Topographic Habitat Research Articles

Fine‐scale habitat heterogeneity influences browsing damage by elephant and giraffe

AbstractEffects of large mammalian herbivores on woody vegetation tend to be heterogeneous in space and time, but the factors that drive such heterogeneity are poorly understood. We examined the influence of fine‐scale habitat heterogeneity on the distribution and browsing effects of two of the largest African terrestrial mammals, the elephant and giraffe. We conducted this study within a 120‐ha (500 x 2,400 m) ForestGEO long‐term vegetation monitoring plot located at Mpala Research Center, Kenya. The plot traverses three distinct topographic habitats (“plateau,” “steep slopes,” and “valley”) with contrasting elevation, slope, soil properties, and vegetation composition. To quantify browsing damage, we focused on Acacia mellifera, a palatable tree species that occurs across the three habitat categories. Overall tree density, species richness, and diversity were highest on the steep slopes and lowest on the plateau. Acacia mellifera trees were tallest and had the lowest number of stems per tree on the steep slopes. Both elephant and giraffe avoided the steep slopes, and their activity was higher during the wet season than during the dry season. Browsing damage on Acacia mellifera was lowest on the steep slopes. Elephant browsing damage was highest in the valley, whereas giraffe browsing damage was highest on the plateau. Our findings suggest that fine‐scale habitat heterogeneity is an important factor in predicting the distribution of large herbivores and their effects on vegetation and may interact with other drivers such as edaphic variations to influence local variation in vegetation structure and composition.

Logging and topographic effects on tree community structure and habitat associations in a tropical upland evergreen forest, Ghana

We determined the response of tree community structure to logging disturbance and topography, and the patterns of tree-habitat associations in Tano Offin Forest Reserve, Ghana. We sampled trees in 27 20 m × 20 m plots randomly and equally distributed in three topographic habitats (slope, valley, and hilltop) in each of two forests: logged and unlogged. Two topographic features, altitude and degree of slope, were measured and related with species composition. Overall, there were significant effects of logging and topographic habitat and their interaction on species diversity and composition, with the unlogged forest and valley habitat supporting higher diversity. Tree diversity varied among the topographic habitats in the logged but not in the unlogged forest. There were topographic effects on abundance of individual species but not on tree community abundance and basal area. Logging and its interaction with topographic habitat showed significant effects on tree abundance and basal area. Some species were associated with specific topographic habitats or a combination in the logged and unlogged forests. However, the patterns of habitat associations of the species differed between the logged and unlogged forests.

Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls.

The construction of artificial structures, such as seawalls, is increasing globally, resulting in loss of habitat complexity and native species biodiversity. There is increasing interest in mitigating this biodiversity loss by adding topographic habitat to these structures, and/or seeding them with habitat-forming species. Settlement tile experiments, comparing colonisation of species to more and less complex habitats, have been used to inform eco-engineering interventions prior to their large-scale implementation. Most studies have focused on applying one type of intervention (either adding habitat structure or seeding with native organisms), so it is unclear whether there are greater benefits to biodiversity when multiple interventions are combined. Using a fully orthogonal experiment, we assessed the independent and interactive effects of habitat structure (flat vs. crevice/ridges) and seeding with native oysters (unseeded vs. seeded) on the biodiversity of four different functional groups (sessile and mobile taxa, cryptobenthic and pelagic fishes). Concrete tiles (flat unseeded, flat seeded, complex unseeded and complex seeded) were deployed at two sites in Sydney Harbour and monitored over 12 months, for the survival and colonisation of oysters and the species density and abundances of the four functional groups. The survival of seeded oysters was greater on the complex than flat tiles, at one of the two sites, due to the protective role of crevices. Despite this, after 12 months, the species density of sessile invertebrates and the percentage cover of seeded and colonising oysters did not differ between complex and seeded tiles each of which supported more of these variables than the flat unseeded tiles. In contrast, the species density of mobile invertebrates and cryptobenthic fishes and the MaxN of pelagic fishes, at 1 month, were only positively influenced by seeding with oysters, which provided food as well as habitat. Within the complex seeded and unseeded tiles, there was a greater species density of sessile taxa, survival and percentage cover of oysters in the crevices, which were more humid and darker at month 12, had lower high temperature extremes at months 1 and 12, than on the ridges or flat tiles. Our results suggest that eco-engineering projects which seek to maximise the biodiversity of multiple functional groups on seawalls, should apply a variety of different microhabitats and habitat-forming species, to alter the environmental conditions available to organisms.

Tree community patterns along pond-upland topographic gradients, upper Uruguay River basin, southern Brazil

We investigated the interrelations between environmental variables (proxies for water availability and soil physico-chemical properties) and tree community patterns (diversity, composition and structure) along pond-upland topographic gradients within depressional watersheds in the upper Uruguay River basin in southern Brazil. Sampling plots were allocated in four topographic habitats: (1) inside ponds (wetland sites) and (2) immediately outside them, and in areas with (3) 60 cm and (4) 120 cm of elevation in relation to their limits (upland sites). Environmental conditions and forest composition varied along pond-upland gradients, but the diversity and structure of tree communities did not. Species turnover was associated with changes in topography-driven environmental conditions, notably water availability. Depressional watersheds show regionally unique abiotic and biotic features but are virtually unprotected by Brazilian private land legislation.

The phylogenetic structure of a subtropical evergreen broad-leaved forest community before and after an ice-snow storm in Gutianshan, eastern China

<p indent=0mm>Ice-snow storms are a major natural disturbance, and it is an important driver of forest regeneration in middle and high latitudes. Previous studies have focused on the impact of ice-snow storm on forest dynamics|however, few studies have investigated the impact of ice-snow storms on the phylogenetic structure of a forest community. In this study, we studied the phylogenetic structure of a 24-ha subtropical evergreen, broad-leaved forest in Gutianshan National Nature Reserve in Zhejiang Province before (2005) and after (2010) an ice-snow storm. We compared NRI and NTI of the community at three grain sizes, four topographic habitat types, and three size classes between before and after the ice-snow storm. Furthermore, we simulated random mortality during 2005–2010 and compared MPD and MNTD between observed values and random mortality in 2010. We found that (1) NRI and NTI were significantly higher after the storm across grain size, habitat, and size class. (2) Additionally, MNTD were not significantly different between random mortality and observed values in 2010 for the three size classes and two habitats, whereas all other cases showed that MNTD and MPD were significantly larger for random mortality than that for observed values. The results indicate that community phylogenetic structures of the study forest were more clustered after the storm, and they were little affected by grain size, vertical layer and habitat condition. This study improves our understanding of ice-storm disturbance on subtropical evergreen broad-leaved forest community phylogenetic structure.

Latitudinal limit not a cold limit: Cold temperatures do not constrain an endangered tree species at its northern edge

AbstractAimA strong influence of climate on species’ range is often assumed, and forms the basis for projecting many future range shifts with changing climate. Particularly at poleward latitudinal or elevational edges, abiotic conditions are thought to play a major role in limiting distributions. We estimated the roles of climate and landscape features in shaping habitat at the northern distributional edge of a critically threatened mountain tree species.LocationBritish Columbia and Alberta, Canada.TaxonPinus albicaulis (Engelm.)MethodsWe used a hierarchical Bayesian model (HBM) to combine multiple scales, sources, qualities and types of species data. We jointly examined the climate influence on occupancy across scales, and on juvenile and adult abundance to quantify habitat quality at two life history stages.ResultsWe found that cold temperature was the strongest predictor of whitebark pine occurrence at regional scales, with colder areas being better (i.e. the sign was negative). Occupancy at local scales was best predicted by low growing degree‐days and declining precipitation as snow. These relationships with occupancy across scales indicate that suitable climatic and topographic habitats currently exist beyond the northern edge of whitebark pine's current range. We found high adult abundance was predicted in sunny, cool habitats with little climatic drought, whereas high juvenile densities were associated with higher precipitation as snow and more climatic drought.Main conclusionsThe negative relationship to temperature and the ample suitable habitats predicted to exist poleward of the current species’ range limit indicates whitebark pine is not limited by cold temperatures. We suggest that not all species’ ranges are cold limited at high latitudes or elevations. For whitebark pine this means warming temperatures may not directly result in a northern range expansion as a result of warming habitat.

Surviving in the shadows: Light responses of co-occurring Rubiaceae species within a tropical forest understory

The question of how closely related species coexist in the heavily shadowed understory of tropical forests is still not properly clarified. We studied the magnitude of trait variation in twelve morphological, nutritional, and physiological traits related to light response in 85 individuals belonging to nine co-occurring Rubiaceae species in the understory of a tropical forest in Southeast Brazil. Three topographic habitats were chosen from shadowed and more fertile soils in the valley to more open canopies and less fertile soils at slopes and ridge, which were characterized using soil and canopy cover variables. Light curves (A/PAR) were adjusted in five leaves (one leaf per individual plant) for each species and habitat. We compared soil and canopy variables among the topographic habitats, quantified trait dispersion among the nine species, and tested for differences in trait values among habitats and species. Ridge habitat showed the lowest resource availabilities, soil nutrients and water, and a more open canopy. Species distributions in the gradient were best explained by specific leaf area (SLA) and leaf thickness. The Rubiaceae species showed a great investment in leaf nitrogen, regardless of habitat or species. Ridge species maintained low dark respiration rates and light compensation points. Higher SLA values for valley plants allowed greater light-saturated net photosynthetic rates on a mass basis, providing higher positive net carbon balance when compared to the plants in the slope and ridge. Living and survival of these shade plants are possible by very low mean light compensation points. We show that trait differences among topographic habitats in coexisting species of Rubiaceae are important to understanding the organization of tropical forests understory “species swarms”.

Neither species geographic range size, climatic envelope, nor intraspecific leaf trait variability capture habitat specialization in a hyperdiverse Amazonian forest

AbstractMany plant species exhibit strong association with topographic habitats at local scales. However, the historical biogeographic and physiological drivers of habitat specialization are still poorly understood, and there is a need for relatively easy‐to‐measure predictors of species habitat niche breadth. Here, we explore whether species geographic range, climatic envelope, or intraspecific variability in leaf traits is related to the degree of habitat specialization in a hyperdiverse tropical tree community in Amazonian Ecuador. Contrary to our expectations, we find no effect of the size of species geographic ranges, the diversity of climate a species experiences across its range, or intraspecific variability in leaf traits in predicting topographic habitat association in the ~300 most common tropical tree species in a 25‐ha tropical forest plot. In addition, there was no phylogenetic signal to habitat specialization. We conclude that species geographic range size, climatic niche breadth, and intraspecific variability in leaf traits fail to capture the habitat specialization patterns observed in this highly diverse tropical forest.

The Relationships among Topographically-driven Habitats, Dominant Species and Vertical Layers in Temperate Forest in China

The relationships among topography, topographically-driven habitats, dominant species and vertical layers in forest ecosystem are often complicate. In this study, based on the five 1 ha forest dynamics plots in China, we examined these relationships in different communities of temperate forest in China through structural equation model and torus-translation test. Our results showed that (1) the dominant species is an important factor that affects the forest vertical layers; (2) interspecific competition is prevalent between dominant species and canopy layer; (3) the species in each vertical layer have varying topographical habitat preferences. These findings shed new light to the species coexistence among forest vertical layers in temperate forest.

Habitat filtering of six coexistingHeliconiaspecies in a lowland tropical rain forest in Amazonian Ecuador

AbstractHerbaceous plants are often under-studied in tropical forests, despite their high density and diversity, and little is known about the factors that influence their distribution at microscales. In a 25-ha plot in lowland Amazonian rain forest in Yasuní National Park, Ecuador, we censused six species ofHeliconia(Heliconiaceae) in a stratified random manner across three topographic habitat types. We observed distribution patterns consistent with habitat filtering. Overall, more individuals occurred in the valley (N = 979) and slope (N = 847) compared with the ridge (N = 571) habitat. At the species level,Heliconia stricta(N = 1135),H.spathocircinata(N = 309) andH.ortotricha(N = 36) all had higher abundance in the valley and slope than ridge. Further,H. vellerigera(N = 20) was completely absent from the ridge. Conversely,H. velutina(N = 903) was most common in the drier ridge habitat. The two most common species (H.strictaandH.velutina) had a reciprocal or negative co-occurrence pattern and occurred preferentially in valley versus ridge habitats. These results suggest that taxa within this family have different adaptations to the wetter valley versus the drier ridge and that habitat partitioning contributes to coexistence.

Field surveys and revegetation experiments show that simulation of topographical habitat features could improve the chances of successful restoration for the threatened succulent Juttadinteria albata

Species with highly restricted ranges are disproportionately at risk of extinction, particularly where habitat loss occurs as a result of mining. Postmining restoration of rare species populations is considered as an appropriate response to counter such threats, but requires a careful, evidence‐based, and information‐driven approach. The economically important diamond mining at Sendelingsdrif in the southern Namib Desert occurs in the highly diverse Succulent Karoo Biome and threatens a significant part of the population of the narrow endemic succulent plant species Juttadinteria albata. To decrease the inherent risks of restoring such a rare species, we studied the habitat features of premining J. albata populations and experimentally tested whether some features could assist future reintroductions in postmining substrates. Plots where J. albata occurred were mostly south‐ to west‐facing and had among others higher rock cover and steeper slopes than plots where J. albata plants were absent. A revegetation experiment, with J. albata cuttings that were established on postmining substrate mounds, revealed that plants on steeper slopes, facing south to west, grew faster than plants on other slopes and aspects. Slope and aspect are therefore important habitat properties to recreate when restoring J. albata populations. These, and other preferred habitat properties such as higher levels of organic C, should now be tested in larger‐scale field trials. Validation of habitat requirements of J. albata through the revegetation experiment has decreased the risks at least partially and provides additional empirical evidence of the importance of establishing reference conditions to enhance ecological restoration.

Quaternary range-shifts of arboreal rodents of the genus Habromys (Cricetidae, Neotominae) in Mesoamerica and their evolutionary consequences

Arboreal mice of the genus Habromys (Cricetidae, Neotominae) comprise a clade of seven monotypic and endangered species that inhabit Mesoamerican cloud forests, one of the most threatened of the world’s ecosystems. Previous genetic analyses have suggested that closely related lineages are poorly differentiated, which is probably an evolutionary consequence of distributional shifts driven by Quaternary climate oscillations through a complex topography. Here, we assess the potential role of distributional shifts through time on current patterns of genetic differentiation in Habromys. Following recent best-practices recommendations for estimating ecological niches and paleodistributions for montane species, we estimated the potential distribution of the genus across the last glacial-interglacial cycle under three alternative Global Circulation Models to explore the more recent evolutionary history of this genus. Distributional shifts and population connectivity among populations of Habromys were greatly affected by past climate change coupled with topographic habitat heterogeneity. The effectiveness of some barriers to dispersal has changed over time as result of the climatic oscillations. Poorly differentiated genetic lineages might be an evolutionary consequence of intermittent connections and corridors for montane species motivated by glaciations. Some valleys and lowland habitats probably served as previously unrecognized barriers to dispersal during warmer periods. This study emphasizes the dynamic nature of species’ geographic distributions and potential barriers to create current patterns of genetic variation in Mesoamerican rodents.

Optimization of sensor deployment for acoustic detection and localization in terrestrial environments

AbstractThe rapid evolution in miniaturization, power efficiency and affordability of acoustic sensors, combined with new innovations in smart capability, are vastly expanding opportunities in ground‐level monitoring for wildlife conservation at a regional scale using massive sensor grids. Optimal placement of environmental sensors and probabilistic localization of sources have previously been considered only in theory, and not tested for terrestrial acoustic sensors. Conservation applications conventionally model detection as a function of distance. We developed probabilistic algorithms for near‐optimal placement of sensors, and for localization of the sound source as a function of spatial variation in sound pressure. We employed a principled‐GIS tool for mapping soundscapes to test the methods on a tropical‐forest case study using gunshot sensors. On hilly terrain, near‐optimal placement halved the required number of sensors compared to a square grid. A test deployment of acoustic devices matched the predicted success in detecting gunshots, and traced them to their local area. The methods are applicable to a broad range of target sounds. They require only an empirical estimate of sound‐detection probability in response to noise level, and a soundscape simulated from a topographic habitat map. These methods allow conservation biologists to plan cost‐effective deployments for measuring target sounds, and to evaluate the impacts of sub‐optimal sensor placements imposed by access or cost constraints, or multipurpose uses.

Canopy openness and topographic habitat drive tree seedling recruitment after snow damage in an old-growth subtropical forest

We investigated the relative and combined effect of topography and light environment on the recruitment of seedlings in a subtropical forest after snow damage to the canopy. The tree seedling community in an old-growth subtropical forest was monitored using 500 2 m × 2 m seedling plots at six-month intervals for 2 years. With a focus on recruitment following canopy damage, we related abiotic and biotic environmental variables to seedling dynamics, and we tested if significant topographic and light habitat associations were present for seedlings recruiting via a torus translation test. Then, we used variance partitioning to examine the relative effects of spatial, topographic and light variables on the temporal assemblages of seedlings. A total of 3047 seedlings from 58 species recruited in the first 2 years following snow damage. At the community level, increases in seedling abundance and richness were positively correlated with canopy openness and negatively correlated with elevation. At the species level, both pioneer and late-successional tree species had more recruits in high light environment than in low light environment. 84.3% of the recruiting seedlings were significantly associated to either light environment (35.7%), topography (26.5%) or both (22.1%). Despite this, at the plot level, spatial variables (PCNM) explained the majority of the variability in seedling composition over time. Our results suggest that snow damage to the canopy increases species richness and abundance via light-facilitated seedling recruitment, and that the composition of recruiting seedlings was largely spatially dependent. Topographic habitat filtering acts as a persistent force in determining the recruitment of seedlings and increases in strength with increased light-facilitated seedling recruitment. Our results highlight that, in this subtropical forest, both light requirement and topographic specialization interact over time to play a key role in promoting coexistence of tree species through selection of individuals at the seedling stage. We also suggest exploring the possibilities of management intervention to speed up the recovery of this forest.

Effects of light and topography on regeneration and coexistence of evergreen and deciduous tree species in a Chinese subtropical forest

Abstract Evergreen broad‐leaved forests are widely distributed in eastern Asia with evergreen broad‐leaved (EBL) and deciduous broad‐leaved (DBL) tree species coexisting under the same climatic regime, raising questions as to the underlying mechanisms. Since EBL and DBL species differ in leaf life span, a key component of resource economic strategies, their coexistence might be attributed to regeneration niche partitioning across habitats varying in resource supply. We investigated the effects of variation in insolation and topography on regeneration of EBL and DBL species in a subtropical EBL forest of eastern China after an ice storm that caused severe canopy disturbance. Using a mixed‐effects modelling framework and census data from 2011 to 2014 on 8,548 wild seedlings of 123 species, we quantified habitat preferences of EBL and DBL species during post‐disturbance regeneration and how their survival and height relative growth rates varied among habitats. The relative density of DBL seedlings (proportional to all seedlings) was greater in habitats with greater (canopy gaps) compared to habitats with lesser (understorey) insolation and increased with canopy gap size. However, DBL seedlings were not more frequent in higher (valleys) compared to lower (ridges) fertility habitats. Although DBL seedlings exhibited larger differences in growth between higher and lower resource habitats than EBL seedlings, their growth rates did not increase with canopy gap size. Seedlings of EBL species had high survival in all habitats, but larger DBL seedlings survived equally well on ridges. Consequently, the relative density of DBL seedlings declined in valleys, so that by 2014 it became more similar in valley and ridge habitats, whereas it remained higher in gaps than in the understorey, and especially in larger gaps. Synthesis. Specialization on contrasting topographic habitats is considered the primary mechanism mediating coexistence between deciduous broad‐leaved and evergreen broad‐leaved species. Our results, however, suggest this may not always be true, since seedlings of deciduous broad‐leaved and evergreen broad‐leaved species partitioned regeneration niches based on light more so than topography. We propose that coexistence of deciduous broad‐leaved and evergreen broad‐leaved species can strongly depend upon canopy disturbance to create a mosaic of habitat patches, including high light gaps favouring regeneration of deciduous broad‐leaved species.

Abiotic niche partitioning and negative density dependence drive tree seedling survival in a tropical forest.

In tropical tree communities, processes occurring during early life stages play a critical role in shaping forest composition and diversity through differences in species' performance. Predicting the future of tropical forests depends on a solid understanding of the drivers of seedling survival. At the same time, factors determining spatial and temporal patterns of seedling survival can play a large role in permitting species coexistence in diverse communities. Using long-term data on the survival of more than 45 000 seedlings of 238 species in a Neotropical forest, we assessed the relative importance of key abiotic and biotic neighbourhood variables thought to influence individual seedling survival and tested whether species vary significantly in their responses to these variables, consistent with niche differences. At the community level, seedling survival was significantly correlated with plant size, topographic habitat, neighbourhood densities of conspecific seedlings, conspecific and heterospecific trees and annual variation in water availability, in descending order of effect size. Additionally, we found significant variation among species in their sensitivity to light and water availability, as well as in their survival within different topographic habitats, indicating the potential for niche differentiation among species that could allow for species coexistence.

South-west Sri Lanka: a floristic refugium in South Asia

In South Asia, south-west Sri Lanka alone represents the sole surviving perhumid region where Mixed Dipterocarp Forests (MDF) harbour a number of relict signature taxa providing evidence of ancient plant migration routes from Gondwana to perhumid Far Eastern Sunda land, at least partially via southern Laurasia. Palynological and phylogenetic evidence now confirm that the Malesian rain forest flora overwhelmingly immigrated from tropical Africa/Madagascar and South Asia along an ever-wet equatorial corridor sometime before the collision of the Indian Plate with the Southern Laurasian coast in middle Eocene (c. 45 Ma). This ever-wet climatic and geological history has led to the evolution of a characteristic species composition and dynamics within SW Sri Lanka’s rain forest communities, which exhibit remarkable patterns of species distribution and habitat specialization, and which parallel those of Far Eastern sister taxa. We here examine the patterns of floristic variation in each of three MDF sites using classification and ordination methods to analyse tree data collected in 62 random plots, each 0.25ha, along an altitudinal gradient. In addition, a 25 ha forest dynamics plot (FDP) was established in the Sinharaja World Heritage Site, within the CTFS-ForestGEO network that addresses issues related to community ecology and phylogenetics incorporating large-scale biogeographic patterns of major clades of both plants and animals. In the above studies, ecologically distinct floristic assemblages were revealed, i) among the three MDF sites as well as, ii) among the ridge-, slope-, and valley habitats within each forest site. In the FDP too, >80% of the 125 species, with individuals >1 cm dbh and >100 individuals per species, are significantly more associated with one or more of eight topographic habitat categories than in others. These results suggest that ecological ranges and dispersion of tree species in SW Sri Lanka is primarily mediated by soil water and nutrient levels in topographically different catenal habitats mirroring those in the Far East. Many of these species are threatened relict endemics, and their distribution patterns have important conservation implications. Their spatial distributional features provide useful criteria in site-species matching or environmental filtering in forest restoration efforts.

Understory host plant and insect gall diversity changes across topographic habitats differing in nutrient and water stress in the Brazilian Amazon rainforest

ABSTRACT Topographic gradients in terra firme forests are associated with pronounced changes in soil texture, soil nutrients and distance to the water-table, thereby creating different hydric and nutritional conditions for plants and their associated herbivore community. The aim of this study was to investigate galling species and host plant richness and gall species composition across topographic habitats differing in nutrient and water stress in the Brazilian Amazon rainforest. Nineteen 250 x 3 m plots were randomly sampled in the valley, slope, plateau-slope transition, and plateau habitats in terra firme forests. All individual dicotyledonous plants 1 to 3 meters high were examined for the presence of insect galls. Galling species and host plant richness differed significantly among the studied habitats and were higher in slope habitats (drier habitats), as expected. More humid areas (valleys) showed the lowest richness of galling species, and a lower number of understory host plants. PERMANOVA and PERMDISP results demonstrated that the gall species composition differed significantly in the valley, slope, plateau-slope transition, and plateau areas. However, these structural differences in species composition could be due to unequal dispersion of variability among forest habitats. Our data suggested that spatial heterogeneity, such as a mosaic of local water status and nutrient availability resulting from the topographic conditions, can affect host plants and their associated galling insects.

Distribution comparée du Dauphin commun (Delphinus delphis) et du Dauphin bleu et blanc (Stenella coeruleoalba) en Méditerranée occidentale

Comparative distribution of Common Dolphin (Delphinus delphis) and Striped Dolphin (Stenella coeruleoalba) in western Mediterranean Sea. The summer distribution and habitat of the Striped Dolphin Stenella coeruleoalba and Common Dolphin Delphinus delphis in the western Mediterranean Sea were investigated with small boat survey data collected from 1988 to 2012. An effective survey effort of 57 912 km enabled to collect 2300 sightings of Striped Dolphin and 46 of Common Dolphin, in most regions of the western basin. High Striped Dolphin relative abundance indices (RAI) were obtained in the northernmost regions and the Alboran Sea, about one dolphin per km of effort, and very low Common Dolphin RAI were obtained off the French continental coast. Common dolphin relative abundance was at the highest in the Alboran Sea (about 0.38 individual/km), moderate in the western Sardinia regions, and significant in other southern regions. Striped and Common Dolphins did not share the same topographic habitat, the former favouring slope and oceanic waters (mean bottom depth 1839 m) and the latter being more frequent in the shelf and slope domains (mean bottom depth 649 m). However, Striped Dolphin was frequently observed in the three habitat types, when Common Dolphin was almost not recorded in the oceanic domain. French stranding statistics were in agreement with our survey results and suggested that Striped Dolphin frequency increased during the 20th century, when Common Dolphin strandings decreased, in particular on French continental shores. However, both species were present in records since the 19th century. While some scientific accounts mentioned Common Dolphin as ''common'' and Striped Dolphin as ''rare'', erroneous identifications and dubious mentions were identified in the literature. The overall picture suggests that the Striped Dolphin is, and was, a common species in oceanic waters of the western Mediterranean Sea, while the Common Dolphin occupies and occupied a more specific habitat in shallower waters, certainly in relation to its preferred diet. Common Dolphins might have disappeared from regions where their preferred habitat has been overexploited. This study brings new elements to understand Common Dolphin current conservation issues in the Mediterranean Sea.

Linking trait similarity to interspecific spatial associations in a moist tropical forest

AbstractAimCommunity assembly theory predicts that niche differentiation promotes the spatial clustering of functionally dissimilar species, whereas habitat filtering has the converse effect. We used these predictions to assess the relative effects of habitat filtering and niche differentiation on recruit community assembly over spatial (5‐ and 30‐m neighbourhoods) and temporal (20‐yr) scales in the Forest Dynamics Plot at Barro Colorado Island.LocationBarro Colorado Island, Panama.MethodsWe integrated data on the spatial patterns of ≥1 cm DBH (diameter at 1.3 m above ground) recruits with data on seven functional traits for 64 species. First, we quantified the interspecific association patterns of all species pairs i and j using the K‐function Kij(r) and the nearest‐neighbour distribution function Dij(r). Second, for those pairs with significant spatial associations, we calculated an index of interspecific spatial association using the results of these two summary statistics. Finally, we examined the relationship between interspecific spatial association and trait similarity using simple and partial Mantel tests.ResultsIn all censuses, almost one‐half of species pairs had no spatial associations, but for pairs that were significantly spatially associated, positive relationships between trait similarity and spatial association occurred in 5‐m and 30‐m neighbourhoods, whereas significant negative relationships only appeared in 5‐m neighbourhoods. This suggests that habitat filtering was more important than niche differentiation in assembling recruit communities at 5‐ and 30‐m scales. Habitat filtering mainly acted upon traits related to topographic habitat preferences and dispersal mode, whereas spatial association was inversely related to similarity in terms of wood specific gravity and shade tolerance.ConclusionsOur findings suggest that both stochastic and deterministic processes operate in species‐rich ecological communities, but the role of habitat filtering and niche differentiation as determinants of community assembly vary over spatial and temporal scales. Species co‐occurrence was driven by habitat filtering at small and large scales, but also by a combination of niche differentiation and weaker‐competitor exclusion effects at small scales. Temporal variations in the importance of habitat filtering and niche differentiation could be related to the occurrence of disturbances such as tree falls. Our results emphasize the role of trait‐based processes in plant community assembly.