Here is our cute little ecosystem of free spirited people being one with nature. Every organism here interacts with themselves and the abiotic factors of their environment. Little did they know that they were about to be introduced to something called parasitism.
John had something called a brain-eating amoeba, which was a parasite that ate his brain tissue. Since he needed his brain, and he didn't know about it until it was too late, he eventual died. This is an example of parasitism.
Parasitism is a symbiotic relationship where one organism benefits and the other organism is harmed. Many things like deadly bacteria can be examples of parasitism, and it can cause a lot of damage to many different organisms.
But without the parasite, John would live, and be able to live with nature like he did before .
Without the host, the parasitic amoeba would probably be able to get food somewhere else and it may not be as effected.
On a lighter note, let's explore the symbiotic relationship of commensalism, and the effects of it.
A good example of this is with the cow and the cattle egret. The bird waits for a cow to move when grazing, and when it moves, it kicks up several bugs from the grass for the birds to eat. This way the cow isn't effected at all, and the bird benefits from all the food that they are getting.
Commensalism is a relationship between two species where one species benefits from it, and the other species isn't bothered at all.
Without the cattle egret, the cow would be completely unaffected, living on normal life, but if the cow was gone, it would be harder for these birds to find food, and you would likely see a decrease in population.