Saturday, July 23, 2016
Caterpillars, Food Webs and Doug Tallamy
Note: This program first aired July 23, 2016.
Ecosystems are like salad bars, you fill your plate with lots of leafy greens and then sprinkle lesser amounts of more concentrated food items on top (nuts, olives, bacon bits). Ecosystems have a similar food or trophic structure, at the bottom are the primary producers, the plants and other photosynthetic organisms, as you move up at each level there are fewer and fewer non plant individuals ( herbivores, omnivores and carnivores).
Land plants are the primary producers of terrestrial ecosystems, doing the work of capturing the sun’s energy on dry ground, they are the lettuce on the salad bar of the forest. And it turns out there are bacon bits here in the forest as well, in the form of caterpillars. As herbivores these organisms are able to transfer the energy that plants capture from the sun to higher trophic levels in the forest food web. That is what I learned listening to a talk from University of Delaware entomologist Doug Tallamy when he came to Maine to speak earlier this summer.
Tallamy’s work looks at the relationship between the primary producers, insects and the upper trophic levels, mainly birds. And what he has discovered has profound implications for the way we manage the landscapes directly under our control. It turns out that if you want birds around, particularly migratory song birds, you need insects, specifically caterpillars. These birds migrate thousands of miles to northern North America to breed, because of the plentiful food sources here.
Caterpillars, which are made up of fat and protein, are very nutrient dense and make up a huge proportion of that food for many of these birds. Tallamy recalled watching a pair of birds feeding their young, calculating that the nest of young birds were fed hundreds of caterpillars a day, thousands over the two week period they spent in the nest. And here is where it gets interesting, because if you want caterpillars around, caterpillars being the larval stage of lepidoptreans, the butterflies and moths, you need plants, because caterpillars eat plants.
But not just any plants, caterpillars generally eat specific plants, plants they have a long evolutionary relationships with. Plants do everything they can to not be eaten, evolving elaborate chemical warfare against hungry insects. Insects do everything they can to evolve physiologic means of evading the plant defenses, in an ever escalating evolutionary game, the relationship between the eater and the “eat-ee” gets more and more specific. We have all heard about the monarch butterfly and its host plant common milkweed, but many lepidopterans have this level of specificity with their target food source. Others are less specific and play the field, having relationships with plants in more than one genus. And plants are not monogamous, as the “prey species” in this relationship, they have many different insects trying to eat them, and can play host to tens to hundreds of different kinds of caterpillars.
What Tallamy and his lab have quantified is the number of different lepidopteran species (in the form of the caterpillars—which are the stage in which these insects do the majority of their primary production energy transfer) various genera of plants support. The results are astonishing. In my area, native tree genera like willow (Salix) and oak (Quercus) can support between 300 and 350 different types of caterpillars! Other native tree genera support nearly that many. The corallary to this is that nonnative trees and shrubs, frequently planted as part of “normal” landscaping, support virtually no caterpillars, because no native insects have evolved ways to evade these plants’ defenses. None. Insects are the animal that is responsible for transferring the majority of primary productivity from plants to the rest of the terrestrial food web. Birds, reptiles, amphibians, spiders, and rodents all rely on caterpillars as a food source. So if you don’t have plants that support insects, you don’t have anything else in the food web either. Biodiversity is our life support system, and what I learned from Doug Tallamy is that it starts from the ground up, in an elegant and fundamental relationship between those that eat and those that are eaten.
Doug Tallamy’s book on native plant landscaping in the suburban environment: http://www.bringingnaturehome.net/
Download the data from Doug Tallamy’s studies here: http://udel.edu/~dtallamy/host/
One of Doug’s popular science articles (slightly dated) but with interesting stats: https://www.americanforests.org/magazine/article/backyard-biodiversity/
Our local native plant advocacy group, Wild Seed Project, has lists of appropriate native plants (to Maine) for different environmental conditions: http://wildseedproject.net/using-natives-in-the-landscape-a-comprehensive-plant-list/
The native plant finder at the National Wildlife Federation site: http://www.nwf.org/nativeplants-beta/About (its a beta version--still a little buggy, no pun intended, for example, when I searched my zip code some high scoring trees did not appear, but when I searched them individually they showed up in my zip code…)
Nutritional info on caterpillars: http://healthland.time.com/2013/08/21/why-eating-bugs-is-good-for-you-its-about-the-nutrients/slide/caterpillar/
Want to eat some bugs yourself? Here’s a list of internet resources: https://edibug.wordpress.com/where-to-get-bugs/