Heterogeneity–diversity relationships in sessile organisms: a unified framework

The hypothesis that environmental heterogeneity promotes species richness by increasing opportunities for niche partitioning is a fundamental paradigm in ecology. However, recent studies suggest that heterogeneity–diversity relationships (HDR) are more complex than expected from this niche‐based perspective, and often show a decrease in richness at high levels of heterogeneity. These findings have motivated ecologists to propose new mechanisms that may explain such deviations. Here we provide an overview of currently recognised mechanisms affecting the shape of HDRs and present a conceptual model that integrates all previously proposed mechanisms within a unified framework. We also translate the proposed framework into an explicit community dynamic model and use the model as a tool for generating testable predictions concerning how landscape properties interact with species traits in determining the shape of HDRs. Our main finding is that, despite the enormous complexity of such interactions, the predicted HDRs are rather simple, ranging from positive to unimodal patterns in a highly consistent and predictable manner.     

Postglacial recolonization of North America by spadefoot toads: integrating niche and corridor modeling to study species’ range dynamics over geologic time

Understanding the factors that shape species’ distributions is a key topic in biogeography. As climates change, species can either cope with these changes through evolution, plasticity or by shifting their ranges to track the optimal climatic conditions. Ecological niche modeling (ENM) is a widespread technique in biogeography that estimates the niche of the organism by using occurrences and environmental data to estimate species’ potential distributions. ENMs are often criticized for failing to take species’ dispersal abilities into consideration. Here, we attempt to fill this gap by combining ENMs with dispersal and corridor modeling to study the range dynamics of North American spadefoot toads (Scaphiopodidae) over the Holocene. We first estimated the current and past distributions of spadefoot toads and then estimated their past distributions from the Last Glacial Maximum (LGM) to the present day. Then, we estimated how each taxon recolonized North American by using dispersal and corridor modeling. By combining these two modeling approaches we were able to 1) estimate the LGM refugia used by the North American spadefoot toads, 2) further refine these projections by estimating which of the putative LGM refugia contributed to the recolonization of North America via dispersal, and 3) estimate the relative influence of each LGM refugium to the current species’ distributions. The models were tested using previously published phylogeographic data, revealing a high degree of congruence between our models and the genetic data. These results suggest that combining ENMs and dispersal modeling over time is a promising approach to investigate both historical and future species’ range dynamics.     

Aerial seed bank in a cold desert annual-ephemeral species: Role of anatomical structure of stem and delayed fruit dehiscence in timing of seed dispersal

Why the fruits are retained on dead upright herbaceous plants and how this relates to seed dispersal and timing of germination remain unclear. Stems of the annual Euclidium syriacum (Brassicaceae) with infructescences bearing indehiscent silicles remain upright after plants die in the spring. We investigated the effect of anatomical structures of stem and pedicle and delayed silicle dehiscence on seed dispersal phenology of this species. For comparison, sections were made of the stem of the annual Goldbachia laevigata (Brassicaceae), which has stems that fall over when plants die. Compared to G. laevigata, the stem of E. syriacum has vascular bundles that are closer together, a thicker xylem and phloem, more fibers, a thicker perimedullary zone and a smaller pith diameter:stem diameter ratio. The thickened pedicle did not form an abcission layer. By late October, 5–20% of seeds were dispersed, depending on the position of infructescences on the plant. Snow covered the plants in late autumn and when it melted in mid-April many of the plants had fallen over, with a high number of seeds germinating in attached silicles; seedlings became rooted in soil. After snowmelt, 14–15% of the silicles on the remaining upright plants contained seeds; all seeds were dispersed by early July. The anatomical structures of the stem and pedicle plus the delayed dehiscence of silicles explain the presence of an aerial seed bank in E. syriacum and delay of germination of many of seeds until spring. Further, pieces of upright plants are broken off and dispersed by wind, which helps to explain the wide distribution of E. syriacum in the cold desert.     

Spatial graphs highlight how multi-generational dispersal shapes landscape genetic patterns

Current approaches that compare spatial genetic structure of a given species and the dispersal of its mobile phase can detect a mismatch between both patterns mainly due to processes acting at different temporal scales. Genetic structure result from gene flow and other evolutionary and demographic processes over many generations, while dispersal predicted from the mobile phase often represents solely one generation on a single time-step. In this study, we present a spatial graph approach to landscape genetics that extends connectivity networks with a stepping-stone model to represent dispersal between suitable habitat patches over multiple generations. We illustrate the approach with the case of the striped red mullet Mullus surmuletus in the Mediterranean Sea. The genetic connectivity of M. surmuletus was not correlate with the estimated dispersal probability over one generation, but with the stepping-stone estimate of larval dispersal, revealing the temporal scale of connectivity across the Mediterranean Sea. Our results highlight the importance of considering multiple generations and different time scales when relating demographic and genetic connectivity. The spatial graph of genetic distances further untangles intra-population genetic structure revealing the Siculo-Tunisian Strait as an important corridor rather than a barrier for gene flow between the Western- and Eastern Mediterranean basins, and identifying Mediterranean islands as important stepping-stones for gene flow between continental populations. Our approach can be easily extended to other systems and environments.     

东南亚生物多样性热点地区的形成与演化

东南亚地处热带,生物多样性极为丰富,可分为4个热点地区:印度-缅甸区的中南半岛、巽他区(含马来半岛、婆罗洲、苏门答腊岛)、菲律宾区(菲律宾群岛)、华莱士区(苏拉威西岛、爪哇岛、马鲁古群岛、小巽他群岛等)。中南半岛在泥盆纪便已是欧亚大陆的一部分,在印度板块撞击欧亚大陆之后受挤压而出;巽他区来自于冈瓦纳古陆和澳洲古陆;菲律宾群岛部分来自于劳亚古陆的碎片向南漂移,部分来自于太平洋西南岛弧的向北迁移;华莱士区则是劳亚古陆碎片、太平洋西南岛弧以及澳洲古陆北侧碎片的组合。巽他区地处赤道,常年温湿;菲律宾区、华莱士区、中南半岛则都受到不同程度的季风气候决定的干湿季变动。地质历史和季风气候影响程度的不同,奠定了东南亚4个生物多样性热点地区的雏形。华莱士区保存有大量的早期被子植物原始类群如睡莲目(Nymphaeales)和木兰藤目(Austrobaileyales),是现代被子植物起源地和冰期避难所之一。巽他区(婆罗洲)和中南半岛是亚洲热带植物的现代分布中心和"进化前沿",是整个东南亚地区重要的种源;而华莱士区的爪哇岛和小巽他群岛主要是物种迁入和中转的种库。这样的物种形成历史与迁移格局,塑造了东南亚4个生物多样性热点地区物种多样性水平与地理范围的基本格局。巽他区和印度-缅甸区曾在冰期通过陆桥相连,使得东南亚成为周边植物扩散交汇的一个"十字路口"。但是,人们对东南亚生物多样性热点地区的物种长距离扩散规律及植物地理学分区仍存在分歧;东南亚与邻近生物多样性热点地区如新几内亚岛、西高止山脉-斯里兰卡、中国横断山区的历史联系,还尚待深入解析。利用现代分子生物学技术,覆盖整个东南亚地区进行全域取样开展代表性类群的物种迁移与生物地理学研究,有望进一步揭示东南亚生物多样性热点地区的形成过程与演化趋势。     

植物大年结实及其与动物贮食行为之间的关系

大年结实(mast seeding)是多生年植物种群周期性同步大量繁殖的一种自然现象。大年结实作为植物适应环境条件、提高繁殖能力的一种策略而备受关注, 但其驱动机制和进化意义尚存在较大争议。在依赖动物扩散种子的植物中, 大年结实被认为是一种调控动物贮食行为、提高种子扩散效率, 并最终增加繁殖成功率的一种策略; 动物介导的植物间互作可能是促进植物共存的进化驱动力。本文简要梳理了大年结实现象的各种假说, 提出了一个包括气候、资源、动植物互作的理解大年结实机制的概念框架, 并着重讨论了大年结实和动物贮食行为之间的关系及其进化和生态意义。建议未来研究需要借助长期生态监测和分子生物学方法, 揭示植物大年结实与动物贮食行为之间的生态与进化过程。     

Habitat fragmentation shapes natal dispersal and sociality in an Afrotropical cooperative breeder

It remains poorly understood how effects of anthropogenic activity, such as large-scale habitat fragmentation, impact sociality in animals. In cooperatively breeding species, groups are mostly formed through delayed offspring dispersal, and habitat fragmentation can affect this process in two opposite directions. Increased habitat isolation may increase dispersal costs, promoting delayed dispersal. Alternatively, reduced patch size and quality may decrease benefits of philopatry, promoting dispersal. Here, we test both predictions in a cooperatively breeding bird (placid greenbul, Phyllastrephus placidus) from an Afrotropical cloud forest archipelago. Males born in fragmented forest dispersed about 1 year earlier than those born in continuous forest. Contrary to females, males also started to reproduce earlier and mostly settled within their natal patch. Females only rarely delayed their dispersal for more than 1 year, both in fragmented and continuous forests. Our results suggest that early male dispersal and reproduction is jointly driven by a decrease in the value of the natal territory and an increase in local breeding opportunities in fragmented forest. While plasticity in dispersal strategies of cooperative breeders in response to anthropogenic change is believed to optimize reproduction-survival trade-offs, to what extent it shapes the ability of species to respond to rapid environmental change remains to be studied.     

Beyond the landscape: Resistance modelling infers physical and behavioural gene flow barriers to a mobile carnivore across a metropolitan area

Urbanization affects key aspects of wildlife ecology. Dispersal in urban wildlife species may be impacted by geographical barriers but also by a species’ inherent behavioural variability. There are no functional connectivity analyses using continuous individual‐based sampling across an urban‐rural continuum that would allow a thorough assessment of the relative importance of physical and behavioural dispersal barriers. We used 16 microsatellite loci to genotype 374 red foxes (Vulpes vulpes) from the city of Berlin and surrounding rural regions in Brandenburg in order to study genetic structure and dispersal behaviour of a mobile carnivore across the urban‐rural landscape. We assessed functional connectivity by applying an individual‐based landscape genetic optimization procedure. Three commonly used genetic distance measures yielded different model selection results, with only the results of an eigenvector‐based multivariate analysis reasonably explaining genetic differentiation patterns. Genetic clustering methods and landscape resistance modelling supported the presence of an urban population with reduced dispersal across the city border. Artificial structures (railways, motorways) served as main dispersal corridors within the cityscape, yet urban foxes avoided densely built‐up areas. We show that despite their ubiquitous presence in urban areas, their mobility and behavioural plasticity, foxes were affected in their dispersal by anthropogenic presence. Distinguishing between man‐made structures and sites of human activity, rather than between natural and artificial structures, is thus essential for better understanding urban fox dispersal. This differentiation may also help to understand dispersal of other urban wildlife and to predict how behaviour can shape population genetic structure beyond physical barriers.