Conceptual Life Science

CONCEPTUAL LIFE SCIENCE

Ecology

DEFINITION OF ECOLOGY

Ecology is the science that deals in the interrelationships between organisms and their environment. The environment contains physical factors and living factors.

THE ENVIRONMENT

Physical factors

The physical environment consists of the soil and rocks in the surroundings of the organism. The light, heat and water in the surroundings are also part of the physical environment.

Living factors

The living environment consists of all plants and animals nearby which interact with the organism. This is a community. All of the organisms of the same species within the community constitute a population.

Energy in the environment

Figure E-1. The pyramid of energy in ecology.

E-1 E-2

Food chains

All energy on the surface of the Earth is derived from the Sun. Photosynthesis occurs in the primary producers that are green plants. Primary consumers eat the plants. Predators eat the primary consumers. The predators are secondary consumers. This constitutes a food chain. There are higher levels of a food chain. There may be tertiary or quaternary consumers. In North America most food chains end with a predatory bird such as the eagle.

Flow of energy in the food chain

Energy in food chains always begins with the Sun. Sunlight is absorbed by green plants. The green plant uses photosynthesis to trap solar energy and convert it to glucose. The glucose molecule contains stored chemical energy.

The primary consumer eats the green plant and gains some of the energy from it. The primary consumer is a herbivore because it eats the plants. Most of the energy in the plant is lost when it is eaten.

The secondary consumer eats the primary consumer. It is a carnivore. The carnivore obtains some of the energy stored in the animal it eats but most of the stored energy is lost. Thus, energy is lost as the food chain grows longer. In order to obtain the maximum amount of energy stored by the original producer plant, you have to eat the plant directly.

Food webs

Food webs are more complicated than food chains because some consumers may be eaten by several predators. This causes branches in food chains.

Figure E-2. Diagram of a food web. E-3

Succession of plants

Newly exposed rock will first be colonized by lichens. These are joined by small plants like mosses and liverworts. After some soil begins to form, ferns and grasses move in. More soil forms and then shrubs and pine trees begins to grow. The thickness of the soil continues to increase. The shrubs and pine trees are replaced by oak trees, beech trees and finally maple trees. In the temperate forests of the United States, the climax vegetation will be maple trees. The sequence from bare rock to mature forest takes about 100 years.

Table XIII. Ecological succession.

Early (Pioneer) Plants 1. Lichens 2. Liverworts and mosses 3. Ferns 4. Grasses and shrubs Intermediate Plants 5. Pine trees 6. Oak trees 7. Beech trees Climax Forest Plants 8. Maple trees

Biomes

A biome is a collection of many communities which is characterized by the same form of climax vegetation. Terrestrial biomes are listed in Table XIV. `The climax vegetation is a botanical community. For example, the northeastern United States is in the biome known as the Temperate Deciduous Forest. “Deciduous” means that the leaves fall off the trees in the autumn.

The terrestrial biomes depend on climate. The climate changes as you travel toward the North Pole. It changes in a similar fashion as you go high up into the mountains. Climates similar to Tundra and Taiga are found on high mountains.

The marine biome includes all of the communities that are found in the ocean. The types of living things that are present in a given part of the marine biome depend on the water temperature, availability of food and nutrients for the sea life, and other factors. E-4

Table XIV. Terrestrial biomes.

Biome Climate Vegetation Tundra Very cold, long winters, Lichens, mosses, grasses, very short growing season, small flowering plants only the topsoil thaws (permafrost) Taiga Cold winters, moderate growing Conifers (pine, spruce and season (no permafrost) fir trees) Temperate Deciduous Forest Cold winters, hot summers, plentiful Deciduous trees Rainfall, long growing season many flowering plants Grassland Cold winters, hot summers, not Mainly grasses Enough rainfall to support trees Desert Hot days, cool nights Plants adapted to growing very little rainfall with little water Tropical Rain Forest Hot days all year long; frequent, Plants with large leaves Heavy rain, very humid adapted for heavy rainfall

MATERIAL CYCLES

Nature works in cycles. There are a number of cycles in the ecosystem that serve to regenerate and renew materials for use by living things.

Figure E-1. The carbon-hydrogen-oxygen cycle. E-5

The carbon-hydrogen-oxygen cycle

Photosynthesis by green plants releases oxygen into the atmosphere because oxygen is the waste product of photosynthesis. Living things use oxygen for their respiration and release water and carbon dioxide into the atmosphere. These materials are then used by the plants for more photosynthesis.

The nitrogen cycle

Nitrates from the soil are absorbed by the green plants and are used to make plant proteins. Animals make proteins from amino acids derived from plant proteins. Dead plant and animal tissue is decomposed by bacterial action. Other bacteria in the soil transform nitrogen products to produce more soil nitrates for plants.

Figure E-2. The Nitrogen cycle.

RELATIONSHIPS BETWEEN ORGANISMS

Between organisms of the same species there can be sexual contact, cooperation and social organization, especially in a large population. Between different species, the interactions are commensalism, mutualism, parasitism and predation. All of these interactions are forms of symbiosis. E-6

Symbiosis

“Symbiosis” means living together. The term does not indicate the nature of the relationship. Life scientists prefer to use one of the other terms that give a more precise indication of the relationship.

Commensalism

Commensalism is where one organism benefits but does not cause any harm to the other. An example is Spanish Moss. It hangs from trees in Florida. It is not a parasite; all it needs is a place to hang. It gets its water from the air.

Mutualism

In mutualism, a mutually beneficial relationship exists where both partners benefit. An example is the relationship between the legumes (beans, peas, clover, alfalfa) and the nitrogen-fixing bacteria. The bacteria produce usable nitrogen for the plant and the plant gives them a place to live.

Termites eat wood that they cannot digest. There are bacteria and protozoa in their digestive systems that can digest cellulose to produce glucose. The microorganisms benefit because they have a place to live and a constant supply of cellulose to digest. The termites get the benefit from the glucose released from the cellulose by the bacteria and protozoa.

A cow eats grass that it cannot digest. In the multichambered stomach of the cow are bacteria and protozoa that digest the cellulose for the cow. In return they get a place to live and a food supply.

Parasitism

In parasitism, the parasite benefits but the host does not. Parasites are generally internal. In the lower phyla of the animal kingdom, most groups contain one or more parasites of various kinds. Parasites require the host for reproduction.

Predation

A predator is a free-living and feed on other organisms. Some, like lions, kill and eat an entire organism. Others, like mosquitoes, take only a small part.