Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 408

ing species (May, 1972). Since then, empirical (Frank and McNaughton,
1991) and modeling (Dodds and Henebry, 1996) approaches have sug-
gested that increased diversity begets greater stability. I am not aware of
any data from aquatic systems that can resolve the controversy regarding
food web complexity and stability.
The degree of connectivity (the number of links per species) and how
it changes with community size are also controversial. It has been sug-
gested that connectivity increases as food webs become more complex
(Havens, 1992), but this analysis is controversial (Martinez, 1993; Havens,
1993). Interesting results have been obtained that indicate that the pro-
portion of species in each trophic category does not change with food web
size (Havens, 1992). However, even this idea of constant proportions of
species at each trophic level independent of food web size has been demon-
strated to be false in one stream community (Tavares-Cromar and Williams,
1996).
As a convenience, scientists conducting community analyses of food
webs lump organisms into trophic categories. For example, in benthic food
webs, one of the primary food sources is often detritus. In reality, detritus
is a complex microbial and invertebrate community associated with de-
caying organic material. Analysis of a highly resolved food web from Lit-
tle Rock Lake, Wisconsin, suggests that lumping or aggregation of food
webs into functional groups strongly influences properties such as the pro-
portion of species at each trophic level and the connectivity of the food
webs (Martinez, 1991).
Food webs can have variable structure over space and time. Descrip-
tive characteristics of a detritus-based food web varied over time where the
food web was more or less complex across seasons depending on the life
cycle of the benthic invertebrates (Tavares-Cromar and Williams, 1996).
Temporal variation in stream food webs has been linked to seasonal hy-
draulic cycles (Power et al., 1995). Spatial variation also has clear effects,
and benthic and pelagic food webs in lakes are distinct (Havens et al.,
1996a), pools and riffles in streams contain different organisms, and there
are strong effects of water depth on species composition in wetlands.
SUMMARY
1. Some species from most major groups of aquatic animals can eat
phytoplankton, periphyton, macrophytes, detritus, or other animals.
Omnivory is common in freshwater invertebrates.
2. Stable isotopes are commonly used to assess trophic interactions.
3. Prey can respond to chemical, visual, and hydromechanical cues.
4. Protective adaptations to predation include mechanical (size and
spines), chemical, and behavioral.
5. Predators forage optimally to maximize their efficiency; this may lead
to one of several functional responses to prey numbers.
6. Predators can sense their prey by using chemical, visual, tactile, and
hydromechanical cues.
7. Trophic cascades are an important part of aquatic food webs and may
occur in all freshwater habitats. Effects can be transmitted through