I guess I was thinking along the lines of:
Introduction - why did zoos get created in the first place, and how their aims have changed. At the end, give the reasons why they exist now.
Middle section: a series of paragraphs explaining their current aims and how they fulfil them
Point 1: Conservation and funding for conservation (Examples of conservation successes ex-situ)
Point 2: Education for people
Point 3: Enjoyment and seeing animals they otherwise wouldn't see.
Point 4: Providing jobs and acting as a frontrunner for in-situ conservation.
Paragraph 5: What will happen to zoos in the future? Will they still have a role to play if the planet recovers?
Point 6: Arguments against PETA - how zoos are actually accommodating their animals and examples of fantastic exhibits.
Conclusion: Zoos are essential for the survival of species and their role cannot be replaced.
So I guess I would like conservation story stats (populations before and after), stats on how people learn from zoos, and other juicy numbers like that

I could also do with a few stats against zoos so I can argue against them but still be seen to have a two-sided views - essays shouldn't be too one-sided (although I would love it it if they could)
Thanks is advance