Can the co-existence of the cactus and the shrub in the community be driven by a cyclic succession mechanism?Location:
Mapimí Biosphere Reserve (southern Chihuahuan Desert). The vegetation is scrub, dominated by Larrea tridentata.Methods:
Interspecific spatial association between adult L. tridentata and differently sized Cylindropuntia leptocaulis plants was analysed using Ripley′s K function, exploring the hypothesis that facilitation causes differential recruitment of cactus below the shrub and a subsequent interspecific association, the intensity of which declines with the cactus size due to competitive elimination of the shrub. Competition for water among adult plants was investigated by irrigating solitary and associated plants of both species. The proportion of vigorous (measured by the percentage of dead branches) and dead L. tridentata plants associated with adult cacti was compared to that of solitary L. tridentata plants, testing the hypothesis that competition among associated adults reduces the vigour and increases the proportion of dead shrubs, relative to that of solitary plants.Results:
In 14 out of 16 spatial analyses (four cactus sizes in four plots), C. leptocaulis was found to clump around the L. tridentata adults. Larrea tridentata suffered the effects of interspecific competition for water, but these diminished with increased water provision. In contrast, C. leptocaulis was unaffected by interspecific competition for water. The proportion of vigorous shrubs was higher in solitary than in associated individuals; however, the proportion of dead shrubs was lower in associated than in solitary individuals.Conclusion:
The irrigation experiment and the proportion of dead branches suggest interspecific competition. However, spatial association of the shrubs with every cactus size and the proportion of dead shrubs associated with the cacti both suggest that replacement of the shrub by the cactus is an infrequent event. Cyclic succession therefore appears to be the exception rather than the rule, and does not account for the co-existence of the species in the community. The possible scenarios in which cyclic succession may occur are discussed.