Environmental Engineering Reference
In-Depth Information
collectors. Collectors may remove fine material that interferes with shredders
or excrete nutrients that stimulate the microbes and make litter usable for
shredders. Such facultative links merit additional study.
OTHER SPECIES INTERACTIONS
In addition to predation (
/
), competition (
/
), and mutualism
(
/
), other species interactions [neutralism (0/0), amensalism (
/0), and
commensalism (
/0)] may be important but are rarely studied. Several ex-
amples are given here to explain potentially important interactions in which
one species has an influence on a species that does not have an influence in
return. First, tadpoles of a common frog
(Rana temporaria)
have a negative
effect on the snail,
Lymnaea stagnalis,
by competing for microalgae (Brön-
mark
et al.,
1991). The snail then utilizes lower quality
Cladophora
and ex-
cretes nutrients that stimulate microalgal growth. The tadpole has a strong
negative effect on the snail, but the snail has a weak positive effect on the
tadpole. Second, macrophytes in lakes and ponds may provide a vital habi-
tat for survival of small fishes. The macrophytes receive little direct benefit
from the fish that live in them. Third, nitrogen-fixing microbes may leak ni-
trogen and stimulate nearby organisms that are unable to utilize N
2
. Most
interactions of aquatic organisms with humans are amensal. Humans have
negative effects on many aquatic species, and the effects on humans are neg-
ligible. With some thought, the reader could identify more examples.
COMPLEX COMMUNITY INTERACTIONS
Disturbance
Defining
disturbance
is difficult. White and Pickett (1985) suggest that
a disturbance is “any relatively discrete event in time that disrupts ecosys-
tem, community, or population structure and changes resources, substrate
availability, or the physical environment.” This definition is very broad and
allows many important aspects of disturbance to be included: spatial dis-
tribution, frequency, return interval, predictability, area, and intensity. As
with any ecological aspect, the appropriate scale considered needs to be rel-
evant to the organisms being studied. One thorny issue in disturbance ecol-
ogy is how to classify natural events. For example, a benthic stream alga
may be adapted to flooding and actually grow better after a flood; thus, is
a flood a disturbance for the alga?
Disturbance has a strong influence on some aquatic communities. In
general, groundwaters and lakes are affected least by disturbance. Streams,
rivers, and some wetlands are more prone to disturbance. Flooding is an in-
tegral part of stream ecosystems and riparian wetlands, and recovery from
flooding and adaptation to such a disturbance are important to many
aquatic organisms. A nonequilibrium view of streams and rivers has become
essential to understanding their ecology (Palmer and Poff, 1997). The idea
that flooding is a natural part of flowing water communities is being ex-
tended to habitat management, including operation of dams and reservoirs.
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