Environmental Engineering Reference
In-Depth Information
species [
36
]. The apparent diversity-ecosystem function relationship can thus
be partly caused by a greater chance of an influential species with particular traits
being present in more diverse communities than in species-poor communities.
If it is possible to predict and identify a priori a set of species traits that determine
keystone interactions in a system, this would greatly benefit management and conser-
vation purposes. Species' traits determine how species contribute to ecosystem pro-
cesses, so the presence and distribution of such traits can be utilized to indicate aspects
of ecosystem functioning [
37
]. To identify keystone species various methods have
been used ranging from experimental removal or addition manipulations to compara-
tive studies and natural history observations [
5
]. Partly because of these methodologi-
cal issues, identifying keystone species has so far proved elusive [
5
,
38
] although some
progress has been made and its concept now widely investigated in the context of
complex ecological networks [
25
,
39
-
41
]. Some examples of specific traits are for
instance trophic level, body size, connectance, or traits concerning tolerance and
resilience to disturbances. Organisms that influence their environment strongly and
contribute disproportionately to the functioning of ecosystems often seem to occupy
higher trophic levels in food webs [
5
]. Top predators have been described as highly
interactive keystone species [
42
], have been shown to play an important role in
stabilizing food webs [
43
], and play important roles in marine ecosystems [
44
]and
terrestrial ecosystems [
45
].
Also, the loss of top predators has been linked to secondary extinctions [
46
,
47
].
This has been attributed to their ecological role as suppressors of medium-sized
predators (mesopredators) (e.g., [
48
,
49
]) and generalist herbivores [
50
,
51
]. In
terrestrial ecosystems, organisms that influence their environment strongly also
often seem to be large bodied (e.g., [
52
]). Larger bodied organisms require a high
resource and energy use per individual [
53
,
54
] and have greater mobility, home
ranges, and longevity [
55
,
56
] and, thereby, control more resources over greater
and coarser spatial scales [
52
,
57
]. It is also proposed that well-linked and
interacting species as key interactors are more important for the community [
28
,
58
-
62
]. This approach characterizes the interaction structure of species placed in
an ecological network. Among plants, on the other hand, some studies have shown
that species within the same functional types but with different requirements and
tolerances may provide insurance to the system in the form of long-term resilience
against changes in environmental factors, such as global warming, grazing, drought
or frost [
35
].
The latter example indicates that the keystone status of a species often appears to
be context dependent, and may change with successional status, productivity, diver-
sity, and other ecosystem traits [
63
]. It is therefore important to identify how the
importance of traits that define keystone species change across a gradient of
conditions, measuring environmental factors, community composition, trophic
dynamics, and distribution of strong and weak links in the community (e.g., [
24
]).
Without droughts, a specific plant species may not play an important role in
maintaining community composition or ecosystem functioning. The Australian
brushtail possum (
Trichosurus vulpecula
) may function as a keystone species in
rata-kamahi forests by defoliating and killing canopy trees, but not in beech-
dominated forests where floristic composition, but not forest structure, is typically
