Communities seem to be rather stable through time. This may happen because the individual species co-evolve with one another, and it may also be part of the incumbency effect.

In American politics, it is difficult for a Congressional Representative to lose his or her seat. The incumbent has two very great advantages: name recognition, and ability to raise money. A challenger does not typically have these. So most incumbents that seek another term are elected.

Ecological rules are different, of course. But any species that might replace another will have to do its ecological job better than the incumbent already does, a very difficult thing to do when the incumbent has been honed by natural selection for a long time for that very job.

Even so, there are times when an apparently small change in a community can have a huge effect.

Adding to a community

What if a member of the community evolves some new character? Example: for much of the later Paleozoic, no land vertebrate could breathe while it ran. Its chest was twisted side to side as it walked on sprawling limbs, and any attempt to run would flex the chest so much that breathing would be impossible. This remains true for almost all lizards today.

So Paleozoic vertebrate ecosystems would have looked rather bizarre, with predators and prey both unable to breathe while they ran.

Then a lizard evolved the ability to run on its hind limbs, which avoided the constraint. This was so successful that it became the ancestor of dinosaurs, which dominated the Mesozoic vertebrate world on land. All the sprawlers became extinct except the tiny ones that lived in burrows and became our ancestors, and terrestrial communities were totally reconstructed.

Subtracting from a community

A community may be very dependent on a particular species. The best example is a coral reef. ALL the members of the community are ultimately dependent on the dinoflagellates that are symbiotic with the corals. An environmental change that interferes with the symbiosis will have a catastrophic effect on the rest of the community.

So reef ecosystems are typically long-lasting in time, but seem to suffer worse than most ecosystems in mass extinctions.

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