Connections: The Deer, the Bumblebee, and the Trillium
By
Dick Byers
http://www.westmorelandconservancy.org/Newsletter2003July.html#connections
Francis Thompson (1860-1907) wrote a short poem about Nature's interconnectedness:
All things by immortal power,
Near or far,
Hiddenly to each other, linked are
That thou canst not stir a flower
Without troubling a star.
The web of life in Thompson's poem is a bit overstated, but he gets his point across, that everything is related in one way or another, directly or indirectly, and by various degrees. The science that studies these relationships is called ecology, one of the youngest branches of biological science. Research in this field requires years to control all the variables, consequently, a human understanding of what really goes on in nature is still in its infancy. Ecologists are still gathering, sorting and analyzing information. Sound theories are still in the process of being formed.
Every now and then a relationship is discovered and added to the puzzle. Such is the case with the slow disappearance of trillium grandiflorum in Western Pennsylvania.
Trillium grandiflorum is the large white flower that covers our forested slopes and moist woodlands in mid-April through most of May. When I first moved to Murrysville in 1966 the slopes below my house were literally covered with this wildflower. When I moved away in 1998 there were just scattered patches of this species left. The huge expansive beds were mostly gone. The reduction of the abundance of this plant was almost imperceptible. I knew they were declining, but looking at my old slides taken in the late 60's made me acutely aware of the extent of the depletion that had taken place over the years.
A seed of trillium grandiflorum takes two years to germinate and 17 years to flower. The plant can live for 70 years. (Growth rings can be counted on the rootstock). Large trillium beds are, therefore, quite old. Deer, rabbits and groundhogs browse on the leaves and flowers. There is nothing wrong with this. Herbivores have to eat. A browsed trillium might stay dormant or in a non-flowering stage for a year, but it will rebound and bloom again. Each trillium flower, if pollinated, can produce up to 30 seeds, but germination success is only about 20%, so it takes some time for trillium to spread. Also trillium is not a self-pollinating species. It must be visited by an insect for fertilization to occur. Bumblebees are the main pollinators. For centuries, trillium reproduced enough to offset any browsing by wildlife, so why were they suddenly disappearing?
Tiffany Knight, a graduate student at Pitt, decided to find out. She studied 12 natural trillium populations in the northern tier of the state over a period of several years. Deer were being blamed at the time of her study as the main culprit in the decline. At this point, one should understand the difference between browsers and grazers. Browsers eat every here and there and never deplete the entire food source at any one place. Grazers eat everything in sight at a single location and therefore do much more damage to the environment. Deer and birds are browsers. Cattle and sheep are grazers.
Grazers can quickly clear out the understory of a forest. Browsers leave most of the understory intact. However, an unusually high population of browsers, given enough time, can make the forest understory look grazed. That's the current situation with our deer herd.
Tiffany noticed trillium were abundant and spreading on steep slopes where deer are inhibited from browsing. Trillium reproduction at such sites was normal. Where deer browsed, however, trillium reproduction was inadequate, even though many plants were left uneaten. This seemed strange and contrary to most evolution scenarios. Reproduction usually increases when mortality is high, and falls when the population exceeds the carrying capacity. Trillium seemed an exception. Why were the plants left uneaten by the deer not reproducing?
Anyone who has read Bernd Heinrich's Bumblebee Economics might guess the answer to this riddle. Bumblebees, the main trillium pollinator, must refuel often in their foraging flights. The payback in nectar and pollen from small patches of flowers is not worth the expenditure in flight energy. Bumblebees seek out huge patches of flowers so they can visit many plants with the least investment in energy. Were there fewer bees visiting trillium sites where deer where browsing? To test this idea Tiffany transplanted trillium to plots containing 1 to 160 plants per plot. Plots with lots of plants had more bees and therefore more seeds. Also, by artificially pollinating some small plots by hand, she found there were more seeds produced than in small plots left to natural pollination. Deer browsing in trillium beds were simply decreasing the flowering plants enough that bees were not visiting them and reproduction dropped off. In heavily browsed areas, with no reproduction, trillium populations, even though they can live for 70 years, faced eventual extinction.
The situation could be solved by either finding a better pollinator for trillium or bringing the deer population back into balance with the land. The latter is hard to do humanely in suburban areas.
The Trillium Trail in Fox Chapel borough was famous for its spring wildflower bloom for years. But during the 1980's the trail was nearly completely depleted of trillium. Erecting a deer exclosure fence produced a swift recovery of the flowers. Not only did the trillium population recover inside the fence, they pushed out the garlic mustard that had invaded in their absence. Ecological balance is always delicate. Populations of organisms normally fluctuate up and down depending on conditions, but they usually stay around a mean level. Unusual and continued disturbance usually brings about permanent change. Whether the change is good or bad depends on your point of view. In this case a high deer population means less biodiversity and ecologists are learning that a certain degree of biodiversity is much more important than they previously thought over the long term.
