| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Systematics and Phytogeography |
2Instituto de Biología, Universidad Nacional Autónoma de México, Departamento de Botánica, Tercer Circuito s/n, Ciudad Universitaria, Copilco, Coyoacán A.P. 70-367, México, Distrito Federal, C.P. 04510, Mexico; 3Jardín Botánico y Herbario, Facultad de Ciencias Químicas y Biológicas, Universidad Autónoma de Guadalajara, Av. Patria 1201, Apdo. Postal 1-440, 44100 Guadalajara, Jalisco, Mexico
Received for publication March 2, 2004. Accepted for publication December 6, 2004.
ABSTRACT
The type locality of the slipper spurge Euphorbia conzattii has been in doubt because the 1917 type is a mixed collection with vague label data. In recent field work, the species was found on Cerro Espino, Pochutla District, Oaxaca, Mexico. We used the Method for Evaluation of Risk of Extinction for Mexican Wild Species (MER), required to protect a species under Mexican law, to assess the conservation status of E. conzattii and found it to be endangered. We discuss the mixture in the type of this species with E. calcarata and present an updated description and an illustration of E. conzattii. Preliminary MER assessments of the other Mexican Pedilanthus clade species show two species to be extinct (E. cyri, E. dressleri) and four threatened (including E. colligata, E. finkii, E. tehuacana). The remaining eight have more favorable outlooks. We comment on MER robustness and aspects vulnerable to confusion and offer clarifications. Extinction risk is not distributed evenly throughout the clade, with a subclade of leafy treelets from a variety of habitats having the greatest number of endangered species. Extinction risk is distributed across all Pedilanthus-clade habitats; the strongest association noted is that both species from moist highlands are endangered.
Key Words: Cerro Espino • Euphorbiaceae • Euphorbia • extinction • Oaxaca • Pedilanthus • threatened plants
Less than a quarter of vascular plants have been evaluated for their conservation status, with 33 000 of these classified as rare or threatened with extinction (Walter and Gillett, 1998
). Detailed knowledge of a species is needed before effective management decisions can be made, yet such documentation is extremely heterogeneous. Whereas some species are represented by thousands of specimens, most biologists can name species known from few collections or few localities. For example, in a survey of 317 neotropical plant species in 15 genera, nearly 55% were known from two or fewer specimens (Madriñán-Restrepo, 1996
; fide Donoghue and Alverson, 2000
). In many cases, these collections are old and the locality data on the labels may be extremely vague. Such species are for all practical purposes lost to management.
One example of a plant "lost" until recently is the slipper spurge Euphorbia conzattii V. W. Steinm. We present an account of the rediscovery of this species on a mountain in Oaxaca, Mexico, along with assessment of its conservation status, using the Method for Evaluation of Risk of Extinction for Mexican Wild Species (MER; SEMARNAT, 2002
). We also provide preliminary evaluations of the conservation status of the other species of the slipper spurge clade (formerly Pedilanthus) in Mexico. All of the narrow endemic slipper spurges are restricted to Mexico, and thus their conservation there is of central interest. Of the 15 species of slipper spurge, a MER assessment has been conducted only for E. coalcomanensis (Croizat) V. W. Steinm., which was found to be threatened (Lomelí-Sención and Sahagún-Godínez, 2002
). We show that other species in the group range from critically endangered to relatively risk-free.
We selected the MER for our study because it has been required by Mexican law since 2002 for listing organisms for protection. Many methods for assessing extinction risk have been proposed, from diversity-driven "hotspot" methods (e.g., Meijaard and Nijman, 2003
) to those that build on population genetic studies (e.g., Cavers et al., 2003
). These methods were rejected as the sole basis for the MER in favor of one that would provide a reasonably reliable way to identify species of conservation concern in the face of rapid extinction rates in a large, extremely diverse country. We note aspects of the MER resulting from this strategy that can make it vulnerable to subjectivity or misinterpretation and suggest that the MER nevertheless likely can meet its intended goals of facilitating timely conservation decisions and generating testable hypotheses. We also make specific suggestions that attempt to clarify points that are vague in the original formulation of the MER.
The slipper spurges were until recently grouped in the genus Pedilanthus. This neotropical group forms a well-supported clade nested within Euphorbia, a finding that necessitated subsuming within Euphorbia (Table 1; Steinmann and Porter, 2002
; Steinmann, 2003
). We refer to these species informally as the Pedilanthus clade. Mexico is the only country in which all 15 species may be found; 12 are endemic (Dressler, 1957
; Webster, 1994
). Most species occur in seasonally dry tropical habitats. In Mexico as in most of the world, such areas are densely populated, and thus are subject to intense pressure from human activities. In addition to being of conservation concern, the Pedilanthus clade is an attractive study system for evolutionary studies. The remarkable array of life forms, from leafy treelets in evergreen moist forests to small-statured leafless stem succulents in true deserts, makes the Pedilanthus clade exceptionally diverse morphologically (Fig. 1A–C). Likewise, the spurred zygomorphic involucre that is an unequivocal synapomorphy of the group varies from bird- to insect-pollination (Fig. 1D–G; Dressler, 1957
; Sahagún-Godínez and Lomelí-Sención, 1997
). Economic uses include rubber and waxes from several species (Sternburg and Rodriguez, 1982
; Sternburg, 1984
), and E. tithymaloides is a widely cultivated houseplant.
|
|
and modifications from ongoing molecular phylogeny reconstructions, the Pedilanthus clade consists of four major groups. Euphorbia finkii is apparently the sister taxon to the rest of the group, which consists of two large clades and one small one. The small clade is made up of E. tithymaloides and E. personata. The two large clades both have six species. One is a clade of mostly leafless succulent species with minimal amounts of xylem and large, water-storing pith and cortical regions (Fig. 1C). The final clade is made up of leafy shrubs or treelets that have more conventional proportions of xylem (Fig. 1A, B). The relationships within and between these clades have yet to be resolved, but this level of resolution is sufficient for the question of whether or not extinction threat in the Pedilanthus clade is more likely in one subclade than another. Similarly, we examine associations between extinction risk and habitat type, testing the idea that extinction risk is greater in species that are restricted to flat country prone to urbanization, whereas species in rugged low country or highlands should be at less risk. Finally, we provide comments on the implications of the conservation status of the species of the Pedilanthus clade for the areas that they occur in as a whole.
Euphorbia conzattii was first collected in April 1917 from a locality specified only as "Cerro Espino, Dto. Pochutla, Oaxaca; Elevation 1200 m." There is little doubt as to the general locality, which is in a well-known coffee-growing area in southern Oaxaca. However, as Dressler (1957)
noted, both the type specimen (US) and the isotype (MEXU) are mixed collections, with well-dried samples of E. conzattii mounted with browned, overdried twigs of E. calcarata Schltdl. Dressler (1957
, p. 112) notes that, "There must thus remain some doubt as to whether they were collected together and as to the correctness of the data." We present evidence that Cerro Espino is the type locality of E. conzattii and discuss the mixture of the type with E. calcarata.
MATERIALS AND METHODS
MER methodology (SEMARNAT, 2002
) consists of four risk criteria, each of which is divided into risk categories with numerical scores, with higher numbers denoting higher risk. The total score is calculated by summing the results from the following four criteria scored as follows: (A) Geographical distribution is assessed as a percentage of the total area of Mexico (
1 953 162 km2, SEP, 1996
). Species that have a distribution that is <5% of the national territory are considered highly restricted and are assigned a risk factor of 4; those between 5 and 15% are considered restricted, risk 3; those between 15 and 40% are considered widely distributed or only moderately restricted, risk 2; those >40% are considered widely distributed, risk 1. (B) Habitat condition is an estimate of the current biotic and abiotic factors that are known or thought to be important for the species being assessed. Note that this assessment is carried out with respect to the taxon under consideration and is not an assessment of the general state of the habitat. The following scores correspond to the risk scores in parentheses: hostile or highly limiting (3); intermediate to limiting (2); favorable or only slightly limiting (1). (C) The intrinsic biological vulnerability of a species includes factors such as reproductive strategy, life history, phenology, habitat requirements, genetic variation, recruitment rates, and the need for nurse plants, and has three score levels: 3, high; 2, medium; and 1, low. (D) Finally, the impact of human activity reflects factors such as proximity to human settlements, habitat fragmentation, pollution, commercial or other uses, land use patterns, invasive species, and the construction of highways and power lines. Like the previous criterion, there are three risk categories, but the scores begin at 2 because it is assumed that all species are affected to some extent by human activity: 4, high; 3, medium; and 2, low. Total scores between 12 and 14 are considered in danger of extinction; those with a score of 10 or 11 are threatened, and those of 9 or below are considered to be of little current risk.
We evaluated E. conzattii using these criteria on four field trips to the Cerro Espino area in January 1999, June 2002, December 2002, and January 2003. We measured the geographical distribution of E. conzattii on a 1: 50 000 scale topographic map (Instituto Nacional de Estadística, Geografía e Informática, Mexico) using the image analysis software Carnoy 2.0 (Laboratory of Plant Systematics, Katholieke Universiteit, Leuven, Belgium). Five measurements were taken and the mean used as our estimate.
We also applied the MER procedure to the 13 other species of the Pedilanthus clade that had not been assessed to date. We offer these assessments as preliminary but feel that the obviously dire conservation situation of many of the species justifies them. The basis for the estimates of range were herbarium specimen labels and maps of geographic distribution generated with this information. Habitat condition and intrinsic biological vulnerability were estimated based on our field work and herbarium label information. We examined material from the following herbaria: ARIZ, C, CHAPA, CHIP, CIDDIR, CIMI, CICY, CR, CREG, ENCB, F, FCME, FLAS, G, GH, GUADA, HCIB, HUAP, IBUG, IEB, IJ, JBSD, K, MAPR, MEXU, MICH, MO, NY, OAX, P, PH-PENN, RSA, S, TEFH, U, UAMIZ, UC, UPR, US, W, and XAL. Herbarium abbreviations follow Holmgren et al. (1990)
. Critical cited specimens are listed in Appendix 1 (see Supplemental Data accompanying online version of this article).
RESULTS
MER assessment for E. conzattii
Geographical distribution
Euphorbia conzattii is known only from the peak of a single mountain, known as Cerro Espina to local residents (but Cerro Espino on maps), in the municipalities of Pochutla and Pluma Hidalgo, Oaxaca, Mexico. Our GPS readings put the elevation of the peak at 1420 m; we encountered the plants only within an elevational band reaching from the peak to 1380 m. Though E. conzattii is by no means evenly distributed throughout this area, we present a generous estimate of the range of E. conzattii as the area of Cerro Espino above 1380 m (Fig. 1). The forest in this area is dense but low, with few trees exceeding 8 m. Our visit was in the dry season, but the area receives abundant moisture in the rainy season, as suggested by abundant ephiphytic ferns, bromeliads, orchids, Peperomia, Anthurium, mosses, and lichens. We estimate the peak area above 1380 m to be 0.02 km2. The southern, northern, and eastern shoulders of Cerro Espino are below the elevation at which we encountered E. conzattii, but they do share to some extent the lower, drier vegetation of the high peak that distinguishes it from the tall forest on the lower slopes. Nevertheless, we did not observe E. conzattii in these areas. Though the Sierra Madre del Sur rises abruptly several kilometers to the north of Cerro Espino, reaching and surpassing 1400 m a.s.l., E. conzattii is apparently unknown in the vicinity of Pluma Hidalgo and the neighboring coffee plantations in the Sierra closest to the Cerro Espino locality. This microendemic species thus occupies a range that is approximately 0.000001% of the area of Mexico and thus it is rated as 4, the highest risk factor.
Condition of habitat
Euphorbia conzattii grows only in shaded areas in pockets of soil on the otherwise rocky limestone surface of the mountain, conditions absent over most of the peak area. Because of this apparent scarcity of suitable microsites, we assess the condition of habitat as intermediate and assign it a score of 2 (Table 2).
|
Impact of human activity
Human use of the peak is currently minimal. No trails exist to the summit, and hunting and wood collecting are limited to the slopes. The forest of the peak area is in good condition, with naturally fallen trees permitted to rot in place. However, as with the lower slopes of Cerro Espino and indeed the entire region, coffee plants are frequently encountered all the way to the peak. We received conflicting reports regarding the origin of these peak plants. Some reported that coffee was planted all over the mountain, including the summit area, whereas others reported that only the lower slopes were utilized, implying that the coffee plants at the summit are feral. In either case, coffee is firmly established on the summit of Cerro Espino. Because a significant invasive plant population is present in the tiny habitat of E. conzattii, our assessment is that the impact of human activities is intermediate (risk 3). This assessment assumes that E. conzattii has always been restricted to the high peak. If it truly was present at 1200 m, as reported by Conzatti, Reko, and Makrinius on the type specimen, then its range has been restricted from ca. 1 km2 to 0.02 km2 (a 50-fold reduction in area) and the impact would be assessed as high. Because the report of 1200 m was either based on an estimate or on a barometric altimeter that was subject to pressure fluctuations, for the purposes of the MER, we prefer the more cautious estimate of an intermediate impact level.
Taxonomic, cultural, and economic significance
In addition to being of interest for the genus-wide characteristics discussed earlier, E. conzattii is of particular interest for phylogenetic studies. In habit, it is most similar to E. calcarata or some populations of E. peritropoides, although it is much smaller, more delicate and intricately branched. Dressler (1957)
notes similarities to E. coalcomanensis in that the ovary remains within the involucre instead of being exserted at anthesis as in most other species of this clade. Dressler (1957)
also notes similarities to E. tithymaloides L. and comments that E. conzattii provides a "link" between these species groups. The availability of fresh material of E. conzattii makes it possible to test these ideas. Local residents were not aware of its presence on Cerro Espino, even when shown samples of the plant. Therefore, we assume that there are no current local uses for the plant. However, because of its delicate habit, brightly colored inflorescences, slow growth, and tendency to flower when very small, E. conzattii has great horticultural potential. The wild population of this species is so small that live material of E. conzattii should be collected in an extremely responsible manner. We have found that cuttings readily root, so it should not be necessary to remove the scarce established plants.
The mixed type of E. conzattii
We consider the uncertainty Dressler (1957)
noted with regard to the type locality of E. conzattii to be resolved by the finding that the species is restricted to the summit of Cerro Espino. Likewise, the mixing of the type of E. conzattii with E. calcarata is understandable, given the overlapping flowering times and proximity of localities. Even though it occurs as close as 2 km to the summit locality of E. conzattii, E. calcarata is only found at lower elevations. We collected E. calcarata in three localities on the upper-middle elevations of Cerro Espino (see critical specimens in Appendix 1; see Supplemental Data accompanying online version of this article), where it grows under the canopy of tall tropical subdeciduous forest or tropical deciduous forest at an elevation of 1040–1320 m. How the tropical deciduous vegetation differs from that of the peak and why the species are allopatric is not clear. Dressler (1957)
cited two specimens from somewhat lower on the mountain (Reko 3574 and Morton and Makrinius 3678). In addition, coincidence in the flowering period of E. conzattii and E. calcarata easily could have led to their being mixed on the type of E. conzattii. Euphorbia conzattii was collected in flower on 12 April 1917, and Reko's collection E. calcarata 7 d before (3574 US!) was also in flower. Thus, though the species are allopatric, they grow on the same mountain and probably were confused and collected as the same species. The differences between the species are less conspicuous than their similarities, and the distinctness of E. conzattii was not recognized until 1957.
Description of E. conzattii
Because of our recent collection, we can now supplement the original description with features that are unavailable from the type (Appendix 2, see Supplemental Data with online version of this article).
MER assessments of other Mexican species of the Pedilanthus clade
The results for the other species examined are summarized in Table 2; explanations for the decisions behind the risk factors assigned are given in Appendix 3 (in Supplemental Data with online version of this article). We found that E. dressleri is probably entirely extirpated and E. cyri is extinct in the wild but persisting in cultivation. Three other species are threatened. The distribution of risk ranks with habitat preference is summarized in Table 3, and the distribution of risk ranks in the clades within the Pedilanthus clade is summarized in Table 4.
|
|
There is no substitute for population genetic and demographic information in the management of threatened species. But in a country as diverse as Mexico, such detailed studies cannot encompass the entire flora. The MER protocol appears to offer a realistic balance between traditional methods in which conservation risk is assessed subjectively with unspecified criteria and methods that provide more information but also require time and funding that may not be available. The assessments summarized here were based on 10 field trips and a review of herbarium specimens; identifying priority species for further study was a generous return for this time investment.
Robustness of MER assessments
Our assessments raise the question of how trustworthy MER results are. We comment here on MER performance based on our work and make suggestions to resolve points that are particularly prone to confusion.
Geographical distribution, measured as a percentage of the total area of Mexico, is the criterion that is most amenable to quantification. This category is also the one most directly afflicted by lack of knowledge about extant populations. Discovery of additional populations similar to known ones would not necessarily change scores in the other criteria substantially, but could result in a lower score for the distribution criterion. This is a crucial point, because a small distribution is a major factor contributing to the status of species of conservation concern (e.g., Table 2). On the other hand, discovering more populations of the many microendemic species in Mexico is unlikely to put them in a lower risk rank, because their ranges are so small that it is unlikely that they will be found to have distributions over 5% of the national territory. For example, finding another population of E. conzattii similar to the known one would double its known range, which would still be exceptionally small.
Habitat condition and impact of human activity are the most difficult criteria to separate. SEMARNAT (2002)
gives little guidance on the distinction between these categories, beyond what is summarized in the Materials and Methods and the rather vague note that they are independent "in general terms." Clarifications, based on our experience, could include the following: In general, the habitat of a species is favorable in the middle of its distribution. In most cases, if anthropogenic degradation is ignored, it is not surprising to find that, with respect to the known or hypothesized needs of a plant, conditions are favorable. Cases in which all populations of a species have been destroyed or leave only marginal peripheral populations or scattered waif individuals is a situation in which the condition of habitat could be assessed as hostile. We assigned species to the intermediate risk category if the conditions of the microhabitat were favorable, but such microsites were rare within the range (e.g., for Euphorbia personata). Intrinsic biological vulnerability must examine only factors that are inherent to the biology of a species independent of the condition of habitat. Euphorbia conzattii is an example of a species with high intrinsic vulnerability that occurs in a habitat of intermediate condition and low human impact.
Given subtle delineations between categories and ultimately subjective assignment of risk score, what effect might these potential pitfalls of the MER have? That is, how likely is it that species assessed to be endangered are not, and how likely is it that species that are endangered are assessed as being safe? Table 2 reveals few surprises: the species with the highest overall risk scores all have very small ranges, many have intrinsic factors that conspire against them in resisting disturbance, and all are impacted by humans. For reasons detailed later, other populations of E. conzattii are not likely to be found; none have appeared in 80 years of botanical exploration of southern Oaxaca. Even if other populations are found, they are probably on similarly small mountaintops and E. conzattii will remain a microendemic in need of protection. Likewise, based on extensive documentation of the flora of the well-studied Valley of Oaxaca, E. cyri is clearly being decimated if not already extirpated. It is not clear what discoveries, apart from the extremely unlikely event of locating previously overlooked large, healthy populations, could change the conservation status of such species, as assessed by MER or other methods. On the other hand, our low risk assessments for species such as E. tithymaloides and E. calcarata are also likely to be robust; these species have wide ranges, moderate capacity to tolerate disturbance, and recruit well.
If, as we suggest here, high MER scores do indeed reflect situations of urgent conservation concern, then scores should be roughly comparable across all taxa. This is likely true: virtually any species with a tiny range, low population number, slow growth, low recruitment, and co-occurring with an invasive species is likely to be threatened. Likewise, species that are no longer found in the wild are obviously of great conservation concern. The MER scores reflect these expectations.
Clade- and habitat-related patterns
Extinction risk is not uniformly distributed across clades or habitats, and the patterns recovered here differed notably from our expectations. The clearest habitat-related trend is that both species in moist highlands are threatened (Table 3). Though such habitats may be found throughout tropical America, patches are often very limited in size and are under heavy pressure from agriculture, resulting in small populations that are easily eliminated. Within dry habitats, we expected flatland species, which are extremely vulnerable to development, to be much more likely to be menaced than those from rugged areas. Although we noted this trend, it was less intense than expected; two of the five flatland species were threatened relative to two of the six rugged dryland species. Likewise, we expected more species from the succulent clade to be threatened than those from the leafy clade (Table 4). This expectation was based on habitat preferences: 67% of the succulent clade are found in flatlands, whereas none of the leafy clade species occur in these habitats. Also, succulent clade species receive some pressure from collection for ornamental use. However, just 33% of the succulent clade species are threatened, compared with 67% of the leafy clade, which occur in moist canyons, rugged drylands, and moist highlands. All of the threatened species have very small ranges, and a tendency for small ranges is apparent in the leafy clade.
The future of E. conzattii
We remain hopeful that E. conzattii will be located elsewhere. Where it might be is not clear. Cerro Espino is an imposing peak rising from the warm coastal lowlands; based on its observed elevational range, E. conzattii presumably requires a minimum elevation of ca. 1400 m. However, the other isolated peaks in the vicinity of Cerro Espino are well below this elevation. Cerro Sinaloa rises a few kilometers SSE of Cerro Espino but is nearly 500 m lower; Cerro Huatulco lies approximately 8 km E of Cerro Espino, but fails to reach the elevation of E. conzattii by 300 m. No other isolated peaks in the area approach the elevation of Cerro Espino. In the wake of Hurricane Paulina and recent economic downturns, coffee cultivation is currently on the decline in this area. A potential benefit is the return of native vegetation. A chilling prospect is the introduction of crops that require complete destruction of the forest such as maize.
Conclusion
Alhough we feel that our MER assessments are likely robust, we regard them as hypotheses for further testing through searches for additional populations and studies of population genetics and demographics. Our hypotheses could be falsified by any number of findings, such as thriving populations of E. dressleri or E. cyri. The most important test is a long-term one; the loss of any more species of the Pedilanthus clade will mark a failure to recognize vulnerable species or implement effective protection.
We are aware of the problems associated with the assessment of threats at community or higher scales based on exemplar species (e.g., Lindenmayer and Fischer, 2003
). However, we do suggest that the threats outlined against Pedilanthus clade species may be symptomatic of the overall state of conservation of the areas in which they are found. For example, the generalized perturbation apparently responsible for the extinction of E. dressleri is also quite likely to be adversely affecting many other species. Likewise, other species that share the plains or low-lying habitats of E. cyri and E. tehuacana are certainly suffering as well. Pedilanthus clade species serve well to highlight areas of conservation concern because they are relatively easy to locate and local people are often aware of their presence. If the unique inflorescence fails to catch attention, then the abundant latex that pours from the slightest wound impresses itself upon locals. Far from being weedy, most species tolerate only a moderate degree of disturbance. Therefore, our intent is not only to call attention to species such as E. dressleri, E. cyri, and E. conzattii as being of conservation concern, but also to highlight that the biological communities in which these species are found are also likely in danger.
|
|
1 The authors thank Pablo Carrillo R., René León M., Elizabeth Ramírez M., and Raymundo and Vicky Ramírez for their help in the field, the directors of herbaria who provided the specimen loans for this study. Fieldwork was supported by National Geographic Society Committee for Research and Exploration Grant 7400-03, the Dirección General de Asuntos del Personal Académico/Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica, UNAM, the Universidad Autónoma de Guadalajara, the Idea Wild Foundation, and the Cactus and Succulent Society of America. 
4 Current address: Department of Botany, University of Wisconsin-Madison, Birge Hall, 430 Lincoln Dr., Madison, WI 53706 USA
5 Author for reprint requests (E-mail: explore{at}explorelifeonearth.org
)
LITERATURE CITED
Anonymous. 2002 Global strategy for plant conservation. The Secretariat of the Convention on Biological Diversity and Botanic Gardens Conservation International, Surrey, UK
Carauta J. P. P. 1989 Ficus (Moraceae) no Brasil: conservaçao e taxonomia. Albertoa 2: 321-322
Cavers S. C. Navarro A. J. Lowe 2003 A combination of molecular markers identifies evolutionarily significant units in Cedrela odorata L. (Meliaceae) in Costa Rica. Conservation Genetics 4: 571-580
De Ávila B A. 1998 El nombre de la yerba. Citas y apuntes para acompañar la dendrología de fray Juan Caballero. Biblioteca Francisco de Burgoa, Exconvento de Santo Domingo, Oaxaca, Mexico
Donoghue M. J. W. S. Alverson 2000 A new age of discovery. Annals of the Missouri Botanical Garden 87: 110-126[CrossRef][ISI]
Dressler R. L. 1957 The genus Pedilanthus (Euphorbiaceae). Contributions from the Gray Herbarium of Harvard University 182: 1-188
Holmgren P. K. N. H. Holmgren L. C. Barnett [eds.] 1990 Index herbariorum; part I. The herbaria of the world, 8th ed. New York Botanical Garden, Bronx, New York, USA
Lindenmayer D. B. J. Fischer 2003 Sound science or social hook— a response to Brooker's application of the focal species approach. Landscape and Urban Planning 62: 149-158[CrossRef][ISI]
Lomelí-Sención J. A. E. Sahagún-Godínez 2002 Rediscovery of Pedilanthus coalcomanensis (Euphorbiaceae), a threatened endemic Mexican species. American Journal of Botany 89: 1485-1490
Madriñan-Restrepo S. 1996 Systematic studies in Lauraceae. Monograph of Rhodostemonodaphne. Ph.D. dissertation, Harvard University, Cambridge, Massachusetts, USA
Meijaard E. V. Nijman 2003 Primate hotspots on Borneo: predictive value for general biodiversity and the effects of taxonomy. Conservation Biology 17: 725-732[CrossRef][ISI]
Rzedowski J. 1983 Vegetación de México, 2nd ed. Limusa, México City, México
Sahagún-Godínez E. J. A. Lomelí-Sención 1997 Pedilanthus diazlunanus (Euphorbiaceae): pollination by hymenopterans in a bird-pollinated genus. American Journal of Botany 84: 1584-1587[Abstract]
SEMARNAT (Secretaría del Medio Ambiente y Recursos Naturales). 2002 Norma oficial mexicana NOM-059-ECOL-2001. Protección ambiental. Especies nativas de México de flora y fauna silvestres. Categorías de riesgo y especificaciones para su inclusión, exclusión o cambio. Lista de especies en riesgo. Anexo normativo I, método de evaluación del riesgo de extinción de las especies silvestres en México (MER). Diario oficial de la Federación, Mexico City, Mexico
SEP (Secretaría de Educación Pública). 1996 Atlas de México. Educación Primaria. SEP, Mexico City, Mexico
Steinmann V. W. 2003 The submersion of Pedilanthus into Euphorbia (Euphorbiaceae). Acta Botanica Mexicana 65: 45-50
Steinmann V. W. J. M. Porter 2002 Phylogenetic relationships in Euphorbieae (Euphorbiaceae) based on ITS and ndhF sequence data. Annals of the Missouri Botanical Garden 89: 453-490[CrossRef][ISI]
Sternburg C. F. 1984 Biochemical evolution of hydrocarbons in Pedilanthus (Euphorbiaceae). Ph.D. dissertation, University of California, Irvine, California
Sternburg C. F. E. Rodriguez 1982 Hydrocarbons from Pedilanthus macrocarpus (Euphorbiaceae) of Baja California and Sonora, Mexico. American Journal of Botany 69: 214-218[CrossRef][ISI]
Walter K. S. H. J. Gillett [eds.] 1998 1997 IUCN Red list of threatened plants. IUCN-The World Conservation Union, Gland, Switzerland
Webster G. L. 1994 Synopsis of the genera and suprageneric taxa of Euphorbiaceae. Annals of the Missouri Botanical Garden 81: 33-144[CrossRef][ISI]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
