Urbanization, introduction of non-native species, and nest predation are factors that decrease the Western pond turtle population.
Habitat loss due to urbanization

This is the figure that I created by combining two sources. The data about the population of western pond turtle is taken from the fifth page of Jeff Lovich's article (3). The data about the population of Ventura County is taken from "Census"(4). 

Graph: Human population is an important factor for urbanization. There is a correlation between the rapid human population growth and urbanization and change on the population of Western pond turtles. As human population increases, urbanization increases, Western pond turtle population decreases.
Without taking proper precautions, urbanization in aquatic habitats can have drastic effects on turtle survival. This is due to increased channelization, reduction of aquatic vegetation, and destruction of basking sites. The development of waterways can destroy basking sites by disturbing surfacing logs and branches that turtles normally bask on, resulting in competition with other species for remaining (and often unfavorable) basking sites. Urbanization in terrestrial habitats can also increase the number of encounters with humans, pets, and predators, which can have a detrimental effect on nesting sites and turtle survival rate. Fortunately, if the habitat is managed properly, urbanization can have little to no impact on the habitat.

Introduction of non-native species

Bullfrog- non-native species

Bullfrogs, a non-native species, are the biggest threat to Western pond turtles. Non-native turtles are another stress for the Western pond turtle. Non-native turtle species occupy the basking sites, displacing Western pond turtles and causing them to look for new basking sites. Besides overtaking native turtle’s niches, non-native species can also introduce diseases and further disrupt the ecosystem. Non-native species also eat hatchlings and young turtles, leading to a reduction in turtle population. (3)
Nest Predators

Raccoon-nest predator
Nest predators include animals such as raccoons, otters, rats, and coyotes. While small sized predators like rats mainly decrease the survival rate of young turtles, meso-predators like coyotes cause a decrease in the adult turtle population. Therefore, if larger predators are absent, adult turtles have a higher chance of survival. (5)

Ways to Protect the Western Pond Turtle Population
  • Monitoring the sites where bullfrogs are present
  • Being aware of the introduction of non-native species and turtle
  • Developing water treatment plans
  • Taking measures to ensure a healthy habitat for the pond turtle population(5)

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