The chapter discusses the significant impact of changes in species abundance and diversity on the functioning of ecosystems. It highlights that species with different functional traits, such as resource processing rates, effects on the physical environment, and interactions with other species, can alter ecosystem processes through changes in their functional traits. These changes can affect processes like nitrogen uptake, nutrient cycling, and regional processes like methane emissions. The text also emphasizes the importance of species diversity in increasing the probability of including species with strong ecosystem effects and enhancing resource use efficiency. It notes that current global environmental changes, such as habitat conversion and the introduction of exotic species, are profoundly altering the biosphere's functioning. The chapter further explores how species differences in environmental response can provide stability or make ecosystems more vulnerable to change. It concludes by discussing the need for comparative and experimental studies to better understand the role of high levels of natural diversity and the importance of protecting keystone species and functional groups to maintain ecosystem resilience and long-term sustainability.The chapter discusses the significant impact of changes in species abundance and diversity on the functioning of ecosystems. It highlights that species with different functional traits, such as resource processing rates, effects on the physical environment, and interactions with other species, can alter ecosystem processes through changes in their functional traits. These changes can affect processes like nitrogen uptake, nutrient cycling, and regional processes like methane emissions. The text also emphasizes the importance of species diversity in increasing the probability of including species with strong ecosystem effects and enhancing resource use efficiency. It notes that current global environmental changes, such as habitat conversion and the introduction of exotic species, are profoundly altering the biosphere's functioning. The chapter further explores how species differences in environmental response can provide stability or make ecosystems more vulnerable to change. It concludes by discussing the need for comparative and experimental studies to better understand the role of high levels of natural diversity and the importance of protecting keystone species and functional groups to maintain ecosystem resilience and long-term sustainability.