Over the past few billion years, life has persisted through countless geologic, atmospheric and extraterrestrial disturbances through its ability to change with the environment. Ecosystems exist in their present state because they have evolved to be as such. It took trillions of events - biotic and abiotic - for these complex systems to weave their thick web of dependence.
One way for ecologists to define and correlate these varied environments is by categorizing these areas by the types of plants that inhabit them. These categories are called biomes. Categorizing each biome by plant life is not an end in itself; instead, indigenous plant life acts as an indicator of the animal life, soil composition and the climate of an area.
Most of us already know the biomes - desert, rain forest, savannah, tundra, etc. - but how exactly are they defined?
Life within a terrestrial biome is founded by its soil. Soil is created by the weathering of rock (inorganic content) and by the decay of tissue from animal, plant and microbial life (organic content). A mature soil - one that has been undisturbed by geologic or human activity - is characterized by distinct layers called soil horizons that will vary from biome to biome. Deserts have a very different soil stratification than rain forests due to the varying organic and inorganic composition of the biome. The horizons will be immediately apparent and distinct with a deep core sample.
In Western Maryland, there are large swaths of land reclaimed from strip mining operations and repopulated with native or analogous plant life. If you take a core sample in these reclaimed areas, there is no stratification. You will see the same trend in park land that was once used for agriculture. The soil will not become mature again for some centuries, perhaps even millennia.
Soil horizons are typically defined as such:
- The O horizon, as shown above, is the "organic" layer, a mixed bag of leaf litter and other organic bits found on the biome floor. This layer will vary with particular biomes, obviously, with deserts or tundra having very little litter at all.
- In the topsoil layer, or A horizon, is usually associated with humus, mostly decayed organics that have been compacted and mixed with fine inorganic particulates such as clay, silt and sand.
- The B and C horizons are mostly composed of inorganic rock in different sizes; C will have larger "chunks" than B.
- Below C is the bedrock.
So, plants form the foundation of each biome, and soil provides a suitable anchor and a source of nutrients for the plants (not to mention for the trillions of microbes and fungi feeding and assisting plant growth).
Climate is also an essential factor in understanding the distinctions made between environments. The Earth's "average" climate is determined by an enormous number of factors, but generally speaking, we can zero in on a few.
Heat from the sun is what drives the circulation of air on Earth, and with the circulation of air, the factors of precipitation. Much of the basis of climatic variation seen between biomes is created by uneven heat from the sun due to the Earth's tilt on its axis and its ovoid orbit around the sun. In short, these variations combined with the ever changing geology have created our extant biomes, which lie within more or less predictable latitudes.
Take a look at the map below:
The middle dotted line is the equator, while the outer two both lie at 30 degrees. Notice that the dark green areas (tropical rain forest) are pretty much bound to the equator, while the yellow areas (deserts, generally) skirt along the 30 degrees north and south. This is caused by the constant heating and cooling of air at the equator leading to almost daily rainfall.
With the moisture expelled in the form of rain, this now dry air mass rises, moving north and/or south. It sinks into the areas at 30 degrees N & S, reabsorbing moisture, and in the process, robbing the area of rain. The mass continues its journey, eventually dumping moisture into the temperate zones. This is a generalized model, but it proves the point.
So, biomes are categorized by the plants that live within certain areas, but those plants exist in their present state only because of the atmospheric and geologic pressures placed upon ancestral organisms. With recent evidence confirming that human beings are indeed causing the global climate to rise prematurely, ecologists will rely on plants to continue their role in indicating climatic influences.
In the next few posts, we'll briefly review the different types of biomes on Earth, making distinctions in precipitation levels, geography and indigenous life.