Effects of Habitat Loss on the Ecology of Pachyphytum caesium (Crassulaceae), a Specialized Cliff-Dwelling Endemic Species in Central Mexico

Abstract

Cliff-dwelling plant species are highly specialized and adapted to a vulnerable, fragmented, and are mostly endemic, narrowly-distributed and threatened. As a contribution to the conservation efforts of endemic cliff-dwelling species, this study provides an overview of the effects of habitat loss on the abundance and distribution of Pachyphytum caesium (Crassulaceae) due to human disturbances. To achieve this objective, we first conducted a retrospective analysis from 2003–2013 to assess the effects of land use change on the abundance of P. caesium. Secondly, we estimate the abundance and distribution of P. caesium throughout the study area, as well as analyze the effect of rock-climbing activities on the density and population structure of P. caesium. The results suggest differences in population abundance among sites is due to the adverse effects of habitat loss. P. caesium presents a very restricted distribution with small and fragmented populations. In addition, guava agriculture has a significant impact on the chemical soil properties of the hillsides, causing a significant effect on the occurrence of P. caesium, while sport activities remove both the soil and the plants from the cliffs. According to the results, P. caesium is classified as a plant species with extremely small populations (PSESP), and it is highly vulnerable to habitat disturbance. Its conservation is thus a priority to ensure its permanence.

A Change in Conservation Status of Pachyphytum caesium (Crassulaceae), a Threatened Species from Central Mexico Based on Genetic Studies

“…The study area is located in central Mexico, between 22 • 40 00 N and 102 • 30 00 W in the southwestern portion of the state of Aguascalientes (Figure 1). The only six known sites with P. caesium are located in this region [21], whose natural range is marginal (less than 20 ha) and fragmented in that territory. The study area has a subtropical climate, with rains in the summer (from July to November) with an average annual rainfall of 591.4 mm, and a temperature ranging from 15 to 29.5 • C. The altitudinal distribution at the study area ranges from 1600 to 2400 masl (meters above sea level) (Figure 1).…”

“…In this context, the vulnerability to extinction is the primary concern in Plant Species with Extremely Small Populations (PSESP). PSESP species are characterized mainly by small population sizes, low genetic variation, restricted distribution, habitat loss, and high vulnerability to environmental changes that make them particularly vulnerable to extinction [21][22][23].…”

“…Habitat loss has been particularly worrying in central Mexico, due to land-use change, as more than 80% of its original vegetation has been transformed into agriculture and cattle raising [21,24]. This activity has been threatening an area with high species diversity [21,25], including species of Pachyphytum, an endemic genus of the Crassulaceae [26]. Knowledge of this genus is mainly limited to morphological descriptions [26][27][28][29] and a general description of the current distribution of their species [29].…”

“…Knowledge of this genus is mainly limited to morphological descriptions [26][27][28][29] and a general description of the current distribution of their species [29]. Among these species, Pachyphytum caesium Kimnach & Moran is a cliff-dwelling species endemic to the dry tropical forest of Aguascalientes in central Mexico, and has been categorized as PSESP due to its highly specialized habitat, small and fragmented populations, and a high vulnerability to environmental changes [21]. Based on the risk of extinction evaluation method of wild species (MER in Spanish), Clark-Tapia et al [21] classified the status of this species as threatened and recommended incorporating the species into the Mexican federal protection list.…”

“…Because an appropriate estimation of extinction risk is one of the key components of any conservation plan, in this work we evaluated the diversity and genetic structure of the Biology 2022, 11, 379 3 of 16 currently know populations of Pachyphytum caesium reported by Clark-Tapia et al [21]. In addition, we assessed the effect of habitat loss on genetic diversity by simulating the loss of individuals in a population or local population extinction in the data set to evaluate its effect on genetic parameters in the MER and provide recommendations for in situ conservation.…”