The Living World Fourth Edition GEORGE B. JOHNSON Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display PowerPoint

The Living WorldFourth Edition

GEORGE B. JOHNSON

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

PowerPoint® Lectures prepared by Johnny El-Rady

32 Populations andCommunities


32.1 Population Growth

A population is a group of individuals of the same species living together

Critical properties of a population include

Population sizeThe number of individuals in a population

Population densityPopulation size per unit area

Population dispersionScatter of individuals within a population’s range

Population growthHow populations grow and the factors affecting growth


Assumes a population is growing without limits at its maximal rate

Rate is symbolized r and called the biotic potential

The Exponential Growth Model

Growth rate = dN/dt = riN No. of individuals in a population

Intrinsic rate of increase

Change over time

The actual rate of population increase is

r = (b – d) + (i – e)

Birthrate Deathrate Net immigration

Net emigration


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

Carrying Capacity


As a population approaches its carrying capacity, the growth rate slows because of limiting resources

The Logistics Growth Model

Growth rate begins to slow as N approaches K

It reaches 0 when N = K

Fig. 32.2

dN/dt = rN K – NK

( )

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

Fig. 32.3


32.2 The Influence ofPopulation 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


Effects that are dependent on population size and act to regulate growth


Density-dependent effects

These effects have an increasing effect as population size increases Song

sparrow

Fig. 32.4

Reproductive success decreases as population size

increases


The goal of harvesting organisms for commercial purposes is to maximize net productivity

The point of maximal sustainable yield lies partly up the sigmoid curve


Maximizing population productivity

Fig. 32.5


32.3 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

K-selected adaptations

Populations experience competitive logistic growth

Favor reproduction near carrying capacity


Most natural populations exhibit a combination of the r/k adaptations


32.4 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 structureCohort = A group of individuals of the same age

Has 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 ratioThe proportion of males and females in a population

The number of births is usually directly related to the number of females


Survivorship curves

Type I

Mortality rises in postreproductive years

Type II

Mortality constant throughout life

Type III

Mortality low after establishment Fig. 32.7

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


32.5 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


32.6 The Niche and Competition

A niche is the particular biological role of an organism in a community

It is a 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

Fig. 32.9 Competition among two species of barnacles limits niche use


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

Fig. 32.10


However, P. caudatum declined to extinction when grown with P. aurelia

Fig. 32.10

Gause formulated the principle of competitive exclusion

No two species with the same niche can coexist

But is one competitor always eliminated?

No, as we shall soon see!

The two shared the same realized niche and the latter was better!


P. caudatum and P. bursaria were able to 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

Fig. 32.10

The two have different realized niches and thus avoid competition


Persistent competition is rare in natural communities

Either one species drives the other to extinction

Or natural selection reduces the competition between them

Resource Partitioning

Fig. 32.11

Five species of warblers subdivided a niche to avoid direct competition with one another


Sympatric species occupy same geographical area

Avoid competition by partitioning resources

Allopatric species do not live in the same geographical area and thus are not in competition

Sympatric species tend to exhibit greater differences than allopatric species do

Character displacement facilitates habitat partitioning and thus reduces competition



Fig. 32.12 Character displacement in stickleback fish


Feeds on plankton

Feeds on both resources

Feeds on larger prey


32.7 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

Fig. 32.14

Ants and Aphids

Ants transport the aphids and protect them from predators

Aphids provide the ants with food in the form of continuously excreted “honeydew”


Fig. 32.15

Ants and Acacias

Symbiotic relationship in which both species benefit

Mutualism

Acacias provide the ants with food in the form of Beltian bodies

Beltian body

Ants provide the acacias with organic nutrients and protect it from herbivores and shading from other plants


Fig. 32.16a

Symbiotic relationship that is a form of predationThe predator (parasite) is much smaller than the prey

The prey does not necessarily die

Parasitism

External parasitesEctoparasites feed on the exterior surface of an organism

Dodder is a chlorophyll-less parasitic plant

Parasitoids are insects that lay eggs on living hosts

Wasps


Cuckoo

Meadow pipit

Internal parasites

Brood parasitismBirds lay their eggs in the nests of other species

Brood parasite

Foster parent

Fig. 32.16

Sarcocystis

Endoparasites live within the bodies of vertebrates and invertebrates

Marked by much more extreme specialization than external parasites

Brood parasites reduce the reproductive success of the foster parent hosts


Symbiotic relationship that benefits one species and neither harms nor benefits the other

Commensalism

Clownfishes and Sea anemonesClownfishes gain protection by remaining among the anemone’s tentacles

They also glean scraps from the anemone’s food

Fig. 32.17


Fig. 32.18

Cattle egrets and African cape buffalo

Note:

No clear distinction between commensalism and mutualism

Difficult to determine if second partner benefits at all

Indeed, the relationship maybe even parasitic

Egrets eat insects off of the buffalo


Fig. 32.20

32.8 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!



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

1. Food plantsWillow and birch twigs

2. PredatorsCanada lynx (Lynx canadensis)

Fig. 32.21a



Fig. 32.21b



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


32.9 Plant and Animal 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

The Evolutionary Response of Herbivores

These insects were able to use a new resource without competing with other herbivores for it

Cabbage butterfly caterpillars

Fig. 32.23

Adult

Green caterpillar


Some animals receive an added benefit from eating plants rich in secondary chemical compounds

Caterpillars of monarch butterflies concentrate and store these compounds

Animal Defenses

They then pass them to the adult and even to eggs of next generation

Birds that eat the butterflies regurgitate them

Fig. 32.24

Blue jay

I’m not eating this again!


Fig. 32.26

Dendrobatid frog

Defensive coloration

Cryptic colorationColor that blends with surrounding

Aposematic colorationShowy color advertising poisonous nature

Fig. 32.25

Inchworm caterpillar

Camouflage!

Warning!

Chemical defensesStings – Bees and wasps

Toxic alkaloids – Dendrobatid frogs


32.10 Mimicry

Many non-poisonous species have evolved to resemble poisonous ones with aposematic coloration

Two types of mimicry have been identified

Batesian mimicryAfter Henry Bates, a 19th century British naturalist

Müllerian mimicryAfter Fritz Müller, a 19th century German biologist


A harmless unprotected species (mimic) resembles a poisonous model that exhibits aposematic coloration

Batesian Mimicry

If the mimics are relatively scarce, they will be avoided by predators

Monarch butterfly

Fig. 32.27

Viceroy butterfly


Two or more unrelated but protected (toxic) species come to resemble one another

Müllerian Mimicry

Thus a group defense is achieved

Yellow jacket

Fig. 32.28

Masarid wasp

Sand wasp Anthidiine bee


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


32.11 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


Three dynamic critical concepts

1. ToleranceFirst to come are weedy r-selected species that are tolerant of the harsh abiotic conditions

2. FacilitationHabitat changes are introduced that favor other, less weedy species

3. InhibitionHabitat changes may inhibit the growth of the species that caused them

Why Succession Happens


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

Why Succession Happens


32.12 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

Latitude


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

Ecosystem Size


The number of species in the tropics is far more than that in the arctic region

Latitude

Fig. 32.32

Two principal reasons

1. Length of growing season

2. 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 sizeLarger islands have more species than smaller ones

Distance from mainlandDistant islands have less species than those near the mainland


Fig. 32.33 The equilibrium model of island biogeography

Equilibrium

Shifting equilibrium

Small distant islands have fewer

bird species

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The Living World Fourth Edition GEORGE B. JOHNSON Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display PowerPoint