Community Ecology Chapter 53

Community An assemblage of populations in an area or habitat Communities differ in Species richness # they contain Relative abundance of diff. species

Different Views of Communities An individualistic hypothesis depicts a community as a chance assemblage of species found in the same area because they happen to have similar abiotic requirements.(Gleason.1900s)

An interactive hypothesis depicts a community as an assemblage of closely linked species locked in by mandatory biotic interactions. (Clements 1900s) In most actual cases, the composition of communities does seem to change continuously

Rivet Model & Redundancy Model The rivet model of communities is a reincarnation of the interactive model. The redundancy model states that most species in a community are not closely associated with one another. No matter which model is correct, it is important to study species relationships in communities.

Interspecific Interactions Populations may be linked by Competition, Predation, Mutualism and Commensalism

Interspecific Interactions

A. Interspecific Competition Resources are in short supply Potential for competition between any two species that need the same limited resource.

Competitive Exclusion Principle 1934, G.F. Gause P. aurelia & P. caudatum, grown separately-did well but tog. P. aurelia drove P. caudatum to extinction Two species with similar needs for same limiting resources cannot coexist in the same place The better adapted of the two will replace the other

Ecological Niche All the biotic and abiotic factors needed to maintain the species. An organism’s niche is its role in the environment. Hawks and Owls have the same niche but can coexist b/c hawks are diurnal, owls are nocturnal The competitive exclusion principle can be restated to say that two species cannot coexist in a community if their niches are identical.

Resource Partitioning Differentiation of niches that enables two similar species to coexist in a community

Character Displacement The tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species. Hereditary changes evolve that bring about resource partitioning

B. Predation (-/+) A predator eats prey. Herbivory, in which animals eat plants. In parasitism, predators live on/in a host and depend on the host for nutrition. Predator adaptations: many important feeding adaptations of predators are both obvious and familiar. Claws, teeth, fangs, poison, heat-sensing organs, speed, and agility

Plant Defenses Chemical compounds that are toxic Thorns Secondary compounds not part of normal met, ex poisonous berries

Animal Defenses b. Mechanical defenses include spines. Behavioral defenses include fleeing, hiding, self-defense, noises, and mobbing. a. Camouflage includes cryptic coloration, deceptive markings. b. Mechanical defenses include spines. c. Chemical defenses include odors and toxins Aposematic coloration is indicated by warning colors, and is sometimes associated with other defenses (toxins).

Fig. 53.6 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mimicry- organisms resemble other species Batesian mimicry is where a harmless species mimics a harmful one Snake Hawkmoth larva

2. Müllerian Mimicry Two or more unpalatable species resemble each other Both produce a toxin& predators learn quickly to avoid them Yellow jacket Cuckoo Bee

Batesian mimicry Convergent evolution Monarch male poisonous Viceroy male edible

Parasites and pathogens as predators A parasite derives nourishment from a host, which is harmed in the process. Endoparasites live inside the host and ectoparasites live on the surface of the host. Parasitoidism is a special type of parasitism where the parasite eventually kills the host. Pathogens are disease-causing organisms that can be considered predators.

C. Mutualism (+) Two species benefit from their interaction Acacia & ants Acaciafood & cover Ants sting any intruder

D.Commensalism (+/o) One species benefits from the interaction, but other is not affected. An example would be barnacles that attach to a whale Bird’s nests in trees

E. Coevolution Two species with intertwined life history- the evolution of one affects the other When one species evolves, it exerts selective pressure on the other to evolve to continue the interaction Ex humming bird & flower

F.Trophic structure Feeding relationships Food chains food chain usually 4 or 5 links = trophic levels length of food chain limited by inefficiency of energy transfer

Food webs Food chains are hooked together into food webs Who eats whom? a species may weave into web at more than 1 trophic level bears “there’s always a bigger fish” What limits the length of a food chain?

G. Dominant & Keystone Species Dominant species most abundant species or highest biomass (total weight) community If we remove a dominant species from a community, it can change the entire community structure. Keystone species exert important regulating effect on other species in community

Keystone species Influential ecological role not necessarily dominant or most abundant keystone species increases diversity of habitat

Keystone species Sea otter is keystone predator in North Pacific What is the impact of the whale? Structure of a community may be controlled bottom-up by nutrients or top-down by predators

II. Disturbances Most communities are in a state of non-equilibrium due to disturbances fire, weather, human activities, etc. not all are negative

Disturbances Disturbances are often necessary for community development & survival

Ecological cycle fire as part of a natural community cycle

IIB. Ecological succession The sequence of community changes after a disturbance transition in species composition over ecological time years or decades Mt. St. Helens

Primary succession Begins with virtually lifeless area without soil, then… bacteria lichens & mosses grasses shrubs trees { make soil

Succession from mosses & lichens = pioneer species to shrubs & trees

Secondary succession Existing community cleared, but soil is intact

Climax forest Plant community dominated by trees representing final stage of natural succession for specific location stable plant community developed through stages remains essentially unchanged in species composition for as long as site remains undisturbed birch, beech, maple, hemlock oak, hickory, pine

Climax Forest The species mix of climax forest is dependent on the abiotic factors of the region solar energy levels temperature rainfall fertility & depth of soil birch, beech, maple, hemlock

III. Biodiversity of Communities Two components Species Richness Relative abundance Greater diversity = greater stability Which is more diverse?

Species richness is related to a community’s geographic size The species-area curve : the larger the geographic area, the greater the number of species

Patterns of Biodiversity Equatorial/Polar gradient: greatest at equator, less &less as move to poles, because Climate: poles frozen at times,long growing season @eq, more solar energy & precip.@ eq Age: eq comm. Are older, more established

Patterns of Biodiversity Island Biogeography: diff islands have diff # of species b/c of immigration & extinction. Immigration & extinction determined by the size of the island & distance from the mainland

Factors that determine the number of species that eventually inhabit the island The rate at which new species immigrate to the island. The rate at which species become extinct.

New Island

Size of Island

Distance from Mainland