1. Community Ecology
Chapter 54

2. Community
an assemblage of populations of various species living close enough for potential interaction

3. Interspecific Interactions
interactions between species in a community

4. Interspecific Interactions – (SYMBIOSIS)
Competition (-/-)
Predation (+/-) includes parasitism
Mutualism (+/+)
Commensalism
EFFECTS ON POPULATION DENSITY
The interaction is detrimental to both species.
The interaction is beneficial to one species and detrimental to the other.
The interaction is beneficial to both species.
One species benefits from the interaction but the other is unaffected.

5. Competition (–/–)
interspecific competition – occurs when species compete for a particular limited resource
strong competition can lead to the elimination of 1 of the 2 competing species = competitive exclusion
b/c one species will use the resource(s) more efficiently and, thus, reproduce more rapidly than the other
competition affects an organism’s niche = its ecological role (how it “fits into” an ecosystem)
fundamental niche = an organism’s potential niche
realized niche = the niche an organism actually occupies due to competition for limited resources

6. Competitive Exclusion Principle
The concept that when populations of two similar species compete for the same limited resource, one population will use the resources more efficiently and have a reproductive advantage that will eventually lead to the elimination of the other population.

7. The Niche
A niche is the full range of physical and biological conditions in which an organism lives and the way in which the organism uses those conditions
An organism’s niche includes:
Its place in the food web
The range of temperatures that the organism needs to survive
The type of food the organism eats, how it obtains that food, and which other species use the organism as food
When and how the organism reproduces
If an organism’s habitat is its address, its niche is its occupation.
The combination of biotic and abiotic factors in an ecosystem often determines the number of different niches in that ecosystem.
No 2 species can share the same niche in the same habitat, but different species can occupy niches that are very similar. Pg. 92 (warbler niches) 7

8. Niche v/s Habitat
If an organism’s habitat is its address, its niche is its occupation.
The combination of biotic and abiotic factors in an ecosystem often determines the number of different niches in that ecosystem.
No 2 species can share the same niche in the same habitat, but different species can occupy niches that are very similar (competitive exclusion principle). 8

9. resource partitioning
the modification of one species niche that allows two species with identical niches to coexist
character displacement
tendency for characteristics of populations of the same species to be more divergent in sympatric populations (geographically overlapping) than in allopatric populations (geographically separate) b/c of competition

10. Predation (+/–)
relationship in which one species (the predator) kills and eats the other (the prey)
includes seed predators
predators possess adaptations for securing prey
acute senses
claws, teeth, fangs, stingers, poison
prey have adaptations that help them avoid predators
behavioral defenses (hiding, fleeing, self-defense, alarm calls)
cryptic coloration (camouflage)
aposematic coloration (bright warning coloration)
Batesian mimicry (harmless species mimics a harmful one)
Müllerian mimicry (two unpalatable species mimic each other)

11. Herbivory (+/–)
an herbivore eats part of a plant or alga

12. Defense Mechanisms to Predation in Plants
Chemical toxins
Strychnine produced by Strychnos toxifera
Morphine produced by opium poppy
Nicotine produced by tobacco plants
Mescaline produced by peyote cacti
Tannins produced by a variety of plant species
Spines & thorns

13. Parasitism (+/–)
symbiotic relationship in which one organism (the parasite) derives its nourishment from another organism (the host) which is harmed in the process
types of parasites:
endoparasites – live within the body of their host
ectoparasites – feed on the external surface of their host
parasitoidism – insects lay eggs on or in living hosts and, then, the larvae feed on the host

14. Disease (+/–) disease-causing agents = pathogens
typically bacteria, viruses, or protists
most microscopic
many inflict lethal harm

15. Mutualism (+/+)
symbiotic relationship that benefits both species involved
ex: ants & acacia tree

16. Commensalism (+/0)
symbiotic relationship that benefits one of the species involved and neither helps nor harms the other species
difficult to document a true example

17. Co-evolution and Interspecific Interactions
Coevolution refers to reciprocoal evolutionary adaptations of two interacting species.
A change in one species acts as a selective force on another species, in which counteradaptation in turn acts as a selective force on the first species.
There is actually LITTLE evidence for coevolution in most cases of interspecific interactions.

18. Community Structure

19. Species Diversity
the variety of different kinds of organisms that make up a community
two components:
species richness = total # of different species in the community
relative abundance = the proportion of the total # of individuals in the community that each species represents
measuring species diversity is essential for understanding community structure & for conserving biodiversity

20. Trophic Structure
feeding relationships between organisms in a community
food chain = the transfer of food energy up the trophic levels
food chains are usually only a few links long (≤5)
two hypotheses why:
energetic hypothesis – suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain
dynamic stability hypothesis – proposes that long food chains are less stable than short chains because population fluctuations at lower trophic levels are magnified at higher levels
most of the data available support the energetic hypothesis
another factor that may limit food chain length is that animals in a food chain tend to be larger at successive trophic levels
food webs – made up of linked food chains

21. food web

22. Dominant Species
those species in a community that are most abundant or have the highest biomass
exert a powerful control over the occurrence and distribution of other species
ex: sugar maples are the dominant plant species in many North American forest communities

23. Keystone Species
species that are not necessarily the most abundant but exert strong control on community structure by their pivotal ecological roles (niches)

24. Foundation Species
organisms that influence community structure by causing physical changes in the environment that affect the structure of the community
act as facilitators that have positive effects on the survival and reproduction of some of the other species in the community

25. Two Models of Community Organization
bottom-up model
postulates a unidirectional influence from lower to higher trophic levels
nutrient availability  plant #s  herbivore #s  predator #s
thus, to change the community structure, you need to alter biomass at lower trophic levels
top-down model
postulates that it is mainly the predators that control community organization
predators limit herbivores which limit plants which limit nutrient levels
a change at the higher trophic levels moves down the trophic structure as a series of +/– effects (trophic cascade)

26. Disturbance
an event that changes a community, removes organisms from it, & alters resource availability

27. Intermediate Disturbance Hypothesis
suggests that moderate levels of disturbance can create conditions that foster greater species diversity than low or high levels of disturbance b/c:
high levels of disturbance reduce species diversity by creating environmental stresses that exceed the tolerance of many species
frequent disturbances may also exclude slow colonizing or slow growing species
low levels of disturbance can reduce species diversity by allowing competitively dominant species to exclude less competitive species

28. Ecological Succession
in a process called ecological succession, a disturbed area may be colonized by a variety of species which are, in turn, replaced by other species until a climax community is reached
two types:
primary succession – occurs in a virtually lifeless area, where there were originally no organisms and where soil has not yet formed
secondary succession – occurs where an existing community has been cleared by some disturbance that leaves the soil intact

29. Primary Succession Biodiversity increases as succession proceeds:
(plants | animals | decomposers
On land, succession that occurs on surfaces where no soil exists is called primary succession.
Example: on the surfaces formed as volcanic eruptions build new islands or cover land w/ lava and ash or on bare rock exposed when glaciers melt.
Stages:
No soil, just ash and rock
First species to populate is called the pioneer species (often lichen)
As lichen grows, it can break up rock, and when they die, they add organic material to help form soil in which plants can grow 29

30. The Pioneers of Succession
Pioneer species – first individuals to inhabit an area of primary succession – break down rock, erode surfaces, and turn it into soil.
Lichen and moss
Climax Community – the final community of any succession.

31. What causes increase/decrease in biodiversity as succession progresses?
Some populations facilitate biodiversity/succession (by developing conditions more suitable for other species and/or developing conditions less suitable for their progeny.
Some populations inhibit biodiversity/succession (by developing conditions less suitable for other species and/or developing conditions more suitable for their progeny.
Increase in plant stratification (increased layering of plants; ex: canopy, understory.
More niches/habitats formed (plants, animals, decomposers).
Pioneer plant species – dominants (more shade-tolerant plants emerge).
Increase in producer diversity brings about increase in consumer diversity.
Shift from more opportunistic (r) to more equilibrium (K) species.

32. Abiotic Factors Can Cause Succession

33. Successions Can Have Immediate & Long-Term Effects
Examples:

34. Biogeographic Factors
geographic location and size are correlated with
species diversity

35. Size species-area curve
shows that, with all other factors being equal, the larger the geographic area of a community, the greater the number of species
likely explanation is that larger areas offer greater diversity of habitats and microhabitats than smaller ones