Communities in Motion A biological community is an assemblage of populations of various species...

Communities in Motion• A biological community is an assemblage of

populations of various species living close enough for potential interaction

For example, the “carrier crab” carries a sea urchin on its back for protection against predators

© 2011 Pearson Education, Inc.

Community interactions are classified by whether they help, harm, or

have no effect on the species involved• Ecologists call relationships between species in a

community interspecific interactions• Examples are competition, predation, herbivory,

symbiosis (parasitism, mutualism, and commensalism)

• Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)


Competition

• Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply


Competitive Exclusion

• Strong competition can lead to competitive exclusion, local elimination of a competing species

• The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

Ecological Niches and Natural Selection• Ecological niche can also be thought of as an

organism’s ecological role (it’s use of biotic and abiotic factors)

• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches; however, TWO SPECIES CANNOT OCCUPY THE EXACT SAME NICHE! (One will be excluded through competition.)


• Resource partitioning of a niche enables similar species to coexist in a community

Figure 54.2 A. distichus perches onfence posts and othersunny surfaces.

A. insolitus usuallyperches on shadybranches.

A. ricordii

A. aliniger

A. insolitus

A. distichusA. christophei

A. cybotesA. etheridgei

Predation• Predation (+/– interaction) refers to

interaction where one species, the predator, kills and eats the other, the prey

• Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison

Prey display various defensive adaptations:•include hiding, fleeing, forming herds or schools, self-defense, and alarm calls

• Animals also have morphological and physiological defense adaptations

• Cryptic coloration, or camouflage, makes prey difficult to spot


http://www.youtube.com/watch?v=PmDTtkZlMwM

• Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration

• Predators are particularly cautious in dealing with prey that display such coloration

• In some cases, a prey species may gain significant protection by mimicking the appearance of another species

• In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model


• In Müllerian mimicry, two or more unpalatable species resemble each other

Symbiosis• Symbiosis is a relationship where two or more species

live in direct and intimate contact with one another

- Parasitism• In parasitism (+/– interaction), one

organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process

• Parasites that live within the body of their host are called endoparasites (worms), or on the external surface of a host are ectoparasites (ticks)

Mutualism

• Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species

• A mutualism can be– Obligate, where one species cannot survive

without the other– Facultative, where both species can survive

alone


Figure 54.7

(a) Acacia tree and ants (genus Pseudomyrmex)

(b) Area cleared by ants at the base of an acacia tree

Commensalism

• In commensalism (+/0 interaction), one species benefits and the other is neither harmed nor helped

• Commensal interactions are hard to document in nature because any close association likely affects both species


• An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interact

• Regardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cycling

Remember: Energy flows through an ecosystem…while matter is recycled!

Physical laws govern energy flow and chemical cycling in ecosystems

Trophic structure is the feeding relationships between organisms in a community

• Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) to secondary consumers (carnivores) to tertiary consumers (carnivores that feed on other carnivores)

Trophic level refers to where a species falls in the food chain

Food chains link trophic levels from producers to top carnivores

Energy, Mass, and Trophic Levels

Heterotrophs depend on the biosynthetic output of other organisms


Every food web must contain a PRIMARY PRODUCER! It is responsible for making all of the organic materials that will be passed along the food chain. Primary producers “capture” all the ENERGY the ecosystem will have!

Primary production is the amount of light energy converted to chemical energy by autotrophs during a given time period The extent of photosynthetic production sets the spending limit for an ecosystem’s “energy budget”

Key

Chemical cycling

Energy flow

Sun

Heat

Primary producers

Primaryconsumers

Secondary andtertiary consumers

Detritus

Microorganismsand other

detritivores

Figure 55.4

• Tropical rain forests, estuaries, and coral reefs are among the most productive ecosystems per unit area

• Marine ecosystems are relatively unproductive per unit area, but contribute much to global net primary production because of their volume

Energy transfer between trophic levels is typically only 10% efficient


The other 90%: •gets used by the organism for cellular process & growth• some is lost as heat•some eliminated as waste


• In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level

• Most biomass pyramids show a sharp decrease at successively higher trophic levels

Trophic Efficiency and Ecological Pyramids• Trophic efficiency is the percentage of

production transferred from one trophic level to the next

• It is usually about 10%, (with a range of 5% to 20%)


• Approximately 0.1% of chemical energy fixed by photosynthesis reaches a tertiary consumer

• *Food chains rarely go beyond quaternary consumers because there simply isn’t enough energy left to sustain that level of consumer.

Food Webs• A food web is a branching

food chain with complex trophic interactions

• Species may play a role at more than one trophic level

Be able to identify the primary producers, primary consumers (herbivores), etc…

How would a decomposer fit into a food web?

• Detritivores, or decomposers, are consumers that derive their energy from detritus, nonliving organic matter

• Prokaryotes and fungi are important detritivores

• Decomposition connects all trophic levels


Species with a Large Impact

• Certain species have a very large impact on community structure

• Such species are highly abundant (Dominant species) or play a pivotal role in community dynamics (Keystone species)


Dominant Species


• Dominant species are those that are most abundant or have the highest biomass

• Dominant species exert powerful control over the occurrence and distribution of other species

– For example, sugar maples have a major impact on shading and soil nutrient availability in eastern North America; this affects the distribution of other plant species

Keystone Species

• Keystone species exert strong control on a community by their ecological roles, or niches

• In contrast to dominant species, they are not necessarily abundant in a community

• Keystone predators usually increase diversity in a community by holding down prey populations.


For example: It could be a small predator that keeps down a population of herbivores and prevents them from wiping out a plant species. They're effective especially when the other animal has no other predators.

Figure 54.17 EXPERIMENT

RESULTS

With Pisaster (control)

Without Pisaster(experimental)

Year

’73’72’71’70 ’69 ’68 ’67 ’66 ’65 ’64 1963 0

5

10

15

20

Nu

mb

er o

f sp

ecie

sp

res

ent

Ecological Succession• Ecological succession is the sequence of

community and ecosystem changes after a disturbance

• Primary succession occurs where no soil exists when succession begins. Retreating glaciers provide a valuable field-research opportunity for observing primary succession.

• Secondary succession begins in an area where soil remains after a disturbance


Early arrival species may facilitate appearance of later species by making the environment favorable


The first stage of succession is called the “pioneer” stage.• Succession is the result of changes induced by the vegetation itself• A CLIMAX COMMUNITY is a mature, stable community that is the

final stage of ecological succession

Biological and geochemical processes cycle nutrients and water in ecosystems

• Life depends on recycling chemical elements• Nutrient circuits in ecosystems involve biotic and

abiotic components and are often called biogeochemical cycles


• A model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirs

• All elements cycle between organic and inorganic reservoirs

Biogeochemical Cycles

• Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally

• Less mobile elements include phosphorus, potassium, and calcium


In terrestrial ecosystems, nitrogen is the most common limiting nutrient.

• Water is essential to all organisms• Liquid water is the primary physical phase in which

water is used• The oceans contain 97% of the biosphere’s water;

2% is in glaciers and polar ice caps, and 1% is in lakes, rivers, and groundwater

• Water moves by the processes of evaporation, transpiration, condensation, precipitation, and movement through surface and groundwater


Movement overland by wind

Precipitationover landPrecipitation

over ocean

Evaporationfrom ocean

Evapotranspira-tion from land

Runoff andgroundwater

Percolationthroughsoil

Figure 55.14a

The Water Cycle

The Carbon Cycle• Carbon-based organic molecules are essential to

all organisms

• Photosynthetic organisms convert CO2 to organic molecules that are used by heterotrophs

• Carbon reservoirs include fossil fuels, soils and sediments, plant and animal biomass, the atmosphere, and sedimentary rocks


• CO2 is taken up and released through photosynthesis and respiration; additionally, volcanoes and the burning of fossil fuels contribute CO2 to the atmosphere

CO2 inatmosphere

Photo-synthesis

Burningof fossil

fuels andwood Phyto-

plankton

Photosynthesis

Cellularrespiration

Consumers

Consumers

Decomposition

The Nitrogen Cycle• Nitrogen is a component of amino acids, proteins,

and nucleic acids

• The main reservoir of nitrogen is the atmosphere (N2), though this nitrogen must be converted to other usable forms for the plant this is done through nitrogen fixation by bacteria


Terrestrialcycling

Decom-position

Denitri-fication

NO3–

NO2–

N2

Assimilation

Fixationin root nodules

Ammonification Nitrification

NH4+NH3

Uptakeof amino

acids

Decomposition and Nutrient Cycling Rates

• Decomposers (detritivores) play a key role in the general pattern of chemical cycling

• Rates at which nutrients cycle in different ecosystems vary greatly, mostly as a result of differing rates of decomposition

• The rate of decomposition is controlled by temperature, moisture, and nutrient availability


Communities in Motion A biological community is an assemblage of populations of various species...

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