ABSTRACT:
Natividad Island, situated 7 km off Punta Eugenia, has an area of 7.2 square km. At least 70 native and naturalized vascular plant taxa have been documented for the island, representing 29 families and 62 genera. Although none are restricted to Natividad Island, at least 2 taxa are endemic to the California Islands. Desert scrub vegetation, dominated by drought­resistant shrubs and cacti, covers most of the island. Spectacular displays of native annuals like Chaenactis lacera and Coreocarpus parthenioides var. involutus can be seen in years with adequate rainfall. Disturbed areas on the south end of the island are periodically dominated by non­native plant taxa like Chenopodium murale, Malva parviflora, and Mesembryanthemum crystallinum. Off­road motor vehicle use and the recent introduction of goats and sheep represent serious threats to the terrestrial ecosystem.

ABSTRACT:
The San Benito Islands, comprising 3 islets with a combined area of 6.4 square km, are located 27 km west of Cedros Island and 66 km from the Baja California peninsula. A total of 48 native and naturalized plant taxa, representing 25 families and 46 genera, have been documented for the 3 islets. At least 4 taxa are endemic to the San Benito Islands (Cryptantha patula, Dudleya linearis, Hemizonia streetsii, and Mammillaria neopalmeri), while 3 additional taxa are found only on 2 or more of the California Islands. Desert scrub vegetation covers most of the island and annual wildflowers like the endemic Hemizonia can be abundant in years with adequate rainfall. Introduced burros, goats, and rabbits are serious threats to the terrestrial ecosystem. Rabbits are currently decimating populations of native plants, especially Dudleya linearis and Euphorbia misera.

ABSTRACT:
Islands in the Gulf of California have been explored sporadically. An inventory was published by Ivan M. Johnston, who explored most of them in the spring of 1921. The most intensive exploration was made by Reid V. Moran, who made many trips in all of the islands between 1950 and 1964, and whose work represents the main reference for the following study.

There are about 37 islands greater than 1 km² surface area, and almost 60 little islets of relative small dimensions. All of them sustain plant life in variable diversity, depending on their size.

Most of the islands are geological remnants of the peninsula's separation from northwest Mexico. This is a continuing process that began some million years ago when the peninsula began to separate from the coast of northwestern Mexico. Some of them have volcanic origin. The first cause of formation noted means the islands were part of the peninsula, whose separation was caused when a sinking coastline, or a rising sea, inundated a projection of land and left a seaward area isolated.

The relative low rate of endemism of plant life is in accordance with that hypothesis. But, the relative nearness of the islands from the peninsula could allow a continuing flow of genes between the island populations and those on the peninsula. This may have avoided an effective genetic isolation, the first condition for speciation and endemism developing.

Since late 1995, the botany group of CIBNOR has made several explorations on the islands of Baja California Sur in the Gulf of California. The islands visited have been: Cerralvo, Espíritu Santo (including Partida), San José (including San Francisco), San Diego, Santa Cruz, Catalina, Monserrat, Danzante, Carmen, Coronados, and Tortuga. The present research embodies the results from collections, field observations, vegetation sampling, bibliographic references, and herbarium studies.

The study of the islands is clearly a longterm project. The present research shows only the achievements of less than three years of study.

The floristic richness on each island depends on the distance to the mainland, the size, variety of habitats, and elapsed time from any catastrophic event, but it always is poorer, to some extent, if compared with the nearest mainland area. Because of the relative poverty of species, the islands are susceptible to modifications by aliens, either grazing animals or other plants, or indirectly by the elimination of a pollinator, whose absence depresses the reproductive capacity of one or several populations.

Our contribution to the knowledge of the plant diversity in these islands has been increased about a 7% in relation with the previous report. We thought San José, the largest island, might contain the highest plant diversity, but Espíritu Santo and Cerralvo, more influenced by tropical storms, have revealed the greatest diversity. Two new species of legume herbs were found. They are now in the process of description.

One of the more appropriate aspects to consider in the structure of the plant communities is the study of the physiognomy or the form of the plants that make up the community. One of the most important classes or kinds of form in plants is referred to as "life forms or growth forms". The life-form rate reveals the influence of the environment as the main force that selects the adaptations of the plant life in a plant community. One of the most practical life-form classifications is that proposed by Shreve & Wiggins (1964) for the plants of the Sonoran Desert. On the basis that, for each species there is a corresponding specific life-form, the rates for each island considered have been obtained.

The annuals (or ephemerals) and the perennial herbs (i. e. plants perennial from rootstocks, whose stems and leaves are renewed annually) are the most abundant life-form groups. Shrubs and trees, the most evident forms, are moderate in number.

We took vegetation samples on most of the islands following standardized techniques, which consisted of plots 5m wide and 200m long (1000 m2) following the rise in height of a typical hill on each island. The floristic and structural data in each sampled plot consisted of obtaining from all of the woody plants (some perennial herbs, shrubs, and trees) the following data: species abundance, mean height per species, mean crown cover per species, and growth form. We measured the dominance of each plot as the product of NI (number of individuals for each species), its mean height, and crown cover.

The "torote" (Bursera microphylla) is the only species which is structurally dominant in all of the samplings, "matacora" (Jatropha cuneata) appeared in four samplings; and "palo adán" (Fouquieria diguetii), "pitaya ágria" (Stenocereus gummosus), "palo fierro" (Olneya tesota), and "palo blanco" -(Lysiloma candida) appeared in two samplings. These could be considered as the physiognomically more important species.

Despite the increase in the knowledge of the plant diversity on the islands studied, we think the basic inventory of flora is still incomplete.

Compared with the flora of the peninsula, the spatial arrangement of the plants in the islands is ruled by a handful of species. The natural balance in structure and composition seems to be more fragile than in any peninsular community. This fact should be of special concern for conservationism. On other islands, such San Pedro Nolasco, in recent decades the natural population of the cacti Opuntia bravoana has practically disappeared because of an infestation of a species of parasitic dodder and also apparently because of some kind of insect. No one knows if this is a natural process or induced indirectly by human traffic.

Human activities have altered the natural resources of the all the islands in the Gulf of California over the past four centuries, and intensely during the last few decades. The populations of domestic goats on the islands should be considered a threat for many plants, especially the succulents. The population of the domestic cat seems to be high enough to be a threat to the reptiles (most of the species are endemic) and to a lesser extent, birds.

We hope the regulations for the islands, which began with the protection for migratory waterfowl on a single island early the 1960s, and giving extensive status for all of them in the late 70s, and finally the decree of the islands as a Biosphere Reserve, a few years ago, can be effective efforts to protect all of their biota.

The isolation that formerly protected the unique biota and its natural setting is gone. It is our responsibility to work for conserving this area for the future.

ABSTRACT:
The San Felipe Desert in the northeastern part of the Baja California peninsula is part of the Sonoran Desert region. Its arid vegetation corresponds to that of the flora of the Lower Colorado Valley.

The vegetation of this area was studied in order to describe it floristically and to analyze the plant communities by using-point centered quadrants and lineal interception methods. The results include absolute and relative frequency parameters as well as density, cover and importance values based on this data. The program SYNTAX was applied and a cluster analysis demonstrated floristic relationships and variants.

Taking into account ecological structure, floristic composition and distribution, seven plant communities are described: salt marsh, coastal dunes, basaltic, desert pavement, desert washes, martirense and sanfelipense.

Floristically, 356 taxa of vascular plants were registered in 69 families and 213 genera, of which 20 are endemic. The families Asteraceae and Fabaceae were found to be the most diverse with 14.32% (51) and 8.7% (31) respectively. With respect to life forms present in this area, herbaceous annuals proved to be the most abundant (142 taxa), followed by shrubs (126), then perennial herbs (69), trees (10) and vines (9).

A map of these vegetation communities is provided (1:50,000) which also indicates the areas either characterized by floristic richness or particular nature of habitat requiring conservation.

RESUMEN:
El Desierto de San Felipe ubicado en la parte noroeste del península de Baja California, queda ubicado dentro del llamado Desierto Sonorense, estando su vegetación formada por comunidades de zonas áridas, florísticamente ésta región se incluye dentro de la subdivisión del Valle Bajo del Colorado (Lower Colorado Valley).

La vegetación del áreas es estudiada con el propósito de describir y analizar las comunidades vegetales aplicando los métodos de cuadrantes centrados en un punto e intercepción en línea, obteniendo los parámetros absolutos y relativos de frecuencia, densidad y cobertura y el valor de importancia de las mismas. Los datos obtenidos fueron tratados con el programa SYNTAX y a partir de análisis de cluster se detectaron algunas asociaciones y variantes florísticas.

Se describen siete comunidades vegetales. saladares, dunas costeras, basaltos, pavimentos, aluviones, martirense y sanfelipense; tomando en cuenta sus aspectos ecológicos, estructura, composición florística y distribución.

Dentro del componente florístico se lograron registrar 356 taxa de plantas vasculares agrupadas en 69 familias, 213 géneros y 20 taxa endémicos. Las familias con mayor diversidad son Asteraceae y Fabaceae, las cuales constituyen un 14.32% (51) y un 8.70% (31) respectivamente del total de la flora. Dentro de las diferentes formas de vida que se presentan, las hierbas anuales son las más numerosa con 142 taxa, seguidas por arbustos (126), hierbas perennes (69), árbol (10) y trepadoras (9).

Se incluye un mapa de vegetación y comunidades (1:50,000) donde se muestran además, las áreas que por su riqueza florística encontrada y calidad del hábitat, son susceptibles a conservar.

RESUMEN:
Se presentan resultados parciales del proyecto "Flora y vegetación de la cuenca del Río Fuerte, Estado de Sinaloa, México", iniciado por los autores en 1994, en colaboración con la Universidad Autónoma de Nuevo León y la Universidad de Occidente-campus Los Mochis. Este estudio se desarrolló en la cuenca baja, donde más del 80% se encuentra desprovista de su vegetación original debido principalmente al gran desarrollo agrícola iniciado en la década de los cincuentas.

El área se ubica en el norte del Estado de Sinaloa, en la región fisiográfica de la Planicie Costera Noroccidental (Rzedowski, 1978), a los 25 grados 50 minutos de Latitud Norte y 109 grados de Longitud Oeste. Aunque algunas cimas alcanzan los 350 msnm, la mayor parte no sobrepasa los 15 msnm. Su clima es del tipo seco-árido (Bw) según García, 1981. Los suelos varían desde litosoles en lugares cerriles, regosoles eutricos, vertisoles crómicos hasta solonchaks hacia la línea de costa.

Se sitúa en la Región Xerofítica Mexicana, Provincia Florística de la Planicie Costera del Noroeste (Rzedowski, 1978). Los principales tipos de vegetación reconocidos son el bosque espinoso, vegetación acuática y de galería.

Desde la perspectiva histórica, social y étnica la región en épocas prehispánicas formaba parte de una confederación cuyo propósito era la defensa conjunta y el intercambio comercial, esto último basado en los propios recursos naturales y en su agricultura (Morales-Garduño, 1982). Como otras etnias del noroeste de México, los "mayos" fueron de difícil sometimiento, lo que permitió preservar relativamente sus tradiciones culturales, mismas que aún se manifiestan en un profundo conocimiento del ambiente, en particular de las plantas silvestres regionales, aprovechadas para complementar sus necesidades de alimento, medicina, techo, instrumentos de caza, ornato e inclusive para comerciar algunas de ellas. Conocimientos que en la actualidad son aprovechados por los mestizos y algunos miles de auténticos "mayos" que habitan el norte del Estado.

Como producto de la exploración, colecta, identificación botánica y entrevistas abiertas con pobladores de la región en el período 1994 a 1996, además de la revisión de literatura y herbarios mexicanos, se presenta un listado de 25 especies de cactáceas y otras suculentas utilizadas por pobladores de esta región para fines ornamental, medicinal y alimenticio. Se anotan sus nombres científicos, familia, nombres locales y usos. Los sitios visitadas fueron Los Mochis, Topolobampo, San Miguel Zapotitlán, Barobampo, Juricahui, Zapotillo, El Porvenir, El Guayabo, Villa de Ahome e Higuera de Zaragoza.

ABSTRACT:
Each of the Pacific coast islands of Southern California and Baja California are unique with their own particular attributes. They vary greatly in size and distance from the mainland. Traditional analysis of island species diversity is based on the relationship between area and the number of species. However the widely differing topography, climatic conditions and geology which forms the structural basis of the islands, also have a very strong influence on the diversity of plant species. The islands are also home to a large number of species which are endemic to one or more of the islands, and not found on the mainland. The existence of these plants in the flora of the islands is the result of biogeographical effects in which many of the endemic species were once common on the mainland as evidenced by fossils. Certain genera of plants, particularly Dudleya, Cryptantha, Galium, Lavetera, Malacothrix, Phacelia, Quercus, Arctostaphylos, Lotus, Hemizonia and Eschscholzia are well represented in the endemic flora. Unfortunately, most of the islands have been subjected to serious environmental damage from long periods of vegetation destruction by feral herbivores. Recent successes in feral animal removal provides hope, but there is still a need for the removal of rabbits from Todos Santos and San Benito, and goats from Los Coronados where they are recently impacting the vegetation, and from Guadalupe where they have caused the extinction of plants and bird species.

ABSTRACT:
Islas Los Coronados consists of a group of four small islands in Mexico, near the International Border with the United States. Together they comprise approximately 2.5 square km and lie 13 km from the mainland. They range in length from 0.3 km for Middle Rock to 3.3 km for South Island. Vegetation is predominantly Maritime succulent scrub dominated by Encelia californica, Euphorbia misera, Hazardia berberidus, Atriplex canescens, Bergerocactus emoryi, Opuntia littoralis and Lycium californicum. However, on the South Island, the north facing slope takes on the aspect of Coastal sage scrub. Middle Rock and Middle Island have barren areas covered with bird guano, however, even they support patches of Maritime succulent scrub. Botanically, the South Island is the most diverse, supporting a number if insular endemics including Eschscholtzia ramosa, Dudleya anomala, Lavatera occidentalis, and Malacothrix foliosa. The Lavatera is noteworthy since it is also found on Isla Guadalupe which lies approximately 350 km to the south. Islas Los Coronados support two endemics, Galium coronadoense, which has only been found on the South Island and Dudleya candida which has been found on all four of the islands. Unfortunately, the South Island has a small herd of goats that are kept by the members of the military garrison. The herd has had relatively recent effect in reducing the areas of shrubs including the Lavatera.