1. Big Idea 4 - Ecology
Ch. 53 – community ecology

2. Levels of Organization
To study relationships within the biosphere, ecologists have organized it into smaller pieces.
The levels are:
Organism (a single individual – one fish, for example)
Population (when organisms of a single species share the same geographic location at the same time)
Biological community (a group of populations that interact and occupy the same space at the same time)
Ecosystem (a biological community plus all of the abiotic factors that affect it)
ex – trees plus water and soil
5. Biome (a large group of ecosystems that share the same climate and have similar communities)
Ex – marine or desert biomes
6. Biosphere (all the biomes on Earth combined)

3. Communities
A community is a group of organisms of different species that live in a particular area

4. Overview
Ecologists call relationships between species in a community interspecific interactions
Interspecific interactions affect species survival and reproduction
Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and disease

5. Ecological Niches
An organism’s niche is the specific role it plays in its environment
All of its uses of biotic and abiotic resources in its environment
Example: oak tree in a deciduous forest
Provides oxygen to plants, animals, etc.
Home for squirrels
Nesting ground for blue jays
Takes water out of the soil
Etc., etc.
Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

6. Fundamental v. Realized Niche
Fundamental Niche includes resources an organism could theoretically use (if no competition)
Realized Niche includes resources it actually does use given competition from other species.

7. Resource partitioning
Resource partitioning is differentiation of ecological niches
enables similar species to coexist in a community
Similar species develop ways to partition/divide resources in order to coexist.

8. Interspecific Competition
Competition between organisms of different species
occurs when species compete for a resource in short supply
can lead to competitive exclusion (local elimination of a competing species)
The Competitive Exclusion Principle:
Two species with similar needs for the same limiting resources cannot coexist in the same place
Niches may overlap but they may not be identical.

9. Interspecific Interactions
There are 4 major interspecific interactions (symbiotic relationships):
Parasitism (includes predation and herbivory)
Competition
Commensalism
Mutualism

10. Predation (and Parasitism)
(+ -)
The interaction is beneficial to one species and detrimental to the other
Predation:
When a predator eats its prey
Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison
Example in picture:

11. Herbivory
Herbivory refers to an interaction in which an herbivore eats parts of a plant or alga
It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

12. Plant Defenses Against Herbivores
“Plants Fight Back!”
Plants have 2 major mechanisms by which they defend themselves against being eaten
Mechanical Defenses
Thorns, hooks, etc.

13. Plant Defenses Against Herbivores
Chemical Defenses
Produce chemicals that are distasteful or harmful to an herbivore
Morphine (opium poppy)
Nicotine (tobacco)

14. Animal Defenses Against Predators
Animals defend themselves against predators passively (hiding) or actively (fleeing)
Cryptic coloration (camouflage) makes prey difficult to spot
Aposematic coloration (warning coloration)warns predators not to each animals that may be toxic or may sting.

15. Animal Defenses Against Predation
Mimicry
When one species “imitates” or “mimics” another
Batesian mimicry
When one edible or harmless species mimics an bad-tasting (unpalatable) or harmful species
Example: hawkmoth mimics a snake

16. LE 53-7
Green parrot snake
Hawkmoth larva

17. Animal Defenses Against Predation
Mimicry
Mullerian mimicry
Two species, both of which are unpalatable (taste bad) or harmful, resemble each other
Example: monarch butterfly (unpalatable) and queen butterfly (unpalatable) resemble each other

18. Parasitism
One organism (the parasite) gets its nourishment from another organism (the host), which is harmed in the process
Usually do not kill their hosts
Endoparasites:
Live within host tissues (tapeworms)
Ectoparasites:
Feed on external surfaces (mosquitoes)

19. Mutualism (++) relationship
Both partners benefit from the relationship
“You scratch my back, I’ll scratch yours”
Examples:
Ants & acacia tree
tree provides high protein food in beltian bodies & habitat for nests inside thorns; ant protects against predators
Mycorrhizae-fungal extentions on plant roots
Plant gets increased water/nutrition, fungi gets food
Hummingbirds & flowers
Hummingbirds get food, flowers can reproduce

20. Commensalism
Commensalism – one organism benefits and the other is neither helped nor harmed
Ex – shark and remora fish; sea anemones and clownfish

21. Dominant & Keystone Species
In general, a few species in a community exert strong control on that community’s structure
Two fundamental features of community structure are species diversity and feeding relationships
Dominant Species:
Species in a community that have the highest abundance or highest biomass
One hypothesis suggests that dominant species are most competitive in exploiting resources
Another hypothesis is that they are most successful at avoiding predators
American Chestnut tree and the chestnut blight
Keystone Species:
Important to a community because of their ecological roles (niches), not by numbers
In contrast to dominant species, keystone species are not necessarily abundant in a community
Wolves control deer populations
Bees help pollinate local plants
Elephants eat small trees, which preserve grasslands
Prarie dogs eat grass which keeps water in the soil and prevents evaporation

22. LE 53-17 100 80 Otter number (% max. count) 60 40 20
Sea otter abundance
400
300
Grams per
0.25 m2
200
100
Sea urchin biomass 10 8
Number per
0.25 m2 6 4 2
1972
1985
1989
1993
1997
Year
Total kelp density
Food chain before
killer whale
involvement in
chain
Food chain after
killer whales started
preying on otters

23. Ecosystem “Engineers” (Foundation Species)
Some organisms exert influence by causing physical changes in the environment that affect community structure
Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community
For example, beaver dams can transform landscapes on a very large scale

24. With Juncus Without Juncus
LE 53-19 8 6
Number of plant species 4 2
With
Juncus
Without
Juncus
Salt marsh with Juncus
(foreground)
Conditions

25. Ecological Succession
Ecological succession is a change in the species that live in a given area over a period of time
One community replaces another
Primary succession = occurs in places where soil is not yet formed (bare bedrock)
Secondary succession = occurs in places where there is soil, but where some disturbance has eliminated the previous community

26. Ecological Succession
The first organisms to inhabit an area undergoing succession are known as pioneer organisms
These are usually small organisms (bacteria, lichens, algae, etc.)
The ecosystem goes through a number of stages, with each new stage usually consisting of larger organisms than the last one
Once a community has become stable and is not changing much, it is known as a climax community

27. Ecological Succession cont.
Early-arriving species and later-arriving species may be linked in one of three processes:
Early arrivals may facilitate appearance of later species by making the environment favorable
They may inhibit establishment of later species
They may tolerate later species but have no impact on their establishment

28. Causes of Ecological Succession
There are 3 major causes of ecological succession:
Human Activities
- logging, mining, development, etc.
Natural Disasters/Disturbances
- fires, volcanic eruptions, etc.
3. Natural Competition Among Species
- Fictitious example:
- turtles and frogs both eat crickets
- frogs are faster, turtles are slower
- frogs eat more crickets, turtles starve
- turtle population dies out, frog population
gets bigger

29. Species Diversity
Species diversity of a community is the variety of organisms that make up the community
It has two components:
species richness
relative abundance
Species richness is the total number of different species in the community
Relative abundance is the proportion each species represents of the total individuals in the community

30. Species diversity cont.
Two communities can have the same species richness but a different relative abundance
A community with an even species abundance is more diverse than one in which one or two species are abundant and the remainder are rare

31. Trophic Structure
Trophic structure is the feeding relationships between organisms in a community
It is a key factor in community dynamics
Food chains link trophic levels from producers to top carnivores

32. A terrestrial food chain
LE 53-12
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Primary
producers
Plant
Phytoplankton
A terrestrial food chain
A marine food chain

33. Food Webs
A food web is a series of connected food chains with complex trophic interactions

34. Limits on Food Chain Length
Each food chain in a food web is usually only a few links long
Two hypotheses attempt to explain food chain length:
the energetic hypothesis and
the dynamic stability hypothesis

35. Limits on food chain links cont.
The energetic hypothesis suggests that length is limited by inefficient energy transfer
Most data support the energetic hypothesis
The dynamic stability hypothesis proposes that long food chains are less stable than short ones

36. Bottom-Up and Top-Down Controls
The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels
In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers

37. The top-down model proposes that control comes from the trophic level above
In this case, predators control herbivores, which in turn control primary producers

38. Long-term experimental studies have shown that communities can shift periodically from bottom-up to top-down controls
Pollution can affect community dynamics
Biomanipulation can help restore polluted communities

39. What Is Disturbance?
A disturbance is an event that changes a community, removes organisms from it, and alters resource availability
Fire is a significant disturbance in most terrestrial ecosystems
It is often a necessity in some communities

40. LE 53-21 Before a controlled burn.
A prairie that has not burned for several years has a high propor-tion of detritus (dead grass).
During the burn. The detritus serves as fuel for fires.
After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

41. The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster higher diversity than low levels of disturbance
The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

42. LE 53-22
Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.
One year after fire. This photo of the same general area taken the following year indicates how rapidly the com-munity began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

43. Human Disturbance Humans are the most widespread agents of disturbance
Human disturbance to communities usually reduces species diversity
Humans also prevent some naturally occurring disturbances, which can be important to community structure

44. Biogeographic factors affect community diversity
Equatorial-Polar Gradients
Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics
The greater age, climate, and amount of light found in tropical environments may account for the greater species richness

45. Biogeographic factors affect community diversity cont.
Climate is likely the primary cause of the latitudinal gradient in biodiversity
Two main climatic factors correlated with biodiversity are:
solar energy and
water availability
They can be considered together by measuring a community’s rate of evapotranspiration
Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

46. Area Effects
The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species
Species richness on islands depends on island size, distance from the mainland, immigration, and extinction

47. Rivet and Redundancy Models
The rivet model suggests that all species in a community are linked in a tight web of interactions
It also states that loss of even a single species has strong repercussions for the community
The redundancy model proposes that if a species is lost, other species will fill the gap
Community hypotheses and models represent extremes; most communities probably lie somewhere in the middle