The two grassland ecosystems may have similar biomass but different species composition, which can affect their ability to resist decline due to a sudden infection by a lethal fungus and pathogenic bacteria. For example, one ecosystem may have a greater diversity of plant species, including some that are resistant to the lethal fungus and pathogenic bacteria, while the other ecosystem may have a more homogenous plant community with less resistance to the diseases.
Additionally, the two ecosystems may differ in their soil characteristics, such as nutrient availability and pH, which can also affect their ability to resist disease. The ecosystem with better soil quality and more favorable conditions for plant growth may be better able to resist decline due to disease.
Moreover, the two ecosystems may differ in their microclimate, such as temperature and moisture, which can affect the growth and spread of the lethal fungus and pathogenic bacteria. For instance, an ecosystem with warmer and drier conditions may be more favorable for the growth and spread of the diseases than a cooler and wetter ecosystem.
Overall, multiple factors can affect an ecosystem's ability to resist decline due to disease, and it is essential to consider the ecological context to understand why two ecosystems may have similar biomass but differ in disease resistance.