Michigan moose munch hardwood leaves but turn up their noses at firs. Harvester ants in California may prefer "Microseris" seeds one year but target goldfield flowers the next.
Grizzly bears crave the tasty bulbs of glacier lilies, even when it means digging up huge chunks of subalpine meadow to find them.
Traditionally, ecologists have focused upon climate as the major factor determining where plants grow. On the large scale, nothing could rival the roles of temperature and precipitation in shaping plant communities: Palm trees clearly couldn't survive North Dakota winters.
But the distributions of plants on smaller scales, within a forest or on a grassland, may depend strongly on the direct and indirect effects of plant-harvesting bugs and beasts.
"Most people have focused on the bear or moose themselves and not what it is they are doing in the landscape," says Jack Stanford, a University of Montana ecologist who is studying how foraging grizzly bears affect Montana meadows. "Everyone who is looking at animals these days finds some large effect."
Understanding these effects can aid in preservation and management of intact ecosystems.
Complex changes necessary
For example, if you want to maintain the natural forests of northern Michigan, with their patches of spruce, fir and hardwoods, a large and stable population of moose won't help. Population fluctuations are key, explains John Pastor, an ecologist at the University of Minnesota in Duluth.
Moose rummaging around in northern Michigan and Minnesota forests target the juicy leaves of hardwoods, like the guest who digs through the bowl of mixed nuts for the macadamias.
When the macadamia nuts are gone, the guest is unlikely to starve. But moose populations may collapse when their food plants are gone.
With fewer moose around, birch and aspen can become re-established in an area, and moose from adjacent areas eventually drift back in.
"The system has to oscillate like this to maintain itself," says Mr. Pastor, who has been studying the moose on Michigan's Isle Royale for more than 10 years.
Those oscillations also include wolves, which prey upon moose, and fire, which helps hardwoods repopulate a burned area.
Mr. Pastor's recent work suggests that moose do more than simply remove young hardwood seedlings by eating them. The indirect effects of such leaf- and twig-nibbling, or "browsing," make it even harder for the few overlooked seedlings to grow up.
Moose eat leaves that would otherwise drop to the ground and decompose. Mr. Pastor found that heavily browsed areas had much less leaf litter on the ground than unbrowsed or less-browsed areas. Moreover, the chemical quality of the soil differed.
The tender leaves of hardwood decompose more easily than the tough leaves of spruce and fir. So nutrients that plants need, such as nitrogen, are returned to the soil more quickly when there are a lot of hardwood leaves around.
Germinating hardwoods need lots of nitrogen to get started, while firs and spruce can thrive early on with much less nitrogen.
So more moose browsing means fewer hardwoods, which means poorer-quality leaf litter, which means even fewer hardwoods. With few hardwoods around, firs and spruce don't have much competition, and they can grow bigger.
Digging sets up cycle
Bears, however, can have their cake and eat it, too.
"Glacier lilies are candy for bears," says Sandra Tardiff, a University of Montana graduate student who studies the effects of bear digging on subalpine meadows.
Foraging bears churn up the meadow, and the exposed soil heats up in the sun. Microbes -- tiny bacteria and fungi that decompose dead bits of plants -- thrive in the heat.
Water moves more easily through the loosened soil, and ensuing wet-dry cycles make microbes better at decomposing dead plant material.
And decomposition releases nutrients that plants need. Lilies that sprout on bear digs are particularly nutrient-rich. With those extra nutrients, glacier lilies on bear digs produce far more seeds than their ordinary-soil bound neighbors, Ms. Tardiff says. Extra nutrients may make the lilies even tastier for bears.
Lilies are also better than many other meadow plants at recolonizing newly exposed soil. So after an area is churned up by bears, lilies sprout back.
But it's not only big animals such as bear and moose that affect what plants can grow where.
Pocket gophers, and even ants, are key components of certain grassland systems, says Richard Hobbs, an Australian ecologist who has worked on a large patch of serpentine grassland near Stanford University in California for 15 years.
In the spring, California's serpentine grasslands, named after the rocky soils on which they grow, are patchwork quilts of color. Some areas are carpeted with bright yellow goldfield flowers, while in other areas, the purple of owl's clover and Linanthus poke through. Mint green "bunch grasses" are numerous in one area, but sparse nearby.