These sorts of studies can help researchers determine the relative importance of nurture and nature in determining a trait. Nature here would refer to genetics passed down to an individual from its parents, whereas nurture refers to the environment the individual is exposed to.
A parasite invades the species and causes its population to decline because a biotic factor is a living factor that affects a species
It causes eutrophication which means that the algae have too much nutrients and take over the pond, use the oxygen so there is not enough left for marine life, which die as a result.
D. lactobacillus is the right answer
