Communities move intact in response to climate change. Modern ecosystems do not provide modern analogues for all ecosystems in the recent past. Some pollen types are poorly preserved. Not all pollen types can be identified at the species level. The chronological sequence is often disturbed due to the mixing of material from different depths by physical processes.
Some species produce large amounts of pollen, so they are over-represented in the diagram.
All of the above. The flora of the British Isles is less diverse compared to that of continental Europe. A rise in sea level in the early Holocene. Climatic constraints limiting the distribution of species. The heavy use of fertilizers resulting in a loss of species richness.