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

• Mutualism is a symbiotic relationship in which both species _benefit .

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

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.

Above & Below: A Clown fish makes its

home and hides from its predators in an

anemone and in return keeps the anemone clean from parasites

and safe from its predators.

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).

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

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

Commensalism• Commensalism is a

symbiotic relationship in which one species benefits and the other species is not affected

Parasitism• Parasitism is a symbiotic

relationship in which one species benefits (parasite) and the other species is harmed (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.

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.

Right: Ergot is a fungus that

grows on grains such as

barley, breaking down

the grain and getting food

from it.

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 InteractionsInteraction Direct

Effect on Species 1

Direct Effect on Species 2

Neutral relationship 0 0

Predation of Sp2 by Sp1

Mutualism of Sp1 & Sp2 *

Commensalism of Sp1 w/ Sp2 *

Parasitism by Sp1 on Sp2 *

Competition betw. Sp1 & Sp2

* Symbiotic relationship

+ -

+ +

+ 0

+ -

- -

“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 ExclusionThere 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 WolfEx2: 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 wordsuccession mean??

There’s Royal succession…

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

Primary Ecological SuccessionPrimary 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 SuccessionSecondary 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

Sec

onda

ry S

ucce

ssio

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