Aster as a Keystone Species
In late summer and early autumn, flowering goldenrod (Solidago spp.) washes yellow over many Pennsylvania meadows, poking out of ditches and growing along streambeds and roadsides. Occasionally, I'll notice a dab of purple in the landscape, too; a cluster of asters (family Asteraceae) is sprinkled here and there. Over the last several years in the acre of old field near my house, where once there was a handful of clumps, I have noticed the aster diminishing. Now, while the goldenrod is doing well, even on the increase, asters are no longer growing in my meadow. I’m not certain what species of aster I once observed, having not recorded it as a thoughtful naturalist or gardener might. The yellow center disk, I remember, suggests New England aster (Symphyotrichum novae-angliae).
Deer, rabbits, and other mammals nibble at the flower buds. They don't generally eat the whole aster plant because of its pungent smell, bitter taste, and the rough-textured leaves found on most species. The plant tolerates nibbling, and the nibbling even serves as beneficial pruning. Year after year of overgrazing, however, too many mammals competing for too little food might be doing more than suppressing blooms. Perhaps that eliminated the aster, but this analysis is mere speculation.
What I do know for a fact is that the existence of native plants, in general, has been under threat from habitat loss for decades. Lawns and exotic species of plants consume much of the landmass across this country, and invasive species are overwhelming many of the few natural areas remaining, protected or otherwise, especially in the East. Asters are an important food and nesting source for wildlife, and I wonder if I should cultivate a few species. In eastern temperate forests, or what is often referred to as Ecological Region 8, scientists have even labeled asters a "keystone species." Simply put, the term means any species integral to an ecosystem.
Asters underwent a revision in taxonomy several years back, and the genus Aster now applies only to plants native to Europe and Asia. The name remains within Asteraceae, which has risen to the rank of family. Formerly called the Compositae, the Asteraceae is still called the daisy or composite family. The common name aster is informal but is still used by all manner of people who work with plants, from botanists to gardeners.
Most asters native to North America are placed in the genera Symphyotrichum and Eurybria. As far as perennials are concerned, both asters and goldenrods play an oversized role in invertebrate survival. According to the National Wildlife Federation, the aster genus Symphyotricum feeds at least 100 types of larval caterpillars. Perhaps this is not as many hungry caterpillars as supported by woody plants, like oaks or even blueberries, but being a host plant is only one role that asters play.
Bloom time is particularly critical. In autumn, as cold weather looms and other perennials have faded away, asters grab the last of the summer's heat and light to set flower. A wide range of pollinators depend on asters for nectar and pollen. Their resulting seeds then provide sustenance to both migrating and resident birds. The Connecticut Audubon Society reports that seeds from asters attract orange-crowned warblers (Leiothlypis celata), dark-eyed juncos (Junco hyemalis), indigo buntings (Passerina cyanea), American goldfinches (Spinus tristis), and American tree sparrow (Spizelloides arborea), to name a few. Other creatures benefit from the seeds, like mice, which help feed snakes and raptors. Not only a food source, but once winter arrives, an array of insects takes shelter in their depleted stalks.
Being a keystone species, I wonder theoretically if the loss of asters from the landscape would cause environmental collapse, as the name "keystone" suggests. Biodiversity sustains life. In his influential book, Bringing Nature Home, first published in 2007, entomologist Doug Tallamy states, "For the last 60 or more years, scientists have been studying the complexity of ecosystems, the degrees of biodiversity necessary to keep an ecosystem functioning and to support essential food webs that sustain all levels of creatures, from the tiniest micro-organism to higher level predators, like us." He describes how redundancy is integrated into ecological systems. Goldenrod also holds the title to the keystone species designation. While goldenrod remains a vital genus, would it fill the gap if asters were to disappear completely, sustaining the same food web? So many factors, both known and unknown, come into play. But I have no doubt that the loss of any species causes environmental degradation.
In addition to their ecological value, asters provide many planting options and satisfy a range of aesthetic sensibilities, providing a wide selection of colors and growing conditions. Despite their often scruffy foliage, I am partial to the daisy-like purple and lavender flowers of the New England aster (S. novae-angliae). Cutting back stems in early summer results in a more compact form, and planting them in the background helps to hide their browning foilage. The nectar of aster attracts monarch butterflies (Danaus plexippus).
Smooth blue aster (S. laeve) is a species I have propagated successfully from seed. It is host to the pearl crescent butterfly (Phyciodes tharos), according to Butterflies and Moths of North America. Aromatic aster (S. oblongifolium) grows in low mounds and is a host for the silvery checkerspot butterfly (Chlosyne nycteis). White wood aster (Eurybia divaricata) attracts butterflies and is found all around the woods of Pennsylvania in shade and dry, rocky sites. Fortunately, much is already in evidence where I live. Blue wood aster (S. cordifolium) also likes shade and is a prolific seed producer.
Varieties of calico aster (S. lateriflorum) possess the unusual attribute of presenting flower heads of more than one color simultaneously, from white or light yellow to pink and purple. Cultivars of aster often fail to attract the same numbers of pollinators as the straight species. However, in a 2002 aster trial, the Mount Cuba Center reported that 'Lovely,' a calico aster cultivar, "hums with insects" while in flower.
In just the last decade, I have observed rapid change to the five-acre plot of land I attempt to garden and steward in northeast Pennsylvania, from deer pressure to the ever-increasing numbers of invasive species. I garden to surround myself with the beauty and the serenity gardens engender, but as much as I am able, I also garden to support wildlife.
