Community Structure

Factors that affect Community Structure: • Predation • Symbiosis – Mutualism – Commensalism – Parasitism • Competition – Keystone predators • Succession – Primary and Secondary

Predation

• Predators feed on other living organisms -- their prey • Predators do

not

take up residence in or on prey (unlike parasites) • Prey may or may not die from relationship

Symbiosis

• A symbiotic relationship is one in which there is an intimate, long-term relationship between two (or more) species – Involves one species living on or in another species for at least part of life cycle • A symbiotic relationship could be: – Mutualistic – Commensalistic – Parasitic

Mutualism

Above

: Impala being “groomed” by Oxpecker bird, which gets a meal of insects.

• Mutualism is a

Above & Below

: A Clown fish makes its symbiotic relationship home and hides from its predators in an anemone and in return in which both species keeps the anemone _

benefit

.

clean from parasites and safe from its predators.

Left

:

Lichen

is really a mutualistic symbiosis between photosynthetic

algae

(which makes sugars for both species) and a

fungus

, which “houses” the algae and protects it from water loss.

Commensalism

Above

: Remoras get a free ride from a Lemon shark, but do no harm to the shark.

• Commensalism is a symbiotic relationship in which one species

benefits

and the other species is

not affected Below

: Man-O-War fish (Nomeidae) find shelter and safety among the tentacles of a Portuguese Man-O War jellyfish.

Right

:

Epiphytes

are plants that live on tropical tree limbs and obtain sunlight & rainwater without harming their “home” since their roots remain in the air (not inside tree bark or tissue).

Parasitism

• Parasitism is a symbiotic relationship in which one species

benefits

(

parasite

) and the other species is

harmed

(

host

) .

Right

:

Ergot

is a fungus that grows on grains such as barley, breaking down the grain and getting food from it.

Left

: Tapeworms inhabit animal digestive tracts, living off of the nutrients that are intended for the host. Over time, this weakens and can kill the host.

Above

: Cowbirds are “brood parasites” they lay their eggs in another bird’s nest and leave them to be hatched & raised by the host species. Cowbird nestlings grow rapidly, frequently out-competing the host's offspring for food and parental care. This adult Yellow-throat is feeding a cowbird fledgling that's more than twice its size.

Competition

• Competition occurs when two or more individuals attempt to use an essential common resource that is in limited supply (food, water, shelter, sunlight, etc.) • Can be

intraspecific

(among individuals in a population) or

interspecific

(between different species)

Ex) Oak trees and pine trees may compete for sunlight and space in a New England forest.

Species - Species Interactions

Interaction

Neutral relationship

Direct Effect on Species 1 0 Direct Effect on Species 2 0

Predation of Sp2 by Sp1 Mutualism of Sp1 & Sp2 *

+ -

Commensalism of Sp1 w/ Sp2 * Parasitism by Sp1 on Sp2 *

+ + + + 0 -

Competition betw. Sp1 & Sp2 * Symbiotic relationship

-

“Ancient Farmers of the Amazon”

video clip

• Leaf cutter ants • Leaves (from trees) • Cultivated Fungus (Ant “garden”) • Mold • Bacteria -- that produce “antibiotic” chemicals (really anti-mold)

Competition & the Niche

• Every org. has its own ecological role or “job” in its community; this is called its

ecological niche

.

• This role description includes all the biotic & abiotic aspects of the organism’s existence; it’s physical habitat, it’s prey, it’s predators, it’s competitors, and how it is influenced by light, wind, water, etc.

• When resources are abundant, different species can share a niche or role. Unlimited resources are not often found in mature, stable communities, however…

Inter-specific Competition

• More typically, species compete for resources and a particular niche.

• Inter-specific competition between species with overlapping niches can lead to

competitive exclusion

. (ie removal of the less-fit species from the shared niche in question…)

Competitive Exclusion

Laboratory Experiment by Gause (1934):

Two species of paramecium reach high population densities in given conditions when grown

separately

.

When grown under the same conditions

together

, however, one species is out-competed by the other, and eventually goes extinct.

http://www.sci.sdsu.edu/classes/bio100/Lectures/Lect21/Image294.gif

more

Competitive Exclusion

There once were two species of barnacle that inhabited the same rock… For the story, visit: http://nortonbooks.com/college/biology/animations/ch35a02.htm

•

Potential niche

v.

Realized niche

Species-Species Interactions:

Practice Questions

Communities are shaped by all the interactions that occur within them

• Ecologists used to think that

competition

was the most important relationship that determined community structure (both the number of species in a community and the size of each population).

• Further study has proven that this is not the case, all types of species-species interactions, as well as biotic-abiotic interactions, are highly influential in determining community structure.

Community

stability

is an outcome of forces that have come into

“uneasy balance”

• Resources are

sustained

,

as long as populations do not grow beyond capacity.

• Predators & prey

coexist

,

as long as neither wins.

• Competitors have

no sense of fair play

.

• Mutualists are

stingy.

– Plants produce as little nectar as needed to attract pollinators – Pollinators take as much nectar as they can for the least effort

Disturbance

can

destabilize

a community

• Short-term, disturbance can slow growth of some populations • Long-term changes (like climate) can also destabilize • If instability becomes great enough, community may change in ways that

persist

even when disturbance ends or is reversed • If some community member species are rare or weak competitors, they may become

extinct.

Keystone Species are “key” to maintaining community structure

• • •

Keystone Species:

a species that is critical in determining the nature of an entire community, usually by influencing amount of available food, water, or other resource.

Keystone species

may be present in relatively small numbers, but still affect whole community structure.

Ex: Fig trees in tropical rainforests of C. & S. Am.

– – – – –

Fig trees produce a continuous crop, year-round Figs sustain fruit-eating vertebrates at times of year when other fruits less available No figs



no fruit-eating vertebrates No fruit-eaters



less seed dispersal in fruited plants Less seed dispersal



reduced distribution of fruit-bearing plants

Keystone Predators

• What is the role of a keystone predator?

–

reduces the density

of the strongest competitors in a community – helps

maintain species diversity

in a community

by preventing competitive exclusion of weaker competitors

, which in turn contributes to

community stability

(think about why this is so).

Ex1: Gray Wolf Ex2: Sea Star (

Pisaster

) Ex3: Common periwinkle (

Littorina littorea

)

Examples of Keystone Predators

• Ex1: Gray Wolf – Wolves hunted to extinction in some areas  elk, deer pops explode due to loss of key predator  overgrazing of vegetation by deer  decline of smaller animals (rodents, rabbits, insects) which rely on vegetation  decline of foxes, hawks, owls, badgers which rely on small animal prey  also see decline in scavengers (ravens, eagles) that eat wolf kill

Examples of Keystone Predators

• Ex2: Sea star (

Pisaster

) preys on blue mussels, limpets, chitons, & various barnacles – When

Pisaster

was removed from community, saw decline from 15 species to 8 species overall, because

Mytilus

was left to dominate and crowd out other invertebrates!

Another Example

• Ex3: Common periwinkle

(a type of snail)

– Periwinkes live in the rocky intertidal zone and eat algae.

– In tidepools, they eat the dominant algal species, like the yummy tender filamentous green algae.

– This predation of filamentous green algae by periwinkles keeps this dominant species

in check

and allows other less competitive species, like the tough, unpalatable red algae Irish moss to survive.

Common periwinkle

Filamentous green algae

“Irish moss” – a red algae

Species-Species Interactions:

More Questions for Thought & Practice

What does the word succession mean??

There’s

Royal

succession…

Ecological Succession

A series of predictable changes in community structure over time, typically culminating in a mature, relatively stable “climax community.”

Primary Ecological Succession

Primary succession describes the process by which life can colonize virgin territory and turn barren land (

no true soil present

) into a thriving ecosystem, over a period of time.

Pioneer species:

the first species to inhabit an area

Primary Ecological Succession

Secondary Succession

Secondary Succession occurs where

vegetation has been removed

but

soil is intact

(instead of starting with bare rock).

Succession vocab: Disturbance, Pioneer Species, Climax Community

Photo Credits

http://www.world builders.org/lessons/less/biomes/introbiomes.html

http://www.micro.utexas.edu/courses/levin/bio304/ ecosystems/ecology.html

http://www.aloha.com/~lifeguards/portugue.html

Pioneer species :

first species to inhabit an area

Climax Community :

mature, stable community

Secondary Succession occurs where vegetation has been removed but soil is intact.