A retrospective study of climatic suitability for the tick Rhipicephalus (Boophilus)microplus in the Americas

Aim Climate change has the potential to have an impact on the distribution of ticks and tick‐borne diseases. This paper identifies the changes in climate suitability for the tick Rhipicephalus (Boophilus) microplus in the Americas by analysing climate data for the period 1950–99. Location The model was applied to the American continent. Methods A model based on Environmental Niche Modelling was used on a gridded (0.5°) long‐term (1950–99) climate data set. A map of the core range of the species was constructed, and areas where habitat suitability (HS) changes suddenly over short time periods were identified as regions of high sensitivity. Tendency of climate in the continent was evaluated and scenarios constructed for 2025 and 2050. Results Regions of high sensitivity included the southern USA, Mexico and western and central Argentina. Analysis of climate variables in these regions identified water vapour pressure deficit and evaporation as underlying the high sensitivity of habitat suitability in the USA and Mexico, and showed that episodes of high variability are linked to the El Niño Southern Oscillation. Projections of the tendency of HS as observed for the 1950–99 period point to an increase in this value in parts of the southern USA and in central Argentina, a finding that can be attributed to the progressive increase in minimum and yearly averaged temperatures. Conclusions Short‐term changes in climate may drive the system into unstable situations with sudden changes in habitat suitability for the target tick in specific zones of the Americas. Results suggest an increased abiotic (climate) suitability for R. microplus in areas whose habitat is currently unsuitable for this species.     

Changes in Habitat Suitability for the Tick Ixodes ricinus (Acari: Ixodidae) in Europe (1900–1999)

Climatic changes may lead to drastic changes in the distribution of arthropods important in human health. We tracked changes in habitat suitability for the tick Ixodes ricinus in Europe from 1900 to 1999, using a geographically extensive gridded climate data set. For the whole period, 52% of the territory was always unsuitable for the tick. In the grid, 6.11% of the cells were classified as having a deterministic drift with positive trend and 7.4% as deterministic drift with a negative trend. A total of 17.25% of cells were classified as exhibiting a random walk behavior, with a trend to increase of habitat suitability (9.57%) or decrease (7.68%). Zones of deterministic trend extend into most of Ireland and parts of the United Kingdom and France. Total and summer rainfall primarily drive changes in habitat suitability in these sites. Areas of random walk are common in Scandinavia, central Europe, and the Balkans, with summer rainfall and temperature largely directing the changes. Sites of reported increased abundance of I. ricinus coincided with areas of increased habitat suitability over the last 20–30 years, but this feature showed a long-term random walk negative trend. Habitat suitability for I. ricinus remains relatively stable in Europe, with no sites showing permanent changes in habitat suitability (negative to sustained positive or vice versa). However, some zones in the continent showed a clear trend to increase or decrease.     

The effect of rainfall on tick challenge at Kyle Recreational Park,Zimbabwe

The effect of rainfall pattern on tick challenge was investigated at Kyle Recreational Park, Zimbabwe, from 1991 to 1992 using drag and removal plot methods to sample environmental tick density. The abundance of adults and nymphs of the brown ear-tick Rhipicephalus appendiculatus and larvae of the bont tick Amblyomma hebraeum was positively correlated with monthly rainfall, whereas no relationship with rainfall was revealed for larval R. appendiculatus, adults of the red-legged tick R. evertsi, or larvae of the blue tick Boophilus decoloratus. A comparison between 1991 (490 mm rainfall) and the drought year of 1992 (161 mm) revealed significant differences in the abundance of R. appendiculatus, A. hebraeum, and B. decoloratus. During the wet season, R. appendiculatus adults were 2–3 time more numerous in the environment during the higher rainfall year of 1991. A. hebraeum larval abundance exhibited a similar pattern to that of R. appendiculatus adults, but B. decoloratus larvae were more abundant in the drought year of 1992 during both the wet and dry seasons. Comparable tick abundance data collected at Kyle during the above-average rainfall years of 1975–1977 (mean = 1029 mm) were compared with tick challenge during the below-average rainfall years of 1991–1992 (mean = 326 mm). In grassland sand habitat and all habitats combined R. appendiculatus adults, nymphs, and larvae were much more abundant during the high rainfall years. In contrast, larvae of B. decoloratus were more numerous during the drier years. A. hebraeum larvae were also more abundant during the drier years. The strong positive correlation of adult R. appendiculatus abundance with rainfall and the coincidence of increased adult tick challenge with increased rainfall indicates that adult R. appendiculatus tick burden on hosts would be heaviest during the wet season and high rainfall years.     

Cattle ticks of the genera <Emphasis Type="Italic">Rhipicephalus</Emphasis> and <Emphasis Type="Italic">Amblyomma</Emphasis> of economic importance in Tanzania: distribution assessed with GIS based on an extensive field survey

In order to implement a robust integrated tick and tick-borne disease control programme in Tanzania, based on ecological and epidemiological knowledge of ticks and their associated diseases, a national tick and sero-surveillance study was carried out in all 21 regions of the mainland, as well as on Mafia Island, between 1998 and 2001. The current distributions of Rhipicephalus appendiculatus, R. pravus, Amblyomma variegatum, A. gemma, and A. lepidum are illustrated and discussed. Tick distribution maps were assessed using the Weights-of-Evidence method (WofE), and employing temperature, humidity, NDVI, rainfall, and land-cover predictive data. Ground-truthing was done to check correspondence both of the data employed in prediction with land-cover characteristics discerned in the field as well as of the surveyed and predicted tick distributions. Statistical methods were used to analyse associations of the tick species with their environment, cattle density, and other ticks. Except for R. appendiculatus, no appreciable changes were demonstrated in the predicted and observed tick distributions compared to the existing maps that originated in the 1950–1960s. Cattle density influenced the distribution of A. variegatum and, to a certain extent, of A. lepidum, but had no appreciable influence on the distribution of any of the other ticks discussed in this paper, neither did livestock movement. Distinct differences for environmental requirements where observed between different tick species within the same genus. The predictive maps of R. appendiculatus and R. pravus suggest their mutually exclusive distribution in Tanzania, and simultaneous statistical analysis showed R. pravus as a greater specialist. Of the three Amblyomma species, A. variegatum is the most catholic tick species in Tanzania, while both A. gemma and A. lepidum belong to the more specialized species. Despite dissimilar habitat preferences, all three Amblyomma spp. co-exist in central Tanzania, where very heterogeneous habitats may simultaneously satisfy the environmental requirements of all three species. The current study, conducted about 4 decades after the last major survey activities, has shown that changing livestock policies, unrestricted livestock movement and a continuous change in climatic/environmental conditions in Tanzania have brought about only limited changes in the distribution patterns of R. appendiculatus, R. pravus and the three Amblyomma species investigated. Whether this observation indicates a relative indifference of these ticks to environmental and/or climate changes allows room for speculation.     

Shifts in the distributional ranges of <Emphasis Type="Italic">Boophilus</Emphasis> ticks in Tanzania: evidence that a parapatric boundary between <Emphasis Type="Italic">Boophilus microplus</Emphasis> and <Emphasis Type="Italic">B. decoloratus</Emphasis> follows climate gradients

Based on a national tick survey conducted in Tanzania between 1998 and 2001, predictive maps of tick distribution for B. microplus and B. decoloratus were computed and compared with historical maps that originated about 40 years ago. The new data collected showed that except for extremely cold and dry areas B. microplus has extended its distribution range and is now present in all the northern regions of Tanzania, and that high suitability is currently recorded for most of the previously non-occupied areas. In contrast, B. decoloratus, once abundant in Tanzania, has largely retreated to highlands in north and central Tanzania and several other high-altitude refuges. Geostatistical analysis revealed a dissimilar character of distribution for the two species at a local and regional scale, as well as an antagonistic relationship between them in a spatial context. The Tanzania data suggest that an equilibrium phase with a stable parapatric boundary that follows a climate gradient has been achieved. The zone of overlap in the distribution ranges of B. microplus and B. decoloratus is characterized by a decreased infestation rate as well as mutual exclusion on infested cattle. This finding matches studies on parapatry with hybrid zones, where the latter zones that separate species tend to rest in density troughs. Analysis of the Tanzania data shows that the critical factor for the advance of B. microplus and the retreat of B. decoloratus is associated with the 58 mm isohyet and the 22–23°C isotherm and indicates a well developed higher-temperature tolerance for B. microplus. It can be anticipated that climate changes may enhance the spread of B. microplus and consequently Babesia bovis into new areas of the African continent.     

Understanding the Relationships between Landscape Connectivity and Abundance of Ixodes ricinus Ticks

The density of Ixodes ricinus ticks in a heterogeneous landscape of northern Spain was determined and associated with some aspects of habitat topology. The habitat mosaic was used to quantify connectivity between patches of different tick density. The analysis revealed that patches with high tick abundance are ‘stepping-stone’ territories that, when removed from the landscape, cause large changes in connectivity. Sites with medium tick abundance do not cause such a critical transition in connectivity. Patches with low tick abundance, but optimal abiotic conditions for survival are located within the minimum cost corridors network joining the patches, while those sites where the tick has been intermittently collected are located at variable distances from this network. These results suggest that tick distribution in a zone is highly affected not only by abiotic variables (vegetation and weather) but also by host movements. Whether these high-density ‘stepping-stone’ patches occur in other tick species needs to be evaluated because of the potential implications of these foci for human health. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of acaricide resistance on cattle-fever tick (Boophilus spp.) infestations in semi-arid thornshrublands: a simulation approach

Cattle-fever tick (Boophilus microplus and B. annulatus) populations that develop acaricide resistance become more difficult to control or eradicate. We used a simulation model to assess the direct and indirect effects of interactions among season, habitat type, grazing strategy, and acaricide resistance on the ability to eradicate Boophilus infestations in semi-arid thornshrublands of Texas, USA. Season of infestation appeared to have the strongest effect, with infestations begun on 27 September (autumn) tending to die out sooner than those begun on 1 March (spring) and to remain undetected. Habitat type had the next strongest effect, with infestations surviving much longer as canopy cover increased from uncanopied buffelgrass (Cenchrus ciliaris) habitats to mesquite (Prosopis glandulosa)-canopied grass habitats. Acaricide resistance had a moderate effect; as expected, highly resistant tick populations survived longer than those with no acaricide resistance. The importance of grazing strategy varied with changes in habitat type: as canopy cover increased, infestation duration increased faster under continuous grazing than under rotational grazing strategies. Importance of grazing strategy also varied with acaricide resistance: detected tick populations with no and slight acaricide resistance subjected to acaricide treatments tended to survive longer under rotational grazing than continuous grazing, due to reduced contact with a treated host. Populations with moderate and high resistance behaved more like untreated populations, tending to survive longer under continuous, rather than rotational, grazing, because they experienced less mortality on a treated host. Assuming acaricide treatments at 2-week intervals and maintenance of cattle in infested pastures, results indicate that, for each habitat type, infesting ticks have a threshold of acaricide resistance below which one can eradicate them faster with continuous grazing than with rotational grazing. As canopy cover increases, this threshold appears to shift from high resistance (in grass) to slight resistance (in mesquite).     

Using ground‐derived data to assess the environmental niche of the spinose ear tick,Otobius megnini

Four layers of environmental information, namely 1) monthly atmospheric temperature and rainfall, 2) annual ground and underground moisture, evaporation, surface adjacent specific humidity, and temperature, 3) monthly Normalized Derived Vegetation Index (NDVI), and 4) soil physical attributes, were used separately to define the expected geographical distribution and environmental niche of the spinose ear tick, Otobius megnini (Dugès) (Acari: Ixodida: Argasidae), an endophilic argasid, in both tropical and neotropical regions. The best predictive values were obtained from ground‐derived climate. Air‐derived features ranked second. The remaining environmental information had poor discriminatory abilities. The most informative variables in the distribution of neotropical populations are ground temperatures, with surface humidity ranking second. In the tropics, surface humidity is the most important factor delineating the distribution of O. megnini. Marginality scores are similar for tick populations in both biogeographical regions, but specialization factors are different, supporting the findings that both populations are regulated by different variables. Similarly, models trained with records of one biogeographical region and projected into the other one, resulted in poorer predictions than when trained with the homologous set of records. Populations of the tick in the tropics experience a different range of temperatures than their neotropical relatives, whereas marginality scores are similar. The conclusion is that each population uses particular portions of the environmental niche, probably because of different climate or competitor constraints on either biogeographical region.     

The Known Distribution and Ecological Preferences of the Tick Subgenus Boophilus (Acari: Ixodidae) in Africa and Latin America

A compilation of the known distribution of Boophilus ticks in Africa and Latin America is presented, together with details on climate preferences. B. annulatus is recorded mainly in the western part of a strip from the equator to parallel 20° N. It associates with woodlands and forests (lowland rain forest and secondary grassland). This species is also present in the Mediterranean region, associated to woodland and open areas. B. decoloratus extends southern to parallel 20° N, in woodland with montane vegetation and Zambezian miombo; some records have been collected in the highveld grassland. B. geigyi is mainly collected in the western range of a stripe extending between parallels 5° N and 18° N, associated with Sudanian woodland, lowland rain forest with secondary grassland and woodland. Confirmed records of microplus in Africa are restricted to Malagasy region and south and eastern Africa, being predominant in the Zambezian miombo, deciduous forest with secondary grassland, and woodland. In Latin America, microplus is abundant in the Mesoamerican corridor to Venezuela and Colombia, and southern in Brazil and Argentina. The tick is mainly associated to the biomes of Chaco and Pampas in Argentina, the North-central moist Andes, the Atlantic forest (southern range) and the moist Meso-American vegetation (northern range). Most collections of B. annulatus and B. geigyi came from areas where winter minimum temperature is above 15 °C, maximum temperatures remain between 33 and 36 °C and maximum rainfall is recorded between June and September. B. decoloratus and African B. microplus are recorded in sites with low temperatures in May–September. Minimum temperature requirements are similar for both B. decoloratus and African B. microplus, and both are around 4 °C less than the value recorded for collections of Latin-American B. microplus. The rainfall pattern observed for decoloratus shows a minimum in May and June. The requirements of total rainfall are highest for B. microplus in Latin America, while records of African B. microplus are concentrated in areas of low rainfall between May and October, and high rainfall between November and March (low rainfall in the same period for B. decoloratus). Statistical analysis revealed the existence of populations (demes) with ecologically different requirements within each tick species. Both B. annulatus and B. decoloratus showed many different demes clearly associated to defined areas. The collections of Latin American B. microplus are very homogeneous according climate preferences and well separated from the African counterpart.     

宝天曼自然保护区不同针叶树径向生长对气候的响应

树木年轮资料能够提供区域内过去长时间的环境和气候信息,成为获取过去气候变化信息的重要手段之一。利用采自宝天曼自然保护区的两种针叶树种油松和华山松树木年轮样本分别建立了油松和华山松树轮宽度标准年表PT和PA,并将油松和华山松样本合并建立了联合树种的区域年表(RC)。3种年表分别与不同气候要素(月平均气温、月平均最高气温、月平均最低气温、月降水量)及其不同月份组合进行相关分析,结果表明,油松年表PT和华山松年表PA都包含较高的气候信息,且都和生长季不同月份温度显著负相关和降水显著正相关。其中,油松和华山松都与当年4月和5月降水显著正相关,油松还与当年5月的平均最高温度和上年11月最低温显著负相关,与当年3月平均最低气温显著正相关;华山松与上年10月和当年4月的平均最高温度显著负相关,与上年12月和当年7月平均最低温度显著正相关;联合年表RC包含了单个年表PT和PA共同的气候信息,与当年4—5月降水和3月最低温显著正相关,与当年7月最低温和4、5月最高温显著负相关。不同年表与生长季(3—8月)内气候要素月份组合的相关分析也表明联合年表RC包含和单物种年表PT、PA相似的气候信息并加强了PT、PA受当年生长季气候变化影响的公共信号。可见,同一地区的多树种联合年表一定程度上能体现出区域性树木生长对气候变化响应的生态生理特征,为同地区建成多树种联合年表来探讨当地区域性气候变化提供了可行性的理论基础和一定的参考作用.     

Microsatellite Loci of the Cattle Tick Boophilus Microplus (Acari: Ixodidae)

This brief communication reports the identification of microsatellite loci in the economically important tick species Boophilus microplus. The data are potentially useful in distinguishing different strains of B. microplus. Eight polymorphic loci were isolated in larvae, male and female adults analysed individually from 12 field isolates and laboratory strains from Australia (n = 8), Brazil, Mexico, Papua New Guinea and Zimbabwe. Nucleotide sequencing of alleles at these microsatellite loci revealed that non-repeat bases interrupted dinucleotide and tetranucleotide repeats in some loci. Loci with non-repeat bases interrupting them were shorter compared with loci that were not interrupted. Thus the presence of non-repeat bases in a repeated sequence seems to constrain the evolution of additional repeats by slip-strand misparing at these loci.     

Anatomical wood variation of Buddleja cordata (Buddlejaceae) along its natural range in Mexico

Buddleja cordata is an evergreen species of wide distribution in Mexico that is represented by shrubs and trees. Wood variability of B. cordata was evaluated in relation to plant size as well as latitude, altitude, soils, and climatic data. Canonical correlation analysis (CCA) showed that two canonical correlations are significant (Wilks' λ, p<0.0001) and explained 76% of total variance. Redundancy analysis revealed that the first pair of canonical variates are significant, thus the canonical variate, named distribution, represents a gradient of maximum temperature of the warmest period, annual temperature range, and latitude in its area of distribution; whereas the canonical variate named wood represents vessel density, fiber length, and plant size, best associated to the environmental gradient. Vessel density expressed by its distribution in latewood and porosity type showed that ring-porosity is common in individuals from high latitudes. Temperatures below zero or lack of rainfall during several months might induce porosity variability in B. cordata as suggested by CCA, but was not related to phenology since the species is evergreen along its latitudinal range. Plant size was also influenced by extreme temperature and rainfall. Shorter plants are distributed in the northern population or driest sites located in north-central Mexico, and in addition, fiber length followed an allometric relation with individuals' height. Wood characters in B. cordata as for simple perforation plate, helical thickenings, type of intervascular and vessel-ray pits, scanty paratracheal parenchyma, and heterogeneous type IIB rays were not correlated with plant size, climate, and soil parameters or species distribution. These features are common with other species of Buddleja.An erratum to this article can be found at     

基于GIS和Maxent模型的赤水河地区濒危植物桫椤生境适宜性评价

桫椤有"蕨类植物之王"赞誉,有"活化石"之称,曾是地球上最繁盛的植物,与恐龙同期。由于地质变迁和气候变化,目前只能在极少数的"避难所"才能寻其踪迹,而赤水河地区是当下桫椤种群较为集中分布的区域。所以,了解桫椤在赤水河地区的适生区分布,对于桫椤种质资源保护、桫椤种群的恢复重建具有重要意义。基于桫椤在赤水河流域的80个桫椤分布点位、20个样地信息和22个环境因子变量,借助ArcGIS运用Maxent模型软件预测了桫椤的潜在地理分布,综合环境因子变量贡献率、刀切法检验和土地利用现状变化对赤水河地区桫椤的生境适宜性进行评价分析,确定桫椤的生境适宜潜在地理分布区域和面积。研究表明：（1）Maxent模型的预测准确度极高,模型的预测结果为"极好";（2）最暖季的降水（39.65%）、温度日平均范围（18.21%）、温度季节性变化标准差（12.69%）和降水量季节性变异性系数（6.87%）是影响桫椤生长和分布的主导环境因子,累积贡献率达77.42%,可见桫椤在生长过程中对降水、温度的变化较为敏感;（3）模型预测高适生区主要集中在四川（泸州、宜宾、自贡）、重庆、贵州（遵义）,在空间上存在明显连续性,从高适生区到不适生区呈辐射状由中心向外围逐渐递减;虽桫椤高适生区面积高达49842 km²,但现状生境破碎化,人为干扰,人工植被较多等的影响会对桫椤种群造成不利影响。综上,赤水河地区桫椤适生生境面积大、连通性好,为本地区的桫椤提供了优质的生存环境,森林的人工抚育可促进桫椤种群的延续。     

Temporal Distribution of the Annual Nymphal Stock of Ixodes Ricinus Ticks

The human risk of contracting Lyme disease or other tick borne diseases transmitted by the tick species Ixodes ricinus is broadly linked to the tick nymph density. The study was performed in Rambouillet forest (Yvelines, France), a known focus of Lyme borreliosis, from January 1997 to December 1999. We used a nymph sampling methodology which permitted us to obtain a monthly nymph density index (from 0 to 5). Studying the seasonal nymph and larval activity patterns and estimating the larval developmental duration, we demonstrate the existence of an annual nymphal stock. Secondly, we elucidate how this stock is distributed throughout the year, month by month. Its distribution is principally dependent on two factors: the monthly mean ambient temperature and the proportion of active nymphs which find a host each month. Expected monthly nymph densities derived from a theoretical model describing the temperature-dependent stock distribution gave a good fit to the observed densities, accounting for between 76–86% of the monthly variation in observed nymph densities. Predicting the temporal distribution of nymph activity within a stable Lyme borreliosis focus enables more precise identification of risk periods.     

The Effects of Climate Change on the Long-Term Conservation of Fagus grandifolia var. mexicana, an Important Species of the Cloud Forest in Eastern Mexico

We examined the effects of climate change on the future conservation and distribution patterns of the cloud forests in eastern Mexico, by using as a species model to Fagus grandifolia Ehr. var. mexicana (Martínez) Little which is mainly located in this vegetation type, at the Sierra Madre Oriental. This species was selected because it is restricted to the cloud forest, where it is a dominant element and has not been considered for protection in any national or international law. It is probably threatened due to the fact that it plays an important social role as a source of food and furnishing. We used a floristic database and a bioclimatic modeling approach including 19 climatic parameters, in order to obtain the current potential distribution pattern of the species. Currently, its potential distribution pattern shows that it is distributed in six different Mexican Priority Regions for Conservation. In addition, we also selected a future climate scenario, on the basis of some climate changes predictions already proposed. The scenario proposed is characterized by +2 °C and −20% rainfall in the region. Under this predicted climatic condition, we found a drastic distribution contraction of the species, in which most of the remaining populations will inhabit restricted areas located outside the boundaries of the surrounding reserves. Consequently, our results highlight the importance of considering the effects of possible future climate changes on the selection of conservation areas and the urgency to conserve some remaining patches of existing cloud forests. Accordingly, we believe that our bioclimatic modeling approach represents a useful tool to undertake decisions concerning the definition of protected areas, once the current potential distribution pattern of some selected species is known.     

The mycorrhizal status of Welwitschia mirabilis

This is the first report of the mycorrhizal status of Welwitschia mirabilis, a gymnosperm endemic to the Namib Desert. Like all other gymnosperms except the Pinaceae and Gnetaceae, W. mirabilis is associated with vesicular-arbuscular mycorrhizal (VAM) fungi. Mycorrhizal colonization of roots and the diversity and abundance of VAM species were determined at seven sites. Six sites received annual rainfall of 0–100 mm, varying widely from year to year. The seventh site experienced more predictable annual rainfall of 150–200 mm. Perennial vegetation was sparse at the six low-rainfall sites. Dry annual grasses from previous rain events were present at only three of these six sites and mean mycorrhizal colonization levels of W. mirabilis at these three sites were as high as 18%. W. mirabilis was not mycorrhizal at sites where grasses were absent. The seventh site, receiving higher rainfall, supported small trees and annual grasses in addition to W. mirabilis. Mycorrhizal colonization levels of W. mirabilis at this site were significantly higher than at the other six sites, closely paralleling those of the surrounding annual grasses. The mycorrhizal flora of W. mirabilis consisted of four Glomus species. These taxa were not unique to W. mirabilis, having been found with Stipagrostis and Cladoraphis grasses throughout the Namib and Kalahari deserts.     

Impact of landscape predictors on climate change modelling of species distributions: a case study with Eucalyptus fastigata in southern New South Wales,Australia

Aim We consider three questions. (1) How different are the predicted distribution maps when climate‐only and climate‐plus‐terrain models are developed from high‐resolution data? (2) What are the implications of differences between the models when predicting future distributions under climate change scenarios, particularly for climate‐only models at coarse resolution? (3) Does the use of high‐resolution data and climate‐plus‐terrain models predict an increase in the number of local refugia? Location South‐eastern New South Wales, Australia. Methods We developed two species distribution models for Eucalyptus fastigata under current climate conditions using generalized additive modelling. One used only climate variables as predictors (mean annual temperature, mean annual rainfall, mean summer rainfall); the other used both climate and landscape (June daily radiation, topographic position, lithology, nutrients) variables as predictors. Predictions of the distribution under current climate and climate change were then made for both models at a pixel resolution of 100 m. Results The model using climate and landscape variables as predictors explained a significantly greater proportion of the deviance than the climate‐only model. Inclusion of landscape variables resulted in the prediction of much larger areas of existing optimal habitat. An overlay of predicted future climate on the current climate space indicated that extrapolation of the statistical models was not occurring and models were therefore more robust. Under climate change, landscape‐defined refugia persisted in areas where the climate‐only model predicted major declines. In areas where expansion was predicted, the increase in optimal habitat was always greater with landscape predictors. Recognition of extensive optimal habitat conditions and potential refugia was dependent on the use of high‐resolution landscape data. Main conclusions Using only climate variables as predictors for assessing species responses to climate change ignores the accepted conceptual model of plant species distribution. Explicit statements justifying the selection of predictors based on ecological principles are needed. Models using only climate variables overestimate range reduction under climate change and fail to predict potential refugia. Fine‐scale‐resolution data are required to capture important climate/landscape interactions. Extrapolation of statistical models to regions in climate space outside the region where they were fitted is risky.     

Effects of long-term rainfall variability on evapotranspiration and soil water distribution in the Chihuahuan Desert: A modeling analysis

We used the patch arid land simulator (PALS-FT) – a simple, mechanistic ecosystem model – to explore long-term variation in evapotranspiration (ET) as a function of variability in rainfall and plant functional type (FT) at a warm desert site in southern New Mexico. PALS-FT predicts soil evaporation and plant transpiration of a canopy composed of five principal plant FTs: annuals, perennial forbs, C₄ grasses, sub-shrubs, and evergreen shrubs. For each FT, the fractional contribution to transpiration depends upon phenological activity and cover as well as daily leaf stomatal conductance, which is a function of plant water potential, calculated from root-weighted soil water potential in six soil layers. Simulations of water loss from two plant community types (grass- vs. shrub-dominated) were carried out for the Jornada Basin, New Mexico, using 100 years of daily precipitation data (1891–1990). In order to emphasize variability associated with rainfall and fundamental differences in FT composition between communities, the seasonal patterns cover of perennials were held constant from year to year. Because the relative amount of year to year cover of winter and summer annual species is highly variable in this ecosystem, we examined their influence on model predictions of ET by allowing their cover to be variable, fixed, or absent.Over the entire 100-yr period, total annual ET is highly correlated with total annual rainfall in both community types, although T and E alone are less strongly correlated with rainfall, and variation in transpiration is nearly 3 times greater than evaporation and 2 times greater than variation in rainfall (CV of rainfall = 35%). Water use shows a relatively high similarity between the grass- and shrub-dominated communities, with a 100-yr average T/ET of 34% for both communities. However, based on a year-by-year comparison between communities, T/ET was significantly greater in the grass-dominated community, reflecting the fact that over the long term more than half of the rain occurs in the summer and is used slightly more efficiently (T¿E) by the C₄-grass community than the shrub community, although we found some rainfall patterns that resulted in much greater T/ET in the shrub community in a given year. Percent of water lost as transpiration (T/ET) suggests that while there is a general trend toward increased T/ET with rainfall in both community types, T/ET is extremely variable over the 100-yr simulation, especially for normal and below normal amounts of rainfall (T/ET values range from 1 to 58% for the grass-dominated site and 6 to 60% for the shrub-dominated site).These predictions suggest that because of the relatively shallow distribution of soil water, there is little opportunity for vertical partitioning of the soil water resource by differential rooting depths of the plant FTs, in contrast to the two-layer hypothesis of Walter (1971). However, functional types may avoid competition by keying on particular `windows' of moisture availability via differences in phenologies. We found very little differences in average, long-term model predictions of T, E, and ET when annual plant cover was variable, fixed, or absent. The results of our simulations help reconcile some of the disparate conclusions drawn from experimental studies about the relative contribution of transpiration vs. evaporation to total evapotranspiration, primarily by revealing the great year-to-year variability that is possible.