Tallamy, Burghardt, Narango, Shropshire, and others emphasize that Lepidoptera (butterflies and moths) are keystone species, because caterpillars contribute so much energy to the food web. Preserving and increasing Lepidoptera host plants is therefore an effective and efficient method to sustain or restore the health of an ecosystem.
Sam Droege, wildlife biologist and bee specialist, broadens that category to include a greater range of insects and trees. Even trees that host relatively few lepidopteran species can be critical, if those trees are the sole host for a group of insects.
Sam Droege has spent most of his career at the USGS Patuxent Wildlife Research Center. He has coordinated the North American Breeding Bird Survey Program, developed the North American Amphibian Monitoring Program, the BioBlitz, Cricket Crawl, and FrogwatchUSA programs and worked on the design and evaluation of monitoring programs. Currently he is developing an inventory and monitoring program for native bees, online identification guides for North American bees at www.discoverlife.org, and investigating bee-plant relationships.
Perhaps if you were to choose only one insect group, you would choose lepidoptera. But one could argue that bees also play key roles in maintaining ecosystem health. It gets tricky to decide where to make the cut.
What do you think of the biodiversity indicator as a measure of the contribution of tree species to sustaining biodiversity?
Doug Tallamy’s work is excellent and important but looks at things mostly through a caterpillar lens. There are leaf hopper lenses, tree hopper lenses, myriads of leaf miner lenses, gall lens,….oh, and a bee lens.
The general point is that local trees represent literally millions of years of co-occurrence with local insects and each tree species hosts a huge array of insects. Because of that, there is a great deal of specialization by the insects and accommodation by the trees.
This is not the case for species of trees from other continents or the West Coast, though, depending on the insect, there will be exceptions. (Nature is not as black and white as we would wish.)
For the specifics you must ask the insects: each will have its own list of trees it prefers or needs. Just because one species of tree appears to host more species of insects does not make it a superstar. While some trees may host smaller numbers, they are still critical to those species of insects and therefore superstars to that smaller group. Do we largely ignore the minority, or do we cast a big net?
We have the additional problem that some trees and insects are understudied and creating a limited dance card of trees may inadvertently leave some wallflowers to wither.
Is there an alternative or additional indicator that you would recommend?
I would add additional ones, bees, galls, cankers, miners, treehoppers, beetles, etc.
How important are native trees to sustaining native bees?
Very important. They present boatloads of pollen and nectar early in the year when other plants are not contributing (plants can be selfish). It depends on the tree species as to how much of a contribution (some are zero).
You would want a diversity of trees to sustain generalist bee communities throughout the spring to early summer. Most trees do not bloom for a long period of time in the spring (most often, a couple of weeks only), so it is important to provide a parade of tree bloom. Thus, a diversity of trees is important.
There are a few tree species which support highly specialized species of bees, as opposed to the generalist bees, but not as many as the shrubs and forbs plant groups.
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