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Lecture 27 Populations, Communities, & Ecosystems. 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 - PowerPoint PPT Presentation
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Lecture 27Populations, Communities, & Ecosystems
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
5 Levels of Ecological Organization
1. Populations Individuals of the same species living together
2. Communities Populations of different species living together
3. Ecosystems Combination of communities and associated non-living factors
4. Biomes Major terrestrial assemblages of organisms that occur over wide
geographical areas
5. The Biosphere All biomes together with marine and freshwater assemblages
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
Assumes a population is growing without limits at its maximal rate Rate is symbolized by r and called the biotic potential
The Exponential Growth Model
The actual rate of population increase is
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
The Logistics Growth Model
History of a fur seal population on St. Paul Island, Alaska
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
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
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
Survivorship Curves
Type I Mortality rises in
postreproductive years
Type II Mortality constant throughout
life
Type III Mortality low after establishment
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
Population Demography
Demography is the statistical study of populations
Greek demos, “people”
Greek graphos, “measurement”
It helps predict how population sizes will change in the future Growth rate sensitive to:
Age structure Sex ratio
Age structure 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
Sex ratio The proportion of males and females in a population
The number of births is usually directly related to the number of females
Population Pyramids
A population’s age structure and sex ratio can be used to assess its demographic trends
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
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
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
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
Competitive Exclusion
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
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
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
Resource Partitioning
Allopatric species do not live in the same geographical area and thus are not in competition
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
Symbiotic relationship in which both species benefit
Mutualism
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
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
Parasitism
Symbiotic relationship that benefits one species and neither harms nor benefits the other
Commensalism
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
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!
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
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
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
Animal Defenses
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
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
Self Mimicry
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
Nutrients & Chemicals Cycle
Raw materials are not used up when organisms die
They are recycled back into the ecosystem for use by other organisms
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
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
Succession involves three dynamic critical concepts
1. Tolerance First to come are weedy r-selected species that are tolerant of
the harsh abiotic conditions2. Facilitation
Habitat changes are introduced that favor other, less weedy species
3. 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
The Process of Succession
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
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