Community Ecology

I. Introduction

A. Definitions of Community

- broad: a group of populations at the same place and time

“old-hickory community”

Community Ecology

I. Introduction

A. Definitions of Community

- broad: a group of populations at the same place and time

“old-hickory community”

- narrow: a “guild” is a group of species that use the same resources in the same way.

Community Ecology

I. Introduction

A. Definitions of Community

- broad: a group of populations at the same place and time

“old-hickory community”

-narrow: a “guild” is a group of species that use the same resources in the same way.

-complex: communities connected by migration or energy flow

complex: communities connected by migration or energy flow

Dragonflies eat pollinators and reduce plant reproduction rates. Fish reverse these effects, increasing plant reproduction.

I. Introduction

A. Definitions of Community

B. Development of the Community Concept

Clements: “superorganism” concept

Discrete transitions between communities

Development of the ‘climax’ community through time, like development of an organism to the ‘mature’ adult

I. Introduction

A. Definitions

B. Development of the Community Concept

Clements: “superorganism” concept

Gleason: “individualistic” concept

“Communities” are just assemblages of species that happen to have overlapping ranges; but they are responding independently to the environment.

Whittaker – ’70’s:

Species respond independently to environmental gradients, but steep gradients will create abrupt transitions between community types.

For example, the transition in community type at a ‘serpentine boundary’.

Serpentine soils have very high chromium, nickel, and magnesium.

There is usually an abrupt change in soil concentrations, creating an abrupt change in community type.

Where environmental changes are more gradual, community transitions will be less abrupt, too.

For example, although each species is most abundant under certain moisture and elevation conditions in the Smokies, they co-occur over a wide range of conditions. Only their relative abundances may vary.

I. Introduction

A. Definitions

B. Development of the Community Concept

Clements: “superorganism” concept

Gleason: “individualistic” concept

Whittaker: gradient analysis

Pickett and White: Patch-Dynamic Theory

Variation in community type may NOT just be a function of changes in environmental conditions; it may be function of changes in disturbance regime, time since the last disturbance, and successional stage of the community.

Difference between pine and oak communities may not be due to moisture; it could be due to time since last fire.

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors – what is measured and compared?

1. Species RichnessHabitat 1 Habitat 2

species A

species B

Richness

50 1

2 2

50 99

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors

1. Species Richness

2. Species Diversity

- relative abundance

- Diversity Indices

Simpson’s = 1/Σ(pi)2

Habitat 1 Habitat 2

species A

species B

Richness

Simp. Div.

50 1

2 2

2 1.02

50 99

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors

1. Species Richness

2. Species Diversity

- relative abundance

- Diversity Indices

Simpson’s = 1/Σ(pi)2

3. Membership

- species list

Habitat 1 Habitat 2

species A

species B

Richness

Simp. Div.

50 1

2 2

2 1.02

50 99

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors

1. Species Richness

2. Species Diversity

- relative abundance

- Diversity Indices

Simpson’s = 1/Σ(pi)2

3. Membership

4. Trophic Relationships

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors4. Trophic Relationships

- Food webs: define trophic relationships between species/taxa

- quantities:

nodes: species or 'trophic species' (ate and eaten by same group of species) (S)

links: connections between trophic species (L)

connectance: C = L/(S(S-1)/2) tends to be constant across webs with different richness... this is really the observed links over the maximum number of links possible (S(S-1)/2)

4. Trophic Relationships - Common Trophic Patterns:As species richness increases, the number of trophic levels tends to increase and the number of guilds tends to increase. But the links/species stays about the same for a given community type.

- Omnivory is rare- ‘Loops’ are rare

Community Ecology

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors

D. Conceptual Models

D. Conceptual Models

1. Elton - numerical and biomass pyramids

E. Conceptual Models

1. Elton - numerical and biomass pyramids

Numerical and biomass pyramids can be "inverted":

- one tree can be preyed upon by thousands of insect herbivores

E. Conceptual Models

1. Elton - numerical and biomass pyramids

Numerical and biomass pyramids can be "inverted":

- one tree can be preyed upon by thousands of insect herbivores

- a lower trophic level can support more biomass at a higher level IF the rate of biomass production in lower level is high

E. Conceptual Models

1. Elton - numerical and biomass pyramids

Numerical and biomass pyramids can be "inverted":

- one tree can be preyed upon by thousands of insect herbivores

- a lower trophic level can support more biomass at a higher level IF the rate of biomass production in lower level is high

- but a productivity pyramid (new biomass/area/time) can't be permanently inverted

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

- energetic conversion rates determine biomass transfer:

- endotherm food chains are short; only 10% efficient

Only 10% of the biomass consumed by herbivores is converted into herbivore biomass that is available to predators.

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

- energetic conversion rates determine biomass transfer:

- endotherm food chains are short; only 10% efficient

- ectotherm food chains can be longer, because energy is transfered more efficiently up a food chain (insects - 50% efficient).

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

- energy available in lower level will determine the productivity of higher levels... this is called "bottom-up" regulation.

not enough energy to support another trophic level

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

3. Hairston, Slobodkin, and Smith (HSS) - 1960

- Observation: "The world is green" - there is a surplus of vegetation

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

3. Hairston, Slobodkin, and Smith (HSS) - 1960

- Observation: "The world is green" - there is a surplus of vegetation

- Implication: Herbivores are NOT limited by food... they must be limited by something else...predation?

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

3. Hairston, Slobodkin, and Smith (HSS) - 1960

- Observation: "The world is green" - there is a surplus of vegetation

- Implication: Herbivores are NOT limited by food... they must be limited by something else ....predation?

- If herbivore populations are kept low by predators, they must be the variable limiting predator populations - as food. SO:

Top Pred's: Limited by Competition

Herbivores: Limited by Predation

Plants: Limited by Competition

E. Conceptual Models

1. Elton - '20's - numerical and biomass pyramids

2. Lindeman - 40's - energetic perspective

3. Hairston, Slobodkin, and Smith (HSS) - 1960

- Observation: "The world is green" - there is a surplus of vegetation

- Implication: Herbivores are NOT limited by food... they must be limited by predation.

- If herbivore populations are kept low by predators, they must be the variable limiting predator populations - as food. SO:

Top Pred's: Limited by Competition

Herbivores: Limited by Predation

Plants: Limited by Competition

Community structured by "top-down effects" and ‘trophic cascades’

Community Ecology

I. Introduction

A. Definitions

B. Development of the Community Concept

C. Key Descriptors

D. Conceptual Models of Trophic Structure

E. Empirical Tests of Trophic Models

E. Empirical Tests of Trophic Models 1. Leibold et al. (1997)

As primary productivity increases, herbivore biomass increases, consistent with bottom-up theory.

When fish were added, herbivores (zooplankton) declined and phytoplankton were released from herbivory and increased; indicating top-down effects once the third level (predators) were added.

Adding fish reduces zooplankton and RELEASES phytoplankton (“top-down”)….

E. Empirical Tests of Trophic Models 1. Leibold et al. (1997) 2. Hansson et al. (1998)

Adding nutrients in 3-level systems pumped up zooplantkon, NOT phytoplankton (consistent with L-V predator-prey models and bottom up effects).

No effect

In 4-level systems, adding nutrients pumped up FISH, who ate the zooplantkton, and RELEASED algae. Alternating effects as top-down predict.

No effect