Ecosystems, Food Webs, Symbiotic Relationships, Population & Extinction



·   Ecosystems or Biomes:  a community of plants and animals with interconnected relationships that live in a particular climate (arctic, desert, tropical) or environment (land, freshwater river or lake, ocean).  Land biomes are usually classified according to the dominant vegetation of the biome.  While there are a number of different biome classification systems, the following are some of the main land biome types, with associated plants and climates, and an example of each:

·        Grassland / Savanna: grasses and low shrubs, a few trees at rivers.  Climate: wetter than desert, dryer than forest, usually with cold or dry seasons.  Example: Great Plains of United States.  Dallas is at the boundary between grassland and temperate forest.

·        Temperate Forest: mixed evergreen and deciduous forests; Climate: cool to cold winters, warm to hot summers, year-round rainfall.  Example: forests in Southeastern United States, including East Texas.

·        Desert: plants and animals that need little water, such as cactus and lizards.  Climate: very dry, can be cold or hot.  Example: Chihuahuan Desert in West Texas.

·        Mediterranean: mixed grasslands and shrub land; Climate:  mild wet winters and hot, dry summers.  Example: Southern California.

·        Tropical Rainforest: dense forests; Climate: no winters, very humid and hot, sometimes wet and dry seasons.  Example: Amazon Jungle.

·        Boreal Forest:  cone bearing evergreen forests.  Climate: very cold winters, short wet summers.  Example: Northern Canada and much of southern Alaska.

·        Tundra: low plants and shrubs, no trees.  Climate: long, cold winters, short cool summers.  Example: mountains in United States above tree line (11,000’ above sea level in Colorado).

·        Polar:  no plant life, all animal life associated with ocean.  Climate: very cold and dry year-round.  Example: Antarctica.


Food Webs

·   Predator and Prey Relationships:  a predator is an animal that eats another animal, called the prey, as a food source. 

·        Food Chains or Webs:  a description of the predator-prey and food source relationships between (1) plants, which are the producers of food supporting life on Earth, (2) animals that eat the plants, called primary consumers, and (3) other animals that eat animals (secondary consumers, third level consumers, etc.).  Each level of the food chain is called a trophic level.  Because most plants and animals have many different predators and prey, a complex food web better describes the relationships than a simple food chain.  The majority of life on Earth occupies the bottom two levels of the food chain – producers and primary consumers.  There are relatively few second level consumers, and even less third and fourth level consumers.  Food chains in the ocean usually have more levels: small fish are eaten by bigger fish, which are eaten by even bigger fish, etc.

·        Disruption of Food Webs:  When one species in a food web is removed, it has an effect on all other species both up and down the food web.  In general, the prey of that species will benefit, at least for a while, and the predators of that species will be harmed, especially if an alternate food source can’t be found.  For example, if a grasshopper that eats wheat is removed from the food web, the wheat population will probably go up, but the population of birds and mice that eat the grasshoppers will go down.  If a species of bird only eats that grasshopper, it may also die off completely.


Symbiotic Relationships

·        Symbiotic Relationships are ecological relationships between different species that are in direct contact with one another.  There are three kinds of these relationships:  mutualism, commensalism, and parasitism.

·   Mutualism is a relationship between two different species in which both species benefit.  An example is the relationship between nitrogen fixing bacteria and legume plants.

·   Commensalism is a relationship between two different species in which one species benefits while the other is neither helped nor harmed.  An example is cowbirds following cattle, to eat the insects the cattle stir up as they graze.

·   Parasitism is a relationship between two species in which one species benefits while the other species is harmed.  An example is a tick on a dog.  For a parasitic species to be successful, it cannot harm the host species so much as to kill the species off.

·        Disruption of Symbiotic Relationships:  Like the disruption of food webs, removing one species from a biome can harm other species if they had a symbiotic relationship with at species.  For example, a population of ticks cannot survive without any host animals on which to feed.  Life on earth cannot survive without nitrogen fixing bacteria that provide nitrates for all other plant and animal life.


Factors Affecting Species’ Population and Extinction

·        Identified Species on Earth:  There are about 1,600,000 identified and named species of life on Earth.  Scientists estimate there may actually be between 10 and 80 million different species.

·        Factors Affecting Populations:  There are two kinds of factors affecting the distribution (range) and abundance (population) of each species of life on Earth, and a species ability to survive:  Biotic (living) factors and Abiotic (non-living) factors.

·   Biotic factors are relationships with other life that affects the ability of a species to survive.  Examples of biotic factors are interactions with other species (food chain, predator-prey and symbiotic relationships) and competition for food and shelter between species.

·   Abiotic factors are non-living conditions that affect the ability of a species to survive.  They are classified as either physical factors or chemical factors.  Physical factors include climate, temperature, moisture, light, wind and soil structure.  Chemical factors include water, salinity, oxygen, pH and soil nutrients.

·        Habitat Changes and Populations:  Changes in habitat, such as those created by climate change or by human development, can help some species while hurting others.  For example, global warming has created warmer summers in Alaska, which in turn has created a population explosion of beetles which in turn is killing entire forests.  The beetles are the winner, and the trees the loser, in this habitat change.  When humans drain a swamp to build a city, species that need a swamp habitat may die off or move away, while other species which like living in the city, like pigeons or sparrows, may move in.

·        Threatened and Endangered Designations:  When the population of a species is reduced to a level where it may eventually become extinct, it is designated as threatened.  When the population is further reduced to the point where it may soon become extinct, if steps are not taken to save the species, it is designated as endangered.  Only recently have humans realized that preservation of biodiversity (maintaining a large variety of life forms) is valuable.  Threatened and endangered species get special protection under the law of many countries, including the United States (the U.S. Endangered Species Act).

·        Extinction:  the end of a species. When all of the population of a species dies off, and the species no longer exists on Earth, it is said to be extinct.

·        Causes of Species Extinction:  There are both natural (short and long-term) and human-made causes of extinction.  Most extinctions are caused by humans either directly or indirectly.

·        Long-term natural causes include tectonic plate movement, natural long-term climate change, habitat changes related to climate change (forest to grassland to desert), and species migration that upsets existing food webs with new predators.

·        Short-term natural causes include volcanic eruptions, forest fires, droughts, floods, diseases, meteoric impacts, and sudden-onset climate changes such as ice ages.

·        Human-made causes include habitat destruction (for example, cutting down forests to make farms and cities, draining swamps), unintended disruption of food webs, uncontrolled hunting, pollution, ozone-layer destruction, human-created global warming, competition for limited resources (using up all the available water) and accidental injury (road-kill).