The article presents a new theory on plant invasibility, proposing that fluctuating resource availability is the key factor determining an environment's susceptibility to invasion by non-native species. The theory is based on the idea that invading species thrive when there is an increase in unused resources, such as light, nutrients, and water, and when they face less competition from resident species. This theory is mechanistic and quantitative, leading to testable predictions. It suggests that invasibility is not a static property but a condition that fluctuates over time, influenced by factors such as disturbance, resource supply, and uptake. The theory explains how disturbances, such as fires or grazing, can reduce resource uptake, increasing resource availability and thus invasibility. It also highlights the role of nutrient availability, with studies showing that increased nutrient supply can enhance invasibility. The theory also addresses the complexity of invasibility, noting that it is not necessarily linked to species diversity or productivity. The theory is supported by various studies showing that resource availability, whether through increased supply or reduced uptake, can lead to invasion. The theory also suggests that invasibility can be predicted based on resource availability, making it a valuable tool for understanding and managing invasive species. The article emphasizes the need for a predictive science of invasions, integrating resource availability, disturbance, and fluctuating environmental conditions.The article presents a new theory on plant invasibility, proposing that fluctuating resource availability is the key factor determining an environment's susceptibility to invasion by non-native species. The theory is based on the idea that invading species thrive when there is an increase in unused resources, such as light, nutrients, and water, and when they face less competition from resident species. This theory is mechanistic and quantitative, leading to testable predictions. It suggests that invasibility is not a static property but a condition that fluctuates over time, influenced by factors such as disturbance, resource supply, and uptake. The theory explains how disturbances, such as fires or grazing, can reduce resource uptake, increasing resource availability and thus invasibility. It also highlights the role of nutrient availability, with studies showing that increased nutrient supply can enhance invasibility. The theory also addresses the complexity of invasibility, noting that it is not necessarily linked to species diversity or productivity. The theory is supported by various studies showing that resource availability, whether through increased supply or reduced uptake, can lead to invasion. The theory also suggests that invasibility can be predicted based on resource availability, making it a valuable tool for understanding and managing invasive species. The article emphasizes the need for a predictive science of invasions, integrating resource availability, disturbance, and fluctuating environmental conditions.