Ecology

Ecology is the study of the

relationships between organisms and

their physical and biotic environment:

Relationships involve interactions with the physical world as well as interrelationships with other species and individuals of the same species.

O2

Nutrients

CO2

Living organisms can be studied at

different levels of complexity.

From least to most complex, these

levels are (in an ecological

context):

Individual

Population

Community

Ecosystem

Biome

Biosphere

Biological Complexity

Biosphere

Biome

Ecosystem

Community

Population

Individual

The biosphere is the

region within which all

living things are found on

Earth.

It is the narrow belt around

the Earth extending from

the bottom of the oceans

to the upper atmosphere.

The Biosphere

Ecosystems Light intensity varies

Flow rate varies Rainfall level varies

An ecosystem includes:

all of the organisms(the community) …

and their physical environment.

There are many different

sorts of ecosystems from

natural to artificial, and

they range in size from large

to small.

Still water habitatFast flowing water habitat

Rock habitat Stream bank habitat

A community is a naturally occurring group of organisms living together

as an ecological entity; the biological part of the ecosystem.

Communities

A nudibranch snail feeding on rock

encrusting organisms

Abiotic (physical) factors are the

influences of the non-living parts of the

ecosystem.

Examples include pH, salinity, temperature, turbidity, wind speed and direction, humidity, precipitation, water pressure, and light intensity and quality.

Biotic factors are the influences of the

living parts of the ecosystem. Producers and

consumers interact as competitors,

parasites, pathogens, symbionts, and

predators.

Factors Affecting Ecosystems

Physical environment

Community

Abiotic factorsatmosphere,soil,water,wind speed wind direction,current velocity

Biotic factorscompetitors, symbionts, predators, parasites, pathogens

Ecosystem

The physical environment refers

to the physical surroundings of any

organism, including:

the medium, e.g. water

substrate, e.g. soil

climatic (atmospheric) conditions

light …

and other physical properties.

Environments

The type and extent of

vegetation in a particular

ecosystem is determined by

physical factors on both a large

scale and on a very localized

(microclimate) level.

Vegetation patterns are governed largely by climate (which is broadly related to latitude) and altitude.

Climate and Vegetation

Temperate climate

High latitude climate

Tropical evergreen forests are

found in equatorial regions where

total annual rainfall exceeds 250 cm

and the dry season lasts for no

more than 2-3 months. These

forests are species-rich.

The climate is warm and rainy all

year round.

Tropical Rainforests

Rainforest Communities

Dominant plantsTrees and vines

Floral richnessExtremely high; the richest

of all biomes.

Faunal richnessExtremely rich in mammals, birds,

amphibians, and arthropods.

Soil biotaVery rich, but not well known.

Temperature range: 2.2°C

Annual total rainfall: 262 cm

Example: Iquitos, Peru 3°S

Physical Factors in Tropical Rainforests

The high species diversity of

tropical rainforests can be

supported because of the wide

variety of microhabitats provided

by the layered structure of the

forest.

The physical conditions at the

uppermost level are quite

different to those at the forest

floor with respect to light

intensity (and quality), wind

speed, and humidity.

A Tropical Rainforest

Canopy

Subcanopy

Understorey

Ground layer

Rainforest FactorsLight: 70%Wind: 15 kmh-1

Hum: 67%

Light: 50%Wind: 12 kmh-1

Hum: 75%

Light: 12%Wind: 9 kmh-1

Hum: 80%

Light: 6%Wind: 5 kmh-1

Hum: 85%

Light: 1%Wind: 3 kmh-1

Hum: 90%

Light: 0%Wind: 0 kmh-1

Hum: 98%

Light: light intensityWind: wind speedHum: humidity

The ecological niche describes the

functional position of an organism in its

environment.

A niche comprises:

the habitat in which the organism lives.

the organism’s activity pattern: the periods of time during which it is active.

the resources it obtainsfrom the habitat.

Ecological Niche

Adaptations

Physical conditions

Activity patterns

Presence of other organisms

Habitat

The physical conditions influence the habitat in which an organism lives. These

include:

substrate

humidity

sunlight

temperature

salinity

pH (acidity)

exposure

altitude

depth

Each abiotic (or physical) factor may be well suited to the organism or it may present

it with problems to overcome.

Physical Conditions

The law of tolerance states that “For each abiotic factor, an organism has a

range of tolerances within which it can survive.”

Law of Tolerance

Examples of abiotic factors that influence size of the realized niche:

Tolerance range

Optimum range

Unavailable niche

Marginal niche

Num

ber o

f org

anis

ms

Preferred niche

Marginal niche

Unavailable niche

An organism’s habitat is the physical place or environment in which it lives.

Organisms show a preference for a particular habitat type, but some are

more specific in their requirements than others.

Habitat

Lichens are found on rocks, trees, and bare ground.

Most frogs, like this leopard frog, live in or near fresh water, but a few can survive in arid habitats.

An organism’s habitat is not always of a single type. Some organisms occupy a range of

habitats. There are various reasons why:

Highly adaptable in habitat requirements.

Different, but equivalent, resources available in different habitats.

Reduced competition for resources in sub-optimal habitats.

Habitat extremes may influence growth form, especially in plants.

Habitat Range

Dingoes are a highly adaptable

species found throughout

Australia in ecosystems as

diverse as the tropical rainforests

of the north and the arid deserts

in the central Australia.

Within each of these ecosystems, they may occupy a range habitats, each one offering slightly different resources.

Dingo Habitats

A microhabitat describes the precise

location within a habitat where a species

is normally found. It is a small, often

highly specialized, and effectively isolated

location.

The term microhabitat generally applies to invertebrates which do not forage widely.

Example: Within a woodland habitat, woodlice may be found in the microhabitat provided beneath the bark of the rotting wood.

Microhabitats

Woodlouse

An adaptation (or adaptive feature)

is an inherited feature of an

organism that enables it to survive

and reproduce in its habitat.

Adaptations are the end result of the

evolutionary changes that a species

has gone through over time.

Adaptations may be:

behavioral

physiological

structural (morphological).

Adaptations

Osprey: a diurnal bird of prey

Spotted owl: a nocturnal bird of prey

Organisms have adaptations for:

Biorhythms and activity patterns, e.g. nocturnal behavior

Locomotion (or movement)

Defense of resources

Predator avoidance

Reproduction

Feeding

These categories are not mutually exclusive.

Purposes of Adaptations

Structural adaptations: physical features

of an organism, e.g. presence of wings for

flight.

Behavioral adaptations:

the way an organism acts, e.g. mantid

behavior when seeking, capturing, and

manipulating prey.

Functional (physiological) adaptations:

those involving physiological processes,

e.g. the female mantid produces a frothy

liquid to surround and protect the groups

of eggs she lays.

Types of Adaptations

Praying mantis

The adaptations found in plants reflect

both the plant’s environment and the type

and extent of predation to which the plant

is subjected.

Many plant adaptations are concerned with maintaining water balance. Terrestrial plant species show a variety of structural and physiological adaptations for water conservation.

Plants evolve defenses, such as camouflage, spines, thorns, or poisons, against efficient herbivores.

Plant Adaptations

Mangrove Adaptations

Water level at high tide

Prop roots descend from the trunk to provide additional support.

Salt may accumulate in older leaves before they fall.

Specialized root membranes in some mangroves prevent salt from entering

their roots (salt excluders).

Salt glands in the surface layers of leaves secrete salt

(salt excretors).

Cable roots radiate from the trunk. Fine feeding-roots grow off these radial

roots and create a stable platform.

Oxygen diffuses through the spongy tissue of the pneumatophore to the

rest of the plant.

Pneumatophores (breathing roots) arise from the cable roots.

Tropical forest plants live in areas

of often high rainfall. Therefore, they

have to cope with high transpiration

rates.

Tropical Forest Plants

Shallow fibrous root system

Funnel shaped leaves channel rain

Water table high

Water loss by transpiration

Ocean margin plants, e.g. intertidal

seaweeds and mangroves, must cope with

high salt content in the water.

Ocean Margin Plants

Mangrove pneumatophores

Some mangrove species take in brackish water and excrete the salt through glands in the leaves.

Seaweeds growing in the intertidal zone tolerate exposure to the drying air every 12 h.

Structural Adaptations in RabbitsStructural adaptations

Widely spaced eyes gives a wide field of vision for surveillance of the

habitat and detection of danger.

Long, mobile ears enable acute detection of sounds from many angles for predator detection.

Long, strong hind legs andlarge feet enable rapid movement

and are well suited to digging.

Cryptic coloration provideseffective camouflage in

grassland habitat.

Rabbits are colonial mammals that

live underground in warrens and feed

on a wide range of vegetation.

Many of their more obvious

structural adaptations are

associated with detecting

and avoiding predators.

Functional Adaptations in RabbitsFunctional (physiological) adaptations are associated with physiology.

The functional adaptations of rabbits are associated with detecting and avoiding predation, and maintaining populationsdespite high losses.

Functional adaptations

High reproductive rate enables rapid population increases when food is

available.

Keen sense of smell allows detection of potential threats from predators and from rabbits from

other warrens.

Microbial digestion of vegetation in the hindgut enables more efficient

digestion of cellulose.

High metabolic rate and fast response times enables rapid

response to dangers.Hawks are major predators of rabbits

Behavioral Adaptations in RabbitsThe behavioral adaptations of rabbits

reflect their functional position as herbivores

and important prey items in many food webs. Behavioral adaptations

Freeze behavior when startled reduces the possibility of detection by

wandering predators.

Thumps the ground with hind legs to warn others in the warren of

impending danger.

Lives in groups with a well organized social structure that facilitates

cooperative defense.

Burrowing activity provides extensive underground habitat as refuge from

predators.

Freezing is a typical behavior when threatened

Competition describes the active

demand between two or more

organisms for a resource.

Competition may be:

Intraspecific: between individuals of the same species.

Interspecific: between individuals of different species.

Each competitor is inhibited in some

way by the interaction.

Competition

Interspecific competition on a reef

Intraspecific competition: hyaenas

Competition affects the size of a competitor’s

realized niche.

The effect is dependent on the intensity and type of

the competition.

Niches are narrower with moderate interspecific competition (Fig. 1).

Intense interspecific competition results in a

very narrow realized niche as species specialize

to exploit a narrower range of resources (Fig. 2).

Intense intraspecific competition results in a

broader realized niche as individuals are forced to

occupy suboptimal conditions (Fig. 3).

Competition and Niche Size

Fig. 1

Fig. 2

Fig. 3

Narrower niche

Broader niche

Possible tolerance range

Realized niche of species

Gause’s competitive exclusion principle states:

“two or more resource-limited species, having identical patterns of resource use, cannot coexist

in a stable environment:

one species will be better adapted and will out-compete or otherwise eliminate the other(s)”.

If two species compete for some of the same resources (e.g. food items of a particular

size), their resource use curves will overlap. In the zone of overlap, interspecific

competition is the most intense.

Gause’s Principle

Zone of overlap

Species B

Resource use as measured by food item size

Am

ount

eat

en

Species A

Interspecific competition is usually less intense than intraspecific competition because

niche overlap between species is not complete.

Species with similar ecological requirements may reduce competition by exploiting different

microhabitats within the ecosystem.

Example: Ecologically similar damsel fish at Heron Island, Queensland, Australia exploit different resources or regions over the coral reef.

Niche Differentiation

Sea levelReef crest

Pw Pomacentrus wardiPf Pomacentrus flavicaudaPb Pomacentrus bankanensisSa Stegastes apicalisPl Plectroglyphidodon lacrymatusEf Eupomacentrus fasciolatusEg Eupomacentrus gascoyneiGb Glyphidodontops biocellatus

In the eucalypt forests of eastern Australia different bird species forage at different heights in the forest.

This selective foraging behavior reduces niche overlap between species that might otherwise compete directly.

Competition in Eucalypts

Key to bird speciesYellow-throated scrubwrenBrown thornbill

Spine-tailed swift

Striated thornbill

Leaden flycatcher

Ground thrush

Rufous fantail

White-throated treecreeper

Ys

Bt

Sw

Lf

St

Gt

Rf

Wt

Organisms do not generally live alone. A

population is a group of organisms from

the same species occupying in the same

geographical area.

This area may be difficult to define

because:

A population may comprise widely dispersed individuals which come together only infrequently, e.g. for mating.

Populations may fluctuate considerably over time.

Populations

Migrating wildebeest populationMigrating wildebeest population

Tiger populations comprise Tiger populations comprise widely separated individualswidely separated individuals

Populations are dynamic and exhibit

attributes that are not shown by the

individuals themselves.

These attributes can be measured or

calculated and include:

Population size: the total number of organisms in the population.

Population density: the number of organisms per unit area.

Population distribution: the location of individuals within a specific area.

Features of Populations 1

Features of Populations 2Population composition provides

information relevant to the dynamics of

the population, i.e. whether the

population is increasing or declining.

Information on population composition

(or structure) includes:

Sex ratios: the number of organisms of each sex.

Fecundity (fertility): the reproductive capacity of the females.

Age structure: the number of organisms of different ages.

The study of changes in the size and

composition of populations, and the factors

influencing these changes, is population

dynamics.

Key factors for study include:

Population growth rate: the change in the total population size per unit time.

Natality (birth rate): the numberof individuals born per unit time.

Mortality (death rate): the number of individuals dying per unit time.

Migration: the number moving into or out of the population.

Population Dynamics

Population size is influenced by births…Population size is influenced by births…

……and deathsand deaths

MigrationMigration is the movement of organisms

into (immigration) and out of

(emigration) a population. It affects

population attributes such as age and sex

structure, as well as the dynamics of a

population.

Populations lose individuals through deaths and emigration.

Populations gain individuals through births and immigration.

Migrating species may group together to form large mobile populations

WildebeestWildebeest

Canada geeseCanada geese

The number of individuals per unit area

(for terrestrial organisms) or volume

(for aquatic organisms) is termed the

population density.

At low population densities, individuals are spaced well apart.

Examples: territorial, solitary

mammalian species such as tigers

and plant species in marginal

environments.

At high population densities, individuals are crowded together.

Examples: colonial animals, such

as rabbits, corals, and termites.

Population Density

High density populations

Low density populations

A crude measure of population density tells us

nothing about the spatial distribution of individuals in

the habitat.

The population distribution describes the location

of individuals within an area.

Distribution patterns are determined by the

habitat patchiness (distribution of resources) and

features of the organisms themselves, such as

territoriality in animals or autotoxicity in plants.

Individuals in a population may be distributed

randomly, uniformly, or in clumps.

Population Distribution

More uniform distribution in cacti

Clumped distribution in termites

A population’s distribution is considered random if

the position of each individual is independent of the

others.

Random distributions are not common; they can

occur only where:

The environment is uniform and resources are

equally available throughout the year.

There are no interactions between individuals or

interactions produce no patterns of avoidance

or attraction.

Random distributions are seen in some invertebrate

populations, e.g. spiders and clams, and some

trees.

Random Distribution

Spider populations appear to show a random distribution

Uniform or regular distribution patterns occur

where individuals are more evenly spaced

than would occur by chance.

Regular patterns of distribution result from

intraspecific competition amongst members of

a population:

Territoriality in a relatively homogeneous environment.

Competition for root and crown space in forest trees or moisture in desert and savanna plants.

Autotoxicity: chemical inhibition of plant seedlings of the same species.

Uniform Distribution

Saguaro cacti compete for moisture and show a uniform distribution

Clumped distributions are the most common in

nature; individuals are clustered together in

groups.

Population clusters may occur around a a resource

such as food or shelter.

Clumped distributions result from the responses of

plants and animals to:

Habitat differences

Daily and seasonal changes in weather and

environment

Reproductive patterns

Social behavior

Clumped Distribution

Sociality leads to clumped distribution

Calculating Population Change

Births, deaths, and net migrations determine the numbers of individuals in a

population

Emigration (E)

Births (B) Immigration (I)

Deaths (D)

Rates of Population ChangeEcologists usually measure the rate of

population change.

These rates are influenced by

environmental factors and by the

characteristics of the organisms themselves.

Rates are expressed as:

Numbers per unit time,e.g. 2000 live births per year

Per capita rate (number per head of population),e.g. 122 live births per 1000 individuals (12.2%)

Many invertebrate populations increase rapidly in the right conditions

Large mammalian carnivores have a lower innate capacity for increase

Populations becoming established in a new

area for the first time are often termed

colonizing populations.

They may undergo a rapid exponential (logarithmic) increase in numbers to produce a J-shaped growth curve.

In natural populations, population growth rarely

continues to increase at an exponential rate.

Factors in the environment, such as available

food or space, act to slow population growth.

Exponential GrowthColonizing Population

Here the number being added to the population per unit time is large.

Exponential (J) curve Exponential growth is sustained only when there are no constraints from the environment.

Here, the number being added to the population per unit time is small.

Lag phas

ePo

pula

tion

num

bers

(N)

Time

Logistic GrowthAs a population grows, its increase will slow, and it will stabilize at a level that

can supported by the environment.

This type of sigmoidal growth produces the logistic growth curve.

Environmental resistance increases as the population

overshoots K.

Environmental resistance decreases as the population

falls below K.

Established Population

Carrying capacity (K)The population density that can be

supported by the environment.

The population tends to fluctuate around an 'equilibrium level'. The fluctuations are caused by variations in the birth rate and death rate as a result of the population density exceeding of falling below carrying capacity.

In the early phase, growth is exponential (or nearly so)

Lag phase

Logistic (S) curveAs the population grows, the rate of population increase slows, reaching an equilibrium level around the carrying capacity.

Popu

latio

n nu

mbe

rs (N

)

The population encounters resistance to exponential growth as it begins to fill up the environment. This is called environmental resistance.

Time

Two parameters govern the logistic growth of populations.

The intrinsic rate of natural increase or biotic potential. This is the maximum reproductive potential of an organism, symbolized by the letter r.

The saturation density orcarrying capacity of theenvironment, representedby the letter, K.

We can characterize

species by the relative

importance of r and K

in their life cycles.

‘r’ and ‘K’ Selection

r-selected speciesThese species rarely reach carrying capacity (K). Their populations are in nearly exponential growth phases for much of the year. Early growth, rapid development, and fast population growth are important.

K-selected speciesThese species exist near asymptotic density (K) for most of the time. Competition and effective use of resources are important.

Time

Popu

latio

n nu

mbe

rs (N

)

r-Selected SpeciesSpecies with a high intrinsic capacity for

population increase are called r-selected

or opportunistic species.

These species show certain life history features and, to survive, must continually invade new areas to compensate for being displaced by more competitive species.

Opportunists include algae, bacteria, rodents, many insects, and most annual plants.

Climate Variable and/or unpredictable

Mortality Density-independent

Survivorship Often type III(early loss)

Populationsize

Fluctuates wildly. Often below K.

Competition Variable, often lax. Generalist niche.

Selectionfavors

Rapid development, high rm, early

reproduction, small body size, single

reproduction (annual)

Length of life Short, usually less than one year

Leads to: Productivity

K-Selected SpeciesSpecies that are K-selected exist

under strong competition and are

pushed to use available resources

more efficiently.

These species have fewer

offspring and longer lives.

They put their energy into

nurturing their young to

reproductive age.

K-selected species include most

large mammals, birds of prey,

and large, long-lived plants.

Climate Fairly constant and/or predictable

Mortality Density-dependent

Survivorship Usually types I and II(late or constant loss)

Population sizeFairly constant in time. Near equilibrium with

the environment.

Competition Usually keen.Specialist niche.

Selection favors

Slower development, larger body size,

greater competitive ability, delayed

reproduction, repeated reproductions

Length of life Longer (> one year)

Leads to: Efficiency

No organism exists in isolation. Each participates in interactions with other organisms and

with the abiotic components of the environment.

Species interactions may involve only occasional or indirect contact (predation or

competition) or they may involve a close association between species. Symbiosis is a

term that encompasses a variety of such close associations, including parasitism (a form

of exploitation), mutualism, and commensalism.

Species Interactions

Oxpecker birds on buffaloCanopy tree with symbionts attached

Types of Interaction

ParasitismMany animal taxa have representatives that have

adopted a parasitic lifestyle.

Parasites occur more commonly in some taxa than in others. Insects, some annelids, and flatworms have many parasitic representatives.

Parasites live in or on a host organism. The host

is always harmed by the presence of the parasite,

but it is not usually killed. Both parasite and host

show adaptations to the relationship.

Parasites may live externally on a host as

ectoparasites, or within the host’s body as

endoparasites.

Tick ectoparasite on bird wing

Many birds and mammals use dust bathing to rid themselves of external parasites

Mutualistic relationships occur between

some birds (such as oxpeckers) and

large herbivores (such as zebra, Cape

buffalo, and rhinoceros). The herbivore

is cleaned of parasites and the oxpecker

gains access to food.

Lichens are an obligate mutualism

between a fungus and either a green

alga or a cynobacterium. The fungus

obtains organic carbon from the alga.

The alga obtains water and nutrient

salts from the fungus.

Mutualistic Relationships

Lichen: an obligate mutualism

Cape buffalo and oxpecker birds

In commensal relationships, one party (the

commensal) benefits, while the host is

unaffected.

Epiphytes (perching plants) gain access to

a better position in the forest canopy, with

more light for photosynthesis, but do no

harm to the host tree.

Commensal anemone shrimps

(Periclimenes spp.) live within the tentacles

of host sea anemones. The shrimp gains

protection from predators, but the anemone

is neither harmed nor benefitted.

Commensal Relationships

Competition is one of the most familiar of species

relationships. It occurs both within (intraspecific)

and between (interspecific) species.

Individuals compete for resources such as food,

space, and mates. In all cases of competition, both

parties (the competitors) are harmed to varying

extents by the interaction.

Neighboring plants compete for light, water, and

nutrients. Interactions involving competition

between animals for food are dominated by the

largest, most aggressive species (or individuals).

Competition

Intraspecific CompetitionEnvironmental resources are finite. Competition within species for resources increases as

the population grows. At carrying capacity (K), it reduces the per capita growth rate to

zero.

When the demand for a resource (e.g. water, food, nesting sites, light) exceeds supply, that

resource becomes a limiting factor.

Animals compete for resources such as water (left) or mates (right), especially when these are in short supply or access to them is restricted.

Most predators have more than one prey species, although one may be preferred. As one

prey species becomes scarce, predation on other species increases (prey switching), so

the proportion of each prey species in the predator’s diet fluctuates.

Where one prey species is the principal food item, and there is limited opportunity for prey

switching, fluctuations in the prey population may closely govern predator cycles.

Predator-Prey Interactions

The Role of Prey SwitchingVertebrate predators rarely control

their prey populations. Prey species

tend to show regular population

cycles in response to other factors

and predators track these cycles.

Predators usually have a preferred

prey species, but will switch to other

prey when that species is rare.

Generalist predators can maintain

stable populations by prey switching

in response to changing prey

densities.

Voles are the preferred prey of red foxes, but they will take other prey as well

Brown bears are true generalists and feed according to availability

Predator-Prey CyclesMammals frequently exhibit marked population cycles of high and low density that

have a certain, predictable periodicity.

Regular trapping records of the Canada lynx over a 90 year period revealed a cycle of

population fluctuations that repeated every 10 years or so (below). These oscillations

closely matched, with a lag, the cycles of their principal prey item, the snowshoe hare.