5.5 Biogeography  (Page 6/6)

Other aquatic factors

Some abiotic factors, such as oxygen, are important in aquatic ecosystems as well as terrestrial environments. Terrestrial animals obtain oxygen from the air they breathe. Oxygen availability can be an issue for organisms living at very high elevations, however, where there are fewer molecules of oxygen in the air. In aquatic systems, the concentration of dissolved oxygen is related to water temperature and the speed at which the water moves. Cold water has more dissolved oxygen than warmer water. In addition, salinity, current, and tide can be important abiotic factors in aquatic ecosystems.

Other terrestrial factors

Wind can be an important abiotic factor because it influences the rate of evaporation and transpiration. The physical force of wind is also important because it can move soil, water, or other abiotic factors, as well as an ecosystem’s organisms.

Fire is another terrestrial factor that can be an important agent of disturbance in terrestrial ecosystems. Some organisms are adapted to fire and, thus, require the high heat associated with fire to complete a part of their life cycle. For example, the jack pine—a coniferous tree—requires heat from fire for its seed cones to open ( [link] ). Through the burning of pine needles, fire adds nitrogen to the soil and limits competition by destroying undergrowth. Closer to home, the tallgrass prairie ecosystem of the Kansas Flint Hills is dependent on fire and grazing by large herbivores (formerly bison, now cattle). In the absence of such disturbances, the grasslands of the Flint Hills become scrubby cedar forests in just a few decades.

Abiotic factors influencing plant growth

Temperature and moisture are important influences on plant production and the amount of organic matter available to other organisms (net primary productivity). Net primary productivity is an estimation of all of the organic matter available to organisms in other trophic levels; it is calculated as the total amount of carbon incorporated into plant tissues per year minus the amount that is used during plant metabolism. In terrestrial environments, net primary productivity is estimated by measuring the aboveground biomass per unit area, which is the total mass of living plants, excluding roots. This means that a large percentage of plant biomass which exists underground is not included in this measurement. Net primary productivity is an important variable when considering differences between biomes. Very productive biomes have a high level of net primary productivity, i.e., a large amount of energy at the primary producer trophic level.

Annual biomass production is directly related to the abiotic components of the environment. Environments with the greatest amount of biomass have conditions in which photosynthesis, plant growth, and the resulting net primary productivity are optimized. The climate of these areas is warm, wet, and usually stable year-round. Photosynthesis can proceed at a high rate, enzymes can work most efficiently, and stomata can remain open without the risk of excessive transpiration. Together, these factors lead to the maximal amount of carbon dioxide (CO ₂ ) moving into the plant, resulting in high biomass production. This biomass produces several important resources for other living things, including habitat and food. Conversely, dry and cold environments have lower photosynthetic rates and therefore less biomass. The animal communities, and the complexity of the food webs, will also be affected by the decrease in available energy at the primary producer level.