Have you ever wondered why it is, that to reproduce Monarch butterflies need milkweed; and why there actually are plants deer dislike; and why northeastern squirrels prefer white oak acorns to those of red oaks? Why, field guides of plants, insects and butterflies, use the term ‘host plant’? What is the connection between the evolutionary kingdoms?
This is an excerpt from the Wild Ones Journal
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In the May/June 2008 we launched a series, Mysteries Explored, in which we offered to explore the validity of our claim that working with native plant communities lets us practice ecologically sound landscaping and helps us preserve biodiversity. It is also our way of exploring the nature of native plants.
In the article titled Buckthorn, Birds and Diarrhea we looked at how one particular genus of fruit producing plants interacts with birds that are its evolutionary cohorts and others, on a different continent, that are, in essence, evolutionary strangers. One set of birds has adapted to the defenses of the plant; the other set has not. The plant is ‘native’ to one set and ‘alien’ to another set.
The critical aspect of these interspecies interactions came down to a chemical that the buckthorn produces. This chemical is in no way vital to the plant’s life-critical cellular function; the plant could perform photosynthesis without it; it could bear fertile fruit and thereby reproduce itself: this is not a ‘primary metabolite’. And yet the plant expends energy to produce this chemical—this ‘secondary metabolite’. For what purpose?
Since plants are firmly rooted in the ground and are not able to voluntarily knock an enemy for a loop, they have developed another avenue of defense. The secondary metabolites defend against other organisms, predators that endanger the plant — organisms that might eat its leaves, burrow into its fluid-transport systems, chew on its roots; organisms that might prematurely take its fruit. They attract pollinators and seed dispersers.
The process of producing these chemicals is something that evolves in a plant species over evolutionary time. Plants that have more or less of this chemical, upon interacting with their predators, have different survival rates, and pass on their varying production rates to their off-spring.
The organisms against which the plant is defending (let’s continue with the birds in this example, but you may also think in terms of insects and mammals), in their turn, are dependent on the plant for food, and need to evolve ways to deal with/adapt to the defensive strategies of the plant. Over evolutionary time this give and take is smoothed out and the two very different species, from two separate evolutionary Kingdoms, come to serve each other’s purposes to the benefit of both. They may even become entirely reliant on each other for survival.
The birds that co-evolved over millennia in the same geographic space as the buckthorn have learned to recognize when the fruit is ready to eat — when it will no longer cause life threatening diarrhea; and, incidentally, coincident with the best time to spread the plant’s seeds far and wide.
The chemical that at one time was a defensive mechanism, is no longer necessary for that purpose and the plant reduces the rate of production, resulting in a lower concentration (this reaction, too, may be the result of interacting hormonal/chemical levels within the plant).
In the bird, this lower concentration has a paradoxical effect: what once, earlier in the season, caused debilitating diarrhea, now causes prolonged gut retention. The bird is rewarded by being able to extract more food value from the fruit; the seeds, being retained longer, are removed and deposited at a distance from the mother plant, decreasing chances for competition within the plant family. Both plant and animal benefit.
When this same plant is thrust into the presence of bird species, in a different ecoregion that have not coevolved with it, the evolutionary adjustments and balances are tossed out the window! The plant continues to produce its secondary metabolites but those birds without the advantage of covergent evolution are without the mechanisms necessary to deal with these metabolites and are, in fact, damaged by them.
Another plant/animal interaction that involves secondary metabolites is one we are familiar with. That is the relationship between the Monarch butterfly and the milkweed plant. While many insects are able to take and use the nectar of the milkweed, the larva of the Monarch butterfly is one of the few that is able to feed on the leaves of the plant. In fact the milkweed is the only plant the monarch caterpillar can feed on. Lacking milkweed the caterpillar will starve to death. The very chemicals in the milkweed that are toxic to other insects (and birds that eat the insects that feed on milkweed) are necessary to the monarch’s body chemistry, and thereby for its survival.
Secondary metabolites may well be the basis for most of the food preferences of which we are aware in insects and mammals. Almost invariably they are products of covergent evolution that took place over many thousands of years.
Here we have another aspect of the definition of what is a native plant: it is a plant that plays a supportive role in its environment—a role that has co-evolved over time, interacting with other organisms in the same environment.
It is also one more reason why reproducing native plant communities in our gardens and natural areas is important.
By Maryann Whitman