Environmental effects on Neotropical liana species richness

Aim Lianas differ physiologically from trees, and therefore their species‐richness patterns and potential climate‐change responses might also differ. However, multivariate assessments of spatial patterns in liana species richness and their controls are lacking. Our aim in this paper is to identify the environmental factors that best explain the variation in liana species richness within tropical forests. Location Lowland and montane Neotropical forests. Methods We quantified the contributions of environmental variables and liana and tree‐and‐shrub abundance to the species richness of lianas, trees and shrubs ≥ 2.5 cm in diameter using a subset of 65 standardized (0.1 ha) plots from 57 Neotropical sites from a global dataset collected by the late Alwyn Gentry. We used both regression and structural equation modelling to account for the effects of environmental variables (climate, soil and disturbance) and liana density on liana species richness, and we compared the species‐richness patterns of lianas with those of trees and shrubs. Results We found that, after accounting for liana density, dry‐season length was the dominant predictor of liana species richness. In addition, liana species richness was also related to stand‐level wood density (a proxy for disturbance) in lowland forests, a pattern that has not hitherto been shown across such a large study region. Liana species richness had a weak association with soil properties, but the effect of soil may be obscured by the strong correlation between soil properties and climate. The diversity patterns of lianas and of trees and shrubs were congruent: wetter forests had a greater species richness of all woody plants. Main conclusions The primary association of both liana and tree‐and‐shrub species richness with water availability suggests that, if parts of the Neotropics become drier as a result of climate change, substantial declines in the species richness of woody plants at the stand level may be anticipated.     

Tree species richness in primary and old secondary tropical forest in Singapore

If secondary succession can accumulate species rapidly, then tropical secondary forests may have an important role to play in the conservation of biodiversity. Data on the floristic composition of forest stands in the Central Catchment Nature Reserve, Singapore, have been analysed to investigate the diversity of approximately 100-year-old tropical secondary forest. Classification using TWINSPAN indicated that three floristic communities could be recognized from 59 0.2 ha plots enumerated for trees >30 cm gbh. These were two types of secondary forest, both dominated by Rhodamnia cinerea (Myrtaceae), and dryland primary forest. The secondary forest was developed on land abandoned after use for agriculture at the end of the 19th century. The 16 primary forest plots contained a total of 340 species, more than the 281 recorded from the 43 plots of the two secondary forest types combined. The mean species number per plot in the more diverse of the two secondary forests was only about 60% of the primary forest. Thus the secondary forest, despite a century or so for colonization by species and the presence of contiguous primary forest, was still significantly less diverse than primary forest areas. It is concluded that secondary forest cannot be assumed to accrete biodiversity rapidly in the tropics, and may not be of direct value in conservation. However, other indirect roles, such as providing resources for native animals, and buffering and protecting primary forest fragments may make the protection of secondary forest worthwhile.     

Can high tree species richness be explained by Hubbell’s null model?

We examine several features of Hubbell’s nonequilibrium, or “null”, model of tree dynamics, which holds that species-rich tropical tree communities are maintained on a local scale by a balance of extinction and immigration, and on a global scale by a balance of extinction and speciation. All species are held to be ecologically equivalent, such that species having equal initial abundances have equal probabilities of extinction or fixation. We show here that the null model is not robust to relaxation of the assumption of ecological equivalence. Recently, 32 ; J. Theor. Biol. 188: 361–367) showed that persistence times decrease when unequal colonization rates are allowed, but their results still permit very long persistence times in stands of hundreds of thousands of stems or more. We extend their work by allowing tree mortality rates to differ across species, as is seen in all natural tree communities. As a result, persistence times drop dramatically, and forest composition becomes highly deterministic, such that long-lived species drop out of the community much more slowly than short-lived species. We also note that the use of tree deaths (instead of years) as a measure of time inflates estimates of persistence times. In summary, calculated persistence times of tropical tree species, even those in very large stands, no longer reach time scales plausible for speciation.     

Reconciling topographic and climatic effects on widespread and range-restricted species richness

Aim To identify the reasons behind differing geographical species richness patterns of range‐restricted and widespread species. Location The Western Hemisphere. Methods We used regression to determine the strongest environmental predictors of richness for widespread and range‐restricted mammal species in 10,000 km² quadrats in the continental Americas. We then used range‐placement models to predict the expected correlation between range‐restricted and widespread species richness were they to be determined by identical, random, or contrasting environmental factors. Finally, to determine the reasons underlying deviations from these predictions, we divided the Americas into 5% quantiles based on temperature and topographic heterogeneity and correlated richness of these two assemblages across quantiles – an approach that avoids constraints on statistical testing imposed by low potential for range overlap among range‐restricted species. Results Minimum annual temperature was the strongest predictor of widespread species richness while topographic heterogeneity was the best, although weak, predictor of range‐restricted species richness in conventional regression analysis. Our models revealed that the observed correlation between range‐restricted and widespread species richness was similar to what would be observed if both range‐restricted and widespread species richness were determined by temperature. Patterns of range‐restricted and widespread species richness were highly correlated across temperature quantiles, but range‐restricted species uniquely showed an increasing pattern across heterogeneity quantiles. Main conclusions Species richness gradients among range‐restricted species differ from those of widespread species, but not as extensively or for the reasons reported previously. Instead, these assemblages appear to share some but not all underlying environmental determinants of species richness. Our new approach to examining species richness patterns reveals that range‐restricted and widespread species richnesses share a common response to temperature that conventional analyses have not previously revealed. However, topographic heterogeneity has assemblage‐specific effects on range‐restricted species.     

Drivers of understorey biomass: tree species identity is more important than richness in a young forest

林下生物量影响因素：幼龄林树种特性比丰富度更重要 生物多样性与生态系统功能的正相关关系已被广泛报道,其主要来源于对草原生态系统的研究。然而,该结论并不一定适用于更复杂的环境,例如具有不同垂直层次的森林。举例而言,已有研究表明上层乔木树种丰富度与林下生产力降低有关。树种丰富度是否会通过增加(由于生境异质性)或降低(通过增强竞争)资源的可利用性进而影响林下生产力,以及林下生产力是否受树种特性的影响更大,这些影响机制都可能会随着时间的推移而改变。此外,研究还表明,丰富度-生产力关系随着环境背景的变化而改变。本研究利用可以操控树种丰富度的实验林场研究了这些不同垂直层位里的时间和环境动态。在中国亚热带森林生物多样性与生态系统功能(BEF-China)研究计划的框架下,我们在3年时间里沿树种丰富度梯度反复采集林下生物量样本,研究了不同环境处理中树种丰富度、树种特性和时间对林下生物量的影响。尽管我们发现乔木层特性对林下生物量有显著和一致的影响,但是树种丰富度对后者却不具有这种影响。另外,在森林结构层之间,可能并不存在单一的、具有普遍性的上层乔木树种丰富度与林下生产力的相关关系,并且与上层乔木相关的环境因素(如透光率)对林下生产力的贡献程度会随着时间而变化。总体而言,我们的结果表明,在研究森林结构层之间的关系时应将时间动态变化考虑在内。     

Regional patterns of diversity and estimates of global insect species richness

The total number of insect species in the world is an important if elusive figure. We use a fresh approach to estimate global insect species richness, based on biogeographic patterns of diversity of well or better documented taxa. Estimates generated by various calculations, all variations on a theme, largely serve to substantiate suggestions that insect species are likely to number around 10 million or less.     

Patterns of species richness and biogeographic regionalization of the avifaunas of the seasonally dry tropical forest in Mesoamerica

Biogeographic patterns of avifaunas associated with seasonally dry tropical forests in Mesoamerica are poorly understood despite their high levels of species richness and endemism. Through the parsimony analysis of endemicity, we analyzed biogeographic relationships of 650 resident species of birds associated with seasonally dry tropical forests from Mexico to Panama, based on potential distributions obtained through ecological niche modeling. Results show two general avifaunal groups, east and west of the Isthmus of Tehuantepec. Patterns of biogeographic distribution and species richness also helped illuminate the importance of key areas for birds associated to this habitat in the region.

Los patrones biogeográficos de las avifaunas asociadas a los bosques tropicales estacionalmente secos en Mesoamérica están pobremente entendidos, a pesar de que estas áreas poseen una gran riqueza de especies y endemismo. Analizamos las relaciones biogeográficas con base en distribuciones potenciales, hechas a partir de modelos del nicho ecológico usando el análisis de parsimonia de endemismos, de 650 especies de aves residentes asociadas a los bosques tropicales estacionalmente secos desde México hasta Panamá. Los resultados muestran dos grupos generales de la avifauna, al este y oeste del Istmo de Tehuantepec. El contexto biogeográfico y la riqueza de especies resalta también la importancia de áreas clave para las aves asociadas a este tipo de hábitat en la región.     

Different evolutionary histories underlie congruent species richness gradients of birds and mammals

Aim The global species richness patterns of birds and mammals are strongly congruent. This could reflect similar evolutionary responses to the Earth’s history, shared responses to current climatic conditions, or both. We compare the geographical and phylogenetic structures of both richness gradients to evaluate these possibilities. Location Global. Methods Gridded bird and mammal distribution databases were used to compare their species richness gradients with the current environment. Phylogenetic trees (resolved to family for birds and to species for mammals) were used to examine underlying phylogenetic structures. Our first prediction is that both groups have responded to the same climatic gradients. Our phylogenetic predictions include: (1) that both groups have similar geographical patterns of mean root distance, a measure of the level of the evolutionary development of faunas, and, more directly, (2) that richness patterns of basal and derived clades will differ, with richness peaking in the tropics for basal clades and in the extra‐tropics for derived clades, and that this difference will hold for both birds and mammals. We also explore whether alternative taxonomic treatments for mammals can generate patterns matching those of birds. Results Both richness gradients are associated with the same current environmental gradients. In contrast, neither of our evolutionary predictions is met: the gradients have different phylogenetic structures, and the richness of birds in the lowland tropics is dominated by many basal species from many basal groups, whereas mammal richness is attributable to many species from both few basal groups and many derived groups. Phylogenetic incongruence is robust to taxonomic delineations for mammals. Main conclusions Contemporary climate can force multiple groups into similar diversity patterns even when evolutionary trajectories differ. Thus, as widely appreciated, our understanding of biodiversity must consider responses to both past and present climates, and our results are consistent with predictions that future climate change will cause major, correlated changes in patterns of diversity across multiple groups irrespective of their evolutionary histories.     

中国不同地理区域鸟兽物种丰富度的相关性

生物类群之间物种丰富度的相关性研究是当前物种多样性研究中的热点问题之一,目前,中国尚无相关的研究报道。我们收集了中国三种区域类型：动物地理亚区、行政区和保护区的鸟兽名录,分析了行政区与保护区、动物地理区和经纬度带中鸟兽物种数比值及其相关性。 结果表明：不同区域、动物地理区和经纬度带中鸟兽物种数都显著相关。保护区尺度鸟兽物种 数的相关系数为0.818和动物地理区中的华北区为0.768,其他所有区域和地理区域的鸟兽物 种数的相关系数都高于0.850。因此,鸟兽物种数的相关关系在一定程度上具有预测价值。我们发现不同区域鸟兽物种数比值无显著性差异;但是,不同区域间鸟兽物种数 比值差异显著。该比值在中国呈中间低四周高的分布趋势,其中东北地区最高。我们还利用历史累积调查数据与非历史累积调查数据进行了鸟兽物种数比值及其相关性分析,发现利用累积数据计算的相关性低于非累积数据计算的相关性,但利用累积数据计算的鸟兽物 种数比值高于非累积数据计算的比值。最后,探讨了为什么鸟类与兽类的物种数目会相关。我们根据物种－面积公式,S=CA^Z,导出了两个生物类群物种丰富度的相关关 系式。利用全国不同区域数据拟合,得到Z₁/Z₂＝0.913,Z₁/Z₂接近于1。于是 ,C₁/C2可视为近似等于R_am。本研究可推广到其他不同生物类群物种。物种数量的相关关系为快速评估区域的物种多样性提供了一条途径。     

A central Amazonian terra firme forest. I. High tree species richness on poor soils

Tree size, density, and species richness were established for three one-hectare plots of terra firme forest in central Amazonian Brazil. In the three hectares, 1916 individual trees with DBH 10 cm were sampled. A total of 58 families, 181 genera, and 513 species were determined. Hectare A had 285 species, 138 genera, and 47 families; hectare B 280 species, 123 genera, and 48 families; and hectare C 280 species, 125 genera, and 44 families. Comparably high species richness in Amazonia has heretofore only been reported from western Amazonia. This dispels the idea that high species richness can only develop in areas with rich soils and relatively high rainfall. It is suggested that such high species richness is the result of a combination of habitat heterogeneity and geological history. These high diversity forests, because they occur on nutrient poor soils, can be protected with little or no impact on development in the region because the soils are essentially useless for agriculture and for supporting long-term cattle pasture.     

Pteridophyte species richness in Andean forests in Bolivia

I assessed the magnitude and distribution of pteridophyte species richness on the eastern Andean slope in Bolivia based on 676 study plots of 400 m² each in forest habitats at 65 study sites. In total 755 species were recorded, including 426 (56%) epiphytes and 598 (79%) terrestrials, with 266 species (35%) recorded under both groups. Mean number of species per plot at a given site varied from 0 to 31.1 for epiphytes, 0 to 20.9 for terrestrials, and 0 to 47.9 for all species combined. The highest numbers of species recorded at a given study site were 110 epiphytes, 101 terrestrials, and 167 species in total. While overall there were more terrestrial than epiphytic species, at individual sites and plots the reverse was true, indicating that terrestrial species tended to be more patchily distributed than epiphytes. Despite high survey intensity, many species went unrecorded; the minimum estimate of total species richness obtained through extrapolation was 975 species overall, including 559 epiphytes and 880 terrestrials. A correlation analysis of species richness to 14 environmental parameters revealed a highly positive correlation to mean annual precipitation and bryophyte cover on tree branches (a proxy for air humidity). Significant correlations to other parameters (e.g. human impact, canopy height, etc.) reflected the covariance of these factors with precipitation and bryophyte cover. Despite a lack of data on the pteridophyte communities from much of the Bolivian Andes, it appears that in most of the countries, pteridophyte diversity can be protected by focussing the most humid parts of the Andean forests.     

Delving into the variations in tree species composition and richness across South American subtropical Atlantic and Pampean forests

Aims We analyse here the variations in species composition and richness and the geographic ranges of the tree species occurring in South American subtropical Atlantic and Pampean forests. Our goals were to assess (i) the floristic consistency of usual classifications based on vegetation physiognomy, climate and elevation; (ii) the leading role of temperature-related variables on the variations in species composition and richness; (iii) the predominance of species with tropical–subtropical ranges, possibly as a result of forest expansion over grasslands after the Last Glacial Maximum (LGM); (iv) the restriction of most subtropical endemics to stressful habitats as a possible result of past forest refuges during the LGM.Methods The region was defined by the Tropic of Capricorn to the north, the Rio de la Plata to the south, the Atlantic shoreline to the east and the catchment areas of the upper Paraná and Uruguay Rivers to the west. Multivariate analyses, multiple regression modelling and variance partition analyses were performed on a database containing 63 994 occurrence records of 1555 tree species in 491 forest sites and 48 environmental variables. All species were also classified according to their known geographic range.Important findings A main differentiation in species composition and richness was observed between the eastern windward coastlands (rain and cloud forests) and western leeward hinterlands (Araucaria and semi-deciduous forests). Pre-defined forest types on both sides were consistent with variations in tree species composition, which were significantly related to both environmental variables and spatial proximity, with extremes of low temperature playing a chief role. Tree species richness declined substantially towards the south and also from rain to seasonal forests and towards the highland summits and sandy shores. Species richness was significantly correlated with both minimum temperature and actual evapotranspiration. About 91% of the subtropical flora is shared with the much richer tropical flora, probably extracting species that can cope with frost outbreaks. The 145 subtropical endemics were not concentrated in harsher habitats.     

Patterns of tree species richness in relation to environment in southeastern New South Wales,Australia

Abstract We present regression models of species richness for total tree species, two growth forms, rainforest trees (broadleaf evergreens) and eucalypts (sclerophylls), and two large subgenera of Eucalyptus. The correlative models are based on a data set of 166 tree species from 7208 plots in an area of southeastern New South Wales, Australia. Eight environmental variables are used to model the patterns of species richness, four continuous variables (mean annual temperature, rainfall, radiation and plot size), plus four categorical factors (topographic position, lithology, soil nutrient level and rainfall seasonality). Generalized linear modelling with curvilinear and interaction terms, is used to derive the models. Each model shows a significant and differing response to the environmental predictors. Maximum species richness of eucalypts occurs at high temperatures, and intermediate rainfall and radiation conditions on ridges with aseasonal rainfall and intermediate nutrient levels. Maximum richness of rainforest species occurs at high temperatures, intermediate rainfall and low radiation in gullies with summer rainfall and high nutrient levels. The eucalypt subgenera models differ in ways consistent with experimental studies of habitat preferences of the subgenera. Curvilinear and interaction terms are necessary for adequate modelling. Patterns of richness vary widely with taxonomic rank and growth form. Any theories of species diversity should be consistent with these correlative models. The models are consistent with an available energy hypothesis based on actual evapotranspiration. We conclude that studies of species richness patterns should include local (e.g. soil nutrients, topographic position) and regional (e.g. mean annual temperature, annual rainfall) environmental variables before invoking concepts such as niche saturation.     

Bird species richness in vegetation fences and in strips of residual rain forest vegetation at Los Tuxtlas, Mexico

Fragmentation of the lowland tropical rain forest has resulted in loss of animal and plant species and isolation of remaining populations that puts them at risk. At Los Tuxtlas, Mexico, lowland rain forests are particularly diverse in the avian fauna they contain and while most of the forests have been fragmented by human activity, many of the fragments still harbor diverse assemblages of bird species. In these landscapes, linear strips of residual rain forest vegetation along streams as well as linear strips of vegetation fences (live fences) crossing the pastures might provide some connectivity to bird populations existed in forest fragments. We investigated bird species richness and relative abundance in one 6-km long section of live fences (LF) bordering a dirt road and in two 6-km long sections of residual forest vegetation along a river (MR) and one permanent stream (BS). We used point count procedures which resulted in the count of 2984 birds representing 133 species. At the LF site we detected 74% of the species, 72% at the BS site and 57% at the MR site. Only 38% of the species were common among sites. Neotropical migratory birds accounted for 34–41% of the species counted at all sites. While edge and open habitat birds accounted for 6–10% of the species and for 50% of the records at the three vegetation strips, about 90% of the species were forest birds. Distance to forest fragments and degree of disturbance of the vegetation seemed to negatively influence bird species presence at the BS and MR strips. Rarefaction analysis indicated that the LF strip was richer in species than the other two sites, but the occurrence of the three vegetation strips in the landscape seem to favor the presence of many more species. We discuss the value of these vegetation strips to birds as stepping stones in the fragmented landscape.     

Functional group dominance and not productivity drives species richness

Background: There is a lack of consensus about the productivity–richness relationship, with several recent studies suggesting that it is not productivity but other factors that are the important drivers that determine species richness.

Aims: Here, we examine the relationship between productivity, functional group dominance and plant species richness at the plot scale in Tibetan Plateau meadows. These alpine meadows are ideal to examine the species productivity-richness relationship because they have a very high species richness, a large gradient in productivity, and can be dominated by either graminoids (grasses and sedges) or forbs.

Methods: We measured plant species richness and above-ground biomass along a natural gradient of functional group abundance in 44 plots distributed across five natural, winter-grazed but otherwise undisturbed sites in the eastern part of the Qing-Hai Tibetan Plateau, in Gansu province, China in 2008.

Results: Graminoid abundance (i.e. graminoid biomass as percent of the total above-ground biomass) explained 39% of plot differences in species richness while neither productivity nor the biomass of the three most abundant plant species, either individually or combined, were a significant predictor of species richness.

Conclusions: Our results show that within these alpine meadows, a shift from graminoid to forb dominance, rather than the individual dominant species or productivity itself, is strongly correlated with species richness. Thus, differences in functional group abundance can be a strong driver of observed plant species richness patterns.     

Biogeographic anomalies in the species richness of Chilean forests: Incorporating evolution into a climatic – historic scenario

Broad‐scale richness gradients are closely associated with temperature and water availability. However, historical and evolutionary processes have also contributed to shape current diversity patterns. In this paper we focus on the potential influences of Pleistocene glaciation and phylogenetic niche conservatism (the tendency for traits to be maintained during diversification) on the tree diversity gradient in Chile, and we quantify its primary climatic correlates. Tree species richness is greatest at mid latitudes, particularly in the Andes and Coastal ranges, and decreases abruptly to the south and north. Regression tree analysis identified annual precipitation and annual temperature as the primary probable drivers of this gradient. Ice cover during the Last Glacial Maximum was also identified as an ‘important’ variable, but the contemporary and historical predictors are strongly collinear. Geographically weighted regression indicated that the relationships between richness and environmental variables vary regionally: the relationship between tree richness and precipitation is stronger in north‐central Chile, whereas tree richness and temperature are most strongly associated in south‐central Chile. By assigning each species the age of the family to which it belongs and averaging all species in each geographical unit, we also found that species from the oldest families are distributed mainly in mid to high latitudes and species from younger families are distributed mainly at lower latitudes. This pattern is closely associated with annual precipitation. Thus, the ecological component of tree richness follows contemporary climatic gradients of both energy and water, but the aridification of the Atacama Desert was an important driver over evolutionary time. The influence of recent Pleistocene glaciation remains unresolved but it cannot be discounted.     

Exploring the phylogenetic history of mammal species richness

Aim At broad geographical scales, species richness is a product of three basic processes: speciation, extinction and migration. However, determining which of these processes predominates is a major challenge. Whilst palaeontological studies can provide information on speciation and extinction rates, data are frequently lacking. Here we use a recent dated phylogenetic tree of mammals to explore the relative importance of these three processes in structuring present‐day richness gradients. Location The global terrestrial biosphere. Methods We combine macroecological data with phylogenetic methods more typically used in community ecology to describe the phylogenetic history of regional faunas. Using simulations, we explore two simple phylogenetic metrics, the mean and variance in the pairwise distances between taxa, and describe their relationship to phylogenetic tree topology. We then use these two metrics to characterize the evolutionary relationships among mammal species assemblages across the terrestrial biome. Results We show that the mean and variance in the pairwise distances describe phylogenetic tree topology well, but are less sensitive to phylogenetic uncertainty than more direct measures of tree shape. We find the phylogeny for South American mammals is imbalanced and ‘stemmy’ (long branches towards the root), consistent with recent diversification within evolutionarily disparate lineages. In contrast, the phylogeny for African mammals is balanced and ‘tippy’ (long branches towards the tips), more consistent with the slow accumulation of diversity over long times, reflecting the Old World origin of many mammal clades. Main conclusions We show that phylogeny can accurately capture biogeographical processes operating at broad spatial scales and over long time periods. Our results support inferences from the fossil record – that the New World tropics are a diversity cradle whereas the Old World tropics are a museum of old diversity.