Five things you need to know about plant ecology:
1) Succession:
http://www.life.umd.edu/classroom/bsci124/lec34.html
http://www.env.duke.edu/forest/sucession.htm
This is the natural change of the species composition of a community over time. It happens in a predictable way although the exact sequence of species that appear usually depends on the local conditions. Patches of open ground very near to each other with the same physical characteristics are usually colonized by the same species in about the same order.
Primary succession is where pioneer species arrive at a totally bare site and begin to colonize it. An example might be the colonization of a new lava flow after it has cooled. Mosses are important pioneer species.
Secondary succession is the replacement of species already in a given area by new species, usually triggered by some change in the local conditions. The species replace one another in a definite sequence. Each one tends to make the conditions difficult for those that proceeded it, leading to an irreversible shift in the dominant species composition of the area.
Eg:
Crabgrass > horseweed > broom sedge > pine forest
Hydrarch succession is the primary succession that occurs when newly formed ponds are colonized by aquatic plants . Typically this leads to the pond slowly turning into a marsh, then disappearing altogether as it is slowly drained and filled by the roots and foliage of the successive plant species invasions.
A climax community is one that forms at the end of a secondary succession event. At one time was considered the permanent end point of succession that remained unchanged until some kind of disturbance event (like a fire) cleared new space. In temperate areas the climax community is usually considered to be oak or pine forest. Many modern plant biologists now think that a climax community is an illusion caused by short human life spans. In reality, communities are always changing in ways that reflect slow trends in climate change and evolution.
2) Biomes
http://www.ucmp.berkeley.edu/glossary/gloss5/biome/
These are the so-called typical "climax" communities of plants that appear in various parts of the world. For example "savana", "rain forest" and "desert". The problem with these names is that they are generalizations that do not reflect the uniqueness of each and every community on Earth. This would be like saying that all towns in America are "American". While this is true, this generalization does not really tell us much about the unique characteristics and population demographics of each town, which may all be very different (and always changing).
3) Causes of species diversity
http://ecology.botany.ufl.edu/ecologyf03/biogeography.html
Why are there the number of species that there are?
Why not more? Why not less?
There several factors that control how diverse a community is i.e., how many different species there are. I will mention just a few.
Gause's principle of competitive exclusion
Definition:
http://en.wikipedia.org/wiki/Competitive_exclusion_principle
Essentially this says that only one species can live in one niche. E.g. Cows and sheep cannot permanently live together in the same field.
If you see two or more species in the same niche, one of them is always on its way to driving the others into extinction OR the dominant species is being prevented from excluding the other species by some form of disturbance - eg grazing or mowing in plant communities.
It follows that disturbance can also be an important factor that can affect diversity. If there is too much disturbance the diversity mat be reduced. I f there is too little disturbance then Gause's principle will tend to reduce the diversity of a given community. However, just the right amount of disturbance can dramatically increase diversity. This is called the intermediate disturbance theory. Rainforests and coral reefs are thought to be diverse because (among other reasons) they receive just the right amount of disturbance (in the form of floods and storms respectively) to keep them diverse.
Ecologically speaking, disturbance includes phenomena such as fire, flood, grazing, harsh winters, droughts or other infrequent events that can lead to high mortality rates or the appearance of new habitats.
Incidentally, I keep my model ecosystems diverse by pruning some of the plants (to simulate grazing) and by periodically raising the water level to twice its normal level for a few hours (to simulate floods).
Diversity is also affected by other issues like:
Island biogeography (size and separation of islands as well as there physical characteristics).
Productivity and / or energy input.
Temperature.
Available resources.
Ecological efficiency.
Specialization, competition an niche partitioning.
Predation.
Physical complexity of environment.
and many other factors.
The final answer to this question is:
we don't really know.
Why is the earth primarily green?
Why are there so many exceptions to every rule?
"Colinvaux's law" (tongue in cheek) "Any ecological process or law you can think of probably is important somewhere".
Chaos / complexity theory? New branch of mathematics. Difficulty of understanding complex systems with many interconnected parts. May show "emergent properties"
i.e, the system is somehow more than the sum of its parts.
4)
Ecological efficiency
1st law of thermodtnamics: Energy cannot be created or destroyed. It merely changes form.
2nd
law of thermodynamics: Energy cannot be transformed with 100% efficiciency.
Energy is often lost from a system as heat.
Disorder in a system tends to increase unless energy is expended to prevent
this.
Disorder in the universe tends to increase.
As food moves through a food chain, the second law of thermodynamics prevents
it from moving with perfect efficiency. As food moves from one trophic
level to the next, it typically does so with an efficiency of only
10%. 1000 grams of grass makes 100 grams of cow, which makes only 10
grams of human.
Ecosystems arranged as a pyramid of mass (or energy) is often the result of this physical phenomena.
2nd law has implications for the upper size limit of animals and their diets because an animal must have a territory sufficiently large to provide enough plants to support it, yet it must also be able to visit all of the territory in a reasonable period of time. This is one reason why most large animals are vegetarians and why there are no land animals bigger than the dinosaurs.
http://ridge.icu.ac.jp/gen-ed/ecosystems.html
5) The interconnectedness of living things
Every organism is connected to every other.
For example.......
DoDo (turns out to be untrue, or at least unsupported.)
http://home.conceptsfa.nl/%7Epmaas/rea/dodobird.htm
Better examples are problems with non-native species introduction.
eg Cane toad in Australia
http://www.energyres.com.au/kakadu/cane-toads.shtml
What is the Gaia hypothesis and who suggested it?
http://www.oceansonline.com/gaiaho.htm
Nitrogen and Carbon cycles as well as other nutrient cycling, as evidenced in Rob's artificial ecosystems. Know the basics of the diagrams on these two web pages.
