A researcher at Colorado State University has shown that the brighter it gets, the better bugs remove slime from streams. Todd Wellnitz, research associate in Colorado State’s biology department, conducted experiments in a clear, Rocky Mountain trout stream to examine the effects of light and insects on periphyton, the slime layer on streambed rocks.
It turns out, Wellnitz told participants at the 85th annual meeting of the Ecological Society of America Aug. 6-10 in Snowbird, Utah, that different levels of light affected how insects-in this case, mayfly nymphs-consumed periphyton.
And this and other research, he said, strongly reinforce a notion often ignored in ecological investigations: that environmental gradients (changes in an environmental factor over time or distance) are ubiquitous in nature and can affect the outcomes of species interactions in streams and elsewhere.
Although a nuisance to clumsy fly fishermen, periphyton is vital stuff for streams. Consisting of algae, bacteria, protozoa, fungi and detritus, periphyton is the food of choice for mayfly nymphs, caddisfly larvae and other "grazing" insects beloved by trout and imitated by fly-tying fishermen.
Wellnitz varied the density of grazing mayfly nymphs and altered light levels, using 36 streambed-mounted troughs (which appeared vaguely boatlike and were dubbed "The Armada.") Shading cloth controlled light levels and window screens, attached to either end of the troughs, controlled the mayfly numbers inside. After three weeks, Wellnitz found that the amount of periphyton removed by the mayflies depended on the intensity of light.
"The more light, the bigger the impact these little critters had," he said. The mayflies ate negligible amounts of periphyton at 10 percent sunlight, ate more at 50 percent and had their greatest effect at full sunlight, consuming nearly half the periphyton.
"It’s just one example of how the environmental context matters," Wellnitz said.
For example, Wellnitz said, the importance of predators in shaping a community can change with temperature. Competitive contests between plants vying for the same resource may have opposite outcomes in different soils, he said.
Wellnitz underscored his point with results from work that he and N. Leroy Poff, assistant professor of biology at Colorado State, are conducting on the upper Colorado River.
In one experiment, Wellnitz and Poff turned loose three insect grazers-two mayfly nymphys and a caddisfly larvae-into artificial stream channels that manipulated current velocity to five, 15 and 35 centimeters per second. The insect grazers were given periphyton to eat.
As current went from slow to medium to fast, the insects differed in their ability to remove periphyton. One kind of mayfly beat the other mayfly nymph and caddisfly larvae feeding in slow current. The other mayfly had no net effect on algae at medium current speeds but ate more than the other two grazers at fast current speeds. Wellnitz said the point is that current speed not only affected each species’ performance but their relative performance as well.
Environmental conditions may also determine whether species help or hurt one another. In a third experiment, Wellnitz and Poff demonstrated that caddisfly larvae, which construct small domes of sand grains around their stubby-legged bodies, gained weight in slow current in the presence of potentially competing mayfly nymphs. The mayfly nymphs are bigger, have longer legs and knock bits of thick, gummy periphyton loose. This enables the smaller, less mobile caddisfly that would otherwise get stuck in the gummy periphyton to move about more easily and feed.
This positive interaction, or facilitation, disappears at faster currents because the periphyton is thinner, less sticky and more easily consumed by caddisfly larvae whether or not mayfly nymphs are present, Wellnitz said.
It is important to recognize how environmental gradients influence interacting species in nature, he said. Understanding how herbivory, competition and predation structure natural communities requires that ecologists examine these interactions across the gradient of conditions where they appear.
"It doesn’t matter whether one is studying grazers on the African Serengeti, butterfly species competing for nectar or lynx chasing their rodent prey," said Wellnitz.
Wellnitz, who earned his doctorate form the Swiss Federal Institute of Technology in Zurich, works with Poff under a National Science Foundation grant.
The lesson, he believes, is that experiments that examine ecological relationships under a restricted set of environmental conditions will only reveal an incomplete picture of how species and their interactions shape communities.
"The reality is more complex and interesting," he said.