Last time, we discussed the various ways in which populations interact with one another and how such interactions benefit or harm the species involved.
Groups of interacting populations at a particular location comprise what is known as a community -- might consider the plant community of a tropical rainforest, the fish community of a freshwater lake, or the bird community of a saltwater marsh.
Communities have many characteristics that are of interest to ecologists. One of the most important of these is the number of species present in a community or the species diversity of the community.
There are many factors that determine species diversity of communities -- here are a few:
1. climate and other physical factors &emdash; harsh environments (e.g. arctic tundra, deserts) typically have lower diversity of plants and animals than other environments.
2. interactions between populations (e.g. competition, predation).
Keystone species concept: a keystone species is one that determines community structure through biotic interactions &emdash; e.g. a predator species may be very important in determining diversity of prey community.
Example: Pisaster starfish in intertidal communities &emdash; diversity of prey (shellfish) community increased by predator (Pisaster) because predation keeps prey densities low thereby reducing competition and competitive exclusion &emdash; removal of Pisaster results in shellfish populations competing and some are lost through competitive exclusion.
Global patterns of species diversity -- generally speaking, communities at low latitudes (e.g. tropics) have higher species diversity than communities at higher latitudes. There are a number of reasons for this pattern:
1. evolutionary time -- tropical communities are older and speciation has been going on longer there-- development of temperate communities has been disrupted by ice ages, etc. and greater extinctions than in the tropics.
2. climatic stability -- tropics have less fluctuations in temperature, rainfall, etc. -- longer growing season provides more resources which can support many specialized species.
Another important pattern of species diversity involves islands -- islands always have lower species diversity than mainland communities -- two important variables determine species diversity on islands:
1. island size -- large islands are physically more complex and usually have a variety of habitats -- this allow more species to exist -- small islands may have only a single habitat type -- also extinction rates are higher on small islands because populations are small and they are more vulnerable to random events.
2. distance to source (mainland) communities -- islands close to mainland are more likely to be colonized by species from the mainland than are more distant islands.
So, large islands close to the mainland will have the most species, while small islands distant from mainland will have the least.
Let’s now discuss highest and most complex of all levels of biological organization: the ecosystem.
In a given area, all populations (the community) and the physical environment comprise an ecosystem -- note that the ecosystem has both biotic and abiotic components.
Ecosystems are structured -- can be viewed as a series of biotic components that are linked together and thus interact with one another.
The fact that ecosystem components are linked has an important ramification: disturbances to one component impact on all other components of the ecosystem to varying degrees.
Interactions between ecosystem components involve two general processes:
1. Energy flow.
2. Nutrient cycling.
Ecosystems are structured according to how different populations acquire energy -- species obtaining energy in a similar way are grouped into trophic levels -- there are three primary trophic levels:
1. primary producers
2. consumers
3. decomposers
Primary producers -- autotrophic organisms capable of photosynthesis -- make food for themselves and indirectly for other components -- these are primarily green plants.
Consumers-- heterotrophic organisms dependent on other organisms for food -- can subdivide consumers into more specific trophic levels --those feeding directly on producers are called primary consumers (herbivores) -- secondary and tertiary consumers (carnivores) eat other consumers -- decomposers are organisms that obtain energy and nutrients from remains of dead producers and consumers -- primarily bacteria and fungi -- extremely important to the process of nutrient cycling (more later).
Let's look at how energy and nutrients move through ecosystems.
Energy Flow -- a one-way process in ecosystems -- in order to persist, ecosystems require a constant input of energy.
1. the sun is the ultimate source of energy for most ecosystems.
2. primary producers capture a fraction of energy in sunlight striking the earth and convert it into chemical energy (carbohydrate) that is stored in tissues of the primary.
3. energy in tissues of primary producers transferred to consumers as each consumes tissue of other organisms -- about 90% - 95% of energy present in one component is lost as heat at each transfer -- very inefficient process -- very little energy left when decomposers get to it.
4. important point is that energy does not cycle through ecosystems -- ecosystems require constant energy input from sun or some other source.
The rate at which primary producers remove carbon from the atmosphere and convert it into biomass (living tissue) is called the primary productivity of the ecosystem -- productivity varies greatly from ecosystem to ecosystem -- here are some representative values:
|
Ecosystem |
g C/m2/year |
|
tropical rainforest |
2200 |
|
grassland |
600 |
|
deserts |
90 |
|
coral reefs |
2500 |
|
lakes and streams |
250 |
|
open ocean |
125 |
|
|
|
Trophic relationships -- we can sometimes describe "who-eats-whom" in an ecosystem as a food chain -- more often, however, food chains are "cross-linked" into more complicated structures called food webs.
Notice that as ecosystem diversity (e.g. number of species) increases, the complexity of these food webs also increases -- as complexity increases so does stability -- e.g. disturbance or extinction of one or two species can be compensated for -- in simple food webs or chains, extinction of one species may lead to the collapse of the entire system.
Human agricultural ecosystems are good examples of simple, unstable ecosystems -- monocultures of single crop plants consumed by livestock and/or humans -- because they are unstable, they require additional energy inputs to prevent them from collapsing -- e.g. insecticides, herbicides, fertilizers, irrigation water, etc. -- Irish potato famine is an example of the instability of human agricultural ecosystems.
Another way to summarize trophic structure
of ecosystems is in the form of ecological pyramids -- three
types of pyramids based on three different kinds of information:
1. pyramid of numbers -- numbers of individuals in each
trophic level.
2. pyramid of biomass -- weight of living material in each trophic level.
3. pyramid of energy -- energy content of each trophic level.
Regardless of what information is used, values generally get smaller as you go from producers to primary consumers to secondary consumers, etc. -- this is due again to the fact that energy transfer between trophic levels is very inefficient (ca. 5% - 10%).
Figure 41.6
Now, let's consider nutrient cycling in ecosystems and see how it contrasts with energy flow.
Nutrients: molecules required by living organisms -- e.g. carbon, nitrogen, phosphorus.
Major Point: unlike energy, there is no major input of nutrients from outside the ecosystem -- thus, nutrients are used over and over again -- the carbon, nitrogen, etc. atoms in your body have been used over and over again in ecosystems.
Nutrients move through ecosystems by way of biogeochemical cycles --name indicates that these cycles include biological, geological and chemical processes.
Nutrients exist in either a reservoir or exchange pool:
Reservoir -- storehouse for a nutrient -- nutrients in the reservoir are not directly available to organisms in the ecosystem -- e.g. reservoirs for carbon include limestone, coral reefs, and fossil fuels such as coal and natural gas.
Exchange pool -- source of nutrients in a form that can be taken up by primary producers -- e.g. carbon in the form of atmospheric carbon dioxide is the exchange pool form -- nutrients tied up in biomass are released by decomposition back into exchange pool.
Nutrients taken up by primary producers and passed from trophic level to trophic level like energy -- not lost as heat, but are incorporated into biomass of each trophic level.
Decomposers -- in obtaining energy from dead organic material, decomposers release nutrients back into exchange pool.
Some of this dead material may not be decomposed and nutrients may become part of reservoir -- e.g. carbonification of dead organic materials not decomposed produces reservoir forms such as natural gas, oil and coal.
Let's look at one of the more important biogeochemical cycles: the carbon cycle.
Reservoirs -- limestone, reefs, sediments, fossil fuels.
Exchange pool -- carbon dioxide in atmosphere.
1. carbon dioxide removed from exchange pool by producers through photosynthesis.
2. moves from trophic level to trophic level.
3. some released back into atmosphere through respiration and decomposition.
4. some becomes part of reservoir through carbonification.
Figure 41.13. The carbon cycle.
Human activity has had a significant impact on the carbon cycle that is only now being understood -- we will discuss this topic in a later lecture &emdash; e.g. combustion of fossil fuels, destruction and burning of vegetation.
Other nutrients also have cycles -- e.g. nitrogen, phosphorus.
Next time, now that we have some feeling for ecosystem structure and function, we will look at some of the characteristics of various kinds of ecosystems found on earth.
Next time: Ecosystems of the World.