An ecological niche can be define as the functional role played by the organism in an ecosystem so as to lead it's life cycle, grow, survive and reproduce. Like the resources used by the organism like food source, water source, habitat, type of mating season chosen and others.
In accordance to the competition exclusion principle, when two species or organisms having the same ecological niche, thriving for the same kind of resource, and lives in a same region or ecosystem, then the one which is more superior over the other derives the resource. The other one is excluded from the region by either decline in population, extinction or migration.
The seminal vesicle and the prostate glands add their secretions so that the sperm are in a fluid that makes their transport easier, and this fluid also provides nutrition.
The cell membrane is said to be selectively permeable is because it allows a select few things to go through while stopping the passage of other stuff.
