1. Lecture 27 Populations, Communities, & Ecosystems

2. What is Ecology? Evolution and ecology are two key concepts
Evolution: Changes that occur in organisms’ traits over time
Ecology: How organisms live in their environment
The great diversity of life on earth is the result of evolution
And evolution can be said to be the consequence of ecology over time
The term was coined by Ernst Haeckel (1866)
Thus, ecology is the study of how organisms interact with their environment

3. 5 Levels of Ecological Organization
Populations
Individuals of the same species living together
Communities
Populations of different species living together
Ecosystems
Combination of communities and associated non-living factors
Biomes
Major terrestrial assemblages of organisms that occur over wide geographical areas
The Biosphere
All biomes together with marine and freshwater assemblages

4. Population Growth
A population is a group of individuals of the same species living together
Critical properties of a population include
Population size
The number of individuals in a population
Population density
Population size per unit area
Population dispersion
Scatter of individuals within a population’s range
Population growth
How populations grow and the factors affecting growth

5. The Exponential Growth Model
Assumes a population is growing without limits at its maximal rate
Rate is symbolized by r and called the biotic potential
The actual rate of population increase is

6. The Logistics Growth Model
No matter how fast populations grow, they eventually reach a limit
This is imposed by shortages of important environmental factors
Nutrients, water, space, light
The carrying capacity is the maximum number of individuals that an area can support
It is symbolized by k
As a population approaches its carrying capacity, the growth rate slows because of limiting resources
The logistic growth equation accounts for this
A graphical plot of N versus t (time) gives an S-shaped sigmoid growth curve
History of a fur seal population on St. Paul Island, Alaska

7. The Influence of Population Density
Density-independent effects
Effects that are independent of population size but still regulate growth
Most are aspects of the external environment:
Weather: droughts, storms, floods
Physical disruptions: Fire, road construction
Density-dependent effects
Effects that are dependent on population size and act to regulate growth
These effects have an increasing effect as population size increases

8. The Influence of Population Density
Maximizing population productivity
The goal of harvesting organisms for commercial purposes is to maximize net productivity
The point of maximal sustainable yield or optimal yield lies partly up the sigmoid curve

9. Life History Adaptations
Life history = The complete life cycle of an animal
Life histories are diverse, with different organisms having different adaptations to their environments
r-selected adaptations
Populations favor the exponential growth model
Have a high rate of increase
Followed by rapid decrease
K-selected adaptations
Populations experience competitive logistic growth
Favor reproduction near carrying capacity
Most natural populations exhibit a combination of the r/k adaptations

10. Survivorship Curves
Provide a way to express the age distribution characteristics of populations
Survivorship is the percentage of an original population that survives to a given age
Type I
Mortality rises in postreproductive years
Type II
Mortality constant throughout life
Type III
Mortality low after establishment

11. Population Demography
Greek demos, “people”
Demography is the statistical study of populations
Greek graphos, “measurement”
It helps predict how population sizes will change in the future
Growth rate sensitive to:
Age structure
Sex ratio
Cohort = A group of individuals of the same age with a characteristic
Birth rate or fecundity
Number of offspring born in a standard time
Death rate or mortality
Number of individuals that die in that period
The relative number of individuals in each cohort defines a population’s age structure
The proportion of males and females in a population
The number of births is usually directly related to the number of females

12. Population Pyramids
A population’s age structure and sex ratio can be used to assess its demographic trends

13. Human Populations
Throughout most of our history, human populations have been regulated by
Food availability
Disease
Predators
Two thousand years ago, the human population was ~ 130 million
It took one thousand years for it to double
And another 650 years for it to double again
Starting in the 1700s, technological changes gave humans more control over their environment
These changes allowed humans to expand the carrying capacity of their habitats
Currently, the human population is growing at a rate of ~ 1.3% annually
Doubling time at this rate is only 54 years!
Human population growth is not uniform

14. Communities
All organisms that live together in an area are called a community
The different species compete and cooperate with each other to make the community stable
A community is often identified by the presence of its dominant species
The distribution of the other organisms may differ a good deal; however, the ranges of all organisms overlap

15. The Niche and Competition
A niche is the particular biological role of an organism in a community
Habitat  place
Niche  pattern of living
Competition is the struggle of two organisms to use the same resource
Interspecific competition occurs between individuals of different species
Intraspecific competition occurs between individuals of a single species
Because of competition, organisms may not be able to occupy their fundamental (theoretical) niche
Instead, they occupy their realized (actual) niche

16. Competitive Exclusion
In the 1930s, G.F. Gause studied interspecific competition among three species of Paramecium
P. aurelia; P. caudatum; P. bursaria
All three grew well alone in culture tubes
However, P. caudatum declined to extinction when grown with P. aurelia
The two shared the same realized niche and the latter was better!
P. caudatum and P. bursaria were able to coexist
The two have different realized niches and thus avoid competition

17. Resource Partitioning
Gause formulated the principle of competitive exclusion
No two species with the same niche can coexist
Gause’s principle of competitive exclusion can be restated
No two species can occupy the same niche indefinitely
When niches overlap, two outcomes are possible
Competitive exclusion or resource partitioning
Persistent competition is rare in natural communities
Either one species drives the other to extinction
Or natural selection reduces the competition between them

18. Resource Partitioning
Sympatric species occupy same geographical area
Avoid competition by partitioning resources
Sympatric species tend to exhibit greater differences than allopatric species do
Character displacement facilitates habitat partitioning and thus reduces competition
Allopatric species do not live in the same geographical area and thus are not in competition

19. Coevolution and Symbiosis
Coevolution is a term that describes the long-term evolutionary adjustments of species to one another
Symbiosis is the condition in which two (or more) kinds of organisms live together in close associations
Major kinds include
Mutualism – Both participating species benefit
Parasitism – One species benefits while the other is harmed
Commensalism – One species benefits and the other neither benefits nor is harmed

20. Mutualism Symbiotic relationship in which both species benefit
Ants and Aphids
Aphids provide the ants with food in the form of continuously excreted “honeydew”
Ants transport the aphids and protect them from predators
Ants and Acacias
Acacias provide the ants with food in the form of Beltian bodies
Ants provide the acacias with organic nutrients and protect it from herbivores and shading from other plants

21. Parasitism Symbiotic relationship that is a form of predation
The predator (parasite) is much smaller than the prey
The prey does not necessarily die
External parasites
Ectoparasites feed on the exterior surface of an organism
Parasitoids are insects (wasps) that lay eggs on living hosts
Endoparasites live within the bodies of vertebrates and invertebrates
Marked by much more extreme specialization than external parasites
Brood parasites (birds) lay their eggs in the nests of other species
Brood parasites reduce the reproductive success of the foster parent hosts

22. Commensalism
Symbiotic relationship that benefits one species and neither harms nor benefits the other
Clownfishes and Sea anemones
Clownfishes gain protection by remaining among the anemone’s tentacles
They also glean scraps from the anemone’s food
Cattle egrets and African cape buffalo
Egrets eat insects off of the buffalo
Note: There is no clear distinction between commensalism and mutualism
Difficult to determine if second partner benefits at all
Indeed, the relationship maybe even parasitic

23. Predator-Prey Interactions
Predation is the consuming of one organism by another, usually of a similar or larger size
Under simple laboratory conditions, the predator often exterminates its prey
It then becomes extinct itself having run out of food!

24. Predator-Prey Interactions
In nature, predator and prey populations often exhibit cyclic oscillations
The North American snowshoe hare (Lepus americanus) follows a “10-year cycle”
Two factors involved
Food plants: Willow and birch twigs
Predators: Canada lynx (Lynx canadensis)
Predator-prey interactions are essential in the maintenance of species-diverse communities
Predators greatly reduce competitive exclusion by reducing the individuals of competing species
For example, sea stars prevent bivalves from dominating intertidal habitats
Other organisms can share their habitat
Keystone species are species that play key roles in their communities

25. Plant Defenses
Plants have evolved many mechanisms to defend themselves from herbivores
Morphological (structural) defenses
Thorns, spines and prickles
Chemical defenses
Secondary chemical compounds
Found in most algae as well
Mustard oils
Found in the mustard family (Brassicaceae)
Mustard oils protected plants from herbivores at first
At some point, however, certain insects evolved the ability to break down mustard oil
These insects were able to use a new resource without competing with other herbivores for it
Cabbage butterfly caterpillars

26. Animal Defenses
Some animals receive an added benefit from eating plants rich in secondary chemical compounds
Caterpillars of monarch butterflies concentrate and store these compounds
They then pass them to the adult and even to eggs of next generation
Birds that eat the butterflies regurgitate them
Cryptic coloration: Color that blends with surrounding
Aposematic coloration: Showy color advertising poisonous nature
Chemical defenses
Stings – Bees and wasps
Toxic alkaloids – Dendrobatid frogs

27. Mimicry
Many non-poisonous species have evolved to resemble poisonous ones with aposematic coloration
Batesian mimicry
A harmless unprotected species (mimic) resembles a poisonous model that exhibits aposematic coloration
If the mimics are relatively scarce, they will be avoided by predators
Müllerian mimicry
Two or more unrelated but protected (toxic) species come to resemble one another
Thus a group defense is achieved

28. Self Mimicry
Involves adaptations where one animal body part comes to resemble another
This type of mimicry is used by both predator and prey
Example: “Eye-spots” found in many butterflies, moths and fish

29. A Closer Look at Ecosystems
Ecosystems: the fundamental units of ecology
All organisms in an ecosystem require energy
Almost all energy comes from the sun
Energy flows
Energy is lost at each step of the food chain
This limits the number of steps

30. Nutrients & Chemicals Cycle
Raw materials are not used up when organisms die
They are recycled back into the ecosystem for use by other organisms

31. Biomes
Rainfall and temperature are the two most important factors limiting species distribution
These physical conditions with their sets of similar plants and animals are called biomes

32. Ecological Succession
Succession is the orderly progression of changes in community composition that occur over time
Secondary succession: Occurs in areas where an existing community has been disturbed
Primary succession: Occurs on bare lifeless substrates, like rocks
The first plants to appear from a pioneering community
The climax community comes at the end

33. The Process of Succession
Succession involves three dynamic critical concepts
Tolerance
First to come are weedy r-selected species that are tolerant of the harsh abiotic conditions
Facilitation
Habitat changes are introduced that favor other, less weedy species
Inhibition
Habitat changes may inhibit the growth of the species that caused them
As ecosystems mature, more K-selected species replace r-selected ones
Species richness and total biomass increase
However, net productivity decreases
Thus, agricultural systems are maintained in early successional stages to keep net productivity high

34. Biodiversity
Biologically diverse ecosystems are in general more stable than simple ones
Species richness refers to the number of species in an ecosystem
It is the quantity usually measured by biologists to characterize an ecosystem’s biodiversity
Two factors are important in promoting biodiversity
Ecosystem size
Larger ecosystems contain more diverse habitats and therefore have greater number of species
A reduction in an ecosystem size, will reduce the number of species it can support
Faunal collapse (extinction) may occur in extreme cases
Latitude
The number of species in the tropics is far more than that in the arctic region
Two principal reasons
Length of growing season
Climatic stability

35. Island Biodiversity
In 1967, Robert MacArthur and Edward O. Wilson proposed the equilibrium model
The species richness on islands is a dynamic equilibrium between colonization and extinction
Two important factors
Island size
Larger islands have more species than smaller ones
Distance from mainland
Distant islands have less species than those near the mainland