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