Resource Availability:
* Loss of food: If the primary food source for a population disappears or becomes scarce (due to disease, overgrazing, climate change, etc.), the population will decline as individuals starve.
* Lack of water: Water is essential for survival. A drought or changes in water availability (like pollution) can cause a population to shrink.
* Limited shelter: If suitable nesting sites, burrows, or other shelter become unavailable, animals may be more vulnerable to predators or harsh weather, leading to population decline.
Predation and Competition:
* Increased predation: If the number of predators in an area increases, or if a new predator is introduced, the prey population will suffer.
* Increased competition: If other species competing for the same resources become more abundant, or if the population itself becomes overcrowded, competition for food, shelter, and mates can drive down survival rates.
Disease and Parasitism:
* Disease outbreaks: An outbreak of a contagious disease can decimate a population, especially if the animals have weakened immune systems due to other stressors.
* Parasite infestation: Heavy parasite loads can weaken individuals, making them more susceptible to disease, predators, and starvation.
Environmental Changes:
* Climate change: Changes in temperature, precipitation, and extreme weather events can disrupt breeding cycles, food availability, and overall habitat suitability, leading to population declines.
* Pollution: Air, water, and soil pollution can directly harm organisms, making them sick or infertile, or indirectly affect their food sources, leading to population decline.
* Habitat destruction and fragmentation: Loss of habitat due to deforestation, development, or agriculture reduces the space available for populations to thrive, causing decline or extinction.
Other Factors:
* Natural disasters: Fires, floods, and other natural disasters can wipe out entire populations or significantly reduce their numbers.
* Human activity: Hunting, fishing, and trapping can directly reduce populations if not managed sustainably.
It's important to note that these changes can interact and compound each other. For example, habitat loss might lead to increased competition for resources, making the population more vulnerable to disease and predation.