Population Biology: A summary

• The term "population growth" refers to how the number of individuals in a population increases (or decreases) with time. This growth is controlled by the rate at which new individuals are added to the population -- the birth rate, and the rate at which individuals leave the population -- the death rate.

• There are many types of plants and animals, and different types show different kinds of population growth.

Population Biology: Geometric and Exponential Growth

• If a population has a constant birth rate and is not limited by food or disease, it grows geometrically or exponentially – the birth rate alone controls

how fast (or slow) the population grows.• Population declines (r < 0) • Population increases (r > 0)

Population does not change (r = 0)

Population Biology: Logistic Growth

• In most populations both food and disease become important as conditions become crowded. There is an upper limit to the number of individuals the environment can support. The carrying capacity (K)

• Populations in this kind of environment show what is known as logistic growth.

                                           

Announcements

• Lectures posted on DISL share site• Sample test questions will be posted next

week (feel free to remind me!)• Midterm Thursday July 24

– Runs from 9AM to 12– Lab notebooks due by 4 PM that afternoon

Population Interactions

• Competition (--) when both species suffer from an association

• Predation (+-) when one benefits and one suffers• Commensalism (+0) when one species benefits

from another and it is unaffected• Amensalism (-0) when one species negatively

affects another and it is unaffected • Mutualism (++) when both species benefit from

another

Competition

Liebig’s Law of the minimum

• Guiding principle: Liebigs's Law of the Minimum• The distribution of a species will be controlled

by that environmental factor for which the organism has the narrowest range of adaptability or control.

• The growth of a population of organisms increases until the supply of a critical resource becomes limiting

Liebig’s Law of the Minimum: an example

• “growth of phytoplankton is dependent on the minimum amount of nutrients/light present”

• whatever is in shortest supply will limit (and may stop) growth – nitrate, silica, phosphate, or iron limitation – light limitation

• if a nutrient (or light) is at low levels, it may be limiting growth

Limiting Resources: an example

Space is a limiting resource to these colonies of colonial ascidians

Intraspecific competition: Background

• Individuals of same species

• Shared resource demands and use of a limiting resource- food, shelter, mates

• As individuals compete for resources some deprived- results in reduced fitness, reflected by lower growth, fecundity and survivorship

Intraspecific competition: an example

Patella cochlear

Intraspecific Competition

• Limpets

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Intraspecific Competition (Self Thinning)

• Reduced individual fitness

• Reduced body size

• Increased mortality

Gause’s Competitive Exclusion Principle

When two species compete for identical resources, one will be more successful and will eventually eliminate the other

Forms of Interspecific Competition

• Interference (Contest) Competition– access to a resource is limited or denied by

the dominant species– examples include antibiotics secreted by

microorganisms, or territorial behavior• Exploitative (Scramble) Competition

– the direct use of a resource before a competitor can use it, thus reducing its availability simply by elimination

Nature of the interspecific competitive interaction

• Direct competition- Interference competition– Dominate resource – Need for space - e.g. Sessile & territorial– Prevent use by others, “winner takes all”

• Indirect competition- Exploitation competition – Competitors can not dominate the resource – Reduces resource availability by using it up– Resource is “shared” - no outright “winner”

Competition

• Where do you see evidence of competition in the oceans?

– Overgrowth– Aggressive behavior

• What’s the result of interspecific competition?– Dominance or monopoly by a single species in a

given habitat– Competitive exclusion

Persistence among competing species

• Behavioral acclimations- learn to feed when competitors are not present

• Character displacement-through time two closely related species tend to be more distinct morphologically and therefore use different portions of limiting resources

• Change in habitat utilization

Competition in unusual forms

• Overgrowth competition-one species overgrows a second species– Some corals and sponges

• Chemicals are used to defend access to a shared resource– Big impacts on settlement– Allelopathy in some sponges

How to assess competitive effects

• Measure: – Resource dynamics – Number of competing individuals

• Disadvantageous impacts on: – survival rate– growth rate– adult weight – fecundity

How do we measure competition in the field?How do we measure competition in the field?

1. Observe the patterns of distribution and see if they conform to predictions of competition theory1. Observe the patterns of distribution and see if they conform to predictions of competition theory

white shrimpwhite shrimpbrown shrimpbrown shrimp

Estuarine salinity gradientEstuarine salinity gradient

One conclusion is that white and brown shrimp are in competition with each other (competition theory would predict this distribution)One conclusion is that white and brown shrimp are in competition with each other (competition theory would predict this distribution)

Weak argument: there are multiple explanations for this distributionWeak argument: there are multiple explanations for this distribution

Low High

Classic study in Experimental EcologyClassic study in Experimental Ecology

Connell (1961)

Experiments with intertidal barnacles Connell (1961)

Experiments with intertidal barnacles

Observations

Two species – Chthamalus stellatus and Balanus balanoides (now Semibalanus balanoides)

Chthamalus adults in upper zone, juveniles in both upper and lower zone.

Balanus in lower zone

Observations

Two species – Chthamalus stellatus and Balanus balanoides (now Semibalanus balanoides)

Chthamalus adults in upper zone, juveniles in both upper and lower zone.

Balanus in lower zone

ObservationsObservations

Chthamalus juvenilesChthamalus juveniles

Chthamalus adultsChthamalus adults

BalanusBalanus

ModelModel

Ha: Competition for space with Balanus prevents adult Chthalamus from occurring in the lower area

Ha: Competition for space with Balanus prevents adult Chthalamus from occurring in the lower area

H0: When present together Balanus has no effect on Chthalamus

H0: When present together Balanus has no effect on Chthalamus

ExperimentExperiment

• Transplanted stones with Chthamalus to lower level

• Followed settlement of Balanus, removed them from one half of each stone

• Recorded the fate of individual barnacles

• Transplanted stones with Chthamalus to lower level

• Followed settlement of Balanus, removed them from one half of each stone

• Recorded the fate of individual barnacles

BalanusBalanus

ChthamalusChthamalus

ResultsResults• Chthamalus survival was much greater where Balanus was excluded.

• Most Chthamalus killed by being overgrown or undercut by Balanus

• Chthamalus survival was much greater where Balanus was excluded.

• Most Chthamalus killed by being overgrown or undercut by Balanus

Experimental conclusions• Balanus

– upper limit set by physical environment– lower limit set by Thais predation

• Chthamalus – upper limit probably set by physical environment– lower limit set by interspecific competition

• Asymmetry

growthrate

Location in intertidal zone

low highmiddle

Chthamalusalone

competitive release – niche of the competitively-inferior species expands in the absence of the

competitively-superior species

fundamental niche

realized niche

Chthamalus withBalanus

competitiverelease

Character displacement

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• When two species occur in sympatry natural selection should favor the evolution of mechanisms that reduce competition if resources are limiting

• This often takes the form of character displacement, where the two competing species diverge in a trait that reduces the strength of interspecific competition

Use

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Resource

Character displacement: mud snails

Hydrobia ventrosa Hydrobia ulvae A

A

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The importance of intra- and interspecific competition

• Can have strong negative impact on the population growth of inferior competitor

• Reduces the geographic distribution of competing species

• Alter evolutionary trajectories.

The Niche Concept

• Niche - the role of a species in a community, defined in practice by measuring all possible resources used and tolerance limits; Elton (1927) and Hutchinson (1958).

• Niche Breadth - The amount of a resource used by an organism; this amount may change when new species are introduced or removed from a community

The Niche Concept (cont)

• Fundamental: all the environmental factors relevant to a species survival and reproduction. – Also called the n-dimensional hypervolume

• Realized: The actual set of factors observed to determine the density of a species.

Niches and Types of Species

• Generalist species have large niches; tolerate wide range of environmental variations; these do better during changing environmental conditions

• Specialist species have narrow niches; they are more likely to become endangered; these do better under consistent environmental conditions

Diet breadth

consumes only one prey type

consumes many prey types

broad diet

narrow diet

specialist

generalist

Niche Breadth (width or size)

• Some plants and animals are more specialized than others, and measures of niche breadth attempt to measure this quantitatively

• It is typically measured by observing the distribution of individual organisms within a set of resource states

• Information is collected and presented in a resource matrix

Niches and Natural Selection

Region of niche overlap

Generalist specieswith a broad nicheSpecialist species

with a narrow nicheNiche

breadth

Nicheseparation

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Resource use

Measuring an Ecological Niche

• To measure the niche, we must determine all the biotic and abiotic factors that influence a species’ abundance.

• Such factors can be plotted against species abundance on graphs. The axes are called niche axes.

• Niche breadth can be visualized on such a niche axis graph.

Competition

• Intraspecific competition – Competition between members of the same species;

all species exhibit this

• Interspecific competition – Competition between members of different species.

Limiting Factors: Some additional examples

• Classes of abiotic variables• Direct effect • Temperature • Wind • Salinity • Resources (indirect effect) • Nutrients • Shelter • Light

Interspecific Competition

• Between different species

• Both have equal access but one exploits better

• Some individuals control access

Fundamental niche depends on physical (abiotic) conditions.

Realized niche depends on biotic as well as abiotic conditions.

What is the realized niche of each barnacle?What is the fundamental niche of each?

growthrate

Location in intertidal zone

low highmiddle

How can we determine the realized niche of each barnacle?

Where do they grow when allowed to compete?

Balanusrealized

niche Chthamalus realized niche

Balanus and

Chthamalus

Liebig’s Law: Continued

• A limiting factor is a factor that restricts the fundamental niche of an organism– temperature is one example- animals and plants are all limited

to some temperature zone of tolerance (tropical, temperate species)

• Snook (Centropomus undecimalis) northern limit is at 15 oC isotherm

• Pinfish (Lagodon rhomboides) cannot tolerate low temperature (12 oC C)

– currents - marine invertebrate larvae are carried by currents to suitable settlement sites; if carried to an inappropriate area by the current, the larvae die

– salinity - copepod species differ in salinity tolerances

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