Conservation challenges in a threatened hotspot: agriculture and plant biodiversity losses in Baja California,Mexico

Modern agricultural practices pose serious threats to biodiversity worldwide. Species losses from habitat conversion are well documented, but indirect impacts such as reduced water availability to adjacent ecosystems are less known. San Quintín is an important agricultural valley in the mediterranean climate region of Baja California, Mexico. The region is also a hotspot of plant species richness and endemism. Plant species in the region are here analyzed by comparison of the contemporary flora to historical botanical collections to identify extirpations. Historical collections indicate that habitat loss to agriculture has been a direct cause of species losses. As importantly, the unsustainable extraction of groundwater has apparently led to salt water intrusion, resulting in the loss of 22 native plant taxa, including 13 rare plants. Seventy-eight percent of all the vernal pool taxa have been lost from the flora (including 85 % of the rare taxa) and 11 % of plants of riparian and pond habitat (including 25 % of the rare taxa) are no longer found in the region. Unsustainable agricultural practices continue to threaten fragile coastal ecosystems and are a serious challenge to current and future conservation efforts. Ironically, these same practices frequently result in abandonment of cultivated areas. Owing to indirect impacts, conservation of biodiversity and large-scale agricultural operations are even less compatible on a regional scale than indicated by direct impacts. It is vital that sustainable agricultural practices be adopted locally and globally to avoid further losses of biodiversity.     

Water relations of stem succulent trees in north-central Baja California

Summary Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 to 4 kg H₂O/m³ of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H₂O/m³. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m^–2 s^–1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of elastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanism allowing for the maintenance of leaf turgor in these stem succulent trees. The low transpiration rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.     

A transitional xerophytic tropical plant community of the Cape Region,Baja California

Abstract. Our main objective was to use comparative floristic and structural criteria in order to understand the differences in assemblages brought about by the meeting of arid and dry‐tropical environments. We sampled 33 vegetation plots and recorded all perennial species. The data were subjected to multivariate analysis, including Principal Component Analysis (PCA). The floristic variation detected by the PCA was strongly related to altitude and rainfall. The gradient detected by the ordination procedure had Tropical deciduous forest (TDF) and Sonoran Desert (SD) at opposite extremes and Cape sarcocaulescent shrubland (CSCS) in an intermediate position. The numerical classification algorithm detected six distinct groups of species with clearly identifiable field distributions. Vegetation structure also differed significantly between the regions. SD had the lowest species‐richness (α‐diversity) and inter‐site similarity in the CSCS was highest, indicating that this environment is relatively uniform and has low species turnover (β‐diversity). It is concluded that the CSCS is distinctly different from the sarcocaulescent shrublands of the more northern plant communities of the peninsula, where Sonoran Desert floristic affinities prevail. The floristic composition of the CSCS is rich in dry‐tropical affinities, its species richness is higher, it is homogeneous in its species content and turnover and is more dense with a taller canopy than the northern desert scrubs. It is proposed that the boundary line separating the Cape sediments of granitic origin from the basalt‐derived sediments of the northern‐lying Sierra de la Giganta should be used as an easily identifiable landscape trait to delimitate this unique community.     

Detection of coccidioidal antibodies in serum of a small rodent community in Baja California,Mexico

Coccidioidomycosis (Valley Fever) represents a serious threat to inhabitants of endemic areas of North America. Despite successful clinical isolations of the fungal etiological agent, Coccidioides spp., the screening of environmental samples has had low effectiveness, mainly because of the poor characterization of Coccidioides ecological niche. We explored Valle de las Palmas, Baja California, Mexico, a highly endemic area near the U.S.–Mexico border, where we previously detected Coccidioides via culture-independent molecular methods. By testing the serum from 40-trapped rodents with ELISA, we detected antibodies against Coccidioides in two species: Peromyscus maniculatus and Neotoma lepida. This study comprises the first report of wild rodent serum tested for coccidioidal antibodies, and sets the basis to analyze this pathogen in its natural environment and explore its potential ecological niche.     

Xerie Mediterranean-type shrubland associations of Alta and Baja California and the community/continuum debate

A survey of the xeric shrublands of Pacific coastal North America from San Francisco to El Rosario (Mexico), including the inner Channel Islands, was conducted using 99 sample sites of 0.063 ha size. TWINSPAN classification and DECORANA ordination confirmed the existence of two plant formations, distinguishable physiognomically: coastal sage scrub and coastal succulent scrub. Within coastal sage scrub, four floristic associations were recognized: Diablan, Venturan, Riversidian and Diegan. Within coastal succulent scrub, two floristic associations were defined: Martirian and Vizcainan. These associations occur in distinct geographical regions following the coastline, with the Riversidian association occurring in the basin inland from Venturan and Diegan regions. Their locations are strongly correlated with differences in evapotranspirative stress regimes. The Channel Island sites show affinities to several of the mainland associations. The Venturan association can be further subdivided floristically into two subassociations, dominated by Salvia mellifera and S. leucophylla respectively. These subassociations which are coextensive geographically at a regional scale, typically do not intermingle at a local scale but often meet along sharp boundaries in the landscape. The dominant species segregate by moisture preference, S. mellifera preferring coarser-texture soils and more southerly aspects than S. leucophylla. Richness and equitability of these sites are depressed relative to other xeric shrubland sites, reflecting the fact that the two subassociations partition the Venturan flora into substantially non-overlapping subsets of species. This segregation of associates between the two Salvia dominance types strongly suggests biotic influence of the dominants on subordinate species, perhaps mediated by allelopathy. This biotic interaction, leading to relatively strong floristic subassociations segregating independently in the landscape, would provide an example of the holistic community structure referred to by Clements and his followers, embedded within a larger pattern of continuity in species distributions.Nomenclature follows Munz & Keck (1959), Munz (1974) and Wiggins (1980).I am grateful to the following for research assistance: S. Coon, E. Hobbs, S. Lavinger, J. F. O'Leary, K. R. Preston, B. Rich and A. Troeger. I also thank the numerous public and private land owners who permitted access to the study sites. This research is based upon work supported by the National Science Foundation under grant DEB 76-81712.     

Ecological effects of seaweed harvesting in the Gulf of California and Pacific Ocean off Baja California and California

The ecological effects of harvesting were investigated for two species, the giant kelp Macrocystis pyrifera and Eucheuma uncinatum, by using harvest records and aerial surveys that show yield changes and by using studies of recruitment, survivorship and community interactions. There were no ecological effects found for M. pyrifera harvested with contemporary methods, and there are no recommendations for changing current management practices for this species. It is recommended that E. uncinatum, which on occasion produces commercially significant standing crops, not be harvested unless special precautions are taken to leave some plants for regeneration of the harvested populations.     

Deep genealogies and the mid-peninsular seaway of Baja California

Geological forces and long-term climate changes can have profound effects on species. Such effects may be manifested in the pattern and magnitude of genealogical diversity, as revealed by mitochondrial DNA (mtDNA) lineages. The relative importance of the different forces on a regional biota must be evaluated along with a good understanding of geological and climatological history. The peninsula of Baja California of north-western Mexico is one area where both geology and climate have affected the historical biogeography of the regional biota. Molecular studies based on the genealogical relationships among mtDNA lineages have contributed greatly towards elucidating the historical biogeography of Baja California. Perhaps most noticeably, numerous concordant breaks in mtDNA genealogies half-way along the peninsula suggest a vicariant history in which the mid-peninsula was temporarily submerged. This vicariant explanation has recently been criticized, as no conclusive geological evidence exists for a continuous submergence of the mid-peninsula. As an alternative, a scenario based on climatological factors has been suggested. Here we discuss the validity of the hypothesized mid-peninsular vicariance event and the climate-based alternative in explaining the concordant genealogical breaks. We argue that, despite the significant changes in climate brought about by the glacial cycles throughout the Quaternary, a vicariant history involving a mid-peninsular seaway remains the most parsimonious explanation of the observed patterns in mtDNA genealogies.     

Social-ecological Resilience Modeling: Water Stress Effects in the Bighorn Sheep Management System in Baja California Sur,Mexico

Bighorn sheep (Ovis canadensis) management involves ecological and socioeconomic aspects, creating a social-ecological system (SES). Social-ecological thresholds can be identified in the system to assess its specific resilience in response to climate stressors. Thus, the aim of this study is to build a dynamic model to assess whether this system is resilient to a particular stressor (water stress). In this study, the SES is considered resilient if the bighorn population is sufficiently large to provide economic revenue to landowners and promote conservation action. We validate and formalize this model by conducting semistructured interviews to Bonfil ejido landowners located in Baja California Sur (BCS), Mexico, and to experts in the field of recreational hunting and wildlife management. To explore the changes in specific resilience in this SES, we conduct simulations to assess the impact of rainfall variability patterns on the system. Our results indicate that rainfall variations with respect to the historical record have the potential to disrupt both the species and the local economy and that the lack of adaptive capacity in both harvest and conservation strategies may affect the dynamics of the whole SES. Finally, this paper explores how adaptive wildlife conservation management strategies can enhance the resilience of both subsystems in this SES.     

The peninsula effect on species diversity: a reassessment of the avifauna of Baja California

The peninsula effect, a decrease in species diversity from the base to the tip of peninsulas, has been proposed to explain the relatively poor species diversity of mammals on North American peninsulas. Subsequent work has questioned both the existence of peninsular declines in diversity, as well as the proposed cause (immigration-extinction dynamics). Previous studies of the Baja California avifauna have shown a gradual decrease in the diversity of breeding birds from the base to the tip of the peninsula. Using newly published data on the breeding land birds, I found a decrease only from the base to the middle of the peninsula, with a slight increase in diversity from the middle to the tip. This result is similar to that for other highly vagile taxa (e.g., Chiroptera. Lepidoptera) and is largely due to the coneave diversity gradient of montane species along the peninsula. Habitat associations of the Baja avifauna and the location of potential source populations suggest that: 1) local habitat heterogeneity is likely the single most important factor influencing the avian diversity gradient along the peninsula; and 2) limited immigration of Neotropical species from mainland areas, and of Nearctic species from the base of the peninsula to the montane southern tip is partly responsible for the form of the diversity gradient along the southern half of the peninsula. My results along with those from previous studies, suggest that rather than colonization/extinction dynamics, habitat heterogeneity and the vagility of the taxa considered have the greatest impact on the observed patterns of species diversity along peninsulas,     

Cyanobacterial diversity in extreme environments in Baja California, Mexico: a polyphasic study

Cyanobacterial diversity from two geographical areas of Baja California Sur, Mexico, were studied: Bahia Concepcion, and Ensenada de Aripez. The sites included hypersaline ecosystems, sea bottom, hydrothermal springs, and a shrimp farm. In this report we describe four new morphotypes, two are marine epilithic from Bahia Concepcion, Dermocarpa sp. and Hyella sp. The third, Geitlerinema sp., occurs in thermal springs and in shrimp ponds, and the fourth, Tychonema sp., is from a shrimp pond. The partial sequences of the 16S rRNA genes and the phylogenetic relationship of four cyanobacterial strains (Synechococcus cf. elongatus, Leptolyngbya cf. thermalis, Leptolyngbya sp., and Geitlerinema sp.) are also presented. Polyphasic studies that include the combination of light microscopy, cultures and the comparative analysis of 16S rRNA gene sequences provide the most powerful approach currently available to establish the diversity of these oxygenic photosynthetic microorganisms in culture and in nature. Electronic Publication     

The Mexican tropical deciduous forest of Baja California Sur: a floristic and structural approach

A quantitative study is presented of the tropical deciduous forest located in the Sierra de La Laguna in the southern part of the peninsula of Baja California, Mexico with data on structure, species composition, diversity, density, and abundance of perennial plants.4 study plots were selected to represent the predominant geomorphologic units, and to include topographic and climatic variations reflected by the distribution of this vegetation on the lowlands of slopes facing the Gulf of California and the Pacific Ocean.25 families containing 67 perennial species were found on the lowlands, with Leguminosae, Cactaceae, and Euphorbiaceae best represented. A high family diversity was found in the plots, but there was a low number of species per family. Dissimilarities between sites were found to be reflected significantly in growth-form abundances as well as in structural features and species diversity. Results show that the xeric environment, the low number of species, and the high incidence of dominant shrub species confer the vegetation of the lowlands simpler structural traits than those described for other tropical dry forests.     

The stratified microbial community at Laguna Figueroa, Baja California, Mexico: a possible model for prephanerozoic laminated microbial communities preserved in cherts

The microbial mat community of the evaporite flat at North Pond, Laguna Figueroa (Baja California, Mexico) was actively involved in the production of laminated sediments prior to 1978. Heavy rains in 1979 and 1980 flooded the mat with 1 and 3 meters of meteoric water respectively. The flooding deposited up to 10 cm of silicoclastic sediment over the Microcoleus-dominated mat and resulted in the cessation of laminated sediment deposition. In 1982, the surface had been recolonized by species of cyanobacteria (Spirulina, Oscillatoria) and purple photosynthetic bacteria (Chromatium, Thiocapsa). The silicoclastic sediments and residual evaporites, which overlaid the laminated sediment, had been reworked into an anaerobic, sulfide-rich mud and contained well preserved sheaths of filamentous and coccoid bacteria. The Swaziland Sequence in the Barberton mountain land (which includes the Onverwacht and Fig Tree Group as well as the Swartkoppie zone between them) contains laminated sediments and carbon-rich chert. Structurally preserved microfossils have been found in the smooth black chert but not in the laminae. We concur with others who suggested that the laminated sediments from the Swaziland Sequence were deposited by an active stratified microbial community. However, we propose that these organisms which were preserved were originally buried in the associated sulfide-rich muds and were subsequently silicified.     

Water versus spacing: A possible growth preference among young individuals of the giant cardon cactus of the Baja California Peninsula

Mature columnar cardon (Pachycereus pringlei) and saguaro (Carnegiea gigantea) cacti sometimes grow in very dense stands without apparent effect on growth; their seedlings and young plants are commonly found in clusters under legume nurse trees. The potential preference between space and water of young cardon was quantitatively measured under controlled environments for 30 months. The assessment used two types of experiments, one with different plant densities and the other of two plant densities combined with different irrigation regimes. Increases in population density reduced height and dry weight of the plants, but increased their volume and hydration; the water potential of the plants and the soil was less negative for denser populations. Addition of water above the minimum moisture required for growth made water potential less negative in plants and soil. The denser the population in a pot, the less soil surface was exposed to hot air. We conclude that water evaporation from soil surface from exposure to hot air during cultivation of this cactus was significantly reduced when the population density increased. Since loss of soil water from transpiration by cacti is very limited, evaporation directly from soil surface becomes dominant. With less evaporation under high plant density (shading), more water remains in the plant-soil system to be available for storage in the plant tissue.     

Metabarcoding reveals seasonal and spatial patterns of arthropod community assemblages in two contrasting habitats: Desert and oasis of the Baja California Peninsula,Mexico

Aim

Desert springs or oases are the only permanent mesic environments in highly water-limited arid regions. Oases have immense cultural, evolutionary and ecological importance for people and a high number of endemic and relic species. Nevertheless, they are also highly vulnerable ecosystems, with invasive species, overexploitation and climate change being the primary threats. We used the arthropod communities' spatiotemporal diversity and distribution patterns as a proxy to understand biodiversity dynamics in two geographically close but ecologically contrasting and highly threatened ecosystems: deserts and oases.

Location

Baja California Peninsula, Mexico.

Methods

Arthropod communities at five oases and surrounding desert scrub areas were sampled in two seasons. Using DNA metabarcoding and traditional taxonomic surveys, we tried to identify what biotic and abiotic characteristics of the habitat are important drivers of arthropod diversity and how these characteristics can change across spatial and temporal scales.

Results

Over 6200 individuals representing 23 orders were collected. In oasis samples, the community composition fluctuated more in space (i.e. among sites) than in time (i.e. seasons). Thus, seasonal changes did not affect oasis community diversity and composition, but the dissimilarity among sites increased with geographic distance. Moreover, anthropic activities negatively correlated with arthropod diversity in oases. On the other hand, the season, geography (e.g. latitude) and biotic characteristics of the habitat (e.g. sampled scrub species) significantly affected the diversity and composition of the desert arthropod communities.

Main Conclusions

Neutral dynamics (e.g. historical climatic events, dispersal limitation and spatial component) and human impact significantly influenced the biodiversity patterns of each oasis. In contrast, the habitat's seasonal variation and biotic characteristics were the most important variables influencing the diversity of the desert communities. Baja California oases harbour distinct invertebrate communities; therefore, each oasis should be conserved individually to preserve these unique assemblages.     

Natural disturbance and treefalls in a pine-oak forest on the Peninsula of Baja California,Mexico

The magnitude of natural disturbances by treefalls and their spatial occurrence were studied in a pine-oak forest of Sierra de La Laguna, located at the southern part of the Peninsula of Baja California in Mexico.Twenty transects covering 4 ha, perpendicular to north- and south-facing slopes were sampled. The percentages of rocky outcrops, slope, orientation, and gap size created by treefalls were recorded. The mode of tree death, treefall direction, trunk length, and basal diameter were also measured. Data were analyzed using principal component analysis, one-way ANOVA, multiple comparison tests by contrasts, and chi-square independence tests.Results suggest that exogenous disturbances have an important effect in this community. A mean number of 80 gaps per ha was recorded comprising 18.1% of the observed area. Gap sizes were fitted to a lognormal distribution as 2.3±1.4 and patches were found to be created by single treefalls. Analysis of dead material shows that there are significant differences between numbers of fallen trees on N vs S facing slopes, and that forms of dead trees are significantly associated with species. Snapped trees represent 39.5% of treefalls, dead standing trees 26.4%, uprooted trees 20.5% and cut-down trees 0.9%. The frequency of occurrence of various forms of the dead trees suggests that the cause of mortality is primarily due to the high incidence of tropical hurricanes, and secondly to the combined effect of wind and natural fire which occurs during the rainy summer season.     

Cryptic vicariance in Gulf of California fishes parallels vicariant patterns found in Baja California mammals and reptiles

Comparisons across multiple taxa can often clarify the histories of biogeographic regions. In particular, historic barriers to movement should have affected multiple species and, thus, result in a pattern of concordant intraspecific genetic divisions among species. A striking example of such comparative phylogeography is the recent observation that populations of many small mammals and reptiles living on the Baja California peninsula have a large genetic break between northern and southern peninsular populations. In the present study, I demonstrate that five species of near-shore fishes living on the Baja coastline of the Gulf of California share this genetic pattern. The simplest explanation for this concordant genetic division within both terrestrial and marine vertebrates is that the Baja Peninsula was fragmented by a Plio-Pleistocene marine seaway and that this seaway posed a substantial barrier to movement for near-shore fishes. For some fish species, the signal of this vicariance in mtDNA has been eroded by gene flow and is not evident with classic, equilibrium measures of population structure. Yet, significant divisions are apparent in coalescent analyses that jointly estimate divergence with gene flow. The genetic divisions within Gulf of California fishes also coincide with recognized biogeographic regions based on fish community composition and several environmental factors. It is likely that adaptation to regional environments and present-day oceanographic circulation limit gene exchange between biogeographic regions and help maintain evidence of past vicariance.