Here's why:

* Competition: Competition arises when organisms need the same resources, like food, water, shelter, or mates, but there isn't enough to go around.

* Limited Resources: When resources are scarce, organisms must compete to get what they need to survive and reproduce. This competition can be direct (fighting for the resource) or indirect (using the resource more efficiently).

* Other Factors: While a decrease in resources is the most direct factor, other changes can also lead to increased competition, such as:

* Increased population density: More organisms in a limited space will increase competition.

* Introduction of a new species: A new species might compete for the same resources as existing ones.

* Environmental changes: Changes in climate, habitat, or other environmental factors can make resources more scarce.

In summary: A decrease in available resources creates a situation where organisms are forced to compete for the essentials, leading to increased competition among them.