A sustainable resource is a resource that is constantly renewed. For example water would be a renewable resource because of the water cycle.
Substainability
Hopefully the sun will go on shining for a few years yet
Do you mean - Sustainability?'Substainability' is not a English word.Sustainability (noun)The act/ability to sustain something (physical or mental).The test for the duration it can sustain for.From 'sustain'To keep up, experience (such as an injury), to withstand, to support, to confirm.
  Sustainable development is a pattern of resource use that aims to meet human needs while preserving the environment so that these needs can be met not only in the present, but also for future generations  
When the number of organisms in an area reaches the food source sustainability of its habitat, the population may stabilize, leading to a balance between birth and death rates. If the population exceeds the carrying capacity, resource depletion can occur, resulting in competition for food and other necessities, which may lead to starvation, disease, or increased mortality. In some cases, this can trigger a population decline, allowing the ecosystem to recover and restore balance. Ultimately, the health of the habitat and its resources determines the long-term viability of the organisms living there.