Dec. 5, 2000 No self-respecting parent would raise a new baby on a diet of sugar alone. Instead, good parents are careful to give mother’s milk or to mix formula containing all the energy and nutrients baby needs to grow.
Unfortunately, Mother Nature is not such a doting parent. In a study to be published in the November 30 issue of Nature, Arizona State University ecologist Jim Elser and his colleagues found that plant-feeding insects are provided with a diet so poor in nutrients that it impairs growth.
The researchers used basic chemical principles to evaluate the un-nutritious menu available for insects living in terrestrial habitats. They then compared it to the food available to plant-eating animals in lakes, where the animals seem to be literally swimming in nutrient-rich foods. The high quality of the aquatic diet gives diners the green light on rapid growth.
Elser, an aquatic ecologist, teamed with eleven other experts in aquatic and terrestrial ecology to do the study. “The main goal was to find a means to compare aquatic and terrestrial food webs with each other,” he says. Ecologists already know a lot about how nutrient balance affects food webs in lakes. They wanted to find out if the principles of aquatic nutrient balance could be applied to land environments.
The easiest place to start with this comparison is at the base of the food web – plants and the animals that eat them. By looking at the interactions between plants and herbivores in lakes and on land, the researchers hoped to determine whether nutrient balance in food webs varies in different environments.
A fair comparison of these contrasting habitats, and the plants and animals that live in them, must be based on a common currency. “They seem so different when you think about them – microscopic algae in lakes and trees on land – how do you compare these things?” Elser asks. “You have to find something they have in common, and what they have in common is their elemental composition, because all organisms use the same elements to build their bodies with.”
They looked at the content of the elements nitrogen, phosphorus, and carbon in aquatic and terrestrial organisms. This type of research uses the principles of chemical stoichiometry, the study of the balance of multiple elements in chemical reactions.
The elements nitrogen and phosphorus are central in applying stoichiometry to living things, because plant and animal cells need these elements to make new proteins, DNA, and other molecules during growth. In fact, animals with very high growth rates seem to have more phosphorus in their bodies than slower growers. Therefore, nitrogen- and phosphorus- rich plants are considered nutritious.
The carbon in molecules provides the main structural building blocks of new body materials, as well as a source of energy to power growth. But foods with excess carbon don’t provide a good source of the other important nutrient elements. “In stoichiometry that means that those foods are poor quality. It’s like eating marshmallows all the time. You can certainly satisfy your daily energetic requirements by eating marshmallows, but you can’t grow because you need more than energy to grow,” Elser explains.
“You’ve got to have a nutritionally balanced diet. Stoichiometry is a way of deciding what a chemically balanced diet looks like.”
Elser and his collaborators gathered reams of data on the chemical composition of plants and animals from previously published studies. Their literature search yielded information on 501 land plants such as trees and grasses, algae and other microscopic organisms from 226 lakes, 130 species of terrestrial insects like butterflies and grasshoppers, and 43 species of planktonic crustaceans (tiny insect-like animals that inhabit the water column).
An important new step was to analyze the nitrogen, phosphorus, and carbon content of the plant-eating animals, Elser explains. The amounts of the elements in their bodies indicates how much of each nutrient they need in their diet. “It’s not enough just to consider what the food is. You have to consider what the animal needs to build its biomass,” he says.
They discovered that, as Elser puts it, “a bug is a bug is a bug.” Within each habitat, animals varied slightly in their composition, but, overall, the nutrient content of the herbivores’ bodies were very similar in both habitats. This is valuable knowledge because “whatever we’ve learned about stoichiometry or growth or nutrient balance in lakes we probably can move over to land insects without having to worry that they have drastically different requirements,” says Elser.
The same is not true for the bugs’ food source, however. “Food webs across different habitats are really built on different bases as far as their nutritional quality,” Elser discovered.
Nitrogen and phosphorus are abundant in algae. These aquatic plants are like baby formula for crustaceans and support rapid growth. But land plants are more like sugary junk food, containing more carbon and about three times less of the nitrogen and phosphorus necessary for growth. Insects eating the nutrient-poor leaves may have no other option but to grow slowly.
“The success of herbivores and their dynamics and what nutrients they’re limited by probably differs quite a lot in lakes and terrestrial systems.” And this could be a determinant of how plant-eating animals evolve in the two habitats, Elser says. “The evolutionary implication is that a high-growth-rate lifestyle may be more feasible in phosphorus-rich environments” like lakes.
A fast-growing insect on land will be susceptible to unstable population fluctuations or even extinction, Elser says. “As you grow faster, your nutrient requirements go up, but your environment may not be presenting those nutrients at levels that can sustain you. Such an organism would suffer from severe food quality problems, and therefore not be evolutionarily feasible.”
These findings could also have important implications for agriculture. Farmers fertilizing their crops get the benefit of producing nutrient-laden, fast-growing plants. But their nitrogen- and phosphorus-rich crops are more attractive to pest insects seeking high-quality diets. It might be possible for farmers to minimize crop losses to insects by using fertilizers that are high enough in nitrogen and phosphorus to stimulate crop growth, but too low in these nutrients to create an ideal diet for pests.
Co-authors on this paper are: Robert F Denno, Andrea Huberty (Dept of Entomology, Univ of Maryland, College Park), Sebastian Interlandi, Susan S Kilham School of Environmental Science, Drexel Univ, Philadelphia), Edward McCauley (Ecology Division, Univ of Calgary), Kimberly L Schultz (Dept of Environmental Biology, SUNY, Syracuse), Evan H Siemann (Dept of Ecology, Rice Univ, Houston), Robert W Sterner (Dept of Ecology, Univ of Minnesota, St Paul)
A complete summary of the data sets and original citations can be viewed at http://www.nceas.ucsb.edu/ecostoichiometry.
For more on research in this area, see: http://lifesciences.asu.edu/irceb/stoichiometry/
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The above story is based on materials provided by Arizona State University College Of Liberal Arts & Sciences.
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