Any abiotic or biotic factor in an ecosystem that causes a population’s size to slow or decrease is a factor.

The correct answer for the fill in the blank is Limiting factor.

A limiting factor is any physical ( abiotic)  or biological factor which limits the growth of an organism in an ecosystem. Thus, it causes population size to slow down or decrease within an ecosystem.  

It is described under the Liebig’s Law of the Minimum. According to this law, the growth of an organism is not regulated by the total resource available in the ecosystem, rather it is controlled by the resource which is scarcest or limiting factor.

It is broadly divided into- density-dependent ( effect dependent on population size) and density independent ( effect independent of population size) factor.

Example- Drought. It is an abiotic factor in an ecosystem which has severe consequences on all members of a population.