The article proposes a new theory to explain the invasion of habitats by non-native plant and animal species, which has significant ecological, economic, and social implications. The theory, based on the concept of fluctuating resource availability, suggests that invasibility, or the susceptibility of an environment to invasion, is primarily controlled by the availability of resources such as light, nutrients, and water. The theory posits that an invading species will have greater success if it does not face intense competition for these resources from resident species. The availability of resources can increase due to either a decline in resource uptake by resident vegetation or an increase in resource supply. Disturbances, such as disturbances from herbivores, pests, or diseases, can reduce resource uptake and thereby increase invasibility. The theory also predicts that environments with pronounced fluctuations in resource supply will be more susceptible to invasions, and that invasibility will increase following disturbances, diseases, or pest outbreaks that enhance resource availability. The theory is mechanistic and quantitative, providing clear testable predictions and a framework for understanding and managing invasive species.The article proposes a new theory to explain the invasion of habitats by non-native plant and animal species, which has significant ecological, economic, and social implications. The theory, based on the concept of fluctuating resource availability, suggests that invasibility, or the susceptibility of an environment to invasion, is primarily controlled by the availability of resources such as light, nutrients, and water. The theory posits that an invading species will have greater success if it does not face intense competition for these resources from resident species. The availability of resources can increase due to either a decline in resource uptake by resident vegetation or an increase in resource supply. Disturbances, such as disturbances from herbivores, pests, or diseases, can reduce resource uptake and thereby increase invasibility. The theory also predicts that environments with pronounced fluctuations in resource supply will be more susceptible to invasions, and that invasibility will increase following disturbances, diseases, or pest outbreaks that enhance resource availability. The theory is mechanistic and quantitative, providing clear testable predictions and a framework for understanding and managing invasive species.