- Why is there Life on Earth.
- Biomes: concept, main types on Earth.
- Ecosystems: concept and components.
- Habitats and ecological niches.
- Land ecosystems: main types and features. The formation of soil.
- Aquatic ecosystems: main types and features.
- Anthropogenic ecosystems: main types and features.
- Feeding interactions: producers, consumers and decomposers; food chains and food webs; trophic pyramids.
- Other biotic interactions: interspecific and intraspecific.
- Major human influences in the ecosystems.
One ecosystem is any area that happens to have an specific set of features (living species and physical-chemical conditions) that make it significantly different from its surroundings. Thus, a forest is an ecosystem, but a lagoon or a puddle of water in that forest, are themselves ecosystems too. A city is also an ecosystem: a man-made ecosystem.
The limits of many ecosystems are difficult to define. For instance, a lagoon in a forest can be fed with water from an aquifer that extends throughout an area much bigger than the forest itself, and that also feeds with water the farm-land that surrounds the forest. This way, the lagoon, the forest and the farm-land are connected. Ecosystems are not isolated in Nature: there are connections and transitional areas between them.
When an ecologist tries to describe an ecosystem, he/she has to describe the following sorts of facts:
- The set of living species in it, that is, the biocenosis. For instance, in a forest there could be pines, oaks, mice, doves, butterflies, kites, ants…
- The set of physical and chemical conditions, that is, the biotope. In a forest: the average temperature in the coldest month, the average temperature in the hottest month, the rainfall rate, the kind of salts that exist in the soil…
- The relationships between the living species: the pines holding the nests that doves make, the kites eating the mice, the mushrooms feeding on the fallen leaves…
- The relationships between the physical-chemical conditions: how temperatures alter the soil's humidity through evaporation, how wind wears away soil particles…
- The relationships between the physical-chemical conditions and the living species: the way in which the animal activity is affected by the day-night cycle, the way in which temperature affects the loss of water vapour by plants through evapotranspiration, the way plants retain the soil particles with their roots and prevent them from being worn away by the wind, the way in which the decomposition of fallen leaves or dead animals adds new salts and minerals to the soil…
Every ecosystem can have other ecosystems inside of it. The Earth itself is one ecosystem and comprises all the other ecosystems. The Ecosphere is the name we give to our planet when we think of it as an ecosystem, and it has a biotope and a biocenosis. The biotope of the Ecosphere is made up by the Geosphere, the Hydrosphere and the Atmosphere. The biocenosis of the Ecosphere is called Biosphere, and comprises all the living beings of the Earth.
If we wanted to split the Ecosphere in the biggest possible ecosystems, we would split it into the biomes. A biome is a large ecosystem made up of all the ecosystems belonging to the same climatic zone. There are, for instance, the tropical rainforest biome, the mediterranean forest biome and the desert biome.