The highly endemic diversity of the Yun-Gui plateau lakes has decreased severely in recent decades. Protecting these native species is a challenge, in part because their biology and population ecology remain unexplored. We investigated 1 focal endangered gastropod species (Margarya melanioides) in 1 plateau lake: 1) to quantify spatiotemporal variation in population abundance and size structure, and 2) to identify the key variables influencing these variations. Density was 0.068 individuals/m² with an estimated total population size of 2.1 × 10⁷ individuals. The size-frequency distribution was skewed toward larger, mature individuals (40.0–60.0 mm), a result indicating that the population is aging. Population size structure showed high spatial heterogeneity, and density was highest in the north-central region of the lake. Redundancy analysis indicated that human-induced environmental degradation strongly controlled the spatial patterns in size-structure. This species has experienced a remarkable decrease in range and population size over the past 50 y, and the population probably will collapse by 2015. The decline could be attributed to demographic drift, environmental filtering, human harvesting, and its limited dispersal ability. Our studies should give useful information for building efficient conservation strategies for M. melanioides and other threatened plateau gastropods. Further studies are needed on the population ecology and reproductive biology of M. melanioides, particularly on its reproductive behavior and size-specific survivorship.