Environmental Engineering Reference
In-Depth Information
species, have a crisis-like impact on plant-pollinator
systems. Specialist relationships are, in themselves,
much more vulnerable than generalist relationships,
but plant-pollinator interactions are only seldom
specific to the species level; relatively few plant-
pollinator interactions are absolutely obligate in a strict
sense, in particular in much of Europe and the east-
ern and northern parts of North America (Johnson &
Steiner 2000). Many flowers show specialization in
floral traits, yet they are often visited by diverse
assemblages of animals. Mutualism is mainly an
interest of the plant community and the pollinator
community, rather than that of two specific plant-
pollinator populations. In their review, Kwak
et al.
(1998) illustrated that pollen and gene flow in frag-
mented habitats not only depend on the investigated
plant populations as such, but also on the neighbouring
species of the plant communities and the flowering
phenologies of the component species. In small habitat
fragments, less-attractive plant species may receive
fewer pollen visits and a higher proportion of hetero-
specific pollen grains, thereby reducing pollination
efficiency and gene flow.
are not permanent but may last for periods of a few
minutes to several months, and the benefits often
differ in kind and amount. Individual birds and
mammals often achieve a higher rate of energy
intake in mixed-species associations than when for-
aging alone or with conspecifics, and mixed-species
flocks of bird may confuse predators.
4.5 Concluding remarks
The dynamics of seemingly distinct systems are
intimately related by spatial flow of matter and
organisms, which has led Polis
et al.
(1997) to try
and integrate landscape ecology and food-web eco-
logy. Indeed, ecosystems are closely bound to one
another. The relationship between species diversity
and ecosystem functioning is complex. For example,
studies of interactions between below-ground her-
bivores have shown both competition and facilitation
(Mortimer
et al.
1999). Facilitation may occur through
substrate modification or the increased attractive-
ness of damaged root to other herbivores. Specialist
herbivores cause changes in plant community com-
position through their effects on specific host plants,
primarily as a result of the alteration of competitive
interactions between coexisting plant species. Gen-
eralists can also affect plant community composition,
as a result of differences in their preference for plant
species or host plant susceptibility.
Within food webs, the large array of indirect
interactions may be at least as important as the direct
interactions (Wootton 1994): for example exploitation
competition, trophic cascades, apparent competition,
indirect mutualism or interaction modifications. Indir-
ect effects occur when the impact of one species on
another requires the presence of a third species. They
can arise in two general ways: through linked chains
of direct interactions and when a species changes the
interactions among species.
Simulation models indicate that some indirect
effects may stabilize multi-species assemblages. On the
one hand, such complexity may stress the notions of
chaos and unpredictability. On the other hand, this has
not given rise to hopelessness, but to a better recog-
nition of choosing adequate temporary and spatial
scales for the analysis of mechanisms. Grover (1994),
recognizing that the assembly history of nearly all
4.4.3 Mutualism among animal species
Associations between tick birds, or oxpeckers, and their
mammalian hosts are among the best known of
mutualisms between vertebrates. Another example is
fossorial blind snakes (
Leptotyphlops dulcis
) eating
insect larvae in the nests of screech owls (
Otus asio
),
and hence potentially reducing larval parasitism on
nesting owls. Interactions between insect species are
numerous, for instance between ants and aphids.
Among plant-insect mutualisms, I refer to the mutu-
alistic system of anti-herbivore defence in acacias
brought about by their ant inhabitants. According to
Dickman (1992), asymmetric commensal and symmetric
mutualistic interactions occur frequently between
species of terrestrial vertebrates. I do not refer to com-
mensal interactions in terms of mutualism, but call it
facilitation (see Chapter 5). Potential advantages for
individuals in mixed-species associations are very
diverse, and include reduction in parasite load,
reduced risk of predation and increased access to food
and other resources. In contrast to obligate mutualisms,
associations between species of terrestrial vertebrates
