Why are organisms absent some places and abundant in others?

·         Abiotic Forces: plants have certain limits regarding environmental tolerances. Example : Temperature, moisture, sunlight, pH, substratum, salinity, atmosphere, etc.

·         Biotic Forces: Organisms compete for space and resources, and some eat others. Example : competition, predation, symbiosis, nest sites, habitat modification.

·         Opportunity (history): organisms are absent where there is no geographical access.

a.       Sometimes when they are introduced into areas where they were previously absent, havoc ensues.

b.      Introduce (exotic) species are the second most important cause of extinction.

·         Habit destruction is the first.

Examples: kudzu, fire ants, killer bees, zebra mussels, Asiatic clams, Japanese honeysuckle, water hyacinth, Hydrilla, Egeria, Japanese beetles, honeybee mites, giant slugs, Brazilian pepper, tamarisk, thistles, Australian pine, privet.

Liebig’s Law of the Minimum

·         Liebig recognized that plant growth is controlled by plant nutrients, and whichever nutrient is in most limited supply controls the plant_s growth.

·         This concept was later expanded to cover other environmental factors such as water, light, temperature.

·         He noted that plants had varying ranges of tolerance for a given factor. “The growth or distribution of a plant is dependent on the one environmental factor most critically in demand.”.

Shelford’s Law of Tolerance

·         Shelford in 1913 noted a weakness in Liebig_s general law which came to be known as the Law of Tolerance. And this in turn was modified by Ronald Good, a plant geographer: “Each and every plant species is able to exist and reproduce successfully only within a definite range of environmental conditions.

·         Good rated climatic factors above edaphic factors.

·         Some Examples:

a.       Salinity tolerance in plants (intertidal zonation)

b.      Thermal constraints on activity (sparrow microclimates, shown in the diagram from Smith_s Ecology and Field Ecology.)

•         The distribution of a species along an environmental gradient generally approximates a Gausian distribution, with the optimal occurrence somewhere near the midpoint of the distribution.