Last time, we covered the topic of population ecology &emdash; discussed various characteristics of populations (e.g. distribution, dispersion, age structure, etc.) and how rates of natality and mortality influence population growth.
Populations are the components of the next highest level of biological organization: the community.
Community: assemblage of populations interacting with one another in the same environment.
Biological interactions within and among populations may have important influences upon mortality and natality rates and, in turn, on population growth and size -- these interactions may have negative, neutral or even positive influences on interacting populations.
So, today, we are going to consider some of the interactions and how they effect the community.
Competition -- resources such as food, space, water, etc. in limited supply in the environment and play a major role in determining a population's carrying capacity -- competition may be intraspecific (competition between members of population) or interspecific (competition between members of different species populations &emdash; both have impacts on population growth and size.
Let’s first look at competition among individuals of the same populations (intraspecific) before considering this interaction from a community standpoint.
Intraspecific competition -- individuals of one population compete with one another for food, space, access to mates, etc. -- such interactions are important in determining carrying capacity of population -- increasing competition may result in lower population growth rates -- as individuals compete, mortality may increase and natality may decrease.
In some cases, intraspecific competition leads to direct physical contact -- e.g. aggressive behavior such as fighting -- in many species, natural selection has led to the evolution of "ritualistic" behaviors that take the place of direct aggression.
Several phenomena reflect the action of intraspecific competition:
1. emigration (dispersal) -- competition may cause individuals to leave in search of unutilized resources -- as individuals emigrate, population growth rates decrease -- has same effect on population size as mortality.
2. territoriality -- in some species, obtaining and defending a territory may be the prerequisite for reproduction -- as population size increases fewer territories are available and more individuals fail to reproduce because they don’t have a territory -- e.g. in some birds, females "select" their mates on the basis of the quality of his territory -- males without territories make up a "floater" subgroup of the population that doesn’t breed.
3. dominance hierarchies -- through behavioral interactions, individuals form a "pecking order" -- dominant individuals command major share of resources and, perhaps, access to mates -- subordinate individuals may not breed -- hierarchy reduces aggression among individuals.
Interspecific Competition -- if two or more populations are utilizing the same set of limited resources, then there are fewer resources available for each population -- carrying capacity is less for each than if they were alone -- interaction is negative for both populations.
If competitors are very similar in their resource needs, they may be unable to coexist together -- one or both may go extinct as a result of their competitive interaction = competitive exclusion principle.
Competitive exclusion principle: "complete competitors cannot coexist."
Competitive exclusion first proposed by a Russian biologist named Gause -- studied lab populations of two species of microorganisms called Paramecium (P. caudatum; P. aurelia) -- grown alone and in mixed culture.
When grown alone, both species displayed logistic growth and reached carry capacity (64 for P. caudatum; 105 for P. aurelia).
In mixed culture, two important events observed:
1. neither grew to size observed when grown alone (effect of competition).
2. P. caudatum was driven to extinction = competitive exclusion -- after extinction of P. caudatum, P. aurelia grew to its carrying capacity.
Figure 40.4.
Species introductions by man have also demonstrated the negative effects of competition and the reality of competitive exclusion.
e.g. tamarisk (salt cedar) trees native to desert regions of Africa, Middle East -- rapid growing with large water requirements -- introduced into deserts of U.S. -- have out-competed and excluded native vegetation along streams (Grand Canyon) -- Anza-Borrego Desert State Park instituting a tamarisk eradication program.
e.g. introduction of mainland birds (e.g. English sparrow, European starling) to Hawaii led to the extinction of much of the island’s endemic bird fauna due to competitive exclusion.
In interspecific competition, the fitness of organisms is negatively affected -- therefore, natural selection will favor phenotypes that are able to reduce or escape competition -- the evolution of many species is guided by competition.
Evolution in species that have been competing for long periods of time may result in a phenomenon called character displacement.
character displacement: increased morphological differences between species when they occur together in the same habitat -- taken as evidence of competition.
e.g. Darwin’s finches in the Galapagos Islands &emdash; bill size is an important adaptation to size of food items &emdash; species eating same size food have same size bills -- similar bill sizes where they occur by themselves in allopatry, very different bill sizes where they occur together in sympatry.
Predation -- interaction in which one member (predator) kills and utilizes the other (the prey) as a food item -- interaction clearly positive for the predator but negative for the prey.
Notice that there are several other interactions that have similar outcomes to predation (e.g. one species benefits, the other is harmed) and can be thought of as special cases of predation:
Parasitism and disease &emdash; parasite/disease uses host for food, but does not immediately kill the host.
Herbivory &emdash; herbivorous animals (insects, mammals, etc.) remove parts of plants for food &emdash; plant may or may not die, but is adversely effected.
The dynamics of predator and prey populations are often linked:
If prey populations increase, predator populations respond in several ways:
1. functional response -- when prey density increases, predators will eat more prey per unit time until some satiation threshold is reached -- above this threshold density, prey eaten per unit time is more or less constant.
2. numerical response -- increased prey density will increase the population size of the predator -- increased reproduction by predator as well as immigration of predators to areas of high prey density.
Predator-prey interactions may produce cyclic changes in population size of predator and prey.
Figure 40.7
e.g. 10 year cycles in snowshoe hare and Canadian lynx populations --hares cycle with herbivory-induced changes in their food resources (the vegetation) and lynx populations follow the hare populations. (See Figure 40.8)
Like competition, predatory-prey interactions are often important in directing the evolution of the predator and prey species -- recall the story of the pepper moth, Biston betularia -- predation by birds responsible for the rapid change in coloration of moth populations.
Predators and their prey evolve adaptations in response to their interactions.
In predators, natural selection favors traits that increase their efficiency in obtaining prey and turning them into offspring -- keen senses, rapid locomotion, "weapons" like teeth, claws, venom, etc.
At same time, natural selection favors adaptations in prey that help them escape predators -- camouflage, rapid movement, group living.
Prey species may evolve anti-predator defenses that make them noxious or otherwise unpalatable -- stings, quills, chemical defenses -- e.g. wasps, porcupines, skunks, monarch butterfly -- often advertise noxiousness by aposematic coloration (bright contrasting colors such as red, yellow, black, white).
Non-noxious species may obtain protection from predation by evolving to look like noxious species = Batesian mimicry -- e.g. stingless flies mimic bees and wasps; viceroy butterfly mimics noxious monarch.
Effects of predation have repeatedly been demonstrated by human disturbances of ecosystems -- e.g. introduction of mongoose to control rats on Caribbean Islands, Hawaii -- e.g. elimination of predators such as wolves has "released" prey species from an important population control mechanism resulting in their over-population.
Symbiosis: "living together" -- term usually refers to pairs of species that live together without harming one another.
Specific kinds of symbiosis:
1. Neutralism (0,0) -- two species live together, but have no effect on one another -- not very common and not very interesting.
2. Commensalism (+,0) -- relationship in which one species benefits, while the other is neither harmed nor benefitted -- commensal species usually employs host speicies as a home and/or transportation.
Remora -- fish with dorsal fin modified into a suction cup --attach to underside of shark and feed on remains of shark kill -- remora clearly benefiting, while shark is neither helped nor hindered.
Insectivororous birds (e.g. egrets) may have commensal relationship with large grazing animals (e.g. cattle) -- birds follow cattle around and eat insects stirred up as cattle graze.
Mutualism -- interaction that benefits both participants -- some very remarkable examples of coevolution.
Animal-Bacteria mutualisms -- e.g. bacteria in intestines of humans, cows, termites, etc. allows host to utilize food more efficiently and may provide molecules host is unable to synthesize -- bacteria has nice warm home and a regular food supply.
Plants-Pollinators -- plants have evolved a number of adaptations to facilitate their pollination by bats, flies, bees, birds -- e.g. colors, odors, rich energy sources (nectar), landing platforms -- animals have evolved traits that allow them to be effective pollinators -- e.g. hovering flight, structures for removing nectar (e.g. bill of hummingbird), host plant fidelity -- many pollination vectors are very specific in terms of species they pollinate.
Benefit to Plant: cross-fertilization is accomplished.
Benefit to Animal: plant supplies vector with a energy-rich food.
Lichens -- "organism" found growing on tree-trunks and rocks -- actually consists of mutualistic algae and fungi &emdash; fungi release enzymes that dissolve minerals from rock and make them available to algae, while algae photosynthesize and make carbohydrates from photosynthesis available to fungi.
Ants and Bullhorn Acacia -- tropical tree and ant species that display an amazing mutualistic relationship -- trees have large thorns ("bullhorns") in which ants nest and lay eggs-- plant also has nectaries which supply adult ants with food and fat and protein-filled structures on leaves called Beltian bodies that are fed to young ants.
What does the plant get from all this? Ants diligently protect "their" acacia from herbivorous animals ranging from insects to cows -- will also attack any vines, shrubs or other trees that come in contact &emdash; ants are active 24hrs a day.
This relationship is obligatory for both -- acacias grown over and out-competed by other plants without ants -- ants require acacia for nest and food.
Next time: Ecosystem Structure and Function