Possible latitudinal clines in Antarctic intertidal and subtidal zone communities encrusting ephemeral hard substrata

Aim Encrusting faunal communities on rocks were examined from Southern Ocean intertidal and subtidal (6 m) zones to investigate potential change with latitude. Location The site locations were South Georgia (54°S [sub-Antarctic]), Signy Island (60.5°S [maritime Antarctic]) and Adelaide Island (68°S [high Antarctic]). Methods The number of taxa, degree of colonization and bryozoan growth and mortality were measured. Results The communities present were relatively simple in terms of composition and interactions. Spirorbid polychaetes (Annelida) and cheilostomatid bryozoans were the principal components. The community simplicity and dominance of unitary (compared to colonial) fauna increased with latitude. Mean annual mortality of subtidal cheilostomatids also significantly increased with latitude. The number of bryozoan species and potential aspects of community complexity increased with latitude. The growth of the bryozoan Inversiula nutrix increased with latitude reversing the general trend of decreased growth rate. Conclusions These findings suggest that disturbance may significantly increase with latitude within the Southern Ocean. The apparent reversal of typical latitudinal growth trends is probably a result of classical island biogeography, with species decreasing away from a centre of high diversity.     

Competition, mortality and diversity in South Atlantic coastal boulder communities

Boulder shores are common at all latitudes and dominate the intertidal and subtidal zones of sub-Antarctic coastlines. The encrusting benthos of boulders was examined on similar shore types at four locations: Tierra del Fuego, East Falkland, West Falkland and Bird Island (off South Georgia). Bird Island is unusual in experiencing high trampling and organic enrichment from fur seals. The results were compared to a Patagonian site and a non-trampled South Georgia site (Husvik) and other sites taken from the literature. Principal Component Analysis revealed South Atlantic/Southern Ocean encrusting faunas formed a distinct cluster when compared to assemblages from elsewhere at similar latitudes. Bray Curtis cluster analysis of the South Atlantic-Southern Ocean sites showed the major division was between Southern Ocean and South Atlantic Ocean sites, beyond which there were three distinct clusters centred around Patagonia (Magellanic), the Falklands and Southern Ocean sites. The organisation of competitive interactions between species was mostly determinate and transitive (essentially hierarchical). The transitivity index scores were higher than most similar assemblages studied to date. The diversity of encrusting assemblages ranged from Shannon Weaver H values of 2.38–0.77 (East Falkland and Bird Island, respectively) in the intertidal to 1.27–0.73 (Patagonia and South Georgia, respectively) in the subtidal zone. Annual mortality (of bryozoan colonies) varied from 85–97% in the intertidal to 65–92% in the subtidal, being higher in the Southern Ocean than South Atlantic sites, largely due to ice scour and wave action. The Bird Island mortality values may be high even for a Southern Ocean site. Accepted: 5 October 2000     

Clade perseverance from Mesozoic to present: a multidisciplinary approach to interpretation of pattern and process

Two clades of marine bryozoans, cyclostomes and cheilostomes, exemplify the benefits of applying a multidisciplinary approach to the interpretation of long-term evolutionary patterns. The cyclostome bryozoans were dominant in the Mesozoic; since that era, they have decreased in absolute terms and the cheilostomes have come to exceed them in both abundance and diversity. Many studies of living assemblages of the encrusting members of these two clades indicate that cheilostomes are superior space competitors, but paleontological studies suggest that competition between the two taxa has not been escalating over geological time. Both clades occur throughout the world's oceans and seas, and recent work in the geographical extremes has shown that the relative success of the clades varies markedly from place to place. In this study, the importance of differential patterns of recruitment and cumulative space occupation in the two clades was evaluated over four years and in two environments, one temperate and one polar. In both of these environments, peaks of recruitment and space occupation by the two clades were out of phase. The different strategies and outcomes of spatial competition are examined, largely using data from the literature. Only recently has it been realized that tied outcomes of competition are stable alternative results and not simply transitory phases. Many competitive encounters involving cyclostomes result in ties, implying that their strategy is based on persistence rather than dominance. When different indices and models are used to analyze competition data from the two clades, the interpretation varies markedly with methodology. The differences in patterns of recruitment, space occupation, and spatial competition have influenced both our understanding of how the two clades have persisted alongside each other and our perception of cheilostome superiority. Analysis of fluid dynamics has shown that small differences in the mechanical structure of typical members of each clade lead to fundamental differences in water movement. For animals that rely on water motion for transport of nutritional and excretory elements (suspension feeders), small changes in current velocity and direction can have a major impact. Preliminary chemical analysis of the excurrent stream leaving cheilostome colonies has shown it to be laden with excretory products, which can interfere and mix with a neighbor's feeding currents. Clearly, spatial competition involves more than a simple mechanical "showdown."     

Polarization of competition increases with latitude

Many organisms overlap in their use of resources in space and time. Where and when resources are restricted, species must compete for them. Living space, often a critical resource controlling food and mate availability, is directly contested by organisms in most habitats. The ensuing animal interactions generally result in a winner gaining space and a loser, which may die. Contact matrices from studies of interference competition in encrusting marine Bryozoa (clonal and colonial animals), spanning at least 60 degrees latitude in both hemispheres, were analysed and subjected to a modern transitivity index. Only data for Bryozoa were used because (i) use of a single taxon with restricted ecology simplifies the scope for types of encounters, (and therefore) interpretation; and (ii) ecological bias is reduced because bryozoans are abundant at all latitudes. The analysis shows that assemblage competition is more hierarchical towards both poles. Thus, poorer competitors fail more frequently in interactions with increasing latitude. The cause of this trend is the simplification of overall outcomes between competitors, such as fewer ties, reversals in outcome or competitive loops (where low-ranking competitors beat those of higher ranking). The implication of such a trend is that the maintenance of biological diversity at high latitudes may principally be by physical rather than biological (competition) processes. Certainly, ocean surface energy increases with latitude through wind and wave action (and ice scour in polar regions).     

Sneaking up on ‘enemies’: alleviating inherent disadvantages in competitive outcomes in a nearly 3-million-year-old palaeocommunity from Florida,USA

Bryozoans offer one of the few systems in which competitive interactions for living space can be studied in the fossil record. Here, we describe the outcome of competitive overgrowths in a 3-million-year-old bryozoan palaeocommunity encrusting shells of the bivalve Anomia simplex from the lower Tamiami Formation in Florida (upper Pliocene, Piacenzian). We found that win–lose overgrowths are more common than stand-offs in interspecific encounters, while stand-offs are more common than win–lose overgrowths in intraspecific encounters. We observed more intraspecific encounters and fewer interspecific interactions than expected under a null hypothesis, suggesting that bryozoans of the same species are likely to be clustered. For some species, intraspecific encounters are more likely to result in the apparent fusion of the two colonies, with the development of rows of kenozooids along the contact edge, probably reflecting relatively low dispersal. We also identified some clear winners and some clear losers. A negative correlation was found between the number of colonies observed and the probability of winning for a species, resulting in a dominance of loser species in the assemblage, a pattern previously described as typical for early colonizers of hard substrates. Our results also confirm the finding of earlier studies that having large zooids and, subordinately, multilayered growth are key traits for success in overgrowth competition, with angle of encounter also having an effect for both poor and good competitors that take advantage of ‘attacking’ colonies of other species from the rear and the flank.     

Biodiversity and biogeographic affiliation of Bryozoa from King George Island (Antarctica)

King George Island (KGI), which is located between the Antarctic and South American continents, may play a crucial role in the exchange of Bryozoa amongst the various Antarctic sectors and across the Polar Front. Knowledge regarding the biological diversity of this area could help us understand the evolution of the Antarctic ecosystem and its connectivity to the South American continent as well as the colonization ability of particular species. Here, we investigate the patterns of diversity and biogeographic affiliation of the cheilostome Bryozoa from KGI and the surrounding areas. Of 114 identified taxa from a depth range of 6–492 m, 26 species were reported for the first time in KGI. The most speciose genera were Camptoplites, Osthimosia, Smittina, and Cellarinella. Species richness at KGI consisted of 70% of the total bryozoans at the South Shetland Islands (SSI). Fifty-nine per cent of the bryozoans from KGI are endemic to Antarctica, which closely reflects the previously estimated endemism rate for bryozoans and other Antarctic taxa. Cluster analysis indicated that the strongest faunal links of SSI bryozoans were with Antarctic Peninsula assemblages, corresponding to the physical distance between both locations. The biogeographic similarities between SSI and South America confirm the broad trend of existing Antarctic–South American faunal links previously observed in bryozoans and many other taxa and indicate that SSI might be an important transitional zone between Antarctica and South America.     

Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell’s neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.     

Diversity,recruitment and competition on island shores at south-polar localities compared with lower latitudes: encrusting community examples

Comparisons of temperate and tropical shores have yielded considerable debate as to whether the former really are less benign, diverse and structured by different ecological processes. Studies of comparable boulder communities have shown high within region variability. Equivalent polar assemblages, from island shores compared here, show much reduced within region variability and considerably reduced numbers of phyla and species encrusting boulders. The rate of colonisation (compared from settlement panel studies) was an order of magnitude higher in warmer water, but did vary with isolation (near vs offshore islands). Comparison of the most ubiquitous taxon, the bryozoans, between polar and non polar sites shows a decrease in the proportion of inter-specific competition, indeterminate competitor (species) pairs and incidence of tied outcomes in competition. These three parameters all increased with depth at the localities studied, whilst no obvious influence of isolation was found.     

The degree of intransitivity and its measurement in an assemblage of encrusting cheilostome bryozoa

The outcome of spatial encounters between 12 species of encrusting bryozoans is largely dependent on the angle formed between the direction of growth of the two species. An index has been devised which describes the degree to which the pattern of interspecific overgrowth is an hierarchical, perfectly transitive one, where each species invariably overgrows those beneath it in the hierarchy or an intransitive one, where all of the species have an equal probability of being overgrown; these two states therefore form quantifiable and opposite ends of a continuum. Frontal encounters alone produced a value closer to the transitive end than did encounters from all directions. Since the latter are more likely in nature, the underlying pattern of overgrowth in assemblages of encrusting bryozoans will be a highly intransitive one.     

Ophiuroid community structure of the South Shetland Islands and Antarctic Peninsula region

During the BENTART 95 survey, epifauna was sampled with an Agassiz trawl at 24 stations ranging from north of Livingston Island (South Shetland Islands) to the Antarctic Peninsula at depths from 40 to 850 m. Four assemblages of ophiuroids were identified and correspond to Deception Island, Trinity Island, and the southern and northern zones of Livingston Island. In addition to these regions, gradients related to depth and filter-feeder biomass were found. The brittle-star assemblages were dominated by Ophionotus victoriae Bell, 1902, which to a large extent determine the biogeographic structure in the South Shetland Islands.     

Extensive genetic differentiation in Gobiomorphus breviceps from New Zealand

Partial mitochondrial DNA sequences for parts of the cytochrome b gene and control region were obtained for 89 upland bullies Gobiomorphus breviceps from 19 catchments in New Zealand. There were two highly distinctive mtDNA clades: a northern clade corresponding to the North Island, northern South Island and west coast South Island, and a south‐east clade, in the southern and eastern South Island. Within these major clades there were further distinct clades that correlated with geographic sub‐regions and catchments. The marked genetic differentiation has occurred in the absence of obvious morphological divergence. Based on cytochrome b sequence divergences and the molecular clock hypothesis, the northern and southeastern clades correspond with the uplift of the Southern Alps during the Pliocene, while populations in the North Island and northern South Island were estimated to have diverged during the Pleistocene. The widescale geographic divergences were similar to those observed in the galaxiids, Galaxias vulgaris and Galaxias divergens , but biogeographic management boundaries may not be the same, reflecting different evolutionary histories for non‐diadromous species occupying the same areas.     

Spatial competition among clonal organisms in extant and selected paleozoic reef communities

Summary Occurrences of densely packed benthic organisms in extant reefs are of two types: 1) live-live interactions, where two living organisms interact, and 2) live-dead associations, where only one is alive and uses the other as a substrate. The latter are common in reef deposits due to biostratinomic feedback, i.e. dense skeletal accumulations provide hard substrates for clonal recruitment, thus facilitating greater frequency of live-dead encounters than in lower biomass level-bottom communities dominated by solitary organisms. Differentiating between these two types in ancient reefs is difficult, often impossible. Most live-live interactions among clones in extant reef communities involve competition for space. Clonal spatial competition is divisible into four types: 1) direct-aggressive: encrusting overgrowth; 2) indirect-passive: depriving neighbors of resources, chiefly sunlight, by growth above them; 3) stand-off: avoidance of competition by organisms adopting positions that avoid or minimize direct polyp/zooid contact; and 4) overwhelming: one clone/ species volumetrically or numerically overwhelms the other, meeting minimal resistance. Despite class-order level differences in taxa, our results indicate that extant analogs, based on the arrangement and distortion of skeletons, are valuable for recognizing live-live interactions in Silurian and Carboniferous reefs and interpreting the types of spatial competition represented. Comparison of overhead (plan) views of live-live coral competition in Polynesian reefs with vertical sections of Silurian and Carboniferous sponge-dominated reefs and biostromes suggests that direct-aggressive competition is more common among extant than among Paleozoic reef-builders. Stand-offs showing clone margin distortion and overwhelming with minor skeletal distortion are most common in our fossil examples and probably relate to the dominance of these reefs by sponges. Success by extant sponges in spatial competition is largely due to allelochemical deterrence which may explain the predominance of stand-off and overwhelming confrontations in fossil sponges rather than tentacle-mesentery based direct aggression among extant corals and bryozoans.     

Occupation of patches in the epifaunal communities on pier pilings and the bivalve Pinna bicolor at Edithburgh,South Australia

Summary The reoccupation of artificially cleared patches in a subtidal epifaunal community was investigated in two field experiments on the pilings of Edithburgh pier, South Austrlia. In most cases, the greatest proportion of the patch was reoccupied by the vegetative extension of established sponge and tunicate colonies adjacent to it. Larval recruitment by sponges, bryozoans, tunicates and serpulids contributed to the reoccupation but resulted in only a small proportion of the mean percentage cover. The relative abundances of individual species established in any patch were shown to be a function of the (1) position in space, (2) age, (3) time of creation, (4) initial size of the patch.There was a large amount of between-patch variation in all cases. Overgrowth interactions occurred frequently within patches, and for many pairs of species, neither species consistently overgrew the other. Overgrowth interactions were tested statistically, and a large number of pairs of species were found to be competitively equivalent. This represents a possible situation additional to the alternatives recognized in the literature, namely competitive hierarchies or networks. Interactions between species should be regarded as stochastic, with a wide range of possible outcomes. The situation at Edithburgh is likely to produce greater between-patch variability than either a network or a hierarchy.Despite this large variation, super-specific taxa differ fairly consistently in capacity for overgrowth. Tunicates overgrow sponges, which overgrow bryozoans, which overgrow serpulids. The occupation of most patches was directional in the sense that bryozoans and serpulids invaded first, but tunicates and sponges excluded them and came to dominate the patch. These realtionships are used to predict patterns of abundance for substrata which are small and isolated, and these predictions are compared with the epifauna of the bivalve Pinna bicolor, which provides such substrata adjacent to the pier.     

Effects of two marinas on the composition of fouling assemblages

The composition of fouling assemblages was surveyed inside and near two fully enclosed marinas using settlement plates. The location of a plate, inside or outside the marina, influenced the abundances of four functional groups of fouling organisms (solitary ascidians, arborescent bryozoans, encrusting bryozoans and colonial ascidians). Transplantation of mature assemblages revealed that reductions in the abundance of arborescent bryozoans inside marinas might be explained by increased growth and recruitment of these bryozoans outside the marina. Surveys of settlement revealed decreased recruitment of encrusting bryozoans inside the marinas, a result consistent with patterns of adult abundance. It is proposed that an increased abundance of solitary ascidians inside marinas may be due to decreased competition. A second survey of adult assemblages was performed with multiple ‘Outside’ sites per marina. Effects of location consistent with the first survey were found for arborescent bryozoans, and in one marina area for solitary ascidians and encrusting bryozoans, but not in the other. Although mechanisms can be proposed to explain the effects of the marina (inside or outside) on the abundances of solitary ascidians, arborescent bryozoans and encrusting bryozoans, the second survey revealed that the effects may vary among marinas.     

A latest Cretaceous to earliest Paleogene dinoflagellate cyst zonation from Antarctica,and implications for phytoprovincialism in the high southern latitudes

The thickest uppermost Cretaceous to lowermost Paleogene (Maastrichtian to Danian) sedimentary succession in the world is exposed on southern Seymour Island (65° South) in the James Ross Basin, Antarctic Peninsula. This fossiliferous shallow marine sequence, which spans the Cretaceous–Paleogene boundary, has allowed a high-resolution analysis of well-preserved marine palynomorphs. Previous correlation of Cretaceous–Paleogene marine palynomorph assemblages in the south polar region relied on dinoflagellate cyst biozonations from New Zealand and southern Australia. The age model of the southern Seymour Island succession is refined and placed within the stratigraphical context of the mid to high southern palaeolatitudes. Quantitative palynological analysis of a new 1102 m continuous stratigraphical section comprising the uppermost Snow Hill Island Formation and the López de Bertodano Formation (Marambio Group) across southern Seymour Island was undertaken. We propose the first formal late Maastrichtian to early Danian dinoflagellate cyst zonation scheme for the Antarctic based on this exceptional succession. Two new late Maastrichtian zones, including three subzones, and one new early Danian zone are defined. The oldest beds correlate well with the late Maastrichtian of New Zealand. In a wider context, a new South Polar Province based on Maastrichtian to Danian dinoflagellate cysts is proposed, which excludes most southern South American marine palynofloras. This interpretation is supported by models of ocean currents around Antarctica and implies an unrestricted oceanic connection across Antarctica between southern South America and the Tasman Sea.     

Environment and life cycles influence distribution patterns of hydromedusae in austral South America

We analysed hydromedusa assemblages of South America (from 22°S to 56°S and from 040°W to 080°W), their association with water masses and the influence of the life cycle on medusa distributions. The geographic distribution of 130 species of hydromedusae was compiled from literature reports (62 publications between 1913 and 2012). Seven areas were defined: Atlantic Magellanic, Argentinean, Pacific intermediate zone, Pacific Magellanic, Peruvian–Chilean, South Brazilian and Oceanic. The variance of the species–environment relationship was explained by depth and temperature. Distribution patterns of Atlantic hydromedusae are associated with neritic water masses, supporting previously proposed biogeographical provinces. Assemblages on the Pacific side of South America are under the influence of the Humboldt Current system, with a break in species distribution around Chiloé Island. Only the oceanic assemblage contained the same species in both the Pacific and Atlantic zones. We found that meroplanktonic medusae contributed more to define the neritic assemblages, while the oceanic assemblage was better defined by holoplanktonic medusae. Therefore, our data suggest that meroplanktonic hydromedusae appeared to be more restricted in distribution than holoplanktonic ones.     

Spatial competition induces the mobilization of morula cells in the colonial ascidian Didemnum perlucidum (Tunicata: Didemnidae)

Abstract. The effects of spatial competition among colonial marine organisms are often evident in the contact zones between colonies. These effects are especially pronounced when the interaction results in overgrowth or necrosis of one of the competitors. Ascidians, one of the dominant taxonomic groups in subtidal sessile communities, have specialized morula cells that provide a defense against microbial infections. Injuries resulting from interspecific competitive interactions might also act as a stimulus for this defensive mechanism. Therefore, we expected to see the recruitment of morula cells in tissues near competitor contact zones. To test the hypothesis that spatial competition elicits this immune response, we placed colonies of the ascidian Didemnum perlucidum from southeastern Brazil in four different types of competitive situations: (1) overgrowth of the competitor, (2) stand‐off interactions, (3) overgrowth by the competitor, and (4) free of competitors. Our results indicate that competitive interactions increase the population of morula cells in contact zones, as more cells were observed in interactions that resulted in the overgrowth of individuals of D. perlucidum, and fewer cells were observed in colonies that were free of competitors. We identified the defensive function of the morula cells by showing the presence of the enzyme phenoloxidase within its vacuoles. Phenoloxidase is a widespread enzyme among animals and plants, and is frequently used in defense by synthesizing toxic quinones from polyphenol substrates. This is the first study to document the presence of morula cells in didemnid ascidians and the mobilization of these cells by spatial competition by heterospecifics, and one of the first studies to identify phenoloxidase activity in morula cells.     

Early Ordovician bryozoans from north-western Russia

ABSTRACT. A bryozoan assemblage from the Billingen Stage of north-western Russia (Ingria) is apparently the oldest known in the world. It consists of six species distributed among the trepostomate genera Esthoniopora Dianulites, Revalotrypa Phragmophora and Hemiphragma . Two species are new: Phragmophora lavaense Pushkin, sp. nov. and Hemiphragma priscum Pushkin, sp. nov. The bryozoans are characteristic of the medium diversity benthic fauna of uncertain origin which migrated into the Baltic Basin during Billingen time (late Prioniodus elegans — early Oepikodus evae zones). The Billingen bryozoans have a close similarity to the late Arenig bryozoan faunas of Baltoscandia that are also dominated by trepostomates, but differ significantly from the low diversity late Arenig bryozoan assemblages of North America, Ireland, North China, Vajgach and Novaja Zemlja.     

Molecular Systematics of Threatened Seed Plant Species Endemic in the Caribbean Islands

A review of available Caribbean Island red-lists species (CR and EN categories based on the IUCN guidelines from 2001, and E category established according to the IUCN guidelines from 1980) is presented. A database of over 1,300 endemic species that are either Critically Endangered or Endangered sensu IUCN was created. There are molecular systematic studies available for 112 of them. Six of these species (in six genera) are the only members of early divergent lineages that are sister to groups composed of a large number of clades. Seven of the species (in seven genera) belong to clades that have a small number of taxa but are sister to species/genus-rich clades. Ten of the species (in six genera) are sister to taxa restricted to South America or nested in clades endemic to this region. Fifty-seven of the species (in 35 genera) are sister to Caribbean Island endemic species. Erigeron belliastroides, an Endangered (EN) Cuban endemic, is sister to the Galapagos genus Darwiniothamnus. The phylogenetic placement of four of the threatened species resulted in changes in their taxonomic placement; they belong to polyphyletic or paraphyletic genera.     

Latitudinal gradient of nestedness and its potential drivers in stream detritivores

Understanding what mechanisms shape the diversity and composition of biological assemblages across broad‐scale gradients is central to ecology. Litter‐consuming detritivorous invertebrates in streams show an unusual diversity gradient, with α‐diversity increasing towards high latitudes but no trend in γ‐diversity. We hypothesized this pattern to be related to shifts in nestedness and several ecological processes shaping their assemblages (dispersal, environmental filtering and competition). We tested this hypothesis, using a global dataset, by examining latitudinal trends in nestedness and several indicators of the above processes along the latitudinal gradient. Our results suggest that strong environmental filtering and low dispersal in the tropics lead to often species‐poor local detritivore assemblages, nested in richer regional assemblages. At higher latitudes, dispersal becomes stronger, disrupting the nested assemblage structure and resulting in local assemblages that are generally more species‐rich and non‐nested subsets of the regional species pools. Our results provide evidence that mechanisms underlying assemblage composition and diversity of stream litter‐consuming detritivores shift across latitudes, and provide an explanation for their unusual pattern of increasing α‐diversity with latitude. When we repeated these analyses for whole invertebrate assemblages of leaf litter and for abundant taxa showing reverse or no diversity gradients we found no latitudinal patterns, suggesting that function‐based rather than taxon‐based analyses of assemblages may help elucidate the mechanisms behind diversity gradients.