Chapter 53Community Ecology

Community

• Any assemblage of populations in an area or habitat

• Has a set of properties defined by its species composition, with a structure determined by the interactions between species

• Species richness: the number of species a community contains

Individualistic Hypothesis

• H.A. Gleason in 1900• Depicted a plant community as a chance

assemblage of species found in an area simply because they happen to have the same abiotic requirements– Temperature, rainfall, and soil type

• Emphasizes studying single species

Interactive Hypothesis

• F.E. Clements in 1900• Saw a community as an assemblage of closely

linked species, locked into association by mandatory biotic interactions that cause the community to function as an integrated unit– A superorganism

• Emphasizes entire assemblages of species as the essential units for understanding the interrelationships and distribution of organisms

Rivet Model

• Suggests that most of the species in a community are associated tightly with other species in a web of life– Reducing or increasing the abundance of one species

in a community will affect many other species• Not all the rivets in an airplane wing are

required to hold the wing together, but if you take out the rivets one by one it would cause a problem

Redundancy Model

• Most of the species in a community are not tightly associated with one another, and the web of life is very loose

• An increase or decrease in one species has little effect on others

• Species are redundant– If one predator disappears, another predatory

species in the community will take its place

Interspecific Interactions

• Relationships between different species of a community– Competition– Predation– Mutualism– Commensalism

Competition

• Competitive Exclusion Principle– 1934- Gause studied effects on interspecific

competition in the lab with two species of Paramecium.

– Cultured protists under stable conditions with constant amount of food added every day• When grown separately, each population grew rapidly and

leveled off at its K• When cultured together, one had the competitive edge in

obtaining food and drove the other to extinction

– Two species that are similar enough that they compete for the same limiting resources cannot coexist in the same place

Ecological Niche

• Sum total of a species’ use of the biotic and abiotic resources in its environment

• If an organisms’ habitat it its address, the niche is that habitat plus the organism’s occupation– Where it lives, what ranges it tolerates, what time

of day it is active, what it eats• Two species cannot coexist in a community if

their niches are identical

Resource Partitioning

• Two possible outcomes of competition:– Less competitive species will be driven to local

extinction– One species may evolve enough through natural

selection to use a different set of resources• Resource Partitioning

– Ex: Warblers

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

• Ex: Galapagos finches beak– Enables two species to avoid competition by

feeding on seeds of different sizes and probably represents an evolutionary outcome of past competition

Predation

• Includes herbivory, parasitism, and carnivory• Predator adaptations– Acute senses that enable them to locate and

identify potential prey– Claws, teeth, fangs, stingers, poison– Heat-sensing, vibration-sensing organs– Many herbivorous insects locate food by using

chemical sensors in their feet

Plant Defenses Against Herbivores

• Chemical toxins– Strychnine from tropical vines– Morphine from poppies– Nicotine from tobacco

• Antipredator spines and thorns• Could be distasteful to animals, but used as spices

for humans– Cinnamon, cloves, and peppermint

• Some produce chemicals that imitate insect hormones and cause abnormal development in some insects that eat them

Animal Defenses Against Predators

• Hiding, escaping or defending themselves, alarm calls

• Cryptic coloration: camouflage, is a passive defense that makes them difficult to spot

Animal Defenses Against Predators

• Mechanical or chemical defense– skunks and porcupines– Poisonous toads– Some acquire toxins from the food they eat• Monarch butterflies store poison from milkweeds they

eat

Animal Defenses Against Predators

• Aposematic coloration: warning coloration for organisms with chemical defenses– Black with yellow or red stripes are unpalatable

animals• Batesian mimicry: a harmless species mimics

a harmful model– Larva of hawkmoth puffs up like a snake• Even weaves back and forth and hisses

Animal Defenses Against Predators

• Mullerian mimicry: two unpalatable species resembles each other

Animal Defenses Against Predators

• Predators will mimic others also– Snapping turtles have tongues that resemble a

worm that lures in fish

Parasites and Pathogens as Predators

• Endoparasites: parasites that live within their host– Tapeworms and malarial parasites

• Ectoparasites: parasites that feed on the external surface of a host– Mosquitoes and aphids

• Parasitoidism: lay eggs in living hosts, larva feed on the host and kill it– Small wasps

Mutualism

• Mutualism: interspecific interaction that benefits both species– Sometimes require the coevolution of adaptations

in both species• Ex: cellulose-digesting bacteria in stomach of termites

– Most angiosperms have adaptations that attract animals that function in pollination or dispersal

Commensalism

• Commensalism: an interaction between species that benefits only one of the species involved– Sometimes involve one species obtaining food

that is inadvertently exposed by another• Ex: cattle egrets and grazing animals

– Eat bugs flushed out by grazing

Trophic Structure

• Trophic structure: transfer of food energy from its source in plants through herbivores to carnivores and decomposers– Food chain

• Four or five links, or trophic levels, make up a food web

Food Webs

• It is not a chain because a given species may weave into the chain at different levels

• Animals at each successive level tend to be larger with each link (except parasites)

Length of a Food Chain

• Energetic Hypothesis: is the most widely accepted reason– Length is limited by the inefficiency of energy

transfer• Dynamic stability hypothesis: long food

chains are less stable, fluctuations are magnified at higher levels

Dominant Species

• Species that have the highest abundance or highest biomass (sum weight of all individuals in a population)

• Exert powerful control over the occurrence and distribution of other species

• Species that are the most competitive in exploiting nutrients become the dominant species

Keystone Species

• Exert control by their ecological niches• Figure out what it is by removing certain

species and looking at the effects

Bottom-Up Model

• Mineral nutrients (N) control the community organization because the nutrients control the plant (V) which control herbivores (H), which control predator numbers (P)– N V H P

• If you want to change the biomass, you have to add more nutrients for the producers and then all others will increase as well

Top-Down Model

• It is mainly predation that controls community organization because predators control herbivores, which control plants, which control nutrient levels – N V H P

• Also called the trophic cascade model– Increasing predators will depress numbers on

lower levels

Biomanipulation

• Lake Vesijarvi in Finland– Huge algae blooms due to toxic waste dumping– Primary consumer fish (roach fish) were eating all

herbivorous zooplankton, which led to an increase in algae

– They took away 1,018 tons of fish, and added fish that prey on roach

– Algae went back to normal

Disturbance and Community Structure

• Disturbances: events such as storms, fire, floods, drought, overgrazing, or human activities that damage communities, remove organisms from them, and alter resource availability

• Humans are the most widespread agents of disturbance– Logging and clearing, mining, farming, overgrazing– We currently use about 60% of Earth’s land

Ecological Succession

• Most apparent when a disturbance, such as a large fire or volcanic eruption strips away the existing vegetation

• Disturbed area may be colonized by new species which are succeeded by other species = ecological succession

• Primary succession: begins in a virtually lifeless area where soil has not been formed– Volcanic island or where a glacier has melted– Bacteria lichens and mosses soil development

grasses, shrubs and trees community’s prevalent vegetation• Process takes hundreds or thousands of years

Secondary Succession

• Occurs where existing community has been cleared by some disturbance that leaves the soil intact

• Begins to return to something like its original state

Processes Involved with Succession

• The early arrivals facilitate, or contribute to, the appearance of the later species by making the environment more favorable for the later species– May make the soil more fertile

• The early species may inhibit establishment of later species, so that the later species colonize successfully in spite of, rather than because of, the earlier species

• Early species may tolerate the later species but do not help or hinder colonization

Processes Involved with Succession

• Increases soil quantity– More detritus, decreased erosion because more plants hold soil

in place, provides more anchoring for plants• Improved soil quality

– Soil gains organic matter, provides more nutrients for plant growth

• Increase in water retained in soil– Increased matter retains water, shading reduces evaporation,

more water is available for plants• Higher humidity

– Caused by more transpiration• Decrease in temperature

– Caused by shading

Biodiversity

• Species richness: total number of different species

• Relative abundance: proportion of total abundance that each species makes up

• Heterogeneity: measurement of biodiversity that includes species richness and relative abundance

Species-Area Curve

• The larger the area, the more species there will be– Larger areas offer a variety of different habitats

and microclimates