Jonathan Belmaker

Contrasting changes in the abundance and diversity of North American bird assemblages from 1971 to 2010

Although it is generally recognized that global biodiversity is declining, few studies have examined long-term changes in multiple biodiversity dimensions simultaneously. In this study, we quantified and compared temporal changes in the abundance, taxonomic diversity, functional diversity, and phylogenetic diversity of bird assemblages, using roadside monitoring data of the North American Breeding Bird Survey from 1971 to 2010. We calculated 12 abundance and diversity metrics based on 5-year average abundances of 519 species for each of 768 monitoring routes. We did this for all bird species together as well as for four subgroups based on breeding habitat affinity (grassland, woodland, wetland, and shrubland breeders). The majority of the biodiversity metrics increased or remained constant over the study period, whereas the overall abundance of birds showed a pronounced decrease, primarily driven by declines of the most abundant species. These results highlight how stable or even increasing metrics of taxonomic, functional, or phylogenetic diversity may occur in parallel with substantial losses of individuals. We further found that patterns of change differed among the species subgroups, with both abundance and diversity increasing for woodland birds and decreasing for grassland breeders. The contrasting changes between abundance and diversity and among the breeding habitat groups underscore the relevance of a multifaceted approach to measuring biodiversity change. Our findings further stress the importance of monitoring the overall abundance of individuals in addition to metrics of taxonomic, functional, or phylogenetic diversity, thus confirming the importance of population abundance as an essential biodiversity variable.

Large but uneven reduction in fish size across species in relation to changing sea temperatures

Abstract Ectotherms often attain smaller body sizes when they develop at higher temperatures. This phenomenon, known as the temperature–size rule, has important consequences for global fisheries, whereby ocean warming is predicted to result in smaller fish and reduced biomass. However, the generality of this phenomenon and the mechanisms that drive it in natural populations remain unresolved. In this study, we document the maximal size of 74 fish species along a steep temperature gradient in the Mediterranean Sea and find strong support for the temperature–size rule. Importantly, we additionally find that size reduction in active fish species is dramatically larger than for more sedentary species. As the temperature dependence of oxygen consumption depends on activity levels, these findings are consistent with the hypothesis that oxygen is a limiting factor shaping the temperature–size rule in fishes. These results suggest that ocean warming will result in a sharp, but uneven, reduction in fish size that will cause major shifts in size‐dependent interactions. Moreover, warming will have major implications for fisheries as the main species targeted for harvesting will show the most substantial declines in biomass.

Non-stationarity in the co-occurrence patterns of species across environmental gradients

Summary nQuantifying the role of biotic interactions in driving community assembly often relies on analyzing species co-occurrence patterns, where segregated patterns are taken to indicate antagonistic interactions such as competition. It is unknown, however, if co-occurrence patterns are stationary across environmental gradients, as it is possible that the strength of biotic interactions that drive these patterns also depends on the environment. In this study, we aim to understand how patterns of co-occurrence change when species move from their environmental range center towards their range periphery while isolating the potential signal of biotic interactions from confounding factors. nWe used two separate statistical approaches (null models and joint species distribution models) to quantify pairwise co-occurrence patterns for tree species sampled in 9,382 plots distributed across the conterminous US. We also analyzed co-occurrence patterns that emerged from a simple meta-community model. We then assessed how patterns of species segregation and aggregation change in relation to habitat suitability while accounting for multiple factors known to confound co-occurrence analyses. nWe found strong non-stationarity in co-occurrences, with patterns shifting from segregated at the environmental range center towards aggregated at range margins for the majority of tree species. Patterns were in full agreement between model simulations and both empirical analyses. Model simulations suggest that this pattern is at least partly driven by variation in the relative abundances of non-focal species even when no direct biological interactions are present. nSynthesis: patterns of tree species co-occurrence vary across environmental gradients, with increased segregation when environmental conditions are optimal and increased aggregation when the environment is less suitable. This pattern may originate from a tradeoff between the abundance of the focal species pair, which decreases towards the environmental range margin, and the increasing abundance of non-focal species to which the environment is more suitable. The strong dependence of co-occurrence patterns on environmental conditions might limit the predictive ability of joint species distribution models, which couple species co-occurrences and their environmental responses, because co-occurrence patterns and environmental responses are confounded. n n nThis article is protected by copyright. All rights reserved.

Global reef fish richness gradients emerge from divergent and scale-dependent component changes

Biodiversity varies from place to place due to environmental and historical factors. To improve our understanding of how history and the environment influence observed patterns, we need to address the limitations of the most commonly used biodiversity metric, species richness. Here, we show that scale-dependent dissections of species richness into components of total abundance, species relative abundances and spatial aggregations of species reveal that two well-known biogeographic reef fish species richness gradients emerge from very different underlying component patterns. Latitudinal richness is underpinned by scale-independent patterns of total and relative abundances, suggesting ecological constraints scale up to determine abundances within communities. In contrast, the longitudinal gradient of species richness typically attributed to historical biogeography only emerges at the largest scale and is accompanied by a similar pattern of relative abundances, suggesting that site-to-site compositional variation leading to species aggregation (i.e. a component of β-diversity) underlies this gradient. Examining relationships among the components that underpin biodiversity gradients reveals new patterns that can better identify processes influencing patterns of biodiversity.

Comparison of wormlions and their immediate habitat under man-made and natural shelters: suggesting factors making wormlions successful in cities

Wormlions are fly larvae that construct pit-traps in loose soil and ambush prey that fall into their pits. They occur in high numbers in cities, below any man-made shelter providing protection from direct sunlight, such as a concrete roof with a thin layer of sand at the ground. Their natural habitat is either caves or any natural structure that provides full shade. We characterized a large urban habitat and compared it to two natural habitats, where wormlions occur in caves. Wormlions were abundant in all studied habitats. Our goals were to understand whether wormlions in the urban habitat perform better than in the natural habitats, and to suggest differences between the habitats that may contribute to their success under man-made shelters. Wormlions in the city reached larger size before pupation, and wormlion clusters there were larger. The studied urban habitat contained more concrete and perennial plants, while the natural habitats comprised of more annuals. We suggest that this concrete, covered with a thin layer of sand, leads to large areas suitable for wormlions. Furthermore, ants were more common in the urban habitat than the natural habitats, referring to their relative proportion of all arthropods collected. We suggest that these small ants provide suitable prey for wormlions, especially in the early stages of their development, when wormlions are limited by prey size. This could explain why they reach larger size prior to pupation. Pits were probably larger because they were constructed by larger individuals. In conclusion, we suggest that wormlions present an interesting case of an insect pre-adapted to urban life.

Among‐Species Overlap in Rodent Body Size Distributions Predicts Species Richness Along a Temperature Gradient

1718 ––––––––––––––––––––––––––––––––––––––––

Tropical bird species have less variable body sizes

Ecologists have often predicted that species niche breadths should decline towards the Equator. Dan Janzen arrived at this prediction based on climatic constraints, while Robert MacArthur argued that a latitudinal gradient in resource specialization drives the pattern. This idea has some support when it comes to thermal niches, but has rarely been explored for other niche dimensions. Body size is linked to niche dimensions related to diet, competition and environmental tolerance in vertebrates. We identified 68 pairs of tropical and nontropical sister bird species using a comprehensive phylogeny and used the VertNet specimen database to ask whether tropical birds have lower intraspecific body-size variation than their nontropical sister species. Our results show that tropical species have less intraspecific variability in body mass (; p = 0.009). Variation in body-size variability was poorly explained by both abiotic and biotic drivers; thus the mechanisms underlying the pattern are still unclear. The lower variation in body size of tropical bird species may have evolved in response to more stable climates and resource environments.

Habitat niche breadth predicts invasiveness in solitary ascidians

Abstract A major focus of invasion biology is understanding the traits associated with introduction success. Most studies assess these traits in the invaded region, while only few compare nonindigenous species to the pool of potential invaders in their native region. We focused on the niche breadth hypothesis, commonly evoked but seldom tested, which states that generalist species are more likely to become introduced as they are capable of thriving under a wide set of conditions. Based on the massive introduction of tropical species into the Mediterranean via the Suez Canal (Lessepsian migration), we defined ascidians in the Red Sea as the pool of potential invaders. We constructed unique settlement plates, each representing six different niches, to assess ascidian niche breadth, and deployed them in similar habitats in the native and invaded regions. For each species found on plates, we evaluated its abundance, relative abundance across successional stages, and niche breadth, and then compared (1) species in the Red Sea known to have been introduced into the Mediterranean (Lessepsian species) and those not known from the Mediterranean (non‐Lessepsian); and (2) nonindigenous and indigenous species in the Mediterranean. Lessepsian species identified on plates in the Red Sea demonstrated wider niche breadth than non‐Lessepsian species, supporting the niche breadth hypothesis within the native region. No differences were found between Lessepsian and non‐Lessepsian species in species abundance and successional stages. In the Mediterranean, nonindigenous species numerically dominated the settlement plates. This precluded robust comparisons of niche breadth between nonindigenous and indigenous species in the invaded region. In conclusion, using Red Sea ascidians as the pool of potential invaders, we found clear evidence supporting the niche breadth hypothesis in the native region. We suggest that such patterns may often be obscured when conducting trait‐based studies in the invaded regions alone. Our findings indicate that quantifying the niche breadth of species in their native regions will improve estimates of invasiveness potential.

Remarkable size-spectra stability in a marine system undergoing massive invasion

The Mediterranean Sea is an invasion hotspot, with non-indigenous species suspected to be a major driver behind community changes. We used size spectra, a reliable index of food web structure, to examine how the influx of Red Sea fishes into the Mediterranean Sea has impacted the indigenous species community. This is the first attempt to use changes in the size spectra to reveal the effect of biological invasions. We used data from trawl catches along Israels shoreline spanning 20 years to estimate changes in the community size spectra of both indigenous and non-indigenous species. We found that the relative biomass of non-indigenous species increased over the 20 years, especially for small and large species, leading to a convergence with the indigenous species size spectra. Hence, the biomass of indigenous and non-indigenous species has become identical for all size classes, suggesting similar energetic constraints and sensitivities to fishing. However, over this time period the size spectrum of indigenous species has remained remarkably constant. This suggests that the wide-scale invasion of non-indigenous species into the Mediterranean may have had little impact on the community structure of indigenous species.