Chapter 21

SOILFERTILITYANDARBOREAL

MAMMALBIOMASSINTROPICAL

FORESTS

Carlos A. Peres

OVERVIEW

Tropicalforestshavebeencharacterizedacrossawiderangeof primaryplantproductivity,whichispartlydependenton
soil properties.Yet the relationships between soil fertility and plant productivity and herbivore biomass remain poorly
understood in tropical forests. Here I review the evidence regarding the relationship between soil fertility and mammal
assemblage biomass from the perspective of primates spanning a wide dietary spectrum. I also present new data based
on a comprehensive compilation of available community-wide estimates of neotropical primate biomass density in
Amazonian and Guianan forests. A composite index of soil fertility, based on both chemical and physical properties of
soils, explained 37% of the variation in total diurnal primate biomass in a set of 60 undisturbed Amazonian forest sites
that had not been affected by hunting pressure and anthropogenic habitat disturbance. I discuss the mechanisms by
which tropical forest soil nutrient availability may constrain bottom-up trophic cascades from green plant producers
to primary and secondary consumers.These include edaphic effects on the quality and amount of resources produced
by individual food plants, as well as wholesale changes in floristic composition. Measures of soil fertility and other
environmental gradients affecting forest productivity can serve as an efficient framework for predicting the diversity
and population sizes of vertebrate species that can be protected by potential reserve polygons.

INTRODUCTION

Early perceptions that evergreen tropical forests

must be sustained by fertile soils (Wallace 1853) –

which is reinforced by the paradoxical high

phytomass supported by highly efficient nutri-

ent capture and cycling in nutrient-poor soils –

have been unequivocally demystified by a vast

body of evidence on the limited agropastoral

potential of the humid tropics (Goodland and

Irwin 1975, Irion 1978, Sioli 1980). Yet the

physical and chemical properties of unfertil-

ized tropical forest soils are remarkably variable

(Projeto RADAMBRASIL 1972–1978, Sanchez

1981, Uehara and Gilman 1981, Cochrane and

Sanchez 1982, Vitousek and Sanford 1986,

Jordan 1989, Furley 1990, Richter and Babbar

1991).The local to regional scale heterogeneity in

soilage,texture,drainage,depth,parentmaterials,

pH, and macro- and micronutrient content there-

fore presents a highly diverse set of consequences

to forest primary productivity.

Most soils in the humid tropics are highly

weathered and relatively nutrient poor (Irion

1978, Uehara and Gilman 1981). Tropical for-

est plants are often able to prevent nutrient

loss through leaching and herbivory using a

number of above- and below-ground strategies.

However, retention of scarce nutrients may be

less imperative in fertile soils that can replace

nutrients lost through leaching and herbivory.

Given the fundamental laws of thermodynamics,