Photo: Sagebrush habitat, Yellowstone NP. © 2016 Delena Norris-Tull
How ecosystems maintain diversity
Summary of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
In a review of research on how ecosystems maintain species diversity, Chesson (2000) stated that, “The most common meaning of diversity maintenance is coexistence in the same spatial region of species having similar ecology… Another meaning of diversity maintenance refers not to coexistence of fixed sets of species but to the maintenance of species richness and evenness over long timescales, necessitating consideration of speciation and extinction rates, and infrequent colonizations.”
Chesson, 2000, points out that “species coexistence… may be stable or unstable. Stable coexistence can be quantified by the long-term rates at which community members recover from low density… Coexistence mechanisms function in two major ways: They may be (a) equalizing because they tend to minimize average fitness differences between species, or (b) stabilizing because they tend to increase negative intraspecific interactions relative to negative interspecific interactions. Stabilizing mechanisms are essential for species coexistence.”
Chesson, 2000, found that, “Models of unstable coexistence, in which species diversity slowly decays over time, have focused almost exclusively on equalizing mechanisms. These models would be more robust if they also included stabilizing mechanisms… Models of unstable coexistence invite a broader view of diversity maintenance incorporating species turnover.” Chesson describes the Lotka-Volterra competition model, the gold standard for mathematical calculations of species coexistence. “The defining feature of Lotka-Volterra competition is that per capita growth rates are linear decreasing functions of the densities of each of the species… ‘Species j cannot competitively exclude species i if the effect that species j has on itself is more than the effect that species j has on species i….’ [In other words], The criteria for species coexistence in a two-species system… can be read very simply as ‘intraspecific competition must be greater than interspecific competition.’”
Chesson, 2000, points out that, “models with nonlinear per capita growth rates can be written [using the Lotka-Volterra equations]… by making the competition coefficients functions of density…, and the results…remain true, provided the competition coefficients are evaluated for the resident [species] at equilibrium and the invader at zero. Lotka-Volterra models and their nonlinear extensions may be thought of as models of direct competition: Individual organisms have immediate direct negative effects on other individuals.”
Chesson, 2000, then explains models with explicit resource dynamics, which “are not defined by a mechanism of competition. A mechanistic understanding of competition usefully begins with Tilmans’ resource competition theory in which species jointly limited by a single resource are expected to obey the R* rule. For any given species, R* is the resource level at which the species is just able to persist. The winner in competition is the species with the lowest R* value. A species’ R* value, however, reflects not the ability of members of the species to extract resources when they are in low concentration, but their ability to grow and reproduce rapidly enough, at low resource levels, to compensate for tissue death and mortality, which are affected by such factors as grazing and predation… It is the overall fitness of a species that leads to it's R* value.”
Chesson, 2000, points out that, “Tradeoffs play a major role in species coexistence: Advantages that one species may have over others are offset by compensating disadvantages. However, examined in relation to the R* rule there is clearly more to stable coexistence. With just a single resource as a limiting factor, tradeoffs may make the R* values of different species more nearly equal, but that would not lead to stable coexistence... Similarity in average fitness does not lead to stable coexistence,… [as] only one [species] can increase from low density in the presence of the other.”
“Coexistence with resource partitioning contrasts with this. Using MacArthur’s mechanistic derivation of Lotka-Volterra competition… The two species coexist if the stabilizing term is greater in magnitude than the fitness difference term because then both [species] have positive growth as invaders… The assumptions of the model entail that doing well on some resources means doing less well on others. Each species has density-dependent feedback loops with its resources that limit it intraspecifically and limit other species interspecifically. However, limited resource overlap and tradeoffs in resource benefits means that intraspecific limitation is enhanced relative to interspecific limitation. This concentration of intraspecific effects relative to interspecific effects is the essence of stabilization.”
Chesson, 2000, points out that MacArthur’s equation “generalizes to multispecies communities involved in diffuse competition, that is, where competition between species involves comparable interaction strengths for all pairs of species… [In the model’s equation] the first term is an average fitness comparison and the second term is a stabilizing term. Without this term, the first term of necessity leads to loss of all species… However, if the stabilizing term is larger in magnitude than the relative average fitness term for the worst species, then all species coexist. These two general terms may involve different mechanisms. Those [terms] reducing the magnitude of the fitness difference term will be referred to as equalizing mechanisms, while those increasing the magnitude of the stabilizing term will be referred to as stabilizing mechanisms. In the absence of the stabilizing term, equalizing mechanisms can, at best, slow competitive exclusion; but in the presence of stabilizing mechanisms, equalizing mechanisms may enable coexistence.”
Chesson, 2000, reviewed research on niche space, “conceived as having four axes: resources, predators (and other natural enemies), time, and space.” For example, “A species consumes resources, and therefore has an effect on resource density. Individuals of a species may also reproduce, grow, or survive in response to resources. The essential way in which stabilization occurs is most clearly seen with resource competition. If a species depends most on a particular resource (strong response), and also reduces that resource (strong effect), then it has a density-dependent feedback loop with the resource and is limited by it. If a second species has a similar relationship with a different resource, then even though the species each consume some of the resource on which the other depends most strongly (limited resource overlap) each species depresses its own growth more than it depresses the growth of other species. The result is stable coexistence.”
Chesson, 2000, points out that “stable coexistence mechanisms may be fluctuation dependent or fluctuation independent… Fluctuation-independent mechanisms are resource partitioning and frequency-dependent predation.” Two examples of resource partitioning are spatial partitioning of soil moisture and grazing. “These mechanisms can function in the presence of environmental fluctuations… We can think of the operation of the mechanism as independent of the [environmental] fluctuations in the system.”
Reference:
Next Sections on the role of diversity:
Next Sections on the research on the success of invasive species:
How ecosystems maintain diversity
Summary of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
In a review of research on how ecosystems maintain species diversity, Chesson (2000) stated that, “The most common meaning of diversity maintenance is coexistence in the same spatial region of species having similar ecology… Another meaning of diversity maintenance refers not to coexistence of fixed sets of species but to the maintenance of species richness and evenness over long timescales, necessitating consideration of speciation and extinction rates, and infrequent colonizations.”
Chesson, 2000, points out that “species coexistence… may be stable or unstable. Stable coexistence can be quantified by the long-term rates at which community members recover from low density… Coexistence mechanisms function in two major ways: They may be (a) equalizing because they tend to minimize average fitness differences between species, or (b) stabilizing because they tend to increase negative intraspecific interactions relative to negative interspecific interactions. Stabilizing mechanisms are essential for species coexistence.”
Chesson, 2000, found that, “Models of unstable coexistence, in which species diversity slowly decays over time, have focused almost exclusively on equalizing mechanisms. These models would be more robust if they also included stabilizing mechanisms… Models of unstable coexistence invite a broader view of diversity maintenance incorporating species turnover.” Chesson describes the Lotka-Volterra competition model, the gold standard for mathematical calculations of species coexistence. “The defining feature of Lotka-Volterra competition is that per capita growth rates are linear decreasing functions of the densities of each of the species… ‘Species j cannot competitively exclude species i if the effect that species j has on itself is more than the effect that species j has on species i….’ [In other words], The criteria for species coexistence in a two-species system… can be read very simply as ‘intraspecific competition must be greater than interspecific competition.’”
Chesson, 2000, points out that, “models with nonlinear per capita growth rates can be written [using the Lotka-Volterra equations]… by making the competition coefficients functions of density…, and the results…remain true, provided the competition coefficients are evaluated for the resident [species] at equilibrium and the invader at zero. Lotka-Volterra models and their nonlinear extensions may be thought of as models of direct competition: Individual organisms have immediate direct negative effects on other individuals.”
Chesson, 2000, then explains models with explicit resource dynamics, which “are not defined by a mechanism of competition. A mechanistic understanding of competition usefully begins with Tilmans’ resource competition theory in which species jointly limited by a single resource are expected to obey the R* rule. For any given species, R* is the resource level at which the species is just able to persist. The winner in competition is the species with the lowest R* value. A species’ R* value, however, reflects not the ability of members of the species to extract resources when they are in low concentration, but their ability to grow and reproduce rapidly enough, at low resource levels, to compensate for tissue death and mortality, which are affected by such factors as grazing and predation… It is the overall fitness of a species that leads to it's R* value.”
Chesson, 2000, points out that, “Tradeoffs play a major role in species coexistence: Advantages that one species may have over others are offset by compensating disadvantages. However, examined in relation to the R* rule there is clearly more to stable coexistence. With just a single resource as a limiting factor, tradeoffs may make the R* values of different species more nearly equal, but that would not lead to stable coexistence... Similarity in average fitness does not lead to stable coexistence,… [as] only one [species] can increase from low density in the presence of the other.”
“Coexistence with resource partitioning contrasts with this. Using MacArthur’s mechanistic derivation of Lotka-Volterra competition… The two species coexist if the stabilizing term is greater in magnitude than the fitness difference term because then both [species] have positive growth as invaders… The assumptions of the model entail that doing well on some resources means doing less well on others. Each species has density-dependent feedback loops with its resources that limit it intraspecifically and limit other species interspecifically. However, limited resource overlap and tradeoffs in resource benefits means that intraspecific limitation is enhanced relative to interspecific limitation. This concentration of intraspecific effects relative to interspecific effects is the essence of stabilization.”
Chesson, 2000, points out that MacArthur’s equation “generalizes to multispecies communities involved in diffuse competition, that is, where competition between species involves comparable interaction strengths for all pairs of species… [In the model’s equation] the first term is an average fitness comparison and the second term is a stabilizing term. Without this term, the first term of necessity leads to loss of all species… However, if the stabilizing term is larger in magnitude than the relative average fitness term for the worst species, then all species coexist. These two general terms may involve different mechanisms. Those [terms] reducing the magnitude of the fitness difference term will be referred to as equalizing mechanisms, while those increasing the magnitude of the stabilizing term will be referred to as stabilizing mechanisms. In the absence of the stabilizing term, equalizing mechanisms can, at best, slow competitive exclusion; but in the presence of stabilizing mechanisms, equalizing mechanisms may enable coexistence.”
Chesson, 2000, reviewed research on niche space, “conceived as having four axes: resources, predators (and other natural enemies), time, and space.” For example, “A species consumes resources, and therefore has an effect on resource density. Individuals of a species may also reproduce, grow, or survive in response to resources. The essential way in which stabilization occurs is most clearly seen with resource competition. If a species depends most on a particular resource (strong response), and also reduces that resource (strong effect), then it has a density-dependent feedback loop with the resource and is limited by it. If a second species has a similar relationship with a different resource, then even though the species each consume some of the resource on which the other depends most strongly (limited resource overlap) each species depresses its own growth more than it depresses the growth of other species. The result is stable coexistence.”
Chesson, 2000, points out that “stable coexistence mechanisms may be fluctuation dependent or fluctuation independent… Fluctuation-independent mechanisms are resource partitioning and frequency-dependent predation.” Two examples of resource partitioning are spatial partitioning of soil moisture and grazing. “These mechanisms can function in the presence of environmental fluctuations… We can think of the operation of the mechanism as independent of the [environmental] fluctuations in the system.”
Reference:
- Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual Review of Ecological Systems, 31, 343-366.
Next Sections on the role of diversity:
- Fluctuation Dependent Mechanisms
- Competition-based coexistence mechanisms
- Niche Differences
- Species Richness
Next Sections on the research on the success of invasive species: