Carbon mineralization in the southern Sonoran Desert

We measured carbon mineralization in four different desert habitats (Arroyos, Hillsides, Canopies-Plains and Open-Plains) and the separate effect of litter addition from annual and perennial plants on soil microbial respiration using two laboratory soil incubation experiments. The differences in total aboveground phytomass among habitats correlates with soil nutrient content, soil particulate organic matter (POM) and consequently, C mineralization. The Arroyos habitat with the highest perennial plant phytomass and litter production, had the highest soil nutrient content, soil POM and C mineralization. Litter from annual plants had twice the P concentration than litter from the perennials, but only half the N concentration. Soil microbial respiration was higher with annual plant litter than with perennial plant litter in the Hillsides and Canopies-Plains, suggesting that microbial activity in both habitats was improved by litter with a higher C quality. In contrast, in the poorest habitat, the Open-Plains, the better response to the addition of perennial plant litter suggests that microbial activity may have been constrained by N input.     

A new species of Peltula from the Sonoran Desert,Mexico

Abstract: The new species Peltula sonorensis is described from the Sonoran Desert. It is found in several localities throughout the Mexican part of the Sonoran Desert and belongs to a distinct group within the genus Peltula , which is characterized by fairly large (up to 15 mm diam.), umbilicate-peltate squamules, growing individually and having ellipsoid to bacilliform spores.     

Leaf litter decomposition in a southern Sonoran Desert ecosystem, northwestern Mexico: Effects of habitat and litter quality

Leaf litter decomposition of dominant woody perennial species in the three most common habitats of the southern Sonoran Desert was studied using the litter-bag method. Our objective was to assess the influence of litter quality on decomposition rates in three contrasting desert environments. The hypotheses were: (1) decomposition rates within the same litter type are faster in more mesic habitats, (2) decomposition rates are lower in higher lignin content or lower nutrient quality substrates, and (3) species-rich substrates enhance decomposition rates. For all litter types and habitats, a rapid loss of mass occurred during the summer rains at the start of the experiment, but total loss within the same litter type differed significantly among habitats. Decay rates were not higher in the more mesic habitat, but in the dry plains where solar irradiance and termite activity were highest. While termite activity was less important in the arroyos and absent in the hillsides habitats, proliferation of fungal mycelium in these sites was much higher than in the plains, suggesting that biotic and abiotic factors act both independently of litter richness. Lignin content seems to be an important factor controlling the loss of litter, because decay rates were inversely related to litter initial lignin content in all three habitats. Leaf litter diversity did not enhance rates of decomposition. The leaf litter mixture had k-values similar to the most recalcitrant monospecific litter in all three habitats, indicating a neutral or even antagonistic role of species-specific compounds in decomposition rates.     

Biogeography of the octocorallian coelenterate fauna of southern Africa

The geographical and bathymetric distribution of southern African octocorals is analysed. Of the over 200 estimated species of regional octocorals, 81 confirmed and adequately described species are studied using a radial sector method. Two primary faunal components are recognized–endemic (53.3% of the fauna by numbers of species) and Indo-Pacific (39.4%). An Atlantic component contributes only minimally (about 1.7%), while the remaining fauna is made up of cosmopolites (2.8%) and scattered species (2.8%). A subantarctic component is not evident for the present-day, although evidence for previous contact is presented. A sister-group analysis using genera as a guide to sister species, shows the biogeographic affinities for the present-day fauna as a whole to be 45% Indo-Pacific, 31% cosmopolitan, 10% endemic, 10% Atlantic and 4% southern oceans (subantarctic). Applying the same method to only those genera with endemic species shows the affinities of the present-day endemic fauna to be 27.5% Indo-Pacific, 27.5% endemic, 24% cosmopolitan, 14% Atlantic and 7% subantarctic. Clearly defined boundaries for west, south, and east coast faunas (as recognized by previous authors in describing various intertidal faunas) are found not be present with regard to the octocoral fauna (largely due to its overwhelmingly subtidal nature). Instead two primary zoogeographic provinces are recognized–the Cape Endemic Province (extending from Liideritz to Inhaca Island) and the south-western fringe of the Indo-Pacific Province from East London north-eastwards. An overlap zone between these two is recognized between East London and Inhaca Island, with the region in the vicinity of Richards Bay having an essentially evenly mixed fauna (roughly 50% Cape Endemic Province and 50% Indo-Pacific). Of the 84 octocoral genera recorded for the region, seven (or 8.3%) are endemic, and of these, five are monotypic while two are ditypic. The fauna is shown to be predominantly sublittoral (about 95% by numbers of species), the shallow sublittoral (< 100 m in depth) being the region with highest species richness. Pennatulaceans are eurybathic (intertidal to 4756 m) and clearly show a high proportion of cosmopolites (20% of presently identified species). Soft corals are stenobathic and restricted to the intertidal, continental shelf and uppermost portion of the continental slope (<500m), while gorgonians are intermediate in depth distributon (intertidal to 1200 m). No cosmopolitan alcyonaceans are presently recorded. The centre of the Cape Endemic Province is the Agulhas Bank–an extensive region of shallow continential shelf (< 200 m in depth) between Cape Town and East London. Two regions of octocoral radiation for southern Africa are postulated–the Agulhas Bank and the western Indian Ocean.     

Biogeography and conservation of the genus Ficus (Moraceae) in Mexico

Aim The main objective of this study is to document the biogeographical patterns, endemism and degree of conservation of the species of Ficus (Moraceae) in Mexico. There are over 750 species of the genus Ficus distributed worldwide, and Mexico practically represents its northernmost limit in the American continent. Detailed studies at regional scales may help to understand the biogeography of large genera such as Ficus. Location Mexico. Methods The biogeographical patterns of Mexican Ficus were obtained from information of fig specimens available in two of the main herbaria of Mexico (2140 vouchers), collecting figs throughout this country, and revising the specialized literature. The presence of each species of Ficus was recorded for every one of Mexico's states and several tropical countries of America. Besides, the Mexican territory was divided into cells of 1° × 1° and the presence or absence of all species of the genus was recorded. Rarity of species was classified based on the width of geographic distribution, habitat specificity and population size. Results A total of 21 species of Ficus occur in Mexico, including six species (28.6%) that are endemic to this country. Five species are included in subgenus Pharmacosycea and 16 species are documented under subgenus Urostigma. Affinities of Ficus flora with other tropical countries in America generally decreased as geographical distances from Mexico increased. Mexican states and cells with highest values of Ficus species richness (both total and endemic species) were located. Ten species, including three endemics, presented a wide distribution. Five species, including two endemics, possess the three attributes of rarity (narrow geographical distribution, high habitat specificity and scarce local populations). Three species of Ficus, including the endemic and very rare Ficuslapathifolia (Liebm.) Miq., are not recorded in any protected area existing in Mexico. Main conclusions Most of the Mexican Ficus show a great morphological variation and occupy different habitats along their geographic distribution. The biogeographical patterns described here establish a fundamental scenario for ongoing studies on Ficus–pollinator interactions. However, many local populations are considered to be at risk, as there have been significant reductions in the number and size of local populations. Further studies are needed to understand the process of colonization, maintenance and persistence of fig–pollinator mutualism in species with different patterns of geographic distribution. Mexican Ficus require special policies for conservation due to their complex degree of rarity, particularly their geographic distribution in different types of vegetation, ranging from dry scrublands to tropical rain forests.     

Summer water relations of the desert phreatophyte Prosopis glandulosa in the Sonoran Desert of southern California

Summary Two shoot populations of the rhizomatous, patchforming herb Solidago canadensis were studied throughout a developmental cycle in two abandoned pasture sites in southern Ontario. The shoot cohorts that emerged in spring dominated the two populations; subsequent recruitment was very low. Shoot mortality was highest in June and was concentrated in the smallest size classes. Both populations showed a pronounced bimodal size structure for most of the growth cycle. Relative growth rate of shoots declined as the growing season progressed and tended to be lowest in the smallest size classes. Inflorescence production depended on shoot size. The calculated relationship between log mean weight and log density of shoots was not constant during the growth cycle and the calculated maximum biomass values do not transgress the ultimate thinning line suggested from previously published data.Address for proofs and present address: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX. England     

Biogeography and history of the Mediterranean bird fauna

With 366 species of breeding birds, the Mediterranean region is a "hot spot" of species diversity. Many biogeographic realms contributed to the establishment of the extant fauna, which makes this region a crossroads for birds, but the two most important realms are the large forest blocks that extend today over Eurasia and the semi-arid belts of the southern and southwestern Palaearctic. The few groups that presumably differentiated within the Mediterranean basin are mostly birds of open habitats and shrublands (e.g. Sylvia spp.), whereas few species evolved in Mediterranean forests dominated by sclerophyllous evergreen tree species. We suggest this results from the history of vegetation belts and their associated faunas during the Pleistocene. On the whole, in contrast to other groups of vertebrates, the bird fauna is fairly homogeneously distributed all over the basin although there are some regional-specific trends in species assemblages, mostly on the basis of habitat selection and biogeographic origin. Many species of eastern and southeastern origin invaded the Mediterranean basin on the northern side of the sea up to the Balkan peninsula and the southern side to the Atlantic coast. The extant biogeographic patterns of the Mediterranean bird fauna are interpreted in the light of the Quaternary history shared by the biotas of the western Palaearctic in relation to the cycle of climatic changes which produced periodic huge spatiotemporal migrations of communities and populations. The severe human impact that started c. 8000-10,000 years ago resulted not so much in species extinctions as in dramatic changes in distributional patterns, complicating the reconstruction of biogeographic scenarios.     

Growth and demography of a declining,endangered cactus in the Sonoran Desert

Increases in the incidence and severity of drought threaten the viability of rare plants in arid regions. The endangered Nichol's Turk's head cactus (Echinocactus horizonthalonius Lemaire var. nicholii L. Benson) occurs only in four small, isolated populations in the Sonoran Desert of North America. Since 1995 we have monitored a population in southeastern Arizona (USA). Here we report 23 years of observations on abundance, growth, mortality, flowering and recruitment. Abundance of plants decreased from 132 in 1996 to 40 in 2017, with 100 individuals recruited and 203 dying during the study. Individual plants grew slowly, increasing annually by an average of 0.22 cm (95% confidence interval, 0.18–0.26 cm) in diameter and 0.27 cm (0.20–0.33 cm) in height. Growth was slowest when drought was most severe and slowed as plants reached maximum size. Annual mortality increased markedly across the study period and did not vary with plant size. Annual probability of flowering increased as plants increased in diameter but not in height, and varied with precipitation and drought but not with mean annual temperature. Recruitment was higher when average temperature was higher and the number of recruits per capita increased across the study period. The annual rate of change in abundance averaged −6%, but shifted markedly from −1% during 1995–2008 to −11% during 2008–2017. Our results indicate that the population's decline was not a consequence of failed recruitment but of increased mortality, which we discuss in the context of climate and herbivory.     

Vegetation ordination at the southern Chihuahuan Desert (San Luis Potosi,Mexico)

Cover data for 93 perennial plant species from fifty 1 ha sites, were used to ordinate desert vegetation in relation to 50 environmental variables at El Huizache Corridor. Cumulative variance recovered in the Bray and Curtis variance-regression ordination was substantial (80%). Community structure of desert plant communities at El Huizache Corridor may be influenced primarily by a combination of landscape and edaphic variables, which in turn may determine the distribution and abundance of moisture and nutrients, and perhaps promote habitat specialization and or competitive exclusion. Secondly, to a lesser extent, climate variables could be influencing community organization at small scale gradients, the longer the gradient the more relevant climatic factors become. First axis represented a landscape gradient; it was positively correlated to exposure, geology, slope angle, rocks, stoniness, iron, January mean temperature, and organic matter content; it was negatively correlated with latitude, longitude, soil depth, and potassium content. The second axis represented mainly a climatic gradient; it was positively correlated with mean precipitation of January, February, July, August, September, November, December, annual mean precipitation, Lang's Index, organic matter content, and stoniness. The third axis represented an edaphic gradient; it was positively correlated with electrical conductivity, Mn, Zn and elevation, and negatively correlated with pH, nitrates, Ca, and disturbance. These findings should guide conservation efforts to maintain species diversity and endemism at this area. This revised version was published online in June 2006 with corrections to the Cover Date.     

Coexistence and divergence of tropical dry forests and savannas in southern Mexico

Aim The purpose of the study was to assess the degree of floristic differentiation between tropical dry forest (TDF) and savanna occurring in a single landscape. This comparison provides information on the responses of vegetation to the prevailing environmental conditions, while it also allows us to make inferences about large‐scale events and processes, both biogeographical and evolutionary. Our approach included three levels of analysis: (1) taxonomic, (2) morphological and (3) vegetational. Location The seasonal dry tropical landscape in the Nizanda region, Oaxaca State, southern Mexico. The landscape comprises a complex vegetation mosaic in which tropical dry forest and savannas are the most conspicuous components. Methods Comparisons between TDF and savanna were based on inventories for these communities produced after 8 years of botanical survey. At the taxonomic level, the relative representation of taxa of different hierarchical levels in each community was examined. Morphological analyses required the classification of species on each of three criteria: (1) growth form, (2) life form and (3) growth habit. Vegetation level analysis was based on the frequencies of taxa in one hundred 100‐m² composition plots with which matrices of binary data were constructed for species, genera and families. These were subjected to classification analysis with Ward's method and using Euclidean distances as the dissimilarity algorithm. Results The combined flora for both communities comprised 600 species, 375 genera and 94 families; between them they shared 31, 40 and 34 taxa, respectively. The corresponding Sørensen similarity values were 10%, 21% and 72%, respectively. Ranking genera and families according to their species richness displayed large differences between savanna and TDF. Large differences between these communities were observed for Acanthaceae, Cactaceae, Euphorbiaceae and Mimosaceae, whereas Fabaceae and Asteraceae had similar high ranks according to the species richness in the two systems. The growth form spectrum diverged between the two communities, with TDF having more trees, shrubs and climbers. Savanna was characterised by forbs and graminoid herbs. Growth habit spectra revealed a clear dominance of herbaceous and suffruticose plants in savanna, and of woody elements and epiphytes in TDF. Regarding Raunkiaer's life forms, savanna had relatively more hemicryptophytes, and TDF more phanerophytes. Classification analyses showed that savanna and TDF forest samples kept their identities, regardless of taxonomic level (species, genera and families) at which the analyses were performed. Main conclusions The TDF and savanna of Nizanda represent two floristic systems with a large degree of differentiation at all taxonomic levels and patterns of morphological attributes. This suggest that the two floristic sets have evolved independently for extended periods of time, despite their close proximity. One important implication of this floristic differentiation is the large joint contribution made by these communities to the regional flora.     

Diversity and Coexistence of Sonoran Desert Winter Annuals

Abstract Annual plants make up ca. 50% of local floras in the Sonoran Desert. As with most plant communities, there is no shortage of potential coexistence generating mechanisms, and several mechanisms are likely contributors to coexistence at different spatial scales in the Sonoran Desert, e.g. spatial heterogeneity and the behaviors of predators and grazers. We explore one mechanism of likely importance for desert annuals: temporal environmental variation. It is widely recognized that coexistence is promoted by temporal variation if species such as desert annuals have "temporal niches" in the sense that each has years in which it out-performs the others. It is usually suggested that some resistent life-history stage, such as a seed bank, is also necessary to buffer each species from the negative population dynamic impact of unfavorable years. Using ten years of demographic data, we document the large year-to-year variation in population dynamics of desert annuals and show that ten species respond differently to temporal variation. Competition experiments document reversals in competitive superiority. Also, all species have a between-year seed bank, such that only a proportion of the seed bank germinates in any given year. Thus this system meets our intuitive requirements for variance-based coexistence. Dynamic models of this system demonstrate that subtle aspects of the species biology determine whether coexistence criteria are actually met. Specifically, variable germination fractions are required and coexistence is most readily favored with "predictive" germination. Germination fractions in this system do vary among years in a species specific fashion. Also, for the three years of available data, germination was predictive, in that each species had greater germination fractions in year of greater demographic success. Thus all of the population dynamic elements necessary for temporal variance mediated coexistence seem to be present in this system.     

Ecology and population genetics of Sonoran Desert Drosophila

Three species of cactophilic Drosophila endemic to the Sonoran Desert of North America, D. nigrospiracula, D. pachea and D. mettleri, experience marked differences in spatial resource availability, and the first two of these display significant differences in dispersal behaviour. We employed starch gel and cellulose acetate electrophoresis for eight allozyme loci to test for a relationship between these variables and genetic differentiation among geographical populations of each species. No evidence was found for population structure in any of the three species, populations of which were separated by geographical distances of up to 475 km. Allele frequencies for two loci, Mdh-1 and Est-2, in D. nigrospiracula and D. pachea were very similar to those obtained approximately 30 years ago by other workers, indicating that the polymorphisms are remarkably stable under the stressful and variable conditions of the desert environment. High longevity, dispersal and multiple female remating are likely to contribute to the apparent high level of gene flow in all three species.     

Seasonal reversals of upland-riparian diversity gradients in the Sonoran Desert

Flood disturbance and water resource availability vary sharply over time and space along arid‐region rivers and can interact in complex fashion to shape diversity patterns. Plant diversity showed spatial patterning along a topogradient from the floodplain of the San Pedro River (Arizona, USA) to the arid upland, but the patterns shifted temporally as the suite of limiting factors changed. During two of three sampling times, spatial diversity patterns were shaped primarily by gradients of water availability, the regional limiting factor. In the summer dry season, microscale diversity (species richness per 1 m²) and mesoscale diversity (cumulative species and functional types in 20, 1‐m² plots) of herbaceous plants decreased along the topogradient from floodplain to upland, reflecting the greater water availability on the low surfaces. During a summer wet season with moderate rains and flooding, diversity increased in all hydrogeomorphic zones (floodplain, terrace, upland), but the spatial pattern along the topogradient persisted. Following a very wet winter, patterns along the topogradient reversed: scour from large floods limited diversity on the floodplain and competitive exclusion limited the diversity on undisturbed river terrace, while abundant rains allowed for high microscale diversity in the upland. Disturbance and resource availability thus interacted to influence plant species diversity in a fashion consistent with the dynamic‐equilibrium model of species diversity. In contrast to the microscale patterns, mesoscale diversity of species and functional types remained high in the floodplain during all sampling times, with 58% more plant species and 90% more functional types sampled in low floodplain than arid upland for the year as a whole. Species with a wide range of moisture and temperature affinities were present in the floodplain, and seasonal turnover of species was high in this zone. The floodplain zone of a perennial to intermittent‐flow river thus had greater plant diversity than arid Sonoran Desert upland, as measured at temporal scales that capture seasonal variance in resource and disturbance pulses and at spatial scales that capture the environmental heterogeneity of floodplains. Although periodically limited by intense flood disturbance, diversity remains high in the floodplain because of the combination of moderate resource levels (groundwater, seasonal flood water) and persistent effects of flood disturbance (high spatial heterogeneity, absence of competitive exclusion), in concert with the same climatic factors that produce seasonally high diversity in the region (temporally variable pulses of rainfall).     

Prey refugia and the distributions of two Sonoran Desert cacti

Summary Two small Sonoran Desert cacti, Mammillaria microcarpa and Echinocereus englemannii, are commonly found beneath canopies of the larger, tree-like cactus Opuntia fulgida. The mechanism leading to this distribution pattern is incidental to the mode of reproduction in O. fulgida. Opuntia fulgida propagates by means of easily-detached, spine-covered stem joints that accumulate beneath the parent plant. These accumulations of spines apparently deter mammalian herbivores that otherwise consume succulent tissues of the smaller cacti. Such incidental effects are little studied, but they may contribute substantially to structure within plant communities.     

Spatial and temporal variation in islands of fertility in the Sonoran Desert

In many arid and semi-arid ecosystems, canopy trees and shrubs have a strong positive influence on soil moisture and nutrient availability, creating islands of fertility where organic matter and nutrients are high relative to areas outside the canopy. Previous studies of canopy effects on soil processes have rarely considered how landscape context may modulate these effects. We measured the effects of velvet mesquite trees (Prosopis velutina) on soil moisture and the biogeochemistry of nitrogen at different positions along a topographic gradient from upland desert to riparian zone in the Sonoran Desert of central Arizona. We also examined how landscape position and patterns of precipitation interact to determine the influence of P. velutina on soil moisture, N availability assessed using ion exchange resins, net N mineralization and net nitrification, and microbial biomass C and N. P. velutina clearly created islands of fertility with higher soil organic matter, net N mineralization and net nitrification rates, and microbial biomass under mesquite canopies. These effects were consistent across the landscape and showed little temporal variability. Magnitude and direction of effect of mesquite on soil moisture changed with landscape position, from positive in the upland to negative in the terrace, but only when soil moisture was >4%. Resin N showed responses to mesquite that depended on precipitation and topographic position, with highest values during wet seasons and under mesquite on terraces. We suggest changes in proximity of P. velutina to groundwater lead to shifts in biogeochemical processes and species interactions with change in landscape position along a topographic gradient.     

Biogeography of limno-terrestrial Tardigrada, with particular reference to the Antarctic fauna

The Tardigrada is a cosmopolitan phylum of pre-Pangaean origin, yet tardigrade families and genera show distinct biogeographic components isolated by two major geological events. Separate Laurasian and Gondwanan familial clusters correlate with the Triassic disintegration of Pangaea, while discrete Antarctic, Australian and New Zealand familial/generic clusters relate to the subsequent Jurassic/Cretaceous disintegration of Gondwana.     

Nurse Plants, Mycorrhizae, and Plant Establishment in a Disturbed Area of the Sonoran Desert

Arbuscular-mycorrhizal (AM) fungi stabilize the soil and enhance plant growth by alleviating nutrient and drought stress. Their contributions to agriculture are well known, but their role in desert ecosystems has received less attention. The AM status of perennial plants in disturbed and undisturbed plots were investigated in the Sonoran Desert near La Paz, Baja California Sur, Mexico to determine if AM fungi contribute to resource-island stability and plant establishment. All perennial plants (46 species) in the study plots were AM, but root colonization varied widely (<10 to> 70%). Roots of plants that established in greatest numbers in plant-free zones (colonizers) of disturbed areas were highly AM. Plants with trace (<10%) root colonization (cacti of the tribe Pachycereae: Pachycereus pringlei, Machaerocereus gummosus, and Lemaireocereus thurberi; and Agave datilyo) established preferentially in association with nurse trees. The pachycereid cacti grew under Prosopis articulata and A. datilyo under Olneya tesota canopies. Of the nine species of trees and arborescent shrubs in the area, the mature (>20 yr) nurse-legumes P. articulata and O. tesota supported the largest number of under-story plants. Younger plants had only occasional associates. AM propagule densities in plant-free areas were lower than under plant canopies (40 vs. 280 propagules/kg soil). Occurrence of soil mounds (islands) under plants owing to soil deposition was related to the nature of the canopies and to the AM status of the roots. Island soils were enmeshed with AM-fungal hyphae, especially in the upper layer (approximately 10 cm). Seedlings of P. pringlei, growing in a screenhouse for six months in soil collected under P. articulata, had a biomass ten times greater than plants growing in bare-area soil. The results are consistent with the proposition that AM fungi contributed to the plant-soil system of our study area by: (1) helping to stabilize windborne soil that settles under dense plant canopies; (2) enhancing the establishment of colonizer plants in bare soils of disturbed areas; and (3) influencing plant associations through differences in the mycotrophic status of the associates.     

Chemical Interactions in the Cactus-Microorganism-Drosophila Model System of the Sonoran Desert

The Cactus-Microorganism-Drosophila Model System of the SonoranDesert represents an excellent paradigm of the role of chemistryin plant-animal interactions. In this system, four species ofendemic Drosophila feed and reproduce in necrotic tissue offive species of columnar cacti. Studies over the past 35 yrhave characterized a myriad of interactions between the threemajor components of the model system. The cacti contain a varietyof allelochemicals which are primarily responsible for the highlyspecific pattern of host plant utilization exhibited by thedesert Drosophila. Plant chemistry, through its effect on themicrobially produced volatile patterns, is further involvedin host specificity because the flies use the volatile patternto cue in on necroses in the appropriate species of cactus.The metabolic activities of microorganisms (bacteria and yeasts)living in the necrosis can affect the substrate chemistry inboth positive and negative ways (i.e., acting to increase orto decrease the toxicity of the substrate). Finally, cactuschemistry may affect drosophilid mating behavior since larvalrearing substrate has been shown to influence adult hydrocarbonepicuticular composition. In D. mojavensis, adult hydrocarbonprofile has been implicated as a determinant of mate choiceleading to premating isolation between geographically isolatedpopulations that use chemically different cactus substrates.Current research is focused on the evolution and regulationof genes whose products (cytochrome P450 enzymes) are involvedin the specific insect-host plant relationships which existbetween the Drosophila species and the cactus species. There are many reasons why investigators choose to focus theirresearch efforts on what are referred to as "model systems."Typically included among these would be the idea that modelsystems are easier to study because they are less complex thanother scientific situations. At the same time, model systemsshould be representative of more complex, natural systems sothat information that is obtained from their study is broadlyapplicable. For almost a century, the fruit fly, Drosophilamelanogaster, has served as a model organism for the study ofgenetics. As a genetic paradigm, Drosophila is more tractableto scientific investigation than most organisms and has providedimportant insights into a wide variety of human maladies fromalcohol abuse to neurological brain disorders (Bellen, 1998).Similarly, the interrelationships of the columnar cacti andthe cactophilic Drosophila species of the Sonoran Desert have,for the past 35 yr, provided an excellent model system withwhich to study relevant questions in evolution, ecological genetics,and chemical ecology. The intent of this article is to brieflyreview and characterize the chemical interactions between theplants (cacti) and animals (Drosophila) of this model system,and, in addition, provide some thoughts on possible future directionsfor integrative approaches in this research area.     

Distribution and sporulation phenology of myxomycetes in the Sonoran Desert of Arizona

All pith samples from 68 dead saguaro cacti in 3 plots and 11 isolated dead plants in Saguaro National Monument, Arizona, produced at least one species of myxomycete upon incubation at 20 or 30°C. Three species,Badhamia gracilis (Macbr.) Macbr.,Physarum straminipes Lister, andDidymium eremophilum M. Blackwell et Gilbertson, developed at high frequencies on the substrates in moist chamber culture.Perichaena corticalis (Batsch) Rost, andProtophysarum phloiogenum M. Blackwell et Alexopoulos were also present. Although previous literature reports [9] indicated that Myxomycetes grow best at low pH, these species all tolerated substrates of pH 8.7–10.4.Didymium eremophilum andP. phloiogenum had peaks in sporulation within 6 days; other species were slower. There was no difference in time of sporulation ofB. gracilis orD. eremophilum at 20 and 30°C; however, sporulation ofP. straminipes was significantly later at 30°C. Reduced spore germination and slower buildup of critically sized amoebal populations ofP. straminipes at 30°C may be a factor.