Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities - Freshwater Ecology: Concepts and Environmental Applications - page 414

Zooplankton species compete for food resources. In Chapter 18, I dis-
cussed the idea that ratios of nutrients may alter competitive ability or lead
to coexistence of phytoplankton species. It has been hypothesized that sim-
ilar resource partitioning can lead to coexistence of potentially competing
zooplankton species (Lampert, 1997; DeMott, 1995). Such application has
not been explored for many species but may be a productive area of re-
search.
In some cases, a trade-off may occur in competitive ability. For exam-
ple, one species of rotifer (Keratella cochlearis) may compete better at a
lower food concentration and the other (Keratella earlinae) at higher con-
centrations (Fig. 20.2). Both species could coexist in a spatially or tempo-
rally variable environment.
Competitive interactions among zooplankton species probably vary
over space and time. In Chapter 19, I discussed the idea that large zoo-
plankton are superior competitors for phytoplankton cells. However, this
competitive interaction may not be so simplistic. Large zooplankton may
slow or cease feeding in blooms of large inedible algae while smaller zoo-
plankters continue to feed unhindered (Gliwicz, 1980). Similarly, clado-
cerans may be superior competitors for food compared to rotifers, but sus-
pended clay can decrease the growth of cladocerans while having little
effect on rotifers. Thus, the clay allows rotifers to be released from com-
petition with cladocerans (Kirk and Gilbert, 1990).
There are situations in which competitive exclusion should operate but
may not be strong enough to drive out weaker competitors. An example
occurred with five species of herbivorous zooplankton in a small humic
lake (Hessen, 1990). These species coexisted, but bottle experiments showed
no evidence of significant predation or a single dominant competitor. Weak
competition for abundant nutrient-poor food in this lake was hypothesized
to allow coexistence of the species. This study and those noted previously

0.3

0.2

0.1

0.0 Keratella earlinae

K. cochlearis

-0.1

-0.2

-0.3

0.0 0.5 1.0 1.5

Cell concentration (mg L -1 )

FIGURE 20.2 Competitive ability of two species of rotifers ( Keratella cochlearis and K. ear-
linae ) that feed on the cryptomonad Rhodomonas. Note that K. cochlearis is able to grow
more rapidly at lower concentrations of food, but K. earlinae grows better at higher concen-
trations of food (redrawn from Stemberger and Gilbert, 1985).