COMMUNITY CHANGE COMMUNITY CHANGE (SUCCESSION)(SUCCESSION)

Krebs cpt. 21; pages 403-424; 431Krebs cpt. 21; pages 403-424; 431

Connell and SlatyerConnell and Slatyer 19771977

Succession driven by interactions Succession driven by interactions between species:between species:

FACILITATIONFACILITATION

TOLERANCE (COMPETITION)TOLERANCE (COMPETITION)

INHIBITIONINHIBITION

Facilitation

SUCCESSION PATHWAYSSUCCESSION PATHWAYS

A B C D

A B

CD

Tolerance & Inhibition

Succession models (Krebs 5th Fig. 21.4)

++

++

++

-

-

-

o

o

o

Species interactions + - o

FACILITATION …only certain pioneer species can establish after disturbance. Early species enhance the ability of later species to establish.

INHIBITION …any species can invade. Once established, species resist further invasion and are displaced only by death or further disturbance.

TOLERANCE …any species can invade. Early species have little or no effect on abilities of later species to invade. Later species can displace early species by reducing resources to levels below those tolerated by earlier occupants.

A study from abandoned farmland illustrates A study from abandoned farmland illustrates some aspects of Facilitation, Tolerance and some aspects of Facilitation, Tolerance and

Inhibition (see Krebs pp 419-422) Inhibition (see Krebs pp 419-422)

FIELD ABANDONED IN FALLFIELD ABANDONED IN FALL

1. INITIAL INVASION:1. INITIAL INVASION:

• HorseweedHorseweed

• a winter annuala winter annual

• produces abundant seedproduces abundant seed

• self-allelopathicself-allelopathic Conyza canadensis

2. NEXT SEASON:2. NEXT SEASON:

• Asters Asters

• More susceptible to More susceptible to decaying roots of decaying roots of horseweed, than horseweedhorseweed, than horseweed

• Tolerant of dry conditionsTolerant of dry conditions

3. SECOND AND THIRD SEASONS3. SECOND AND THIRD SEASONS:

• BluestemBluestem

• Seedlings invading since initial abandonmentSeedlings invading since initial abandonment

• BroomsedgeBroomsedge

• Seedlings invading since initial abandonmentSeedlings invading since initial abandonment

• More tolerant of dry conditions than AstersMore tolerant of dry conditions than Asters

• Decaying roots of Horseweed promote growthDecaying roots of Horseweed promote growth

Bluestem

Broomsedge

FACILITATION:

• Broomsedge is promoted by decaying roots of horseweed & aster

TOLERANCE:

• Broomsedge displaces aster through competition for water

INHIBITION:

• Horseweed litter prevents aster from persisting

SUMMARYSUMMARY

FORCIER 1975

Succession driven by interaction of disturbance & life history traits of species

Studied trees in a New Hampshire forest

• Trees with dbh >2 cm (400 plots)

• 90% of canopy:

• Sugar maple

• American beech

• Yellow birch

Eastern deciduous hardwood forests

Several tree species

Grey Birch, Betula populifera

Black Gum, Nyssa sylvatica

Red Maple, Acer rubrum

Beech, Fagus sylvatica

YELLOW BIRCH

SUGAR MAPLE BEECH

r K

SPECIES Density

(#/m2)

(dmin/dmax )

x 100

% plots with at least 1 seedling

Mass/ind (mg)

Yellow birch

21.3 2 92 8

Sugar maple

10 57 73 268

Beech 3.9 66 55 569

Many small offspring (r-type)

Fewer large offspring (K-type)

Variable density and high dispersal

More even density and lower dispersal

Number of cohort alive at beginning of interval

Age

interval

Yellow birch

Sugar maple

Beech

0-1

1-2

2-3

3-4

4-5

5-6

6-7

7-8

8-9

9-10

1000

70

<30

1000

583

317

225

166

118

84

54

48

42

1000

410

262

188

138

121

110

92

87

83

CANOPY SEEDLINGS SAPLINGS

< 0.5m tall 0.5 – 2 m tall

Yellow birch Yellow birch

Sugar maple

Beech

+

+0

-

+

-

Sugar maple Yellow birch

Sugar maple

Beech

0

+

+

0

0

++

Beech Yellow birch

Sugar maple

Beech

0

0

+

0

0

Sprouts + (seeds -)+ = significantly present

0 = no relation

- = significantly absent

CATASTROPHIC DISTURBANCE

Yellow birch

Sugar Maple

Beech (seed)

Beech (sprouts)

MINOR DISTURBANCE

Successional communities