Interspecific Interactions

Reading: Freeman, Chapter 49

The NicheThe 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 Effect on Effect on Species 1 Species 2

Neutralism 0 0Competition - -Commensalism + 0Amensalism - 0Mutualism + +Predation, - +Parasitism, Herbivory

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

CompetitionCompetition 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 CompetitionExploitation 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 CompetitionExploitation 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 PrincipleEarly 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. 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 DisplacementSympatric 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 DisplacementThe 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 ChaparralBlack 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.

MutualismMutualism 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 MutualismMost 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 SymbiosisMutualistic 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 DiscussionDescribe the interaction of human beings withA: Domestic Livestock.B: Atlantic SwordfishC: 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 CoevolutionThe 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 DynamicsPredation 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.