Environmental Engineering Reference
In-Depth Information
Definition of the Subject
This entry focuses on two ecological phenomena. The first is “keystone species”
which is defined by Paine [
1
] as a species (mostly of high trophic status) whose
activities exert a disproportionate influence on the patterns of species occurrence,
distribution, and density in a community. The second is the concept of “ecosystem
engineers” defined by Jones et al. [
2
] as organisms that directly or indirectly
modulate the availability of resources (other than themselves) to other species by
causing physical state changes in biotic or abiotic materials.
Introduction: Keystone Species and Ecosystem
Engineers: Analysis of Concepts
Paine's definition of keystone species was inspired from the large effects of the
removal of the carnivorous starfish (
Pisaster ochraceus
) from intertidal habitat,
which reduced prey species diversity due to intense competition from mussel prey
[
3
], and represents now a classic textbook in ecology. The original keystone species
concept of Paine [
1
,
4
] thus identified a very specific mechanism: the top-down
regulation of community structure and diversity by a top predator (
Fig. 4.1
). The
concept of keystone species has been later extended to a broader definition and now
includes any species whose effect on ecosystems is disproportionately large relative
to its low biomass in the community as a whole [
5
]. Keystone species are thus
species which have large effects on communities and ecosystems through many
different processes such as trophic interactions, pollination, or habitat modification
[
6
,
7
]. Examples include rabbits that can increase abundance and diversity of lizards
Pisaster
(starfish)
Sea otter
Thais
(snail) 1 sp.
Sea urchin
Chitons
2 sp.
Limpets
2 sp.
Mytilus
(bivalve)
1 sp.
Acorn barnacles
3 sp.
Mitella
(goose
barnacle)
Kelp
a
b
Loss in diversity of
pisaster
's preys
Loss of species associated to kelp
Fig. 4.1 Two examples of keystone species impacts. (a) Effects of the removal of
Pisaster
on prey
species diversity as a consequence of mussel population explosion. (b) Consequences of the
removal of sea otters on species diversity due to overgrazing of kelp by sea urchins. Keystone
species are represented in
grey boxes
. Small
grey arrows
indicate the direction of species
abundance changes following the removal of the keystone species. The large
grey arrows
indicate
the global consequences of keystone species loss on the ecosystem
