An ecosystem is not a still life. Even where everything looks stable—a woodland, a lake, the soil—the internal "bookkeeping" keeps changing: how many individuals belong to which species, and for how long. Some populations expand, others crash. That dynamism is part of what we call biodiversity, but it also carries risk: when numbers are very low, chance events and short spells of unfavorable conditions can increase the likelihood that a species disappears locally.
