Ecological pyramids are of

What are the three types of ecological pyramids? What information is conveyed by each pyramid with regard to structure, function, and energy in the ecosystem?

Consider the following statements about ecological pyramids:
$I.$ Charles Elton developed the concept of ecological pyramid.
$II.$ After his name,these pyramids are also called as Eltonian pyramids.
$III.$ It is a graphical representation or pyramid-shaped diagram which depicts the number of organisms,biomass,and energy at each trophic level.
Which of the statements given above are correct?

Which is an example of a true pyramid in an ecosystem?

Which type of ecological pyramid is always upright when representing the functional aspects of an ecosystem?

In an ecological pyramid,what does the base represent?