8. The Chesson Framework Flashcards
Structure of the lecture
- Fitness differences
- Niche differences
- A comprehensive model for coexistence
How do fitness differences drive coexistence?
1.1
Fitness differences are density-dependent traits between species that drive niche formation. When life-history trade-offs act on traits related to fitness, these trade-offs will be stabilising and lead to niche formation
How do fitness differences lead to life-history trade-offs?
1.2
Resources invested into one trait are not able to be invested in another. The resulting trade-offs offer a potential niche aces along which species can differentiate.
Chasing an entirely fictional status of ‘Darwinian Demon’, which is a species that is able to balance all trade-offs to the optimum value
What are the 3 major models we can use to understand seed number/size trade-offs?
1.3
Seed number/size trade-offs can be used to show tht fitness differences drive niche formation through life-history trade-offs, AND that fitness differences are niche dependent.
Can be modelled using metapopulation models to understand how sites occupied depends on mortality and colonisation. Large sites leave more patches for other species.
Can be modelled using Equalising models that argues that seed-masses are equivalent despite mass/number, and that differences are ecologically meaningless
Can be modelled using competition, which argues that each species distributes the same mean seed mass, but this is not meaningless/equalising. Small-seeded species place more ‘bets’ but large-seeded have less intra-specific competition. Not neutral.
A resolution: Oberved differences in seed size may not be a source of niche differences, but may instead be what allows for stable coexistence. Fitness trade-offs are stabilising at a population levels
Summary of how fitness differences link to life-history
1.4
Life-history trade-offs are not perfectly equalising.
The moment two species are different, fitness differences are inevitably introduced.
Without a stabilising mechanism, this inevitably leads to competitive exclusion.
However, life-history trade-offs allow for stabilisation since species undergo intra-specific competition.
How do niche differences drive coexistence?
2.1
Niche differences are a negative density-dependent force, and are stabilising. They help to prevent competitive exclusion, and mean that intra-specific competition is greater than inter-specific competition
Describe an empirical tewst for niche differences
2.2
Niches cause species to limit their own individuals more than other species. Artificial niche removal would mean competitive exclusion occurs, and diversity is lost.
Method: Control mixture plots allowed to develop naturally. Niche removal plots grow for 10 years.
Hypothesis: Niche removal plots should lost diversity fastest if niches are important for stabilisation
Results: Shannon diversity round 50% higher for control plants. Niche removal plots had low diversity
Levine and HillRisLambers, 2009
How are niches important for stabilisation?
2.3
Niches aid in community stability by preventing competitive exclusion. Niches lead to highly diverse communities
Controversies in testing for niche differences
2.4
- Hard to account for all biological factors on the scale of small plots
- Requires an immense number of assumptions
- Only a model! Can be inappropriate/limiting
Controveries regarding Chesson framework
3.1
This theory is still based on phenomena only. Does not contain biological mechanisms
Essay plan on ‘reconciling niche and neutral perspectives’
- Niche perspectives (fail to account for stochasticity/heterogeneity)
- Neutral perspectives (fail to account for fitness differences or population density dependence)
- Reconciliation via the Chesson framework (incorporation of fitness differences, a niche for neutrality and density dependence): Exploration via fitness differences (density-dependent, stabilised by life-history trade-offs), and niche differences (prevent competitive exclusion, allow stable coexistence)
