Epithelantha. Soil and the boundaries of desert species

A small genus with separate environments

The range of Epithelantha covers the southwestern United States and northeastern Mexico. This area includes desert plains, mountain slopes, foothills and transitions towards thorn scrub. Current POWO accepts ten species. Two names clearly illustrate the changed scope of older identifications. E. potosina (formerly E. greggii subsp. potosina) and E. pulchra (formerly E. pachyrhiza subsp. pulchra) are now separate species. [2-WWW]

In 2021, Aquino and co-authors compared sites of all ten species with information on soil, landforms and climate. The comparison also included Ariocarpus, Kadenicarpus, Rapicactus, Strombocactus and Turbinicarpus. The question covered two scales. These were environmental characteristics distinguishing Epithelantha among related genera and differences between species within this genus. Answering it required site maps to be combined with the history of relationships. [Aquino et al. 2021, pp. 4520–4522]

What exactly was compared

The geographical material came from fifteen herbaria and field observations in Coahuila, Nuevo León, San Luis Potosí and Tamaulipas. The authors excluded duplicates, imprecise locations and sites too close to other points, using a distance of at least 5 km. After filtering, 101 Epithelantha sites remained. Numbers were uneven in the other genera, ranging from six Kadenicarpus sites to 91 Ariocarpus sites. The sampling scale affects the precision of comparisons between genera. [Aquino et al. 2021, pp. 4521–4522]

Each point was assigned values from maps with a resolution of approximately 1 km². The dataset included nineteen bioclimatic variables, soil pH and category, and landscape form. Strongly correlated climate indices were removed to reduce repetition of the same information. Six climate characteristics and pH remained for further comparisons. A site in a rock crevice could therefore receive a value applying to the entire surrounding map cell. Distinguishing microhabitats would require additional measurements directly around the plants. [Aquino et al. 2021, pp. 4521–4523]

Soil and landforms

In the comparison of six genera, Epithelantha was frequent in foothill zones and on typical kastanozems, termed haplic kastanozems in the source classification. The distribution of sites among soil and landform categories also differed between its species. A soil category describes a set of properties and the history of profile formation. A landscape category expresses position, with which slope, drainage conditions and aspect co-occur. In this study, these were two separately analysed map variables. [Aquino et al. 2021, pp. 4523–4525, Fig. 3]

Associations were assessed by comparing observed site numbers with expected frequencies in a contingency table. The Pearson residual indicates the direction and magnitude of the discrepancy. A positive value means an excess of observations. The example of 3,662 for E. bokei on calcaric regosols is such a residual. The overall comparison of soil categories between species showed a significant difference in distributions. The interpretation concerns an occurrence pattern and remains dependent on site numbers and completeness. [Aquino et al. 2021, pp. 4522, 4524–4525]

Two lineages and ancestral environments

The tree of relationships was based on four plastid DNA regions, petL–psbE, psbA–trnH, trnL–F and trnQ–rps16. The set of twenty taxa included Epithelantha species and outgroups, and the sequences compared contained 3736 positions in total. Within the genus, one larger lineage grouped E. bokei, E. cryptica, E. ilariae, E. micromeris, E. potosina and E. spinosior. The second included E. greggii, E. pachyrhiza, E. polycephala and E. pulchra. [Aquino et al. 2021, pp. 4522–4524, Fig. 2]

Such a tree allows phylogenetic signal to be tested, assessing whether more closely related species are also similar in a particular environmental characteristic. In the paper, soil pH, isothermality, temperature seasonality and annual precipitation showed significant signal. Isothermality relates the daily temperature range to the annual range, and seasonality describes fluctuations over the year. Mean annual temperature and mean temperatures of the driest and coldest quarters did not have the same outcome in signal tests. Different variables therefore described different parts of ecological history. [Aquino et al. 2021, pp. 4522–4526, Fig. 4]

According to the maps used, soils at most Epithelantha sites were alkaline. In the same comparison, some related genera occupied substrates closer to neutral. The authors considered pH as a factor that might contribute to lineage differentiation. The demonstrated signal links a site characteristic with relationships, while the selection mechanism requires plants to be studied under comparable conditions. Spatial data do not define a single optimum pH for potting substrate. [Aquino et al. 2021, pp. 4525–4527]

Similar DNA, different habitat boundaries

The most interesting case is the pair E. micromeris and E. cryptica. They were very similar in the plastid DNA regions studied. Morphological and geographical differences nevertheless co-occurred with different environments. E. micromeris occupied four soil categories and three landscape categories, and E. cryptica a narrower set of sites. The climate assigned to E. micromeris was cooler and drier. The authors regarded environmental data as additional evidence for retaining two species. [Aquino et al. 2021, pp. 4524–4527]

Other species show further differences. E. ilariae lies at the transition from the Chihuahuan Desert to Tamaulipas thorn scrub. Annual precipitation assigned to its sites was approximately 544 mm. E. greggii and E. polycephala occupied drier environments. Southern E. potosina sites had lower temperature seasonality and higher isothermality than many northern sites of the genus. Each of these results comes from a specific set of points and map layers. [Aquino et al. 2021, pp. 4525–4527, Fig. 4]

What the ecological pattern means

Environmental separation of species may arise through local adaptation, differences in colonisation ability and the history of population movement. The authors propose that soil type and landforms act as ecological barriers helping maintain distinct lineages. Their material provides map-based and phylogenetic evidence for this interpretation. Distinguishing the contributions of selection, dispersal limitation and gene flow would require reciprocal transplant experiments and denser genetic sampling of populations. [Aquino et al. 2021, pp. 4526–4527]

For a species description, assigning data to the correct plant and scale is particularly valuable. A soil name on a map covers a larger area, while a specific specimen uses a crevice, local drainage and shelter. A plastid tree indicates relationships, while species boundaries also involve morphology and distribution. The Epithelantha analysis shows how this information can be combined into an account of the causes of the high number of local endemics among desert cacti. [Aquino et al. 2021, pp. 4521–4522, 4526–4527]