Environmental Engineering Reference
In-Depth Information
species which have disproportionate effects on community structure and ecosystem
functioning (“outcome focused” sensu [
22
]) whereas the ecosystem engineering
concept considers organisms which influence the abiotic environment with
consequences on other species and related ecosystem processes (“process focused”
sensu [
22
]). These differences between the two concepts are reflected in the
literature: these concepts generally appear in distinct studies as less than 5% of
the studies on these topics refer to both ecosystem engineers and keystone species
(source: ISI Web of Science).
The keystone species concept has been strongly related to food web theory since
its first definition [
1
,
23
]. In particular, the identification of keystone species in food
webs is an important issue. Theoretical studies have tried to pin down the
characteristics of keystone species through food web models [
24
,
25
] and several
indices based on food web topology have been developed to identify keystones
[
26
]. Models have shown that the loss of species with a large number of trophic
interactions can trigger high numbers of secondary extinctions with serious
consequences for species persistence; a result which highlights the potential key-
stone role of highly connected species in food webs [
24
,
27
,
28
].
In contrast, the concept of ecosystem engineering has been rarely related to food
web studies. Recent studies acknowledge that ecosystem engineers may also play
an important role in the network of trophic interactions but separating the trophic
effects from the engineering effects to determine their relative importance is
difficult [
20
,
29
-
31
].
The importance of keystone species can also be strongly linked with ecosystem
engineering. For example, the large impact of sea otters in kelp forest ecosystems
results from the coupling between engineering effects and a trophic cascade [
32
]. In
these ecosystems, kelp provides habitat for many species and dampens wave action;
the keystone effect of sea otters is thus mediated through their indirect trophic effect
on kelp densities which is a main ecosystem engineer.
Issue-1: How to Find Keystone Species and Ecosystem
Engineers in Communities?
Keystone species and ecosystem engineers may affect ecosystem processes, such as
nutrient cycling, and thereby ecosystem functioning. In the face of rapid biodiver-
sity loss, a considerable amount of studies were dedicated to investigate a possible
link between species richness and ecosystem function [
33
] and the threat of
diversity loss on the loss of ecosystem services to man. First indications show
positive relationships between species richness and ecosystem productivity, stabil-
ity, and sustainability, with more species being able to fully and complementarily
run ecosystem functions due to niche differentiation and facilitative interactions
(reviewed by [
34
]). However, there is now a growing consensus that functional
diversity, rather than species numbers per se, strongly determines ecosystem func-
tioning [
35
]. This means that the presence of a particular species with specific traits
may play a larger role in determining ecosystem function than merely the number of
