neutralism the most common type of
interspecific interaction. Neither population
affects the other. Any interactions that do
occur are indirect or incidental.
 Example: the tarantulas living in a desert
and the cacti living in a desert
Competition
 Competition occurs when organisms in the same
community seek the same limiting resource. This
resource may be prey, water, light, nutrients, nest
sites, etc.
 Competition among members of the same species is
intraspecific.
 Competition among individuals of different species
is interspecific.
 Individuals experience both types of competition, but
the relative importance of the two types of
competition varies from population to population and
species to species
Types of Competition
 Exploitation competition occurs when
individuals use the same limiting resource
or resources, thus depleting the amount
available to others.
 Interference competition occurs when
individuals interfere with the foraging,
survival, or reproduction of others, or
directly prevent their physical establishment
in a portion of a habitat.
Example of Interference Competition
 The confused flour beetle, Triboleum confusum,
and the red flour beetle, Triboleum castaneum
cannibalize the eggs of their own species as well as
the other, thus interfering with the survival of
potential competitors.
 In mixed species cultures, one species always
excludes the other. Which species prevails
depends upon environmental conditions, chance,
and the relative numbers of each species at the start
of the experiment.
Outcomes of Competition
 Exploitation competition may cause the exclusion of
one species. For this to occur, one organism must
require less of the limiting resource to survive. The
dominant species must also reduce the quantity of the
resource below some critical level where the other
species is unable to replace its numbers by
reproduction.
 Exploitation does not always cause the exclusion of
one species. They may coexist, with a decrease in their
potential for growth. For this to occur, they must
partition the resource.
 Interference competition generally results in the
exclusion of one of the two competitors.
The Competitive Exclusion Principle
 Early in the twentieth century, two mathematical
biologists, A.J. Lotka and V. Volterra developed
a model of population growth to predict the
outcome of competition.
 Their models suggest that two species cannot
compete for the same limiting resource for long.
Even a minute reproductive advantage leads to
the replacement of one species by the other.
 This is called the competitive exclusion
principal.
Evidence for Competitive Exclusion.
 A famous experiment by the Russian
ecologist, G.F. Gausse demonstrated that
Paramecium aurellia outcompetes and
displaces Paramecium caudatum in mixed
laboratory cultures, apparently confirming the
principle.
 (Interestingly, this is not always the case. Later
studies suggest that the particular strains involved
affect the outcome of this interaction).
Other experiments...
 Subsequent laboratory studies on other
organisms, have generally resulted in
competitive exclusion, provided that the
environment was simple enough.
 Example: Thomas Park showed that, via
interference competition, the confused flour
beetle and the red flower beetle would not
coexist. One species always excluded the
other.
Resource Partitioning
 Species that share the same habitat
and have similar needs frequently use
resources in somewhat different ways
- so that they do not come into direct
competition for at least part of the
limiting resource. This is called
resource partitioning.
 Resource partitioning obviates
competitive exclusion, allowing the
coexistence of several species using
the same limiting resource.
 Resource partitioning could be an
evolutionary response to interspecific
competition, or it could simply be that
competitive exclusion eliminates all
situations where resource partitioning
does not occur.
Example of Resource Partitioning
 One of the best known cases of resource partitioning
occurs among Caribbean anoles.
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As many as five different species of anoles may exist in
the same forest, but each stays restricted to a particular
space: some occupy tree canopies, some occupy trunks,
some forage close to the ground.
When the brown anole was introduced to Florida from
Cuba, it excluded the green anole from the trunks of trees
and areas near the ground: the green anole is now
restricted to the canopies of trees:the resource (space,
insects) has been partitioned among the two species
– (for now at least, this interaction may not be stable in the long
run because the species eat each other’s young).
Character Displacement
 Sympatric populations of similar species
frequently have differences in body
structure relative to allopatric populations
of the same species.
 This tendency is called character
displacement.
 Character displacement is thought to be
an evolutionary response to interspecific
competition.
Example of Character Displacement
 The best known case of character displacement
occurs between the finches, Geospiza fuliginosa and
Geospiza fortis, on the Galapagos islands.
 When the two species occur together, G. fuliginosa
has a much narrower beak that G fortis. Sympatric
populations of G fuliginosa eats smaller seeds than
G fortis: they partition the resource.
 When found on separate islands, both species have
beaks of intermediate size, and exploit a wider
variety of seeds.
 These inter-population differences might have
evolved in response to interspecific competition.
Competition and the Niche
 An ecological niche can be thought of in terms
of competition.
 The fundamental niche is the set of
resources and habitats an organism could
theoretically use under ideal conditions.
 The realized niche is the set of resources and
habitats an organism actually used: it is
generally much more restricted due to
interspecific competition (or predation.)
Two organisms cannot occupy
exactly the same niche.
This is sometimes called Gausse’s
rule(although Gausse never put it exactly that
way).
-Experiments by Gausse (Paramecium), Peter Frank
(Daphnia), and Thomas Park (Triboleum) have confirmed
it for simple laboratory scenarios.
-This creates a bit of a paradox, because so many species
exist in nature using the same resources.
-The more complex environments found in nature may
enable more resource partitioning.
Amensalism
 Amensalism is when one species suffers
and the other interacting species
experiences no effect.
 Example: Redwood trees falling into the
ocean become floating battering-rams
during storms, killing large numbers of
mussels and other inter-tidal organisms.
 Allelopathy involves the production and
release of chemical substances by one
species that inhibit the growth of another.
These secondary substances are chemicals
produced by plants that seen to have no
direct use in metabolism.
 This same interaction can be seen as both
amensalism, and extremely one-sided
interference competition-in fact it is both.
Example: Allelopathy in the California Chaparral
 Black Walnut (Juglans nigra) trees excrete an antibiotic
called juglone. Juglone is known to inhibit the growth of
trees, shrubs, grasses, and herbs found growing near black
walnut trees.
 Certain species of shrubs, notably Salvia leucophylla
(mint) and Artemisia californica (sagebrush) are known to
produce allelopathic substances that accumulate in the soil
during the dry season. These substances inhibit the
germination and growth of grasses and herbs in an area up
to 1 to 2 meters from the secreting plants.
Commensalism
 Commensalism is an interspecific interaction where one species
benefits and the other is unaffected.
 Commensalisms are ubiquitous in nature: birds nesting in trees are
commensal.
 Commensal organisms frequently live in the nests, or on the bodies,
of the other species.
 Examples of Commensalism:
 Ant colonies harbor rove beetles as commensals. These beetles
mimic the ants behavior, and pass as ants. They eat detritus and
dead ants.
 Anemonefish live within the tentacles of anemones. They have
specialized mucus membranes that render them immune to the
anemone’s stings. They gain protection by living in this way.
Mutualism
 Mutualism in an interspecific interaction
between two species that benefits both
members.
 Populations of each species grow, survive
and/or reproduce at a higher rate in the presence
of the other species.
 Mutualisms are widespread in nature, and occur
among many different types of organisms.
Examples of Mutualism
 Most rooting plants have mutualistic associations
with fungal mychorrhizae. Mychorrhizae
increase the capability of plant roots to absorb
nutrients. In return, the host provides support and
a supply of carbohydrates.
 Many corals have endosymbiotic organisms
called zooxanthellae (usually a dinoflagellate).
These mutualists provide the corals with
carbohydrates via photosynthesis. In return, they
receive a relatively protected habitat from the
body of the coral.
Mutualistic Symbiosis
 Mutualistic Symbiosis is a type of mutualism in
which individuals interact physically, or even live
within the body of the other mutualist. Frequently,
the relationship is essential for the survival of at
least one member.
 Example: Lichens are a fungal-algal symbiosis (that
frequently includes a third member, a cyanobacterium.) The
mass of fungal hyphae provides a protected habitat for the
algae, and takes up water and nutrients for the algae. In
return, the algae (and cynaobacteria) provide carbohydrates
as a source of energy for the fungus.
Facultative vs. Obligate Mutualisms
 Facultative Mutualisms are not essential
for the survival of either species.
Individuals of each species engage in
mutualism when the other species is
present.
 Obligate mutualisms are essential for the
survival of one or both species.
Other Examples of Mutualisms
 Flowering plants and pollinators. (both
facultative and obligate)
 Parasitoid wasps and polydna viruses. (obligate)
 Ants and aphids. (facultative)
 Termites and endosymbiotic protozoa. (obligate)
 Humans and domestic animals. (mostly
facultative, some obligate)
Short Discussion
 Describe the interaction of human beings
with
 A: Domestic Livestock.
 B: Atlantic Swordfish
 C: The roof rat, Rattus rattus.
 D: The influenza A virus.
 E: The American Cockroach, Periplaneta
americanum.
 F: The migratory locus, Locusta
migratoria.
Predation, Parasitism, Herbivory
 Predators, parasites, parasitoids, and herbivores
obtain food at the expense of their hosts or prey.
 Predators tend to be larger than their
prey, and consume many prey during
their lifetimes.
 Parasites and pathogens are smaller
than their host. Parasites may have one
or many hosts during their lifetime.
Pathogens are parasitic microbes-many
generations may live within the same
host. Parasites consume their host
either from the inside (endoparasites)
or from the outside (ectoparasites).
 Parasitoids hunt their prey like predators,
but lay their eggs within the body of a host,
where they develop like parasites.
 Herbibores are animals that eat plants.
This interaction may resemble predation, or
parasitism.
Predator-Prey and Parasite-Host
Coevolution
 The relationships between predator and
prey, and parasites and hosts, have
coevolved over long periods of time.
 About 50 years ago, an evolutionary biologist named
J.B.S. Haldane suggested that the interaction between
parasite and host (or predator and prey) should resemble
an evolutionary arms race:
 First a parasite (or predator) evolves a trait that allows it to
attack its host (or prey).
 Next, natural selection favors host individuals that are able
to defend themselves against the new trait.
 As the frequency of resistant host individuals increases,
there is natural selection for parasites with novel traits to
subvert the host defenses.
 This process continues as long as both species survive.
 Recent data on Plasmodium, the cause of malaria, support
this model.
Example of Parasite-Host Coevolution
 The common milkweed, Asclepias syriaca has
leaves that contain cardiac glycosides: they are very
poisonous to most herbivores. This renders them
virtually immune to herbivory by most species.
 Monarch butterfly larvae have evolved the ability to
tolerate these toxins, and sequester them within their
bodies. They are important specialist hervivores of
milkweeds.
 These sequestered compounds serve the additional
purpose of making monarch larvae virtually inedible
to vertebrate predators.
Predator-Prey Population Dynamics
 Predation may be a density-dependent mortality factor to
the host population-and prey may represent a limiting
resource to predators.
 The degree of prey mortality is a function of the density of
the predator population.
 The density of the prey population, in turn, affects the
birth and death rates of the predator population.
 i.e, when prey become particularly common, predators
increase in numbers until prey die back due to increased
predation, this, in turn, inhibits the growth of prey.
 Typically, there is a time lag effect.
 There is often a dynamic balance
between predators and prey that is
necessary for the stability of both
populations.
 Feedback mechanisms may control the
densities of both species.
Example of Regulation of Host Population by a
Herbivore
 In the 19th century, prickly pear cactus, Opuntia sp. was
introduced into Australia from South America. Because no
Australian predator species existed to control the
population size of this cactus, it quickly expanded
throughout millions of acres of grazing land.
 The presence of the prickly pear cactus excluded cattle and
sheep from grazing vegetation and caused a substantial
economic hardship to farmers.
 A method of control of the prickly pear cactus was
initiated with the introduction of Cactoblastis cactorum, a
cactus eating moth from Argentina, in 1925. By 1930,
densities of the prickly pear cactus were significantly
reduced.
 Sometimes predator species can drive their prey to
localized extinction.
 If there are no alternate prey, the predator then goes
extinct.
 If the environment is coarse grained, this makes the
habitat available for recolonization by the prey species.
 Example: The parasitic wasp Dieratiella rapae is a very
efficient parasitoid. One female can oviposit into several
hundred aphids during its lifetime. Frequently, aphids are
driven locally extinct and the adults must search for new
patches when they emerge. Once the aphid and the host
are gone, the host plants may become re-infested with
aphids.
 In other cases, there are alternate prey
to support the predator and the prey is
permanently excluded.
 Example: Freshwater fish such as
bluegills and yellow perch frequently
exclude small invertebrates such as
Daphnia pulex from ponds. The fish
then switch to other prey such as
insects larvae.
The time-lag effect may lead to predator-prey
oscillations.
 Most predators do not respond instantaneously to the
availability of prey and adjust their reproduction
accordingly.
 If predator populations grow faster than prey populations,
they may overshoot the number of prey that are able to
support them
 This leads to a rapid decline in the prey, followed by a rapid
decline in the predator.
 Once the predator becomes rare, the prey population may
begin growing again.
 This pattern is called a predator-prey oscillation.
Cycles in the population dynamics of the snowshoe hare
and its predator the Canadian lynx (redrawn from
MacLulich 1937). Note that percent mortality is an elusive
measure, it may, or may not, be useful since mortality
varies with environment and time.
•In the 1920s, A. J. Lotka (1925) and V. Volterra (1926)
devised mathematical models representing host/prey
interaction.
•The Lotka-Volterra curve assumes that prey destruction is
a function not only of natural enemy numbers, but also of
prey density, i.e., related to the chance of encounter.
•This model predicts the predator-prey oscillations
sometimes seen in nature. Populations of prey and
predator were predicted to flucuate in a regular manner
(Volterra termed this "the law of periodic cycle").
•Lotka-Volterra model is an oversimplification of reality.
In nature, many different factors affect the densities of
predators and their prey.