Why is spreading of living pteridophytes limited to a narrow geographical region?

Why is use of aspartame limited to cold foods anddrinks?

Assertion: The distribution of bryophytes and pteridophytes is limited. Reason: In bryophytes and pteridophytes water is required for the transport of male gamete and fertilisation.

Why is the use of aspartame limited to cold foods and drìnks?

Why is the length of a food chain in an ecosystem generally limited to 3-4 trophic levels ?