Deterministic extinction happens due to a combination of factors, including:
1. Low fitness in a particular environment: If a species has low fitness in a specific environment, meaning it is poorly adapted to the conditions and unable to compete or survive successfully, it can lead to deterministic extinction. The environment may be outside the species' ecological niche, making it difficult for them to thrive and reproduce.
2. Variability in vital rates: Fluctuations or variability in vital rates such as birth rates, death rates, and reproductive success can contribute to deterministic extinction. If these rates consistently fall below the threshold required for population maintenance and growth, the population may decline and eventually go extinct.
3. Small population sizes: When a population becomes too small, it faces increased vulnerability to various threats. Small populations are more susceptible to genetic drift, inbreeding, reduced genetic diversity, and random demographic fluctuations, all of which can lead to deterministic extinction. Additionally, small populations may struggle to find suitable mates, leading to decreased reproductive success and further population decline.