Chapter 6: Construction

in The nature of the plant community: a reductionist view

J.B. Wilson and A.D.Q. Agnew

Brooke Wheeler and Forbes Boyle

I. Theories: Models of Community “Construction”

Deterministic—Plant interactions and environment control distribution of species

OR

Stochastic—Communities are determined by random chance

Discreet—Communities are spatially seperated

OR

Continuous—There are no clear boundaries, community change is difficult to see

I. Theories

(a)

(b)

(c)

a) Deterministic and Discreet—Clements’ view

b and c) Deterministic but Continuous— Whittaker’s view

b) Stochastic and Continuous—Gleason’s view

b) Stochastic and Discrete

II. Frederick E. Clements

THE INTEGRATED COMMUNITY VIEW

“communities were…nameable and had ‘more or less defining limits’”

“’An association is similar throughout its…general floristic composition’”

“The same seral stage may recur …with the same dominants and codominants.’”

II. Frederick E. Clements

THE INTEGRATED COMMUNITY VIEW

Wilson et al. (1996)

Objective: To test hypothesis that the same community will be found in different locations.

Established “baselines” of neighbor quadrats to determine least similar pairs.

RESULTS: 83% of sites with “similar” vegetation occurred elsewhere…

So communities do occur.

III. Gleason

• Misunderstanding Gleason: his “views were identical to Clements’.”

• Vegetation results from interference.

• Associations have limits and are separated by tension zones (ecotones) (Gleason 1927) and communities have boundaries and uniformity.

• Both Gleason and Clements thought of vegetation as a mosaic of types.

• Gleason stated exact repetition of vegetation won’t occur

IV. Whittaker and Austin• Theory 2: continuous but deterministic

• Structure through competitive exclusion

• Co-evolution leading to avoidance (dominants)

• Continuum Theory- vegetation organized according to continuous change along environmental gradients.

• JB Wilson et al 2004- flaws in analysis cast doubt on continuum of change along environmental gradients

V. Hubbell and Chance• Functional (niche) equivalency

• Hubbell 2001- intended as a null model not a best-fit model

• Section seems tossed in after it has already been discussed at length previously.

QUESTIONS

1) Were Clements’ and Gleason’s ideas about plant communities develop essentially “identical” as Bastow proposes?

2) Is “Clements vs. Gleason” simply a “straw man” for paper intros?

3) There are 4 combinations of how communities develop, but only 3 graphical representations. What’s wrong with this picture???

QUESTIONS

4) What is the importance of gradient analysis?

5) Does it distract from the search for the nature of plant community?

6) Does any statement in community ecology require an appropriate null model?

VI. C-S-R Theory—Philip Grime

C (competition)

S (Stress)(ruderal) Rdi

stur

banc

eproductivity

r

K

productivity disturbance

HABITAT SPECIES

Highly competitive (C)

productivity Stress-tolerant (S)

disturbance Ruderal (R)

VI. C-S-R Theory--Stress

C (competition)

S (Stress)(ruderal) R

dist

urba

nce

productivity

r

K

dist

urba

nce

productivity

Stress

Disturbance

“The external constraints which limit the rate of dry matter production of all or part of the vegetation”

VI. C-S-R Theory--Stress

According to Grime, STRESS is defined on the COMMUNITY, not individual plants

Wilson and Agnew are critical of this:

--Tropical Tree dominants

Hubbell (2005): S species—shade tolerant long life span

resistance to pests

Wilson argues: C or R species—rapid growth post disturbance

VI. C-S-R Theory

C (competition)

S (Stress)(ruderal) R

dist

urba

nce

productivity

r

K

dist

urba

nce

productivity

Stress

Disturbance

“The mechanisms which limit the plant biomass by causing partial or total destruction”

DISTURBANCE:

Little information given also for competition

VI. C-S-R Theory and Succession

Grime: Productivity will influence successional trends

Taraxacum officianale and Salsola kali—secondary pioneers NOT restricted to desert communities

Agriophyllum squarrosum—a secondary pioneer associated with dunes in semi-arid areas

VI. C-S-R Theory and Succession

Wilson and Agnew: Difficult to separate out species in the R and S corner of the Triangle, with regards to early succession

RESULTS VARY!!!!

Only S species would be able to tolerate the S corner at all seral stages of a community

VI. C-S-R Theory Conclusions:

--”USEFUL GENERALIZATION”

--”DIFFICULT TO TEST”>>>NOT A WORKING MODEL

1) Does overlaying MacArthur and Wilson’s r-K line help explain the C-S-R triangle??

2) Why does Grime devote so little of this section to disturbance and competion?

3) Is the species/character test section a useful addition in this chapter?

Questions:

VII. Tilman’s Theory

• “a hard center but woolly edges”• Gaussian exclusion- coexistence of

competing species through different limiting resources

• R* theory- in mixtures, species with the lowest R* will outcompete others

• Deceptively simple• Worked for alga in microcosm with

constant mixing (lab conditions)

R* theory- soil nutrients

• Soil: NPK major nutrients, vary in time and space

• More difficult to support with experiments• Tilman and Wedin (1991 a, b)- didn’t show that winner

could grow at lower N level and loser suffered in high N as well

• Soil nutrients more complicated• Decomposition, leaf leachate, precipitation• Nutrients taken up by bacteria and micro-org.• Animals redistribute nutrients

R* theory- soil nutrients cont.

• pH of soil affects processes and availability of nutrients

• Varies with time- N more abundant in spring

• Varies by nutrient• P is immobile, plants must forage for it• Localized depletion

R* theory- water, light

• Water• Varies through time• Varies with depth (rooting depth important)

• Light• Vertical competition – decreases resource only for

individuals below• Predicts that shade tol. species achieve tolerance by

lower light-compensation points but not supported• Forest regeneration too complex for R*

VII. Succession

• Resource ratio theory of succession• Successional position isn’t clearly related

1.1

1.15

1.2

1.25

1.3

1.35

0 2 4 6 8 10

N status in field: rank

RG

R r

esp

on

se to

X1

0 n

itro

ge

n in

cre

ase

`

Agrostis scabra

Poa pratensis

Schizachyrium scoparium

Fig. 6.6: The experimental response to N compared to the rank of species in a successional/N field gradient.

VII. Tilman conclusion

• These theories aren’t useful in the real world

• While R* theory works well for micro-organisms in labs, reality is complicated

• Tilman was brave to attempt to try to find patterns in ecology

• R* conceived in a controlled, homogenous, lab-tank environment.

QUESTIONS:

1) Are forests too complex for R* ?

2) Are there useful field applications of R* theory?

3) Is it impossible to see how to test or apply R* outside of lab settings?

VIII. GRIME versus TILMAN??or WILSON versus GRIME and TILMAN??

STRATEGY:

Plant Energy Expenditure fits in nicely with both models

C-S-R—no species can occupy all three points

R*--shoot versus root strategy (ALLOCATE model)

SPECIES DIVERSITY:

Humped-back relation between productivity and species richness for BOTH models.

VIII. GRIME versus TILMAN??or WILSON versus GRIME and TILMAN??

COMPETITION:

Grime>>>Low in Stressed Environments

Tilman>>>Equal Across Stressed and Unstressed Environments

Wilson>>Interference limits plant abundance across all environments.--No competition in early succession (Clements)

--Spatial mass effect --Herbivory limits abundance

VIII. GRIME versus TILMAN??or WILSON versus GRIME and TILMAN??

Wilson: Both Grime and Tilman models do not account for resource versus non-resource factors in their environmental gradients.

Difficult to test hypothes between Grime and Tilman models because of the “GROWTH RATE ARTIFACT”:

--vegetation planted in pots will come to competion earlier in high productive environments (Grimes).

VIII. THE WILSON and AGNEW HYPOTHESIS!!!

Beta Niche Gradient (non-resources): competition is constant

Alpha Niche Gradient (resources): competion is strongest when resource is in shortest supply

In High Stress Environments (deserts):

Very similar to Tilman’s idea that competition intensity is constant across ALL communities

VIII. THE WILSON and AGNEW TEST!!!

Test the degree of competition (Relative Growth Rate) along the S-C gradient

C (competition)

S (Stress)(ruderal) R

dist

urba

nce productivity

r

K

CONCLUSIONS:

1) Competition is equally intense along a non-resource gradient

2) Severest competition occurs at low-levels of a “sought after” resource

QUESTIONS:

1) How does Clements’ model of community development differ from Grime’s and Tilman’s?

2) Why does Wilson come to support Clements’ model and refute Grime’s and Tilman? Is a reason made clear in the chapter?

IX. Synthesis

1. Too soon to tell• Community ecologists are in the worst

position• Dismal depiction of how little we know

2. Does vegetation suit our models?• We love Clements (in case you had not

heard)• Variation along gradients is continuous or

discontinuous due to a switch

IX. Synthesis cont.

• Grime’s C-S-R theory- useful generalisation

• Tilman’s R* theory too simplistic

• “Does the vegetation suit our models?” approach to plant ecology

• Complexity makes it difficult for vegetation to fit simple models

Box 6.1: Types of interaction between plants.At the species (or within-species) level

negative effectsinterference (negative effects via reaction)

competition: species X removes resources from the environment, which are then unavailable to species Y

allelopathy: X produces a substance toxic to Yspectral interference: X changes the red/far-red balance, disadvantaging Y switch: X causes reaction in an environmental factor, disadvantaging Y negative litter effects: X produces litter of a type that disadvantages Y

(positive effects are a type of subvention)parasitism: X removes resources directly from Yautogenic disturbance: X disturbs, disadvantaging Y negative effects via heterotrophs: X changes the heterotroph population, disadvantaging

Y Subvention (positive effects)

mutualism = X and Y both benefit relative to their being at the same density on their own benefaction = X benefits Y as above, with no known advantage/disadvantage to itselffacilitation = X benefits Y, to its disadvantage

At the community levelguild/community X gives a relative disadvantage to itself:

the effect is density-independent: facilitation and/or autointerference = relay floristicsthe effect disappears at low density of X (negative feedback) = stability

guild/community X gives a relative advantage to itself = switch

Three Things

1. Plant communities generally have many species.

2. Heterogeneity is a rule.

3. In order for there to be “science in plant community science” we hope there are “rules governing the assembly of species in them”.

IX. Heterogeneity

• Heterogeneity of environment- future community process research should concentrate where allogenic heterogeneity is low

• Call for work on plant/littler effects on soil

• Ignore species area curves because they don’t tell us much

IX. Assembly rules

• Evidence mostly from herbaceous communities- esp. Otago Botany Lawn

• Difficult to search for assembly rules– Don’t know what the rules are– Need character-based rules (careful

selection)

• Preadaption of species is key

IX. Conclusions

• Need for integrated knowledge of plant-plant interactions

• Switch is supreme process in plant communities– Move beyond the “easy task”– Switching causing Alternative stable states

Questions

• Do plant ecologists just produce models and try to make the data fit?

• Do species area curves provide useful insights into community ecology?