Chapter Eight

Why are most species small?

Why study body size and diversity? Body size is the most obvious and one of the most easily measured features of an organism’s biology. It is also one of the most important: body size is strongly connected with the strategies that an organism can employ to obtain resources from the environment, and use them for growth and reproduction. What is fascinating is the idea that a biological trait that figures so prominently in the day to day lives of individual organisms could have a profound influence on large-scale patterns of biodiversity. One of the most clear macroecological patterns is the preponderance of small compared to large species. Yet we still do not fully understand why there are so many species of small size and so few of large size. Explanations range from macroevolutionary processes that play out over large scales of space and time, to the microevolutionary optimization of body size within populations by natural selection.

What are the main points?

• An organism’s body size places severe constraints on its fundamental architecture and physiology, and thereby the way it obtains energy and resources from the environment, assimilates energy into body tissue and uses it for growth and reproduction.
• The frequency distribution (histogram) of body sizes for large groups of species is often right-skewed on a logarithmic scale. This indicates a strong bias in species numbers towards species of smaller size. But it also requires us to explain the low species richness at the very smallest sizes.
• There are many hypotheses for this uneven distribution of body size, though some focus on the relative lack of small species, and some on the low numbers of both very small and large species

What techniques are covered?

• Using life history optimization models to determine optimum body sizes for different species.
• Using phylogenetic methods to test for associations between body size and rates of diversification.

What case studies will be included? 

Testing for a link between body size and diversification rates in insects.

 

Image: Carlos Delgado; CC-BY-SA