Once nitrogen increases in the soil, as when motor-vehicle pollution settles to the ground, nonnative plants take over. Subsequent changes in the soil also increase nitrogen pollution in water supplies. In addition, species diversity in affected areas declines. What's more, observes Wedin, the disruptions caused by air-borne nitrogen may not be restricted only to grasslands, since the same effect is likely in other plant communities. When mating season rolls around, animals respond in various ways.
Deer grow antlers for their annual rut.
Bull elephants become belligerent. And some male birds put on their brightest feathers and grow bigger brains. These differences in size may be the most dramatic examples of nerve growth in any adult animal, Smith says. The change results from increased testosterone, which rises nearly 100-fold between October and late April, when the birds are issuing their mating calls. Smith also found that in the fall, the notes and trills of song sparrow songs are more variable than they are in the spring.