Latitudinal Gradient In Species Richness Research Articles

Latitudinal biodiversity gradients of rocky intertidal assemblages: Spatial scales and complex associations with environmental factors

AbstractLatitudinal biodiversity gradients vary across taxonomic groups and spatial scales, and various environmental factors have been associated with those patterns. However, it remains unknown whether taxonomic groups with strong ecological associations have similar or different latitudinal patterns. Macroalgae are foundation assemblages on rocky intertidal shores and are often inhabited by invertebrates, predominantly polychaetes. This study analysed latitudinal patterns of macroalgae and their associated polychaetes at different spatial scales and determined how environmental factors influenced those patterns. Macroalgae and polychaetes were collected from transects within 38 rocky intertidal shores of Western Australia at 14 latitudes between 18° S and 34° S. Latitudinal gradients in species richness, diversity (Simpson's diversity index) and abundance were evaluated at transect, site and latitudinal scales. Relationships between environmental factors and rocky intertidal assemblages were analysed using piecewise structural equation modelling based on direct, indirect and complex models. Macroalgae showed increases in species richness, diversity and abundance at transect and site scales towards high latitudes, but species richness and diversity patterns were unclear at the latitudinal scale where transect and site data were pooled. In contrast, polychaete diversity decreased towards high latitudes, although this pattern was unclear at the transect scale. Polychaete richness and abundance tended to follow parabolic patterns that peaked at 27° S. Relationships between environmental factors and rocky intertidal assemblages were best described by a complex model, with significant relationships more often at transect and site scales. Sea surface temperatures showed the strongest relationship with macroalgal and polychaete distributions.

Latitudinal distributions of the species richness, phylogenetic diversity, and functional diversity of fleas and their small mammalian hosts in four geographic quadrants

We studied latitudinal patterns in the species richness (SR), the phylogenetic diversity (PD), and the functional diversity (FD) of fleas and their mammalian hosts. We asked whether these patterns in either fleas, hosts, or both 1) conform to a classical latitudinal gradient; 2) vary geographically; and 3) differ between fleas and hosts. We also asked whether the patterns of PD and FD follow those of SR. We collected data on the latitudinal distribution of 1022 flea and 900 mammal species from literature sources and calculated the SR, PD, and FD of both groups in 1° latitude bands. Then, we used broken‐stick regression models to analyse separately the latitudinal variation of 1) each diversity facet and 2) fleas and hosts in each geographic quadrant. The classical latitudinal gradient pattern was not found in either fleas or hosts across any facet of diversity or geographic quadrant, except for the PD of fleas in the southeastern quadrant and the FD of hosts in the southwestern quadrant. Latitudinal patterns of the SR, PD and FD of fleas and hosts differed substantially between geographic quadrants. Furthermore, the latitudinal distributions of flea and host SR were similar in three of four quadrants (except the northeastern quadrant), whereas the latitudinal distributions of flea and host PD were similar in the southwestern quadrant only. No similarity in flea versus host FD was revealed. The latitudinal patterns of flea and host PD and FD mostly did not follow those of their SR. We conclude that latitudinal gradients of species richness and phylogenetic and functional diversity appeared not to be universal phenomena. Instead, the latitudinal distributions of these diversity facets represent an interplay of ecological (current and past) and historical processes. For parasites, the processes acting on hosts add another layer of complexity underlying their latitudinal diversity patterns.

Climatic Niche Dynamics of the Astereae Lineage and Haplopappus Species Distribution following Amphitropical Long-Distance Dispersal.

The tribe Astereae (Asteraceae) displays an American Amphitropical Disjunction. To understand the eco-evolutionary dynamics associated with a long-distance dispersal event and subsequent colonization of extratropical South America, we compared the climatic and geographic distributions of South American species with their closest North American relatives, focusing on the diverse South American Astereae genus, Haplopappus. Phylogenetic analysis revealed that two South American genera are closely related to seven North American genera. The climatic niche overlap (D = 0.5) between South and North America exhibits high stability (0.89), low expansion (0.12), and very low unfilling (0.04). The distribution of the North American species predicted the climatic and geographic space occupied by the South American species. In central Chile, Haplopappus showed a non-random latitudinal gradient in species richness, with Mediterranean climate variables mainly explaining the variation. Altitudinal patterns indicated peak richness at 600 m, declining at lower and higher elevations. These findings support climatic niche conservatism in shaping Haplopappus species distribution and diversity. Two major endemism zones were identified in central Chile and the southern region, with a transitional zone between Mediterranean and Temperate macro-bioclimates. Our results indicate strong niche conservatism following long-distance dispersal and slight niche expansion due to unique climatic variables in each hemisphere.

Pleistocene glaciations caused the latitudinal gradient of within-species genetic diversity.

Intraspecific genetic diversity is a key aspect of biodiversity. Quaternary climatic change and glaciation influenced intraspecific genetic diversity by promoting range shifts and population size change. However, the extent to which glaciation affected genetic diversity on a global scale is not well established. Here we quantify nucleotide diversity, a common metric of intraspecific genetic diversity, in more than 38,000 plant and animal species using georeferenced DNA sequences from millions of samples. Results demonstrate that tropical species contain significantly more intraspecific genetic diversity than nontropical species. To explore potential evolutionary processes that may have contributed to this pattern, we calculated summary statistics that measure population demographic change and detected significant correlations between these statistics and latitude. We find that nontropical species are more likely to deviate from neutral expectations, indicating that they have historically experienced dramatic fluctuations in population size likely associated with Pleistocene glacial cycles. By analyzing the most comprehensive data set to date, our results imply that Quaternary climate perturbations may be more important as a process driving the latitudinal gradient in species richness than previously appreciated.

Geographical distribution of floating aquatic plants in relation to environmental conditions in southern Quebec, Canada

Communities of free-floating aquatic plants dominated by Lemnaceae (duckweeds) are common in ponds and wetlands globally with important ecological functions. In addition to the cosmopolitan common duckweed (Lemna minor), other species of Lemnaceae, liverworts (Ricciaceae), hornworts (Ceratophyllum), and water ferns (Salviniaceae) often coexist over broad geographic areas. Despite these species all sharing a similar mode of life and resource requirements, they have important morphological and physiological differences such as variation in body size, degree of morphological reduction, presence/number of roots, and position in the water column. The degree to which these differences play a role in shaping their distributions is, however, unclear. We report a field survey of floating aquatic plants in southern Quebec, Canada. We find that species distributions are influenced by habitat specialization to water phosphorus levels. High nutrient specialization followed a continual reduction in body size and morphological complexity, with smaller, simpler species found in higher nutrient sites. We also found a latitudinal gradient in species richness with more northerly sites supporting less diversity. Rare species did not show clustering suggesting that dispersal limitation was not an important factor in determining local abundance. Our results suggest that the distribution of these plants is in part driven by competitive sorting of suitable species from a regional species pool.

Patterns of functional diversity along latitudinal gradients of species richness in eleven fish families

AbstractAimAs we enter an era of major biodiversity shifts, understanding large‐scale biodiversity patterns has become crucial for ecological and conservation purposes. Often, conservation priorities are based on concepts derived largely from species richness, yet recent work shows that different facets of biodiversity are also crucial for proper ecosystem continuity, function and services. One facet of biodiversity increasingly relevant to conservation is functional diversity. Here, we aim to improve our understanding of large‐scale patterns of biodiversity by testing the hypothesis that species richness can also accurately estimate functional diversity along the latitudinal gradient of species richness in fish.LocationMarine environments.Time periodContemporary.Major taxa studiedEight hundred and forty‐two species within 11 fish families: Acanthuridae, Blenniidae, Chaetodontidae, Gobiidae, Labridae, Lutjanidae, Pleuronectidae, Pomacanthidae, Pomacentridae, Scombridae and Sparidae.MethodsUsing geometric morphometrics to calculate morphological diversity, a proxy for functional diversity, we estimated the expected functional diversity for a given number of species and compared it with the observed functional diversity in fish families along latitudes. We then fitted a broken‐stick regression model with estimates of functional diversity over absolute degree of latitudes to locate latitudes where significant shifts in functional diversity occur.ResultsWe found that species richness typically over‐ or underestimated functional diversity along the latitudinal gradient of species richness in the evaluated fishes. We also showed that for most families investigated, there was a pattern of stable functional diversity from the equator through the tropics that shifted, with a mean inflection point occurring at absolute latitude 31.7 ± 10.1°. We suggest that this pattern might be linked to changes in environmental factors such as global temperature and/or habitat availability beyond tropical latitudes; however, these concepts require more study.Main conclusionsThis analysis shows the importance of considering functional diversity further, in combination with other biodiversity metrics, when developing conservation priorities and policies.

Can functional leaf traits be used for monitoring wetland restoration? A comparison between commonly used monitoring metrics and functional leaf traits

Monitoring is essential to restoration, but the standard metrics used to monitor wetland restoration do not explicitly account for function in plant communities. Functional traits may be a useful addition to the wetland monitoring toolkit, because they can represent aspects of ecosystem functioning that standard metrics may not. Our objective was to determine how abiotic factors that influence wetland community composition and structure relate to both functional leaf traits and standard vegetation monitoring metrics to determine if functional leaf traits could add a functional component to wetland monitoring. We surveyed 66 100-m2 plots in 22 floodplain wetlands in Illinois that were restored between 1997 and 2010. We used plant species data to calculate the mean coefficient of conservatism (mean C) and richness, and collected leaves to determine community weighted means of specific leaf area (SLA) and leaf dry matter content (LDMC) at each plot. Hydrologic data were used to calculate variables related to frequency, depth, and duration of inundation, and soil samples were collected to determine soil pH, organic matter content, and nitrogen and phosphorous content at each plot. We used structural equation models to understand how predictor variables (hydrological variables, soil variables, canopy cover, time since restoration, and latitude) influenced each other, and ultimately how they influenced response variables (mean C, richness, percent non-native species cover, SLA, and LDMC). LDMC and SLA were poorly explained by predictor variables and had relatively few significant relationships within models. Predictor variables best explained variance in mean C, followed by percent non-native cover, richness, then LDMC and SLA. Mean C was positively influenced by canopy cover and negatively influenced by soil fertility, whereas richness was negatively influenced by latitude. There was a strong latitudinal gradient of species richness from north to south, such that southern plots were significantly more diverse and had less cover by non-native species than northern plots. Our research suggests that SLA and LDMC may not be well suited for use as wetland restoration monitoring tools in Midwestern floodplain wetlands, and that monitoring tools already in place may sufficiently reflect abiotic conditions.

Robust metagenomic evidence that local assemblage richness increases with latitude in ground‐active invertebrates of North America

Biodiversity monitoring is imperative for understanding how changing climate may impact the distributions of taxa from single species to the spatial distribution of biological diversity. Large‐scale and cross‐taxa biodiversity monitoring also allows an empirical understanding of biogeographic patterns across taxa. One such pattern, where in taxonomic richness peaks at tropical latitudes are typically treated as a biogeographical rule with few notable exceptions. Here we leveraged the invertebrate pitfall collections of the National Ecological Observatory Network (NEON) across North America to describe patterns of local taxonomic richness across taxa and across taxonomic scale. We focused on Arthropoda, Annelida and Mollusca. Additionally, we estimated regional species richness using expert‐identified samples of three NEON sentinel taxa: Carabidae, Culicidae and Ixodida. To sample pitfall animals, we filtered storage ethanol and employed environmental DNA‐barcoding methodologies to amplify and sequence extracted DNA from the filtrate for two regions of a mitochondrial gene. We assigned taxonomic names to these sequences at 97% similarity to reference sequences and calculated local taxonomic richness at the levels of species, genus, family and order. We calculated local species richness for 12 common invertebrate taxa. We used generalized linear models to describe the relationships between taxonomic richness and spatial, climatic and abundance predictor variables. At four taxonomic scales, ranging from species to order, taxonomic richnessincreased significantly as a function of latitude. Of the twelve invertebrate taxa weexamined, seven mirrored this positive latitudinal gradient in species richness. At theregional scale, two of three NEON Sentinel Taxa showed positive latitudinal gradientsin species richness. Temperature, precipitation, abundance and sequence read numberplayed minor roles in explaining patterns of taxonomic richness. When considering these mostly temperate sites that span 46 degrees of latitude, we found no support for the expected negative latitudinal gradients across taxa and taxonomic scales. Instead, for many of these taxa and taxonomic scales, we observed significant, positive richness gradients with increasing latitude among ground‐dwelling invertebrate communities. Thus, one of the most ‘general' patterns in biogeography was not found for most invertebrate taxa across temperate latitudes.

Imprints of latitude, host taxon, and decay stage on fungus‐associated arthropod communities

AbstractInteractions among fungi and insects involve hundreds of thousands of species. While insect communities on plants have formed some of the classic model systems in ecology, fungus‐based communities and the forces structuring them remain poorly studied by comparison. We characterize the arthropod communities associated with fruiting bodies of eight mycorrhizal basidiomycete fungus species from three different orders along a 1200‐km latitudinal gradient in northern Europe. We hypothesized that, matching the pattern seen for most insect taxa on plants, we would observe a general decrease in fungal‐associated species with latitude. Against this backdrop, we expected local communities to be structured by host identity and phylogeny, with more closely related fungal species sharing more similar communities of associated organisms. As a more unique dimension added by the ephemeral nature of fungal fruiting bodies, we expected further imprints generated by successional change, with younger fruiting bodies harboring communities different from older ones. Using DNA metabarcoding to identify arthropod communities from fungal fruiting bodies, we found that latitude left a clear imprint on fungus‐associated arthropod community composition, with host phylogeny and decay stage of fruiting bodies leaving lesser but still‐detectable effects. The main latitudinal imprint was on a high arthropod species turnover, with no detectable pattern in overall species richness. Overall, these findings paint a new picture of the drivers of fungus‐associated arthropod communities, suggesting that latitude will not affect how many arthropod species inhabit a fruiting body but, rather, what species will occur in it and at what relative abundances (as measured by sequence read counts). These patterns upset simplistic predictions regarding latitudinal gradients in species richness and in the strength of biotic interactions.

Species packing and the latitudinal gradient in beta-diversity.

The decline in species richness at higher latitudes is among the most fundamental patterns in ecology. Whether changes in species composition across space (beta-diversity) contribute to this gradient of overall species richness (gamma-diversity) remains hotly debated. Previous studies that failed to resolve the issue suffered from a well-known tendency for small samples in areas with high gamma-diversity to have inflated measures of beta-diversity. Here, we provide a novel analytical test, using beta-diversity metrics that correct the gamma-diversity and sampling biases, to compare beta-diversity and species packing across a latitudinal gradient in tree species richness of 21 large forest plots along a large environmental gradient in East Asia. We demonstrate that after accounting for topography and correcting the gamma-diversity bias, tropical forests still have higher beta-diversity than temperate analogues. This suggests that beta-diversity contributes to the latitudinal species richness gradient as a component of gamma-diversity. Moreover, both niche specialization and niche marginality (a measure of niche spacing along an environmental gradient) also increase towards the equator, after controlling for the effect of topographical heterogeneity. This supports the joint importance of tighter species packing and larger niche space in tropical forests while also demonstrating the importance of local processes in controlling beta-diversity.

Global warming is causing a more pronounced dip in marine species richness around the equator

The latitudinal gradient in species richness, with more species in the tropics and richness declining with latitude, is widely known and has been assumed to be stable over recent centuries. We analyzed data on 48,661 marine animal species since 1955, accounting for sampling variation, to assess whether the global latitudinal gradient in species richness is being impacted by climate change. We confirm recent studies that show a slight dip in species richness at the equator. Moreover, richness across latitudinal bands was sensitive to temperature, reaching a plateau or declining above a mean annual sea surface temperature of 20 °C for most taxa. In response, since the 1970s, species richness has declined at the equator relative to an increase at midlatitudes and has shifted north in the northern hemisphere, particularly among pelagic species. This pattern is consistent with the hypothesis that climate change is impacting the latitudinal gradient in marine biodiversity at a global scale. The intensification of the dip in species richness at the equator, especially for pelagic species, suggests that it is already too warm there for some species to survive.

Pycnogonids from marine docks located along the west coast of the Gulf of California, Mexico.

Sea spiders (Class Pycnogonida Latreille, 1810) are chelicerate arthropods, with an extraordinarily reduced body armed with eight to twelve elongated legs. A literature review of taxonomic and ecological studies of pycnogonids from Mexico identified 49 nominal species (~3.6% out of 1,335 species described worldwide). This low species richness is likely caused by limited taxonomic research and intermittent sampling and research efforts initially carried out by foreign scientists (1893-1996) and later by Mexican scientists. The present study investigates the latitudinal gradient of species richness of the pycnogonids associated with nine marine docks located between Cabo San Lucas (22°53'N) and Santa Rosalía (26°58'N) along the west coast of the Gulf of California, 22°53'N Mexico and provides a detailed morphological re-description of each nominal species using light and scanning electron microscopy. Nine nominal pycnogonid species and one unidentified species in the genus Tanystylum were collected and identified from the biological samples collected between 2011-2017. The highest pycnogonid species richness was recorded at Bahía de La Paz (seven species, 24°14'N) and the lowest species richness at Cabo San Lucas (one species) without an evident latitudinal species richness gradient. Anoplodactylus californicus Hall, 1912 and Ammothella spinifera Cole, 1904 were the most abundant species (52% of the total number of individuals). Tanystylum occidentale (Cole, 1904) and Nymphon apheles Child, 1979 were new geographical records for Mexico and Ammothella symbia Child, 1979 and A. californicus were new records for Baja California Sur state. Adding these new geographical records of pycnogonids increased the species richness previously recorded at Baja California Sur from 20 to 24 nominal species and for Mexico, the species richness increased from 49 to 51 nominal species.

Latitudinal gradients in genetic diversity and natural selection at a highly adaptive gene in terrestrial mammals

Identifying where high genetic diversity is located across our planet and what factors affect the geographical patterns of genetic variation not only provides important insights into distributions of biodiversity but is also crucial for human health, animal and plant breeding and biodiversity conservation. Recent studies show that genetic diversity at nearly neutral genes decreases from the tropics to the poles in different taxa, mirroring the oldest recognized latitudinal gradient of species richness, yet functional genetic diversity has received little attention at such broad spatial or taxonomic scales. Here, we investigated latitudinal gradients in genetic diversity at a polymorphic exon 2 (MHC II DRB gene, 1515 sequences) of the major histocompatibility complex that confers resistance to parasites in 93 terrestrial mammal species at the global scale. We also estimated the effects of species traits, positive selection and anthropogenic biomes on genetic diversity. We found clear latitudinal gradients in genetic diversity and the presence of positive selection at exon 2. Absolute latitude, the presence of positive selection and body mass are important predictors of within‐species genetic diversity. A higher occurrence of positive selection, faster evolutionary speed or slower drift at low latitudes may shape the latitudinal gradient in the within‐species genetic diversity of the gene. Our results contribute to a greater understanding of how species traits, selection and drift geographically shape functional genetic diversity, broadening the generality of latitudinal biodiversity gradient. The results highlight the importance of conservation at low latitudes.

Latitudinal gradient of termite diversity indicates higher diversification and narrower thermal niches in the tropics

AbstractAimTo investigate the contributions of ecosystem productivity, temperature‐dependent metabolism and climatic tolerances to the latitudinal gradient in species richness and changes in species composition in tropical and subtropical regions.LocationGlobal.Time periodContemporary.Major taxa studiedTermites.MethodsWe compiled termite occurrence data from papers using the standardized sampling protocol proposed 20 years ago by Jones and Eggleton. Three continents, 28 countries and 494 sampling plots were included in our database. We used structural equation models to determine the direct and indirect effects of productivity, and the average and extreme values of temperature and precipitation on termite species richness. We also compared the observed association between diversity and temperature with predictions from the metabolic theory of ecology and compared how the similarity in species composition relates to geographic distance and environmental differences in tropical versus subtropical regions.ResultsTermite species richness was, directly and indirectly, related to average and extreme temperatures, but not to productivity. However, the association of species richness and temperature was much stronger than predicted by the metabolic theory of ecology. Moreover, species composition changed more rapidly with geographic distance and temperature differences between plots in the tropics than in the subtropics.Main conclusionsThese results suggest that the increase of species richness toward the equator cannot be explained by productivity gradients. Instead, the increase in the speciation rates and the narrow species niches along the temperature gradient in tropical regions likely determine the distribution of termites at broad spatial scales.

Global biodiversity and biogeography of mangrove crabs: Temperature, the key driver of latitudinal gradients of species richness.

Mangroves are ideal habitat for a variety of marine species especially brachyuran crabs as the dominant macrofauna. However, the global distribution, endemicity, and latitudinal gradients of species richness in mangrove crabs remains poorly understood. Here, we assessed whether species richness of mangrove crabs decreases towards the higher latitudes and tested the importance of environmental factors such as Sea Surface Temperature (SST) in creating the latitudinal gradients in species richness of mangrove crabs. A total of 8262 distribution records of 481 species belonging to six families of mangrove crabs including Camptandriidae, Dotillidae, Macrophthalmidae, Ocypodidae, Sesarmidae, and Oziidae were extracted from open-access databases or collected by the authors, quality controlled, cleaned, and analyzed. Species richness was plotted against 5° latitudinal bands in relation to environmental factors. The R software and ArcGIS 10.6.1 were used to analyze the species latitudinal range and richness as well as to map the distribution of mangrove forest, endemic species, species geographical distribution records, and biogeographic regions. The Indo-West Pacific showed the highest species richness of mangrove crabs where more than 65% of species were found in the Indian Ocean and along the western Pacific Ocean. Our results showed that there are 11 significantly different biogeographic regions of mangrove crabs. The highest endemicity rate was observed in the NW Pacific Ocean (29%). Latitudinal patterns of species richness in Macrophthalmidae, Ocypodidae, and Sesarmidae showed an increasing trend from the poles toward the intermediate latitudes including one dip near the equator. However, latitudinal gradients in Camptandriidae, Dotillidae, and Oziidae were unimodal increasing from the higher latitudes towards the equator. Species richness per 5° latitudinal bands significantly increased following mean SST mean (°C), calcite, euphotic depth (m), and mangrove area (km2) across all latitudes, and tide average within each hemisphere. Species richness significantly decreased with dissolved O2 (ml l-1) and nitrate (μmol l-1) over all latitudes and in the southern hemisphere. The climax of global latitudinal species richness for some mangrove was observed along latitudes 20° N and 15°-25° S, not at the equator. This can suggest that temperature is probably the key driver of latitudinal gradients of mangrove crabs' species richness. Species richness and mangrove area were also highly correlated.

Climatic stability drives latitudinal trends in range size and richness of woody plants in the Western Ghats, India.

Understanding the determinants of range location and size is fundamental to our understanding of spatial patterns in species richness. Here, we aimed to test the role of ‘climatic stability’ in determining latitudinal trends in range size and as a consequence on species richness of tropical woody plants. Using primary data from 156 (0.06 ha) plots comprising 20,400 occurrences of more than 400 species of tropical woody plants, we built a biome-wide species database that covers the entire latitudinal extent of the wet-evergreen forests of the Western Ghats (8o to 20o N), India. We consolidated this database using secondary data from other published species inventories. We then calculated the range sizes and climatic niche width of woody plants to test the predictions of the climatic stability hypothesis and examined the relationship between range position and climatic tolerance of species. Our results show a significant latitudinal gradient in species richness and turnover where local and regional species richness increase monotonically from higher latitudes to lower latitudes of the Western Ghats. We found strong support for Rapoport’s Rule with an increase in range size from lower to higher latitudes; our results are consistent with the predictions of the climatic stability hypothesis, where species at higher latitudes exhibited greater tolerance to temperature and rainfall seasonality. Contrary to earlier work, our findings suggest that Rapoport’s Rule and the climatic stability hypothesis can operate over regional scales, and even at lower latitudes. We suggest that latitude associated climatic seasonality through its influence on species ranges, can influence latitudinal patterns in species turnover as well as species richness.

Biogeography of polychaete worms (Annelida) of the world

The global biogeography of polychaete worms has never been assessed previously. In the present study, we studied the world distribution patterns of polychaetes based on datasets obtained from the Global Biodiversity Information Facility, the Ocean Biogeographic Information System and our recently published checklist of Indonesian polychaete species. Polychaete biogeographic regions were visualized using ‘Infomap Bioregions’, and the latitudinal species richness gradient of the animals was examined using 3 metrics, i.e. alpha, gamma and estimated species richness (the last metric was adjusted for sampling bias). We identified 11 major polychaete biogeographic regions. The North Atlantic, Australia and Indonesia were the top 3 species-rich biogeographic regions in the world. The total number of polychaete species was higher in the southern hemisphere (~2100 species, 67 families) than in the northern hemisphere (~1800 species, 75 families) despite significantly more data in the latter (>500000 records compared to >26000 records). Contrary to the classical idea of a unimodal distribution pattern, the latitudinal gradient of polychaetes was generally bimodal with a pronounced dip north of the Equator (15°N). We suggest that the slightly higher peak of species richness in the southern (30°S) than in the northern (60°N) hemisphere reflects higher southern endemicities. These patterns are unlikely to be due to sampling bias but rather represent a natural phenomenon, and we found them most significantly correlated with sea temperature.

A Global-Scale Mid-Domain Effect Cannot Explain the Latitudinal Gradient in Species Richness.

The latitudinal gradient in species richness is perhaps the most fundamental pattern of biodiversity, yet a satisfactory explanation for its existence remains elusive. A geometric "mid-domain effect" is often cited as having potential to help explain the latitudinal gradient in species richness, but the logic underpinning this hypothesis is apparently built on two incorrect assumptions: (1) that a given great circle-usually the Equator-can constitute the geometric "mid-domain" of the Earth's surface, and (2) that geophysical or bioclimatic boundaries are of geometric relevance in the context of a global-scale mid-domain effect. This article gives a brief overview of the relevant literature and history of thought on the subject, and describes in clear and simple terms why a global-scale mid-domain effect cannot arise, and thus cannot possibly represent a mechanistic basis for the latitudinal gradient in species richness. Explicit acknowledgement of this fact is of great importance, as it allows us to dispense with a commonly cited hypothesis for the latitudinal gradient in species richness.

Macroecology of ground beetles: Species richness, range size and body size show different geographical patterns across a climatically heterogeneous area

AbstractAimEcogeographical patterns have been widely studied in endothermic vertebrates, but relatively few studies have simultaneously examined patterns and causes of gradients in species richness, range size and body size in ectothermic insects. We examined patterns in species richness, mean range size and mean body size of ground beetle assemblages across the biogeographical provinces of Northern Europe, a region that was mostly covered by ice sheets during the latest Ice Age and that presents strong contemporary climatic gradients.LocationNorthern Europe.MethodsWe used literature information on the occurrence of ground beetles, and analysed patterns in species richness, mean range size and mean body size across the provinces using generalized linear models and boosted regression tree (BRT) analysis.ResultsWe found a strongly decreasing gradient in species richness with increasing latitude, a strongly unimodal range size‐latitude relationship, and a weak unimodal body size‐latitude relationship in entire ground beetle assemblages. These gradients also varied among four major genera, suggesting that the overall patterns result from the nuances of smaller clades of ground beetles. The relative importance of contemporary environmental drivers also varied between species richness, mean range size and mean body size in BRT analysis. While species richness increased with mean annual temperature, mean range size showed an opposite relationship. Mean body size was most clearly associated with the precipitation of the driest month.Main ConclusionsOur findings showed that the latitudinal species richness gradient was strong, and it was closely related to concomitant variation in temperature, whereas variations in mean range size and mean body size were more complex. These findings suggest that the causes for range size and body size variation in insects may be complex, requiring additional insights from studies conducted at local, regional and continental scales.

A world dataset on the geographic distributions of Solenidae razor clams (Mollusca: Bivalvia).

BackgroundUsing this dataset, we examined the global geographical distributions of Solenidae species in relation to their endemicity, species richness and latitudinal ranges and then predicted their distributions under future climate change using species distribution modelling techniques (Saeedi et al. 2016a, Saeedi et al. 2016b). We found that the global latitudinal species richness in Solenidae is bi-modal, dipping at the equator most likely derived by high sea surface temperature (Saeedi et al. 2016b). We also found that most of the Solenidae species will shift their distribution ranges polewards due to global warming (Saeedi et al. 2016a). We also provided a comprehensive review of the taxon to test whether the latitudinal gradient in species richness was uni-modal with a peak in the tropics or northern hemisphere or asymmetric and bimodal as proposed previously (Chaudhary et al. 2016).New informationThis paper presents an integrated global geographic distribution dataset for 77 Solenidae taxa, including 3,034 geographic distribution records. This dataset was compiled after a careful data-collection and cleaning procedure over four years. Data were collected using field sampling, literature and from open-access databases. Then all the records went through quality control procedures such as validating the taxonomy of the species by examining and re-identifying the specimens in museum collections and using taxonomic and geographic data quality control tools in the World Register of Marine Species (WoRMS) and the r-OBIS package (Provoost and Bosch 2017). This dataset can thus be further used for taxonomical and biogeographical studies of Solenidae.