The main difference between small, controlled lab experiments and ecosystem-scale experiments relates to the applicability of the results.
In lab experiments, conditions can be tightly controlled, allowing for specific variables to be manipulated, but these results may not always translate directly to complex, real-world ecosystems. In contrast, ecosystem-scale experiments involve numerous interacting variables typical of natural environments, making the results more applicable to real-world ecological contexts, albeit with less control over specific factors.
While other options like abiotic factors, number of variables, and species of interest can also vary between the two types of experiments, the key distinction in terms of the overarching impact of the findings is their applicability to real-world scenarios.