An analysis of the correlation between landmass area, species richness, and extreme sizes of mammals and land snails

The largest land snail can reach 200 times the length of the smallest. However, the controls on these size extremes remain poorly understood. To investigate factors limiting body-size, one must partition Earth somehow and ask what extreme body sizes are under specific conditions. Partitioning the Earth into landmasses surrounded by water allowed researchers to investigate the correlation between the area of a landmass and size of the largest and smallest terrestrial mammal on it. With increased landmass area, maximum mammal size increases and minimum decreases. While this could indicate a link between body size and landmass area, it is also consistent with random sampling because larger landmasses support more species. To address this gap in knowledge, I gather landmass area, species richness, and maximum and minimum sizes of both mammals and land snails for 26 landmasses ranging from less than 1 square kilometer to over 30 million square kilometers. Using structural equation modeling, this study disentangles the effects of landmass area and species richness on body-size extremes and compares these results for land snails, mammals, and mammals excluding bats. After accounting for the correlation between species richness and maximum size, area has a statistically significant positive correlation to the size of the largest mammal when bats are excluded but no significant correlation to the size of the largest snail or largest mammal when bats are included. There is also a positive correlation between landmass area and smallest size across the three taxa. Mammals have higher metabolisms and activity than snails, so landmass area is potentially limiting maximum size by limiting food availability or actual space. Additionally, dispersal biases may account for the positive effect of landmass area on minimum size across taxa.