Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 371

of populations of one species in response to another. Trait-mediated inter-
actions are those that are evidenced by evolved traits in response to selec-
tive pressure associated with the interaction. Most cases of species inter-
actions have been explored with experiments that focus on density-mediated
responses; this approach typifies most examples in this chapter and the fol-
lowing two chapters. In general, density-mediated species interactions are
important to those interested in questions of how to manage organisms
and how to predict the immediate effects of disturbances on organisms.
Trait-mediated interactions are important in determining why specific in-
teractions occur and making generalizations about interaction types across
communities. The reason that these two interaction types are separate is
that population regulation does not necessarily define what is causing nat-
ural selection for specific traits.
As discussed in Chapter 7, the interactions that can occur among or-
ganisms are exploitation (mainly parasitism and predation), mutualism,
competition, commensalism, amensalism, and neutralism. The relative oc-
currence of each of these interactions in microbial communities is gener-
ally not well-known. In a study of interactions among the cyanobacterium
Nostoc and bacteria (Gantar, 1985), positive interactions were about as
likely as negative interactions. However, studies of phytoplankton summa-
rized in Hutchinson (1967) show an excess of negative interactions. Twenty-
seven species of tropical and subtropical fungi were mostly inhibitory to-
ward each other when grown together in culture (Yuen et al., 1999). If an
excessive number of negative interactions occurs, amensalism and compe-
tition will be more important. Some possible microbial interactions are dis-
cussed in the following sections.

PREDATION AND PARASITISM

Microbial food webs are central to nutrient cycling and energy trans-
fer in most aquatic systems. Transfer of energy, carbon, and other nutri-
ents through the microbial food web is referred to as the microbial loop
(Fig. 18.4), which occurs in streams, groundwater, wetlands, and lakes.
The idea that microbial assemblages have an active role in transfer of en-
ergy in aquatic systems has changed the way that food webs and energy
transfer are viewed. The microbial loop is essential to scavenging dissolved
organic compounds in water and returning this organic material to the
food web. Bacteria release a large variety of enzymes that allow utilization
of organic carbon (Münster and DeHaan, 1998). Organisms that eat these
bacteria return the organic carbon (or a portion of it) back into the food
web. Without the microbial loop, the bulk of the organic carbon in aquatic
ecosystems would be dissolved organic material or bacteria. The microbial
loop may also be important to understanding how pollutants move into
food webs (Wallberg et al., 1997).
Microbial food webs in lakes include phytoplankton, bacteria, proto-
zoa, viruses, and rotifers. Phytoplankton can be divided into very small
cells (e.g.,

m) and larger cells. The very small cells are referred
to as picophytoplankton and have been demonstrated to be present in a
wide variety of lake types (Søndergaard, 1991). Likewise, very small fla-

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