Garden Biodiversity: The total number of species living in the garden ecosystem. Of course, garden biodiversity cannot be empirically measured as there are many thousands of microscopic species in both the soil and on the garden’s plants. The gardener can, however, keep a list of the macroscopic species found in the garden, including plants, insects, birds, and mammals.
We garden in a frightening period of Earth’s history: Our planet is in the midst of the sixth mass extinction of plants and animals. And for the first time in the history of Earth, humans are witness to such a mass extinction. (The fifth mass extinction, the End Cretaceous, an event that wiped out dinosaurs and 75 percent of all other life forms, occurred 65 million years ago, before humans were on the scene.) The present global species extinction rate is at least 1000 times that of the normal background rate with dozens of species going extinct every day (Center for Biological Diversity, www.biologicaldiversity.org).
Without question, the human species is the cause of the current extinction crisis. Human activities, including habitat destruction, global transport of invasive species, pollution of all types, human population growth, and over-harvesting of Earth’s resources, are the major causes of our planet’s extinction crisis (E.O. Wilson, www.unesco.org).Marjorie and I believe that gardeners can play a significant role in stemming the tide of biodiversity decline, especially for those creatures on the smaller end of the size spectrum, the insects, creatures that E.O. Wilson dubbed “the little things that run the world”. This belief was reinforced this past gardening season when we decided to plant annual milkweed (Asclepias curassavica) in Marjorie’s Garden, a decision that led to the appearance of monarch butterflies in our garden for the first time. In a previous column (“A Monarch Summer, September 2, 2014, http://gardeningintunewithnature.bangordailynews.com/author/rmanley/), I wrote about our first experience with monarchs, including participation in a tagging project to track their fall migration to overwintering forests in Mexico. We also planted a perennial form of milkweed, swamp milkweed (A. incarnata), in our garden last summer. This is the best perennial Asclepias for the home garden as the other possibility, common milkweed (A. syriaca), is a rhizomatous plant that will take over the garden. Common milkweed is the species that you see growing in large roadside colonies. It is a great plant for monarch butterflies and other nectar feeders, just too aggressive for most gardens. Swamp milkweed, on the other hand, is a clump-forming plant that stays where you have planted it. While all of our young swamp milkweed plants grew between two and three feet in height their first year in the garden, only one flowered, producing a small cluster of mauve blossoms. The 25 monarch caterpillars in the garden were content to feed on the annual milkweed plants and showed little interest in the swamp milkweed leaves, leaving them to a horde of milkweed tussock moth larvae (Euchaetes egle). Feeding in large groups, these caterpillars would skeletonize all of a plant’s leaves in a matter of hours. By summer’s end, our swamp milkweed plants had been reduced to slender leafless stalks as the caterpillars grew fat. (E. egle larvae goes through four instar stages as they mature and the first instar stage looks nothing like the fourth stage larva. The first stage is grayish and only slightly hairy, while the fourth stage looks like “Cousin It” of Addams Family fame.) The single cluster of swamp milkweed flowers was repeatedly visited by a female great black wasp (Sphex pensylvanicus). It was the first time we had noticed this species in our garden, although we had watched it nectaring on the same milkweed species in other gardens. At about two inches long and jet black except for wings that are iridescent blue in sunlight, she has a foreboding countenance, yet she must be considered among the gardener’s best friends. When she is not sipping nectar, she spends her time preying on grasshoppers, crickets, and other Orthopterans, dragging their paralyzed bodies to her underground nest where they serve as a first meal for her larvae. Making frequent stops at our little milkweed patch to watch the tussock moths (I was hoping to see a predator at work), I noticed that the color of one of the stems had changed from green to orange. Close inspection revealed a growing colony of oleander aphids (Aphis nerii), so named for their primary host plants in more southern regions of the country, but also called milkweed aphids in New England.
It only took a few days for every swamp milkweed stem to become completely covered with orange aphids, each tapped into the phloem sap of the stem. This is not surprising, since every aphid is a female capable of giving birth to 80 live young, called nymphs, every week, and each nymph, also a female, becomes sexually mature in about a week. It’s an interesting math exercise to start with a single aphid and calculate the number of aphids after just 5 weeks. Of course, aphid population explosions are usually tempered by ladybug beetles and other predators.
I was so captivated by the presence of both herbivores, the tussock moth larvae and the aphids, that I overlooked the possibility that no predators would show up to control their population growth. Both the tussock moth larvae and the aphids were ingesting the same glycosides that are toxic to potential predators of monarch butterflies. The orange color of the aphids and the vivid orange and black coloration of the tussock moth larvae – the same color combination of adult monarchs – serve as warnings to would be predators: Don’t Eat Me!
Thus, over the course of the last few weeks before first frost, our young swamp milkweed plants took a beating. Based on experience with this species at Marjorie’s office garden, we know that mature plants can take the abuse and come back the next spring with vigor, but we may be asking a lot of one year old plants. If we do have to replant, I will take on the role of caterpillar and aphid predator for a year or two, hand picking the former and removing the later with strong streams of water.
These experiences of the past garden season were epiphanies for Marjorie and me. In the course of a single season, the planting of a milkweeds, both annual and perennial, resulted in a four-species increase in garden insect biodiversity. Total garden biodiversity increased by six species, since both of the Asclepias species were new to our garden.We believe that in this unprecedented age of species extinction, each of our gardens, large and small, can provide refuge for countless forms of life, including species of bacteria, fungi, nematodes, and other life forms associated with healthy soils; insects of all kinds, including pollinators, herbivores (plant munchers, like caterpillars and aphids), and the insects that prey on herbivores; mammals such as mice, squirrels, chipmunks, rabbits, deer, and gardeners; birds, reptiles, and amphibians; and, of course, garden plants.
Imagine the gardens of a neighborhood linked together by vegetation corridors and a common will among the gardeners to maximize neighborhood biodiversity. Imagine an excited conversation over the garden fence about a new insect or new bird added to your garden’s species list.