Ephemeral species are plants and animals whose lifespan lasts only a few weeks or months. The most common types of ephemeral species are desert annuals, plants whose seeds remain dormant for months or years but which quickly germinate, grow, and flower when rain does fall. In such cases the amount and frequency of rainfall determine entirely how frequently ephemerals appear and how long they last. Tiny, usually microscopic, insects and other invertebrate animals often appear with these desert annals, feeding on briefly available plants, quickly reproducing, and dying in a few weeks or less. Ephemeral ponds, short-duration desert rain pools, are especially noted for supporting ephemeral species. Here small insects and even amphibians have ephemeral lives. The spadefoot toad (Scaphiopus multiplicatus ), for example, matures and breeds in as little as eight days after a rain, feeding on short-lived brine shrimp, which in turn consume algae and plants that live as long as water or soil moisture lasts. Eggs, or sometimes the larvae of these animals, then remain in the soil until the next moisture event.
Ephemerals play an important role in many plant communities. In some very dry deserts, as in North Africa, ephemeral annuals comprise the majority of living species—although this rich flora can remain hidden for years at a time. Often widespread and abundant after a rain, these plants provide an essential food source for desert animals, including domestic livestock. Because water is usually unavailable in such environments, many desert perennials also behave like ephemeral plants, lying dormant and looking dead for months or years but suddenly growing and setting seed after a rare rain fall.
The frequency of desert ephemeral recurrence depends upon moisture availability. In the Sonoran Desert of California and Arizona, annual precipitation allows ephemeral plants to reappear almost every year. In the drier deserts of Egypt, where rain may not fall for a decade or more, dormant seeds must survive for a much longer time before germination. In addition, seeds have highly sensitive germination triggers. Some annuals that require at least one inch (two to three cm) of precipitation in order to complete their life cycle will not germinate when only one centimeter has fallen. In such a case seed coatings may be sensitive to soil salinity , which decreases as more rainfall seeps into the ground. Annually-recurring ephemerals often respond to temperature, as well. In the Sonoran Desert some rain falls in both summer and winter. Completely different summer and winter floral communities appear in response. Such adaptation to different temporal niches probably helps decrease competition for space and moisture and increase each species' odds of success.
Although they are less conspicuous, ephemeral species also occur outside of desert environments. Short-duration food supplies or habitable conditions in some marine environments lead to ephemeral species growth. Ephemerals successfully exploit such unstable environments as volcanoes and steep slopes prone to slippage. More common are spring ephemerals in temperate deciduous forests. For a few weeks between snow melt and closure of the overstory canopy, quick-growing ground plants, including small lilies and violets, sprout and take advantage of available sunshine. Flowering and setting seed before they are shaded out by larger vegetation, these ephemerals disappear by mid-summer. Some persist in the form of underground root systems, but others are true ephemerals, with only seeds remaining until the next spring.
[Mary Ann Cunningham Ph.D. ]
Whitford, W. G. Pattern and Process in Desert Ecosystems. Albuquerque: University of New Mexico Press, 1986.
Zahran, M. A., and A. J. Willis. The Vegetation of Egypt. London: Chapman and Hall, 1992.
Hughes, J. "Effects of Removal of Co-Occurring Species on Distribution and Abundance of Erythronium americanum (Liliaceae), a Spring Ephemeral." American Journal of Botany 79 (1990): 1329–39.
Went, F. W. "The Ecology of Desert Plants." Scientific American 192 (1955): 68–75.