Richness Gradients Research Articles

Seed size underlies the uncoupling in species composition between canopy and recruitment layers in European forests

Increasing evidence supports that canopy richness favors multifunctionality in European forests. Given that recruitment represents one of the main demographic bottlenecks in plant communities, the provision of similar levels of forest functioning in the future will depend on successful natural regeneration in the present. Here, we evaluated whether canopy diversity enhances recruitment (abundance and richness) by providing a more diverse seed supply and a more heterogeneous environmental context; or, alternatively, if post-dispersal environmental filters outweigh these expected positive effects. For this purpose, we used a large-scale European platform comprising 173 forest plots that follow a canopy species richness gradient (from monoculture to mixed plots up to 5 tree species) in five forest types from Mediterranean to boreal forests. Despite that canopy diversity promoted a more diverse seed supply and a more heterogeneous light environment, this did not translate into a richer or more abundant community of recruits. In fact, we found an important uncoupling in species composition between recruitment and canopy layers. Our results suggest that strong post-dispersal filters, mediated by an interaction between seed size and latitude, drive this mismatch. Large-seeded species were over-represented in the recruitment at both extremes of the European gradient (Mediterranean and boreal forests), while the opposite trend occurred at intermediate latitudes. Altogether, our work shows that preserving a more diverse canopy may not ensure successful recruitment, and advocates future studies analyzing the role of post-dispersal filters on the assembly of European forests.

A comprehensive formula for decomposing change in community similarity into introduction and extinction events

Biological invasion, climate change and urbanization are presently altering species compositions worldwide (McKinney and Lockwood 1999, McKinney 2006, Qian and Ricklefs 2006, Magurran et al. 2015, Gossner et al. 2016, Newbold et al. 2018), while changes in community composition are generally associated with higher community similarities and losses of community distinctiveness (McKinney and Lockwood 1999, Olden et al. 2004). The underlying processes of homogenization are still unclear and especially lack theoretical underpinnings (Olden and Poff 2003, Rosenblad and Sax 2017). It was theoretically and empirically proposed that similarity changes are determined by different events (Olden and Poff 2003, Villéger and Brosse 2012, Rosenblad and Sax 2017). For example, the introduction of the same species to two sites would result in an increase in similarity, while the extinction of same native species from two sites would lead to a decrease in similarity. In complement to earlier works on the mathematical description of homogenization (Olden and Poff 2003, Villéger and Brosse 2012). Rosenblad and Sax (2017) provided a concise framework of six types of introduction and extinction events with formula derivations to qualify the relative importance of these events on changes in community similarity. They found that the effects of these events vary according to event type and initial similarity. However, their conclusions mainly derive from simplified assumptions of species richness and number of events, which prevent us from understanding the precise relationships among initial similarity, different event types and total number of species. Through mathematical derivation and simulations (Fig. 1), we thus aim to: 1) combine previously identified determinants of change in similarity into a single comprehensive formula; 2) illustrate how these determinants interact with each other; and 3) highlight the important role of gradient in species richness in affecting the magnitude of similarity changes.

The functional trait space of tree species is influenced by the species richness of the canopy and the type of forest

Analysing how species modify their trait expression along a diversity gradient brings insight about the role that intraspecific variability plays over species interactions, e.g. competition versus complementarity. Here, we evaluated the functional trait space of nine tree species dominant in three types of European forests (a continental‐Mediterranean, a mountainous mixed temperate and a boreal) growing in communities with different species richness in the canopy, including pure stands. We compiled whole‐plant and leaf traits in 1719 individuals, and used them to quantify species trait hypervolumes in communities with different tree species richness. We investigated changes along the species richness gradient to disentangle species responses to the neighbouring environment, in terms of hypervolume size (trait variance), shape (trait relative importance) and centroid translation (shifts of mean trait values) using null models. Our main results showed differences in trait variance and shifts of mean values along the tree diversity gradient, with shorter trees but with larger crowns in mixed stands. We found constrained functional spaces (trait convergence) in pure stands, suggesting an important intraspecific competition, and expanded functional spaces (trait divergence) in two‐species admixtures, suggesting competition release due to interspecific complementarity. Nevertheless, further responses to increasing species richness were different for each forest type, waning species complementarity in sites with limiting conditions for growth. Our results demonstrate that tree species phenotypes respond to the species richness in the canopy in European forests, boosting species complementarity at low level of canopy diversity and with a site‐specific pattern at greater level of species richness. These outcomes evidence the limitation of functional diversity measures based only on traits from pure stands or general trait database values.

Latitudinal and bathymetrical species richness patterns in the NW Pacific and adjacent Arctic Ocean

Global scale analyses have recently revealed that the latitudinal gradient in marine species richness is bimodal, peaking at low-mid latitudes but with a dip at the equator; and that marine species richness decreases with depth in many taxa. However, these overall and independently studied patterns may conceal regional differences that help support or qualify the causes in these gradients. Here, we analysed both latitudinal and depth gradients of species richness in the NW Pacific and its adjacent Arctic Ocean. We analysed 324,916 distribution records of 17,414 species from 0 to 10,900 m depth, latitude 0 to 90°N, and longitude 100 to 180°N. Species richness per c. 50 000 km2 hexagonal cells was calculated as alpha (local average), gamma (regional total) and ES50 (estimated species for 50 records) per latitudinal band and depth interval. We found that average ES50 and gamma species richness decreased per 5° latitudinal bands and 100 m depth intervals. However, average ES50 per hexagon showed that the highest species richness peaked around depth 2,000 m where the highest total number of species recorded. Most (83%) species occurred in shallow depths (0 to 500 m). The area around Bohol Island in the Philippines had the highest alpha species richness (more than 8,000 species per 50,000 km2). Both alpha and gamma diversity trends increased from the equator to latitude 10°N, then further decreased, but reached another peak at higher latitudes. The latitudes 60–70°N had the lowest gamma and alpha diversity where there is almost no ocean in our study area. Model selection on Generalized Additive Models (GAMs) showed that the combined effects of all environmental predictors produced the best model driving species richness in both shallow and deep sea. The results thus support recent hypotheses that biodiversity, while highest in the tropics and coastal depths, is decreasing at the equator and decreases with depth below ~2000 m. While we do find the declines of species richness with latitude and depth that reflect temperature gradients, local scale richness proved poorly correlated with many environmental variables. This demonstrates that while regional scale patterns in species richness may be related to temperature, that local scale richness depends on a greater variety of variables.

Freshwater conservation assessments in (semi-)arid regions: Testing river intermittence and buffer strategies using freshwater mussels (Bivalvia, Unionida) in Morocco

The IUCN Red List assessments are essentially based on population trends and range, namely Area of Occupancy (AOO) and Extent of Occupancy (EOO). Range estimations are based on fixed grids, but this is likely inappropriate for species living in river networks. Furthermore, AOO and EOO are measured using the whole hydrographic network, therefore disregarding temporary sections, which is particularly problematic in arid and semi-arid regions. Here we mapped the permanent hydrographic network of Morocco using satellite imagery, complemented with field surveys to collect samples for molecular analyses of the five freshwater mussel species present and assess their distribution. The phylogeographic patterns are described for each species and used to identify priority areas and evolutionary significant units for conservation. Permanent hydrographic river sections represent only 18.3% of the whole hydrographic network. A north-to-south gradient of genetic diversity, species richness and distribution range was found, being coherent with water availability and river intermittence. Isolated evolutionary units were detected in southern basins that should also receive particular attention in conservation planning. We propose the mean river width multiplied by the extent of the river network as the best and the most adequate way to estimate both EOO and AOO. Given the worldwide degradation of freshwater systems and biodiversity, an accurate (re)assessment of species conservation status supported with maps of intermittent water bodies will be essential for prioritizing and guiding conservation actions and management plans, especially in arid and semi-arid regions.

Latitudinal effects of anthropogenic factors driving raptor species richness across the American continent

AbstractAimGeographic gradients in species richness have been traditionally attributed to variation in environmental conditions. However, untangling the importance of human activities in explaining spatial patterns in raptor species richness, still remains a challenge. We evaluated the relative importance of how human impacts and ecogeographical conditions shape the latitudinal patterns of resident (n = 140) and migratory (n = 35) raptor species richness in the American continent, and analysed whether these factors differ between the Nearctic and the Neotropical regions.LocationAmerican Continent.TaxonRaptor species.MethodsWe processed BirdLife distributional data for 175 raptor species distributed in America, and a set of three ecogeographical and three human impact variables at a spatial resolution of 96.5 × 96.5 km. We applied simple, multiple, simultaneous auto‐regressive models and partial regression analyses to explore the associations between ecogeographical conditions and human impacts on raptor richness.ResultsAt the continental scale and the Neotropical region, global forest structure is the main driver of richness patterns for all raptors combined, and for resident and migratory species alone. In the Nearctic region, human accessibility was the best explanatory variable to understand the spatial patterns of richness regardless of the raptor group analysed.Main conclusionsEnvironmental features play a leading role in determining raptor species richness. However, the independent contribution of human activities emerges as an important factor in explaining richness patterns of migratory species at the continental scale and within the Neotropical region, whereas humans become particularly important in the Nearctic region for all raptors, residents and migratory species. We show for the first time how human‐related factors influence raptor species richness.

Biomass compensation: Behind the diversity gradients of tidepool fishes

AbstractSpecies richness is unevenly distributed on the Earth, with biodiversity gradients of various spatial scales supposedly being affected by abiotic as well as biotic factors including community traits such as body size spectra and relative abundance patterns. To explore large‐scale spatial variation in species diversity and their processes, tidepool fish communities were investigated through an intensive field work conducted on 55 shore sites in south‐western Japan. Multiple ecological measures were taken into account to assess changes in local community structures with changes in the number of species. Biomass (total fish wet weight) per unit area showed no systematic change with latitude, while taxa richness and number of individuals tended to increase toward lower latitudes. In addition, median fish body weight scaled positively with latitude, which was more conspicuous in Blenniidae than in Gobiidae. The latitudinal gradient of diversity in tidepool fish assemblages appears to be characterized by partitioning of total biomass that tends to stay constant across latitudes, suggesting the phenomenon of “biomass compensation” whereby body size and abundance/diversity change in opposite directions with latitude. Our study highlights that biomass compensation can be part of processes involved in generating gradients of species richness even without an apparent energy/resource gradient.

Diversity, Endemism, Species Turnover and Relationships among Avifauna of Neotropical Seasonally Dry Forests

Neotropical seasonally dry forests (NSDF) are widely distributed across Latin America and the Caribbean. They possess important levels of species richness and endemism but few studies have assessed the diversity patterns and ecological relationships between the entire avifauna of these threatened forests. Thus, in order to analyse the macro-ecological patterns and the community structure of NSDF avifaunas, we generated species distribution models describing the current geographical distribution of 1,298 bird species inhabiting NSDF. We assessed species richness gradients in terms of distance from the Equator using both linear and polynomial regressions. Then, based on a matrix composed of the presence or absence of species in 563 quadrants, we performed cluster analyses (considering the Simpson dissimilarity index [βSIM] as a distance measure) to identify the main NSDF regions and describe the avifaunal affinities among them. For the identified groups, we estimated the dissimilarity values, using both an ANOSIM test and the βSIM index. Overall, we observed the lack of an equatorial peak for species diversity of NSDF avifauna in the latitudinal gradient and identified 12 avifaunistic groups. The βSIM index among the NSDF avifaunal groups ranged from 0.05-0.73, showing statistically significant differences (R = 0.894, p = 0.001) in species composition among them. Species shared between two or three NSDF groups comprised a higher proportion (∼38%) than those exclusive to each group (∼23%). Only 35 species were shared between the 12 groups. This information supports a separation of the NSDF avifauna into two major groups (northern and southern), as well as the idea of connections during recent geological time among the NSDF in southeastern South America (the so-called Pleistocene Arc Hypothesis). We provide a scientific framework to contextualise the importance of each NSDF nuclei in terms of their avifauna, supplying an ecological basis for future conservation decisions in order to protect their diversity. —Prieto-Torres, D.A., Rojas-Soto, O.R., Santiago-Alarcon, D., Bonaccorso, E. & Navarro-Siguenza, A.G. (2019). Diversity, endemism, species turnover and relationships among avifauna of neotropical seasonally dry forests. Ardeola, 66: 257-277.

Latitudinal gradient in intertidal seaweed composition off the coast of southern Brazil and Uruguay

Seaweed latitudinal patterns of diversity present singularities, with implications for ecological, biogeographic and conservation studies. Despite observed global trends, distinct gradients of marine biodiversity occur along the Southwestern Atlantic (SWA) coast. However, the transitional warm-temperate marine province is a major knowledge gap, limiting conclusions about regional seaweed biodiversity. Our study investigated the composition of macroalgal assemblages in rocky intertidal shores between southern Brazil and Uruguay (from 26 °S to 35 °S), accounting for the regional oceanography and coastline features. Integrating historical and new data, we found a significant latitudinal gradient in seaweed species richness and turnover. The complex thermo-haline structure of the coastal waters, summed to large local fluvial discharges (i.e., La Plata River and Patos-Mirim Lagoon), significantly impacted the distributional pattern of macroalgal assemblages. Sea surface temperature and salinity are the main drivers of the species composition and distribution, despite the changes in coastal geomorphology and substrate availability. Whilst the northernmost section (26 °S–29 °S) was characterized by a tropical/warm-temperate phycoflora, the southernmost (32 °S–35 °S) was dominated by eurythermic and euryhaline species. Taking into account this and previous phycogeographic studies, the lowest seaweed species richness seen in SWA rocky shores occurs across 32 °S and 37 °S, while increasing towards both north (into the tropical marine province) and south (into the cold-temperate marine province). Our study fulfills knowledge gaps in Southwestern Atlantic Ocean biogeography, providing baseline information for conservation efforts and for the detection of ongoing human impacts on intertidal macroalgal assemblages.

Inter-regional beta-diversity patterns of the woody flora of Greece

This study aimed to estimate and explain the compositional differentiation and observed beta-diversity (partitioned as turnover and nestedness) patterns of woody species in Greece. Specifically, the 13 phytogeographical regions of Greece were compared at three taxonomic levels: family, genus, and (sub-) species. In Greece, 565 woody taxa have been identified, which are classified into 224 genera and 85 families. Phytogeographical regions were grouped according to their similarity in floristic composition using cluster analysis. Dominant beta diversity gradients were detected using Mantel directional correlation tests applied at the three taxonomic levels. The 13 phytogeographical regions formed three groups; namely, the northern, central, and southern (Aegean Islands) regions. The taxonomic variation among the phytogeographical regions at the species level was related to a gradient of richness of widespread taxa and Balkan endemic taxa. Analysis at the taxonomic levels of species and genus produced similar results, yet, at the family level, floristic differences were less pronounced, with phytogeographical regions being aggregated in two groups: the southernmost island regions versus all other regions. Beta diversity was higher along a northwestern to southeastern gradient, which was related to a dominant climatic gradient throughout Greece. The partitioning of beta diversity between neighboring phytogeographical regions showed that species turnover was the dominant component; yet, some phytogeographical regions (e.g., Kiklades) were characterized by high nestedness. In conclusion, analysis of woody taxa provides different insights regarding the floristic variation of phytogeographical regions.

Impact of soil leachate on microbial biomass and diversity affected by plant diversity

High plant diversity is usually linked with high soil microbial diversity, which is hypothesized to be attributed to a high diversity of components in the soil leachate, but experimental evidence is scarce. The aim of this study was to determine if the variation in soil leachate caused by plant diversity could affect the soil microbial community. A microcosm experiment was conducted to determine the effect of plant diversity on the soil microbial community by measuring soil leachate in a gradient of plant richness from levels 1 (one species) to 3 (three species). Plant richness significantly affected the diversity of soil leachate and microbial communities. The amount and diversity of soil leachate, microbial biomass carbon (C), basal respiration, β-1,4-glucosidase activity, β-1,4-N-acetylglucosaminidase activity, bacterial biomass, fungal biomass, total microorganism biomass, and microbial diversity (Shannon diversity index and evenness) were highest at richness level 3. Changes in the microbial community were best explained by variation in the amount and diversity of leachate. Linear regression and correlation analyses indicated that leachate diversity had a close association with microbial Shannon diversity and evenness, whereas leachate amount had a close association with microbial biomass C, total microbial biomass, bacterial biomass, enzyme activities, and abundance of microbial groups. An ordinary least squares multiple regression and the structural equation model demonstrated that leachate amount had a greater effect on microbial biomass than leachate diversity, which had a greater impact on microbial Shannon diversity and evenness. Our results indicate that plant diversity drives changes in soil microbial communities by altering the amount and diversity of leachate in the soil. The diversity of soil leachate determined the diversity of the microbial community to some extent.

Different responses of avian feeding guilds to spatial and environmental factors across an elevation gradient in the central Himalaya.

Although elevational patterns of species richness have been well documented, how the drivers of richness gradients vary across ecological guilds has rarely been reported. Here, we examined the effects of spatial factors (area and mid‐domain effect; MDE) and environmental factors, including metrics of climate, productivity, and plant species richness on the richness of breeding birds across different ecological guilds defined by diet and foraging strategy. We surveyed 12 elevation bands at intervals of 300 m between 1,800 and 5,400 m a.s.l using line‐transect methods throughout the wet season in the central Himalaya, China. Multiple regression models and hierarchical partitioning were used to assess the relative importance of spatial and environmental factors on overall bird richness and guild richness (i.e., the richness of species within each guild). Our results showed that richness for all birds and most guilds displayed hump‐shaped elevational trends, which peaked at an elevation of 3,300–3,600 m, although richness of ground‐feeding birds peaked at a higher elevation band (4,200–4,500 m). The Normalized Difference Vegetation Index (NDVI)—an index of primary productivity—and habitat heterogeneity were important factors in explaining overall bird richness as well as that of insectivores and omnivores, with geometric constraints (i.e., the MDE) of secondary importance. Granivore richness was not related to primary production but rather to open habitats (granivores were negatively influenced by habitat heterogeneity), where seeds might be abundant. Our findings provide direct evidence that the richness–environment relationship is often guild‐specific. Taken together, our study highlights the importance of considering how the effects of environmental and spatial factors on patterns of species richness may differ across ecological guilds, potentially leading to a deeper understanding of elevational diversity gradients and their implications for biodiversity conservation.

Latitudinal gradients of parasite richness: a review and new insights from helminths of cricetid rodents

The latitudinal diversity gradient (LDG), or the trend of higher species richness at lower latitudes, has been well documented in multiple groups of free‐living organisms. Investigations of the LDG in parasitic organisms are comparatively scarce. Here, I investigated latitudinal patterns of parasite diversity by reviewing published studies and by conducting a novel investigation of the LDG of helminths (parasitic nematodes, trematodes and cestodes) of cricetid rodents (Rodentia: Cricetidae). Using host–parasite records from 175 parasite communities and 60 host species, I tested for the presence and direction of a latitudinal pattern of helminth richness. Additionally, I examined four abiotic factors (mean annual temperature, annual precipitation, annual temperature range and annual precipitation range) and two biotic variables (host body mass and host diet) as potential correlates of parasite richness. The analyses were performed with and without phylogenetic comparative methods, as necessary. In this system, helminths followed the traditional LDG, with increasing species richness with decreasing latitude. Nematode richness appeared to drive this pattern, as cestodes and trematodes exhibited a reverse LDG and no latitudinal pattern, respectively. Overall helminth richness and nematode richness were higher in areas with higher mean annual temperatures, annual precipitation and annual precipitation ranges and lower annual temperature ranges, characteristics that often typify lower latitudes. Cestode richness was higher in areas of lower mean annual temperatures, annual precipitation and annual precipitation ranges and higher annual temperature ranges, while trematode richness showed no relationship with climate variables when phylogenetic comparative methods were used. Host diet was significantly correlated with cestode and trematode species richness, while host body mass was significantly correlated with nematode species richness. Results of this study support a complex association between parasite richness and latitude, and indicate that researchers should carefully consider other factors when trying to understand diversity gradients in parasitic organisms.

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.

Spatialization of Tree Species Diversity in the State of Minas Gerais

ABSTRACT The state of Minas Gerais has a high ecological relevance mainly due to its forest species diversity. Understanding the spatialization of that diversity is of importance to develop environmental public policies. The hypothesis of this study is that the tree species diversity from different forest types, in the state of Minas Gerais, presents distribution with spatial dependence. Thus, the objective of this work was to prove that spatial dependence and to relate it between the forest types. Data from the project called “Forest Inventory of Minas Gerais” were used to calculate indices of Shannon, Simpson and Pielou. We used geostatistical and kriging tools to create spatial maps. As results, the mappings indicated that the state presents well-defined gradients of diversity and richness of forest species, increasing in North-South and from West-East directions. The spatial dependence and the spatialization of the tree species diversity show that the geostatistical modeling is a tool that supports the forest resource management. The maps of diversity can be used as indicators of potential areas for creating Conservation Units, establishing ecological corridors, besides supporting environmental policy development.

Intraspecific trait variability of trees is related to canopy species richness in European forests

Functional diversity informs about biodiversity-ecosystem functioning relationships. The intraspecific component of functional diversity (i.e. the phenotypic space of each species) depicts individual differences in the resource use and fitness among conspecifics, and gives valuable information about the functional similarity (competition) or dissimilarity (complementarity) of coexisting species. Here, we quantified trait differences within tree species along local diversity gradients to shed light on the role that this intraspecific variability exerts on functional complementarity of tree species. We measured architectural traits in 5,036 individuals and leaf traits in 1,403 individuals from nine dominant tree species, surveyed in 92 plots located in three major European forest types (Mediterranean, temperate and boreal forests). In each forest type, plots were positioned along a canopy richness gradient, with every study species present in different species richness levels, including monocultures. Our results showed that the relative magnitude of intraspecific trait variability to community-level variability is high in these forests. At the species level, we found adjustments of species leaf traits (mean shifts) in response to neighbouring trees, suggesting the existence of processes that limit niche overlap. We also found higher variability in architectural traits of conspecific individuals in more diverse canopies, suggesting greater niche packing and a more efficient use of available space as the number of species in the canopy increases. Altogether, our results support the hypothesis that differential responses of individuals within a species promote species complementarity, suggesting that biodiversity-ecosystem functioning relationships cannot be properly estimated without accounting for the intraspecific level of functional variation.

New insights from multidimensional trait space responses to competition in two clonal plant species

Abstract Trait intraspecific variability determines community dynamics and species coexistence. In response to competition, plants can display intraspecific variability to enhance their competitive ability or stabilise their niche differences with competitors. This response is multidimensional because it involves changes along different functional axes and inevitable trade‐offs between traits. Here, we transposed the recent concept of the multidimensional trait space to the analysis of intraspecific plant response to competition. We specifically tested the following: (a) in the absence of competitors, the plant multidimensional trait space will be packed towards strategies promoting plant colonisation; and (b) with competitors, the plant multidimensional trait space will be directed towards competition with its size and shaping characteristics dependent on competitor species richness. We studied trait intraspecific variability of two clonal species, Brachypodium pinnatum (L.) P. Beauv. and Elytrigia repens (L.) Gould, in response to competition. We analysed plant response in the absence of competitors and in competition. Competition treatments included intraspecific and interspecific experimental mixtures with increasing species richness. For each target species and each treatment, we built an hypervolume based on six traits involved in the three‐dimensional competition (i.e., ramet and connection traits). We measured these hypervolumes for their size, similarity and the contribution of traits in their shaping. In the absence of competitors and for both species, we demonstrated a multidimensional trait space packing towards a colonisation strategy. Under competition, the multidimensional trait spaces of the two target species were the widest at the extremes of the richness gradient, that is, intraspecific and interspecific high richness competition treatments. High intraspecific variability either promoted niche differentiation from individuals of similar species or reflected the large range of competitive responses deployed when plants were faced with many different competitor identities. The multidimensional response process was based on fine adjustments of various traits depending on the surrounding neighbourhood composition and more specifically, on the competitor functional similarity with the target species. This study emphasises the multidimensionality of species competitive response, and also underlines the so far neglected importance of competitor species richness for trait intraspecific variability and subsequently community assembly. A plain language summary is available for this article.

Competition, tree age and size drive the productivity of mixed forests of pedunculate oak, beech and red oak

In order to understand the relationship between tree diversity and forest productivity, it is crucial to understand what is going on at the level of individual trees, since each tree responds to the prevailing environmental conditions and interacts with his neighbours. We explored tree interactions in 53 forest plots in northern Belgium (TREEWEB platform) containing all possible combinations of pedunculate oak, beech and red oak. We cored 307 trees and determined the average annual basal area increment for the last 10 years for each sampled tree. We investigated the importance of neighbourhood competition, tree age an size for tree-level growth and then scaled up the species-specific tree growth to stand-level productivity. By exploring how competition, tree age and size varied along the studied gradient of tree species richness, we were able to explain the observed species-specific tree level and stand-level growth along the tree species richness gradient. We found no relationship between tree species richness and forest productivity, due to contrasting growth patterns of the three species along the richness gradient: the growth of pedunculate oak decreased, growth of beech was constant and growth of red oak increased along the tree species richness gradient. Increased heterospecific competition was the main reason for the observed lower growth of pedunculate oak in mixtures, whereas variation in tree age and size along the studied tree species richness gradient were the main drivers for the observed growth patterns of beech and red oak. Species-specific age differences along the tree species richness gradient could be linked with the species composition. We observed a multi-aged stand structure with the light-demanding pedunculate oak being older than the more shade-tolerant beech and red oak, probably a consequence of management interventions needed to enable long-term coexistence of light-demanding and shade-tolerant species. Our study highlights that the management history of mature forests may influence diversity-productivity relationships and should therefore be taken into account during the analysis.

Seasonal variability of copepod community structure and abundance modified by the El Niño-La Niña transition (2010) in the tropical Pacific off central Mexico

Seasonal variability of the community structure of copepods in the Eastern tropical Pacific off central Mexico was studied during three distinct hydrodynamic periods in 2010 using statistical and multivariate analyses. The survey period included the second half of the 2009-2010 El Niño (January), the neutral transition period (May-June), and the first half of the 2010-2011 La Niña (October). Seventy-eight copepod species were identified; richness ranged from 11 to 47 species per station, with seasonal averages from 25 species in May to 35 species in January. Cluster analysis indicated that there were four principal groups present across the surveyed periods, defined by January (El Niño), October (La Niña), May offshore stations, and May upwelling stations (cyclonic eddy and coastal stations). There were no significant differences in abundance, but the January (El Niño) cluster was most diverse with 32 species, May offshore and October (La Niña) clusters each had 25 species, and the May upwelling was the least diverse cluster with 18 species. Mesoscale processes were strongest during May, which was the only period with a significant inshore-offshore gradient of species richness and diversity. Canonical correspondence analysis (CCA) revealed that variability was primarily driven by subsurface (75-200 m) ammonium, and surface (0-50 m) temperature, nitrates+nitrites, salinity and phosphorus. Copepodites and adults of the primarily herbivorous Eucalanidae dominated the stations of the upwelling cluster, while copepodites and adults of the carnivorous Euchaetidae dominated the January (El Niño) station cluster. The higher Chl a levels during the less productive (reduced upwelling) El Niño period were probably due to reduced grazing activities and increased ammonium availability through increased zooplankton metabolism. The horizontal distribution of copepods in the Eastern Tropical Pacific off Mexico appears to be principally defined by mesoscale eddy processes (offshore) and upwelling (coastal). These mesoscale processes were affected by El Niño - La Niña transitions, which subsequently disrupted the inshore-offshore gradient and in the case of El Niño likely caused reductions in copepod abundance across the entire region which persisted for the entire study period, and possibly longer.

To what extents are species richness and abundance of reef fishes along a tropical coast related to latitude and other factors?

This study has employed an integrated statistical approach to determine the extents to which species richness and abundance (catch rate) of fishes over reefs along an extensive coastline were related to various factors. Fish were thus sampled by trapping over deep (~ 22 m) and shallow (~ 12 m) reefs along the ~ 1500 km tropical coast of north-western Australia (NWA). Fish were caught during day and night in both dry and wet seasons at two well-spaced locations in each of the Kimberley (13–16°S), Canning (16–19°S) and Pilbara (20–22°S) bioregions. Species richness and abundance were both typically less at the two locations at the lowest latitude than at the two at the highest latitudes. This trend, which contrasts with the paradigm regarding latitudinal trends for these biotic variables, is attributable to the far more extreme hydrological conditions in the Kimberley than Pilbara. Indeed, both biotic variables peaked in the Canning, presumably reflecting, inter alia, the better development of reef and suspension feeder habitats in that bioregion. The peak in abundance was attributable to particularly high numbers of the NWA endemic Lethrinus punctulatus. A greater species richness and abundance of fishes in deeper than shallow water during the wet season, and particularly in areas of greater cyclonic activity, reflect the movement of species such as Lethrinus laticaudis and Lutjanus carponotatus from shallow to calmer, deeper waters. These two biotic variables were greater during the day than night, with a greater number and abundance of diurnally-active species, e.g. lethrinids, feeding overall in the traps than is the case with nocturnal species, e.g. lutjanids. The results imply that the typical poleward latitudinal gradients in species richness and abundance are over-ridden along the NWA coast by environmental differences among bioregions.