Balances, disturbances and biodiversity

An ecosystem is never static. It oscillates, absorbs disturbances, recovers — up to a point.

Dynamic equilibria

Predators and prey regulate each other, with a characteristic lag:

   numbers
      |      prey            ___              ___
      |                    _/   \__         _/   \__
      |                  _/        \_     _/        \_
      |    predators   ___/  ___      \___/  ___
      |              _/     /   \_        _/    \_
      +--------------------------------------------> time
                       the predator peak FOLLOWS the prey peak

Plenty of prey → predators multiply → prey collapse → predators go hungry → prey recover. This is not a static balance but a sustained oscillation.

Keystone species

Some species have an influence out of all proportion to their abundance. Remove them and the whole system shifts.

The best documented example is the wolves of Yellowstone, reintroduced in 1995 after seventy years of absence:

   wolves return
        |
        v
   elk, fewer and more mobile, stop lingering in the valley bottoms
        |
        v
   willows and poplars grow back along the rivers
        |
        v
   beavers return  ->  dams  ->  wetlands  ->  amphibians, birds
        |
        v
   banks stabilised by roots: the course of the rivers changes

This is a trophic cascade: a predator's effect propagates all the way to geomorphology. The exact scale of the phenomenon is still debated among ecologists — other factors played a part — but the principle is established: a predator does not merely reduce its prey, it changes their behaviour, and that alone can transform a landscape.

Invasive species

Introduced outside its native range, a species may escape its predators and proliferate.

   rabbit in Australia (1859)   24 individuals -> hundreds of millions
   Asian hornet in Europe       predator of honeybees
   water hyacinth               chokes tropical waterways

Biological invasions rank among the leading causes of extinction, particularly on islands, where local species evolved without predators and have no defences.

Resilience and tipping points

   WEAK disturbance   ->  the ecosystem absorbs it and returns to its state
   STRONG disturbance ->  it tips into ANOTHER stable state,
                          from which it does not spontaneously return

   coral reef  --warming-->  algae-covered reef
   forest      --massive deforestation-->  savanna

The existence of tipping points is what makes ecological degradation hard to reverse: past a threshold, easing the pressure no longer suffices to go back. The ecosystem has settled elsewhere.

Biodiversity is the main factor of resilience: the more species occupy similar roles, the more the loss of one can be offset by another. This is the same reasoning as for genetic diversity in a population.

Ecosystem services

pollination        ~ 75 % of crops depend on it at least in part
water purification by wetlands and soils
climate regulation carbon storage by forests and oceans
soil fertility     entirely produced by decomposers

These functions are provided free and become extremely costly to replace. In some regions of China where pollinators have disappeared, orchards are pollinated by hand — a concrete illustration of the real cost of a service once taken for granted.

Summary

• Populations oscillate: the predator peak follows the prey peak.
• A keystone species has disproportionate influence (Yellowstone wolves).
• A trophic cascade propagates from predator to landscape.
• Invasive species are a major cause of extinction, especially on islands.
• Past a tipping point, an ecosystem does not return spontaneously.
• Biodiversity is the main factor of resilience.