Agriculture Reference
In-Depth Information
Rapidly increasing (example: foliage of deciduous trees in relation to herbivores in
temperate climate spring);
Rapidly decreasing (example: rapidly decomposing litter in tropical rainforests in
relation to primary decomposers);
Constantly renewed (example: soil organic matter in relation to geophages);
Constant (example: bird nesting sites, soil pores at certain scales of time in relation to
micro-organisms);
Pulsed (example: root exudates, litter leachates in relation to the soil microflora).
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Variation in resource availability over time is of considerable importance since
interactions between consumers competing for the same resource will depend on both
the timing of its availability and the capacities of consumers to respond to it by increasing
their populations. To illustrate, the exploitation of a rapidly-increasing resource is
unlikely to lead to competition among consumers since their populations will increase
too slowly to create density-dependent effects. In contrast, such competition is more.
likely where resources are regularly-renewed or rapidly-decreasing. However, even in
these circumstances the likelihood of competition taking place may be reduced by
the spatial distribution patterns of the resource (Table I.24; Price, 1984).
The ways in which consumers exploit a resource and the nature of the relationships
they develop with other consumers of that resource is dependent on the
spatial
distribution of the resource
. The spatial distribution of a resource may be evaluated by
relating resource patch size to the home and migratory ranges of its consumers
(Southwood, 1977).
The following three classes of resource spatial distribution may therefore be defined
(Figure I.39):
Evenly distributed and available to all consumers in the area;
Occurring in large, isolated patches located further apart than the normal migratory
ranges of the consumer species; and
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