Why do animal and plant biogeographic regions sometimes have different boundaries?

Biogeographic regions for plants (phytogeographic kingdoms) and animals (zoogeographic realms) often feature different boundaries due to fundamental differences in the biology and evolutionary history of these groups.1

Key factors contributing to these discrepancies include:

• Mobility: Animals are generally more mobile than plants, allowing them to disperse across barriers that might permanently restrict plant populations. Consequently, some faunal divisions that may have existed historically have been absorbed by surrounding, more dominant faunas.1
• Evolutionary Age: Land and freshwater plant groups are typically older than the animal groups with which they coexist. As a result, phytogeographic regions often reflect more ancient phases of Earth's history, whereas zoogeographic regions may reflect more recent evolutionary and geological events.1
• Dispersal Capabilities: While both groups are influenced by climatic factors and geographic barriers, some organisms possess exceptional abilities to "skip" climatic zones or disperse to remote, isolated regions, leading to unique distribution patterns that do not align perfectly between flora and fauna.1

Ultimately, these regions serve as useful tools for understanding evolutionary processes, dispersal mechanisms, and the impact of geologic events on the biosphere.12