Species Competition and Predation - Ecological Systems - page 222

assembly of local communities, and their reassembly following perturbations. Niches
define patterns of linkages between species, their resources, and their natural enemies.
These patterns include how linkages change over time, and between different spatial
locations, and define mechanisms by which similar species are able to coexist by their
effects on competition and predation relationships. The human element in the envi-
ronment has profound effects on these phenomena. Changing the environment shifts
interactions between species, and profoundly modifies the structure of food webs. In
the modern day, there is much community reassembly, potentially involving major
shifts in competition and predation. Humans transport invasive species that act as
predators, prey, and competitors with potentially major effects on the community
reassembly process.
Introduction
Competition and predation are key species interactions that are believed to structure
natural ecosystems and to have major roles in systems dominated by humans. Both
of these interactions involve consumer-resource relationships in one form or
another [ 1 ]. The relationship between a predator and one of its prey species is of
necessity a consumer-resource relationship with the predator being the consumer
and the prey its resource. Competition is mostly commonly resource competition,
where several consumer species share one or more resources and compete for these
resources [ 2 - 4 ]. A resource may or may not be a biological species. When the
resources are biological species that are killed by the consumer, competition
necessary involves predator-prey relationships. However, resource species may
instead be grazed or browsed, parasitized or infected. In these cases, the consumer
species are natural enemies of their resources, harming them without necessarily
killing them, which generalizes the idea of a predator-prey relationship to
a species-enemy relationship.
The resources of plants are generally not biological species, and indeed this is
the case with the resources of most plants, which are instead broadly light energy,
water, and chemical elements [ 5 ]. Plants are often thought of as requiring space to
grow as a resource [ 4 ]. Space then provides their other needs. In general, not all
space is equal, and plant species tend to be somewhat specialized, leading to the
concept of safe sites [ 6 ] (places that satisfy the requirements for establishment,
growth, and reproduction for a particular species), and the regeneration niche [ 5 ]
(an elaboration of the idea of a safe site with a particular view to how species
compete with one another). Sedentary animal species that either settle in
a particular place and do not move, or establish territories, can also be regarded
as having space as a resource [ 7 ]. Animal species require particular places for
particular uses, such as nest holes and wallows, and use various dead organic and
inorganic materials in their lives. These all count as resources if they are used or
occupied by an individual to its benefit.
Both competition and predation are assumed to involve harm. In the case of
predation, of course the predators benefit from the relationship, and prey are harmed