We just learned about all the oceans and lakes and rivers that comprise the aquatic environment,
so let’s make our way onto dry land and examine the terrestrial environment, and we
can do so first in an evolutionary context.
The transition from aquatic to terrestrial habitats was an influential event in the evolution
of vertebrates.
All of a sudden, new land animals diversified in response to new physical and biological
conditions.
As one might imagine, the first stages of this transition presented challenges to ancestral
aquatic organisms.
Among various physiological mechanisms, this included changes to movement, feeding, respiration,
and reproduction.
Perhaps the greatest constraint imposed by terrestrial environments was desiccation,
or the removal of moisture.
Living cells, both plant and animal, are mostly made of water, and unless the air is full
of moisture, water easily escapes from the surfaces of cells via the process of diffusion,
or the net movement of molecules from an area of higher to lower concentrations.
Water that is lost to the air must be replaced if cells are to remain hydrated and continue
to function.
Maintaining this balance of water between organisms and their surrounding environment
has been a major factor in the evolution of life on land.
For example, in adapting to the terrestrial environment, plants have evolved highly specialized
cells for different functions.
Stems and leaves, for example, are coated with a waxy “cuticle” that prevents water
loss.
However, while it reduces the loss of water, the waxy surface also prevents gas exchange
between carbon dioxide and oxygen.
As a result, terrestrial plants evolved pores on the leaf surface, called stomata, that
allow the gases to diffuse from the air into the interior of the leaf.
To stay hydrated, an organism must replace water that it has lost to the air.
Terrestrial animals can get water by drinking and eating.
For plants, however, the process is passive.
Early in their evolution, land plants evolved tissues consisting of cells joined into vessels
that transport water and nutrients throughout the plant’s body.
Because air is less dense than water, it results in a much lower drag, or frictional resistance,
on the movement of organisms, but it greatly increases the constraint imposed by gravity.
The upward force of buoyancy resulting from the movement of water helps organisms in aquatic
environments overcome the constraints imposed by gravity.
In contrast, the need to remain strong against gravity in terrestrial environments results
in a significant investment in structural materials such as skeletons for animals, or
cellulose for plants.
Just think of how tough that stuff is next time you bite into a stalk of celery.
Much more information on the evolution of plants can be found in the botany series.
The basic food for all organisms is produced by plants.
In the process of food production, oxygen is released, which is essential for the majority
of life on the planet.
Plants provide food, fiber, shelter, medicine, and fuel.
Additionally, certain plants are “keystone plants” meaning they have a disproportionately
large effect on the ecosystem, serving as homes for many insects and other species.
We’ll talk about soil shortly, but briefly, soil quality has an effect on the overall
health of an ecosystem.
The roots of plants can keep soil together, preventing erosion.
A plant’s eventual death fertilizes the soil through decomposition.
Through transpiration, as we mentioned earlier, plants move water from the soil to their roots,
through their structures and then back into the atmosphere, where water rejoins the hydrologic
cycle.
Research shows that the presence and types of vegetation - not just temperature or precipitation
- will determine whether we live in a wetter or drier world.
Now, how is it that dirt, or soil, ends up forming on our planet?
The key is weathering, or the physical and chemical breakdown of rocks and minerals,
a concept we discussed in the geology series.
Soil is not just loose material that you can run through your fingers.
For an ecologist, soil is the material that includes organic matter, lies within the top
few tens of centimeters of the surface, and is important in sustaining plant growth.
While the mineral content of soil is variable, it is typically dominated by clay and quartz.
The types of weathering that take place within a region have a major influence on soil composition
and texture.
For example, in a warm climate, where chemical weathering dominates, soils tend to be richer
in clays.
In a nutshell, soil forms through accumulation and decay of organic matter and through physical
and chemical weathering processes.
Some of the physical properties of soil include its color, structure, porosity, density, and
texture.
These properties, in turn, affect processes such as infiltration, erosion, nutrient cycling,
and biological activity.
The main mineral components of soils are sand, silt and clay; it is these relative proportions
that determine a soil’s texture.
Properties that get affected by soil texture include porosity, or the volume of all the
pores in a material; permeability, or the ability of water to flow through the substance;
and water-holding capacity.
Sand and silt are the products of physical and chemical weathering of the parent rock
while clay is most often the product of the precipitation of a dissolved parent rock as
a secondary mineral.
The water-holding capacity of soil is the ability of a certain soil texture like sand,
silt, or clay, to be physically able to hold water against the force of gravity.
The soil particles hold water molecules by cohesive forces.
As an example, a sandier soil has much less water-holding capacity than a silt soil due
to the size of the particles.
Not all soil moisture in a soil is available to plants, given that clay particles have
the ability to physically and chemically “hold” water molecules more tightly than sands or
silts.
Sands “give up” the water between the pores much more easily than silts or clays.
A good portion - upwards of 50% - in any soil remains unusable to plants.
Unbalanced electric charges due to ions within soil particles determine their role in the
fertility of the soil, as measured by cation exchange capacity.
Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules,
or cations, but allow these to exchange with other positively charged particles in the
surrounding soil and water.
This is one of the ways that solid materials can alter the chemistry of the soil.
The carbon, nitrogen, and phosphorus cycles are called “nutrient cycles” and these
pertain to the soil as well.
The carbon cycle includes the uptake of carbon dioxide by plants, its ingestion by animals,
and its release to the atmosphere through respiration and decay of organic materials.
The nitrogen cycle involves the uptake of nitrogen from the atmosphere by a process
called fixation, carried out by microbes or industrial processes, and then decomposition
of biological waste by microbes which can return nitrogen to the soil, and eventually
the atmosphere.
The phosphorus cycle involves the uptake of phosphorus by organisms.
This is mainly found in rocks and natural weathering processes, which can make phosphorus
available to biological systems.
After decomposition of biological waste, it can then accumulate in large amounts in soils
and sediments.
The rate of nutrient cycling can affect how rapidly nutrients are made available to the
ecosystem again.
Overall, cycling of decomposed matter and nutrient cycling are what allow ecosystems
to be self-sustaining.
With the aquatic and terrestrial environments now covered, let’s start looking at the
types of organisms that call these environments home.