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Origins & Maintenance of Diversity “What puts the brakes on demons and prevents their success causing the destruction of diversity?” J. Silvertown (2005), pg. 12 from “Demons in Eden” Origins & Maintenance of Diversity What are What are we we trying tryingtotoexplain? 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 Origins & Maintenance of Diversity 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”? 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 Origins & Maintenance of Diversity What are we trying to explain? A variety of patterns, at multiple scales, including: 2. Origin of “diversity” (actually S) within a site Disappearance Extant S Cumulative S Appearance t Origins & Maintenance of Diversity What are we trying to explain? A variety of patterns, at multiple scales, including: 2. Origin of “diversity” (actually S) within a site Disappearance Extant S Cumulative S Appearance “cradle effect” t For more on “cradle” & “museum” effects, see Chown & Gaston (2000) Origins & Maintenance of Diversity What are we trying to explain? A variety of patterns, at multiple scales, including: 2. Origin of “diversity” (actually S) within a site Disappearance Cumulative S Appearance “museum effect” Extant S t For more on “cradle” & “museum” effects, see Chown & Gaston (2000) Origins & Maintenance of Diversity Appearance (Speciation & Colonization) Disappearance (Extinction & Extirpation) Cumulative S At the largest spatial scales, speciation & extinction dominate At the smallest spatial scales, colonization & extirpation dominate Sregional Slocal t t1 t t1 Origins & Maintenance of Diversity 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? 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) Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 5. Maintenance of gradients in “diversity” (usually S) Is a particular gradient maintained over the long term? If so, what processes maintain the gradient? Origins & Maintenance of Diversity Pianka (1966) – Six explanations for latitudinal gradients 1. Time – Origins; more time = more net speciation events Disappearance of Species Extant S Cumulative S Appearance of Species t t1 t2 Origins & Maintenance of Diversity 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” Disappearance Cumulative S Appearance “cradle & museum effects” t Extant S in small area Extant S in large area Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 2. Andean Uplift Hypothesis Origins & Maintenance of Diversity 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 3. Riverine Barrier Hypothesis Origins & Maintenance of Diversity 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 4. Marine Transgression Hypothesis Origins & Maintenance of Diversity Pianka (1966) – Six explanations for latitudinal gradients 2. Spatial Heterogeneity – Origins through “cradle” effect and maintenance through competitively-expressed niche differences Origins & Maintenance of Diversity 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) Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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) Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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: Diversity 1. Intermediate Disturbance Hypothesis Disturbance frequency Time since disturbance Disturbance intensity Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 communitylevel consequences of species equivalence (“ecological symmetry”) Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity Species-packing mechanisms that illustrate “nichedifferentiation” explanations for differences in diversity between two sites Figure from Remsen (1991) Origins & Maintenance of Diversity Species-packing mechanisms that illustrate “nichedifferentiation” explanations for differences in diversity between two sites Figure from Remsen (1991) Origins & Maintenance of Diversity Species-packing mechanisms that illustrate “nichedifferentiation” explanations for differences in diversity between two sites Figure from Remsen (1991) Origins & Maintenance of Diversity 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) 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 Origins & Maintenance of Diversity 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 Origins & Maintenance of Diversity Janzen’s (1970) graphical representation of the Janzen-Connell model, one potential mechanism of “compensatory mortality” Origins & Maintenance of Diversity 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” Origins & Maintenance of Diversity According to the competitive exclusion principle, “close competitors” share requirements for limiting resources, and “complete competitors cannot coexist” So, how can there be more species than limiting resources? Hutchinson (1961) “The paradox of the plankton” Origins & Maintenance of Diversity 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) Origins & Maintenance of Diversity 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?” Origins & Maintenance of Diversity Tilman & Pacala (1993) – Several explanations for the maintenance of diversity in plant communities 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) (Competitive “dominance”) Two trade-offs are especially important: 1. High light growth vs. low light survivorship 2. “Competition” vs. “colonization” Figure from Pacala et al. (1996) Origins & Maintenance of Diversity 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” Dispersal limitation & competitive equivalence within “guilds” Density- & frequency-dependence also contribute Origins & Maintenance of Diversity A major goal in diversity research is to understand the relative contributions that individual processes make But this is no easy task... Origins & Maintenance of Diversity 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 10 spp. Region 2 50 spp. of snails 5 spp. Origins & Maintenance of Diversity 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 10 spp. 5 spp. Region 2 50 spp. of snails 5 spp. 3 spp. Origins & Maintenance of Diversity 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 5 spp. 10 spp. 5 spp. Region 2 50 spp. of snails 3 spp. 5 spp. 3 spp. Origins & Maintenance of Diversity Palmer (1994) – More than 100 explanations for the origin and maintenance of diversity and diversity gradients Origins & Maintenance of Diversity 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...”