Habitat primary production and the evolution of body size within the hartebeest clade

Local adaptation is a key process in the evolution of biological diversity but relatively few studies have identified the selective forces that drive trait divergence at low taxonomic levels, particularly amongst mammals. Variation in body size across taxa is fundamental as shown by allometric relationships with numerous physiological, morphological and life-history traits. Differences in adult size across cohorts within populations of temperate ungulates are determined by variation in trophic resource availability during growth, suggesting that natural selection might promote the evolution of size divergence across sister taxa through local adaptation to variation in habitat productivity. We tested this hypothesis in the hartebeest ( Alcelaphu s sp.), an antelope lineage including eight extant (or recently extinct) allopatric subspecies that evolved within the last million years and colonized all the African savannahs. We predicted that body size across the subspecies should correlate positively with habitat productivity across taxon ranges. Mean body size of all the hartebeest taxa was quantified using skull length from museum specimens, and climatic variables were used as surrogates of habitat productivity. Body size across subspecies was positively correlated with rainfall, suggesting that variation in habitat primary production may drive morphological evolution between taxa. Focusing at a low taxonomic level has allowed us to identify a critical selective force that may shape divergence in body size, without the confounding effect of variation in trophic niche. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 431–440.     

Phylogenetic analysis and classification of fossil and recent Alcelaphini Mammalia: Bovidae

Cladistic principles are applied to a group of antelope species, three-quarters of which are extinct. Only cranial and mandibular characters are used. The present phylogenetic hypothesis suggests diat the Alcelaphini are monophyletic; that the genera Parmularius and Damaliscus share a recent common ancestry; and that die extant Hunter's hartebeest should not be removed from Damaliscus into a separate genus Beatragus as several authors have done. A re-evaluation is suggested of die systematic position of Lichtenstein's hartebeest, as being phylogenetically closer to me wildebeests, genus Connochaetes , than to the type species of the hartebeest genus Alcelaphus. Some problems arising in the application of cladistic principles to such a low ranking group, treating die fossil majority exactly equivalently with recent taxa, are discussed. Comments on recency of common ancestry as it affects hybridization potential in bovids, on speciation and extinction rates arise.     

Phylogeography of three closely related African bovids (tribe Alcelaphini)

The phylogeography of three species of African bovids, the hartebeest (Alcelaphus buselaphus), the topi (Damaliscus lunatus), and the wildebeest (Connochaetes taurinus), is inferred from sequence variation of 345 sequences at the control region (d-loop) of the mtDNA. The three species are closely related (tribe Alcelaphini) and share similar habitat requirements. Moreover, their former distribution extended over Africa, as a probable result of the expansion of open grassland on the continent during the last 2.5 Myr. A combination of population genetics (diversity and structure) and intraspecific phylogeny (tree topology and relative branch length) methods is used to substantiate scenarios of the species history. Population dynamics are inferred from the distribution of sequence pairwise differences within populations. In the three species, there is a significant structuring of the populations, as shown by analysis of molecular variance (AMOVA) pairwise and hierarchical differentiation estimations. In the wildebeest, a pattern of colonization from southern Africa toward east Africa is consistent with the asymmetric topology of the gene tree, showing a paraphyletic position of southern lineages, as well as their relatively longer branch lengths, and is supported by a progressive decline in population nucleotide diversity toward east Africa. The phylogenetic pattern found in the topi and the hartebeest differs from that of the wildebeest: lineages split into monophyletic clades, and no geographical trend is detected in population diversity. We suggest a scenario where these antelopes, previously with wide pan-African distributions, became extinct except in a few refugia. The hartebeest, and probably also the topi, survived in refugia north of the equator, in the east and the west, respectively, as well as one in the south. The southern refugium furthermore seems to have been the only place where the wildebeest has survived.     

Forage patch use by grazing herbivores in a South African grazing ecosystem

Understanding how different herbivores make forage patch use choices explains how they maintain an adequate nutritional status, which is important for effective conservation management of grazing ecosystems. Using telemetry data, we investigated nonruminant zebra (Equus burchelli) and ruminant red hartebeest (Alcelaphus buselaphus subspecies camaa), use of burnt patches in a landscape mosaic of nutrient-poor, old grassland interspersed with young, recently burnt, nutrient-rich grass patches. The Mkambati Nature Reserve landscape on the east coast of South Africa provided large grazers with a challenge in finding and using appropriate patches in which to forage to meet their nutritional requirements. In Mkambati, grassland fires, mostly ignited by poachers, induce regrowth of young nutrient-rich grass, which subsequently attract grazers. We tested if the study animals foraged more in burnt patches than in the unburned grassland and whether burnt patch use was related to the distance to the previously visited burnt patch, burnt patch size, burnt patch age, and distance to areas with high poaching risk using MANOVA. In general, zebra moved faster than red hartebeest, and both species moved faster in unburnt grassland than in burnt patches. Red hartebeest and zebra patch selection were influenced by interpatch distance, patch age, patch size, and poaching risk. A limited set of intrinsic traits, i.e., body mass, digestion strategy, and muzzle width, yielded different patch use rules for the two species. Large ungulates patch use behaviour varied among species and across conditions and was influenced by anthropogenic impacts such as poaching and changed fire regimes. This could potentially affect biodiversity negatively and needs to be factored into management of conservation areas.     

Cytochrome b phylogeny of the family bovidae: resolution within the alcelaphini, antilopini, neotragini, and tragelaphini

The family Bovidae is characterized by an incomplete fossil record for the period during which most bovid subfamilies emerged. This, coupled to extensive morphological convergence among species, has given rise to inconsistencies in taxonomic treatments, especially at the tribal and subfamilial levels. In an attempt to clarify some of these issues we analyzed the complete mtDNA cytochrome b gene (1140 bp) from 38 species/subspecies representing at least nine tribes and six subfamilies. Specific emphasis was placed on the evolution of the Alcelaphini (hartebeest and wildebeest), the Tragelaphini (kudu, eland, and close allies), the Antilopini (gazelles), and the Neotragini (dwarf antelope). Saturation plots for the codon positions revealed differences between bovid tribes and this allowed for the exclusion of transitional substitutions that were characterized by multiple hits. There was no significant rate heterogeneity between taxa. By calibrating genetic distance against the fossil record, a transversion-based sequence divergence of 0.22% (+/-0.015%) per million years is proposed for cytochrome b clock calibrations in the Bovidae. All evidence suggests that the Alcelaphini form a monophyletic group; there was no support for the recognition of the Lichtenstein's hartebeest in a separate genus (Sigmoceros), and the acceptance of the previously suggested Alcelaphus is recommended for this species. High bootstrap support was found for a sister taxon relationship between Alcelaphus and Damaliscus, a finding which is in good agreement with allozyme and morphological studies. In the case of the Tragelaphini, the molecular data suggest the inclusion of Taurotragus in the genus Tragelaphus, and no genetic support was found for the generic status of Boocercus. Although associations within the Antilopinae (comprising the tribes Neotragini and Antilopini) could not be unequivocally resolved, there was nonetheless convincing evidence of non-monophyly for the tribe Neotragini, with the Suni antelope (Neotragus moschatus) grouping as a sister taxon to the Impala (Aepyceros melampus, tribe indeterminate, sensu Gentry, 1992) and the Klipspringer (Oreotragus oreotragus) falling within the duiker antelope tribe (Cephalophini).     

Seasonal reproductive characteristics of female and male Jackson's hartebeest (Alcelaphus buselaphus jacksoni)

This project examined reproductive characteristics of female and male Jackson's hartebeest (Alcelaphus buselaphus jacksoni), a subspecies that originates in Africa and is currently a model for studies of endangered hartebeest subspecies. Progestagen concentrations were measured in fecal samples collected thrice weekly for 1 year from three non-pregnant, adult females, in the Northern Hemisphere (31 degrees 37'N, 81 degrees 09'W). When their ovaries were active, the females exhibited regular luteal cycles with an overall mean (+/-S.D.) cycle length of 21.4+/-4.1 days (n=31 luteal phases). Peak luteal progestagen concentration was 1.73+/-0.63mug/g, with a nadir concentration of 0.79+/-0.24mug/g. Cyclic activity ceased from 6 April to 28 June, 7 April to 8 July, and from 18 February to 20 August, for the three females, respectively. During this acyclic period, mean progestagen concentration was 0.90+/-0.23mug/g. Ejaculates were collected by electroejaculation from seven males throughout all seasons, with mean (+/- S.D.) 40+/-18% motility, 4.1+/-0.19 progressive motility (scale, 0-5), 1373+/-826x10(6)sperm/mL, and 42+/-28% morphologically normal sperm. These data characterized basic reproductive traits for Jackson's hartebeest and established the females as spontaneously ovulating and seasonally polyestrous when housed in the Northern Hemisphere, whereas males produced apparently viable sperm throughout the year.     

Chromosome homologies between tsessebe (Damaliscus lunatus) and Chinese muntjac (Muntiacus reevesi) facilitate tracing the evolutionary history of Damaliscus (Bovidae, Antilopinae, Alcelaphini)

Genome-wide homologies between the tsessebe (Damaliscus lunatus, 2n = 36) and Chinese muntjac (Muntiacus reevesi, 2n = 46) have been established by cross-species painting with Chinese muntjac chromosome paints. Twenty-two autosomal painting probes detected 35 orthologous segments in the tsessebe. Hybridization results confirmed that: (i) D. lunatus carries the (9;14) reciprocal translocation that has been proposed to be a derived chromosomal landmark shared by all species of the Antilopinae; (ii) the karyotype of D. lunatus can be derived almost exclusively from the bovid ancestral karyotype through 12 Robertsonian translocations involving 24 ancestral acrocentric autosomes; (iii) in addition to the Rb fusions, pericentric heterochromatic amplification has shaped the morphology of several of the D. lunatus chromosomes. Integrated analysis of these and published cytogenetic data on pecorans has allowed us to accurately discern the karyotype history of Damaliscus (D. lunatus; D. pygargus, 2n = 38; D. hunteri, 2n = 44). The phylogenomic relationships of 3 species reflected by specific chromosomal rearrangements were consistent with published phylogenies based on morphology, suggesting that chromosomal rearrangements have played an important role in speciation within the Alcelaphini, and that karyotype characters are valuable phylogenetic markers in this group.     

Evaluation of LH-RH stimulation of testosterone as an index of reproductive status in rams and its application in wild antelope

In rams a positive correlation (P less than 0.001) existed between average testosterone levels from 30-min blood sampling for 18 h and average testosterone levels of samples taken 0, 1 and 2 h after injection of LH-RH administered 90 min after anaesthesia. Attempts were therefore made to assess testosterone status by LH-RH challenge and limited blood sampling in animals immobilized in their natural habitat. In impala (Aepyceros melampus) territorial males had higher plasma testosterone values than did bachelors after LH-RH challenge (8.1 compared with 2.6 ng/ml, P less than 0.05). In blesbok (Damaliscus dorcas), the relationship was less clear, but testicular volume was correlated with plasma testosterone concentration and with testicular responsiveness measured by testosterone produced per unit of LH (P less than 0.001 and P less than 0.05, respectively). The LH-RH challenge technique therefore has value as a measure of testicular function and permits study of ungulates in their natural environment.     

Nematode biomass changes along an elevational gradient are trophic group dependent but independent of body size

Aboveground, large and higher trophic-level organisms often respond more strongly to environmental changes than small and lower trophic-level organisms. However, whether this trophic or size-dependent sensitivity also applies to the most abundant animals, microscopic soil-borne nematodes, remains largely unknown. Here, we sampled an altitudinal transect across the Tibetan Plateau and applied a community-weighted mean (CWM) approach to test how differences in climatic and edaphic properties affect nematode CWM biomass at the level of community, trophic group and taxon mean biomass within trophic groups. We found that climatic and edaphic properties, particularly soil water-related properties, positively affected nematode CWM biomass, with no overall impact of altitude on nematode CWM biomass. Higher trophic-level omnivorous and predatory nematodes responded more strongly to climatic and edaphic properties, particularly to temperature, soil pH, and soil water content than lower trophic-level bacterivorous and fungivorous nematodes. However, these differences were likely not (only) driven by size, as we did not observe significant interactions between climatic and edaphic properties and mean biomasses within trophic groups. Together, our research implies a stronger, size-independent trophic sensitivity of higher trophic-level nematodes compared with lower trophic-level ones. Therefore, our findings provide new insights into the mechanisms underlying nematode body size structure in alpine grasslands and highlight that traits independent of size need to be found to explain increased sensitivity of higher trophic-level nematodes to climatic and edaphic properties, which might affect soil functioning.     

Morphology, constraints, and scaling of frontal sinuses in the hartebeest, Alcelaphus buselaphus (Mammalia: Artiodactyla, Bovidae)

The frontal sinuses of bovid mammals display a great deal of diversity, which has been attributed to both phylogenetic and functional influences. In-depth study of the hartebeest (Alcelaphus buselaphus), a large African antelope, reveals a number of previously undescribed details of frontal sinus morphology. In A. buselaphus, the frontal sinuses conform closely to the shape of the frontal bone, filling nearly the entire element. However, the horncores are never extensively pneumatized, contrasting with the condition seen in many other bovids. This evidence is inconsistent with the hypothesis that sinuses are opportunistic pneumatizing agents, suggesting that phylogenetic factors also play a role in determining sinus size. Both cranial sutures and neurovasculature appear to constrain the growth of sinuses in part. In turn, the sinus also affects the growth of the parietal; apparently this element is not truly pneumatized by the sinus in most cases, but the bone's shape changes under the influence of the sinus. Furthermore, the sinuses present relatively few struts when compared with the sinuses of some other bovids, such as Ovis. By adapting methods previously developed for measuring structural parameters of trabecular bone, it is possible to quantify certain aspects of sinus morphology. These include the number of bony struts within the sinus, the spacing of these struts, and the size of individual cavities within the sinus. Some differences in the number of struts are evident between subspecies. Similarly, significant differences occur in the relative number of struts between male and female A. buselaphus, which may be related to behavior. The volume of the sinus is strongly correlated with the size of the frontal, but less so with overall cranial size. This finding illustrates the importance of choosing variables carefully when comparing sinus sizes and growth between species.     

Environmental change and rates of evolution: the phylogeographic pattern within the hartebeest complex as related to climatic variation

Global climate fluctuated considerably throughout the Pliocene-Pleistocene period, influencing the evolutionary history of a wide array of species. Using the phylogeographic patterns within the hartebeest (Alcelaphus buselaphus (Pallas, 1766)) complex, we evaluated the evolutionary consequences of such environmental change for a typical large mammal ranging on the African savannah. Our results, as generated from two mitochondrial DNA markers (the D-loop and cytochrome b), suggest an origin of the hartebeest in eastern Africa from where the species has colonized other parts of the continent. Phylogenetic analyses revealed an early diversification into southern and northern hartebeest lineages, an event that may be related to the formation of the Rift Valley lakes. The northern lineage has further diverged into eastern and western lineages, most probably as a result of the expanding central African rainforest belt and subsequent contraction of savannah habitats during a period of global warming. The diversification events appear to have coincided with major climatic changes and are highly correlated in time. These observations strongly suggest that large-scale climatic fluctuations have been a major determinant for the species' evolutionary history and that hartebeest evolution has mainly taken place in isolated yet environmentally favourable refugia during periods of global warming. Indications of sudden population expansion for two putative ancestral hartebeest populations provide further support for a refugia-based explanation of the diversification events. Reciprocal monophyly between southern and northern lineages may suggest that reproductive barriers exist and that the hartebeest complex comprises two different species.     

More stable productivity of semi natural grasslands than sown pastures in a seasonally dry climate

In the Neotropics the predominant pathway to intensify productivity is generally thought to be to convert grasslands to sown pastures, mostly in monoculture. This article examines how above-ground net primary productivity (ANPP) in semi-natural grasslands and sown pastures in Central America respond to rainfall by: (i) assessing the relationships between ANPP and accumulated rainfall and indices of rainfall distribution, (ii) evaluating the variability of ANPP between and within seasons, and (iii) estimating the temporal stability of ANPP. We conducted sequential biomass harvests during 12 periods of 22 days and related those to rainfall. There were significant relationships between ANPP and cumulative rainfall in 22-day periods for both vegetation types and a model including a linear and quadratic term explained 74% of the variation in the data. There was also a significant correlation between ANPP and the number of rainfall events for both vegetation types. Sown pastures had higher ANPP increments per unit rainfall and higher ANPP at the peak of the rainy season than semi-natural grasslands. In contrast, semi-natural grasslands showed higher ANPP early in the dry season. The temporal stability of ANPP was higher in semi-natural grasslands than in the sown pastures in the dry season and over a whole annual cycle. Our results reveal that, contrary to conventional thinking amongst pasture scientists, there appears to be no increase in ANPP arising from replacing semi-natural grasslands with sown pastures under prevailing pasture management practices in seasonally dry climates, while the temporal distribution of ANPP is more even in semi-natural grasslands. Neither sown pastures nor semi-natural grasslands are productive towards the end of the dry season, indicating the potential importance of the widespread practice of retaining tree cover in pastures.     

The sensitivity of annual grassland carbon cycling to the quantity and timing of rainfall

Global climate models predict significant changes to the rainfall regimes of the grassland biome, where C cycling is particularly sensitive to the amount and timing of precipitation. We explored the effects of both natural interannual rainfall variability and experimental rainfall additions on net C storage and loss in annual grasslands. Soil respiration and net primary productivity (NPP) were measured in treatment and control plots over four growing seasons (water years, or WYs) that varied in wet‐season length and the quantity of rainfall. In treatment plots, we increased total rainfall by 50% above ambient levels and simulated one early‐ and one late‐season storm. The early‐ and late‐season rain events significantly increased soil respiration for 2–4 weeks after wetting, while augmentation of wet‐season rainfall had no significant effect. Interannual variability in precipitation had large and significant effects on C cycling. We observed a significant positive relationship between annual rainfall and aboveground NPP across the study (P=0.01, r²=0.69). Changes in the seasonal timing of rainfall significantly affected soil respiration. Abundant rainfall late in the wet season in WY 2004, a year with average total rainfall, led to greater net ecosystem C losses due to a ～50% increase in soil respiration relative to other years. Our results suggest that C cycling in annual grasslands will be less sensitive to changes in rainfall quantity and more affected by altered seasonal timing of rainfall, with a longer or later wet season resulting in significant C losses from annual grasslands.     

Population status and human impact on the endangered Swayne’s hartebeest (Alcelaphus buselaphus swaynei) in Nechisar Plains,Nechisar National Park,Ethiopia

The study on the population status and human impacts on the endemic and endangered Swayne’s hartebeest (Alcelaphus buselaphus swaynei) was carried out in Nechisar Plains, Nechisar National Park, between 2009 and 2010. Direct and total count methods were applied in the population census. Data collected since 1967 were also examined to indicate trends of Swayne’s hartebeest populations and their translocation status. In eight illegal Guji Oromo settlements, a total of 134 household samples were identified for group discussion and interview. A trend on livestock population was carried out. The total count of Swayne’s hartebeest was 12, 11 and 12 during the 1st, 2nd and 3rd counts, respectively. The population has been decreasing and nowadays only 12 individuals remain. Most respondents had a negative attitude towards conservation areas. The increase in livestock population from 1985 to 1996 was 47.8%. It also increased by 49.9% and 56.5% during 2006 and 2010, respectively. There were 12,531 heads of livestock that depended on the grass plains illegally. Overstocking rate of livestock, illegal resource exploitation and loss of wildlife habitat were the major problems encountered. Active measures have to be implemented to control the human impact and safeguard the future of Swayne’s hartebeest.     

Contrasting above‐ and belowground sensitivity of three Great Plains grasslands to altered rainfall regimes

Intensification of the global hydrological cycle with atmospheric warming is expected to increase interannual variation in precipitation amount and the frequency of extreme precipitation events. Although studies in grasslands have shown sensitivity of aboveground net primary productivity (ANPP) to both precipitation amount and event size, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and NPP. We conducted a 2‐year experiment in three US Great Plains grasslands – the C₄‐dominated shortgrass prairie (SGP; low ANPP) and tallgrass prairie (TGP; high ANPP), and the C₃‐dominated northern mixed grass prairie (NMP; intermediate ANPP) – to test three predictions: (i) both ANPP and BNPP responses to increased precipitation amount would vary inversely with mean annual precipitation (MAP) and site productivity; (ii) increased numbers of extreme rainfall events during high‐rainfall years would affect high and low MAP sites differently; and (iii) responses belowground would mirror those aboveground. We increased growing season precipitation by as much as 50% by augmenting natural rainfall via (i) many (11–13) small or (ii) fewer (3–5) large watering events, with the latter coinciding with naturally occurring large storms. Both ANPP and BNPP increased with water addition in the two C₄ grasslands, with greater ANPP sensitivity in TGP, but greater BNPP and NPP sensitivity in SGP. ANPP and BNPP did not respond to any rainfall manipulations in the C₃‐dominated NMP. Consistent with previous studies, fewer larger (extreme) rainfall events increased ANPP relative to many small events in SGP, but event size had no effect in TGP. Neither system responded consistently above‐ and belowground to event size; consequently, total NPP was insensitive to event size. The diversity of responses observed in these three grassland types underscores the challenge of predicting responses relevant to C cycling to forecast changes in precipitation regimes even within relatively homogeneous biomes such as grasslands.     

Patterns of sexual dimorphism in the Persian Leopard (Panthera pardus saxicolor) and implications for sex differentiation

Extant felids show a high degree of inter-sexual dimorphism, meaning significant size differences between males and females. Such a differentiation may have various ecological, behavioural and evolutionary implications, at both species and subspecies levels. We have investigated the sexual size differences in one of the most dimorphic felids, i.e. the Leopard (Panthera pardus), based on 63 craniometric and 55 morphometric samples from Iran which belong to the subspecies Persian Leopard (P. p. saxicolor). In order to explore patterns of sexual dimorphism, multivariate statistical analysis on 24 skull variables as well as univariate approaches for two body measurements were applied. We found significant inter-sexual differences in skull size whereas it was not meaningful after removing the effect of size to address skull shape. Moreover, inter-sexual differentiation was also remarkable when comparing morphometric body measurements in adults, showing that the males possess a larger head mass and longer body, but sub-adults did not show any remarkable differentiation between sexes. A combination of craniometric and morphological features is proposed for sex differentiation in Leopards.     

A multivariate statistical study of skull measurements of five taxa of gazelles

A multivariate analysis using canonical variates and the D² test show that five taxa of Palaearctic gazelles examined are distinct. Several forms of gazelles of dubious taxonomic position were also compared on the canonical axes, with conclusions not dissimilar to those of recent workers on the Arabian gazelles, viz. that there are three distinct species in the Arabian peninsula; Gazella dorcas saudiya, Gazella gazella arabica and Gazella subgutturosa marica. Gazella dorcas populations in Somalia have a local form in G. dorcas pelzelni distinct in horns but not in skull measurements from Gazella spekei in the nearby highlands.     

Size, space, and adaptation in some subfossil lemurs from Madagascar

We examine several explanations for the geographic pattern of body size variation exhibited by the subfossil lemur Archaeolemur. Part and partial correlation analysis and multiple regression analysis are applied in a stepwise, hierarchical fashion to help to determine variable interdependencies. Variance in site means for body size is best explained by the richness of the plant community and by several correlated climatic variables (bioclimatic zone and mean annual rainfall). Body size differentiation in Archaeolemur roughly mirrors patterns observed among many other Malagasy lemur species and subspecies groups. This consistency alone suggests that common ecological factors have strongly affected size differentiation in lemurs, most probably (as suggested by our correlation analyses) by uniformly influencing the productivity of their niches. Smaller individuals tend to inhabit arid regions, and larger individuals tend to inhabit wetter regions. The interplay between selective differentiation and allopatric speciation appears to have yielded the concordant pattern of size variation observed in Malagasy lemurs.     

Environmental factors and community dynamics at the southernmost part of the North American Graminetum

We have delineated the spatio-temporal plant assemblages prevailing under contrasting environmental conditions at the southernmost part of the North American Graminetum in central México and discuss the relative importance of these factors in determining plant community structure and composition. 353 line-transect samples were collected during 11 years from exclosures and adjacent grazed areas within three Bouteloua-dominated grasslands and one Acacia schaffneri shrubland and analyzed using TWINSPAN. In addition, eight edaphic variables were utilized to evaluate similarities in soil properties among sites, using unweighted-pair groups method. Results from TWINSPAN were translated into mosaic patterns to show the distribution of communities as a function of environmental factors over time. Under no or moderate grazing, summer precipitation promotes an initial differentiation of vegetation into high, low or average rainfall communities. This situation is altered in grasslands degraded by intensive grazing pressure, where rainfall patterns play a subordinate role to that of livestock herbivory. Soil influences are secondary to those of precipitation and grazing in affecting grassland structure and composition. In shrubland, community stability is related to high floristic and edaphic spatial heterogeneity in the face of climatic variability.