Chapter 9

APPROACHINGECOLOGICAL

COMPLEXITYFROMTHE

PERSPECTIVEOFSYMMETRIC

NEUTRALTHEORY

Stephen P. Hubbell

OVERVIEW

I argue, seemingly paradoxically, that the most rapid path to understanding ecological systems, especially complex
systems such as species-rich tropical tree communities, is through an interaction of empirical science, guided by
strong inference, with theories that start very simply, with few free parameters and assumptions, and add complexity
reluctantly, kicking and screaming, only when absolutely necessary to obtain some desired level of fit to the data.
Neutral theory is one such starting point. Although it is only a first approximation, neutral theory is a remarkably good
approximation to many of the patterns of relative tree species abundance we observe in tropical forests worldwide. In
this chapter, I briefly review some of the major developments in neutral theory since publication of my book in
2001, and try to clear up several persistent misconceptions about neutral theory. One common misconception is that
a finding of density dependence falsifies neutrality, which it does not, provided that all species exhibit approximate
symmetry in their density dependence. I conclude with some new findings about the dynamics of the tropical tree
community on Barro Colorado Island, Panama, over the past quarter century that are more consistent with neutrality
and drift than they are with stable population fluctuations around fixed carrying capacities, the expectation of classical
niche-assembly theory.

INTRODUCTION

Soon after coming to the United States during
World War II, Enrico Fermi, the great Italian
physicist, was told by some US flag officers that
so-and-so was a great general. What is the defini-
tion of a great general, Fermi asked? After some
thought, they agreed that winning five major
battles made a great general. And how many
generals are great? After some more back and
forth, they replied only about 3%. Well, Fermi
replied, suppose armies in battles are equally
matched, and the probabilities of winning or
losing are equal and random. Then you would

find by chance that about 3% of generals win five

battles!

The point of relating this story, paraphrased

from Sagan and Druyan (1997), is not to dis-

cuss whethermilitary intelligenceis an oxymoron,

but whether one always needs complex theory

to explain apparently complex ecological phe-

nomena. In my view, much of the complexity

of contemporary theory in ecology is probably

unnecessary and actually impedes the advance-

ment of the ecological sciences. Nearly half a

century ago, Platt (1964) urged molecular biol-

ogists to pursue a program of strong-inference

driven science, and now we need a comparable