Chapter 4

Explaininggeographicrange

sizebyspeciesage:ATest

UsingNeotropicalPiper Species

John R. Paul and Stephen J. Tonsor

OVERVIEW

Tropical plant species vary dramatically in their geographic range sizes.Theory predicts that narrowly endemic species
may simply be young species that have not had sufficient time to expand their ranges. If two assumptions are met,
namely that new species start with small range sizes and that the probability of extinction is inversely related to range
size, then older species should, on average, have larger range sizes than younger species. This conjecture, originally
formulated by John Willis as the age-and-area hypothesis, and recently predicted by models of neutral community
dynamics, has not been adequately tested in tropical plant taxa. To test this hypothesis, we focused on neotropical
species of the tropical understory shrub genusPiper(Piperaceae). We used published internal-transcribed spacer (ITS)
sequences to infer species’ divergence times using Bayesian relaxed-clock methods and herbarium records to estimate
range sizes. We asked if there is a positive relationship between species age and range size. Using linear regression,
we found that relative species age significantly explains a quarter of the variance in range size among species in
this prominent tropical plant genus. This result confirms that species age can be a significant predictor of range size,
and is notable in light of uncertainties in divergence time estimation using limited sequence data and incomplete
sampling. We discuss the generality of our results with regard to other tropical plant taxa and briefly review the
limited data on species-level age estimates from tropical plants. Furthermore, we discuss the potential limitations and
difficulties of using divergence times as proxies for species ages, particularly when applied to analyses involving range
and population sizes of new species. We suggest that the wealth of new genetic and biogeographic data on tropical
plant species promises broader explorations of the impact of species age on species’ range sizes in the near future.

INTRODUCTION

What accounts for rarity and endemism? Ecol-
ogy, the study of distribution and abundance
of species, remains without a coherent and
consistent answer to this question. In tropi-
cal forest communities, the vast majority of
species have few individuals and small geo-
graphicranges(Wallace1878,Dobzhansky1950,
Hubbell2001a).Explaininghowrarespeciesdiffer

from more common species, and elucidating the

relative importance of various factors that reg-

ulate species’ abundance and distribution, is a

central goal of ecology. However, the complicat-

ing influence of both deterministic and stochastic

forces acting at various levels of biological organi-

zation and temporal duration make this a difficult

task. In this chapter, we concentrate on the

role of evolutionary history in structuring the

abundance and distribution of plant species in

tropical forests. Specifically, we address how the