The atmosphere is the source of oxygen for animals and carbon dioxide for plants both are sufficiently common and well-mixed in the atmosphere that they are essentially never limiting. Air as a medium of support is much less buoyant than water. Thus, an organism need relatively little support to survive in water, but it needs a fairly rigid skeleton to live and move on land. Climate in a terrestrial ecosystem is much more variable than it is in water.
The most important aspects of climate are the temperature and water relations of ecosystems. These relations include mean annual temperature, amount of temperature fluctuation, annual rainfall, degree and time of fluctuation in rainfall and potential evapotranspiration.
Rainfall tends to be somewhat irregular even in the most predictable terrestrial ecosystems and in arid regions or during periods of drought, lack of water may be a source of extreme stress.
The temperature is much more variable on land than in water. This is caused by the fact that aquatic ecosystems are heated by absorption of radiation by water, and water has a high specific heat ; terrestrial ecosystems, on the other hand, are heated by the absorption of heat by soil, rock, and vegetation, whose specific heat are much lower.
A given quantity of heat energy absorbed by solid material can change its temperature two to five times as much as the same energy absorbed by water. Thus heat is gained and lost rapidly by terrestrial ecosystems, with resulting wide fluctuations of temperature, both diurnally and seasonally.
Table 22.1. Specific heat of several common substances, (Clapham, Jr., 1973).
1. Fresh water1.00
2. Sea water0.93
3. Wet mud0.60
4. Moist sandy clay0.33
5. Solid rock0.20
In terrestrial ecosystems, the soil serves two major functions— it provides support for living organisms, and it is the source for all essential nutrients except for carbon, oxygen, and hydrogen. Even its supportive role is different from the way in which lake sediment supports rooted vegetation. Because the entire weight of all living organisms is borne by the soil, there is no partition of support between the soil and the air.
The role of soil as a source of nutrients is unique to terrestrial environments. It is the site of the entire detritus food chain and thus is central to the biogeochemical cycling of nutrient materials. Different types of soils have different properties that affect the availability of nutrients to plants, so the productivity of the terrestrial ecosystems is very closely tied to the chemistry of the soil.
Further, in terrestrial biotic communities interactions between species are real but they are simple, involving relationships like predation, mutualism, parasitism and the like. In contrast to aquatic ecosystems, living organisms of terrestrial ecosystems leave a permanent mark on the system.
For example, the role of plants in breaking down and weathering rock and building soil is much more pronounced than it is in almost all aquatic ecosystems. In addition, the succession that takes place in a terrestrial community is almost entirely a function of the organisms within the ecosystem, unlike aquatic successions such as Lake Eutrophication, which are dependent in large part on materials being washed in from outside.
Lastly, in each of these two ecosystems, namely aquatic and terrestrial ecosystems, several types of major subdivisions can be recognized: thus one can distinguish fresh-water, estuarine and marine aquatic ecosystems, and several major types of terrestrial ecosystems such as prairies, forests, tundra, etc.
The former are distinguished on the basis of a major chemical difference (i.e., salt content), the latter generally on the basis of the predominant type of vegetation (grass, tree, etc.) (See Kormondy, 1976).