Three sites in Cuatro Ciénegas
Portilla-Alonso and coauthors followed Coryphantha werdermannii in the Cuatro Ciénegas valley in Coahuila in 2005–2006. Samples from three populations comprised 87, 101 and 124 plants. Sites differed in the intensity of chronic human impact, identified from accumulated signs of grazing and land use. Each plant was assigned to one of six size classes. A repeat census allowed assessment of survival, size change and participation in reproduction. [Portilla-Alonso and Martorell 2011, pp. 510–512, Table 1–3]
In June 2005, 1000 seeds were sown in each population. By January 2006, four seedlings had been found, two at each of the two more disturbed sites. None were found at the least disturbed site. Such rare appearance of new plants made precise estimation of recruitment, the addition of young individuals, difficult. The authors had to examine how different assumptions about this parameter changed the forecast. [Portilla-Alonso and Martorell 2011, pp. 511–512]
The model depends on rare seedling emergence
In matrix models, probabilities of survival and transitions between size classes were combined with seed production and the probability of seedling establishment. One version assigned all populations a common probability of seedling appearance estimated from the entire sample. The second varied this probability between sites and assumed a small positive value where no emergence had been observed. The forecast of population growth changed with this choice. A rare event early in life therefore has considerable significance, although it is harder to measure than the survival of marked adult plants. [Portilla-Alonso and Martorell 2011, pp. 511–513, Section 2.3, Table 3]
In the least disturbed population, seedling appearance was a major constraint. In the population with intermediate disturbance, adult survival became more important. The authors observed damage associated with trampling and cattle eating plant apices. In the most disturbed population, plant growth was limited. The distribution of these processes differed between the three sites, so a common disturbance indicator does not describe the entire mechanism of change. [Portilla-Alonso and Martorell 2011, pp. 512–515]
Density and reproduction
Martorell and coauthors broadened the assessment by combining data on several threats in a study published in 2015. The density of C. werdermannii was positively associated with signs of grazing and the proportion of bare soil, and negatively associated with shrub cover. Open surfaces with fine mineral material may provide places for seedling emergence. This was, however, a comparison of existing habitats that also differed in other respects. [Martorell et al. 2015, pp. 74–77, Table 1–2]
At the same time, greater human activity was associated with fewer reproductive structures per adult plant. The authors considered damage resulting from harvesting candelilla wax around the cacti. Density and current reproduction can therefore change in different directions. A large population may contain many young plants while its adults contribute less to production of the next generation. [Martorell et al. 2015, pp. 76–79, Fig. 2–3]
Simulated removal of adult plants
In the 2015 demographic models, a small additional loss of adult plants had a much stronger effect than a large loss of seeds. In the version used, removing 3% of adults halted population growth, whereas the model retained growth even when seed input was halved. The greater importance of adults results from their survival over successive years and repeated participation in reproduction. [Martorell et al. 2015, pp. 77–79, Table 3, Fig. 4]
The reported proportions are simulation results under a particular structure and transition probabilities. They describe the model's sensitivity. Applying them to another population would require knowledge of survival, growth, reproduction and rare recruitment in that population. In particular, the result for seed loss depends on the assumed chance of new plant establishment. [Martorell et al. 2015, pp. 75–79]
Potential distribution and climate change
The same study modelled the species' potential distribution for 2030 and 2050. It used six niche modelling methods, three climate models and four SRES emission scenarios available at the time. The results predicted a considerable reduction in the area with suitable climatic conditions, with differing magnitudes between versions. These are historical forecasts based on the data and scenarios available when the publication was prepared. [Martorell et al. 2015, pp. 75–78, Table 2, Fig. 1]
The area predicted to be climatically suitable is a different quantity from the actual range of occupied sites. A climate model indicates where conditions may favour a species. Survival of individual plants also depends on location, damage and local demographic processes. Studies of C. werdermannii show these processes separately, with differing constraints on recruitment, growth and adult persistence at individual sites. [Martorell et al. 2015, pp. 78–80]
Demographic consequences of chronic anthropogenic disturbance on three populations of the endangered globose cactus Coryphantha werdermannii
Portilla-Alonso R. M., Martorell C., Demographic consequences of chronic anthropogenic disturbance on three populations of the endangered globose cactus Coryphantha werdermannii, Journal of Arid Environments, 2011, 75(6):509-515, doi: 10.1016/j.jaridenv.2011.01.015
A comparison of three Coryphantha werdermannii populations in Cuatro Ciénegas with different levels of chronic human pressure. Observations of growth, survival, reproduction and seedling emergence were used in matrix models. The paper shows different effects of disturbance on small and large plants and the model's sensitivity to recruitment assumptions.
Assessing the importance of multiple threats to an endangered globose cactus in Mexico: Cattle grazing, looting and climate change
Martorell C., Montañana D. M., Ureta C., Mandujano M. C., Assessing the importance of multiple threats to an endangered globose cactus in Mexico: Cattle grazing, looting and climate change, Biological Conservation, 2015, 181:73-81, doi: 10.1016/j.biocon.2014.10.035
It combines the density and reproduction of Coryphantha werdermannii with demographic models and climatic niche projections. It analyses grazing, human pressure, plant collection and climate. It emphasises the role of adult persistence and differences between local density, population growth rate and potential area of occurrence.