Life On Land

***The following notes are taken directly from Life on Land— Chapter 2 of  Ecology:  Concepts and Applications, 5th edition, by Manuel C. Molles Jr.***

Chapter 2:  LIFE ON LAND


*Much of earth’s climatic variation is caused by uneven heating of it surface by the sun.

*Because the earth is a sphere, the sun’s rays are most concentrated where the sun is directly overhead.

*Because the earth’s axis of rotation is not perpendicular to its plane, but tilted, the amount of solar energy recieved by the Northern and Sourthern Hemisphere changes seasonally.

*During the Northern Winter Solstice, on December 21, the sun is directly overhead at the Tropic of Capricorn.

*During the Northern Summer Solstice, on June 21, the sun is directly overhead the Tropic of  Cancer

*Heating of the earth’s surface and atmosphere drives circulation of the atmosphere and influences patterns of precipitation.

*The sun heats air at the equator, causing it to expand and rise.  The warm, moist air cools as it rises.  Since cool air holds less water vapor than warm air, the water vapor carried by thiis rising air mass condenses and forms clouds, which produce the heavy rainfall associated with tropical climates.

*Eventually, the equatorial air mass ceases to rise and spreads north and south.  This high-altitude air is dry, since the moisture it once held fell as tropical rains. As this air mass flows north and south, it cools, which increases its density.  Eventually, it sinks back to the earth’s surface at about 30 degrees latitude and spreads north and south.  This air draws moisture from the lands over which it flows and creates deserts in the process.


* Climate diagrams were developed by Heinrich Walter in 1985 as a tool to explore the relationship between the distribution of terrestrial vegetation and climate.


*Soil structure results from the long-term interaction of climate, organisms, topography, and parent mineral material.

*Though soil structure usuall changes gradually with depth, soil scientists generally divide soils into several discrete horizons:  O, A, B, and C.

*The O, or organic horizon lies at the top of the prolile.  The most superficial layer of the O horizon is made up of freshly fallen organic matter, including whole leaves, twigs, and other plant parts.  The deeper portions of the O horizon consist of highly fragmented and partially decomposed organic matter.  At is deepest levels, it merges with the A horizon. *It is usually absent in agricultural soils and deserts.

*The A horizon contains a mixture of mineral materials, such as clay, silt, and sand, and organic material derived from the O horizon.  Both A and O support high levels of biological activity. Clays, iron, aluminum, silicates and humus move down through this horizon and are deposited in the B horizon.

*The B horizon contains the clays humus, and other materials that have been transported by water from the A horizon.  It is occcupied by the roots of many plants.  *It eventially merges with the C horizon.

*The C horizon consists of weathered parent material, which has been worked by the actions of frost, water, and the deeper penetrating roots of plants.  Weathering slowly breaks the parent material into smaller and smaller fragments to produce sand, silt, and clay-sized particles.  Because weathering is incomplete and less intense than in the A and B horizons, the C horizon may contain many rock fragments.  Under the C horizon we often find bedrock.


*The geographic distribution of terrestrial biomes corresponds closely to variation in climate, especially prevailing temperature and precipitation.


*Tropical rain forests straddle the equator in three major regions:  Southeast Asia, West Africa, and South and Central America.

*Annual rainfall ranges from about 2,ooo to 4,000 mm.

*Average temperatures are 25 – 27 degrees celcius.

*Rain forest soils are often nutrient poor, acidic, thin, and low in organic matter because heavy rains gradually leach nutrients from them.

*Rain forest plants are adept at conserving nutrients. 

*Mycorrhizae–are (for the sake of gathering nutrients) mutually beneficial partnerships between fungi and plant roots.

*Rains forests that grow on young, volcanic soils are fertile because they have no yet been leached of their nutrients. *Fertile rain forest soils also occur along rivers.

*Trees dominate the rainforest landscape and average about 40 m in height.

*Tropical rains forests may contain up to 300 species of trees.

*There are many intricate, complex relationships between species; some species unable to exist without the presence of another particular species.

*many othe the world’s staple foods, including maize, rice, bananas, and sugercane, and approximately 25% of all prescription drugs were originally derived from tropical plants.


*The tropical dry forest responds to hte rhythms of the annual solar cycle, which drives the oscillation between we and drey seasons.

*During the dry season most trees in the tropical dry forest are dormant. As the first storms of the wet season arrive, the trees produce their leaves and transform the landscape.

*Dry seasons last 6 to 7 months; wet seasons 5 to 6 months.

*Soils are typically of great age, and tend to be less acidic than those of rain forests, and also contain more nutrients. –Torrential rains make the soils highly vulnerable to erosion.

*Trees are tallest in the wettest areas. –50% of the trees in these areas are also evergreen.

*Heavy human settlement and agricultural development has devastated the tropical dry forest–causing them to all but disappear.


*The Tropical Savanna is the birthblace of humankind.

*Most tropical savannas occur north and south of tropical dry forests within 10 -20 degrees of the equator.

*Tropical savannas, like tropical dry forests, have alternating dry and wet seasons.

*The rains come in summer and are accompanied by intense lightning.  This lightning often stars fires which kill young trees while the grasses survive and quickly resprout.  –This helps to maintain the tropical savanna as a landscape of grassland and scattered trees.


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