Species interact. Species actually interact a lot. One can classify species interactions into categories based on the effect that the interaction has on their fitness (≈ reproductive success). Here is a table of possible biological interactions (comes from [wiki]), where a
+ indicates "beneficial", a
- indicates "detrimental" and a
0 indicates it has no effect on fitness.
Different terms exist for a given relationship. Typically, antagonism is often called parasitism.
Note about symbiosis
Different authors use the word symbiosis differently. From wikipedia:
The definition of symbiosis is controversial among scientists. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to any type of persistent biological interaction (i.e. mutualistic, commensalistic, or parasitic). After 130+ years of debate, current biology and ecology textbooks now use the latter "de Bary" definition or an even broader definition (i.e. symbiosis = all species interactions), with the restrictive definition no longer used (i.e. symbiosis = mutualism)
It may seem pretty obvious that two species should have either one type or another type of interaction. However, the reality is often more complicated than that. An interaction might fall in one category under one set of environmental conditions and fall under another category under another set of environmental conditions. In addition to that, it is not uncommon that due to selection on the host genes due to the presence of a parasite, the host evolve so to tolerate the presence of the parasite up to the point of actually having a lower fitness in the absence of the parasite.
Example of environment-dependent interaction type
Consider an animal that is host to an algea. The host is providing shelter to the algea while the algea is providing sugar (produced through photosynthesis). If there is no light or if the enviroment is loaded with sugar, then the host has no gain of hosting the algea. In such case, the algea may even be parasitic. If, however, there plenty of light and there is few nutrients in the environment, then the algea and the host have a mutualist relationship.
Example of the evolution of a parasitic interaction
Consider for example a snail which is infected by a parasite. Let's assume (I think it is a real world example but I am too lazy to find out a reference) that the parasite need the snail to be eaten in order to reproduce in the body of the predator. In order to improve the chance of its host to be eaten the parasite is selected so to diminish the thickness of the snail's shell. In response to that, there is selection (which intensity depend also on the proportion of infected snails in the population) for having thicker shells. But now, snails that are not infected have shells that are so thick that the can't move as fast as they'd need to forage. So, by the end infected snails may have a higher fitness than non-infected snails. And therefore the interaction between the snail and its parasite is not a parasitic interaction anymore but a mutualistic interaction.