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

2. 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

3. 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

4. Mutualism
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.
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.

5. Commensalism
Commensalism is a symbiotic relationship in which one species benefits and the other species is not affected
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.
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).

6. Right: Ergot is a fungus that grows on grains such as barley, breaking down the grain and getting food from it.
Parasitism
Parasitism is a symbiotic relationship in which one species benefits (parasite) and the other species is harmed (host).
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.

7. 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.

8. Species - Species Interactions
Direct Effect on Species 1
Direct Effect on Species 2
Neutral relationship
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

9. “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)

10. 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…

11. 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…)

12. 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.

13. more Competitive Exclusion
There once were two species of barnacle that inhabited the same rock…
For the story, visit:
• Potential niche v. Realized niche

14. Species-Species Interactions: Practice Questions

15. 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.

16. 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

17. 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.

18. 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

19. 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)

20. 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

21. 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!

22. 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

23. Species-Species Interactions: More Questions for Thought & Practice

24. What does the word succession mean??

25. There’s Royal succession…

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

27. 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

28. Primary Ecological Succession

29. 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

30. Photo Credits http://www. world-builders

31. Pioneer species: first species to inhabit an area
Climax Community: mature, stable community

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