Seasonal and cyclic succession

Unlike secondary succession, these types of vegetation change are not dependent on disturbance but are periodic changes arising from fluctuating species interactions or recurring events. These models propose a modification to the climax concept towards one of dynamic states.

Causes of plant succession

Autogenic succession can be brought by changes in the soil caused by the organisms there. These changes include accumulation of organic matter in litter or humic layer, alteration of soil nutrients, change in pH of soil by plants growing there. The structure of the plants themselves can also alter the community. For example, when larger species like trees mature, they produce shade on to the developing forest floor that tends to exclude light-requiring species. Shade-tolerant species will invade the area.

Allogenic succession is caused by external environmental influences and not by the vegetation. For example soil changes due to erosion, leaching or the deposition of silt and clays can alter the nutrient content and water relationships in the ecosystems. Animals also play an important role in allogenic changes as they are pollinators, seed dispersers and herbivores. They can also increase nutrient content of the soil in certain areas, or shift soil about (as termites, ants, and moles do) creating patches in the habitat. This may create regeneration sites that favor certain species.

Climatic factors may be very important, but on a much longer time-scale than any other. Changes in temperature and rainfall patterns will promote changes in communities. As the climate warmed at the end of each ice age, great successional changes took place. The tundra vegetation and bare glacial till deposits underwent succession to mixed deciduous forest. The greenhouse effect resulting in increase in temperature is likely to bring profound Allogenic changes in the next century. Geological and climatic catastrophes such as volcanic eruptions, earthquakes, avalanches, meteors, floods, fires, and high wind also bring allogenic changes.

Glossary

The first law of thermodynamics is an expression of the principle of conservation of energy.The law states that energy can be transformed, i.e. changed from one form to another, but cannot be created nor destroyed. It is usually formulated by stating that the change in the internal energy of a system is equal to the amount of heat supplied to the system, minus the amount of work performed by the system on its surroundings.

The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and explains the phenomenon of irreversibility in nature. The second law declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind.

Photosynthesis (from the Greek φώτο- [photo-], "light," and σύνθεσις [synthesis], "putting together", "composition") is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea.

Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions that involve the redox reaction (oxidation of one molecule and the reduction of another). Respiration is one of the key ways a cell gains useful energy to fuel cellular reformations.

The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή (trophē) referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism is from the start of the chain is a measure of its trophic level.

A food chain is a linear sequence of links in a food web starting from a trophic species that eats no other species in the web and ends at a trophic species that is eaten by no other species in the web. A food chain differs from a food web, because the complex polyphagous network of feeding relations are aggregated into trophic species and the chain only follows linear monophagous pathways. A food web (or food cycle) depicts feeding connections (who eats whom) in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs.

Linemann’s law of 10% of energy, the thermodynamic interpretation of the circulation of energy flow through trophic levels in an ecosystem. Law, opened by Lindeman (1942), according to this only portion (10%) of energy received at a particular trophic level of ecological community, is transmitted organisms, are on higher trophic levels. Eg. The amount of energy to-heaven comes to tertiary carnivores (trophic level V), is about 4.10 of energy absorbed by producers. This explains the limited number (5 - 6) units (levels) in the food chain regardless of the considered biogenesis.

The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants and algae. Photosynthesis can be described by the simplified chemical reaction H₂O + CO₂ + energy --> CH₂O + O₂, where CH₂O represents carbohydrates such as sugars, cellulose, and lignin.

Total energy produced in photosynthesis. Photosynthesis produces chemical energy in the form of glucose, a carbohydrate or sugar. The glucose produced by photosynthesis is an integral part of the food chain because a great deal of energy is stored in the chemical bonds in the glucose molecule, and this energy can be released during digestion and chemical processing by other organisms. The value of the photosynthetic efficiency is dependent on how light energy is defined. On a molecular level, the theoretical limit in efficiency is 25 percent for photosynthetically active radiation . In actuality, however, plants do not absorb all incoming sunlight and do not convert all harvested energy into biomass, which results in an overall photosynthetic efficiency of 3 to 6 percent of total solar radiation. If photosynthesis is inefficient, excess light energy must be dissipated to avoid damaging the photosynthetic apparatus. Energy can be dissipated as heat or emitted as chlorophyll fluorescence.

Biomass, as a renewable energy source, is biological material from living, or recently living organisms.^[1] As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel.

In ecology, productivity or production refers to the rate of generation of biomass in an ecosystem. It is usually expressed in units of mass per unit surface (or volume) per unit time, for instance grams per square metre per day. The mass unit may relate to dry matter or to the mass of carbon generated. Productivity of autotrophs such as plants is called primary productivity, while that of heterotrophs such as animals is called secondary productivity.

An ecological pyramid of productivity is often more useful, showing the production or turnover of biomass at each trophic level. Instead of showing a single snapshot in time, productivity pyramids show the flow of energy through the food chain. Typical units would be grams per meter per year or calories per meter per year. As with the others, this graph begins with producers at the bottom and places higher trophic levels on top.

The advantages of the pyramid of productivity:

• It takes account of the rate of production over a period of time.

• Two species of comparable biomass may have very different life spans. Therefore their relative biomasses are misleading, but their productivity is directly comparable.

• The relative energy chain within an ecosystem can be compared using pyramids of energy; also different ecosystems can be compared.

• There are no inverted pyramids.

• The input of solar energy can be added.

The disadvantages of the pyramid of productivity:

• The rate of biomass production of an organism is required, which involves measuring growth and reproduction through time.

• There is still the difficulty of assigning the organisms to a specific trophic level. As well as the organism in the food chains there is the problem of assigning the decomposers and detritivores to a particular trophic level