The size of Australian animals, such as the distinct marsupials, results largely from Australia's history of geographic isolation, which allowed these species to evolve without competition from a diverse set of other mammals.
The large size of many Australian animals can be attributed to multiple factors including Australia's long-term geographic isolation, which led to unique evolutionary paths and the emergence of many endemic species.
Australian animals are often big due to Australia's historic geographic isolation fostering unique evolutionary conditions for endemic species such as giant marsupials.
Historically, Australia has been isolated from other continents, allowing for the diversification of marsupials and other fauna without competition from the broad range of species found elsewhere. This lack of competition and predation pressures from other mammals led to the evolution of large marsupials such as the Diprotodon and Zygomaturus, similar in size to modern elephants. This evolutionary trajectory is distinct from that of other continents where diverse animal groups co-evolved. Additionally, in environments like Australia's, larger species often have lower population densities, as shown in enigmatic graphs indicating an inverse relationship (Figure 19.2). This could be due to larger animals requiring more resources or larger territories. Australia's historical abundance of enormous animals also parallels other isolated ecosystems, where large sizes evolved possibly as a strategy against predation or for ecological dominance.
Overall, the significant size of some Australian animals can primarily be linked to the continent’s isolation, leading to evolutionary paths without the pressure of a wide range of competing mammals.