Shrub species exhibit differing long-term responses to a change in the species of ungulate browsing

Abstract

Hyper-arid rangeland vegetation is typically dominated by large woody species which are often overlooked in herbivory studies. Knowledge of long-term large shrub population responses to change in browsing system in the Arabian Peninsula has been anecdotal. Population and size of 1559 individuals from four shrub species were opportunistically assessed over an 11-year period under two browsing regimes, one in which domestic livestock (camels) were replaced by semi-wild ungulates (oryx and gazelles) before, and the other during, the study period. Each shrub species exhibited a different response to the change in herbivory. Populations of Calotropis procera decreased dramatically. Populations of both Calligonum comosum and Lycium shawii increased through sexual reproduction, but the spatial distribution of recruits indicated different modes of seed dispersal. Average lifespans were estimated at 22 and 20 years respectively. The strategy of Leptadenia pyrotechnica was similar to tree species of this habitat, prioritizing vegetative regrowth, and average lifespan was estimated at 95 years. Hyper-arid large shrub populations may take many decades to adjust to a major change of browsing regime if they have adopted a vegetative method of persistence, though the size of surviving individuals may adjust relatively quickly.