Ecosystem Engineers, Keystone Species - Ecological Systems - page 59

Physical state 1 Physical state 2

Creation of resources :
Living space
Modulation of abiotic forces :
Storm impact

KELP
growth

KELP
physical structure

a

Physical state 1 Physical state 2

SOIL
with aggregates
and pores

Creation of resources :
Living space
Control over abiotic forces :
Soil physical processes,
dynamics of soil organic
matter

SOIL

Earthworm
feeding

b

Fig. 4.2 Two examples of ecosystem engineering with kelp (a) and earthworms (b) as ecosystem
engineers. Ecosystem engineering corresponds to changes in physical state (state 1-2) of biotic
(i.e., kelp for a) or abiotic (i.e., soil for b) materials. Ecosystem engineers are represented in grey
boxes .(a) Case of autogenic engineering, the engineer is part of the new physical state (via growth
here). (b) Case of allogenic engineering, the new physical state is caused by the engineer ( , caused
via feeding here), but the engineer is not part of the new physical state

[ 8 ] and sea otters whose hunting in the late nineteenth century caused a population
explosion of their sea urchin prey and consequent overgrazing of kelp which led to
numerous extinctions of local species [ 6 ].
The concept of ecosystem engineering was proposed two decades later than the
“keystone species” concept by Jones and colleagues [ 2 ]. They defined ecosystem
engineers 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. In so doing they modify, maintain and/or create
habitats” [ 2 ]. They further distinguish between two types of ecosystem engineers
( Fig. 4.2 ): autogenic engineers that change the environment via their own physical
structures, i.e., their living and dead tissues, and allogenic engineers that change the
environment by transforming living or nonliving materials from one physical state
to another via mechanical or other means. The idea that organisms can have
important effects on abiotic processes occurring in the environment had been
recognized before; indeed, Darwin devoted a whole topic to the impact of
earthworms on soil formation [ 9 ]. However, since the development of the concept
of ecosystem engineer, engineering effects have been described for many
organisms, from classic examples such as beavers, termites, or earthworms
[ 10 - 12 ] to mollusks [ 13 ], fish [ 14 ], caterpillars [ 15 ], polychaete worms [ 16 ],
grasses [ 17 , 18 ], burrowing shrimp [ 19 ], ants [ 20 ], and many other species (see
Table 1 of [ 21 ]).
Both the keystone species and the ecosystem engineer concepts point out to
species which have important effects in ecological communities and ecosystems.
Although these concepts partly overlap - an ecosystem engineer can be a keystone
species - they however insist on different aspects: the keystone concept focuses on