Mainland size variation informs predictive models of exceptional insular body size change in rodents

Abstract

The tendency for island populations of mammalian taxa to diverge in body size from their mainland counterparts consistently in particular directions is both impressive for its regularity and, especially among rodents, troublesome for its exceptions. However, previous studies have largely ignored mainland body size variation, treating size differences of any magnitude as equally noteworthy. Here, we use distributions of mainland population body sizes to identify island populations as 'extremely' big or small, and we compare traits of extreme populations and their islands with those of island populations more typical in body size. We find that although insular rodents vary in the directions of body size change, 'extreme' populations tend towards gigantism. With classification tree methods, we develop a predictive model, which points to resource limitations as major drivers in the few cases of insular dwarfism. Highly successful in classifying our dataset, our model also successfully predicts change in untested cases.

Keywords: biogeography; body size; decision tree; island; mammal; rodent.

Figure 1.

Density plots comparing distributions of numbers of island populations (y-axis) for various characteristics (x-axis). (a) Size ratios for mainland (black) and island (purple) populations; vertical lines delimit tails of the distributions showing extreme size reduction (left) or increase (right). (b–i) Various attributes whose distributions differ significantly (K–S test, p < 0.05) between ‘normal’ (solid black), Small (dashed blue) and/or Big (dotted red) island populations. (Online version in colour.)

Figure 2.

Classification tree, showing predicted direction of size change for insular rodent populations, and body size ranges corresponding to its terminal nodes. (a) Classification tree showing variables and the ranges of values used in the classification process and the numbers of cases showing size increase/decrease at each node. In the terminal nodes, the numbers of cases correctly predicted are in a larger font. (b) Box-and-whisker plots showing the distribution of species masses for each of the four terminal nodes in the tree. The box extends from the first to the third quartile, with the bold line indicating the median mass for the box. (Online version in colour.)