Coexistence and the niche in a nurse-cactus interaction: is cyclic dynamics justified?

1. The interaction between the nurse plant Larrea tridentata and the cacti Cylindropuntia leptocaulis is thought to follow cyclical-replacement dynamics. However, the required changing nature of their interaction, from facilitation to competition, has not been investigated through their full life cycle.
2. In order to test the hypothesised cyclical dynamics, we compared the demography of four sub-populations (LA = Larrea associated to Cylindropuntia; CA = Cylindropuntia associated to Larrea, LS = solitary Larrea; CS = solitary Cylindropuntia) in the Mapimí Biosphere Reserve, Mexico, over the period 2008-2015. This allowed us to compare their demography at varying levels of the generally low and unpredictable limiting resource, water. Given the protection afforded to C. leptocaulis seedlings by L. tridentata and their presumed increasingly competitive effect on L. tridentata, which would justify the expectation of cyclic replacement dynamics, we expected to find a zero or positive per c...