Lecture Date: 3-10-04Lecturer: Jonathan Maul

Chapter 17Processes Shaping

Communities

Previously you learned about community structure.

Community structure can change through:

1) SPACE

2) TIME

Change in communities through space = ZONATION

The classic example of zonation is a salt marsh.

Zonation on a rocky shore along the North Atlantic

Zonation on a sandy beach along the Mid-Atlantic

2 Types of Transition Zones

1)Edge = where 2 or more veg. communities meet.

2)Ecotone = where 2 or more veg. communities intergrade.

This will be covered in detail in Chapter 19.

EDGE (a) & ECOTONES (b,c,d)

Classification of communities is Scale Dependent – I (Section 13.9)

1) Classifying is a human action to facilitate description and transfer of information.

2) It is very important to identify your scale when classifying communities.

3) Smaller Scale = >> Precision Larger Scale = << Precision

Classification of communities is Scale Dependent - II

4) In the next 3 graphical examples you will see vegetative communities classified at 3 different scales.

5) Each example is actually a smaller part of the next.

Small Scale(Fig. 13.4)

Med.Scale

(Fig. 13.7)

LargeScale(Fig. 13.8)

Classification of communities is Scale Dependent - III

6) Small scale of a single hillside, one can actually discuss individual tree species.

7) Middle scale of several peaks appears to be a range of communities depending on moisture and elevation.

8) Large scale of Great Smokey Mtns. is described as Oak-Chestnut forest.

Community structure can change through:

1) SPACE

2) TIME

Previously you learned about community structure

Change in communities through time =

SUCCESSION

Succession refers to a given point in space.

Succession is common to all environments.

The process goes from early successional to late successional

Timex x x x x

Characteristics of Early Successional Species

1) High growth rate.

2) Small size.

3) Wide dispersal.

4) Fast population growth rate.

Characteristics of Late Successional Species

1) Low rates of dispersal and colonization.

2) Slow growth rates.

3) Larger size.

4) Longer lives.

Figure 21.3 (d)- cut out colored part of figure, put circle around late succ. Section.

Figure 13.10 – Here is an example showing early and late successional algae species

Successional sequence tends to bevery predictable or repeatable

This suggests to Ecologists a common mechanism controlling the process of succession.

Rate at which succession occurs isdependent on the community in question.

e.g.) algae communities have a fast rate: 1-3 yrs.

Notice time scale on x axis.

Rate at which succession occurs isdependent on the community in question.

e.g.) forest communities have a slow rate: 5-80 yrs.

Types of Succession1) Primary Succession = occurs at a site previously unoccupied by a community.Examples: a) Lava flows.

b) Artificial substrate like algae exp.c) Newly formed bodies of water.d) Newly exposed land.

2) Secondary Succession = occurs on previously occupied sites following disturbance.

Examples: a) Forest clearcuts. b) Weather damaged sites.

Disturbance= Discrete event in time that disrupts communities and creates opportunities for new individuals.

1) Can be small: single tree in forest (40m radius).

2) Can be large: miles of clearcutting, fire, or disease.

3) Forest disturbance alters factors such as: light, soil, temperature, nutrients, soil moisture, and relative humidity.

One 3-Dimensional view of disturbance

1. Frequency

2. Severity

3. Ex

tent

Intermediate DisturbanceHypothesis

1) Proposed by J.H. Connell in 1978.

2) Suggests that species diversity is controlled by the frequency and intensity of disturbance.

3) Essentially, he proposed that species diversity is maximal at intermediate levels of disturbance.

Relationship between diversity and an intermediate level of disturbance

Climax Community - I1) Succession appears to approach an endpoint which is called the climax community.

2) Characteristics of a theoretical climax community include: a) Tolerance of ambient conditions. b) Equilibrium between production & respiration. c) Equil. between energy captured and released. d) Equil. between nutrient uptake and return. e) Many species. f) Complex food chain.

Climax Community - II

3) Problem: Imagine no human impacts and all communities at their climax. Would this mean that all early successional species go extinct???

4) Answer: No, the climax community is not at equilibrium. Changes are continuously occurring throughout the climax community.

For example see the following figure -->

Graphical depiction of homogenous climax community w/ Holes consisting of early successional plant species.

Climax Community - III

5) The holes in the climax community of the previous slide are called Gaps.

6) Characteristics of forest Gaps include: a) Light Intensity. b) Soil Availability. c) Temperature. d) Nutrients. e) Soil Moisture. f) Relative Humidity.

Succession affects the distribution and abundance of animals

(Section 13.13)

1) Replacements can be observed in animal species at various stages of succession.

Figure 21.19

NOTE: Meadow Mouse is replaced by White-footed Mouse and Black-throated Green Warbler is replaced by Veery in older forests.

Succession affects the distribution and abundance of animals

2) Overall species diversity changes with successional stage as well.

Notice drop in diversity at the young forest stage.

So far, we have discussed temporal changes on the order of months (algae communities) to decades (forest communities).

Geologic time is another component that must be addressed.

Community over Geologic Time

Succession over Geologic Time

Postglacial migration of 4 tree genera. White lines indicate extent of present ranges.

SPRUCE

OAK MAPLE

WHITE PINE

(Fig. 13.16)

Influences on Community Structure1) Direct Interactions:

1) Competition2) Mutualism3) Predation4) Parasitism

2) Indirect Interactions: 1) Interaction between species 1 and 3 below

Species 1 Species 2 Species 3consumes consumes

Indirect interaction of sp.1 on sp. 3

Influences on Community Structure

1) Direct Interactions2) Indirect Interactions

Can operate through niche overlap

The Fundamental Niche ConstrainsCommunity Structure

1) Certain range of conditions are necessary for a species to survive.

2) Can be shown by the bell-shaped curves for 1 environmental gradient.

Some Basics on Niches= extra info

Section 22.3Changes in Niche Dimensions

1) Fundamental Niche: Hutchinsonian niche in the absence of competitors.

2) Realized Niche: Subset of fundamental niche used when competitors are present because competitors make some resources unavailable.

Resource

# Indi

vidu

als

Fundamental for Sp.1

Realized for Sp. 1

Species 2

In 1958 Robert MacArthur

studied Niches with 5

Warblers

= extra info

How??: Through different adaptations for acquiring resources.