Productivity of Ecosystems
Productivity of ecosystems is the energy flow between trophic (feeding) levels of an ecosystem. Primary productivity is that made from sunlight by photosynthesis. The energy flow between trophic levels limits the food available. Ecosystem productivity changes can be caused by the seasons, disturbances, and biological diversity.
Ecosystems are complex dynamic systems made up of living organisms, their interactions, and their physical environment. These are the plants, animals that eat them and animals that prey on others, together with the soil and rainfall on which they all depend. Examples of ecosystems are ponds, desert, grasslands, forests, and oceans.
Ecosystem Productivity Changes reflect the amount of energy moving through an ecosystem, termed the net primary productivity, and is determined by the plants, algae, water, and sunlight.
The seasons, says Robinson et al. (2013) will change the amount of sunlight and rainfall entering an ecosystem and affect its productivity. The age of an ecosystem will also affect productivity with younger plants and animals actively growing and producing more biomass, or living mass. An ecosystem can be thought of as young if it is regrowing after a disturbance, such as fire or prolonged drought, or rainforest clearing for agriculture.
In rainforests many of the nutrients are held in plant material, as the heavy rainfall leaches nutrients from the soil. When rainforests are cleared and burned valuable nutrients are lost with the vegetation.
Biological diversity, according to Tilman et al. (2012) will also affect the productivity of an ecosystem. More research is needed to determine why but it may be because a greater diversity of species results in more carbon and nitrogen in the soil.
Energy enters ecosystems as sunlight which is converted to chemical energy by photosynthesis, and that chemical energy flows through food chains and webs. This is termed the gross primary productivity GPP.
Chemical elements such as carbon, phosphorus, and nitrogen, also cycle through the ecosystem, being passed between the biotic and abiotic components. For example, decomposition of dead plant and animal matter, the biotic, and back into the soil, abiotic.
The biomass of different parts of an ecosystem is often expressed as grams of dry weight. Grams of different biomass contain slightly varying amounts of joules of energy.
Plants have about 19kJ per gram of dry weight biomass, algae have about 22 kJ, insects about 23kJ, and animals about 24kJ per gram of dry weight.
The energy in biomass can be stored, used for movement, producing heat, or consumed by other organisms.
Productivity is the rate of output of energy, often seen in the biomass or living mass, but is not the total accumulation of biomass.
Net primary productivity NPP is the energy left after accounting for the energy used in respiration to produce the stored energy. It is the proportion of the biomass that is available for harvest by animals such as zebras, lions, and humans, or decomposers such as bacteria.
Energy can be expressed as biomass equivalents. The biomass of plants is often measured as dry weight produced over an area and over time.
Net primary production of an ecosystem is often measured as grams of dry weight of biomass per square metre per year (g m2 year).
Per year measurements can then take into consideration growing and nongrowing seasons, for example
Some parts of a plant may be eaten or die, such as leaves, flowers and fruits, and allowances can be considered when measuring productivity.
Ecosystem productivity can change over time, this will be reflected in a change in the biomass produced and can indicate an environmental change is occurring.
The net primary productivity is the rate at which the main primary producers photosynthesize which is limited by the amount of sunlight, heat, water, and nutrients available.
Ecosystems with ample sunlight, and are warm, wet, and fertile are the most productive. Such as tropical rainforests, estuaries, reefs, and tropical wet dry savanna as seen in the photographs of a dry season in Tanzania.
Less productive ecosystems are generally limited by low sunlight or water, such as tundra and alpine areas, and semiarid and arid scrub areas. As well as open oceans which are often limited by lack of nutrients, in particular phosphorus and nitrogen.
The amount of energy in an ecosystem is determined by the net primary productivity which may change with time, and productivity is generally higher in rainforests than savanna, but both have higher productivity than deserts and open oceans.
References:
Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.
Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (19th Ed.) Canada.: Cengage Learning. Book.
Robinson, T. M., La Pierre, K. J., Vadeboncoeur, M. A., Byrne, K. M., Thomey, M. L., & Colby, S. E. (2013). Seasonal, not annual precipitation drives community productivity across ecosystems. Oikos, 122(5), 727–738. Google Scholar Seasonal, not annual precipitation drives community productivity across ecosystems (si.edu)
Tilman, D., Reich, P. B., & Isbell, F. (2012). Biodiversity impacts ecosystem productivity as much as resources, disturbance, or herbivory. Proceedings of the National Academy of Sciences, 109(26), 10394–10397. Google Scholar Biodiversity impacts ecosystem productivity as much as resources, disturbance, or herbivory | PNAS
