It is advantageous for plants to disperse their seeds away from the parent plant. If all the seeds just fell under the parent, the high density of seeds would be very attractive to seed predators such as rodents and beetles. Seed dispersal also increases the likelihood of at least some seeds landing in a good spot for germination and growth.
Plants disperse their seeds by wind, or water, or hitchhiking in animal fur, or by attracting animals that may carry seeds away in their digestive systems. Various kinds of animals are attracted by so-called fleshy fruits — the fruit pulp is food for the critter, the seeds pass through the gut and are deposited somewhere else. Southeast Alaska has many native species of fleshy fruits that are edible to critters: blueberries, salmonberries, thimbleberries, lingonberries, cranberries, currants, red-osier dogwood, devil’s club, crabapples, rose hips, to mention just some of them. We often see evidence of fruit consumption in the scats of bears and birds. The non-native bird cherry and European mountain ash fall into this category, and dispersal by animals accounts for the appearance of these species in uncommon places away from human development.
Because seeds of fleshy fruits must spend some time in the consumers’ guts, the embryos inside have to be protected during the gut passage time from all the churning around and digestive acids and enzymes. The time needed for gut passage through many fruit-eaters is short, which reduces the danger of damage. For example, experiments have shown that hermit thrushes and robins pass seeds (of several small-seed species) through the gut in about thirty minutes, on average. In some cases, birds regurgitate seeds after the fruit pulp has been removed, so the seeds spend even less time in the digestive tracts (only ten or twenty minutes). This relieves the avian consumer of undigestible ballast, enabling it to fly more effectively and to eat more fruit.
In contrast to the small birds, mammalian fruit-eaters hold ingested seeds for longer periods. For instance, captive martens pass seeds of blueberry and salmonberry in four or five hours. Bears also can evacuate seeds in a few hours; they have a relatively short gut, which sometimes passes whole fruits undigested. Fruit-eating mammals do not normally regurgitate seeds.
A hard seed coat also contributes to seed dormancy — a waiting period that delays germination. Dormancy is a time of lowered physiological activity. It is an adaptation, a means of dispersing in time. Within a single species, variation in the length of dormancy in a crop of seeds means that not all of the seeds encounter the same environmental conditions and at least some of them may encounter good conditions for germination and growth (on the proverbial principle of not putting all the eggs in one basket). Some species have short periods of dormancy, just long enough to delay germination until winter, or an annual drought, or a seasonal inundation, has passed. Other species have very long dormancy periods, a matter of decades or even centuries.
A hard seed coat is not the only means of making a seed dormant: some physiological mechanisms lie within the embryo itself. But the existence of a hard seed coat in fleshy fruits sometimes leads people to say that the seed must be subjected to digestive activity that weakens the seed coat, in order to germinate. That is not the case. Gut passage may break dormancy to some degree and speed up germination, but seeds that do not pass through the gut would germinate eventually — thus dispersing in time. If dormancy is broken (and that adaptation is thwarted), are there compensating benefits? In other cases, gut passage has no detectable effect on germination. As one might expect, the effects of gut passage on seeds depends on the species of seed and the species of consumer, sometimes on other foods in the digestive tract, sometimes on time of year (e.g., whether or not the fruit overwintered), and a variety of other factors.
One such factor might be characteristics of the fruit pulp, an idea prompted during a recent visit to my home state of Wisconsin. After a stroll through a nice oak forest atop a bluff above the Wisconsin River, I was treated to the observation of a flock of cedar waxwings foraging on some small, black berries. The shrub with the berries grew on a rocky headland with some human trampling where no other shrubs were growing. I didn’t think this shrub was a native plant, but I needed some reminding to identify it: European or common buckthorn (Rhamnus cathartica). This is a widely planted non-native ornamental; birds eat the fruits and disperse the seeds, sometimes to unexpected places, such as this bluff. The fruits of buckthorn are reported to have strong effects in speeding up gut passage rates — hence the name ‘cathartica’.
This poses a conundrum: the fruit itself is an adaptation for seed dispersal, but the cathartic effect might limit the number of fruits a bird would eat. Very rapid gut passage would also reduce potential dispersal distances. So why would a plant produce such cathartic fruits? Are buckthorn seeds more delicate than those of other bird-dispersed fruits, so they need to emerge from the gut quickly? Or is there another reason for the strong effect? Some reports suggest that the compounds that produce catharsis have other, important ecological effects, such as protection from microbes, and the cathartic effect is just a byproduct. Some reports say that the cathartic effect is seen mostly in immature fruits (perhaps reducing the risk of premature dispersal of immature seeds), and not in ripe ones. It seems there is some clarification needed.
All of that leads me to wonder about the composition of fruits native to Southeast and possible effects of the fruit pulp on digestion and seed passage.
As so often happens, I end up with questions!
• Mary F. Willson is a retired professor of ecology. Her essays can be found online at www.onthetrailsjuneau.wordpress.com
• Mary F. Willson is a retired professor of ecology. Her essays can be found online at www.onthetrailsjuneau.wordpress.com
