Origins & Maintenance of Diversity

Origins & Maintenance of Diversity
Please do not use the images in these PowerPoint slides without permission. “What puts the brakes on demons and prevents their success 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 What are we trying to explain? 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) Please do not use the images in these PowerPoint slides without permission. Whenever the rate of speciation outpaces extinction, diversity increases

What are we trying to explain? A variety of patterns, at multiple scales, including: 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”? Please do not use the images in these PowerPoint slides without permission. Processes & variables that influence speciation & extinction rates are especially important at regional to continental scales (the presence of a species on a continent is most often due to having arisen as a new species there), whereas ecological processes play increasingly important roles at smaller, local scales (the presence of a species in a 1-m2 quadrat is most often due, in the most proximate sense, to having arrived into that 1-m2 quadrat from elsewhere) 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

What are we trying to explain? A variety of patterns, at multiple scales, including: 2. Origin of “diversity” (actually S) within a site Appearance Extant S Disappearance Please do not use the images in these PowerPoint slides without permission. Cumulative S t

What are we trying to explain? A variety of patterns, at multiple scales, including: 2. Origin of “diversity” (actually S) within a site “cradle effect” Extant S Appearance Disappearance Please do not use the images in these PowerPoint slides without permission. The idea of “cradle” and “museum” effects originated with Stebbins (1974). Cumulative S t For more on “cradle” & “museum” effects, see Chown & Gaston (2000)

What are we trying to explain? A variety of patterns, at multiple scales, including: 2. Origin of “diversity” (actually S) within a site “museum effect” Appearance Extant S Disappearance Please do not use the images in these PowerPoint slides without permission. The idea of “cradle” and “museum” effects originated with Stebbins (1974). Cumulative S t For more on “cradle” & “museum” effects, see Chown & Gaston (2000)

Appearance (Speciation & Colonization) Disappearance (Extinction & Extirpation) At the largest spatial scales, speciation & extinction dominate At the smallest spatial scales, colonization & extirpation dominate Sregional Cumulative S Please do not use the images in these PowerPoint slides without permission. The key to understanding origins (and maintenance) is to understand the processes that drive appearance and disappearance of organisms in the focal site. Comparing the relative importance of dispersal vs. speciation to the presence of species in different scales of areas, dispersal tends to be relatively more important at smaller scales, since nearly every species present must have arrived as a colonist, rather than having been present upon its population becoming genetically isolated (i.e., having speciated). Slocal t t t1 t1

What are we trying to explain? A variety of patterns, at multiple scales, including: 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? Please do not use the images in these PowerPoint slides without permission. 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)

What are we trying to explain? A variety of patterns, at multiple scales, including: 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 Please do not use the images in these PowerPoint slides without permission.

What are we trying to explain? A variety of patterns, at multiple scales, including: 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 Please do not use the images in these PowerPoint slides without permission. 5. Maintenance of gradients in “diversity” (usually S) Is a particular gradient maintained over the long term? If so, what processes maintain the gradient?

Pianka (1966) – Six explanations for latitudinal gradients 1. Time – Origins; more time = more net speciation events Appearance of Species Extant S Disappearance of Species Cumulative S Please do not use the images in these PowerPoint slides without permission. t t1 t2

Pianka (1966) – Six explanations for latitudinal gradients 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” “cradle & museum effects” Extant S in small area Extant S in large area Appearance Please do not use the images in these PowerPoint slides without permission. Disappearance Cumulative S t

Time & area provide opportunities for various processes that could promote speciation For example, four current or formerly leading hypotheses for the origins of bird diversity in South America rely on large areas & long periods of time A. Forest Refugia Hypothesis (largely unsupported – at least for the Pleistocene) Please do not use the images in these PowerPoint slides without permission. Prance, G. T A review of the phytogeographic evidences for Pleistocene climate changes in the Neotropics. Ann. Missouri Bot. Gard. 69: Figure from Prance (1982)

Time & area provide opportunities for various processes that could promote speciation For example, four current or formerly leading hypotheses for the origins of bird diversity in South America rely on large areas & long periods of time B. Riverine Barrier Hypothesis Please do not use the images in these PowerPoint slides without permission. See Alfred Russel Wallace (1852) On Monkeys of the Amazon. See also: Ayers & Clutton-Brock (1992) – hypothesize that river size and speed are influencial towards creating barriers.

Time & area provide opportunities for various processes that could promote speciation For example, four current or formerly leading hypotheses for the origins of bird diversity in South America rely on large areas & long periods of time C. Andean Uplift Hypothesis (note north-south axis) Please do not use the images in these PowerPoint slides without permission. The north-south axis of this extensive mountain chain means a great deal of climate and other geographic variability is traversed by very steep and tall mountains!

Time & area provide opportunities for various processes that could promote speciation For example, four current or formerly leading hypotheses for the origins of bird diversity in South America rely on large areas & long periods of time D. Marine Transgression Hypothesis (e.g., Pebas Sea) Please do not use the images in these PowerPoint slides without permission. Bush, M.B and Oliveira, P.E. The rise and fall of the Refugial Hypothesis of Amazonian Speciation: a paleoecological perspective. Biota Neotrop. Jan/Abr 2006 vol. 6. ISSN Figure from Bush & Oliveira (2006)

Pianka (1966) – Six explanations for latitudinal gradients 2. Spatial Heterogeneity – Origins through “cradle” effect and maintenance through competitively-expressed niche differences Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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) Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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). Spatial Heterogeneity could be either physically based or biotically based Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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 Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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 Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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 Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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) Please do not use the images in these PowerPoint slides without permission.

Pianka (1966) – Six explanations for latitudinal gradients 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 Please do not use the images in these PowerPoint slides without permission.

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 Diversity Please do not use the images in these PowerPoint slides without permission. Mention Grime (1973) and Horn (1975)! This is just one example of many “Goldilocks Hypotheses” in ecology (conditions “just right” in the middle)! See Hixon & Brustoff (1983) Science for good example of biotic disturbance that results in support for IDH. See Collins et al. (1995) Ecology v. 76 for example of species richness decrease with frequency of burning (abiotic disturbance consequences counter to IDH). Disturbance frequency Time since disturbance Disturbance intensity

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 2. Competitive Equivalence or Equal Chance Hypothesis Please do not use the images in these PowerPoint slides without permission.

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 2. Competitive Equivalence or Equal Chance Hypothesis Hubbell (1979, 2001; Hubbell & Foster 1986) – Explore the community- level consequences of species equivalence (“ecological symmetry”) Please do not use the images in these PowerPoint slides without permission.

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 2. Competitive Equivalence or Equal Chance Hypothesis 3. Gradual Climate Change Hypothesis Please do not use the images in these PowerPoint slides without permission.

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 “Equilibrium” Hypotheses: 1. Niche Differentiation Hypothesis Please do not use the images in these PowerPoint slides without permission.

Species-packing mechanisms that illustrate “niche-differentiation” explanations for differences in diversity between two sites Please do not use the images in these PowerPoint slides without permission. Ricklefs (1987) used a “balls in a box” metaphor, whereas Roughgarden (2009) used a “books on a bookshelf” metaphor to illustrate these species-packing ideas. Figure from Remsen (1991)

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 “Equilibrium” Hypotheses: 1. Niche Differentiation Hypothesis 2. Circular Networks Hypothesis. Applied to competitors, but similar to loops in food webs (which ironically destabilize model food webs) Please do not use the images in these PowerPoint slides without permission. 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

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 “Equilibrium” Hypotheses: 1. Niche Differentiation Hypothesis 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 Please do not use the images in these PowerPoint slides without permission.

Janzen’s (1970) graphical representation of the Janzen-Connell model, one potential mechanism of “compensatory mortality” Please do not use the images in these PowerPoint slides without permission.

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” Please do not use the images in these PowerPoint slides without permission.

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? Please do not use the images in these PowerPoint slides without permission.

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) Please do not use the images in these PowerPoint slides without permission.

Tilman & Pacala (1993) – Several explanations for the maintenance of diversity in plant communities “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?” Please do not use the images in these PowerPoint slides without permission.

Pacala et al. (1996) – Observations from the real world (New England forests) suggest to them that a variety of life-history trade-offs maintain diversity… Please do not use the images in these PowerPoint slides without permission.

(High light growth rate)
Two trade-offs are especially important: 1. High light growth vs. low light survivorship Please do not use the images in these PowerPoint slides without permission. (High light growth rate) Figure from Pacala et al. (1996)

(Competitive “dominance”)
Two trade-offs are especially important: 1. High light growth vs. low light survivorship 2. “Competition” vs. “colonization” (Competitive “dominance”) Please do not use the images in these PowerPoint slides without permission. Figure from Pacala et al. (1996)

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… Pioneer vs. non-pioneer “guilds”, defined by trade-offs Habitat-association “guilds” Please do not use the images in these PowerPoint slides without permission. Dispersal limitation & competitive equivalence within “guilds” Density- & frequency-dependence also contribute

A major goal in diversity research is to understand the relative contributions that individual processes make But this is no easy task... Please do not use the images in these PowerPoint slides without permission.

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 Region 1 100 spp. of snails Region 2 50 spp. of snails Please do not use the images in these PowerPoint slides without permission. For an example of local richness related to regional richness, see: WitmanJD_etal_2004_PNAS.pdf 10 spp. 5 spp.

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 Experiment 1: Predator-removal experiments (indicated in pink) demonstrate an additional, ecological, “keystone predator” influence on local diversity Region 1 100 spp. of snails Region 2 50 spp. of snails Please do not use the images in these PowerPoint slides without permission. 10 spp. 5 spp. 5 spp. 3 spp.

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 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) Region 1 100 spp. of snails Region 2 50 spp. of snails Please do not use the images in these PowerPoint slides without permission. 5 spp. 3 spp. 10 spp. 5 spp. 5 spp. 3 spp.

Palmer (1994) – More than 120 explanations for the origin and maintenance of diversity and diversity gradients Please do not use the images in these PowerPoint slides without permission. Notice how Mike Palmer (1994) drew from population genetics concepts just as Mark Vellend (2010) did.

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...” Please do not use the images in these PowerPoint slides without permission.

Origins & Maintenance of Diversity

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