Biogeographic Transition Zone Research Articles

Habitat of the Long-Tailed Wood-Partridge in Central Mexico

The long-tailed wood-partridge (Dendrortyx macroura; hereafter, wood-partridge) is a forest quail endemic to the temperate forests of the Trans-Mexican Volcanic Belt of central Mexico and is considered threatened according to the Secretariat of the Environment of Mexico. We studied 34 sites within the Natural Resources Protection Area River Basins of the Valle de Bravo, Malacatepec, Tilostoc and Temascaltepec in central Mexico to evaluate wood-partridge habitat during September–December 2019. We evaluated attributes of tree, shrub, and herbaceous vegetation, canopy cover, humidity, slope, and altitude. We also identified vegetation used for nesting, food, and shelter. The scant information published for other geographical areas indicates habitat consists of pine (Pinus spp.), pine-oak (Quercus spp.), and cloud forest. We found that the preferred habitat in the Trans-Mexican Volcanic Belt was forest dominated by Alnus firmifolia, Pinus patula, Pinus pseudostrobus, Pinus teocote, and Quercus castanea. Wood-partridge preferred sites with an average of 90% canopy coverage, 100% shrub cover, high percent humidity, slopes >30%, and altitudes that ranged 2,700–3,000 meters above sea level. Additionally, we located an inactive nest in an area with 100% shrub cover and 80% tree cover in forest dominated by Pinus patula, relative humidity of 55%, and a temperature of 19° C. This is the first investigation of this bird within this important biogeographic transition zone between the Nearctic region and the Neotropics. We report the basic characteristics of the habitat used by this species along with the first description of a nest in an area with high rates of deforestation, overgrazing, and forest fires. The “gallinita de monte” represents an important element of the biocultural heritage for the Mazahua and Otomí peoples, who are Indigenous inhabitants of this area. We recommend additional research on wood-partridge to determine the extent of poaching and collection of eggs for food; determine effects of forest management; determine its basic ecology including survival, reproduction, and population density; verify habitat relationship; and corroborate information that describes the traditional knowledge of the species by the native Mazahua and Otomí peoples. We suggest it is most important to learn more about and disseminate traditional knowledge of the species and to preserve its biocultural value and heritage, particularly for Indigenous communities.

Mitochondrial introgression by ancient admixture between two distant lacustrine fishes in Sulawesi Island.

Sulawesi, an island located in a biogeographical transition zone between Indomalaya and Australasia, is famous for its high levels of endemism. Ricefishes (family Adrianichthyidae) are an example of taxa that have uniquely diversified on this island. It was demonstrated that habitat fragmentation due to the Pliocene juxtaposition among tectonic subdivisions of this island was the primary factor that promoted their divergence; however, it is also equally probable that habitat fusions and resultant admixtures between phylogenetically distant species may have frequently occurred. Previous studies revealed that some individuals of Oryzias sarasinorum endemic to a tectonic lake in central Sulawesi have mitochondrial haplotypes that are similar to the haplotypes of O. eversi, which is a phylogenetically related but geologically distant (ca. 190 km apart) adrianichthyid endemic to a small fountain. In this study, we tested if this reflects ancient admixture of O. eversi and O. sarasinorum. Population genomic analyses of genome-wide single-nucleotide polymorphisms revealed that O. eversi and O. sarasinorum are substantially reproductively isolated from each other. Comparison of demographic models revealed that the models assuming ancient admixture from O. eversi to O. sarasinorum was more supported than the models assuming no admixture; this supported the idea that the O. eversi-like mitochondrial haplotype in O. sarasinorum was introgressed from O. eversi. This study is the first to demonstrate ancient admixture of lacustrine or pond organisms in Sulawesi beyond 100 km. The complex geological history of this island enabled such island-wide admixture of lacustrine organisms, which usually experience limited migration.

Characterization and biogeographic affinity of megazoobenthos in the Central Mediterranean Sea

AbstractThe Strait of Sicily (SoS) is a key area in the Central Mediterranean Sea characterized by high biodiversity and demersal fisheries productivity. Further, it is traditionally considered the main biogeographic boundary between the Western and Eastern basins of the Mediterranean. Due to the poor knowledge on the benthos composition and distribution in this area, we created the first inventory of megazoobenthos recorded on the trawlable soft bottoms of the North‐Western SoS. Samples were collected by bottom trawl surveys between 2003 and 2013 within a depth range of 10–800 m. Overall, 374 taxa with 358 species, belonging to Porifera, Cnidaria, Nemertea, Rotifera, Brachiopoda, Bryozoa, Sipuncula, Annelida, Mollusca, Arthropoda, Echinodermata and Chordata phyla, were found. In addition, the biogeographic affinity of megazoobenthos collected in the SoS to species vulnerable to trawl survey found in other zones of the Central Mediterranean was investigated. Excluding the North Adriatic and the Strait of Messina, moderate affinity values have been recorded within all the biogeographic zones. The benthos species of the North‐Western SoS presented the highest affinity with those of the North Tyrrhenian and Ligurian Sea (about 90% of species in common). However, our results show high similarity with species composition of the South‐Eastern SoS/South Ionian, North Ionian and Central Adriatic (about 80% of species in common). Our study confirms the role as biogeographic transition zone of the SoS for the Mediterranean biota. Furthermore, the lower affinity with native species of the South‐Eastern SoS and the absence of Lessepsian species suggest that the cold waters (due to the permanent up‐welling) off the southern coasts of Sicily may act as thermal barrier for regulating species exchange between the Eastern and Western basins. Some concerns about this role of the northern sector of the SoS were discussed in the light of current climate change.

Phylogenetic structure as a predictive component of beta diversity: Lessons from a comprehensive Neotropical biogeographic transition

Understanding the phylogenetic structure provides evidence about the importance of the different processes that shape an ecological community. We addressed the following questions: i) What is the intensity of the role of the phylogenetic component on variations in the tree species composition in a large biogeographic transition zone in the Neotropics? ii) Have forest and savanna habitats similar answers to the above questions? We built an ultrametric phylogenetic tree for 2475 tree species and obtained eigenvectors for each site using the PCPS method (Principal Coordinates of Phylogenetic Structure). We used a variation partitioning framework coupled with Moran’s Spectral Randomization with three explanatory tables - a novel methodological approach - from canonical models expecting that the phylogenetic component would better explain the floristic variation than spatial and environmental components. The model containing the selected phylogenetic, spatial and environmental predictors explained 18.98 % of the floristic variation; the explanation provided by the phylogenetic component, after partitioning out the influence of spatial and environmental factors, accounted for 17.61 % of the total variation. The predominance of phylogenetic component was also found at other spatial levels (habitats and vegetation types). Variations in species composition in tree communities are thus strongly influenced by phylogenetic structure, and this pattern is consistent for both savanna and forest habitats. We confirmed that evolutive relationships are a synthetic indicator that can be correlated with patterns of floristic variation. This approach is unprecedented for tropical communities and brings promising contributions not only to understanding beta diversity in biogeographic transitions, but to the most varied ecological groups.

Effects of a major Amazonian river confluence on the distribution of floodplain forest avifauna

AbstractAimAlthough ‘river color’ or water type is an important determinant of Amazonian floodplain biodiversity, the relevance of mixing distinct water types at river confluences to the distribution of terrestrial floodplain fauna has been largely overlooked. We investigated how the influx of a sediment‐rich whitewater tributary affects the floodplain forest avifauna along the world's largest blackwater river.LocationNorthwestern Brazilian Amazon.TaxonBirds.MethodsWe sampled floodplain avifauna and retrieved estimates of sediment concentration in the water (the main parameter in river‐water classification) at 52 sites, along the Negro (blackwater) and Branco (whitewater) rivers, above and below their confluence. We compared species richness and composition using analyses of variance, ordinations, generalized linear models and indicator species analyses.ResultsBird species composition on the lower Rio Negro (below the confluence) was distinct from both the upper Negro (above the confluence) and the Branco, and species richness on the Negro increased below the confluence. Typical whitewater bird species occurring on the Branco were found exclusively or predominantly along the left side of the lower Rio Negro, where the Branco's muddy waters seem to be channelled. Overall avian compositional variation among sites was correlated with sediment concentration in the water, a determinant of floodplain forest structure.Main conclusionsThe influx of the muddy waters of the Rio Branco into the Rio Negro promotes the co‐occurrence of bird species that are segregated by river type upstream, increasing species richness at the landscape scale. Rather than just representing a potential blackwater barrier between whitewater systems, the lower Rio Negro comprises a unique biogeographical transition zone, with a mixed avifauna occurring in a mosaic of varied floodplain forest types. Our results suggest that confluences of large rivers of distinct water types represent a significant factor determining species distributional boundaries and geographic patterns of Amazonian floodplain biodiversity.

Partitioning of marine transition zone reefs among temperate, sub-tropical and tropical fishes is related more to depth and habitat than temperature

Changes in fish communities as oceans warm and cool and competition for space between established and novel species can be evaluated in biogeographic transition zones such as the west coast of Australia. At ~30° S in this region, a cool anomaly occurred in the 2000s, between marine heatwaves. Over 2 yr of that anomaly, surveyed reef fishes were 57% temperate, 18% sub-tropical and 25% tropical. The most numerous fishes included a wrasse, herring, bullseye, drummer and damselfish. Based on similarities in the composition of fishes, 7 significant clusters of reefs were identified along a gradation from deep, exposed reefs to shallow, protected lagoonal reefs. Endemic sub-tropical and temperate wrasses and damselfishes typified all reefs. Some of these were ubiquitous over exposed and lagoonal reefs and others prevalent in only one reef type, demonstrating habitat preferences and partitioning among closely related species. This was reflected in the differing order of importance of fishes that typified different reefs. Linear modelling indicated that abiotic (depth, distance from shore) and biotic factors (e.g. algae) explained most of the variation in the fish communities among reefs. Additional variation, particularly within lagoonal reefs, was related to relief, turf and corals, rather than water temperature. Occurrence and reproductive activity of a group of tropical/sub-tropical wrasses and damselfish in some lagoonal reefs with abundant tropical habitats (e.g. corals) suggested that they supported novel communities during cool anomalies. Better predictions of future change and interactions between existing and novel species with environmental cycles requires knowledge of species-specific habitat relationships and biology.

Spatially explicit mapping of phenological transition zones: A fuzzy-logic approach

Spatially explicit mapping of the Phenological Transition Zone (PTZ) is the prerequisite to PTZ dynamics monitoring and characterization, and is significant for more effective management of natural resources, biodiversity, and conservation areas in the context of climate change. However, to our knowledge there have been no studies to delineate spatially explicit PTZs and there are some potential issues associated with the mapping of other biogeographical transition zones. This paper developed a method to explicitly delineate PTZs continuously from 2003 to 2017 in the study area of the Forest-Grassland Mosaic Region (FGMR) in northeastern China, highlighting both their transitional and phenological characteristics. Annual Normalized Difference Vegetation Index (NDVI) time series were fed into the dynamic-time-warping (DTW) based fuzzy c-medoids (FCMdd) clustering to produce 12 fuzzy phenoregions. Potential PTZ pixels were identified for each pair of fuzzy phenoregions via the fuzzy intersection operator. The Delaunay tessellation field estimator was applied to convert the discrete potential PTZ pixels to continuous weighted density surface. The 7 consistently and 8 intermittently existing PTZs were exhaustively derived via thresholding. Three different types of gradient, inclusive and mosaic PTZs were identified. The PTZ dynamics were rapid and dramatic due to its phenological and transitional nature. This PTZ mapping method can be easily extended to other areas with no or minor adjustments, and it can also be adapted to map other biogeographical transition zones.

Microbiome community and complexity indicate environmental gradient acclimatisation and potential microbial interaction of endemic coral holobionts in the South China Sea

Regional acclimatisation and microbial interactions significantly influence the resilience of reef-building corals facing anthropogenic climate change, allowing them to adapt to environmental stresses. However, the connections between community structure and microbial interactions of the endemic coral microbiome and holobiont acclimatisation remain unclear. Herein, we used generation sequencing of internal transcribed spacer (ITS2) and 16S rRNA genes to investigate the microbiome composition (Symbiodiniaceae and bacteria) and associated potential interactions of endemic dominant coral holobionts (Pocillopora verrucosa and Turbinaria peltata) in the South China Sea (SCS). We found that shifts in Symbiodiniaceae and bacterial communities of P. verrucosa were associated with latitudinal gradient and climate zone changes, respectively. The C1 sub-clade consistently dominated the Symbiodiniaceae community in T. peltata; yet, the bacterial community structure was spatially heterogeneous. The relative abundance of the core microbiome among P. verrucosa holobionts was reduced in the biogeographical transition zone, while bacterial taxa associated with anthropogenic activity (Escherichia coli and Sphingomonas) were identified in the core microbiomes. Symbiodiniaceae and bacteria potentially interact in microbial co-occurrence networks. Further, increased bacterial, and Symbiodiniaceae α-diversity was associated with increased and decreased network complexity, respectively. Hence, Symbiodiniaceae and bacteria demonstrated different flexibility in latitudinal or climatic environmental regimes, which correlated with holobiont acclimatisation. Core microbiome analysis has indicated that the function of core bacterial microbiota might have changed in distinct environmental regimes, implying potential human activity in the coral habitats. Increased bacterial α diversity may lead to a decline in the stability of coral-microorganism symbioses, whereas rare Symbiodiniaceae may help to retain symbioses. Cladocopium, γ-proteobacteria, while α-proteobacteria may have been the primary drivers in the Symbiodiniaceae-bacterial interactions (SBIs). Our study highlights the association between microbiome shift in distinct environmental regimes and holobiont acclimatisation, while providing insights into the impact of SBIs on holobiont health and acclimatisation during climate change.

Discovery of populations endemic to a marine biogeographical transition zone

AbstractAimBiogeographical transition zones are areas of overlap between the faunas of adjacent biogeographical entities. Particularly the well‐defined transition zones along linear coastlines are interesting natural laboratories to study dispersal and incipient speciation. Few studies have explored whether marine biogeographical transition zones harbour biodiversity that is distinct from that of the biogeographical entities they separate. The Wild Coast in eastern South Africa is a poorly studied transition zone between the region's warm‐temperate and subtropical faunas, and is generally considered to be an area of faunal overlap.LocationThe South African portion of the Western Indian Ocean.MethodsSequences of the DNA barcoding marker COI were generated from 306 estuarine sandprawns (Kraussillichirus kraussi) collected at 13 sites. Genetic structure and evolutionary history were assessed using a haplotype network and a Bayesian discrete phylogeographic analysis.ResultTwo populations were identified whose ranges are centred on the Wild Coast, a rare one in the northern portion and a more common one in the central and southern portion of this biogeographical transition zone. These populations are not closely related to each other, but descend from subtropical and warm‐temperate sister populations, respectively. Although genetic distances between populations were low, they exceeded within‐population distances, indicating the presence of a "barcoding gap."ConclusionsThis is the first study to indicate that the Wild Coast marine biogeographical transition zone is not merely an area of faunal overlap, and one of very few studies to have discovered genetically unique populations within a marine biogeographical transition zone. The Wild Coast may harbour additional unique biodiversity that remains to be discovered, including rare species that require protection. More research is required to understand how this environmentally dynamic marine biogeographical transition zone differs from the adjacent biogeographical provinces.

Signatures of local adaptation in the spatial genetic structure of the ascidian Pyura chilensis along the southeast Pacific coast

The highly heterogeneous Humboldt Current System (HCS) and the 30°S transition zone on the southeast Pacific coast, represent an ideal scenario to test the influence of the environment on the spatial genomic structure in marine near-shore benthic organisms. In this study, we used seascape genomic tools to evaluate the genetic structure of the commercially important ascidian Pyura chilensis, a species that exhibits a low larval transport potential but high anthropogenic dispersal. A recent study in this species recorded significant genetic differentiation across a transition zone around 30°S in putatively adaptive SNPs, but not in neutral ones, suggesting an important role of environmental heterogeneity in driving genetic structure. Here, we aim to understand genomic-oceanographic associations in P. chilensis along the Southeastern Pacific coast using two combined seascape genomic approaches. Using 149 individuals from five locations along the HCS, a total of 2,902 SNPs were obtained by Genotyping-By-Sequencing, of which 29–585 were putatively adaptive loci, depending on the method used for detection. In adaptive loci, spatial genetic structure was better correlated with environmental differences along the study area (mainly to Sea Surface Temperature, upwelling-associated variables and productivity) than to the geographic distance between sites. Additionally, results consistently showed the presence of two groups, located north and south of 30°S, which suggest that local adaptation processes seem to allow the maintenance of genomic differentiation and the spatial genomic structure of the species across the 30°S biogeographic transition zone of the Humboldt Current System, overriding the homogenizing effects of gene flow.

Spatiotemporal variations in density and biomass of rocky reef fish in a biogeographic climatic transition zone: trends over 9 years, inside and outside the only nearshore no-take marine-protected area on the southern Brazilian coast.

Biogeographical transition zones are important areas to investigate evolutionary ecological questions, but long-term population monitoring is needed to better understand ecological processes that govern population variations in such edge environments. The southernmost Brazilian rocky reefs are the southern limit of distribution for 96% of the tropical ichthyofauna of the western Atlantic. The Arvoredo Marine Biological Reserve is the only nearshore no-take marine-protected area (MPA) located in this transition zone. The main aim was to investigate how the populations of rocky reef fish species vary in density and biomass in space and over time, inside and outside the Arvoredo MPA. This study presents results based on a 9 year (2008-2017) underwater visual census monitoring study to evaluate the density and biomass of key fish species. Variations in density and biomass were detected for most species. Factors and mechanisms that may have influenced spatial variation are habitat structural complexity and protection from fisheries. Temporal variations, otherwise, may have been influenced by species proximity to their distributional limit, in synergy with density-dependent mechanisms and stochastic winter temperature oscillations. The MPAs harbour higher density and biomass for most species. Nonetheless, a prominent temporal decline in the recruitment of Epinephelus marginatus calls into question the continuous effectiveness of the MPA.

Multiple facets of marine biodiversity in the Pacific Arctic under future climate

Climate change is triggering a global reorganization of marine life. Biogeographical transition zones, diversity-rich regions straddling biogeographical units where many species live at, or close to, their physiological tolerance limits (i.e., range distribution edges), are redistribution hotspots that offer a unique opportunity to understand the mechanisms and consequences of climate-driven thermophilization processes in natural communities. In this context, we examined the impacts of climate change projections in the 21st century (2026–2100) on marine biodiversity in the Eastern Bering and Chukchi seas within the Pacific Arctic, a climatically exposed and sensitive boreal-to-Arctic transition zone. Overall, projected changes in species distributions, modeled using species distribution models, resulted in poleward increases in species richness and functional redundancy, along with pronounced reductions in phylogenetic distances by century's end (2076–2100). Future poleward shifts of boreal species in response to warming and sea ice changes are projected to alter the taxonomic and functional biogeography of contemporary Arctic communities as larger, longer-lived and more predatory taxa expand their leading distributional margins. Drawing from the existing evidence from other Arctic regions, these changes are anticipated to increase the susceptibility and vulnerability of the Arctic ecosystems, as trophic connectance between biological components increases, thus decreasing the modularity of Arctic food webs. Our results demonstrate how integrating multiple diversity facets can provide key insights into the relationships between climate change, species composition and ecosystem functioning across marine biogeographic regions.

Functional diversity of reef molluscs along a tropical-to-temperate gradient

Global warming is leading to range shifts of marine species, threatening the structure and functioning of ecological communities and human populations that rely on them. The largest changes are seen in biogeographic transition zones, such as subtropical reef communities, where species range shifts are already causing substantial community reorganisation. This causes functional changes in communities over subtropical latitudes, though a baseline functional understanding remains elusive for many taxa. One key marine taxon are molluscs, which provide many ecosystem services, are important prey for fishes and are also fisheries targets themselves, but remain largely unstudied. Here, we examine the trait composition, functional diversity, and functional redundancy of mollusc assemblages along the tropical-to-temperate transition in Japan (25° to 35° Northern latitude). Specifically, we use a trait database of 88 mollusc species from 31 subtropical reefs along the Pacific coast of Japan to show that trait composition of mollusc assemblages changes continuously along the latitudinal gradient. We discover that functional diversity of mollusc assemblages decreases with increasing latitude, a pattern associated with declines in functional dispersion. Moreover, we find a clear distinction between tropical and subtropical mollusc assemblages, with substrate-attached, suspension feeding bivalves more abundant in the tropics and free-living gastropod grazers more prevalent at higher latitudes. Our trait-based evidence in this study shows a contraction and almost complete shift in the functioning of marine mollusc assemblages at biogeographic transition zones and our trait database facilitates further study. Our findings provide evidence of the changing taxonomic and functional composition of extant mollusc communities with latitude, pointing to potential pertinent changes and tropicalisation of these communities with rapid ocean warming.

Depth moderates loss of marine foundation species after an extreme marine heatwave: could deep temperate reefs act as a refuge?

Marine heatwaves (MHWs) have been documented around the world, causing widespread mortality of numerous benthic species on shallow reefs (less than 15 m depth). Deeper habitats are hypothesized to be a potential refuge from environmental extremes, though we have little understanding of the response of deeper benthic communities to MHWs. Here, we show how increasing depth moderates the response of seaweed- and coral-dominated benthic communities to an extreme MHW across a subtropical–temperate biogeographical transition zone. Benthic community composition and key habitat-building species were characterized across three depths (15, 25 and 40 m) before and several times after the 2011 Western Australian MHW to assess resistance during and recovery after the heatwave. We found high natural variability in benthic community composition along the biogeographic transition zone and across depths with a clear shift in the composition after the MHW in shallow (15 m) sites but a lot less in deeper communities (40 m). Most importantly, key habitat-building seaweeds such as Ecklonia radiata and Syctothalia dorycarpa which had catastrophic losses on shallow reefs, remained and were less affected in deeper communities. Evidently, deep reefs have the potential to act as a refuge during MHWs for the foundation species of shallow reefs in this region.

Drivers of floristic variation in biogeographic transitions: insights from the ecotone between the largest biogeographic domains of South America

ABSTRACT Ecological transitions usually represent vulnerable ecosystems and high biodiversity. Investigating their drivers is important from both biogeographic and conservationist perspectives. One of these transitions is situated between the two largest biogeographic domains of South America - the Amazon and the Cerrado. We evaluated variation in tree floristic composition throughout this transition and assessed which factors explain this variation. We used 17,240 records of occurrences of 2,530 tree species from the NeotropTree database. We investigated variation in floristic composition using UPGMA cluster analysis with bootstrap resampling and NMDS ordination, and modeled the distribution of species in relation to environmental and spatial predictors by means of transformation-based Redundancy Canonical Analysis (tb-RDA) and variance partitioning. We found four general floristic groups: 1) flooded forests; 2) white sand dwarf-forests; 3) savannic and dwarf forests; and 4) non-flooded forest types. Floristic variation along the transition was influenced by the environment, especially variables related to fire and soil moisture, and by space, especially variables acting on broader scales. Correspondence between the environmental heterogeneity found throughout the transition and our results demonstrates once again the importance of conserving biogeographical transition zones in general, and the most comprehensive of South America in particular.

Congruence between fine-scale genetic breaks and dispersal potential in an estuarine seaweed across multiple transition zones

AbstractGenetic structure in biogeographical transition zones can be shaped by several factors including limited dispersal across barriers, admixture following secondary contact, differential selection, and mating incompatibility. A striking example is found in Northwest France and Northwest Spain, where the estuarine seaweed Fucus ceranoides L. exhibits sharp, regional genetic clustering. This pattern has been related to historical population fragmentation and divergence into distinct glacial refugia, followed by post-glacial expansion and secondary contact. The contemporary persistence of sharp ancient genetic breaks between nearby estuaries has been attributed to prior colonization effects (density barriers) but the effect of oceanographic barriers has not been tested. Here, through a combination of mesoscale sampling (15 consecutive populations) and population genetic data (mtIGS) in NW France, we define regional genetic disjunctions similar to those described in NW Iberia. Most importantly, using high resolution dispersal simulations for Brittany and Iberian populations, we provide evidence for a central role of contemporary hydrodynamics in maintaining genetic breaks across these two major biogeographic transition zones. Our findings further show the importance of a comprehensive understanding of oceanographic regimes in hydrodynamically complex coastal regions to explain the maintenance of sharp genetic breaks along continuously populated coastlines.

Multidecadal shifts in fish community diversity across a dynamic biogeographic transition zone

AbstractAimA 21‐year fisheries‐independent monitoring dataset was used to explore fish community diversity across a latitudinal gradient to quantify how diversity has changed and relate those changes in diversity to changes in the abiotic environment. Additionally, this study spans a biogeographic transition zone, providing insight into future species assemblages across regions of relatively high species diversity.LocationIndian River Lagoon, FL, USA.MethodsSpatial and temporal beta diversity was quantified latitudinally with “best derived breaks” determined by using chronological cluster analyses. Multiple indices of alpha diversity were quantified, including species richness, Shannon diversity, Simpson diversity and Pielou's evenness. AIC model selection and environmental fit tests were performed to link patterns of diversity and species assemblages with the abiotic environment.ResultsEvidence of a biogeographic transition zone was supported by data spanning the entire study period; the largest break in species assemblage occurred near 28°N. Fine‐scale analyses using small and large seine catches were noisier than broad analyses but indicated a northern shift in location of the biogeographic transition zone. Beta diversity was generally dominated by species turnover/balance versus nestedness/gradient components, implying that changes were driven by species sorting associated with the physical environment. Excluding the summation of all environmental variables, temperature and dissolved oxygen best describe patterns of diversity and species composition.Main conclusionsOver years less affected by disturbances, large and small seine catch data suggest the fish community assemblage and location of the biogeographic transition zone has shifted 9 and 21 km to the north. If the trends observed in these years were to continue from 1999 until the year 2100, a 111–243‐km shift in fish communities could be expected. Variation in rates of movement based on gear type suggests novel species assemblages could ensue.

Distributions of mammals in Southeast Asia: The role of the legacy of climate and species body mass

AbstractAimCurrent species distributions are shaped by present and past biotic and abiotic factors. Here, we assessed whether abiotic factors (habitat availability) in combination with past connectivity and a biotic factor (body mass) can explain the unique distribution pattern of Southeast Asian mammals, which are separated by the enigmatic biogeographic transition zone, the Isthmus of Kra (IoK), for which no strong geophysical barrier exists.LocationSoutheast Asia.TaxonMammals.MethodsWe projected habitat suitability for 125 mammal species using climate data for the present period and for two historic periods: mid‐Holocene (6 ka) and last glacial maximum (LGM 21 ka). Next, we employed a phylogenetic linear model to assess how present species distributions were affected by the suitability of areas in these different periods, habitat connectivity during LGM and species body mass.ResultsOur results show that cooler climate during LGM provided suitable habitat south of IoK for species presently distributed north of IoK (in mainland Indochina). However, the potentially suitable habitat for these Indochinese species did not stretch very far southwards onto the exposed Sunda Shelf. Instead, we found that the emerged landmasses connecting Borneo and Sumatra provided suitable habitat for forest dependent Sundaic species. We show that for species whose current distribution ranges are mainly located in Indochina, the area of the distribution range that is located south of IoK is explained by the suitability of habitat in the past and present in combination with the species body mass.Main conclusionsWe demonstrate that a strong geophysical barrier may not be necessary for maintaining a biogeographic transition zone for mammals, but that instead a combination of abiotic and biotic factors may suffice.

Widespread shifts in the coastal biota of northern California during the 2014\u20132016 marine heatwaves

During 2014–2016, severe marine heatwaves in the northeast Pacific triggered well-documented disturbances including mass mortalities, harmful algal blooms, and declines in subtidal kelp beds. However, less attention has been directed towards understanding how changes in sea surface temperature (SST) and alongshore currents during this period influenced the geographic distribution of coastal taxa. Here, we examine these effects in northern California, USA, with a focus on the region between Point Reyes and Point Arena. This region represents an important biogeographic transition zone that lies <150 km north of Monterey Bay, California, where numerous southern species have historically reached their northern (poleward) range limits. We report substantial changes in geographic distributions and/or abundances across a diverse suite of 67 southern species, including an unprecedented number of poleward range extensions (37) and striking increases in the recruitment of owl limpets (Lottia gigantea) and volcano barnacles (Tetraclita rubescens). These ecological responses likely arose through the combined effects of extreme SST, periods of anomalous poleward flow, and the unusually long duration of heatwave events. Prolonged marine heatwaves and enhanced poleward dispersal may play an important role in longer-term shifts in the composition of coastal communities in northern California and other biogeographic transition zones.

Kelp forests in False Bay: urchins vs macroalgae in South Africa’s southwest coast biogeographical transition zone

Kelp forests along South Africa's southwest coast occur in a region of biogeographical overlap. Commonly referred to as transition-zone kelp forests, these dynamic ecosystems are particularly susceptible to grazing from species such as urchins. This study explores relationships between urchins and macroalgae in transition-zone kelp forests along the western side of False Bay. Six kelp forests in this region were sampled. At each site, the urchin Parechinus angulosus and kelp Ecklonia maxima were counted, percentage cover of understorey algae was recorded, and drift algae were collected. Despite variability in urchin and algal cover across sites, kelps generally increased from north to south, while urchins did the opposite. Urchins were negatively associated with macroalgae, although this relationship was weak. A localised threshold of 50 urchins m–2 was identified, above which kelp density failed to increase above 10 m–2 and percentage cover of understorey algae usually remained below 20%. Surprisingly, no relationship was discovered between drift algae and attached algae. The findings highlight the complexity of these cool-water environments. To better understand the role of urchins in this system, particularly under changing climate trends, experimental research into the feeding behaviour and possible effects of urchins on attached algae in the presence/absence of drift algae is advised.