Why do pelicans and humans require a more complex heart

Pelicans and humans have complex heart because there is need for them to deliver more oxygen per liter blood to the body. Also, there heart is made up of four chamber and they have high metabolic rate which necessitate them to supply efficient blood throughout the body. 

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.