Biotic means living, so anything in the Koala's environment that may affect its ability to survive. For example, predators, the growth and abundance of eucalyptus trees, human disturbance in the area, pathogens, disease outbreak, competition, parasitism, and symbiosis.
Abiotic means non living, so anything in the Koala's environment that is not alive. For example, rain, wind, temperature, altitude, pollution, soil, PH levels, and sunlight.
When other populations enter a the ecosystem of a Koala, a strain is put on the carrying capacity of that ecosystem. Carrying capacity is the largest population size an ecosystem can support without degrading. Population is somewhat self-regulating, and new populations tend to bring new disease and may drastically change the competition, predator-prey interaction, and resource use in an ecosystem. Members of that ecosystem may find new roles, especially in the food chain.
Koalas are endangered animals, so invasive species pose great threat to them. Koalas are mostly impacted by dogs and vehicles, and their natural predators don't play too much of a role in populations. Their natural predators include goannas dingoes, powerful owls, wedge-tailed eagles, and pythons, all of which mostly prey on the young. Feral animals also threaten populations of Koalas. Other factors such as air pollution, and PH levels also play a role.