AP Environmental Science Community Interactions, Species Diversity

and Succession

Community Ecology Community = an association of interacting

species inhabiting some defined area Community structure = includes attributes such

as the number of species and the kinds of species comprising a community

Community function = includes attributes such as energy flow through food webs

Are ecological communities “real” functional units?

Yes! Frederick E. Clements (1874 – 1945)

Holistic community concept (Closed community) Community as a superorganism Based his conclusion on a study of

vegetation zones along altitudinal gradients

Basis of the modern-day “Gaia concept”

Vegetation Zones

Closed Communities

Along an environmental gradient (e.g. temperature, moisture) there are sometimes sharp boundaries between species assemblages

Areas between assemblages are known as ecotones

Best seen where there are sharp physical boundaries between species assemblages Between aquatic and terrestrial areas Between distinct soil types Between north and south facing slopes

Closed Communities

No! Henry A. Gleason (1882-1975)

Individualistic concept (Open community) The appearance of

communities are simply due to the individual interactions of species and don’t reflect any innate organization above the species level

Open Communities

Closer examination of the deciduous forests of eastern North America revealed no sharp boundaries between forest types

There were generally two gradients of forest types: A north-south gradient based on temperature An east-west gradient based on moisture

Gradient analysis is a technique invented by Robert H. Whittaker to analyze changes in (plant) species abundance along gradients Gradients are variables that affect plant community structure (i.e

temperature, light & water availability, soil nutrients, etc.) Put to rest the extreme closed community concept of Clements

Open Communities

Factors Affecting Biological Community Structure

Species diversity – function of the number of different species that a community contains (species richness) and the relative abundance of individuals of each species (species evenness).

Niche structure: J.H. Brown (1981) bases this on ‘capacity rules’ (how many

potential ecological niches occur) and ‘allocation rules’ (how these ecological niches can be divided up among species).

Geographical location Consistent climates foster high diversity but ‘specialist species’ Variable climates foster low diversity but ‘generalist species’

Species Richness Species Richness is a measure of the total

number of different species in a community Observations regarding species richness:

Gradual increase in diversity/richness Pole to Equator

In a given ecosystem, species richness increases over time.

Larger ecosystems tend to have greater species richness.

Usually increases with ecological succession.

Why these trends?

Why These Trends? Productivity? Spatial complexity/heterogeneity? However, an aspect of environmental

structure that is important to one group may not be important to another group

Therefore, you must know something about the ecological requirements of species to predict how environmental structure affects their diversity.

Species Richness

Factors that affect species richness saturation rates of colonization and extinction (MacArthur & Wilson

(1967)• Depend on size of ecosystem and • Distance from a source of colonists

disturbance severity, variability, and predictability (Slobodkin and

Sanders, 1969) • i.e. rainforests – favorable and constant; therefore predictable;

polluted stream – severe and variable; therefore unpredictable

Calculating Species Richness Generally exclude “accidental species” in the count Does not take into account abundance patterns

among the various species Species counts depend on the sample size or

sample area (rarely do you get every one!) “Equilibrium model of island biogeography”

(MacArthur & Wilson, 1967) # of species (species richness) on an “island” is determined

by immigration and emigration of species Larger islands and islands closer to “mainland” will have

greater species richness.

Equilibrium Model of Island Biogeography

Species-Area Curves

• Species-area curve• S = CAz

• log S = log C + z log A• An equation for a straight line

where log c = Y intercept and z = the slope of the line

S Species Richness

C constant

A Area

z constant

Species Area Curves

Diversity IndicesShannon-Weiner Index

Also known as the “Shannon Index” or the “Shannon-Weaver Index”

Based on Information Theory Measures the amount of order in a

system in bits of information Determines rarity or commonness of

species Combines measure or species

richness with species evenness.

Calculating the Shannon-Weiner Index

H = -∑(pi) (ln pi) H = Shannon-Weiner Diversity Index pi = proportion of total sample belong to

the ith species ln pi = the natural log of pi

An Example

Community A: 99 individuals in species 1; 1 individual in species 2

“Uneven” community

• H = -[(p1)(ln p1) + (p2)(ln p2)]

• H = -[0.99 (ln 0.99) + 0.01 (ln 0.01)]• H = -[(-0.0099) + (-0.0461)] = +0.056

Community B: 50 individuals in species 1; 50 individuals in species 2

“Even” community• H = -[(0.5) (ln 0.5) + (0.5) (ln 0.5)]• H = -[(-0.3466) + (-0.3466)] = +0.693

Comparison of Shannon-Weiner Index Values for Even and Uneven Communities

0.0000

0.5000

1.0000

1.5000

2.0000

2.5000

3.0000

3.5000

0 5 10 15 20 25

# of species

H

H - even community H- uneven community

Pielou’s Evenness Index Compares the evenness component of species

diversity in a community with the maximum amount of evenness possible given the same species richness

Also known as Shannon’s equitability (EH) EH = H/Hmax = H/ln S Hmax = ln S Community A--> J = 0.056/0.693 = 0.081 Community B--> J = 0.693/0.693 = 1.000 EH ranges from 0-->1.0 with 1.0 indicating maximum evenness

Comparison of Pielou's Evenness Index for Even and Uneven Communities

1 1 1 1

0.02850.03040.03380.04340

0.2

0.4

0.6

0.8

1

1.2

#of species 5 10 15

# of species

Ev

en

ne

ss

EH - even community EH - uneven community

Use of Diversity Indices

Used in applied ecology to measure differences in diversity between two or more communities

Also used to measure changes in diversity within a community after some environmental modification or disturbance

Robert MacArthur (1930-1972) UPenn & Princeton Studied the diversity

of warblers in northeastern forests

Discovered that different warbler species fed in different parts of the forest canopy

MacArthur’s Warblers

Bird Species Diversity vs. Foliage Height Diversity

Also showed that the greater the Foliage Height Diversity of the forest the greater the Bird Species Diversity

Ecological Succession Plant (and animal) communities

develop in often predictable ways following disturbances, a process known as succession

Natural, gradual changes in the types of species that live in an area

Classified as primary or secondary succession

Indiana Dunes Succcession

Henry Chandler Cowles (1869-1939)

American botanist and ecological pioneer

Born in Kensington, Connecticut, he attended Oberlin College in Ohio and the University of Chicago

He obtained his Ph.D. in 1898 for his study of vegetation succession on the Lake Michigan sand dunes.

Kinds & qualitative characteristics of disturbances that impact communities

Disturbance frequency

Dis

turb

ance

mag

nitu

de

TreefallIce Storm

Fire

Glaciation

Volcanism

Asteroid impact

Landslide Hurricane

Primary SuccessionGeneral characteristics

Begins in a place without any soil Sides of volcanoes Landslides Flooding Remnants of glaciers

Starts with the arrival of living things such as lichens that do not need soil to survive

Lichens (pioneer species) secrete weak acids which in turn break rocks into soil

Called the Pioneer Stage

Lichens: Symbiosis of a fungus & an alga

Example of a mutualism

Primary SuccessionGeneral characteristics

Soil starts to form as lichens and the forces of weather and erosion help break down rocks into smaller pieces

When lichens die, they decompose, adding small amounts of organic matter to the rock to make soil

Primary Succession General characteristics

Simple plants like mosses and ferns can grow in the new soil

Primary SuccessionGeneral characteristics

The simple plants die, adding more organic material

The soil layer thickens, and grasses, wildflowers, and other plants begin to take over

Called Seral Stages

Primary SuccessionGeneral characteristics

These plants die, and they add more nutrients to the soil

Shrubs and trees can survive now Still called Seral Stages

Primary SuccessionGeneral characteristics

Insects, small birds, and mammals have begun to move in

What was once bare rock now supports a variety of life

Called a Climax Community

Climax Community

A stable group of plants and animals that is the end result of the succession process

Does not always mean big trees Grasses in prairies Cacti in deserts Mosses, sedges and lichens in the tundra

Secondary Succession

Begins in a place that already has soil and was once the home of living organisms

Occurs faster and has different pioneer species than primary succession

Example: after forest fires

Intermediate Disturbance Hypothesis J.P. Grime (1973), Henry Horn (1975) and

Joseph Connell (1978) The Intermediate Disturbance Hypothesis is an

ecological hypothesis which proposes that biodiversity is highest when disturbance is neither too rare nor too frequent.

With low disturbance, competitive exclusion by the dominant species arises.

With high disturbance, only species tolerant of the stress can persist.

Role of Disturbance in Succession