The nature of the plant community: a reductionist view

by Wilson and Agnew

Discussion of Chapter 3: Community-level processes

Nick Adams and Jen CostanzaOctober 4, 2006

Overview

Types of vegetation change– Allogenic change– Directional autogenic change (Clements’ “succession”)– Cyclic succession– Switches (including Alternate Stable States)

Is the framework for vegetation change in this chapter logical, helpful, clear? How could it be improved?

Was an appropriate amount of time spent on topics within the chapter?

Was anything left out?

Allogenic Change

Gradual change over long

time scales

OR Short-term changes– The “elusive” Carousel Theory: “a distraction

from progress toward understanding” how coexistence and competitive exclusion interact

- Are descriptive theories valuable in ecology? When or in what ways?

Community Z

Community Y2Community Y1

Community X1 Community X2

Clements’ relay floristics(AKA Succession to climax)

Time

Directional autogenic change

Reasons for community change:

1. Environment changes

2. New species arrive

3. Species modify the environment to promote others – Succession via facilitation

Community YCommunity Z

Community X

Watt’s cyclicsuccession

Time

Cyclic SuccessionWatt (1947): self-generating cycles

are a widespread cause of vegetation mosaics

W&A: “Has never been reliably confirmed by observation over time”

“One of the best examples”: O. leptocaulis and L. tridentata

- Does cyclic succession have to be completely autogenic and/or facilitation-driven?

- What was Watt’s contribution to ecology?

Watt 1947

Time

Space

Switches

• Odum 1971: “positive-feedback process between biota and environment”

• W&A 2006: “the key to plant community ecology”• Four types of switch• Four possible outcomes• Various mediating agents• May evolve via natural selection more than

facilitation does, especially when the whole population does not benefit

- Is this framework helpful? Does it make sense?

Alternative Stable States

• “Difficult” criteria• Need to examine the mechanisms

before we generate theory• Few examples; many have not

observed over long enough time• W&A: “One of the more nauseating but

delightful idiocies that stem from the heart, not the mind, of otherwise respectable scientists”

- Do alternative stable states exist? Is the framework useful?

- Do theories add to our understanding even if we do not know the mechanism? Which should come first?

Community Z1

Community Z2

Community Y1

Community Y2

Community X

A switch, giving alternative stable states

Time

Diversity-------->Productivity

• Outcomes of niche differentiation:– Overyield– Stability

Overyield

• “Overyield is the situation in which higher species richness leads to higher total biomass, perhaps as a result of niche differentiation”– However, this only applies to one of the

figures in the text: Transgressive overyielding

F ig . 3 .8 . In ‘a ’ th e re is a rith m e tic o v e ry i e ld in g : th e a b so lu te g a in b y A in m ix t u re is g re a te r th a n th e

lo ss b y B so R T M > 1 .0 . In ‘b ’ th e re la tiv e g a in b y B is g re a t e r th a n th e lo ss b y A so R Y T >

1 .0 , in ‘c ’ so m e m ix tu re s y ie ld m o re th a n e ith e r m o n o c u ltu re . F o r d e fin itio n s o f R Y M a n d

R Y T se e W ilso n (1 9 8 8 % 2 7 9 ). = th e o re tic a l b io m a ss o f e a c h sp e c ie s w ith n o

in te ra c tio n , = a c tu a l b io m a ss o f e a c h sp e c ie s , = to ta l b io m a ss o f th e m ix tu re .

100% A 50:50 100% B m ix ture

100% A 50:50 100% B m ix ture

100% A 50:50 100% B m ix ture

Biom

ass

of A

Biomass of B

Biom

ass

of A

Biomass of B

Biom

ass

of A

Bioma

ss of B

(a) RYM arithmetic overyielding

(b) R YT relative overyielding

(c) Transgressive overyielding

-biomass is only greatest w/ 50:50 mixuture under transgressive

overyielding (?)

Niche differentiation vs. the selection effect

• s. effect: highly competitive sp. Have higher production, therefore making the mixture more productive– “if the production of the mixture is greater

than that of any of the monocultures (‘transgressive overyielding’: Fig. 3.8 c) the selection effect can be ruled out”

• ...and niche differentiation is favored

Experiments re. Overyielding• Tilman et al.(2001) –mixtures outyield

monocultures• Lambers et al. (2004) – examined Tilman et

al.'s plots, and over/underyielding spp. were determined by regression of yield on diversity

• Whittington and O'Brien (1968) – Transgressive yielding was seen constantly (across grazing treatments) after 3 years, where overyield is attributed to the depth of nutrient uptake

Overyield

• Is it a result of several years of growth?– Sort into niches– Optimal proportions reached– Better competitor dominates, others “pick up

crumbs”--sort of a WEAK selection effect– If some communities are shown to be

overyielding w/ mixtures of 2 species (Roscher et al. 2005), are niches necessary? Does this discredit production based on certain mixtures (NOT monocultures) as a characteristic of a community?

Stability (arbitrary or deterministic?)

1. Global stability, with one state: This is deterministic

2. ASS, with a few states3. A continuous range of states. The present

assemblage exists for historical or even stochastic reasons, and after a perturbation or even as a result of drift another state might take its place.

Test by pulse perturbation

• Recovery can determine types of stability• “we can doubt #3 [the community recovers

to a different state after perturbation]”• Why? Doesn't it depend on the strength of

the perturbation?

Lyapunov stability1. Reliability: Constancy, lack of change probably

in spite of minor perturbation.

2. Stability sensu stricto, i.e. Lyapunov stability: whether the community ever recovers from a small pulse perturbation.

3. Resistance: Lack of change upon a pulse (i.e. temporary) perturbation. We separate it here into resistance to abiotic perturbation and resistance to invasion.

4. Resilience: Rate of recovery from perturbation.• EQUILIBRIUM IS ASSUMED, which”[probably

isn't real], BUT THE CONCEPTS ARE REAL” (?!)

Are communities with more species more stable?

• Reliability- reliability is the reciprocal of variation, and species richness can increase reliability through:– the portfolio effect-community response is an

average of all the species in a community– the covariance effect-species will increase in

number when other species are disfavored

• “...stability is like pregnancy: either one is or one isn’t”– global vs. local: isn't global stability easier to grasp

and broader? Bastow suggests the opposite– Simulations: # species, likelihood of stability– Tests: stability could be contributed to the “transitive

competitive hierarchy found between species” AND apparent mutualism(increases w/ #species inc.)

Resistance

• “Response of a system to a...perturbation,...or lack of response”– Perturbation: it's all relative--– Drought and trampling-- resistance seems to

be individualistic to the species• So, if resistance is more likely individualistic,

stability may be a little more evident: even w/ individualistic response, communities are somewhat predictable in the composition that one sees after recovery

Resistance to invasion• Invaders: occupy an empty niche, split existing

niches, oust a niche-occupier, or construct a new niche by reaction

• In reference to exotic invaders: “This confuses the ability of incomers to establish in a closed community with whether communities contain empty niches and with whether exotic species are somehow different from native ones” --this is a poorly written sentence...thoughts? Is he saying that communities are not closed and incomers can establish?

• Bastow suggests that experiments done on invasion SHOULD be artificial, in order to eliminate confounding environmental/historical effects across sites: Is the anthropogenic factor not valuable for research's sake?

More on invasion...

• Selection effect — here, invasion depends on the resistance of the dominant species

• Niche complimentarity – here, as species richness increases, the total biomass of the invader decreases as a result of filled niches.

• Complimentarity effect is favored, which in turn favors species richness—the more rich the community, the more resistant to invasion

• Exotic vs. native invader: exotics could be worldwide super-species in certain newly-created condition, or there is a greater probability of exotics being ruderals, vs. the probability of a native being a ruderal

Pause: Are exotic invaders more dangerous?

• Why is resistance to invasion important in describing a community?

• How is the invasive ability of any species different than other competitive abilities of species? “Resistance to competition”

• If invasion is shown here as being a perturbation or disturbance, why are natives included as invaders, if presumably invasion by a native is simply succession ocurring?

• I would love to see some studies exploring productivity of communities with exotic invaders present

Resilience

• Defined as the speed of recovery after pulse perturbation

• Redundancy- within functional guilds, unaffected species replace the perturbed species, therefore compensating for community functions—no conclusions..how prominent are functional guilds?

• Resistance and resilience are inversely related, although not always (depends on the community)

Overall Questions

• How useful are descriptive theories versus mechanistic explanations? When is one more useful than the other?

• Is the framework for vegetation change in this chapter logical, helpful, clear? How could it be improved?

• Was an appropriate amount of time spent on topics within the chapter?

• Was anything left out?