Beta(β)-diversity, or site-to-site variation in species composition, generally decreases with increasing latitude, and the underlying processes driving this pattern have been challenging to elucidate because the signals of community assembly processes are scale-dependent. In this meta-analysis, by synthesising the results of 103 studies that were distributed globally and conducted at various spatial scales, we revealed a latitudinal gradient in the detectable assembly processes of vascular plant communities. Variations in plant community composition at low and high latitudes were mainly explained by geographic variables, suggesting that distance decay and dispersal limitations causing spatial aggregation are influential in these regions. In contrast, variation in species composition correlated most strongly with environmental variables at mid-latitudes (20-30°), reflecting the importance of environmental filtering, although this unimodal pattern was not statistically significant. Importantly, our analysis revealed the effects of different spatial scales, such that the correlation with spatial variables was stronger at smaller sampling extents, and environmental variables were more influential at larger sampling extents. We concluded that plant communities are driven by different community assembly processes in distinct biogeographical regions, suggesting that the latitudinal gradient of biodiversity is created by a combination of multiple processes that vary with environmental and species size differences.

Land‐use changes, one of the greatest threats to global biodiversity, can cause underappreciated effects on ecosystems by altering the structures of interspecific interaction networks. These effects have typically been explored by evaluating interaction networks composed of a single type of interaction. Therefore, it remains unclear whether the different types of interaction networks sharing the same species respond to the same land‐use changes in a similar manner.To compare the responses of herbivory and pollination networks to land‐use changes, we investigated both types of interaction networks in seminatural grasslands categorized into three types of agricultural land‐use (abandoned, extensively managed, and intensively managed) in a Japanese agricultural landscape. We quantified the structures of the interaction networks using several indices (connectance, evenness, diversity, generality, network specialization, and robustness) and compared them among different land‐use types. We conducted piecewise SEM to differentiate the direct and indirect effects of land‐use changes on the network structures.Although both land‐use changes (abandonment and intensification) led to reduced plant and insect species richness, the structures of herbivory and pollination networks showed different responses to the land‐use changes. There was a marked contrast in network generality; while, herbivore species were less generalized (i.e., having fewer host plant species) in fields with land‐use intensification, pollinator species were less generalized in abandoned fields.Furthermore, the mechanisms behind the changes in interaction networks were also different between pollination and herbivory networks. The change in herbivory network generality was induced by the decrease in plant species richness, whereas the change in pollination network generality was mainly induced by the effect independent of changes in species richness and composition, which possibly reflect the less number of flowers in shaded environment.The present study demonstrates that agricultural land‐use changes affect herbivory and pollination networks in contrasting ways and suggests the importance of assessing multiple types of interaction networks for biodiversity conservation in plant–insect systems. Our results also highlight the underappreciated importance of maintaining habitats with an intermediate intensity of land‐use.

Ecological communities are assembled through a series of multiple processes, including dispersal, abiotic and biotic filtering, and ecological drift. Although these assembly processes act in concert to structure local communities, their relative importance is considerably variable among study systems. While such contingency of community assembly has been widely appreciated, the empirical and theoretical evidence is scattered around in the literature, and few efforts have been made to synthesize it. In this mini‐review, we summarize the accumulated evidence of the context‐dependency of community assembly rules, to reach a rough generalization of the contingency. Specifically, we argue that spatial and temporal dimensions can serve as general axes that regulate the relative importance of assembly processes. To this end, we synthesize the current understanding of how the relative importance of multiple assembly processes changes with spatial scales and complexity, and with time in the long and short terms. This review concludes that spatial and temporal dimensions can be common currencies of community assembly rules that are shared across various systems.

Unraveling the determinants of herbivorous insect diversity has been a significant challenge in ecology. Despite the strong association between insect and plant species, previous studies conducted in natural systems have shown great variation in the strength of the correlation between their species richness. Such variation could be attributed to the proportion of generalist insect species (generality). However, both higher and lower generality may weaken the correlation because (a) generalist insect species are less dependent on the number of plant species, and (b) specialist insect species utilize only a proportion of the total plant species. To explore these contradictory effects, we studied plant and herbivorous insect communities in seminatural grasslands in Japan. Plant–insect interactions were evaluated in a unique way with a particular focus on the staying and herbivory behaviors of insects, which reflect their habitat use as well as host use. We found that lower generality of insect communities strengthened the correlation between species richness of plants and insects, but this was not the case when plant species that had no interaction with insect species were also considered. This was because lower generality increased the number of plant species that did not interact with insects. The results indicate that insect generality has contradictory effects on the plant–insect diversity relationship, which emphasizes the importance of distinguishing the effects to understand the variation in the relationship between plant and insect diversity in natural systems.

Environmental heterogeneity is one of the most influential factors that create compositional variation among local communities. Greater compositional variation is expected when an environmental gradient encompasses the most severe conditions where species sorting is more likely to operate. However, evidence for stronger species sorting at severer environment has typically been obtained for less mobile organisms and tests are scarce for those with higher dispersal ability that allows individuals to sensitively respond to environmental stress. Here, with the dynamics of fish communities in a Japanese bay revealed by environmental DNA metabarcoding analyses as a model case, we tested the hypothesis that larger environmental heterogeneity caused by severe seasonal hypoxia (lower concentration of oxygen in bottom waters in summer) leads to larger variation of species composition among communities. During summer, fish species richness was lower in the bottom layer, suggesting the severity of the hypoxic bottom water. In contrast to the prediction, we found that although the environmental parameters of bottom and surface water was clearly distinct in summer, fish species composition was more similar between the two layers. Our null model analysis suggested that the higher compositional similarity during hypoxia season was not a result of the sampling effect reflecting differences in the alpha or gamma diversity. Furthermore, a shift in the species occurrence from bottom to surface layers was observed during hypoxia season, which was consistent across species, suggesting that the severe condition in the bottom adversely affected fish species irrespective of their identity. These results suggest that larger environmental heterogeneity does not necessarily lead to higher compositional variation once the environmental gradient encompasses extremely severe conditions. This is most likely because individual organisms actively avoided the severity quasi‐neutrally, which induced mass effect‐like dispersal and lead to the mixing of species composition across habitats. By showing counter evidence against the prevailing view, we provide novel insights into how species sorting by environment acts in heterogeneous and severe conditions.

Herbivorous insect communities are structured by multiple processes operating locally (e.g. bottom–up effects of plants) and regionally (e.g. dispersal limitation). Although the relative strength of these processes has been well documented, how it varies in time is less understood, especially in relation with the temporal dynamics of plant communities. If temporal turnover of local plant species composition is too rapid for insect community assembly to keep up with, bottom–up effects are expected to be weak. Here, in plant and herbivorous insect communities in Japanese grasslands, we studied how the relative importance of local (bottom–up effects of plants, structures of plant communities and top–down effects of predators) and regional (dispersal limitation) processes varies over the growing season. In addition, we tested the hypothesis that larger temporal turnover of plant species composition is related to the weaker bottom–up effects, that is, the lower explanation power of plant communities for insect communities. We found that, throughout the growing season, the insect species composition was mainly explained by local variables (plant species composition, vegetation height and predator abundance), and their explanation power was higher during later phases of the season (late summer). Furthermore, the variation not explained by plant species composition was correlated with the degree of temporal turnover of plants, suggesting that insect communities failed to track the temporal turnover of plant species. These results were pronounced when we focused on leaf sucker insects, whose host plant range is presumably more limited. We conclude that herbivorous insect communities are mainly regulated by local processes, especially bottom–up effects from plants, while stochasticity may have played a role in early phases of the season. Furthermore, we underscore the importance of considering relative time scale of community assembly and environmental shifts, especially in systems characterized by dynamic changes.

Aim: Global flux data analyses have shown a significant positive and linear relationship between site-scale photosynthetic optimum temperature (T opt-s ) and averaged temperature variables. However, as existing studies have not fully considered species composition, it remains unclear to what extent the change in T opt-s is derived from intraspecific plasticity or from a difference in species with a consistent species-specific T opt-s . We tested these two hypotheses using the satellite-derived enhanced vegetation index (EVI).Location: Subalpine and temperate forests in Japan.

Compositional variation among local communities is a result of environmental (e.g., environmental filtering) and spatial (e.g., dispersal limitation) processes. Growing evidence suggests that their relative importance varies temporally, but little is known about the short-time scale dynamics, that is, seasonality. Using marine fish communities in a Japanese bay as a model system, we tested the hypothesis that seasonal changes in the environment induce a shift in the relative importance of environmental and spatial processes. We used one-year monthly monitoring data obtained using environmental DNA and conducted a variation partitioning analysis to decompose the two processes. The relative importance of environmental and spatial processes was comparable averaged over the year but changed seasonally. During summer, when lower dissolved oxygen concentrations may adversely affect organisms, species composition was more explained by space despite larger environmental heterogeneity than in other seasons. This suggests that environmental processes weakened during the season with extremely severe environments, likely due to the random loss of individuals. We conclude that the assembly processes of communities of mobile organisms, such as fishes, can shift even within a year in response to seasonal changes in environmental severity. The results also indicate the applicability of eDNA techniques for community assembly studies.