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Flashcards in Community Ecology Deck (29)
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What is an ecological community?

A group of populations of different species that occur together in space and/or time and generally share the same resources.

Idea of competition is important.


Example of how diversity is hugely variable

In a single hectare in Brazil - 400 different tree species

In USA and Canada - 700 different tree species in total


What is Gause's principle of competitive exclusion?

Two species cannot coexist on a single limiting resource if other ecological factors remain constant.


Example of competitive exclusion

Gause’s paramecium. Both P. aurelia and P. bursaria eat bacteria. When grown together in a well stirred mixture, P.aurelia outcompetes P.bursaria. If the mixture isn’t stirred, both species persist, but P. bursaria is confined to the bottom of the beaker. This is because anoxia sets in at the bottom of the beaker, and P. bursaria with its photosynthetic symbionts can generate oxygen internally.


Tilman's diatoms?

Photosynthetic, planktonic organisms. Diatom cells are contained within a unique silica cell wall. Can be grown in culture such that silica is the limiting factor.


What happens if you add diatoms to a culture that has initially got plenty of resources?

Population grows and reaches a steady state if the populations are alone.

R* is the level of resource at which the population growth rate is zero. Synedra has a lower R* level than Asterionella so can predict this will be the winner.


Define the R* theory?

R* is the minimum resource concentration a species requires for positive population growth.
The species with the lowest R* for the limiting resource is predicted to be the superior competitor.


Assumptions made for R* theory

The species are competing for a single limiting resource.
The resource is labile and the system is well mixed.


Minnesota example of R*?

Lack of nitrogen for grasses.

Assumptions: species are competing for a single limiting resource and nitrogen is the most mobile molecule in soil.

Measure the R* levels and predict the winner.

The theory correctly predicted a number of 2 species competitions.


Single species population model including density dependence formula?

dN1 / dt = r1N1 ( [K1 - N1] / K1 )


What is the population model including interspecific competition?

dN1 / dt = r1N1 ( [K1 - N1 - a12n2] / K1 )

𝛼12 is the competition coefficient.
Interspecific competitors are N2.


What outcomes does the 2 species Lotka-Volterra competition model have?

Many, including a stable coexistence.

Coexistence occurs when
𝐾1 > 𝐾2𝛼12 AND 𝐾2 > 𝐾1𝛼21


When is interspecific competition likely to be weaker than intraspecific competition?

When each species has its own ecological niche.

If species use different food or prefer different habitats, this concentrates competition onto members of their own species.



Between different species


What is neutral theory?

States that all species are identical. Not just similar but really and truly identical. This means that all species have identical per capita birth and death rates.

Probability of dying and being born isn't to do with what species you are.

Hubbell 2001.


Neutral theory example?

If death hits sites at random, the per capita death rate of each species is the same on average.

Replacement takes place via an unbiased lottery in which all seeds have the same chance of winning. makes the per capita birth rate of all species the same on average.

The common species are more likely to die, but also more likely to capture the vacant site. There is no disadvantage when common, and no advantage when rare Dynamics of drift mean this process is random.

Eventually all species except one will drift to extinction. In a large community, this process takes a very long time.


How robust is neutral theory?

Only works if species are truly identical. Not robust to the inclusion of fitness differences, no matter how small.

No fitness differences and no stabilising mechanisms


Equalising mechanism?

Reduces fitness differences between species


Stabilising mechanisms?

Introduces density dependence in niches. Makes the birth/death rate of each species dependent on the density of conspecific individuals.

To modify the birth rate, can make the fecundity of each species density dependent. As N goes up, F goes down. The fecundity of the species is only affected by number of conspecific individuals. Means that fecundity increases very steeply as the species becomes rare, greatly increasing its chance of winning a site. Dynamical effect of niches.



Belonging to the same species


How does density dependent regulation help prevent extinction?

If one species has higher fitness, it experiences density dependent regulation. Will help prevent the extinction of other species.

If the fitness differences are too large, then the density dependence isn't enough to prevent extinctions.

Larger fitness differences require stronger stabilising mechanisms.


Example of niche separation?

Warblers observed by MacArthur in coniferous North American forests. Each species focussed on a particular section of the tree.

Similarly, if the population size of Cape May warblers increases then other Cape May warblers suffer the most as food availability will be reduced most in the tops of trees.


What are plant niches?

They use a handful of non-interchangeable resources.

Their niche is a life history trade off between growth rate and defense. Resources are limited, meaning that the trade offs offer a potential niche axis along which species can be differentiated. Anything not on the line will be eliminated.


What is a Darwinian demon?

A hypothetical organism which can maximize all aspects of fitness simultaneously and would exist if the evolution of species was entirely unconstrained.


Example of plants adapted to different environments?

Birch - classic pioneer tree. Produces lots of seeds and grows quickly, not long lived. Can’t regenerate in the shade.

Beech - classic shade tolerant tree. Long lived and slow growing. Casts a deep shade but can regenerate in the shade.

Pioneers and shade tolerators at either end of a general trade off between growth and survival. They can stably coexist by partitioning the light environment.

Pioneer species will grow fast when gaps appear, but short lived. Shade tolerant species grow up slowly in the shade cast by the pioneer, but long lived. When a shade tolerator dies, it creates a gap which is captured by the pioneer. When the pioneer dies, its place is taken by the shade tolerator.


Can trade offs be equalising?

Although seed size vs seed mass trade off results in same total mass, they allow for different distributions.

Sand dunes are patchy habitats. This means that small seeded species have an advantage because they distribute their seeds more evenly. This is better because the resources available in each patch are capped. Overkill in one patch doesn’t compensate for not having tickets in other lotteries.


Why does the neutral model not work?

Life history trade offs are very unlikely to be perfectly equalising. When you make things different, you will inevitably introduce fitness differences.


Is there a mechanism that can easily lead to very high diversity?

(Janzen Connell effect)

Mechanism which prevents species from re-occupying a site which it had previously occupied.

Powerful way of stabilising populations.


Example of Janzen Connell effect?

Each plant species has a team of specialist herbivores or pathogens that prevent that species from reoccupying the same site. Could occur because specialist herbivores eat all the seeds around the parent tree. Could also be due to soil pathogens. Farmers don’t grow their crops in the same place again - crop rotation. True for all but one species, that crops couldn’t grow as well on their home soil as other soils. Effect disappears when sterilise the soil to kill pathogens.