Coexistence and the niche in a nurse–cactus interaction: Is cyclic dynamics justified?

Abstract

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. In order to test the hypothesised cyclical dynamics, we compared the demography of four subpopulations (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 capita rate of population growth (r ≥ 0) in CA and LS, and a negative one in CS and LA. The overall r over the study period was close to equilibrium (r = 0) for both LA and LS, with no significant difference between them. In contrast, and as expected, r of CA was consistently and significantly larger than that for CS. In both species, λ (=er) was positively correlated with annual rainfall with no significant difference between LA and LS and a significant difference between CA and CS in this relationship. L. tridentata can exist alone and coexist with C. leptocaulis across the levels of precipitation experienced at the study site, while C. leptocaulis cannot persist in the absence of L. tridentata. The cyclical dynamics of replacement does not occur in this system and, given the static nature of the evidence on which the original conclusion was based, it is unlikely to exist as a general rule across the species’ geographic range. Synthesis. Our findings stress the importance of elucidating the demographic mechanisms that allow species competing for a common limiting resource to coexist when the resource varies temporally, and expose the difficulty of identifying a species’ ecological niche in the absence of detailed, long‐term demographic information.

Age-Independent Adult Mortality in a Long-Lived Herb

“…Individuals that grew more generally had higher mortality when considering all ages, indicating a trade-off between growth and survival [43,44]. Interestingly, the effect was reversed for ages above 10 (i.e., from reproductive maturity).…”

Phylogenetic relatedness and competition: a pot experiment with semiarid tree species

“…According to Valiente-Banuet and Verdú (2007), regeneration niche traits are conserved throughout evolution, i.e., similar species require similar conditions to germinate and establish themselves and, therefore, phylogenetically distant species grow under the canopy of nurse species, and not of similar species and much less of the same species. Facilitation between distant species can be easily observed when studying shrub species facilitating cactus species (e.g., Valiente-Banuet et al, 1991;Drezner, 2006;Flores-Torres et al, 2019). Although in our work we studied species with the same growth form and with a certain phylogenetic relatedness, facilitation was also Tavares et al found in species of the same family (Fabaceae, the more distantly related pair).…”

Conceptual and methodological issues in structured population models of plants

“…For instance, in some studies, the elucidation of environmental effects, plant-plant interactions and coexistence mechanisms would have been muddled by the inclusion of density dependence in the model (Verhulst et al 2008, Ferrer et al 2015, Flores-Torres et al 2019. Topping et al (2015) advocate the use of biological narratives when considering what to include, and how to include it, in a model.…”