Assembly RulesJeff Ott and Jackie White

Early history of assembly rulesGotelli (2000)

• Diamond (1975)– Argued that interspecific competition among bird species in the

Bismark Archipelago resulted in observable community assembly rules, e.g.

• forbidden species combinations• checkerboard distributions • incidence functions

• Conner and Simberloff (1979)– Used null model analysis to show that many of Diamond’s

patterns could also arise in competition-free communities assembled by random colonization

• Triggered a debate that has lasted 25+ years

Null Models

• Tokeshi principle: “the null model must include all features of the observed data except the one it is intended to test

• Questions: – How important are null models in ecology? – Do all patterns need to be tested against a null model to be “proven”?

Sp. 1 Sp. 2 Sp. 3 Sp. 4 Sp. 5 Sp. 6 Richness

Plot 1 1 0 1 0 1 0 3

Plot 2 0 1 1 0 0 0 2

Plot 3 1 0 1 1 1 0 4

Plot 4 1 0 1 1 0 1 4

Frequency 3 1 4 2 2 1

Broadening of assembly rule definitions

• Filtering of the regional species pool to produce a local community (Belyea and Lancaster 1999)– Geographic filters– Environmental filters– Biotic filters

• Filtering of species traits due to environmental constraints (Keddy and Weiher 1999)

• The dynamic process by which communities develop (Booth and Swanton 2002)

Assembly rules: Wilson and Agnew’s definition

• “Restrictions on the observed patterns of species presence or abundance that are based on the presence or abundance of one or other species or groups of species (not simply the response of individual species to the environment)”– Emphasizes pattern, not process– Most likely to apply to equilibrium communities– Ignores within-species genetic variation and phenotypic plasticity– Null models required

• Questions:– Can species interactions and environmental responses really be

treated as isolated topics?– Is environmental characterization of species distributions really

the “easy task of community ecology” and therefore uninteresting?

Ruling out environmental variation

• Is variance in richness different from a null model?

• Example 1– Yes, if you look across the entire sample– No, if you look within specific environments

Alternative question: Is richness correlated with environment?

• Example 2– Yes, whether across entire sample or within environments– However, quadrats within environments are pseudoreplicated: only one quadrat allowed per environment… True???

Alternative question: Is composition correlated with environment?

Ruling out environmental variation

• Is variance in richness different from a null model?

• Example 1– Yes, if you look across the entire sample– No, if you look within specific environments

Alternative question: Is richness correlated with environment?

• Example 2– Yes, whether across entire sample or within environments– However, quadrats within environments are pseudoreplicated: only one quadrat allowed per environment

Alternative question: Is composition correlated with environment?

(Pseudoreplication: lack of independence among sample units, e.g. in space or time)

Questions:Is Wilson and Agnew’s approach for ruling out environmental variation adequate? Are there other ways besides the “patch model” for mitigating pseudoreplication?

Patch model

Ruling out environmental variation

• Is variance in richness different from a null model?

• Example 1– Yes, if you look across the entire sample– No, if you look within specific environments

Alternative question: Is richness correlated with environment?

• Example 2– Yes, whether across entire sample or within environments– However, quadrats within environments are pseudoreplicated: only one quadrat allowed per environment… True???

Alternative question: Is composition correlated with environment?

Zonation on gradients

The work of Whittaker and other ecologists has suggested that species boundaries are randomly-spaced rather than clustered along environmental gradients.

Are there theoretical reasons to continue searching for clustered boundaries on gradients?

Species sorting• Hypothesis 1: Species associations will become stronger over the

course of succession as random processes are replaced by deterministic ones.– Pioneer stage: random or weak negative associations– Building stage: some positive and negative associations– Mature stage: strong associations, mostly negative

• Evidence for:– Tussock grasses after burning

• Evidence against: – Sand quarries (negative associations less common over time)– Canyon walls (greater dissimilarity in frequently-disturbed patches than

undisturbed—relevant?)• Uncertain

– Tropical rain forest (non-random throughout succession?)– Pastures in which negative associations increased, but the total number

of significant associations decreased over time

Species sorting• Hypothesis 2: Species composition will converge over

time in similar conditions (“It would be fascinating to see how similar species assembly was in similar conditions.”)

Table 5.x. Dry mass (g m-2) of selected species in six replicate plots in the experiment of Crawley et al. (1999).

– If conditions were truly identical, would the outcome of succession (species sorting) also be identical?

– What factors in natural communities seem to be most responsible for preventing successional convergence?

Limiting Similarity• Hypothesis: If two species are similar in resource use patterns then one will

be excluded unless character displacement occurs.

• Hubbell (2005): “The empirical evidence, in general, has not borne out these predictions… particularly in plant communities… does limiting niche similarity for functional groups exist?... I believe that the answer is no (at least in plants)

• Wilson and Agnew: Hubbell (2005) was too dismissive. Limiting similarity in plant communities can be demonstrated.

• Examples:– Rooting depth in desert and fybnos (Cody 1986)– Leaf shape/length in fybnos (Cody 1986)– Pollinators in tropics (Armbruster 1986, 1994)– Multiple characters in herbaceous riverside vegetation (Weiher et al. 1998)

and sand dunes (Stubbs and Wilson 2004)– Flowering phenology (Stiles 1977 etc.)– Vegetative phenology (Parrish and Bazzaz 1976 etc.)

• Conclusion: greater support for limiting similarity due to vegetative characters (e.g. rooting patterns, leaf water control) than reproductive characters (e.g. pollinators, flowering phenology)

Guild Proportionality• Pianka (1990) – guilds are groups of species that compete more with each

other than with species in other guilds. – same α niche

• Result: relative constancy in the proportion of species from each guild • Within guild differences (“Micro-niches”) are assumed

– Not simply the abundance of individuals within a guild

• Why species and not abundance? Why would we expect to see more than one spp per guild?

Guild Proportionality Tests

• Ratio of variance in guild proportionality observed with that expected under the null model

• Patch and site null models• Test statistic is relative variance in guild proportionality

(RVgp)

Evidence: constancy in space

RV >1 is likely attributed to environmental heterogeneity

Evidence?YES

• Salt Marsh • Inter-tussock

vegetation• Subalpine meadow• Lawn

NO• Beech forest• Riparian habitat

MAYBE• Dune slack

– In forb layer

Is the scale relevant?

• Scale (4 x 4 cm, 10 x 10 cm, 13 x 13 mm)

• “We deliberately started from the smallest scale that seemed ecologically sensible: 1 x 1 m. Smaller scales would not be conceptually realistic in a forest, since extrapolation of the quadrat into the canopy would be meaningless.” Bycroft et al. 1993

• If significant guild proportionality is observed at these scales, is this evidence of a relevant assembly rule in plant communities? What scale is relevant? At what scale do communities occur?

Evidence: removal experiments

• If species from one guild are removed then, in order to maintain proportionality, any increase in species will likely be from the same guild

• Is this a logical argument? What other outcomes might we expect? Will the outcome depend on the type of species removed or the time since removal?

• Herben et al 2003: – Removed Festuca rubra– Grass biomass increased more than dicots

• Symstad 2000– Removed 3 guilds– Additions after 3 years showed only weak evidence of resident

guilds excluding similar species

Removal experiments

• Fargione et al 2003– Addition experiments at Cedar Creek– Resident guilds had an inhibitory effect on invasion by

its own guild

• Von Holle and Simberloff 2004– Removal of species from riparian plots– Additions– No effect

Conclusion: removal experiments give little evidence for guild based assembly rules

Evidence: Successional convergence

Fukami et al. 2005 – convergence in trait groups

Guilds -- Conclusions

• Is there support for guild proportionality?

• What do Wilson and Agnew conclude regarding the existence of guilds and guild proportionality?

• Why is guild proportionality generally only observed in the herb layer?

Intrinsic guilds

• A priori guild selection is arbitrary– Subjective selection– Classification by trait groups

• Solution: ‘interview the plants’– Maximize RVgp -- Distributional data

• Assume there is guild proportionality– Maximize mean RYM – Interference experiments and

the “Jack Spratt effect”– Response to removal

• Evidence of stochastic assembly

• Benefit is that this technique can fail to find significant guilds

Intrinsic guilds

• What do you think about the method for determining intrinsic guilds? Can all species realistically be assigned to two or three guilds?

Texture Convergence

• Functional characteristics• “in comparable habitats in different areas, whilst

the actual species present may be different, the texture may be the same.”

• Mean texture or distribution of functional traits

Texture

• Wilson et al 1994– Wooded fen communities– Texture – traits related to light capture– Convergence in PSU width and area was observed

when species were weighted by photosynthetic biomass

– Each community has representation from the range of functional characteristics

– ‘Strong evidence that species are being sorted by their characters…for their entry into the community’

Time

• No-analogue communities• Combinations of climatic variates differ• Community misfits occur in same place/time as climate

misfits• What is the relevance of this section?

Environmental factor A Environmental factor A

En

viro

fa

cto

r BE

nvi

ro

fact

or

B

Time 1 Time 2

Niche of Species X Species Y

Existing environ-ments

Existing environ-ments

Abundance

• Biomass constancy per unit area– What about biomass constancy in guilds?

• Relative abundance distribution (RAD)– Niche pre-emption model (Whittaker 1965) –

competition – Broken stick (MacArthur 1957) – random assignment

of resources– “fits to an model based on an ecological theory would

be most interesting, and usually ambiguous…’

Abundance

• Sparse species– Are there special rare niches being filled?– Zobel et al 1994 – no difference in the number

of invasions– Do rare species have a distinct effect?– Lyons and Schwartz 2001 – invasion of exotic

species was greater after removal of rare species

Keystone species

• Vital for maintenance of the community

• Greatest effect on others relative to its biomass

Aspen, Saguaro, and Figs

Exotic species

• Is invasion surprising?– Native species should be more adapted to the

local environment– Species should not be intrinsically different

• All spp are native somewhere

• Drivers or passengers of change?

• 5 explanations for successful invasion

Alternative hypotheses

• Depauperate flora– Implies missing guilds

• Weak competitors– Sub-optimal guild members

• Invaders are r species– More disturbed habitats

• Escape from natural enemies• Co-evolution• Why is invasion limited in tropical forests?

Exotic establishment and community assembly

• How exotic species assemble when they reach a new territory• Wilson et al. 2000 – Roadside communities

– Two distinct groups in UK and NZ

• Community assembly by pre-adaptation• Only strong environmental filtering is able to reassemble

communities (Calcareous grasslands)

• Can this provide insight into assembly rules?

• Are there different assembly rules for different communities?

Conclusions

• There are no real conclusions• Plants interact and species differ

– Must be limitations to coexistence

Fig. 5.16: Profile through a part of the Botany Lawn.

• SUMMARY QUESTIONS:

Did any sections of this chapter seem irrelevant or only tangentially related to the topic?

Which assembly rules were best supported?

What are the major conclusions and are you convinced?