Northwestern Baja California Research Articles

Parasitic load of the Pacific mackerel, Scomber japonicus (Pisces: Scombridae) from Northwestern Baja California, Mexico.

Globally, the exploitation of small pelagic fish, like Pacific mackerel is of great importance due to food industry demand. However, there are few studies regarding its parasites load and there are no in this geographic zone. This study aimed to assess the parasitic composition, some temporal changes (during spring and summer) in abundance, prevalence and intensity of infection parasitic of the Pacific mackerel (Scomber japonicus) from Todos Santos Bay, Baja California, Mexico. The parasite fauna of the Pacific mackerel consisted of 1930 parasites (1413 in spring and 517 in summer) distributed in the follow taxa: an Tetraphyllidea (Cestoda), Kuhnia scombri (Monogenea), Didymozoidae (Digenea), Anisakis sp. (Nematoda), Rhadinorhynchus sp. (Acanthocephala) and Caligus pelamydis (Copepoda). The nematodes parasite were the most abundant both in spring with a mean abundance of 27.6 parasites and in summer 8.2 parasites compared with the other taxa like Cestoda, Monogenea, Digenea, Acanthocephala and Copepoda (P = 0.003). The mean intensity of the nematodes in spring and summer was 28.1 and 13.4, respectively. The nematodes prevalence was 90 % in spring and 60 % in summer. In general, the parasite load is more abundant in spring than summer. In summer, absence of taxa as Cestoda and Copepoda were registered. Nematode larvae were present in the fish guts mesentery and inside of the stomach, pyloric caeca, intestine. Also the nematodes were found in the liver, muscle and gonads. The most affected organ by nematodes was the intestine mesentery. The most predominant parasite of this study has been Anisakis sp. during spring.

Transitional Mangrove Scrub North of La Paz and Magdalena Bay in Baja California, Mexico: A Historical Review

The location and topography of the 1,300 km Baja California peninsula west of the Sonoran Desert provide the most extreme conditions for mangrove forest growth in the Northern Hemisphere. Despite their small size, however, the dwarf shrub mangroves (<2 to 4 m) are important for global CO2 sequestration, and monitoring of their expansion or decline is needed as part of assessing global carbon fluxes and biodiversity. Mangrove distributions at 42 coastal sites in 1972/73 are compared to historical accounts and modern data. These data show that there has been almost no northward migration of Rhizophora mangle during the past century although Avicennia germinans and Laguncularia racemosa have spread north on the Gulf Coast. The dwarf mangrove plant biodiversity in 1972/73 is related to geoform, temperature (T) and seasonal precipitation (P), but mangrove species cluster by annual P and T. The present northernmost Baja California mangrove limits are at 26.79° N on the Pacific Coast, and at 29.04° N on the Gulf Coast mainland, close to limits recorded in 1921. However, there are notable (46 to 4,649 ha) increases in areal cover and canopy height at these northern sites, and in other areas remote from urbanization. Case histories for seven locations point to increasing frequency and severity of tropical storms as main drivers of change on both coasts. However, anthropogenic changes from tourism promotion are major hazards to survival of the Gulf Coast forests, despite international, governmental and public efforts to legislate nature preserves. On the Pacific Coast, rising seawater temperatures may create coastal current conditions that potentially allow the extension of red and white mangrove ranges into northwestern Baja California. However, hazards associated with ocean warming that promotes tropical storm frequency and intensity can have predominantly negative impacts on mangrove production. These extreme hazards counteract the benefits of local and regional protection and management efforts

Astragalus tijuanensis and Astragalus brauntonii var. lativexillum, a new species and a new variety in the genus Astragalus (Fabaceae) from extreme northwestern Baja California, Mexico

We describe and illustrate Astragalus tijuanensis, morphologically related to Astragalus palmeri, and Astragalus brauntonii var. lativexillum as a new species and a new variety respectively from extreme northwestern Baja California, Mexico. Both new taxa were discovered growing on specialized clay soils in a recently burned chaparral habitat, both taxa are extremely rare and threatened.

Villages in Prehistoric Southern California: Communities or Locations?

ABSTRACT The term “village” has been variously applied either to specific geographical locations or to the social units that inhabited such locations. In the late prehistoric territories of the Kumeyaay groups in San Diego and Imperial counties and in northwestern Baja California, the ambiguity of the term may lead to significant misreadings of the region’s ethnohistoric, ethnographic, and archaeological records, including underestimating the groups’ mobility and overestimating their population size.

Was the San Quintín kangaroo rat really rediscovered?

The San Quintín kangaroo rat ( Dipodomys gravipes Huey, 1925) is one of the three kangaroo rats in northwestern Baja California and the largest of them in most body dimensions. After a dramatic population decline in the 1980s, it was declared as potentially extinct in the wild by Mexican law. In 2017, the species was claimed to be rediscovered, but critical data supporting the species recognition were not provided. The question remained: Was the San Quintín kangaroo rat really rediscovered? We analyzed individuals identified by us as D. gravipes to validate our identification. We compared cranial morphometric and mitochondrial data of on-purpose collected specimens with those of historic specimens and with those of sympatric kangaroo rats in Baja California, particularly the Dulzura kangaroo rat ( Dipodomys simulans (Merriam, 1904)). Cranial morphometry and body dimensions of current specimens are indistinguishable from those of historic D. gravipes, while being different from D. simulans in most dimensions. External measurements such as cranial traits adequately distinguish both species. The mitochondrial analysis revealed strong support for the species grouping of sequences of our specimens and DNA sequences of D. gravipes from the GenBank repository. This study comprises a morphometric and molecular confirmation of the presence of the species after >25 years of failures to find it.

Population density and habitat selection in the San Pedro Mártir rainbow trout in mountain streams of northwestern Baja California, Mexico

We assessed the population density and habitat selection of the southernmost rainbow trout subspecies, Oncorhynchus mykiss nelsoni, in three stream sites of the Sierra San Pedro Mártir, Baja California, Mexico. Habitat units (sections of the streams visually delimited on the basis of morphological and hydrological features) were sampled between February 2014 and April 2017, along a sample length of 2,980 m corresponding to 105 habitat units, which were classified into ten types. Mean population density (individuals/m2) was similar among the streams sampled (0.035–0.039/m2), as also over time for each stream, except for San Rafael Creek. Low gradient riffle was the habitat unit with the highest mean density of trout (0.151/m2). Of overall manner (dates and sites combined), population density was positively correlated to mean total length (TL), number of trout, and pH levels, and inversely correlated to total of dissolved solids. The total length of the trout was different among sample sites with the lowest and highest values in San Antonio de Murillo Creek and La Grulla Creek, respectively. Also, the total length of individuals among types of habitat units was different, with the highest values for MCP (mid-channel pool) and SRN (step run). Dates and sites pooled, the mean total length of individuals was positively correlated to population density, number of trout, habitat unit area, macrophyte cover, and inversely correlated to temperature. Low population densities (0.011–0.106/m2) combined with its small body size (38–216 mm TL) characterize this endemic trout in the southernmost part of species’ geographic range.

Formation of a subsurface anticyclonic eddy from the California Undercurrent off northwestern Baja California, Mexico

Hydrographic observations off northwestern Baja California in March 2017 revealed the presence of a subsurface anticyclonic eddy in a cross-shelf transect at ∼ 32 ° N. This eddy’s width was roughly 30 km and it was located about 15 km from the shelf, at depths between 100 m and 400 m. The eddy’s core had hydrographic characteristics consistent with those found in the California Undercurrent (CUC), which suggests that the eddy was originated in the CUC. Numerical-modeling simulations were used to elucidate the origin and evolution of the observed eddy. These simulations also showed a subsurface eddy which crossed the cross-shelf transect’s location with size, depth, and hydrographic characteristics similar to those of the observed eddy (but formed in November 2007, in a simulation for the 2004–2011 period). Thus, this numerical eddy is considered as analogous to the observed one, hence the life cycle is assumed to be the same in both cases. The eddy formation occurred roughly 40 km southeast of the cross-shelf transect, in a zone characterized by a submarine point, where the isobaths change their orientation abruptly and where an enhanced eddy-energy production occurs, which shows that the irregular bottom-topography is a factor that triggered such an eddy formation. The eddy formed in a period of weak along-shelf velocities, 4-10 days after an event of the most intense velocities of the year, associated with the passage of a wind-driven coastal trapped wave generated in remote locations south of the study area. The flow disturbances forced by this wave could be an additional physical factor which predisposes the water column to instability. However, at least in the cases herein analyzed, the eddy separation required an additional momentum/vorticity input by an adjacent mesoscale cyclonic gyre. Comparison with numerical and laboratory–experimental results support the notion that a baroclinic instability process is involved in the eddy generation. The path followed by the subsurface eddy described herein shows a link of the hydrographic characteristics between northwestern Baja California and the Southern California Bight. • Hydrographic observations show a subsurface anticyclonic eddy off Baja California. • An eddy detected in numerical simulations shows consistency with the observations. • Numerical results elucidate the formation and evolution of the observed eddy. • The eddy formed in a zone where isobaths change their direction abruptly. • The formation was linked to weak along-shelf flows and an adjacent cyclonic gyre.

MELANISTIC CALIFORNIA GROUND SQUIRRELS (OTOSPERMOPHILUS BEECHEYI)

Although melanism is common in some species of North American squirrels, its occurrence in the California ground squirrel (Otospermophilus beecheyi) was only anecdotal and reported in the early 20th century. Here we report on melanistic individuals in this species in northwestern Baja California in 2013 and 2020.

Spatial and temporal variations in water quality of Todos Santos Bay, northwestern Baja California, Mexico

Coastal pollution levels in Todos Santos Bay (TSB) recreational beaches have increased in recent years due to the uncontrolled disposal of wastewater and pollutants to the coast by human activities (urbanization and tourism). In this work, the concentration of water quality parameters along the coast of TSB was evaluated from February 2012 to January 2013. Sampling was carried out at monthly intervals on 29 beach locations. Results showed that the maximum values of all parameters were always found close to the treated wastewater discharge area at Arroyo El Gallo station. Temporally, both nutrients and biochemical oxygen demand reached notably high levels in winter-spring and close to wastewater discharges. During summer, maximum counts of total coliforms and parasite eggs were detected close to wastewater discharges. Overall, our study provides a diagnostic approach on the spatiotemporal variability of water quality parameters for future studies along the coastal waters of TSB.

FIRST RECORD OF NECROPHILIA IN THE BAJA CALIFORNIA SPINY LIZARD, Sceloporus zosteromus (Phrynosomatidae)

Necrophilia is a reproductive behavior were a live animal attempts to mate with a dead conspecific. This behaviour has been reported in several lizard species. Here, we report the first case of necrophilia of an adult male Sceloporus zosteromus with a dead conspecific female in northwestern Baja California, Mexico.

First steps towards the identification of evolutionarily significant units in Mexican native trout: An assessment of microsatellite variation

In the remote high-elevation streams and rivers in the northwest of Mexico, there is an endemic and highly diverse group of native trout related to the rainbow trout lineage (Oncorhynchus mykiss). Currently, only two species have been formally described from this trout complex, the Mexican golden trout, O. chrysogaster, and the San Pedro Martir rainbow trout, O. m. nelsoni, but only the former occurs in the Sierra Madre Occidental (SMO). South and north of the O. chrysogaster range, there are other undescribed trout that have been recently discovered. Given the various human impacts on aquatic systems occupied by these Mexican native trout, the lack of taxonomic descriptions and the imminent risk of losing populations without their even being documented, we sought to infer a hierarchical population structure of the Mexican native trout, determine the geographic patterns of genetic diversity and gene flow among them, and identify evolutionarily significant units for a prompt conservation strategy. The allelic diversity across eleven microsatellite loci was used to assess genetic divergences of wild trout collected throughout its range in Mexico. Our results reject the old hypothesis that many of the trout in Mexico are introduced or translocated hatchery-reared rainbow trout and strongly support the new hypothesis of distinct genetic populations and lineages in the northwest of Mexico. According to the different statistical analysis of the genetic data, we distinguish between species and subspecies (O. chrysogaster, O. mykiss, and O. m. nelsoni) of the other regional lineages of undescribed trout (northern and southern) inhabiting the SMO. We also found a strong genetic substructure within these large clusters. We propose the presence of seven Evolutionarily Significant Units in the drainage basins of the SMO (two occur in the northern region, four in the southern, and one for the area of distribution of O. chrysogaster) and one in north-western Baja California. Based in microsatellite data a minimum gene flow among populations is evidenced, resulting the presence the highly divergent populations.

Artisanal elasmobranch fisheries of northwestern Baja California, Mexico

Elasmobranch species composition and spatiotemporal variability can often be characterized from landing surveys of local fisheries, with important management implications. In the present study, we describe the elasmobranch landing composition of 4 major artisanal fishing ports in northwestern Baja California, Mexico. Sampling was conducted opportunistically between 1995 and 2010 and included an effort of 684 fishing days. Species composition was gear-dependent and varied with the fishing area, with thresher sharks (Alopias vulpinus) and spiny dogfish (Squalus suckleyi) dominating the catches landed by coastal gill net fisheries, and blue sharks (Prionace glauca) and shortfin mako sharks (Isurus oxyrhinchus) dominating the catches landed by the offshore longline fishery. Size-structure analysis indicated that catches were composed primarily of juveniles for most species, although the catch of mature soupfin sharks (Galeorhinus galeus) increased in longline fisheries during the winter. The results indicate that minor regional differences in elasmobranch species composition occur along the Pacific coast of Baja California and are most likely related to regional oceanographic or habitat differences. The results of this study may form the basis for future efforts to monitor species composition changes over time, to more completely assess elasmobranch populations.

Genetic patterns and changes in availability of suitable habitat support a colonisation history of a North American perennial plant.

Climatic fluctuations during the Pleistocene influenced the geographical distribution of plant species across the southern region of California. Following an integrative approach, we combined genetic data analysis with Environmental Niche Models (ENMs) to assess the historical range expansion of Yucca schidigera, a long-lived desert perennial native of the Baja California Peninsula. We genotyped 240 individuals with seven nuclear microsatellite to investigate genetic diversity distribution across 13 populations. Indeed, we used Environmental Niche Models to examine the changes on the distribution of suitable climatic conditions for this species during the LIG (~120 ka), LGM (~22 ka) and Mid Holocene (~6 ka). We detected high genetic diversity across Y. schidigera populations (AR = 9.94 ± 0.38 SE; Hexp = 0.791 ± 0.011 SE) with genetic variation decreasing significantly with latitude (allelic richness: R 2 = 0.38, P = 0.023; expected heterocigosity: R2 = 0.32, P = 0.042). We observed low, but significant genetic differentiation (FST = 0.0678; P < 0.001) which was consistent with the parapatric distribution of the three genetic groupings detected by the Bayesian clustering algorithm. The ENMs suggest that suitable habitat for this species increased since the LGM. Our results support a range expansion of Y. schidigera across northwestern Baja California during the late Quaternary. Genetic data suggest that colonization of the current distribution followed a southward directionality as suitable climatic conditions became widely available in this region. High genetic variation across our sample suggests large historic effective population sizes for this section of the geographical range.

Effects of the 2013–2016 Northeast Pacific warm anomaly on physical and biogeochemical variables off northwestern Baja California, derived from a numerical NPZD ocean model

Interannual climatic anomalies have important physical and biological consequences for the southern part of the California Current System (CCS). Two examples of these anomalies are “The Blob” and El Niño phenomena. The Blob was an anomalously warm water mass that formed in the Gulf of Alaska during fall 2013 and then affected most of the eastern North Pacific, including the waters off Baja California, Mexico. In this work we use a biogeochemical nutrient-phytoplankton-zooplankton-detritus (NPZD) model coupled to a three-dimensional hydrodynamic model, which allowed the simulation of some of the physical and biological consequences of the anomalous warm 2013–2016 period (The Blob was observed in 2014 and El Niño by mid-2015). A model-observation comparison showed that in some cases the model solution deviates substantially from the observed vertical thermal structure, but the temporal evolution of the spatially-averaged variables diagnosed by the model shows good agreement with the observations, except for the period from late 2015 to mid-2016, when the sea surface temperature (SST) is underestimated and the model failed to reproduce the correct sign of the surface chlorophyll-a (sCHL) anomalies. An EOF analysis of the 15-year (2003–2017) record of monthly mean modeled variables allowed the identification of two main contributions to the model variability: one mode mostly correlated with the Multivariate El Niño-Southern Oscillation (ENSO) Index (MEI), explaining ~50% of the variance of temperature (T) and dissolved nitrogen (N) and ~10% of the variance of chlorophyll-a (CHL), and another mode mostly correlated with the Coastal Upwelling Index (CUI) anomaly, explaining ~50-70% of variance of CHL and only ~10-20% of variance of T and N. In the 2013–2016 period, an El Niño signal diagnosed by the MEI induced a remarkable warming off northwestern Baja California. As a consequence, the thermocline and the nutricline deepened during this period and caused a deepening and weakening of the subsurface CHL maximum (SCM). Around the southern region of the study area (near Punta Baja) these effects were more intense at the surface over the shelf, but in the northern region (south of Southern California Bight) the effects were more intense subsurface seaward of the shelf break. This ENSO-modulated subsurface warming tends to weaken the Southern California Eddy during El Niño events, and intensify it during La Niña episodes. During the 2013–2015 period there was also a weakening of the upwelling-favorable winds, which caused a prolonged negative anomaly of the wind forcing, reducing the nutrient flux and causing negative CHL anomalies. Finally, in addition to the partial relation between the ENSO and the warming observed along the Baja California coast, the anomalies were observed in this area 4–6 months before the onset of the El Niño conditions (MEI>0.5) period, when the Blob was affecting the southern part of the CCS.

Precipitation and topography modulate vegetation greenness in the mountains of Baja California, México.

Vegetation greenness (normalized difference vegetation index, NDVI) showed significant temporal and spatial correlations with precipitation and topography-derived features within the context of slope aspect (South- (SFS) and North-facing slopes (NFS) and an intermountain valley (IMV)) in a semi-arid Mediterranean-climate watershed in northwestern Baja California, México. Rank correlation with annual precipitation (1986-2016) showed a strong positive relationship with wet season NDVI at SFS (Rs = 0.82), IMV (Rs = 0.79), and NFS (Rs = 0.65) but moderate relation and only on hillslopes in the dry season (SFS, Rs = 0.47; NFS, Rs = 0.39). Thus, the vegetation on the more xeric SFS sites was more responsive to intra-annual and inter-annual precipitation than on either IMV or NFS. The correlation of NDVI with six topography-derived environmental attributes (elevation, slope gradient, curvature, drainage density, topographic wetness index, solar radiation) was weak to moderate, varied in degree and significance between years with exceptionally high or low NDVI, and often differed in sign between SFS, NFS, and IMV. Results showed that precipitation controlled vegetation greenness, under the three aspect conditions, more closely than did the other topography-derived features, and the sparse deciduous vegetation of SFS showed stronger associations with precipitation than IMV or NFS. The measurement of these relationships should be continued and complemented by other studies to improve the overall model, because they are important to modeling ecohydrology and productivity, and may be of use for projecting and hindcasting vegetation dynamics.

Genetic and phenotypic diversity of Branchinecta sandiegonensis(Crustacea: Anostraca) in the vernal pools of Baja California, México.

Branchinecta sandiegonensis is a passively dispersed species that occurs in the vernal pool complexes of southern California, the USA, and northwestern Baja California, México. The fragmented distribution of these vernal pool complexes could limit the gene flow, generating high genetic structure and morphometric variation across the landscape. Here we estimate the genetic and phenotypic variation of B. sandiegonensis in the southern part of its range. We sampled 15 vernal pools from four geographic regions of the Baja California Peninsula. We genotyped 150 individuals using nuclear microsatellites and 31 individuals using the mitochondrial COI region. We also conducted a morphometric analysis on a sample of 232 individuals. We found moderate levels of genetic diversity and different patterns of structure depending upon the spatial scale of analysis. Demographic models suggest contrasting trends among populations. Phenotypically, we found high levels of heterogeneity in body size of fairy shrimps within and across the regions. Our findings highlight that vernal pools in Baja California are important reservoirs of genetic and phenotypic diversity for B. sandiegonensis. The interplay between gene flow and genetic drift may have influenced the patterns we detected in the southern part of the range of this species.

A New Species Of Ascarophis (Nematoda: Cystidicolidae) Parasitizing Clinocottus analis (Pisces: Cottidae) From Baja California, Mexico

A new species of nematode, Ascarophis morronei n. sp. (Cystidicolidae), is described from the stomach wall of the woolly sculpin Clinocottus analis (Cottidae) collected in the rocky intertidal from northwestern Baja California, Mexico. Collected nematodes were studied using both light and scanning electron microscopy. Sequence fragments for 18S rDNA molecular markers were obtained from the new nematode species, in order to test its position within the family Cystidicolidae under a phylogenetic context. Main characters distinguishing this new species include the reduced labia and the morphology of the eggs, distances of nerve ring and excretory pore from the anterior end, and left spicule of males. The new species described here is the second for the genus Ascarophis reported as adult in the Southern California Bight, and the first one recorded for the fish genus Clinocottus.

Biochronology, paleoenvironments, and stratigraphic sequences of the late Albian–middle Eocene fore-arc Vizcaino basin, western Baja California, Mexico

The Vizcaino fore-arc basin accumulated approximately 4 km (∼13,000 ft) of upper Albian–middle Eocene siliciclastic marine sedimentary rocks derived from the Peninsular Ranges in Baja California. Data from eight exploratory wells document the micropaleontological content and lithological characteristics of these rocks. The strata studied represent mostly neritic–upper bathyal marine environments and overlie a basement composed of Cretaceous granitic rocks, or Aptian–Albian volcaniclastic sedimentary rocks correlative with the Alisitos Formation. We recognize four major depositional sequences within the basin that are related to the regional geology. The basal Albian–Turonian sequence 1 represents the initiation of fore-arc basin sedimentation, contains continental conglomerates that change to bathyal shales, and correlates with the lower part of the Valle Group of the Vizcaino Peninsula. Sequence 2 is Coniacian–Paleocene, includes basal conglomeratic sandstones grading into Maastrichtian bathyal shales, and usually overlies a Coniacian–Santonian unconformity. Sequence 2 is represented at the surface by the Rosario Group in northwestern Baja California and the upper part of the Valle Group in the Vizcaino Peninsula. Sequence 3 is Paleocene–middle Eocene, represents continuity of fore-arc sedimentation in neritic–upper bathyal conditions, is capped by a major unconformity, and correlates with the Sepultura and Bateque Formations to the north and south of the basin, respectively. The uppermost Miocene–Pliocene sequence 4 is composed of marine sandstone–siltstone unconformably overlying sequence 3 and is correlative with the Tortugas Formation that represents sedimentation after the end of subduction of the Farallon plate beneath the North America plate.

Factors in palaeoclimate analysis based on the marine biota of the Conejo Volcanics (Miocene), Santa Monica Mountains, California

ABSTRACTThe fossils in the middle Miocene Conejo Volcanics lived on a middle to outer shelf at approximately 100 m depth. They were redeposited with their substrate sediment into a basin where submarine volcanic flows and hyaloclastic breccia were accumulating at bathyal depth. The sediments were deposited as lenses of primarily limestone but also sandstone. The complex depositional history of the biota within a tectonically active region provides the opportunity to investigate the diverse important factors essential in determining palaeoclimate. Palaeoclimate along the Pacific Coast during the Cenozoic has been largely determined by comparing the fossils with the biota diagnostic of present-day provinces. This is not possible for the Conejo Volcanics fauna because most of the shallow-water taxa on which the provinces are based are rare or absent in it, and because the regional palaeogeography that determined the Miocene provinces is not known. Geochemical data are essential to compensate for a lack of information about water depth and geography. A δ18O isotopic palaeotemperature of 15°C for a specimen of Amussiopecten in the Conejo Volcanics suggests a shallow-water setting off north-western Baja California. However at the depth at which the fossils lived, the equivalent present-day location would be farther south, off south-western Baja California. Incorporating the climatic effect of local Miocene geography, the modern equivalent latitudinal position of the Conejo Volcanics would be somewhat north of there, off central Baja California at 25 to 26°N. This location is about 9° south of the present location of the Conejo Volcanics and suggests that the marine climate was warmer in the mid-Miocene by about 5°C. However, because approximately one-half of the latitudinal difference is explained by tectonic translation through micro-plate rotation and San Andreas fault movement, the estimate of climatic change is correspondingly reduced to only a few degrees.

Longevity and population age structure of the arroyo southwestern toad (Anaxyrus californicus) with drought implications.

The arroyo southwestern toad is a specialized and federally endangered amphibian endemic to the coastal plains and mountains of central and southern California and northwestern Baja California. It is largely unknown how long these toads live in natural systems, how their population demographics vary across occupied drainages, and how hydrology affects age structure. We used skeletochronology to estimate the ages of adult arroyo toads in seven occupied drainages with varying surface water hydrology in southern California. We processed 179 adult toads with age estimates between 1 and 6 years. Comparisons between skeletochronological ages and known ages of PIT tagged toads showed that skeletochronology likely underestimated toad age by up to 2 years, indicating they may live to 7 or 8 years, but nonetheless major patterns were evident. Arroyo toads showed sexual size dimorphism with adult females reaching a maximum size of 12 mm greater than males. Population age structure varied among the sites. Age structure at sites with seasonally predictable surface water was biased toward younger individuals, which indicated stable recruitment for these populations. Age structures at the ephemeral sites were biased toward older individuals with cohorts roughly corresponding to higher rainfall years. These populations are driven by surface water availability, a stochastic process, and thus more unstable. Based on our estimates of toad ages, climate predictions of extreme and prolonged drought events could mean that the number of consecutive dry years could surpass the maximum life span of toads making them vulnerable to extirpation, especially in ephemeral freshwater systems. Understanding the relationship between population demographics and hydrology is essential for predicting species resilience to projected changes in weather and rainfall patterns. The arroyo toad serves as a model for understanding potential responses to climatic and hydrologic changes in Mediterranean stream systems. We recommend development of adaptive management strategies to address these threats.