Final answer:
Density-independent factors like extreme weather or natural disasters affect mortality rates but do not directly impact the birthrate of a species. Factors that influence birthrates tend to be density-dependent, such as resource availability or predation pressures.
Step-by-step explanation:
The factor that does not directly impact the birthrate of a species is a density-independent factor. Density-independent factors, which are typically environmental or abiotic, like weather, natural disasters, and pollution, can affect mortality rates of populations but do not directly influence birthrates. Instead, they impact mortality regardless of the current population density. As an example, weather events such as extreme cold or heat can affect the survival of individuals within a species but do not directly alter birthrates. So, a cold winter weather may increase mortality but it does not directly cause a change in how often animals of that species give birth.
In contrast, factors that might impact birthrates include availability of resources, level of parental care, predation pressures, and disease, which are often considered density-dependent factors. These are responsive to the density of the population and can affect both birthrates and death rates. Many aspects of human societal behavior, like postponement of maternity due to poverty or concerns over global issues, can also sway birthrates, although in a more complex manner compared to wildlife populations.