The strength of ecological subsidies across ecosystems: a latitudinal gradient of direct and indirect impacts on food webs

Material and energy flows among ecosystems can directly and indirectly drive ecosystem functions. Yet, how populations of consumers respond to allochthonous inputs at a macroecological scale is still unclear. Using a meta‐analysis spanning several biomes, we show that the abundance of recipient populations is 36–57% larger with increased allochthonous inputs. The strength of direct effects on the recipients of these inputs as well as the indirect effects on the consumers of these recipients (i.e. ascending indirect effects) are constant across a latitudinal gradient spanning subtropical, arid, temperate, boreal and arctic ecosystems. However, indirect effect on the in situ resources of the input recipient (i.e. descending indirect effects) decreases with latitude. Our results suggest that the influence of allochthonous inputs can vary across large‐scale gradients of ecosystem productivity and may be driven by the types of trophic interactions within recipient food webs.     

Southern hospitality: a latitudinal gradient in gene flow in the marine environment

In recent years population genetics and phylogeographic studies have become increasingly valuable tools for inferring both historical and present-day genetic patterns within marine species. Here, we take a comparative approach to population-level study, analyzing original mitochondrial DNA data from 969 individuals representing 28 chiton (Mollusca: Polyplacophora) species to uncover large-scale genetic patterns along the Pacific coast of North America. The data reveal a distinct latitudinal connectivity gradient among chitons: species that exist at lower latitudes tend to have more isolated populations. This trend appears to be a product of between-species differences; within species, no significant gradient in connectivity is observed. Lower average annual sea surface temperatures are hypothesized to contribute to longer larval duration (and by extension, greater connectivity) among lecithotrophic species, providing a mechanism for the observed positive correlation between gene flow and latitude. Because increased isolation among populations may lead to speciation, a latitudinal trend in gene flow may contribute to the increased species diversity observed at lower latitudes.     

Rapoport's bathymetric rule and the latitudinal species diversity gradient for Northeast Pacific fishes and Northwest Atlantic gastropods: evidence against a causal link

Aim Few studies have explicitly considered the recurrent pattern of declining species diversity and increasing geographical range size that exists for numerous taxa across a variety of physical gradients. We extend Stevens’ [ Stevens, G.C. (1996) Journal of Biogeography, 23 , 149] work on Rapoport's bathymetric rule, using a more complete latitudinal assemblage of Northeast Pacific fishes and new data from Northwest Atlantic gastropods, to show that bathymetrical range size and species diversity are not causally linked. Location Fishes from the Northeast Pacific (0°–60° N) and gastropods from the Northwest Atlantic (0°–74° N) distributed from the surface to depths greater than 200 m. Methods Species pools were divided into three bathymetrical subgroups: (1) species restricted to shallow waters, between the surface and 200 m, (2) species that occurred in waters, both shallow and deep of 200 m, and (3) species restricted to waters deeper than 200 m. Median bathymetrical range size and total number of species were plotted against latitude (2° bins) using Stevens’ method, for the entire species pool and individual bathymetrical groups. Results For both fishes and gastropods, the apparent link between extratropical diversity and bathymetrical range size is an artefact resulting from the disproportionately high number of shallow restricted species in tropical latitudes, and the loss of these species in temperate latitudes. Furthermore, the extratropical gradient in gastropod diversity and bathymetrical range size are decoupled by approximately 15°, and while the latitudinal pattern for diversity is consistent across bathymetrical groups, median bathymetrical range size is highly irregular. Main conclusions These results suggests that functional groups can contribute disproportionately to patterns apparent at larger scales and that analysis of ecographical patterns by subregion is a novel approach that can help resolve debates over causality when patterns are seemingly coincident.     

Revisiting Jablonski (1993): cladogenesis and range expansion explain latitudinal variation in taxonomic richness

The increase in diversity towards the equator arises from latitudinal variation in rates of cladogenesis, extinction, immigration and/or emigration of taxa. We tested the relative contribution of all four processes to the latitudinal gradient in 26 marine invertebrate orders with extensive fossil records, examined previously by David Jablonski. Coupling Jablonski's estimates of latitudinal variation in cladogenesis with new data on patterns of extinction and current distributions, we show that the present-day gradient in diversity is caused by higher rates of cladogenesis and subsequent range expansion (immigration) at lower latitudes. In contrast, extinction and emigration were not important in the creation of the latitudinal gradient in ordinal richness. This work represents one of the first simultaneous tests of the role of all four processes in the creation of the latitudinal gradient in taxonomic richness, and suggests that low tropical extinction rates are not essential to the creation of latitudinal diversity gradients.     

沿海地区水杉叶片性状的纬度变化机制

在中国东部沿海地区沿纬度梯度选取8个水杉林,研究水杉的叶片长度、叶片宽度、叶片周长、叶片面积、叶片长宽比、比叶面积、叶片干质量等叶片性状指标的纬度变化规律,分析叶片性状与气象、土壤因子的关系.结果表明: 随着纬度的升高,水杉叶片长、宽、周长逐渐增大,叶面积和比叶面积先增加后降低.在纬度梯度上,水杉叶片性状的变化主要受年均温和年均降水量的影响.随着土壤氮含量的增加,比叶面积呈先下降后上升的趋势,叶干质量则呈显著下降的趋势;随着土壤磷含量的增加,比叶面积逐渐增加,而叶干质量则显著下降.     

Food webs in tropical Australian streams: shredders are not scarce

1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low‐order streams in the Queensland wet tropics.