Kr. Blanche et Ja. Ludwig, Species richness of gall-inducing insects and host plants along an altitudinal gradient in Big Bend National Park, Texas, AM MIDL NAT, 145(2), 2001, pp. 219-232
Insect-induced galls are observed on plants throughout the world, bur patte
rns of gall-inducing insect species richness are not random. In the USA and
Brazil, species richness increases with decreasing altitude, which is asso
ciated with increasing temperature and aridity. ht a given elevation the nu
mber of gall-inducing insect species is also higher in drier habitats than
mesic habitats. However, variations in the number of potential host plant s
pecies, related to soil fertility, may be the cause of these patterns, nor
differences in temperature and aridity.
We examined patterns of species richness of gall-inducing insects by counti
ng the number of gall-inducing insect species and plant species, and measur
ing soil phosphorus, in replicate dry and mesic plots at five locations alo
ng an altitudinal aridity gradient in Big Bend National Park, Texas. Almost
all galls were on woody plant species (trees or shrubs). We found the grea
test number of gall-inducing insect species at intermediate elevations, and
in more mesic habitats, rather than at low altitudes or in drier habitats.
The number of woody plant species was also highest at intermediate elevati
ons and in mesic habitats. Soil phosphorus was high at both extremes of the
altitudinal gradient, where few gall-inducing insect species occurred.
Our results show that patterns of species richness of gall-inducing insects
may largely be a function of the number of woody plant species present. Th
e chance of a gall-inducing insect finding its specific host plant species
increases as the number of woody plant species increases. The effect of soi
l fertility requires further study but the findings suggest that high soil
fertility does not favor gall-inducing insects.