Variation in moisture duration as a driver of coexistence by the storage effect in desert annual plants

Temporal environmental variation is a leading hypothesis for the coexistence of desert annual plants. Environmental variation is hypothesized to cause species-specific patterns of variation in germination, which then generates the storage effect coexistence mechanism. However, it has never been shown how sufficient species differences in germination patterns for multispecies coexistence can arise from a shared fluctuating environment. Here we show that nonlinear germination responses to a single fluctuating physical environmental factor can lead to sufficient differences between species in germination pattern for the storage effect to yield coexistence of multiple species. We derive these nonlinear germination responses from experimental data on the effects of varying soil moisture duration. Although these nonlinearities lead to strong species asymmetries in germination patterns, the relative nonlinearity coexistence mechanism is minor compared with the storage effect. However, these asymmetries mean that the storage effect can be negative for some species, which then only persist in the face of interspecific competition through average fitness advantages. This work shows how a low dimensional physical environment can nevertheless stabilize multispecies coexistence when the species have different nonlinear responses to common conditions, as supported by our experimental data.     

Rhodopechys obsoleta (desert finch): a pale ancestor of greenfinches ( Carduelis spp.) according to molecular phylogeny

The evolutionary history of three out of four birds traditionally classified into the genus Rhodopechys birds has been studied by comparing their mitochondrial cytochrome b DNA sequence with that of greenfinches and other genus Carduelis finches. The desert finch (Rhodopechys obsoleta) or a sister extinct species seems to have existed about 6 million years ago in Asian and perhaps African desert-like areas. This bird has no molecular relationship with other Rhodopechys birds and seems to have given rise to the greenfinches radiation, probably by allopatry of marginal or isolated groups; the latter would have evolved to green plumage colours and more simple song modulations (i.e., greenfinches). The possible role of assortative mating and the newly postulated acquired phenotypic characters in greenfinches speciation are discussed.     

Weight changes of cross-desert migrants at an oasis —do energetic considerations alone determine the length of stopover?

Summary Trans-Saharan insectivorous passerine night migrants stopping-over at a small oasis in Sinai were weighed as frequently as possible, throughout the entire length of fall and spring migration passages. Due to the small size of the oasis and the intensive trapping effort, most stopping-over birds were captured and weighed throughout a sizeable portion of their stopover period. Weights at first capture were either similar in both seasons, or greater in fall than in spring. In some species fall migrants that stayed one day were heavier than those staying longer. In other species, and for all species in spring, weights of birds that stayed one day did not differ from those staying longer. In most species the weight of birds that stayed for more than one day did not vary significantly between arrival and departure, and cases of weight gains were commoner in fall than in spring. Trends of weight changes of an individual during stopover were usually inconsistent, but the longer it stayed, more it gained (in fall), or less it lost (in spring). It is proposed that stopping-over birds do not always resume their migration only after their fat reserves have been replenished, but that their decision to take off, or the reappearance of the migration impulse, are also controlled by a time program incorporated into their endogenous migration scheme, which constantly updates the time left for sampling and refuelling. It seems that in spring less time is allotted for the whole migration program, hence the time constraint overrides then all other tactical considerations, such as the state of fat reserves, and the weather.     

Calcium carbonate in termite galleries – biomineralization or upward transport?

Termites and soil calcium carbonate are major factors in the global carbon cycle: termites by their role in decomposition of organic matter and methane production, and soil calcium carbonate by its storage of atmospheric carbon dioxide. In arid and semiarid soils, these two factors potentially come together by means of biomineralization of calcium carbonate by termites. In this study, we evaluated this possibility by testing two hypotheses. Hypothesis 1 states that termites biomineralize calcium carbonate internally and use it as a cementing agent for building aboveground galleries. Hypothesis 2 states that termites transport calcium carbonate particles from subsoil horizons to aboveground termite galleries where the carbonate detritus becomes part of the gallery construction. These hypotheses were tested by using (1) field documentation that determined if carbonate-containing galleries only occurred on soils containing calcic horizons, (2) ¹³C/¹²C ratios, (3) X-ray diffraction, (4) petrographic thin sections, (5) scanning electron microscopy, and (6) X-ray mapping. Four study sites were evaluated: a C₄-grassland site with no calcic horizons in the underlying soil, a C₄-grassland site with calcic horizons, a C₃-shrubland site with no calcic horizons, and a C₃-shrubland site with calcic horizons. The results revealed that carbonate is not ubiquitously present in termite galleries. It only occurs in galleries if subsoil carbonate exists within a depth of 100 cm. ¹³C/¹²C ratios of carbonate in termite galleries typically matched ¹³C/¹²C ratios of subsoil carbonate. X-ray diffraction revealed that the carbonate mineralogy is calcite in all galleries, in all soils, and in the termites themselves. Thin sections, scanning electron microscopy, and X-ray mapping revealed that carbonate exists in the termite gut along with other soil particles and plant opal. Each test argued against the biomineralization hypothesis and for the upward-transport hypothesis. We conclude, therefore, that the gallery carbonate originated from upward transport and that this CaCO₃ plays a less active role in short-term carbon sequestration than it would have otherwise played if it had been biomineralized directly by the termites.     

Convergence of Differentially Invaded Systems toward Invader-dominance: Time-lagged Invasions as a Predictor in Desert Fish Communities

To what extent do patterns of invasion in one region generalize to other regions? Answering this question is a key goal of invasion biology because it underlies whether we can make progress via comparative studies or must instead cope with a large set of unique cases. Here we quantify similarities and differences in the historical development of nonnative fish assemblages of two North American desert drainages, one with many nonnative fishes (Gila Basin, principally southwestern USA), and one with few (Yaqui Basin, principally northwestern Mexico). The two river basins are similar in size, physiography, and ecology, but because of differences in the timing of regional development, we hypothesized that richness and geographic spread of nonnative fishes in the Yaqui are time-lagged relative to the Gila, and that a slow, but steady increase of nonnative fish occurrence is underway in the Yaqui, similar to what has already occurred in the Gila. Using the comprehensive SONFISHES database, we found that increases in the regional richness of extant nonnative species over time have been roughly linear in both basins. Meanwhile, previously established species have continued to spread spatially, such that the cumulative number of reach records for nonnative species has increased roughly exponentially in both systems. The current status of nonnatives in the Yaqui is remarkably similar to what was evident from the Gila in the past at a comparable level of sampling effort. For all measures of invasion dynamics we examined, a time lag of ～ 40–50 years exists between the Gila and Yaqui. The majority of extant nonnative fishes are known piscivores, and many have high levels of parental care, a life history trait that affords considerable advantages over native fishes. These results predict that, absent strong action now, the presently abundant native fish fauna of the Yaqui may become increasingly imperiled over the next several years, with a future similar to the Gila, where most native fishes are either extirpated, threatened, or substantially reduced in range, at least partially due to nonnative fishes. We recommend immediate actions to identify and protect high priority portions of the Yaqui Basin from further nonnative fish invasion before further degradation occurs.     

Ecological consequences of dead wood extraction in an arid ecosystem

Coarse woody debris (CWD) plays a key role in ecosystems, reducing erosion and affecting soil development, storing nutrients and water, providing a major source of energy and nutrients, serving as a seedbed for plants and as habitat for decomposers and heterotrophs. We asked whether removal of CWD affected the structure and functioning of an arid woodland ecosystem in mid-western Argentina. These woodlands are protected by national laws and inhabited by indigenous local pastoralists who have land rights to use natural resources, including wood for fuel and construction material. We hypothesized that removal of CWD affected negatively the populations of wood-nesting pollinators, the reproductive performance of Prosopis flexuosa (the dominant tree species), plant cover, richness and composition, and nutrient cycling in the soil. We conducted a manipulative experiment consisting of four pairs of 70 m radius circular plots, each pair with an experimental (CWD removal) and a control (no removal) plots. Experimental CWD extraction affected negatively flower visitor abundance, although the magnitude of this effect decreased over time. In contrast, extraction had a significant, positive effect on seed production of P. flexuosa. No significant effects of extraction were found on cover, richness and composition of understory plants and soil properties. Thus, CWD did not have the generally negative effects expected under our hypotheses.     

Adaptive difference in daily timing of hatch in two locust species,Schistocerca gregaria and Locusta migratoria: The effects of thermocycles and phase polyphenism

This study examined the effects of temperature and phase polyphenism on egg hatching time in the desert locust, Schistocerca gregaria, and the migratory locust, Locusta migratoria. The two species exhibited differences and similarities in hatching behavior when exposed to different temperature conditions. In 12-h thermocycles of various temperatures, the S. gregaria eggs hatched during the cryoperiod (low temperature period), whereas L. migratoria eggs hatched during the thermoperiod (high temperature period). The eggs of both species hatched during the species-specific period of the thermoperiod in response to a temperature difference as small as 1 °C. Furthermore, the locusts adjusted hatching time to a new thermal environment that occurred shortly before the expected hatching time. In both species, the hatching of the eggs was synchronized to a specific time of the day, and two hatching peaks separated by approximately 1 day were observed at a constant temperature after the eggs were transferred from thermocycles 3 days before hatching. Eggs laid by gregarious females hatched earlier than those laid by solitarious females in S. gregaria but this difference was not observed in L. migratoria.     

Decomposition of skeletal remains in desert dry soil A roentgenological study

Bones from an Arabian site dating back to the 2nd millenium BC have been studied for their special decomposition patterns resulting from desert burial conditions. This bone diagenesis was caused by dissolution and recrystallization processes in the mineral matrix. Also extensive secondary mineralisation by formation of CaCO₃ could be found. This secondary contamination with carbonate has to be taken in account within biochemical analyses of excavated bones. Despite the extensive diagenetic changes routine histology could be done on the samples by the use of microradiography.