Biogeographic processes


1.      concepts

ecosystem: an ecosystem is a self-sustaining association of living plants and animals and their nonliving physical environment.


Ecology: the study of the interactions between life-forms and their environment is the science of ecology



Ecosystems fall into two major groups-aquatic and terrestrial. Aquatic ecosystems include marine environments and the freshwater environments of the lands. We will focus on the terrestrial ecosystems, which are dominated by land plants spread widely over the land surface.


Two basic kinds of processes must occur in the ecosystems: a cycling of chemical elements and a flow of energy.



2. Energy flow in ecosystems.


Food chains


Food chains is the linkage of who feeds on whom. More complicated food chains are called food webs


Individuals in a biological community can transfer energy, chemical elements, and some compounds from creature to creature along food chains and food webs.


The original source of energy in most ecosystems is the sun. Green plants produce sugars through the process of photosynthesis, using only the energy of the sun and CO2 from the air, so they are the primary producers of the food webs.


The primary producers support the consumers-organisms that ingest other organisms as their food source.  At the lowest level of consumers are the primary consumers (the snail, insects, and fishes). At the next level are the secondary consumers.


Herbivores, organisms that feed on plants, are members of the second trophic level;


Carnivors (meat eaters) that feed directly on herbivores are in third trophic level;


Carnivores feeding on third-level carnivores are in the fourth trophic level; and so on.


For examples, in a north temperate woodland food web that existed in N.America before European settlement and includes human beings.



People are omnivores (eaters of both plants and animals) and feed on several levels


Decomposers also feed on detritus, or decaying organic matter,derived from all levels.



Photosynthesis: through photosynthesis co2 combined with water and light become sugar and byproduct oxygen.


Last time, we also talked about the respiration which is essentially a reverse of the photosynthetic process:


C6H12O6 + O2 à CO2 + H2O + energy (heat).


The overall growth of a plant depends on  net photosynthesis which is the difference between photosynthetic production and respiration loss.


The amount of net photosynthesis depends on controlling environmental factors such as light, water, temperature, soil fertility, and the plant’s site, elevation, and competition from other plants and animals.


Plant productivity increases as light availability increases- up to a point. When the light level is too high, light saturation occurs and most plants actually reduce their output in response.


Net primary productivity: the net photosynthesis for an entire plant community us its net primary productivity. This is the amount of stored chemical energy that the community generates for the ecosystem.


Biomass is the net dry weight of organic material.


Net primary productivity is measured as fixed carbon per square meter per year( “fixed” means chemically bound into plant tissues).


Net primary production tends to be highest between 20 N and 20 S at sea level and decreases toward higher latitudes and altitudes.


Precipitation also affects productivity, like tropical rain forest area. Even though deserts receive high amounts of solar radiation, other controlling factors are more imporatnt, namely water availability and soil conditions.



Critical in each ecosystem is the flow of energy and the cycling of nutrients and water in life-supporting systems. These nonliving abiotic components set the stage for ecosystem operations.


Light, temperature, water and climate.


Solar energy powers ecosystems, so the pattern of solar energy receipte is crutial.

Solar energy enters an ecosystem by way of photosynthesis, and heat energy is dissipated from the system at many points. Of the total energy intercepted at the Earth’s surface and available for work, only about 1% is actually fixed by photosynthesis as chemical energy (energy stored as carbohydrates in plants.)


The duration of Sun exposure is the photoperiod. Along the equator, days are essentially 12 hour long year around.however, with increasing distance from the equator, seasonal effects become pronounced. Plants have adapted their flowering and seed germinate to seasonal changes in insolation. Some seeds germinate only when daylenght reaches a certain number of hours.


Other components are important to ecosystem processes. Air and soil temperature determine the rates at which chemical reactions proceed. Significant temperature factors are seasonal variation and duration and the pattern of minimum and maximum temperatures.



a suplus of carbonhydrates beyond what is lost through plant respiration.

Within a community, two concepts are important: habitat and niche.


Habitat is the type of environment in which an orgainism resides or is biologically adapted to live.

(2) Energy efficiency and transfer efficiency


Energy efficiency is the ratio of output to input. How efficiency do living things use energy ? the second law of thermodynamics state that no system can be 100% efficient.


As energy flows through a food web, it is degraded and less and less is usable.


A common ecological measure of energy efficiency is called food chin efficiency, or trophic level effeciency, which is the ratio of production of one trauphic level to the production of the next lower trophic level.


This effeciency is never very high. Green plants convert only 1 to 3 % of the energy received from the sun during the year to new plant tissue. In a natural wildness, the trophic level efficiency of wolves is about 0.01%, because they use most of the energy they take in from eating  for themselves, especially for moving around in the search for prey.


In highly managed ecosystems, such as ranches, the efficiencies may be greater. Cattle are by far the least efficient producers, requiring around 16 lb of vegetable matter to produce 1 lb meat. At the lower end of the scale, it requires about 3 lb of vegetable matter to produce 1 lb of eggs or chicken meat. On average, it takes an 7 lb of human edible vegetable by livestock to produce 1 lb edible meat.


The energy content of a food chain is often represented by an energy pyramid. For the sake of simpilicity, the food chain shown here assumes that each link in the chain has one and only one source of food.


Assume that if a 165 lb person ate frogs, he would need ten a day, or 3000 a year (approximately 660 lbs). If each frog ate 10 grasshoppers a day, the 3000 frogs would require 9,000,000 grasshoppers a year to supply their energy needs, (or about 19, 800 lbs) of grasshoppers. To sustain so many grasshoppers, it needs about 732,600 lbs) of wheat.


If people fed on grasshoppers rather than frogs, each person could probably get by on 100 grasshoppers a day, the 9,000,000 grasshoppers could support 300 people for a year, rather than only one. If, instead of grasshoppers, people ate wheat, then 333,000 kg of wheat could support 666 people for a year.


Today, approximately half of the cultivated land in the US and canada is planted for naimal consumption-beef and diary cattle, hogs, chickens, and turkey. This inlcudes more than 80% of the annual corn and nonexported soybean harvest. Thus, lifestyle and dietary patterns in North Ameirca and Europe are inefficient food chains.


This argument is often extended to suggest that people should become vegetarians and eat directly from the lowest level of all food chains. However, consider that humans can eat only the parts of some plants. By eating herbivores that can eat the parts of plants that humans cannot eat, or those plants that humans cannot eat at all, more of the energy stored in plants becomes available for human consumption.


The most dramatic example of this is in aquatic food chains. Because people cannot digest most kinds of algae, which are the base  of most aquatic food chains, they depend on eating fish that eat algae, or on those fish that eat other fish.


So, if people were to become entirely herbivorous, they would be excluded from many food chains.



3.biological productivity


the total amount of organic matter on Earth or in any particular ecosystem or area is called its biomass. Biomass is usually measured as the amount per unit surface area  (e.g. as grams per square meter or metric tons per hectare).


The change in biomass over a given period of time is called net production


NP = B2 – B1


Where B2 is the biomass at the end of the time period, B1 is the amount of biomass at the beginning of the time period, and NP is net production.


There are three steps in the production of biomass and its use as a source of energy. First, an organism produces organic matter within its body; next, it uses some of this new organic matter as a fuel in respiration; finally, some of the newly produced organic matter is stored for future use.


The first step, production of organic matter before any use, is called gross production.


This suggests another way to think about net production. Net production is what is left from gross production after use. In these terms.


Net production = gross production – respiration.


There are two different kinds of biological production.


Some organisms make their own organic matter from a source of energy and inorganic compounds. These organisms are called autotrophs (meaning self-nourishing). The autotrophs include green plants, such as herbs, shrubs, and trees; algae, which are usually found in water but occasionally grow on land; and certain kinds of bacteria that grow in water.


The production carried out by autotrophs is called primary production.


Other kinds of life cannot make their own organic compounds from inorganic ones and must feed on other living things. These are called heterotrophs. All animals, including human beings, are heterotrophs, as are fungi, many kinds of bacteria, and many other small forms of life.


Production by heteotrophs is called secondary production because it is dependent on the production of autotrophic organisms.


Once an orgaism has obtained new organic matter, it can use the energy in that organic matter to do things: to move, to make new kinds of compounds, to grow, to reproduce, and so forth.


The use of energy in organic matter in both heterotrophic and autotrophic organisms is accompanied through respiration.


In respiration, an organic compound is combined with oxygen to release energy and produce CO2



4. Ecological succession


Ecological succession occurs when older community of plants and animals (usually simpler) are replaced by newer communities (usually more complex).


Succession often requires an initiating disturbance. External exampls include wind storms, severe flooding, a volcanic eruption, a devastating wildfire, or an agricultural practice such as prolonged overgrazing.


Here we discuss the terrestrial succession


An area of bare rock and soil with no vestige of a former community can be a site for primary succession.


Exampls are any new surface created by mass movement of land, areas exposed by a retreating glacier, cooled lava flows, or lands disturbed by surface miningm clear-cut logging, land development, or volcanic eruption.


More common is secondary succession, which begins if the vestiges of a previously functioning community are present. An area where the natural community has been destroyed or disturbed, but where the underlying soil remains intact, may experience secondary succession.


In terrestrial ecosystems, secondary succession begins with pioneer species that form a pioneer community.


For example, in an abondanded farm of formerly plowed fields. Crabgrass and ragweed quickly take hold, seeded by winds and birds, and they do well in the direct sunlight.


These slowly are choked out by taller, sturdies grasses and shrubs that invade and stabilize the soil, adding nutrients and organic matter. After a quarter-century or so, pines come to dominate the land.


The shade created by the pine forest produces conditions in which grass and shrub seed germination necomes more difficult. But shade-tolerent, slow-growing oak and hickory hardwoods eventually grow taller than the pine forest and shade it, so the pines slowly die back under reduced light conditions.


A fairly stable mature forest of oak and hickory is in place after 150 to 200 years.


However, it is noted that a succession of communities are overlapping in time and space.