L14 modern coexistence theory 2 Flashcards
(26 cards)
What distinguishes equalising mechanisms from stabilising mechanisms in coexistence theory?
Equalising mechanisms minimize fitness differences between species; stabilising mechanisms create niche differences that strengthen intraspecific competition relative to interspecific competition, promoting coexistence.
What trait variations are primary sources of fitness differences in plants?
Variation in growth rates (allometry, R* values), fecundity (maximum or average seed/offspring output), and root architecture depth/extent.
Why do trait trade‐offs occur, and what is an example?
Gains in one function incur costs in another; e.g., deeper roots improve resource access but cost shoot growth.
How does the growth–defence trade‐off operate in plant life histories?
Slow‐growing species allocate resources to chemical/structural defences (e.g., spines), while fast‐growing species invest in leaf area and photosynthesis at the expense of defence.
What is the “Darwinian demon” concept?
A hypothetical organism that escapes all trade‐offs by maximizing all traits simultaneously—an impossible ideal.
What is the seed mass trade‐off observed in annual plants?
Coexisting annuals range from many tiny seeds (high colonisation, low competition) to few large seeds (low colonisation, high competition).
In the metapopulation colonisation–extinction model, what drives patch gains and losses?
Gains are driven by c × P × (1 – P) (colonisation), losses by m × P (mortality).
What is the equilibrium occupancy (\hat P) in the patch model?
(\hat P = 1 - \frac{m}{c}), where c is the colonisation rate and m is the mortality rate.
How does the competition–colonisation trade‐off enable coexistence?
Small‐seeded species quickly colonise vacant patches but are poor competitors; large‐seeded species compete well but colonise slowly; their equilibrium curves intersect, allowing multiple species to persist.
What challenge to niche theory does Hubbell’s neutral theory pose?
It posits demographic equivalence among species, with abundance driven purely by stochastic drift rather than niche differences.
What key assumptions underlie the neutral seed mass model simulation?
(1) Each species produces equal total seed mass; (2) resource sharing in patches is proportional to seed‐mass contribution; (3) seeds disperse stochastically, creating random local densities.
What were the main results of the neutral seed mass simulation?
Small‐seeded species always dominated to fixation, large‐seeded species declined to extinction, and equilibrium biomass was negatively correlated with seed mass.
Why does variance in dispersal break neutrality in the seed mass trade‐off?
Small seeds disperse widely with low variance, covering many patches; large seeds clump with high variance, leaving more patches empty under resource caps.
What implication does dispersal variance have for equalising mechanisms?
Life‐history trade‐offs cannot perfectly equalise fitness because variance effects introduce inevitable fitness differences, leading to competitive exclusion without stabilising (niche) mechanisms.
What remains a central empirical challenge in community ecology?
Disentangling equalising effects (minimizing fitness differences) from stabilising effects (promoting niche differentiation) in real communities.
What was the primary aim of the California annual plant field study?
To quantify both fitness differences and niche (stabilising) differences by testing whether removing all niche differences causes rapid competitive exclusion.
What is frequency dependence in the context of niche stabilisation?
It occurs when each species limits conspecifics more than heterospecifics, giving a species higher per capita growth when rare—a necessary and sufficient condition for niche stabilisation.
How did the researchers “remove niches” in their theoretical model?
By setting all inter- and intraspecific competition coefficients equal, thus eliminating any stabilising niche effects.
What five steps did the study use to quantify fitness differences without niches?
(1) Built an annual‐plant competition model; (2) equalised all competition coefficients (“niche removal”); (3) derived per capita growth‐rate expressions; (4) manipulated seed inputs in field plots to match neutralised growth rates; (5) compared diversity outcomes between control and niche‐removal plots.
What were the diversity outcomes in niche removal plots versus control plots?
Control plots maintained species richness for decades, whereas niche removal plots lost species rapidly, with Salvia spp. sweeping to dominance within ~20 years.
What did per capita growth‐rate measurements reveal under niche removal?
Large, consistent fitness differences among the seven focal species—Salvia spp. had the highest rates, and weaker competitors had much lower rates.
How was frequency dependence validated empirically?
By comparing per capita growth when each species was rare versus common; six of seven species showed significantly higher growth when rare (negative slope).
What key conclusion did the niche removal experiment support?
Removing stabilising niche differences dramatically accelerates diversity loss, confirming that niche mechanisms are critical for coexistence.
What are the main limitations of this empirical niche removal study?
It depends on precise model assumptions and parameter estimates, is limited to easily manipulated annual plants, and does not identify the mechanistic basis of the niches (e.g., resource gradients).
