Community ecologyLecture 9

Principles of EcologyCollege of Forestry, Guangxi University

Eben Goodale

Let’s discuss the midterm exam

• Average grade was 80• 3 A (93+)• 1 A- (90-93)• 7 B+ (85-90)• 3 B (80-85)• 4 C+ (75-80)• 2 C (70-75)• 1 D+ (65-70)• 3 D (60-65)• 1 F (< 60)

Discussion about Isack and Reyes (1989)

• This is a fascinating （迷人的） mutualism（共栖） .

• What data do they show to establish that both sides (humans, honeyguides) benefit?

• What data do they show to establish that honeyguides communicate （联系） information about the hive （蜂巢） to humans?

Back to ourlevels slide

Characteristics of Individuals?

Cold resistant, drought resistant, etc.

Also, life history strategies (long life, many offspring) apply to individual.

Back to ourlevels slide

Characteristics of populations?

Abundance, distribution, demography.

Life history traits also apply (the averagevalues of the individuals in the population)

Back to ourlevels slide

Species interactions:-/ - Examples?+ / - Examples?+ / + Examples?

Community

Today’s lecture

• What is a community?– How do we know how many species?– Species relative abundance and diversity– Interactions of more then two species– Special species

• How do communities change over time?– What is a community: a debate

A community is an association （群组） of interacting species inhabiting （栖息） some defined area.

A coral reef community

What is a community?

• Is a community a group of species working together, a “super-organism”（超有机体） ?

• Are community types very distinct （不同） from each other?

• This view held by the ecologist Clements.

Frederick Clements (1874-1945)

#s ofindivi-duals

Environmental gradient （环境梯度）

CommunityType A Type B Type C

What is a community?• Or do communities grade

into each other （平缓地） , without obvious types.

• In this view, the members of a community do not work together.

• It just happens that they live together at a certain time and place.

• This view held by the ecologist Gleason.

Henry Gleason (1882-1975)

#s ofindivi-duals

Environmental gradient

No community types

Generally, ecologists today are more on the side of Gleason, although there are some cases of discrete community types.

Back to ourlevels slide

Characteristics of the community:1) Number of species2) Species relative abundances（相对多度）3) Species diversity （多样性） (1 + 2)

Moving on to the community:

Species number

G. Evelyn HutchinsonYale scientistIn 1958, write “A Homage to Santa Rosalia, Or Why are There So Many Kinds of Animals?”

In limestone caves below a reliquary (church) in Palermo, Italy, Hutchinson found 2 species of waterbugs. Why weren’t there 20 species, or 200?

Species number

• How many species are there?

To determine species number, need to sample

Sampling effort

Species number

These kind of curvesCalled species accumulation（积累） curves.

Idea is that after a certain Amount of time sampling, you get less and less return in your effort (fewer new species), so should stop.

We want to see the line cometo an asymptote （渐近线） (level off).

To determine species number, need to sample

Which group of species here has the largest number?

Relative abundance of species

Species even in abundance

Species uneven in abundance

This relationship can be shown on a rank-abundance curve

Curves for Actual communities

Community a Community b

Abundance rank (#1, #2, #3…)

Proportionalabundance

Relative abundance of speciesGenerally, in any typical communitythere are somevery rare speciessome very abundantspecies, and mostspecies are average

Note that this axis is logarithmic （对数）and thus this pattern is called the “log-normal” distribution

Frank Preston 1896-1989

Spent his life as a engineer for glass manufactureBut also wrote 4 very influential scientific papers

Lognormal distributions of forest birds:real data

Frank Preston 1896-1989

Spent his life as a engineer for glass manufactureBut also wrote 4 very influential scientific papers

Relative abundance of species

B. Shannon-Wiener Index （多样性指数）

ips

ieipH

1log

Proportion of theith species

naturallogarithm

H Number of speciesin the community

Species Diversity Indexes

Claude Shannon1916-2001.Mathematician famous for informationtheory

species # %

log (%)

% log (%)    

1 21 0.84 -0.17 -0.15    

2 1 0.04 -3.22 -0.13    

3 1 0.04 -3.22 -0.13    

4 1 0.04 -3.22 -0.13    

5 1 0.04 -3.22 -0.13    

  25     -0.66 H' = 0.661

species #          

1 5 0.2 -1.61 -0.32    

2 5 0.2 -1.61 -0.32    

3 5 0.2 -1.61 -0.32    

4 5 0.2 -1.61 -0.32    

5 5 0.2 -1.61 -0.32    

  25     -1.61 H' = 1.609

ips

ieipH

1log

I expect you to be able to calculate this index.

Dealing with more than 2 species

Interactions can be linear…or circular.

(linear)

(circular)

Dealing with more than 2 species

There can be indirect interactions.

Trophic cascade （营养级）

Dealing with more than 2 species

Food webs （食物网） and interaction webs show interactions between many species.

Food webs and trophic cascades

Can get very complicated! Estes et al. 1998

Primary reading for Next lecture: “Ecological meltdown（危机） in predator free forest fragments”

What happens tocommunitieswhen big catsremoved?

Special Species （特有种）• Species that have a major role in a community

– Dominant species.– Keystone species.– Ecosystem engineers.

In network （网状） structure,some species may bemore important than others.

Special Species• “Dominant” species

– Are those species in a community that are most abundant or have the highest biomass （生物量）

– Exert powerful control over the occurrence and distribution of other species

Redwood forest

A salt marsh

A mangroveforest

Special Species• Keystone species （关键种） : one that exercises a

large amount of influence on the community compared to its biomass.

Sea otters: topof the trophic cascade

Fig trees:fruit for animalsthoughout the year.

Pisaster starfish:By predation keepsdiversity in system(lowers numbers ofgood competitors)

Special Species

• Foundation species act as facilitators that have positive effects on the survival and reproduction of some of the other species in the community.

• Ecological engineers （工程师） exert their influence by causing physical changes in the environment that affect community structure.

Salt marsh with Juncus (foreground)

With Juncus

Without Juncus

Num

ber o

f pla

nt

spec

ies

0

2

4

6

8

Special Species

The beaver asa ecosystem engineer

Today’s lecture

• What is a community?– How do we know how many species?– Species relative abundance and diversity– Interactions of more then two species– Special species

• How do communities change over time?

Are the physical and biological characteristics of a community static?

Community Dynamics （群落动态）

• Famous example of dunes near a lake.

• Sand continually blown ashore and gradually dunes get bigger.

• Walking away from the coast one encounters older and older communities.

Are the physical and biological characteristics of a community static?

Community Dynamics

• Or, glacier （冰川） retreats over time.

• Walking away from glacier one encounters older and older communities.

• How do communities respond to disturbance?

What is a disturbance （干扰） ?

Where do humanactivities fit?

Mt St. Helens 1980

What is a disturbance?

Primary vs. secondary succession（初级和次级演替）

• Primary succession: Colonization of habitats devoid of life (e.g., volcanic rock).

• Secondary succession: Reestablishment of a community in which some, but not all, organisms have been destroyed.

An old field: secondary succession

After a glacier: primary

What does succession lead to?

• Clements: succession is like the development of an organism with a birth, a middle age and an end stage.

• The end stage is a “climax community （顶级群落）” .

Frederick Clements (1874-1945)

Recently retreated glacier

Dryas mat (~ 30 years)

Alder thicket (50 yrs)

Spruce forest (~ 100 yrs)

What does succession lead to?

But actually not so simple: there can be multiple different kinds of climax communities depending on environmental conditions

Mature hemlock forest (~ 200 yrs)Climax community on slopes

Muskeg (~ 200 yrs)Climax community in wetlands

And how does succession progress?

• Clements: the different species help each other.

• For example, the first “pioneer （先锋种）” plants help make soil richer for what comes later.

Frederick Clements (1874-1945)

• Facilitation （简易化） can occur: early organisms help later ones.

• Tolerance （忍受力） can occur: early organisms don’t have much of an effect on later ones, but later ones live longer.

• Early organisms might even inhibit （抑制） later ones under some circumstances

Connell and Slatyer 1977

But again perhaps more complicated

But again perhaps more complicated

In general, facultative interactions dominate early in succession,And inhibitive late in succession.

Sometimes succession gets “stuck （困住 )”

Fernlands inSri Lankastop succession

This is when typical succession is altered, so that never get to climax community. Some parts of community act to inhibit establishment of others.

Coral reefovergrown by algae

Without interventionthese regime shiftscan be permanent（永恒） .

This particularlyhappens due to human disturbanceand is called a “regime shift”.

Over time, community grews more complex

More species More nutrients

Disturbances range in Frequency （频率 ), intensity （强度）

and extent （范围）

• So far we have talked about large disturbances.

• But small disturbances, like large trees falling over in the rainforest are also important.

Disturbances range in frequency, intensity and extent

• My wife Uromi studies the pioneer trees that live in tree fall gaps.

• Just like weeds, these trees like light, grow fast, and have many small seeds.

• In contrast, “late-successional（推迟演替）” species are more tolerant （忍耐） of shade.

• Tree fall gaps create a mosaic（镶嵌） of places in the forest of different successional ages.

Disturbances also different time scales

Glacier bay

Old temperate field

Inter-tidal succession

1.5 years Sycamore Creek60 days

~ 1500 years

~ 150 yrs

Where we are going

• Today we talked about how to measure species diversity, and how it changes in one place over time.

• On Tuesday Apr 28 we talk about how species diversity varies across big regions (high near equator) and in islands of different sizes (a famous theory called “island biogeography”).

• On Saturday May 2 we talk about how species diversity varies within a region. Why is it higher in some communities than others?

Island Biogeography Theory

Heterogeneous Habitat Theory

Homework

• Chapter 18 summary• Primary reading: Terborgh et al. 2001. Prepare

to discuss in class next lecture.

Key concepts

• We use species accumulation curves to estimate how many species.

• Species differ in their abundances; species diversity indices combine the number of species with their abundance.

• Some species may particularly important role in a community.

• An old viewpoint is that a community is a super-organism made up of species that work together.

• It is true that a community has a predictable succession after disturbance, and that facilitation (species helping each other) is common early in this process.

• But it also true that there are multiple endpoints possible, and that species can also not help (tolerate) or even inhibit each other.

Tips for listening, reading

Listening:http://www.artsci.utoronto.ca/current/advising/ell/pdfs/ELL_Strengthening_Listening_Comprehension.pdf

Reading to write:http://www.artsci.utoronto.ca/current/advising/ell/pdfs/Reading_to_Write_Previewing.pdf

OK, so communities differ in species diversity … but why?

• We will come back to this question, but today just introduce an idea called “heterogeneous environmental theory”.

• First developed by Robert MacArthur• Came out of the idea of niches, and the

competitive exclusion theory• MacArthur generally worked with birds

Robert MacArthurStudent of HutchinsonDied at age of 42 in 1972

Heterogeneous Environments:Opportunities for Specialization

Remember that eachspecies has a niche –Its occupation.

And no two species can have the same niche“competitive exclusion principle”.We talked about this when talking about competition

In these birds thatMacArthur studiedeach species uses differentpart of the tree.

So it makes sense thatthe higher the canopyof forests, the morespecies there will be.This was MacArthur’sfirst result (1958)

Further experiments demonstrate that vertical heterogeneity increases species

diversity

MacArthur hung a ropevertically from the top of the canopy and measuredhow many times vegetationhit the rope.

From this he calculatedvertical profiles of differentforests, and compared themin their diversity (# of different layers and evennessof layers).

MacArthur and MacArthur 1961

Further experiments demonstrate that vertical heterogeneity increases species

diversity

MacArthur and MacArthur 1961

He found that the more diversethe vertical vegetationthe more speciesof birds were present

Species diversityIs a fascinating subjectand we will look at itIn more detail laterIn the course.