Monday, December 06, 2004

Plant Diversity Supports Animal Diversity

I’m interested in understanding the values native plants have for wildlife. The data discussed here was derived from a survey of the larval hosts the microlepidoptera of the San Bruno Mountains on the San Francisco Peninsula. Microlepidoptera are small moths, mostly “leaf miners” -- their tiny caterpillars “mine” tunnels in leaves. Typically, the caterpillars of each species of moth eat particular species of plants. The source paper is “Larval Hosts of the Microlepidoptera of the San Bruno Mountains, California” by John A. Benedictus, David L. Agner, and James B. Whitfield, published in Atala: the Journal of Invertebrate Conservation, published by the Xerces Society.

I “mined” the article to rank the plant species of San Bruno Mountain according to the number of species of microlepidoptera each one supports, with an eye to understanding which plant species support the greatest diversity of animal life.

160 species of leaf miners are listed in the article, with the authors estimating that there are about 300 species present. Many species of leaf miner confine their feeding to only one or a few species of native plants. Thus the diversity of leaf miners is closely tied to the diversity of plants. In general, a diversity of plants supports a diversity of animals.

I listed the plants, tallying the number of species of leaf miner each supports. The researchers had sampled 82 species of plants out of the 540 species present. My theory is that the diversity of leaf miners supported by a given species of plant is an indication of the total diversity of insects that species supports. In England, where biological studies have proceeded more systematically, total numbers of insect species per plant species are much better known than here in California.

The “top end” of my tally list – plant species the researchers found supporting five or more species of microlepidoptera, these:

Larval Host ----- Number of Species of Leaf Miners

Quercus agrifolia – coast live oak -----15
Baccharis pilularis – coyote bush ----- 12
Salix lasiolepis – arroyo willow ----- 12
Ceanothus thyrsiflorus – blue blossom -----10
Monardella villosa – coyote mint ----- 8
Eriogonum latifolium – coast buckwheat ----- 7
Anaphalis margaritacea – pearly everlasting ----- 7
Rhamnus californica – coffeeberry -----7
Eriophyllum staechadifolium – lizard tail ----- 6
Prunus ilicifolia – islay or hollyleaf cherry ----- 6
Holodiscus discolor – creambush or ocean spray ------ 5
Scrophularia californica – figwort ------5
Rubus ursinus – California blackberry ----- 5
Phacelia californica – California phacelia ----- 5
Lupinus arboreus – bush lupine ----- 5

Unidentified cudweeds, Gnaphalium spp., yielded 8 species of leaf miners. A bushtit nest in a creambush yielded 2 additional species of leaf miners – the creambush indirectly “supported” these species! Yet others were discovered in hawk or owl pellets.

Of the 15 species of native plants supporting the greatest diversity of leaf miners in the San Bruno Mountains, one third --5 -- are herbaceous perennials. This is an indicator of the importance of herbaceous perennials for supporting animal diversity.

Many “mitigation” plantings – plantings made to compensate for lost wildlife habitat – have neglected herbaceous perennials. Historically environmental impact reports have generally neglected to quantify the losses of herbaceous perennials and therefore, subsequently, have also neglected them in mitigation revegetation, which generally concentrates on woody plants.

Herbaceous plants are larval hosts for 112 of the 160 known species of microlepidoptera of the San Bruno Mountains. Some leaf miners eat many kinds of plants, both woody and herbaceous, even as other animal life does. Of 82 species of plants listed in the article as larval hosts, 53 were herbaceous and 29 woody. This study and others have convinced me that herbaceous plants make an important contribution to the diversity and abundance of animal life. They should not be neglected in biological impact assessments or mitigation plantings.

“You cannot begin to preserve any species of animal unless you preserve the habitat in which it dwells. Disturb or destroy that habitat or you will exterminate the species as surely as if you had shot it. So conservation means that have to preserve forest and grassland, river and lake, even the sea itself. This is vital not only for the preservation of animal life generally, but for the future existence of man himself—a point that seems to escape many people.”
---- Gerald Durrell,
Nature Conservancy

Earlier versions of this article appeared in the newsletters of the Santa Clara Valley and San Francisco chapters of the California Native Plant Society, many years ago.

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