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FROM Wednesday - end of lecture oncomparative life histories:A laboratory evolution experiment - effectsof different rates of adult mortality on lifehistory traits in Drosophila (fruit flies)

Question: In environments where there ishigh adult mortality (e.g. high predation)what happens to other aspects ofDrosophila life history?

Experimenters set up selection lines - in someremoved adults 2 x weekly (“High adultmortality treatment”), in others didn’t (“Lowadult mortality treatment”) kept all otherfactors constant

Kept selection up from 1993-1998

Some of their predictions: When adultmortality is high, populations shouldevolve

Higher fecundity early in lifeShorter development timesSmaller adult size

Effect of high adult mortality onamount of early reproduction:Which treatment reproduces earlier? Do

they do it from the start of theexperiment?

High mortalityLow mortality

Effect of high adult mortality on larvaldevelopment time:Which treatment has a shorter larval

development time?

Months

HighmortalityLowmortality

Effect of high adult mortality on bodysize:Which treatments’ adults are smaller?

Months

High mortalityLow mortality

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SummaryLife history evolution studies show some

mechanisms by which animal and plantpopulations diversify in response to theirenvironment.

Some patterns found are general to manyorganisms. For example, the manipulativeselection experiment in Drosophila showsa pattern seen in other descriptiveexperiments:

High adult mortality leads to earlierreproduction, shorter development time,and smaller body size

Population regulation andcompetition

I. Population regulationDensity-dependent and density-independent factors

II. Intraspecific competitionIII. Introduction to species interactions

Types of interactionsThe ecological niche

IV. Interspecific competitionCompetitive displacement, exclusion,and dominance

A naturalist observed swifts in his village inEngland...

"I am now confirmed in the opinion that wehave every year the same number of pairsinvariably; at least the result of my inquiry hasbeen exactly the same for a long time past.The number that I constantly find are eightpairs, about half of which reside in the church,and the rest in some of the lowest andmeanest thatched cottages. Now as theseeight pairs - allowance being made foraccidents - breed yearly eight pairs more,what becomes annually of this increase?"

White 1778

In 1983, a couple of naturalistsreturned to the same village…

Fewer thatched cottages, no swifts in thechurch, but a total of twelve pairs overall. A netchange of 4 pairs of swifts in 205 years...

We are likely to focus on dramaticchanges in population densities:

increases such as locusts, gypsymoths, lemmings, or humans

decreases such as buffalo, condors, passenger pigeons

But most populations are, like theswifts, remarkably constant

So, what regulatespopulations? i.e. keeps themwithin certain limits?

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We can categorize the factors thatinfluence populations as

density-dependent factors

and density-independent factors

What’s a density-dependent factor?

What’s a density-dependent factor?

A factor that increases in intensityas density (population size per unitarea) increases.

Specifically, this means theproportion of the populationaffected increases with density.

A (made-up) example.We want to know if flooding acts asa density-dependent mortalityfactor of a certain grass growing ina wash.

From 1989 to 1995 we go out eachyear after the monsoon, count thenumber of grass plants that arealive, and the no. dead fromflooding. Then we plot our data.

Is flooding a density-dependentmortality factor acting on thegrass?

No.dead

Density (population size)

‘88

‘89

‘90

‘91

‘92‘94

‘95

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Because we expect a larger numberto die as the population sizebecomes large. If we change the yaxis to proportion dead...

Prop.dead

Density (population size)

‘88‘89

‘90 ‘91‘92

‘94‘95

We can see there is no increase in proportionalmortality with an increase in density.

If a factor is density-dependentit should affect the population inthis way...

Prop.dead

Density (population size)

Why do we care about density-dependent factors?

Only density-dependent factors canregulate populations. Other factorscan have effects on populationdynamics, but don’t predictably‘rein in’ populations at highdensities.

Factors which may act in a density-dependent manner?

Competition -intraspecific (within species)

interspecific (between species)

Factors which may act in a density-dependent manner?

Disease caused by pathogensContagious diseases are bettertransmitted at high density

Factors which may act in a density-dependent manner?

Predation and herbivorypredators may be more likelyto find abundant prey types

predator reproduction greaterwhen abundant food

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Factors which are generally density-independent?

Weather - rain, wind, drought,freezes

Fire

Density-independent mortality factorscan be important in a population’sdynamics, even if they are notregulating. Example: winter mortality ofherons

No. ofnestingpairs

5000

Year‘28 ‘32 ‘36 ‘40 ‘44 ‘48 ‘52 ‘56 ‘60 ‘64 ‘68

= Severe winters

Experimental demonstration of density-dependent parasitism: Goldenrod gall fly

Adult lays eggs in stems of goldenrod

Larval fly induces the formation of a gall

Fly overwintersin gall

Most importantmortality agent aparasitic waspthat also spendsthe winter in thegall

Goldenrod gall fly - an experimentAll the galls in several fieldsgathered at the end of a season,put in an unheated shed for thewinter

In the spring, galls put at one of twodensities back into fields

Low density fields

High density fields

Goldenrod gall fly - an experiment

Experimenter measured the parasitism of the new season galls in the high and low density fields

If parasitism density-dependent, whatwould pattern would you expect to see?

Goldenrod gall fly - an experiment

Results: Greater proportion parasitized inhigh density fields than in low densityfields

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II. Intraspecific competition

Competition reduces thecontribution of the competitors tothe next generation throughincreased death rates or decreasedbirth rates

Intraspecific competition increasesdeath rates. Example: Winter moth

Numbers of eggs in spring

Prop

ortio

n su

rviv

ing

Not a winter moth

Intraspecific competition reduces birthrates - example in longhorncattle

Youngperfemale

0 110Density of longhorns

1.0

0.8

III. Species interactions - anintroduction

Species interactions can beclassified according to the neteffect on the species involved,positive or negative

Mutualisms are +/+ interactions -both partners benefit

Example: Pollinators and flowersmore examples to come in nextlecture...

In commensalism one partnerbenefits, and the other is unaffected(+/0)

Example - cattle egrets feed on insects that cattle dislodge. Cattle areunaffected by the egrets.

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In amensalism one partner is harmed,and the other is unaffected (-/0)

Examples - Large animals may negativelyinfluence plants near water holes. Largerherbivores may incidentally consumesmaller herbivores like insects, etc.

In predation and parasitism one partnerbenefits, and the other is harmed (+/-)

Predation Parasitism

In competition, both partners areharmed (-/-)

For example, gilawoodpeckers andstarlings compete for nestcavities

Exploitative vs. interferencecompetition

Exploitative competition:competition through exploitation ofa common resource. Winners ofcompetition appropriate more ofthe resource.

Exploitative competition: Example

Competition between trees for light

Seedlings

Maturetrees

Slight disadvantageof some seedlings(e.g. late germination,slower growth rate)becomes exaggeratedas lose access to light.

* * C. Exploitative vs. interferencecompetition

Interference competition:Competitors directly interfere witheach other. The victor is the onethat wins the contest.

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Interference competition. Example:

Young larvae of parasitic wasps are often equipped with large mandibles used to kill competitors in the same insect host.

Interactions with other species mayaffect a species’ ecological niche.

What’s a niche?

The ecological niche

Many definitions in the literature

A simple one: The niche is the sumof the ecological requirements of aparticular species. E.g., habitatrequirements, food requirements,climatic requirements, etc.

Two kinds of niches…

The fundamental niche - that whicha species could use in the absenceof predation or competition

Two kinds of niches…

The fundamental niche - that whicha species could use in the absenceof predation or competition.

The realized niche - what thespecies uses when predators andcompetitors are present

Two kinds of niches…

Which is likely to be bigger? Thefundamental niche or the realizedniche?

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Niches in the presence ofinterspecific competition - a classicexample from the intertidal regionon the coast of Scotland

Intertidal region -area between highand low tides - sometimes underwater, sometimesdry

Interspecific competition inbarnacles

vs.

Barnacles (crustaceans, Arthropoda) start life as free-swimming larval forms. They then settle onrocks and grow in size by molting, but do not move.

Researcher found twospecies:

Balanus - covered bywater most of the time

Chthamalus - exposedmost of the time

From different areas heremoved all adults of oneor other species, andcontinued to remove theirsettling larvae.

Results of manipulation

Effects on Balanus - the one covered bywater most of the time

Distribution didn’t change whenChthamalus removed - couldn’t colonizearea ordinarily occupied by Chthamalusbecause too dry

Results of manipulation

Effects on Chthamalus - the one exposedmost of the time

When Balanus removed, Chthamaluscolonized the entire region. OrdinarilyBalanus would out-compete Chthamalusin the lower regions by growing over them.

So what does this have to do withniches?

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So what does this have to do with niches?

Balanus

Chthamalus

So what does this have to do with niches?

The niche occupied by Balanus is unaffected by ChthamalusThe niche occupied by Chthamalus is reduced in thepresence of Balanus

The barnacle study provides anexample of competitivedisplacement: one species nichewas contracted in the presence ofthe other.

What do you think would havehappened to Chthamalus (theexposed species) if both specieshad the same fundamental niche?

What do you think would havehappened to Chthamalus (theexposed species) if both specieshad the same fundamental niche?

Chthamalus would have becomeextinct wherever Balanus was.

The principle of competitiveexclusion: when two speciesoccupy the same fundamentalniche, they cannot coexist.

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Competitive exclusion: Anexample from the Sonoran desert

Graduate student at the U of Alooked at competition for seedsamong seed feeders

1) Competition among rodents

Put fences around areas with holesthat excluded kangaroo rats,allowed smaller rodents to pass

Competitive exclusion: An examplefrom the Sonoran desertIn plots where the kangaroo rats wereexcluded:

There were more species of smallrodents, and they were more abundant

So, kangaroo rats competitively excludeother rodents

Copyright (c) Grolier Interactive Inc.

Kangaroo rat

Competitive dominance. Perhapsmost commonly, competition leadssimply to the greater abundance ofone species than the other.

Competitive dominance. Anotherexample of competition for seeds inthe desert.

2) Are ants competing with rodentsfor seeds?

Harvester antwith large seed

2) Are ants competing with rodentsfor seeds?

Removed ants from some plots,rodents from others, kept some ascontrol plots.

Rodents Ants Controlremoved removed plots

No. antColonies 543 0 318

No. ofRodents 0 144 122

2) Are ants competing with rodentsfor seeds?

Yes. Both kinds of seed predatorsare more abundant when the otherkind is removed.

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What species dominates in competitionmay depend on the environment. Anexample with flour beetles.

Experimenter put 2 species of beetlestogether at 29°C - A and B

Days

No.beetles

At 29At 29°C, A wins

A

B

Experimenter put the same 2 species ofbeetles together at 32°C - A and B

No.beetles

Days

At 32°C, B wins

B

A

Why the reversal in outcome? A grows faster thanB at 29°C, and B grows faster than A at 32°C