Species richness of gall-inducing insects and host plants along an altitudinal gradient in Big Bend National Park, Texas

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.