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dahlia The Aztecs cultivated the dahlia, which grows wild in Central America and Mexico, and Spanish explorers brought the flower to Europe. It was named for Anders Dahl, a Swedish botanist. Wild dahlias are flat, with a yellow center and eight single scarlet rays. Modern varieties may be globe-shaped and double, or with many petals. Their color may be white, yellow, orange, red, or purple. The plants grow from 18 inches to 20 feet (46 centimeters to 6 meters) high. They bloom in late summer or autumn.

Dahlias may be grown from seed or cuttings, by grafting (to perpetuate rare varieties), or by division of the tuberous roots. Amateur gardeners commonly use the last method. After frost kills the tops, the tubers should be divided and then stored in a cellar. Dahlias form a genus of the family Compositae. Thousands of varieties of dahlias have been developed from hybrids of Dahlia pinnata and D. coccinea.

deciduous Relating to seasonal loss of leaves; relating to teeth that are replaced by others.

deciduous forest A temperate or tropical forest with moderate rainfall and marked seasons. The trees usually shed their leaves during either cold or dry seasons.

demographic Relating to the statistical study of the age and sex distribution and size of a population of animals, and the changes within these parameters.

den A shelter, natural or constructed, used for sleeping, for giving birth and raising young, and/or for providing shelter during winter.

desert The driest habitats, characterized by rainfall that is low (less than 30 cm/year) and unpredictable as to when it falls. Some deserts are very hot, but some are cold. Vegetation is typically sparse, but spectacular "blooms" may take place following rain.

dicotyledon One of the classes of flowering plants, characterized by the presence of two seed leaves in the young plant, and by net-veined, often broad leaves, in mature plants. Includes deciduous trees.

digit A finger or toe.

dimorphism The occurrence of two distinct forms of structure, size, coloring, or other characteristic in a single species. "Sexual dimorphism" occurs where dimorphism exists between the male and female.

dispersal Movement of an animal away from its previous home range. Often refers to the movement of a young animal away from the home range where it was born when it matures.

display Any conspicuous pattern of behavior that conveys information to others, usually to members of the same species; e.g. threat or courtship displays.

distal Farthest from the body.

diurnal Active during daylight hours.

diversity

alpha-diversity.The number of species coexisting within a uniform habitat or a single community (this is the traditional concept of "species diversity").  

beta-diversity.As habitats change along a topographic or climatic gradient, new species are encountered as other species drop out, and this species turnover rate is termed "beta diversity" - a function of changing habitat. An example would be the rate at which the species composition of moss communities changes as you go higher on a mountain slope.

gamma-diversity. The rate at which additional species are encountered as geographic replacements within a habitat type in different localities; i.e., the species turnover rate with distance between sites of similar habitat, or with expanding geographic areas.

dolphin Dolphins and porpoises, often called simply "small whales," are mammals, not fishes, and are thus warm-blooded, keeping their body temperature nearly constant even when they are exposed to different environmental temperatures. The mothers provide milk for the young and nurse them for a year or more. Like other whales, dolphins have lungs and breathe through a single nostril called the blowhole located on top of the head. The blowhole is opened during their frequent trips to the surface to expel and inhale air. In contrast to some of the large whales, dolphins and porpoises have teeth, which they use to seize their food, consisting primarily of marine fishes and larger invertebrates, such as squid.

dormancy Inactive for an extended period. For example, many bears are dormant for a period in winter. As opposed to animals which are hibernating, their pulse rate and body temperature do not decrease significantly.

dorsal On the upper or top side or surface; e.g. "dorsal fin".

dragonfly Among the most beautiful and useful of all insects is the dragonfly. It has thin silvery wings. Its body may be steel blue, purple, green, or copper. The dragonfly eats mosquitoes, flies, and other insects harmful to man. The dragonfly lives on or near the water. It is a quick-darting insect that flies swiftly from place to place. Sometimes it changes its direction so quickly in mid-flight that its sudden movement is hard to follow with the eye. It can also hover over a lake or stream as it looks for food. The dragonfly's wings are from 2 to 5 inches long. The body is about 3 inches long. Its six legs are far forward and close together. The dragonfly can curve its legs to form a basket. It uses this basket to scoop insects from the air. Then it puts them into its jaws. These jaws have strong teeth. Two great eyes cover most of the head. Each eye has from 20,000 to 25,000 tiny eyes joined together. With these big eyes it can see its prey easily

BE AWARE. BE CONCERNED. SAVE THE ANIMALS. SAVE US. SAVE THE EARTH.

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Graphic images concept from Pia Belardo . Since 04.03.1996 Updated 10.30.2001
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