Would anybody be free to give me a bit of feedback on this?
There are many different relationships and interactions between organisms.
Succession begins when a pioneer species colonises a barren habitat, such as lichen starting to grow on the rocks from a lava flow. As the lichen grows, it will make the area more favourable to the growth of other, less hardy, species, such as mosses, small insects, and grasses. As these plants and animals grow and die, being decomposed by saprobionts and detritivores, the depth of the soil increases, and allows further organisms to move in. Shrubs, bushes, and small trees will come, and will out-compete the original pioneering species. As the trees grow and develop into a large forest, the smaller shade-intolerant plants will die out, leaving a rich leaf-litter and humus for worms, nematodes, slugs, etc which other animals can feed on. The ecosystem will eventually reach a stable state, its climax community.
Interspecific relationships will form, such as the symbiosis between the ant and the aphid; the former “farms” the aphids for their honeydew, and in return protects them against predators such as ladybirds. Another is the mutualism between trees and fungi – mycorrhizae. The hyphae of the fungi being much smaller than the roots of the trees, the mycelium can spread much further and can penetrate the soil more efficiently. This allow the fungi to draw up nutrients and water, which it allows the plant to take through its roots in exchange for carbon-containing molecules such as sugars and amino acids.
Competition is also rife, both between different species and intraspecifically. Parasites are extremely ubiquitous, coming in the form of blood-sucking ticks or the tapeworm which tunnels into the digestive tract of larger creatures. Plants compete for nutrients and water, with some smaller plants growing on larger ones in order to reach the sunlight in the canopy (orchids, for example). There is also the important interaction which is that between predator and prey, one hunting to survive and the other needing at all costs to avoid being caught. Competition between members of the same species is especially fierce, as individuals which do not come out top will not survive. Many animals have strict territories for this reason, with any intruders being chased off or killed. Fights occur over mates, sometimes over one individual (most cats) or for dominance of a harem (red deer). Courting is very important, as it maximises reproductive success and the passing on of genes by signalling that an individual is sexually mature and available, and attracts members of the opposite sex. All this competition and the limited resources which keeps populations from growing exponentially.
Humans have had a particular relationship with the various organisms they use in farming and agriculture. Intensive farming of animals minimises energy loss through respiration by restricting the space they have to move around in, keeping them at an optimal temperature, feeding them as much high-nutrient food as possible, and protecting them from predators and competition. Crops are grown in large fields of monocultures, and pesticides and fertilisers are used to maximise the harvest. There are ethical arguments against such battery farming of animals, as it causes a lot of suffering to the animals and takes any semblance of a decent life away from them. And extensive use of herbicides, fungicides and insecticides on crops reduces biodiversity, as does the removal of hedges to make fields bigger. The nutrients found in fertilisers (especially nitrogen and phosphorous, as they are limiting factors to many plants’ growth) can enter bodies of water due to leaching, and cause eutrophication which leads to dead zones. Growing monocultures depletes the soil of the nutrients that species particularly uses, and due to the export of the crops nothing is put back into the earth. Agroforestry is a more productive form of farming, as the trees use their deep taproots to draw up water and nutrients from low in the soil, from which the crops can benefit.
Humans also breed animals as pets, as companions, for show and for sports, crossing individuals with specific favourable traits that they want the offspring to have. Too much inbreeding, however, can result in a loss of hybrid vigour, and can cause deleterious recessive alleles to be expressed. Outbreeding will help make sure the animals remain healthy.