The living part of the ecosystem is the "community"
community: a group of species (with finite population sizes) that occupy a particular habitat or geographic region. The individual members of the species may interact with heterospecifics to a greater or a lesser extent (for instance, the white-tailed deer and the willow tree interact with one another more than do the white-tailed deer and the seed-harvester ant)
What kind of interactions occur between species?
Do interactions determine the distribution and abundance of organisms?
Do interactions between species in a community affect the species diversity
of the community?
Competition (interspecific): members of two species have overlapping resource requirements; the availability of the resources is limiting; activites of one species reduces resource availability for the other species; decrease in abundance or restriction of distribution of one or both species. See notes on Competition Mechanisms and Niche and Competition
Predation: members of one species (the predator) kill and consume/ingest all or part of the bodies of members of another species (the prey). Animals prey upon other animals, but animals prey upon plants also (carnivory and herbivory), and a few plants prey upon animals. Fungi on plants and animals. The ultimate fate of all living organisms is to be consumed by other living organisms.
Parasitoidism: members of one species (the parasitoid) live within the bodies of members of another species (the host), and ingest their internal organs, eventually killing the host.
Parasitism: members of one species (the parasite) live upon or within the bodies of members of another species (the host). The parasite feeds on the tissues of the host or on materials that have been ingested by the host. Ectoparasites vs endoparasites
ecologically, there is not a clear distinction between parasitism and pathogenesis -- it is a matter of the type of organism involved -- eukaryotic ve prokaryotic/viral
Pathogenesis: members of one species (the pathogen) live within the bodies of members of another species (the host or "patient") and attack specific tissues within the host, usually causing what we think of as "disease". Viral, bacterial, fungal, and protozoan "parasites".
These are special cases of a more general phenomenon called "symbiosis" which means simply "living together". It is possible to live together
Commensialism: members of one species (the commensial) live on or within the bodies of members of the other species (the host). The commensial gains a benefit but does not affect the host in any way, either negatively or positively. Sebaceous mites, cattle egrets, E. coli
Mutualism: members of two species live together, and each benefits from the association. "Mutualists". Gut flora of termites and of ruminants, zooxanthellae of corals
Saprobism: members of one species (scavengers, decomposers)
feed on the bodies or parts of the bodies of members of other species after
they are dead. Scavengers (carrion feeders) are usually not terribly
particular about the type of carrion that they ingest. Turkey vultures,
coyotes, flesh flies, fungi.