Arneberg

v ol. 151, no. 6

the american naturalist

june 1998

Parasite Abundance, Body Size, Life Histories, and the
Energetic Equivalence Rule

Per Arneberg,1,* Arne Skorping,1,† and Andrew F. Read1,2,‡

1. Department of Ecology/Zoology, Institute of Biology,
University of Tromsø, 9037 Tromsø, Norway;
2. Institute of Cell, Animal and Population Biology, University of
Edinburgh, West MainsRoad, Edinburgh EH9 3JT, United
Kingdom
Submitted June 3, 1997; Accepted December 29, 1997

abstract: If common processes generate size-abundance relationships among all animals, then similar patterns should be observed across groups with different ecologies, such as parasites and
free-living animals. We studied relationships among body size,
life-history traits, and population intensity(density in infected
hosts) among nematodes parasitizing mammals. Parasite size and
intensity were negatively correlated independently of all other parasite and host factors considered and regardless of type of analyses
(i.e., nonphylogenetic or phylogenetically based statistical analyses,
and across or within communities). No other nematode life-history traits had independent effects onintensity. Slopes of size-intensity relationships were consistently shallow, around 0.20 on
log-log scale, and thus inconsistent with the energetic equivalence
rule. Within communities, slopes converged toward this global
value as size range increased. A summary of published values suggests similar convergence toward a global value around 0.75
among free-living animals. Steeper slopes ofsize-abundance relationships among free-living animals could be related to fundamental differences in ecologies between parasites and free-living animals, although such generalizations require reexamination of sizeabundance relationships among free-living animals with regard to
confounding factors, in particular by use of phylogenetically based
statistical methods. In any case, our analyses cautionagainst simple generalizations about patterns of animal abundance.
Keywords: parasites, free-living animals, body size, abundance, ecological laws.

Identifying the determinants of species abundance is one
of the central problems in ecology (Begon et al. 1996). So
far, body size is the best single predictor of animal abundance, and with size varying from minute crustaceans to
elephants, bodysize and abundance are strongly negatively correlated, with size accounting for about 90% of
the variation in animal species abundance (Damuth
1987). Determining the precise form of this relationship
is now emerging as one of the major issues in macroecological research, and a central question is whether it is
generated by processes related to population energy usage (e.g., Damuth 1981, 1987;Peters and Wassenberg
1983; Lawton 1989; Marquet et al. 1990, 1995; Nee et al.
1991; Griffiths 1992; Blackburn et al. 1993; Currie 1993;
Silva and Downing 1994, 1995).
Underlying this debate is the idea that similar processes may influence abundance in all kinds of animals,
yet, so far, large-scale relationships between body size
and abundance have been studied in free-living animals
only(e.g., Griffiths 1992). The majority of animal species
are parasites (free-living animals almost certainly harbor
at least one host-specific parasite species; Price 1980), so
that parasites form an important challenge to general
theories of animal abundance. At least on the face of it,
they occupy quite different niches from free-living animals. Such differences are exactly what ecologicalprinciple must cut across, making parasites ideal organisms to
test general theories about the processes generating sizeabundance relationships. Here we relate body size to
abundance among parasites, by focusing on a particularly
well-studied group, the nematodes parasitizing mammals.
Size-Abundance Patterns

*To whom correspondence should be addressed; E-mail: pera@ibg.uit.no.
†E-mail:...