7. Niches vs. Neutrality Flashcards
Structure of the lecture
- Niche perspectives
- Neutral perspectives
- Reconciliation via Chesson framework
What is the fundamental idea of niche perspectives?
1.1
Modern coexistene theory (niche theory) surrounds the idea that species are constantly competing for shared, limited resources. Species with lower resources needs are expected to outcompete those with higher resource needs
General acceptance
Incorporation of fitness differences and population-based carrying capacity (not environment though)
What is R-star theory?
1.2
Hypothesis by Tilman that the species with the lower resource needs (probably more generalist) has a lower R-star, so will outcompete other species and rise to dominance.
Shown by Tilman’s diatoms in silica-limiting environments
Shown by Minnesota grasses
What is resource-ratio theory?
1.3
Modelling resource use for two or more species. Coexistence can occur when multiple rsources are limiting, and when resources are present in different ratios within the environment. Different species have different requirements so can co-exist.
ZNGI modelling.
Controversies regarding niche perspectives
1.4
Meta-analysis by Miller et al., 2005.
42 ‘good’ tests
31 provided evidence for Resource-Ration
11 did not.
Not great evidence? Many invalid studies.
Niche perspectives fail to account for stochasticity and heterogeneity
Too simplistic
Whimsical? Each species has ideal conditions and will just grow there whenver the nutrients are right - fails to account for environmental stochasticity
Fundamental idea of neutral perspectives
2.1
All species are ecologically equivalent to each other, and coexistence is determined by the stochastic processes of birth and death. All species aside from one will drift to extinction. A more abundant population will be more likely to dominate
Highly controversial
Hubbell, 2001
What is the mechanism of neutral perspectives?
2.2
Birth and death processes within a population are entirely stochastic. Drift is the only force acting on the population. All species aside from one will drift to extinction. More abundant species will be more likely to dominate. Extinction is balanced by speciation.
What are the assumptions of neutral perspectives?
2.3
- All species are ecologically equivalent (However, this is not robust against even the slightest fitness differences.
- All species have identical per capita growth rates (This does not account for density dependence)
- Widespread recruitment/dispersal limitations. leading to high stochasticity
What are the controversies regarding neutral perspectives?
2.4
- Does not hold true the second that fitness differences are prsent
- No accounting for density dependence, or population-determined carrying capacity. K is only defined by the environment.
What are the merits of neutral perspectives?
2.5
Replication of Species Rank Abundance Curves in highly diverse communities - likely to be quite accurate? Tested in Sarawak National Park in Borneo
Good replacement for McArthur’s Broken Stick Model.
Define the Chesson framework
3.1
Coexistence is a product of fitness differences between species, and is stabilised by niche differences. Niche differences act as a balancing/stabilising force to create diverse communities despite fitness differences
How does the Chesson framework opposite neutral theory?
3.2
Chesson argued that neutral theory is not robust due to its failture to incorporate population-dependent density dependence for niches. Chesson framework incorporates density dependence to allow neutrality to become a vanishing point on a continuum
How does the Chesson framework provide a ‘niche for neutrality’?
3.3
Adler et al., 2007
Neutrality is a vanishing point on a continuum. Including fitness differences and niches into a model for coexistence allows for populations to be close to the vanishing point of neutrality
