Community is the assemblage of populations of different organisms living in an area and potentially interacting. • E.g. pond community, community of decomposers in a rotten log, forest community.
Interspecific interactions • Interactions between organisms of different species are referred to as interspecific interactions. • These include competition, predation, parasitism, herbivory, mutualism and disease.
Some interactions benefit only one participant (e.g. predation, herbivory) but some benefit both participants (mutualism). • In other cases neither participant benefits (competition). Competition has been extensively studied by ecologists.
Competition • Interspecific competition occurs when different species compete for a resource that is in limited supply. Some resources such as air are usually not limiting so there is no competition for them. • Plants compete for water, nutrients and light. • Mussels and barnacles compete for space to settle on rocks in the intertidal zone. • Owls, foxes and weasels compete for small mammal prey.
Competitive Exclusion • If competition between two species is very strong, one species may outcompete the other and competitively exclude it. • Competitive exclusion first well documented by Gause in 1934.
Competitive Exclusion • Gause studied two species of Paramecium: P. aurelia and P. caudatum. • When cultured in separate containers each species thrived and population leveled off at carrying capacity of test tube.
Competitive Exclusion • However, when P. aurelia and P. caudatum were grown together P. caudatum became extinct. • Gause concluded P. aurelia had a competitive advantage at competing for food and outcompeted P.caudatum.
Competitive Exclusion • Gause developed from this and other experiments his CompetitiveExclusionPrinciple: Two species competing for the same limiting resources cannot coexist in the same place. • If the inferior competitor cannot escape the competition it will be driven to extinction.
Ecological Niches • The sum total uses that a species makes of the biotic and abiotic resources in its environment is referred to as its niche. (Pronounced to rhyme with “itch” in the U.S., but elsewhere to rhyme with “sheesh.”).
Ecological Niches • An organism’s ecological niche is analgous to its “profession,” what it does for a living. • Niche includes many components: the food the organism eats, the places it occupies, the time of day it is active, the temperature range it can tolerate, etc.
Ecological Niches • The concept of niches can be used to restate the competitive exclusion principle: Two species cannot coexist if their niches are identical. • Ecologically similar species, however, can coexist if there are significant differences in their niches.
Ecological Niches • As a result of competition a species fundamental niche, the niche potentially occupied by that species may be different from its realized niche the niche it actually occupies in a particular environment.
Connell’s work on barnacles • Joseph Connell demonstrated the effects of competition on niche occupation in barnacles. • He studied Balanus balanoides and Chthamalus stellatus in the Scottish intertidal zone. • These species have a stratified distribution on intertidal rocks.
Connell’s work on barnacles • Chthamalus occurs higher than Balanus in the intertidal zone. • Connell carried out experiments in which he excluded one or other species from rocks and observed what happened.
Connell’s work on barnacles • When Chthamalus was excluded Balanus did not spread higher up the rocks because it apparently cannot tolerate the stress of drying out for long periods. • Balanus’s realized niche is thus similar to its fundamental niche.
Connell’s work on barnacles • In contrast when Balanus was excluded, Chthamalus spread down the rock. The realized niche of Chthamalus in the presence of Balanus is much smaller than its fundamental niche.
Connell’s work on barnacles • Connell’s observations showed that although Chthamalus settled in the lower zone that Balanus smothered or crushed Chthamalus and that the most mortality occurred during the period of most rapid Balanus growth.
Resource partitioning • Competition between species can result in natural selection causing niches to differentiate to escape the effects of competition. • Anolis lizards in the Caribbean all feed on similar prey and are similar in size, but they make use of different foraging perches.
Character displacement • Natural selection can also result in morphological changes in species that reduce competitive effects. • Many competitors have populations that occur sympatrically (overlap their competitor) and allopatrically (geographically separate).
Character displacement • Often in sympatric populations the two species diverge physically. • For example, among Geopsiza finches on the Galapagos Islands sympatric G. fortis and G. fuliginosa populations differ in beak depth, but measurements of allopatric populations overlap greatly.
Character displacement • Because beak dimensions affect the efficiency with which birds can consume different size seeds, the differentiation of bill depths in sympatry appears to reduce the intensity of competition between the two species.
Other interactions • Besides competition organisms engage in a wide variety of other interactions: • Predation • Herbivory • Parasitism • Disease • Mutualism
Predation • Predation is another major interaction between organisms that shapes communities. • Predators and prey both have extensive suites of adaptations designed to enable them to catch prey or avoid being caught.
Adaptations • Predators possess weaponry: teeth, claws poison, etc. They are also usually quick and stealthy. • Prey generally flee or hide to avoid predation, but may also possess defensive structures (horns, armor, spines) or toxins (pufferfish, plants).
Warning colors • Many organisms have effective chemical defenses and signal them by using warning or aposematic colors. • Examples include Monarch Butterfly, Coral Snake, and Poison arrow frog
Mimicry • Some organisms mimic the warning coloration of toxic organisms to gain protection. • E.g. Viceroy Butterfly mimics pattern of Monarch Butterfly • The mimicry of toxic organisms by non-toxic ones is called Batesian mimicry.
Wasp, Hornet moth, Wasp beetle, Hoverfly
Mullerian mimicry In Mullerian mimicry several toxic or dangerous species all display the same or similar warning colors. Result of convergent evolution.
Mullerian mimics on left of red line Batesian on right of line
Herbivory • Plants are subject to grazing by many organisms and defend themselves with mechanical defenses (thorns, silica, hard shells) and by producing toxins (e.g. capascin [substance that makes chilies hot], strychnine, nicotine and tannins).
Parasitism • In parasitism the parasite derives its nourishment from its host. • Parasites may be internal (endoparasites e.g. tapeworm, fluke) or external (ectoparasites e.g. tick, flea). • In addition parasitoids lay eggs in or on prey and when the larvae hatch they consume the prey.
Parasitism • Many parasites have complex life cycles that include several host species (e.g. for malaria humans and mosquito are hosts). • Parasites frequently modify behavior of host species so they are more vulnerable to predation by the next host species (e.g. acanthocephalan worms enter cockroach’s brain and cause it to wander about in the light where it can be caught and eaten by a rat the next host species).
Disease • Just like parasites pathogens which cause disease are harmful to their hosts. Pathogens include bacteria, viruses, fungi and protists (single-celled organisms). Generally microscopic.
Disease Pathogens that gain access to populations that have not been previously exposed to the disease can have devastating effects. • E.g. smallpox introduced to New World, Dutch Elm Disease.
Mutualism • Mutualistic interactions are interactions that benefit both species involved. • Acacias and Pseudomyrmex ants. Tree produces hollow spines to house ants and food in form of sugar and protein –rich nodules. Ants deter herbivores, remove fungal spores and cut back competing vegetation.
Mutualism • Other mutualistic interactions include nitrogen fixing bacteria and legume roots, bacteria that live in the guts of grazing mammals, and mycorrhizae (association between fungi and plant roots).
Trophic structure • Community structure and dynamics strongly influenced by feeding relationships between organisms: the Trophic Structure. • Transfer of energy up through trophic levels called the food chain.
Energy travels from the primary producers (plants and other photosynthesizers) through herbivores and various carnivores.