Our front yard. Photo by D. Sillman
(Click on the following ink to listen to an audio version of this blog …. Ecological anachronisms
(This is updated discussion of the concept of “ecological anachronisms” that I first wrote about in a blog published in 2016)
This summer, Deborah and I were prowling around in the mulched areas of our xeriscaped front yard, looking for weeds and stray plants. We pulled most of these and tossed them into our weed buckets. Along with the usual suspects of kochia, lambs quarter, spurge and thistle, we also came across three or four bur oak (Quercus macrocarpa) seedlings struggling to survive in the heat and dryness of our Northern Colorado summer. Since there are no oaks growing on our property or on any of the nearby neighbors’ yards, I assume that these seedlings grew from acorns that were flown in from some distance away and accidently dropped by a blue jay or crow. A neighbor about a quarter of a mile away does have several bur oak trees in his front yard. Maybe they were the parental trees.
Red oak acorn. Photo by Jamaine Wikimedia Commons
A reproducing oak tree invests a substantial amount of its metabolic energy into its acorns. They make the acorn “package” as appealing as possible to a wide range of birds and mammals. The birds and mammals, of course, look upon these acorns as a high calorie meal. The oak trees count on some of the acorn consumers being clumsy with their nutritious packages, and calculate that they will drop a few of them in a spot where germination is possible. They also count on some of those early oak seedlings surviving the heavy rate of deer and other herbivore browsing (or homeowner weed pulling). We were looking at the winners of a very intense reproduction gauntlet: oak seedlings growing far from the parental tree.
And, that’s why plants have evolved fruit and nuts! Inside these tempting pieces of food are the embryos of the plant’s next generation, and those plants that can use animals to fly or walk at least some of these embryos to more distant habitats are more likely to disperse, thrive and go on to reproduce another generation (and then another, and another, and another!).
Chestnut burr. Photo by fir0002, Wikimedia Commons
There were many examples of this all over my yard and field back in Pennsylvania. Chestnut seedlings grew in several corners of my old, two acres. I assume that they were “planted” by squirrels who carried the chestnuts from some parental tree and then dropped them (or buried them) and then went off to do something else. I have watched gray squirrels carrying chestnuts that are still in their spiky burrs. The squirrels suddenly stop, shake their heads and fling the nut away. My sense is that one of the burr spikes stuck itself into the flesh of the squirrel’s mouth and triggered a reflexive shudder and toss! Could the sharp spikes of these burrs be more than just protective barriers against non-dexterous nut predators? Maybe they are dispersal triggers, too? The latter might even be more important in an evolutionary scheme!
So fruits and nuts are dispersal investments for a plant. Most fruits are designed to attract consumers at just the right time so that the mature plant seeds can go for a ride inside of the fruit consumer’s intestines. They then leave the consumer along with an aliquot of fertilizer (feces has many uses!) and if chance lines up they germinate, grow, survive and then also reproduce. I think about my old Pennsylvania yard and see the raspberry thickets, mulberry trees, grape vines, cherry seedlings, and poison ivy plants and vines growing in perfusion all thanks to the bird dispersal of each species’ edible and enticing fruit!
So, fruits are made to be eaten (seed and all) and the payoff for the plant, as I have said before, is the sudden acquisition of wings and legs!
There are a few apparent glitches in this scheme, though. There quite a few plants (especially some trees) that have fruits that either can’t be eaten or seeds within their fruit that would, quite literally choke a horse. A seed that can’t be swallowed will be very hard to disperse! The plant’s investment into that fruit and seed, then, does not result in a big payoff (it more closely resembles the investments I have in my stock market portfolio!).
Cassia grandis. Photo by haplochromis. Wikimedia Commons
Dan Janzen (University of Pennsylvania) is a renowned topical ecologist (and the author of the 1983 book Costa Rica Natural History which I have enjoyed and relied on as an authoritative reference for decades). Many years ago Janzen noticed that a middle canopy tree of the Costa Rican forest called the “pink shower tree” (or Cassia grandis) had long, woody seed pods that were not eaten by any of the native animals of Costa Rica. These seed pods, though, are avidly consumed by cattle and horses (both of which were alien exotic species in these ecosystems!). He eventually described about forty other tree species of these forests that produced fruits that were not eaten by any of the native animals! Unconsumed fruit meant non-dispersed seeds! The evolutionary “fruit and seed dispersal contract” was not being honored!
Janzen in collaboration with Paul Martin of the University of Arizona developed the idea of “ecological anachronisms.” The Costa Rican trees must have developed their fruits in co-evolutionary relationships with large animals that had since gone extinct! These trees without their giant ground sloths or massive, elephant-like gomphotheres now were investing their precious metabolic energy into a seed disposal system that no longer existed! They had become “ecological anachronisms.”
Osage orange fruit, B. Martin, Wikimedia Commons
A tree that grows closer to home that also reflects a hidden evolutionary history is the Osage orange (Maclura pomifera). The fruits of the Osage orange are large, seed-filled, softball-sized spheres that contain a white, sticky fluid. No animal eats these fruits and, not surprisingly, the Osage orange has a very restricted natural range along the Red River in east-central Texas, southeastern Oklahoma, and northwestern Arkansas. It is said that these large, Osage orange seeds only are dispersed downhill! This tree species, though, is very well adapted to grow throughout the continental United States and southern Canada (it has been planted, often as a living fence, in 39 of the lower 48 states!). The apparent natural range restriction is undoubtedly due to the absence of natural seed dispersers (probably mammoths and giant ground sloths, two species that went extinct in the later Pleistocene when, coincidentally or not, humans first came to the North American continent).
Tree Vitalize tree planting. New Kensngton PA. Photo by D. Sillman
There are many other temperate and tropical trees that have fruit and seeds that are anachronistic: the large seed pods of the Kentucky coffee bean tree and the honey locust (two trees that my students and I planted in conjunction with a decade of “Tree Vitalize” programs back in New Kensington, Pennsylvania) are not readily eaten by native animals. Both tree species have very constricted natural ranges in spite of their robust ability to grow under a wide variety of conditions (as demonstrated by their survival along the streets of New Kensington and many other towns in Western Pennsylvania!)). Avocados, as my son-in-law, Lee Drake, pointed out a number of years ago, look like giant ground sloth or mastodon food to me! So do papayas, pawpaws, persimmons, desert gourds and wild squash! Big fruit and seeds require big mouths and big intestines to be ecologically and evolutionarily functional! Those big mouths and big guts are, sadly, long gone!
Wooly mammoth. Photo by WolfmanSF Wikimedia Commons
There are more observations and ideas here! Whit Brounaugh wrote a wonderful article entitled “The Trees That Miss the Mammoths” in the Winter 2010 issue of American Forests, and George Monbiot wrote “Thinking Like an Elephant” in the June 2015 issue of BBC Wildlife. Growth patterns of trees, the strength of trunks and branches, patterns and sizes of protective thorns, etc. all suggest a long, adaptive evolutionary interaction between the trees and large browsers and dispersers that no longer exist.
Many of our trees have been sculpted by ecological ghosts, and the momentum of evolutionary change continues even long after the “prime movers” are gone!
I put one of the bur oak seedlings that we pulled from our front yard into a vase of water. It grew roots incredibly rapidly! I then planted it out in a large pot just off of my backyard patio. It seems to be thriving. Its leaves just turned reddish-purple and are just barely hanging on to the stem! In the spring we’ll find a spot for it somewhere in the yard!