Data on parasites of rodents, collected over an 18-year period on the
Iberian peninsula, were used to fmd the determinants of parasite speci
es richness. A total of 77 species of helminth parasites (nematodes, c
estodes and digeneans) was identified among 16 species of rodents. Par
asites were classified into groups according to their specificity towa
rds their host and their life-cycle. A working phylogeny of the rodent
s was proposed on the basis of molecular and paleontological data and
for each host the following parameters were recorded: sample size, wei
ght, geographical range, longevity, and life-style. Two comparative me
thods were used, the independent comparisons method of Pagel (1992) an
d the distance matrix method of Legendre, Lapointe & Casgrain (1995).
The second method has the advantage of measuring the relative contribu
tion of phylogeny. Both methods gave similar results. Overall parasite
species richness correlated only with host sample size. Host body siz
e does not correlate with any subset of parasite species richness. How
ever, host phylogeny is a good predicator of specific parasites and th
e species richness of digeneans correlates with host geographical rang
e. A phylogenetic reconstruction of host relations was performed using
the parasites belonging to subgroups in which richness is correlated
with host phylogeny. These parasite species were treated as Dollo char
acters, i.e. we made the assumption that the loss of a parasite specie
s is irreversible. The consensus tree obtained reflects the major phyl
ogenetic divisions of the host group. Finally, this study illustrates
the relative importance of processes acting at different temporal and
spatial scales (evolutionary time and actual geographical range of hos
ts) in determining the structure of helminth parasite fauna.