Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Ecology:   Communities

subtitled "You can never do just one thing"

I.  What is a community?
A group of populations that interact with one another.  Can you think of some examples?  


II. "The Closing Circle
    In his book Barry Commoner described four "laws" of ecology.  There actually not "laws" but they are generally applicable to our ecology unit so this might be a good place to introduce them.  Don't memorize them by number (like the laws of thermodynamics), simply understand their general principles. 

III.  Community Characteristics


A.  Trophic structure.
    In other words, who eats who?  In general, producers (autotrophs) consumers (heterotrophs) which includes herbivores (primary consumer) carnivores (secondary consumer).  Detritivores, decomposers, and parasites feast on them all.

B.  Species diversity
    This is is a measure of:  (a) species abundance (number of individuals of each kind); and (b) species richness (number of different kinds of species).  Example:  imagine two communities each with the same 4 species summarized in the table below:

















    Note that both communities have equal species richness, but different species abundance; and hence, much different diversity. What would you see when you walked in community 1?  2? Which community is more like a cornfield?  rainforest?  


    Species diversity can be impacted by competition.  For example, periwinkles grazing on algae (example in class) increased algal richness and bison grazing on the prairie increased forb diversity.

C.  Stability
    This refers to the ability to resist change.  Some communities are stable (able to perpetuate themselves on the site), others are not.  A stable community that persists on a site is termed a climax community (see succession below).  Community stability is usually associated with: 

D. Inter-specific Interactions
    Species interact with one another in many ways. (note:  intra - between members of the same group; inter - between members of different groups).  These interactions can take a number of forms (competition, predator-prey & parasite-host relationships, mutualism, commensalism, amensalism).

IV. Competition

    Occurs when there are limited resources shared by two species. A resource is anything directly required by an organism for it's survival such as food, space, water.  There are two basic ways in which one species can compete with one another:  

  1. by being "tougher" - in other words, one species will limit the access of the other to the resource.  Example from text:  kangaroo rat experiment, excluding increase overall rodent species richness; and 

  2. by being "smarter" - by more efficiently utilizing (exploiting) a shared resource, getting a greater portion than the other.  Example from text:  competition between rye and clover.

Competition can:  limit range of a species (wasp story in text) and/or reduce species abundance/richness

V.  Ecological niche

A.  Definitions.
    Habitat is the place where a species lives.  Its "address."  It includes both biological and physical components of the environment.  Niche refers to the role and response of the species to the physical and biological habitat.  Its "profession."  Example:  Fast food restaurants.  Habitat = city  (each occupies the same habitat.However, each responds to the habitat in a slightly different way, i.e., has a slightly different niche.  Some sell chicken, others burgers.  Some make double burgers, others hot and juicy ones.  Some have specialty items, etc.  Each has its own unique response to the environment, so it minimizes competition with the others, and therefore has its own niche.

    Niche is determined by the species': (a) physical environment (e.g., light, pH, temperature, moisture); and (b) biological environment (e.g., food size).  

B.  Graphical representation of niche.
The response of a species to some physical or biological factor can be graphically represented (resource utilization curves).  For example, plot # individuals vs. resource.  This provides a visual picture of how the species is responding to a single variable.  But remember, species are actually responding to numerous variables. 

C. Fundamental vs. realized niche.  
    The broadest niche that a species could potentially occupy is the fundamental niche.  The actual niche occupied by the species is the realized niche (resource utilization curves).


VI.  Competitive exclusion

    Each species has its own niche; one species per niche; no two species have the identical niche

B.  Competition occurs when niches overlap.

C.  Graphical Representations (i.e., plot # individuals vs. resource).  When two niches overlap, competition occurs.  There are three potential outcomes of the competitive process:  (1) both populations will shift niche; (2) one population will shift, the other will remain (out competes); or (3) one goes extinct.  This results in "resource partitioning."


  1. Paramecium.  Russian biologist G. F. Gause (1934) studied two species of Paramecia - P. aurelia and P. caudatum.  Either alone grew fine in culture flask.  When together, P. aurelia out-competed P. caudatum (which went extinct in the flask) and the number of both species was lower.  

  2. Barnacles.  Joseph Connell studied two species on the Scottish coast, Chthalamus and Balanus that show distinct zonation.  These barnacles occupy the same habitat, but have different niches - Chthalamus above tidal line, Balanus below.  Realized niche, can be graphically represented by plot:  # individuals vs. distance from shore .  Connell did experiments to determine the fundamental niche of each species.  Conclusions:  the fundamental niche of each species was larger than the realized niches (i.e., Chthalamus could survive lower, and Balanus larvae higher); the fundamental niches overlapped; Balanus could out compete Chthalamus;  Balanus limited at upper limit by physical environment.  

  3. Others:  Flour beetles; Ungulates of the Serengeti; Warblers; Roots of plants

D.  Species Introductions.
    Usually detrimental.  Introduced species typically out compete native species cause of absence of normal checks and balances (environmental resistance) on population growth.   Think of our "problem" species - buckthorn, house sparrows, dandelions, zebra mussels, and so on - they have all been introduced from other places in the world.  What will the result of their competition with native species be? 

VII.  Predator-prey relationships - "eat or be eaten"

A.  Patterns
    Oscillation of populations if time lag in predator response to prey abundance.  More stable if predator maintains prey below carrying capacity.  Lynx and hare example.  Aphids and ladybird beetle example.Note factors other than just predators may be important in controlling prey (such as cycles in availability of food, parasites, palatability of food).  For example, when supplemented with food, hare populations increased several fold indicating that population size was a function of food availability as well as lynx predation.

B.  Defenses.
    Prey typically have a variety of adaptations to reduce the chance of becoming dinner.  These include:

VIII.  Top-Down Control (Trophic Cascade)

    Recall Barry Commoner's first law of ecology - "Everything is connected to everything else". Although biologists have always known this, most assumed it was something of a one-way street.  That the lower levels controlled the upper levels.  In other words, if you knock out a plant (producer) food source then the herbivores reliant on that food will drop out of the community and then the carnivores, and so on.  However, until relatively recently we didn't realize that it is a two-way street - higher level trophic levels have an impact on lower levels like plant communities.  For example:

A.  Birds - have been shown to be required for a healthy forest.  A study in which birds were excluded from trees showed increased predation by insects/caterpillars which causes forest decline.  This is worrisome consider the reduction of bird populations over the past years as a result of forest fragmentation, habitat loss, etc.

B.  Wolves - on Isle Royale, wolf populations keep moose in check.  In years when wolf populations decline there is an increase in moose which eat balsam fir.   


C.  Starfish, periwinkles, bison. 


D.  Keystone species - presence exerts large impact on community.

IX.  Symbiotic Relationships & Coevolution.
    Symbiosis refers to species living together, each affecting the survival of each other. There are a variety of different types of relationships including:  

A.  Mutualism - both partners benefit.  Examples include ants and acacias (story in class) and pollination biology.  Mutualisms, as are many of the interactions described in these community lecture notes, are the result of coevolution - the process by which two species act as  reciprocal selection pressures.  In other words, each species affects the evolution of the other.

B.  Commensalism - one benefits, other unaffected.  Examples include remoras, cattle egrets, epiphytes

C.  Parasitism - one benefits, the other harmed.  Many examples including various diseases/pathogens.  There are even plant parasites like dodder.  As an aside, a good parasite doesn't kill its host.  Evolution favors the selection of resistance by host (recall the effect of malaria in humans and the rabbits in Australia story.  In the latter case, when the Myxoma virus was originally introduced it killed 99% of the rabbits, some resistant.  On second exposure fewer were killed (say 90%).  Now, it has a comparatively mild effect on host).


D.  Amensalism  - one harmed, the other unaffected

X.  Succession

A.  Definitions and general information.   

B.  Why does succession occur?  
One community modifies the environment, makes it less suitable to itself and more suitable for other species.  In class we will look at some data (actually graphs) showing successional changes in the communities around N. Lake Michigan.  In one area there is a series of about 108 ridges that were deposited during the past 3,500 years.  This means that approximately 1 ridge was deposited every 30 year as the lake receded (which occurred at a uniform rate).  We will look at the following data (from Lichter, seminar 11/1995):

C.  Types:  

1.  Primary.  

  • sites not previously vegetated (bare rock, sand dunes, glacial deposits, riverbanks, water); sites where little soil is present

  • slow - because it requires the development of soil.  

  • Pioneer species - allows for the colonization of new sites 

  • Pond Succession (Hydrarch):  Pond rooted aquatics cattail marsh willow shrubs oak/hickory beech/maple

  • Dune Succession (Xerarch)Dune dune grasses cottonwoods pine forest oak woods beech/maple 

  • Rock Succession (Xerarch):  Rock lichens mosses herbaceous annuals herbaceous perennials pines and shrubs forest

2.  Secondary

  • sites previously vegetated

  • replace previous community

  • method to heal environmental wounds (i.e., fire, tree falls forming a light gap in forest, plowing up a field)  

  • faster

  • Old Field annual weeds perennials and grasses pines/shrubs forest

D.  Successional Trends
    The table below summarize a number of trends that occur in the successional process (from early to late).


Early Successional Community

Late Successional Community


Organismal Size







broad, general

narrow, specialized

Life cycle



Selective Strategy



Reproductive Strategy 



Species diversity/species richness




Stability to disturbance



% soil moisture



light penetration



[N, C]



Nutrient Cycles

open, rapid

closed, slow

[Ca, P]



soil pH




E.  Examples:  moraines in Glacier Bay; fungi on needles, dung successions

XI. Island Biology

A.  Distance effect.  
    Farther from mainland, the fewer species.  Because of the reduced number of colonizers.

B.  Area effect.  
    The smaller the island the fewer the number of species.  Because fewer habitats.

C.  Lessons for conservation management


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Last updated: April 20, 2004        � Copyright by SG Saupe