Hi learners, welcome to my eighth lesson of the course Ecology: From Cells to Gaia.

Today we will talk about evolutionary ecology.

To understand the factors responsible for

the population dynamics of even a single species in a single location,

it is necessary to have knowledge of physical chemical conditions, available resources,

the organism life cycle,

and the influence of competitors,

predators and parasites or rate of birth,

death, immigration and emigration.

There are contrasting theories to explain the abundance of populations.

At one extreme, researchers emphasized the apparent stability of

population and point to the importance of forces that stabilize,

such as the density-dependent factors.

At the other extreme those who place more emphasis

on density fluctuation may look at external,

often density independent factors,

to explain the changes.

K factors analysis is a technique that can be applied to

life table studies to draw light both on determination and regulation of abundance.

Movement can be a vital factor in determining and/or regulating the abundances.

A radical change in the way ecologists think about population has

involved focusing attention less on process occuring within population,

and more patchiness: the colonization and extinction of subpopulation,

within an overall metapopulation and dispersal between subpopulations.

Disturbances that open up gaps - patches - are common in all kinds of community.

Founder controlled communities are those in which

all species are approximately equivalent in their ability to evade gaps,

and there are equal competitors that can hold

the gaps against all comers during their lifetime.

Dominance controlled communities are inside those in

which some species are competitively superior to others,

so that an initial colonizer of a patch cannot necessarily maintain its presence there.

The phenomenon of dominance control is

responsible for many example of community succession.

Primary succession as I told you before,

occur in habitats were no seeds or spores remained from previous occupants of the site.

All colonization must be from the outside the patch.

Secondary succession instead occur when existing communities are disarmed,

but some, at least of their seed, remain there.

It can be very difficult to identify when

a succession reaches a stable climate's community,

since this way takes centuries to achieve and in the meantime,

further disturbances are likely to occur.

The exact nature of

the colonization process in an empty patch depends on the size and location of that.

No predator-prey, parasite-host or grazer-plant pair exist in isolation.

Each is part of a complex food web involving other predators, parasite,

food sources and competitors within the various throphic level of a community.

The effect on one species on another,

its herbivorous prey, maybe direct and straightforward.

But indirect effect may also be felt by any of

the myriad species linked more remotely in the food web.

One of the most common is a trophic cascade;

in which a predator reduces the abundance

of a herbivore thus increasing the abundance of plants.

Top-down control of a food web occurs in situation in which the structure, the abundance,

the species number of lower trophic levels

depends on the effect of consumers from higher trophic levels.

Bottom-up control, instead, in a community structure depend on factors such as

nutrient concentration and prey availability that influence the trophic level from below.

The relative importance of these forces varies according to

the trophic level under investigation and the number of trophic levels present.

Some species are more tightly woven in the food web than others.

This species removal would produce a significant effect such as extinction,

or a larger change in density in at least one other species,

may be thought of as a strong interactor.

Removal of some strong interactors leads

to significant changes that spread throughout the food web.

We refer to these as keystone species.

The relationship between food web complexity and stability is uncertain.

Anchor is needed in deciding what is meant by stability.

Mathematical and empirical studies agree in suggesting that if anything,

population stability decreases with complexity whereas the stability of

aggregate properties or work communities

increases with complexity in particular species richness.

So at the end of this lesson I have some question for you.

The first is: what are mean by bottom-up and top-down control?

The second is: how would you expect communities to differ

if they were dominated by founder or dominance control?

So thank you for your attention and see you at the next lecture.

2.3 Evolutionary ecology

Ecology: from cells to Gaia

Meet the Instructors