Ecology of Populations, Communities and Ecosystems

Ecologists' attention can be focused on three levels of organization:
Population. Organisms of the same species that live in a specific area; for example: populations of sparrows or pines in a forest.
Community. Set of organisms of different species that live in an area and interact through trophic and spatial relationships. For example: the desert community includes plants, animals, and microbes that live in the area.
Ecosystem. Community related to its abiotic environment, with which it interacts as a whole; for example: the desert community plus its soil, climate, water, sunlight and others, form the ecosystem called desert.
In the next section, the topic of population ecology will be developed, which is also applicable to communities and ecosystems.
The objective of population ecology (also communities and ecosystems) is to determine the causes that induce the abundance of some species in a given site. It tries to explain growth rates, evolutionary mechanisms, and future prospects.
Its basic element of study is the population (community and ecosystem).

Populations (also communities and ecosystems) interact with each other at their level of organization, so two types of relationships are distinguished: intraspecific relationships and interspecific.
Intraspecific relationships. They are the relationships developed between members of the same population.
Almost all the relationships that occur in clusters tend to increase the number of individuals in the population; when this happens, the relationship is considered positive (+); when the opposite happens, that is, the population decreases due to the increase in the number of deaths or emigrations, the relationships between individuals are negative (-).
In a population there are always positive and negative relationships; If the ecosystem is in equilibrium, these relationships, in combination with different biotic and abiotic factors, maintain a stable number of individuals.
Interspecific relationships. They are the relationships developed between different populations.
Whenever one population interacts with another, one or both of them modify their growth rates. If a population is benefited, its growth speed tends to increase (+), but if it is harmed, this rate tends to decrease (-).
Sometimes the interactions are beneficial for both (+ / +), others have mixed effects (+/-) and still others are detrimental for the two populations involved (- / -). The null effect is marked with 0.
There are seven types of interspecific relationships:

1. Cooperation (+ / +). Both species benefit, but they are not dependent, since they can live in isolation.
2. Mutualism (+ / +). Benefit for both species, but their relationship is so intimate that they can no longer survive if separated. Eg nitrifying bacteria in plant roots.
3. Commensalism (+ / 0). One of the species benefits, but without causing harm to the other.
4. Amensalism (-10). One species inhibits the growth and survival of the other, without undergoing any alteration. It is also called exclusion.
5. Competence (-/-). It occurs when two populations of different species compete for obtaining some environmental resource. If two populations need the same resource, each one tries to counteract the growth rate of the other.
6. Predation (+/-). Relationship in which one species (predator) attacks and kills another (prey) to feed. The predatory population benefits, while the prey population is inhibited. Large predators such as lions, tigers, wolves, pumas, etc. are common.
7. Parasitism (+/-). It is the interaction of two species, one of which (the parasite) feeds at the expense of another (the host). This relationship is necessary for the parasite to survive and sometimes causes the death of the host. Eg: worm in the intestine of man.

Of all these relationships, those of particular interest to population ecology are predation, competition, and parasitism.