Environmental Engineering Reference
In-Depth Information
attract more herbivores, increasing grazing pressure,
maintaining a low standing crop until herbivore
biomass is high enough for a population of carnivores
to be sustained. From that point in the gradient
onwards, the carnivores regulate the herbivore density,
and as a result, standing biomass of plants can increase
again. This reasoning dates back to the 1980s and is
known as the
exploitation ecosystem hypothesis
(EEH),
mathematically analysed by Oksanen
et al
. (1981) . The
model is general and ignores all kinds of variability,
such as differences in competitive ability and in
resource quality, but it has been a useful starting point
for the further development of theories on plant-
herbivore interactions. An underlying assumption is,
for example, that all vegetation is equally palatable.
However, tall plants, and especially the woody parts of
them are generally much less attractive to herbivores
than herbaceous plants or biomass because stem and
wood material is much harder to digest (Fryxell 1991).
This holds especially for small grazers, not for brows-
ers. This results in a decrease in the density of grazing
herbivores in areas with high plant productivity, even
in the absence of carnivores (van de Koppel
et al
. 1996 ;
see Figure 6.3 for an example).
70
(a)
60
50
40
30
20
10
0
120
(b)
100
80
60
40
20
0
70
(c)
60
50
40
Parasitism
30
Host-parasite interactions are considered as a specifi c
type of consumer-resource interaction, as the parasite
consumes tissue of its host, just as in the case of
herbivore - plant and predator - prey interactions.
Similar models have, therefore, been used for these dif-
ferent types of direct interaction. However, the indirect
effects of parasitism on the biotic community may be
more dramatic, as illustrated below (section 6.3.2).
20
10
0
0
100
200
300
400
Standing crop (g m
-2
)
Figure 6.3
Number (means and individual observations)
of annual droppings of (a) hares, (b) rabbits and
(c) barnacle and brent geese in relation to vegetation
standing crop on the coastal salt marsh of the Waddensea
Island of Schiermonnikoog, the Netherlands. (Modifi ed from
van de Koppel
et al
. 1996 .)
Ecosystem e ngineers
Every living organism changes its environment, by pro-
ducing shade, by the use and consumption of resources,
by excreting various products and so on. Some species
create, maintain or modify their environment to such
an extent that they signifi cantly affect the growth or
survival of other species in their community as well as
their own. These plant or animal species, which are
often considered as species with a
keystone
or a
foun-
dation
role or position (see Chapter 2), can be called
ecosystem engineers
(
sensu
Jones
et al
. 1994 ). Note
that this term is different from the notion of
ecological
engineering
, which is used to indicate human engineer-
ing activities (Rosemond & Anderson 2003). Let us now
consider two examples of direct interactions: (1) bio-
physical engineering, such as when beavers make
dams, and (2) chemical engineering of the environ-
ment, also known as allelopathy in plant communities.
Plant and animal species that physically engineer
their respective environments can have long-lasting
effects, even beyond their own lifetime (Hastings
et al
.
2007). The North American beaver (
Castor canadensis
),
