Natural capital refers to ecosystems that provide valuable goods and services. Forests provide timber but also protect from flooding, while tropical seas provide fish but also support tourism. Resources are classified as renewable if replenished at harvest rate, replenishable if replenished by solar energy, and non-renewable if not replenished within the timescale used. What is considered a resource changes over time and depends on economic and supply factors. Assessing sustainability requires considering whether capital is depleted to support a system.