Why is there a limit the number of levels that a food chain can reach?

What are the types of genetic recombination

Compare and contrast cladistics and phylogeny. what are their advantages and disadvantages? which is the most useful in understanding how an organism developed into its current form? which concept most strongly supports your opinion about the origin of any given organism?

The concept of keystone species is controversial among ecologists because most organisms are highly interdependent. if each of the trophic levels is dependant on all others how can we say one is most important