origins & maintenance of diversity “what puts the brakes on demons and prevents their success...

Origins & Maintenance of Diversity

“What puts the brakes on demons and prevents theirsuccess causing the destruction of diversity?”

J. Silvertown (2005), pg. 12 from “Demons in Eden”

What are we trying to explain?

A variety of patterns, at multiple scales, including:

1. Origin of “diversity” (actually S) within lineages


For example, why do some lineages contain more species than others?

This is a macroevolutionary question; relevant processes are speciation & extinction (therefore, including all the processes that influence speciation & extinction rates)

Whenever the rate of speciation outpaces extinction, diversity increases





2. Origin of “diversity” (actually S) within a site

A “site” may be a very large area (e.g., a continent), or a very small area (e.g., a 1-m2 quadrat of ground cover within a Louisiana pine savanna)

By what combination of processes did the species occupying a site “appear” within the site? How did the community “assemble”?

The question may also be restricted to a given taxon, guild, etc.

The relative influences of macroevolutionary processes (e.g., speciation) vs. ecological processes (such as colonization, competition), may differ from case-to-case and are likely to be scale dependent





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SAppearance

Disappearance

ExtantS

t





Cum

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ive

S

“cradleeffect”

Appearance

Disappearance

ExtantS

t

For more on “cradle” & “museum” effects, see Chown & Gaston (2000)





Cum

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ive

S

“museumeffect”

Appearance

DisappearanceExtant

S

t

For more on “cradle” & “museum” effects, see Chown & Gaston (2000)


Appearance (Speciation & Colonization)Disappearance (Extinction & Extirpation)

Sregional

t

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ive

S

t

At the largest spatial scales,speciation & extinction

dominate

At the smallest spatial scales,colonization & extirpation

dominate

Slocal

t1t1




3. Maintenance of diversity (S or D) within a site

Is status quo species richness maintained over the long term? [How to define the long term? Forever? Relative to the lifespans of the organisms?]

If so, is status quo species composition maintained?

In other words, do we need a mechanism that counters species loss that is equilibrial for S alone, or also for species composition?

Speciation & colonization could account for equilibrial S, but additional mechanisms would be required for the maintenance of species composition (species list and relative abundances)




4. Origin of gradients in “diversity” (usually S)

Why do sites differ in diversity?

At the largest spatial scales of sampling (comparisons of regions or continents) the answer probably depends mostly on differences in macroevolutionary & large-scale biogeographic processes, whereas at smaller spatial scales the answer is likely a combination of source-pool sizes & small-scale ecological processes


Is a particular gradient maintained over the long term? If so, what processes maintain the gradient?

5. Maintenance of gradients in “diversity” (usually S)



4. Origin of gradients in “diversity” (usually S)

Why do sites differ in diversity?

At the largest spatial scales of sampling (comparisons of regions or continents) the answer probably depends mostly on differences in macroevolutionary & large-scale biogeographic processes, whereas at smaller spatial scales the answer is likely a combination of source-pool sizes & small-scale ecological processes


Pianka (1966) – Six explanations for latitudinal gradients

1. Time – Origins; more time = more net speciation events

t

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S

Appearance of Species

Disappearanceof Species

ExtantS

t1 t2



Not specifically mentioned by Pianka, but area is also important; Willis (1922) used “Age and Area” to explain the richness of Indian flora; Terborgh (1973) used age and area to explain “favorableness”

Cum

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ive

S

“cradle & museumeffects”

Appearance

Disappearance

ExtantS inlargearea

t

ExtantS in

smallarea


Time & area provide opportunities for various

processes that could promote speciation

For example, four main hypotheses for the

origins of bird diversity in South America rely on

large areas & long periods of time

1. Forest Refugia Hypothesis


2. Andean Uplift HypothesisTime & area provide opportunities for various






3. Riverine Barrier HypothesisTime & area provide opportunities for various






4. Marine Transgression HypothesisTime & area provide opportunities for various







2. Spatial Heterogeneity – Origins through “cradle” effect and maintenance through competitively-expressed niche differences




Temporal Heterogeneity was later recognized for its potential contributions toward maintaining diversity, especially under circumstances in which “lottery competition” & the “storage effect” cause species to increase more in good years than they decrease in bad years (e.g., Chesson & Warner 1981)




Temporal Heterogeneity was later recognized for its potential contributions toward maintaining diversity, especially under circumstances in which “lottery competition” & the “storage effect” cause species to increase more in good years than they decrease in bad years (e.g., Chesson & Warner 1981).

Spatial Heterogeneity could be either physically based or biotically based



3. Competition – Origins through greater species packing (narrower niches due to greater constancy & impact of competitive interactions) & maintenance through competitively-expressed niche differences; requires spatial heterogeneity



3. Competition – Origins through greater species packing (narrower niches due to greater constancy & impact of competitive interactions) & maintenance through competitively-expressed niche differences; requires spatial heterogeneity

4. Predation – Maintenance through “keystone” predators



5. Climatic Stability – Origins; stability was proposed as a mechanism that could increase niche packing

Maintenance – through competitively-expressed niche differences & etc.

Dramatic climatic instability could alternatively increase extinction rates (e.g., ice ages), and in the extreme set the “Time” clock back to zero


Temporal Heterogeneity was later recognized for its potential contributions toward maintaining diversity, especially under circumstances in which “lottery competition” & the “storage effect” cause species to increase more in good years than they decrease in bad years (e.g., Chesson & Warner 1981)


5. Climatic Stability – Origins; stability was proposed as a mechanism that could increase niche packing

Maintenance – through competitively-expressed niche differences & etc.

Dramatic climatic instability could alternatively increase extinction rates (e.g., ice ages), and in the extreme set the “Time” clock back to zero



6. Productivity. Origins & maintenance through influence on trophic levels, population sizes, etc.

Productivity was later championed by Wright (1983) and Currie & Paquin (1987) in the Species-Energy Hypothesis


Connell (1978) – Six explanations for high rain forest & coral reef diversity

Although he used a restrictive definition of “equilibrium”, his paper was a useful foil against the proliferation of equilibrial hypotheses in light of the empirical evidence for abundant non-equilibrial dynamics in nature

“Non-equilibrium” Hypotheses:

1. Intermediate Disturbance Hypothesis

Disturbance frequencyTime since disturbanceDisturbance intensity

Div

ersi

ty






2. Competitive Equivalence or Equal Chance Hypothesis







Hubbell (1979, 2001; Hubbell & Foster 1986) – Explore the community- level consequences of species equivalence (“ecological symmetry”)







3. Gradual Climate Change Hypothesis




“Equilibrium” Hypotheses:

1. Niche Differentiation Hypothesis

Species-packing mechanismsthat illustrate “niche-

differentiation” explanations for differences in diversity

between two sites


Figure from Remsen (1991)






2. Circular Networks Hypothesis. Applied to competitors, but similar to loops in food webs (which ironically destabilize model food webs)

Competitive hierarchy in which A > B > C, but C eliminates A directly, e.g., A overshadows B,

B overshadows C, but C poisons A

A

C B






2. Circular Networks Hypothesis. Applied to competitors, but similar to loops in food webs (which ironically destabilize model food webs)

3. Compensatory Mortality Hypothesis. Rare species advantage


Janzen’s (1970) graphical representation of the Janzen-Connell model, one potential mechanism of “compensatory mortality”


Tilman & Pacala (1993) – Several explanations for the maintenance of diversity in plant communities

“The extraordinary diversity of the terrestrial fauna, which is much greater than that of the marine fauna, is clearly due largely to the

diversity provided by terrestrial plants… on the whole the problem still remains, but in the new form: Why are there so many kinds of

plants?”

Hutchinson (1959) “Homage to Santa Rosalia”

According to the competitive exclusion principle, “close competitors” share requirements for limiting resources,

and “complete competitors cannot coexist”

Hutchinson (1961) “The paradox of the plankton”


So, how can there be more species than limiting resources?


Hutchinson (1959, 1961) criticized the very restrictive models that were being used to attempt to “explain” diversity

The models (Lotka-Volterra-style) assumed a spatially & temporally homogeneous habitat, populations at equilibrium, a 2-tiered trophic

structure, no limiting physical factors, no neighborhood effects, & simple life histories

Most of these assumptions are broken in the real world; each broken assumption could result in the maintenance of unlimited species richness!

(Tilman & Pacala 1993)



“Almost any model that assumes some sort of environmental complexity, and that assumes allocation-based trade-offs in the abilities of organisms to

respond to their constraints, has the potential to predict the existence of many more species than there are limiting resources and physical factors”

Suggested that the question should shift from:

“Why are there so many species?”

To:

“Why are there a particular number of species, and not many, many more?”



Observations from the real world (New England forests) suggest to them that a variety of life-history trade-offs

maintain diversity…

Two trade-offs are especially important:1. High light growth vs. low light survivorship

Figure from Pacala et al. (1996)

(High light growth rate)

Two trade-offs are especially important:1. High light growth vs. low light survivorship2. “Competition” vs. “colonization”

Figure from Pacala et al. (1996)

(Co

mp

etit

ive

“do

min

ance

”)

Habitat-association “guilds”Pioneer vs. non-pioneer “guilds”, defined by trade-offs

Hubbell and Foster (1986) – Several explanations for the maintenance of diversity in plant communities

Observations from the real world (Panamanian forests) suggest to them that a combination of biology, chance

& history determine a site’s diversity…

Density- & frequency-dependence also contributeDispersal limitation & competitive equivalence within “guilds”


A major goal in diversity research is to understand the relative contributions that individual processes make

But this is no easy task...


10 spp. 5 spp.

Region 1100 spp. of snails



Example: Imagine two regions; sampled plots yield a similar ratio of diversity to the larger regional totals, suggesting that differences in regional

species-pool sizes contribute toward determining local species richness

10 spp. 5 spp.



5 spp. 3 spp.


Experiment 1: Predator-removal experiments (indicated in pink) demonstrate an additional, ecological, “keystone predator” influence on local diversity



10 spp. 5 spp.



5 spp. 3 spp.

5 spp. 3 spp.


Experiment 1: Predator-removal experiments (indicated in pink) demonstrate an additional, ecological, “keystone predator” influence on local diversity

Experiment 2: Substrate-homogenization experiments (blue) demonstrate an additional, ecological, influence on local diversity, perhaps due to competition (e.g.,

each species competes for algae best on a specific substrate type)



Palmer (1994) – More than 100 explanations for the origin and maintenance of diversity and diversity gradients


Ricklefs & Schluter (1993) – Much of the traditional emphasis has been on species richness and diversity per se, but what of species

composition? Ricklefs & Schluter (1993) advocate much more “use [of] historical, biogeographic, and systematic (including molecular)

data to reconstruct the development of species assemblages...”


origins & maintenance of diversity “what puts the brakes on demons and prevents their success...

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