High exposure to warming from climate change is expected to threaten biodiversity by pushing many species toward extinction. Such exposure is often assessed for all taxa at a location from climate projections, yet species have diverse strategies for buffering against temperature extremes. We compared changes in species occupancy and site-level richness of small mammal and bird communities in protected areas of the Mojave Desert using surveys spanning a century. Small mammal communities remained remarkably stable, whereas birds declined markedly in response to warming and drying. Simulations of heat flux identified different exposure to warming for birds and mammals, which we attribute to microhabitat use. Estimates from climate projections are unlikely to accurately reflect species’ exposure without accounting for the effects of microhabitat buffering on heat flux.

If the breeding range of a species is limited by biotic or abiotic environmental factors that depress breeding success at the range margin, then range expansion is expected when those limiting factors are alleviated. Over a three‐year period, we measured breeding success of a desert species, black‐throated sparrow Amphispiza bilineata, along a steep elevation gradient between the Peninsular Mountains and Colorado Desert (San Diego County, California) that is undergoing a warming trend. We compared breeding success at geographically marginal locations (higher‐elevation chaparral sites) to more central locations (lower‐elevation desert scrub sites) only a short distance apart. Breeding success was measured at the nest level, territory level, and population level. At each level measured, breeding success tended to be greater at higher‐elevation chaparral sites at the distribution margin compared to lower‐elevation sites where the bird was more common. Black‐throated sparrows had 100% reproductive failure at lower‐elevation sites during the two driest years of our study (2006–2007), but did relatively well at higher‐elevation sites. Only in a wetter year (2008) was breeding success improved at lower‐elevation sites. Surprisingly, there was no evidence of an upward elevational shift in distribution over a 26‐year period despite a clear warming trend and drier conditions. Greater territory density at lower‐elevation sites with reproductive failure during dry years suggests the possibility of an ecological trap in this system, which could prevent or delay climate‐induced range shifts. A common presumption has been that desert species will undergo relatively mild negative impacts due to a warming climate, but it is possible that some desert species are already at or near their temperature and aridity tolerance limits within their current range and shifts may not always be possible.

ABSTRACT. Since the 1940s, populations of Gray Vireos (Vireo vicinior) in California have collapsed, presumably because of parasitism by Brown-headed Cowbirds (Molothrus ater). In 2012 and 2013, we studied the vireo's nesting ecology to assess factors affecting two of California's largest remaining populations in the chaparral of San Diego County. Nest success was extremely low, with a model-averaged probability of nest survival of only 0.08 (N = 95). More nest failures were due to predation (83%) than to cowbird parasitism (13%). Video-recording at 30 nests revealed that California Scrub-Jays (Aphelocoma californica) were the most common nest predator (67%). Of eight variables tested, height of shrubs surrounding the nest had the strongest negative influence on nest survival, but was more strongly correlated with cowbird parasitism than with jay predation. Despite frequent renesting, seasonal productivity was well below the level required to sustain a population, especially in northern San Diego County where we found no Gray Vireos at six of seven sites where they had been present from 1997 to 2001 and where cowbird parasitism was more frequent. The vireo's continuing range collapse contrasts with recent climate-change models predicting a range expansion, highlighting the importance of demographic studies. Low nest success is likely contributing to population declines in California, and the additive effect of cowbird parasitism suppresses productivity. Conservation of Gray Vireos in California will likely require development of alternative approaches to cowbird and scrub-jay control appropriate to sites widely scattered in rugged chaparral.RESUMEN. Bajo exito reproductivo de Vireo vicinior en una poblaci on de California en declive Desde la d ecada de 1940, las poblaciones de Vireo vicinior en California se han desplomado, presuntamente por el parasitismo por Molothrus ater. En 2012 y 2013, estudiamos la ecolog ıa de anidaci on de Vireo vicinior, con el objetivo de determinar los factores que afectan las dos poblaciones m as grandes de California en el chaparral del condado de San Diego. El exito reproductivo fue extremadamente bajo, con una probabilidad promedio de supervivencia del nido de solo 0.08 (N = 95). La mayor cantidad fracasos de nidos fue producto de depredaci on (83%) y no de parasitismo por Molothrus ater (13%). Grabaciones de 30 nidos revelaron que el depredador m as com un fue Aphelocoma californica (67%). De las ocho variables evaluadas, la altura de los arbustos alrededor del nido tuvo la influencia negativa m as fuerte sobre la supervivencia del nido, sin embargo, estuvo m as fuertemente correlacionada con el parasitismo por parte de Molothrus ater que por la depredaci on por parte de Aphelocoma californica. A pesar de los intentos frecuentes de re-anidaci on, la productividad estacional fue considerablemente menor al nivel requerido para sostener una poblaci on, especialmente en el norte del condado de San Diego, donde no encontramos Vireo vicinior en seis de las siete localidades donde hab ...

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to International Journal of Plant Sciences.Pollen of eight species of Cryptantha was studied using scanning electron microscopy. In addition, transmission electron microscopy was used in Cryptantha intermedia to elucidate the ultrastructural basis for their heterocolpate pollen, in which three "true" colporate apertures alternate with three "pseudoapertures." Both apertures and pseudoapertures are regions of the wall in which ektexine is largely absent. However, apertures have an outer band of verrucate exinous deposits, a central region of inner intine wall material, and a concentration of cytoplasmic vesicles. In addition, apertures are shorter in length and wider at the equator than are pseudoapertures. The exine wall structure in C. intermedia consists of a typical inner, homogeneous endexine and, in regions other than apertures and pseudoapertures, an outer, essentially imperforate, tectatecolumellate ektexine. Curious triangular "polar pseudoapertures," devoid of ektexine, occur at each pole and resemble the pseudoapertures in sculpturing and wall structure. The described heterocolpate pollen in the Boraginaceae may represent a major apomorphy for the tribes Eritricheae and Cynoglosseae and possibly other taxa. However, further comparative studies within these groups are needed to assess definitively the extent of this putative apomorphy and to identify and characterize palynological features that may be useful within these groups.

Aim Local-scale processes at species distribution margins can affect larger-scale distribution dynamics, but are rarely studied. The objective of this research was to elucidate the nature of distribution limits by studying the comparative structure, dynamics and environmental associations of breeding bird populations at their distribution margin. We hypothesized that climate is principally responsible for setting distribution limits, whereas biotic habitat features are more strongly associated with distribution patterns within the range.Location Southern California, USA.Methods During 2005-2007 we studied the distribution patterns of breeding birds in three study areas, each spanning a low-elevation (200-1800 m) desert scrub-tochaparral gradient. We used logistic regression with hierarchical partitioning to assess the independent effects of environmental variables (e.g. climate versus habitat) on distributions. We tested for shifts in the relative importance of these environmental variables in determining distribution limits versus within-range patterns, and we also compared higher-and lower-elevation groups of species.Results Distribution patterns were highly variable among species, but were remarkably static over the three study areas and 3-year study period. Across species, habitat floristic variables performed relatively well at explaining distribution patterns. For higher-elevation species (chaparral birds), climate was relatively important in setting their lower distribution limits, and there was a shift to a greater importance of biotic habitat (mainly habitat structural variables) for determining within-range patterns. Relationships were more mixed for lowerelevation species (desert scrub birds), but with respect to distribution limits, biotic habitat variables tended to be more important relative to climate than we observed for chaparral birds.Main conclusions Along this warm, arid elevational gradient, higher-elevation chaparral birds are more limited by climate at their lower margin than are lowerelevation desert birds at their upper margin, suggesting that climate plays a strong role (relative to other values) in excluding non-desert birds from desert. However, given the strong differences among species, predictive distribution models will need to be individually tailored, and for most species biotic habitat variables were of greater importance than climate in determining limits. This research highlights the usefulness of studying environmental relationships at distribution margins and the importance of considering biotic relationships in forecasting distribution shifts under changing climates.