1.4 - biological sciences of ecology with brodie

Topic 1: What determines the number of species in a community?

1. Understand the different types of diversity (alpha, beta, gamma), and how diversity varies across latitude

2. Explain the differences between evolutionary and ecological hypotheses for latitudinal gradients in species richness, and why it is so difficult to determine which of these actually generate the gradient

3. Describe the difference between the museum and cradle hypotheses, and explain what evidence could be used to support each

4. Describe (verbally & mathematically) a species-area relationship5. Explain which processes are important for within-region (nested) versus

between-region species-area relationships6. Use Island Biogeography Theory to explain how species richness varies with

island size and isolation7. Explain a dynamic equilibrium8. Distinguish between processes that are neutral with respect to species identity,

and those that are based on species niches9. Contrast dispersal limitation versus biotic limitation of local species richness10.Explain the difference between a species fundamental and realized niche11.Understand the potential influence of competition, facilitation, and predation

on local species richness 12.Design and interpret experiments to test for species interactions & dispersal

limitation.

Topic 1: Learning outcomes

Species-area curves: one of the oldest statistical patterns in ecology

1859: First species-area curve (H.C. Watson)

Log area (square miles)

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Village

Surrey

S. England Great Britain

The relationship between diversity and area

Species richness increases with area, usually in a very predictable way.

WHY????

Several factors that we now know influence diversity are themselves affected by the amount of habitat (i.e. area):

-Resources-Speciation-Extinction-Immigration

Why you should care about

species-area theory

Determining how many species can live in a given sized patch is increasingly crucial in our fragmented habitats

Area -> Amount of resources - > Number of individuals->

Number of species

The number of individuals sets an

upper limit to how many species

can occur because:

1. A species cannot exist if its

population is not large enough

to be viable. (More individuals

hypothesis: REGIONAL SCALE)

2. Sampling more individuals

means a greater chance of

sampling any given species.

(Sampling effect: LOCAL SCALE)

Beetles in decaying wood

Area and resources

What is the effect of area on immigration rate? (At least for passive dispersers)

A. Larger areas are bigger targets for colonists

B. Larger areas have more resources

C. Larger areas have more births

D. Larger areas may be avoided by colonists

E. No effect

Yes, for passive dispersers

Post-immigration process

Post-immigration process

Area and immigration

-Its pretty unlikely that a species will be lost from an island or

habitat patch because they all just leave

-So for now, these effects can be ignored.

Area and emigration

Speciation rate increase with

region area But only on large islands

isolated for long periods of time

Anolis lizards on Caribbean islands:

speciation rate increases on islands

> 3000 km2, where lizards have

been present for the last 30 million

years.

Speciation rate increase with

region area But only on large islands

isolated for long periods of time

Anolis lizards on Caribbean islands:

speciation rate increases on islands

> 3000 km2, where lizards have

been present for the last 30 million

years.

So area effects on speciation are NOT relevant for smaller areas and shorter

timeframes

Extinction rates decrease with island area

Reason 1: Total population size is smaller in small

regions, so declining populations reach zero faster

Reason 2: Declining or extinct populations on small

islands have less chance of being rescued by other

populations

Reason 3: Small islands may be subject to stronger or

more frequent disturbances

Area and extinction

Reason 1: Total population size is smaller in small regions, so

declining populations reach zero faster.

Maximum population size

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Area and extinction

Daphnia

Area and extinctionReason 2: Extinct populations in small areas are less easily

rescued

Lizards absentLizards present

Before hurricane

7 mo. after

Area and extinctionReason 3: Small islands have higher disturbance rates

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Log10 Area

Clicker questionIf you have a group of moss patches of different sizes, and you want

to maximize species richness in the small patches, which strategy

might you employ?

A) Fence off the small patches to prevent emigration

B) Build bridges between patches to increase immigration

C) Reduce the area of the large patches

D) Increase the area of the large patches

E) Increase speciation rate on the small patches

How to make a species-area relationship (SAR)

Step 1: Pick an ecological system

Here we will look at mites living in the soil beneath moss patches


Step 2:Choose what type of SAR you want to construct

Between regions

Within region(nested)


Step 3:Count the number of species in each area

100 cm2

30 species


Step 4:Plot the number of species against the sampled area

Within-region (nested) SAR: plot the cumulative number of species against the cumulative sampled area!

AreaS

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Species = c(Area)z


Species = c(Area)z

Q2. Why do the within-

region and between-

region lines differ?

Q1. Mathematically,

how do the within-

region and between-


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SARs can be plotted in either linear or log-log space, but it can be easier to see the patterns in log-log space

Area

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S = cAz

between

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Log S = log c+ z*log A

between

within

If a SAR has c = 0.2 and z = 0.25 then:

A. Species increase proportionally with area

B. Species decrease with area

C. There are 0.2 species in a region size 0

D. Species increase with area, at a decelerating rate

E. Species increase exponentially with area

Z = 1 : proportional (linear in linear space)

Z= 0 : no relationship

0 < Z < 1: decelerating increase

S = cAz Log S = log c+ z*log A


Q1. Mathematically,

how do the within-

region and between-


Moss patches:

Between-region z = 0.38Within-region z = 0.22


Q2. Biologically, why do

the within- region and

between-region lines

differ?

Q1. Mathematically,

how do the within-

region and between-


?

Between-region z > within-region z

Log Area

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Log S = log c+ z*log A

between

within

60 ha 30 ha10 ha

Within-region SAR

Between-region SAR

100 ha

60 ha

To keep the comparison fair, well set the total amount of area to be equal between the two regions

10 ha-In both cases, the sampling effect and the increase in resources with area are operating

-But in the between-region SAR, there is also immigration at play!

60 ha 30 ha10 ha

Within-region SAR

Between-region SAR

100 ha

60 ha

Thats why the difference between the between- and within-region SARs is greatest for large areas. (In the within-region SAR, large areas cant get immigration from anywhere!)10 ha

Log Area

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27

Discussion questionYoure biologists in charge of creating some nature reserves. Your

goal is to maximize gamma diversity across your reserves. Does it

make more sense to preserve a Single Large reserve or Several

Small reserves (the SLOSS debate)?

Why?

1. Understand the different types of diversity (alpha, beta, gamma), and how diversity varies across latitude

2. Explain the differences between evolutionary and ecological hypotheses for latitudinal gradients in species richness, and why it is so difficult to determine which of these actually generate the gradient

3. Describe the difference between the museum and cradle hypotheses, and explain what evidence could be used to support each

4. Describe (verbally & mathematically) a species-area relationship5. Explain which processes are important for within-region (nested) versus

between-region species-area relationships6. Use Island Biogeography Theory to explain how species richness varies with

island size and isolation7. Explain a dynamic equilibrium8. Distinguish between processes that are neutral with respect to species identity,

and those that are based on species niches9. Contrast dispersal limitation versus biotic limitation of local species richness10.Explain the difference between a species fundamental and realized niche11.Understand the potential influence of competition, facilitation, and predation

on local species richness 12.Design and interpret experiments to test for species interactions & dispersal

limitation.

Topic 1: Learning outcomes

1.4 - biological sciences of ecology with brodie

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