Photo: Chihuahuan Desert, Big Bend National Park. © 2018 Delena Norris-Tull
Niche differences: Their role in maintaining plant diversity
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
In related research, Adler, Ellner, and Levine (2010) examined the role of niche differences and “their effect on coexistence in natural communities.” In previous research on the function of niches in ecosystems, “many field studies [prior to 2000] have identified niche differences based on resource partitioning, frequency-dependent predation and species-specific responses to spatial and temporal heterogeneity in the environment. These differences stabilize coexistence by causing species to limit themselves more than they limit their competitors.”
Some ecologists have “questioned the potential for niche theory to explain how hundreds of species can coexist on a handful of limited resources” (Adler, et al., 2010). In a modeling study of field data, Adler, et al., 2010, challenged a newer theory, “the neutral theory of biodiversity,… [which proposes that] similarity in species’ competitive abilities, rather than niche differences, might explain the diversity of natural communities.”
Adler, Ellner, and Levine, 2010, attempted to “(1) quantify the stabilizing effect of niche differences in a sagebrush steppe community, (2) compare the relative strength of stabilizing mechanisms and fitness differences and (3) partition the stabilizing affect into contributions from temporal fluctuations and individual life stages (recruitment, growth and survival).” They analyzed 22 years of field demographic data from 1922 to 1957, collected on plants in the U.S. Sheep Experiment Station near Dubois, Idaho, using “a spatially explicit individual-based model (IBM) and a spatially implicit, integral projection model (IPM)…. After showing that the models reproduce historical abundances, size distributions and spatial patterns, we perturbed model parameters to simulate community dynamics in the absence of stabilizing mechanisms. We used the IBM to study extinction times and the IPM to estimate invasion growth rates and species’ equilibrium abundances without the complicating influence of demographic stochasticity…. Both models describe how recruitment, growth and survival depend on interannual variability and interspecific interactions.” [According to Wikipedia, “Demographic stochasticity refers to variability in population growth arising from sampling random births and deaths in a population of finite size.”]
Adler, et al., 2010, found that the four main native species in the sagebrush community (Artemisia tripartita, and the perennial bunchgrasses Pseudoroegneria spicata, Hesperostipa comata, and Poa secunda) “accounted for over 70% of basal cover (grasses) and 60% of canopy cover (shrubs and forbs).” The models they developed enabled them to fit the field data to the models, and then use simulations to remove stabilizing mechanisms over simulated time periods. “Species’ invasion growth rates are decomposed into… fitness differences and stabilizing niche differences. Stabilizing niche differences include all processes that cause species to limit conspecific more than heterospecific individuals, creating an advantage when rare. Fitness differences… are those differences in intrinsic demographic rates that favour one species over another and lead to competitive exclusion. Once niche differences are removed,… fitness differences (and the sensitivity of population growth to these differences) determine the rate of competitive exclusion.” The models allowed them to simulate invasion growth rates, in 50-year increments, up to 3,000 years. While they used native species in their models, their results have important ramifications for the success or failure of non-native invasive plant species.
With the use of data perturbations within the models, Adler, et al., 2010, discovered “an excess of niche differences: Stabilizing mechanisms were not only strong enough to maintain diversity in the system but were much larger than required to overcome the fitness differences. Evidence for strong stabilization comes from the positive invasion growth rates of all four species… These growth rates correspond to population doubling times for the three grasses (when rare) of less than 7 years. The positive invasion growth rates reflect stronger limitation by intraspecific than interspecific competition, the signature of niche differentiation…. The stabilizing mechanisms were strong enough to produce essentially indefinite coexistence among the three grasses. Although A. tripartita did go extinct after c. 240 simulated years, the cause was demographic stochasticity amplified by the limited spatial extent of the simulation. We know that competitive pressure did not cause these extinctions because A. tripartita had positive invasion growth rates” in both models. “Removal of all stabilizing mechanisms caused major qualitative changes in community dynamics… Extinction times for A. tripartita decreased to less than 50 years and the grasses no longer coexisted in stochastic simulations.”
The modeling conducted by Adler, et al., 2010, demonstrates “that niches are essential for stable coexistence. However, [these] results also indicate relatively small fitness differences between species... Because of these small fitness differences, competitive exclusion in the IBM occurred on the order of centuries even in the absence of stabilizing mechanisms.” Their results indicated the following: “(1) Fluctuation-dependent mechanisms are less important than fluctuation-independent mechanisms in this community... The weak effects of interannual variability in precipitation and temperature [fluctuation-dependent mechanisms] are surprising… (2) Fluctuation-independent mechanisms, which might include spatial partitioning of soil moisture, operate throughout the life cycle but appear to be especially powerful at the recruitment stage... (3) [They] found strong links between intra- and interspecific competition and the spatial patterning of individuals.” Their results challenge the validity of the neutral theory of biodiversity. “Stabilizing mechanisms were much stronger than required to overcome the fitness disadvantages of the inferior competitors (in this case, A. tripartita), demonstrating that coexistence in this natural system is stable and that it’s biodiversity cannot be understood without consideration of the niche differences that neutral theory ignores” (Adler, et al., 2010).
Maron and Marler (2007, 2008) in Montana, studied the factors that may cause increased species diversity to be an inhibitor to invasion.
Maron and Marler, 2008, carried out four experiments that yielded the following results:
(1) “High native plant diversity substantially inhibited exotic invasion, and… reduced the impact of invaders on the native community… Interestingly, however, the slope of the diversity-invasibility/impact relationship differed among the three exotics… used [spotted knapweed, Centauria maculosa, dalmatian toadflax, Linaria dalmatica, and sulfur cinquefoil, Potentilla recta]…suggesting that there are inherent differences even among strong invaders in how they respond to native diversity and resource supply.”
(2) “The impacts of exotics on native biomass scaled directly as a function of exotic biomass… Exotics had their greatest impacts in assemblages that were heavily invaded. This suggests that the same mechanisms that can control invasibility affect impact… Interestingly, however, when variation in exotic biomass was statistically ‘controlled,’ there was still a strong negative effect of diversity on impact… It may be that greater resource preemption by natives in more diverse assemblages not only reduces invasion but also limits per capita impacts.”
(3) “…. Knapweed had strong impacts on the abiotic environment (depressing soil moisture and nitrogen). Exotics are often thought to be successful because they preempt resources from natives,… but in our case, knapweed impacts on resource levels did not correlate with its impacts on native plant biomass… Resource supply had differential effects on [knapweed] invasibility and impact. Whereas water additions increased community susceptibility to knapweed invasion, the impact of knapweed on recipient [native plant] assemblages was relatively unaffected by increased resource supply… Gaining insight into how resource supply and changing diversity jointly influence invader impact is of growing importance.”
(4) “The three exotic invaders varied substantially in their impacts on native biomass. Knapweed was clearly the most potent invader… [a likely] explanation for the greater relative success of knapweed… Knapweed has substantial fall germination [when compared to toadflax and cinquefoil]. [Also] knapweed is allelopathic.”
(5) “Aside from differences in germination timing, other differences among the three exotics in the extent of their functional overlap with natives may account for variation in invasive success. For example, the one monoculture that had the greatest resistance to Potentilla recta invasion was that of the native cinquefoil, Potentilla arguta… This suggests that at least for P. recta, the level of functional overlap with natives may play a role in limiting its invasion success.”
(6) “The productivity of uninvaded [native plant] assemblages increased with increasing diversity… The degree to which invasion increased total community productivity was striking. Knapweed invasion, in particular, actually switched the slope of the diversity-productivity relationship from positive in uninvaded assemblages to negative across invaded assemblages… Zavaleta and Hulvey (2004) similarly found that exotic yellow star thistle… increased total assemblage biomass more at low than at high levels of diversity.”
Maron and Marler, 2008, concluded, “Thus, while our results suggest that diversity can provide an important ‘function’ in resisting invasion, using yield as a benchmark for valuing the importance of diversity may be of limited utility since invasion can so dramatically influence yield. The fact that invaders increase community biomass even in diverse assemblages suggests that native plants may not fill all available niches.”
References:
Next Section on research on the role of diversity in preventing invasions:
Next Sections on research on the success of invasive species:
Niche differences: Their role in maintaining plant diversity
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
In related research, Adler, Ellner, and Levine (2010) examined the role of niche differences and “their effect on coexistence in natural communities.” In previous research on the function of niches in ecosystems, “many field studies [prior to 2000] have identified niche differences based on resource partitioning, frequency-dependent predation and species-specific responses to spatial and temporal heterogeneity in the environment. These differences stabilize coexistence by causing species to limit themselves more than they limit their competitors.”
Some ecologists have “questioned the potential for niche theory to explain how hundreds of species can coexist on a handful of limited resources” (Adler, et al., 2010). In a modeling study of field data, Adler, et al., 2010, challenged a newer theory, “the neutral theory of biodiversity,… [which proposes that] similarity in species’ competitive abilities, rather than niche differences, might explain the diversity of natural communities.”
Adler, Ellner, and Levine, 2010, attempted to “(1) quantify the stabilizing effect of niche differences in a sagebrush steppe community, (2) compare the relative strength of stabilizing mechanisms and fitness differences and (3) partition the stabilizing affect into contributions from temporal fluctuations and individual life stages (recruitment, growth and survival).” They analyzed 22 years of field demographic data from 1922 to 1957, collected on plants in the U.S. Sheep Experiment Station near Dubois, Idaho, using “a spatially explicit individual-based model (IBM) and a spatially implicit, integral projection model (IPM)…. After showing that the models reproduce historical abundances, size distributions and spatial patterns, we perturbed model parameters to simulate community dynamics in the absence of stabilizing mechanisms. We used the IBM to study extinction times and the IPM to estimate invasion growth rates and species’ equilibrium abundances without the complicating influence of demographic stochasticity…. Both models describe how recruitment, growth and survival depend on interannual variability and interspecific interactions.” [According to Wikipedia, “Demographic stochasticity refers to variability in population growth arising from sampling random births and deaths in a population of finite size.”]
Adler, et al., 2010, found that the four main native species in the sagebrush community (Artemisia tripartita, and the perennial bunchgrasses Pseudoroegneria spicata, Hesperostipa comata, and Poa secunda) “accounted for over 70% of basal cover (grasses) and 60% of canopy cover (shrubs and forbs).” The models they developed enabled them to fit the field data to the models, and then use simulations to remove stabilizing mechanisms over simulated time periods. “Species’ invasion growth rates are decomposed into… fitness differences and stabilizing niche differences. Stabilizing niche differences include all processes that cause species to limit conspecific more than heterospecific individuals, creating an advantage when rare. Fitness differences… are those differences in intrinsic demographic rates that favour one species over another and lead to competitive exclusion. Once niche differences are removed,… fitness differences (and the sensitivity of population growth to these differences) determine the rate of competitive exclusion.” The models allowed them to simulate invasion growth rates, in 50-year increments, up to 3,000 years. While they used native species in their models, their results have important ramifications for the success or failure of non-native invasive plant species.
With the use of data perturbations within the models, Adler, et al., 2010, discovered “an excess of niche differences: Stabilizing mechanisms were not only strong enough to maintain diversity in the system but were much larger than required to overcome the fitness differences. Evidence for strong stabilization comes from the positive invasion growth rates of all four species… These growth rates correspond to population doubling times for the three grasses (when rare) of less than 7 years. The positive invasion growth rates reflect stronger limitation by intraspecific than interspecific competition, the signature of niche differentiation…. The stabilizing mechanisms were strong enough to produce essentially indefinite coexistence among the three grasses. Although A. tripartita did go extinct after c. 240 simulated years, the cause was demographic stochasticity amplified by the limited spatial extent of the simulation. We know that competitive pressure did not cause these extinctions because A. tripartita had positive invasion growth rates” in both models. “Removal of all stabilizing mechanisms caused major qualitative changes in community dynamics… Extinction times for A. tripartita decreased to less than 50 years and the grasses no longer coexisted in stochastic simulations.”
The modeling conducted by Adler, et al., 2010, demonstrates “that niches are essential for stable coexistence. However, [these] results also indicate relatively small fitness differences between species... Because of these small fitness differences, competitive exclusion in the IBM occurred on the order of centuries even in the absence of stabilizing mechanisms.” Their results indicated the following: “(1) Fluctuation-dependent mechanisms are less important than fluctuation-independent mechanisms in this community... The weak effects of interannual variability in precipitation and temperature [fluctuation-dependent mechanisms] are surprising… (2) Fluctuation-independent mechanisms, which might include spatial partitioning of soil moisture, operate throughout the life cycle but appear to be especially powerful at the recruitment stage... (3) [They] found strong links between intra- and interspecific competition and the spatial patterning of individuals.” Their results challenge the validity of the neutral theory of biodiversity. “Stabilizing mechanisms were much stronger than required to overcome the fitness disadvantages of the inferior competitors (in this case, A. tripartita), demonstrating that coexistence in this natural system is stable and that it’s biodiversity cannot be understood without consideration of the niche differences that neutral theory ignores” (Adler, et al., 2010).
Maron and Marler (2007, 2008) in Montana, studied the factors that may cause increased species diversity to be an inhibitor to invasion.
Maron and Marler, 2008, carried out four experiments that yielded the following results:
(1) “High native plant diversity substantially inhibited exotic invasion, and… reduced the impact of invaders on the native community… Interestingly, however, the slope of the diversity-invasibility/impact relationship differed among the three exotics… used [spotted knapweed, Centauria maculosa, dalmatian toadflax, Linaria dalmatica, and sulfur cinquefoil, Potentilla recta]…suggesting that there are inherent differences even among strong invaders in how they respond to native diversity and resource supply.”
(2) “The impacts of exotics on native biomass scaled directly as a function of exotic biomass… Exotics had their greatest impacts in assemblages that were heavily invaded. This suggests that the same mechanisms that can control invasibility affect impact… Interestingly, however, when variation in exotic biomass was statistically ‘controlled,’ there was still a strong negative effect of diversity on impact… It may be that greater resource preemption by natives in more diverse assemblages not only reduces invasion but also limits per capita impacts.”
(3) “…. Knapweed had strong impacts on the abiotic environment (depressing soil moisture and nitrogen). Exotics are often thought to be successful because they preempt resources from natives,… but in our case, knapweed impacts on resource levels did not correlate with its impacts on native plant biomass… Resource supply had differential effects on [knapweed] invasibility and impact. Whereas water additions increased community susceptibility to knapweed invasion, the impact of knapweed on recipient [native plant] assemblages was relatively unaffected by increased resource supply… Gaining insight into how resource supply and changing diversity jointly influence invader impact is of growing importance.”
(4) “The three exotic invaders varied substantially in their impacts on native biomass. Knapweed was clearly the most potent invader… [a likely] explanation for the greater relative success of knapweed… Knapweed has substantial fall germination [when compared to toadflax and cinquefoil]. [Also] knapweed is allelopathic.”
(5) “Aside from differences in germination timing, other differences among the three exotics in the extent of their functional overlap with natives may account for variation in invasive success. For example, the one monoculture that had the greatest resistance to Potentilla recta invasion was that of the native cinquefoil, Potentilla arguta… This suggests that at least for P. recta, the level of functional overlap with natives may play a role in limiting its invasion success.”
(6) “The productivity of uninvaded [native plant] assemblages increased with increasing diversity… The degree to which invasion increased total community productivity was striking. Knapweed invasion, in particular, actually switched the slope of the diversity-productivity relationship from positive in uninvaded assemblages to negative across invaded assemblages… Zavaleta and Hulvey (2004) similarly found that exotic yellow star thistle… increased total assemblage biomass more at low than at high levels of diversity.”
Maron and Marler, 2008, concluded, “Thus, while our results suggest that diversity can provide an important ‘function’ in resisting invasion, using yield as a benchmark for valuing the importance of diversity may be of limited utility since invasion can so dramatically influence yield. The fact that invaders increase community biomass even in diverse assemblages suggests that native plants may not fill all available niches.”
References:
- Adler, P.B., Ellner, S.P., & Levine, J.M. (2010). Coexistence of perennial plants: An embarrassment of niches. Ecology Letters, 13, 1019-1029.
- Maron, J.L., & Marler, M. (Oct., 2007). Native plant diversity resists invasion at both low and high resource levels. Ecology, 88, 2651-2661. doi/10.1086/588303
- Maron, J.L., & Marler, M. (July, 2008). Effects of native species diversity and resource additions on invader impact. The American Naturalist, 172 (S1), S18-S33.
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