Effects of local negative feedbacks on the evolution of species within metacommunities

Local negative feedbacks occur when the occupation of a site by a species decreases the subsequent fitness of related individuals compared to potential competitors. Such negative feedbacks can enhance diversity by changing the spatial structure of the environment. The conditions, however, involve dispersive, environmental and evolutionary processes in complex interactive ways. We introduce a model that accounts for four mechanisms: colonisation‐competition‐extinction ecological dynamics, evolutionary dynamics, local negative feedbacks and environmental averaging. Three qualitatively distinct dynamics are possible, one dominated by specialists, another dominated by generalists and an intermediate situation exhibiting taxon cycles. We discuss how metacommunity diversity, macro‐ecological patterns and environmental patterning are linked to the three qualitative dynamics. The model provides classical shapes for morph‐abundance distributions, or diversity‐area relationships. Diversity can be high when specialists dominate or when taxon cycles happen. Finally, local negative feedbacks often yield fine‐grain environments for taxon cycle dynamics and coarse‐grain environments when generalists dominate.     

The cryptic role of biodiversity in the emergence of host–microbial mutualisms

The persistence of mutualisms in host‐microbial – or holobiont – systems is difficult to explain because microbial mutualists, who bear the costs of providing benefits to their host, are always prone to being competitively displaced by non‐mutualist ‘cheater’ species. This disruptive effect of competition is expected to be particularly strong when the benefits provided by the mutualists entail costs such as reduced competitive ability. Using a metacommunity model, we show that competition between multiple cheaters within the host's microbiome, when combined with the spatial structure of host–microbial interactions, can have a constructive rather than a disruptive effect by allowing the emergence and maintenance of mutualistic microorganisms within the host. These results indicate that many of the microorganisms inhabiting a host's microbiome, including those that would otherwise be considered opportunistic or even potential pathogens, play a cryptic yet critical role in promoting the health and persistence of the holobiont across spatial scales.     

Contrasting responses of arable spiders to the landscape matrix at different spatial scales

Aim Animal communities can be influenced by the composition of the surrounding landscape through immigration. Depending on habitat preferences, however, the effect of the landscape matrix can be positive or negative and can vary with scale. We tested this idea with arable spiders and tried to infer dispersal distances from relationships between local density and landscape composition at different spatial scales. Location Thirty‐eight landscapes around the cities of Göttingen and Giessen, Germany. Methods Spiders were captured with pitfall traps in one field of winter wheat in each landscape. Landscape composition around the fields was characterized at 11 scales from 95 m to 3 km radius by land‐use mapping and subsequent GIS analysis. Correlation tests were performed between landscape composition and local densities or species richness. Results In both study regions, local species richness was enhanced by non‐crop habitats on a landscape scale. The overall densities of wolf spiders (Lycosidae), long‐jawed spiders (Tetragnathidae), crab spiders (Thomisidae), and dwarf sheet spiders (Hahniidae) increased significantly in landscapes with high percentages of non‐crop habitats. Out of the 40 species tested, 19 responded positively to the percentage of non‐crop habitats in the surrounding landscape, and five responded negatively. Depending on the species, the spatial scales with the highest explanatory power ranged from 95 m to 3 km radius around the study fields, potentially reflecting dispersal distances. Main conclusions Arable spider species showed contrasting responses to the landscape context with respect both to the direction and to the spatial scale of the relationship. The variation in landscape requirements among species ensures high spider densities in a wide range of situations, which contributes to ecosystem resilience. However, species richness of arable spiders depends on heterogeneous landscapes with high percentages of non‐crop habitats.     

The relative importance of dispersal and local processes in structuring phytoplankton communities in a set of highly interconnected ponds

1. The recognition that both local and regional processes act together in shaping local communities makes determining their relative roles in natural communities central to understanding patterns in community structure. 2. We investigated the relative influence of these processes on the phytoplankton communities of a highly interconnected pond system. We sampled the phytoplankton communities of 28 ponds concurrently with 20 local environmental variables. 3. We found that phytoplankton community variation, in terms of both phytoplankton community composition (PCC) and diversity, was only significantly explained by local environmental variables. These were mainly associated with the contrasting clear‐water and turbid ecological states of the shallow ponds studied. Clear‐water conditions favoured only a few taxa, resulting in a significantly lower taxon diversity and richness under these conditions. 4. The failure to explain variation in PCC by a dispersal model based on the water flow between ponds points at very effective species sorting. This is attributed to the high population turn‐over rates and sensitivity to environmental conditions of phytoplankton communities. Some evidence was found, however, that dispersal influences local communities through mass effects between neighbouring ponds. 5. Overall, our results emphasize both the strong selection pressure that components of the food web exert on phytoplankton communities and the high potential of these communities to respond to such environmental change, thereby effectively opposing the homogenizing effects of continuous dispersal.     

Spatial coherence and the persistence of high diversity in spatially heterogeneous landscapes

Our planet hosts a variety of highly diverse ecosystems. The persistence of high diversity is generally attributed to factors such as the structure of interactions among species and the dispersal of species in metacommunities. Here, we show that large contiguous landscapes—that are characterized by high dispersal—facilitate high species richness due to the spatial heterogeneity in interspecies interactions. We base our analysis on metacommunities under high dispersal where species densities become equal across habitats (spatially coherent). We find that the spatially coherent metacommunity can be represented by an effective species interaction‐web that has a significantly lower complexity than the constituent habitats. Our framework also explains how spatial heterogeneity eliminates differences in the effective interaction‐web, providing a basis for deviations from the area‐heterogeneity tradeoff. These results highlight the often‐overlooked case of high dispersal where spatial coherence provides a novel mechanism for supporting high diversity in large heterogeneous landscapes.     

Aquatic islands in the sky: 100 years of research on water‐filled tree holes

Water‐filled tree holes are unique ecosystems that may occur high up in tree crowns and are essentially aquatic islands in the sky. Insect larvae, mesofauna, and other organisms colonize the waterbodies and feed on the accumulating detritus. Water‐filled tree holes are not only important habitats for these species but have been used as model systems in ecology. Here, we review more than 100 years of research on tree‐hole inhabiting organisms and show that most studies focus on selected or even single species (most of which are mosquitoes), whereas only few studies examine groups other than insects, especially in the tropics. Using a vote counting of results and a meta‐analysis of community studies, we show that the effects of tree‐hole size and resources on abundance and richness were investigated most frequently. Both were found to have a positive effect, but effect sizes were modulated by site‐specific environmental variables such as temperature or precipitation. We also show that parameters such as the height of the tree holes above ground, tree‐hole density, predation, and detritus type can be important drivers of organism abundance or richness but are less often tested. We identify several important research gaps and potential avenues for future research. Specifically, future studies should investigate the structure, functions, and temporal dynamics of tree‐hole food webs and their cross‐system interactions, for example, with terrestrial predators that act as a connection to their terrestrial surroundings in meta‐ecosystems. Global observational or experimental tree‐hole studies could contribute pivotal information on spatial variation of community structure and environmental drivers of community assembly. With a better understanding of these unique aquatic habitats in terrestrial ecosystems, natural and artificial tree holes can not only serve as model systems for addressing fundamental ecological questions but also serve as indicator systems of the impacts of environmental change on ecosystems.     

Integrating community assembly and biodiversity to better understand ecosystem function: the Community Assembly and the Functioning of Ecosystems (CAFE) approach

The research of a generation of ecologists was catalysed by the recognition that the number and identity of species in communities influences the functioning of ecosystems. The relationship between biodiversity and ecosystem functioning (BEF) is most often examined by controlling species richness and randomising community composition. In natural systems, biodiversity changes are often part of a bigger community assembly dynamic. Therefore, focusing on community assembly and the functioning of ecosystems (CAFE), by integrating both species richness and composition through species gains, losses and changes in abundance, will better reveal how community changes affect ecosystem function. We synthesise the BEF and CAFE perspectives using an ecological application of the Price equation, which partitions the contributions of richness and composition to function. Using empirical examples, we show how the CAFE approach reveals important contributions of composition to function. These examples show how changes in species richness and composition driven by environmental perturbations can work in concert or antagonistically to influence ecosystem function. Considering how communities change in an integrative fashion, rather than focusing on one axis of community structure at a time, will improve our ability to anticipate and predict changes in ecosystem function.     

栖息地永久性破坏的比例对物种多度稳定值影响的迭代算法

为了分析栖息地破坏程度对集合群落中物种多度稳定值的影响,基于Tilman等[1, 2]提出的多物种竞争的集合群落模型,设计了一种通用的迭代算法用以分析栖息地永久性破坏的比例对物种多度稳定值的影响。针对Tilman等[2]提出的物种多度与其扩散能力或者与其竞争能力相互关系的四种模型,也即：（1）物种竞争力越强则其多度稳定值越大,所有物种死亡率相同;（2）所有物种不论竞争力如何,其多度稳定值相同、死亡率也相同;（3）物种竞争力越弱则其多度稳定值越大,所有物种死亡率相同;（4）物种多度的稳定值相同,但物种竞争力越弱其死亡率越高。先前的研究已经阐明了在前2种模型中栖息地永久性破坏的比例对物种多度稳定值的影响;而对于模型3,因为其数学表达式较为复杂,先前的研究者不得不使用模型3的简化式来考察栖息地永久性破坏的比例对物种多度稳定值的影响;而对于模型4,由于其数学表达式更为复杂,栖息地永久性破坏的比例对物种多度稳定值的影响未能被以往的研究所阐明。本文所使用的迭代算法可以阐明四种模型中任何一种模型条件下栖息地永久性破坏的比例对物种多度稳定值的影响。我们发现对于模型1和2迭代算法所得到的物种多度稳定值与通过数学解析式分析的结果完全一致,同时通过使用迭代算法还阐明了模型4中栖息地永久性破坏的比例对物种多度稳定值的影响。假设栖息地永久性破坏的比例达到了能够导致第s个物种灭绝的水平,起初幸存物种竞争力的排序为s 1 ~ s 3 ~ s 5 ~ … ~ s 6 ~ s 4 ~ s 2,但是随着栖息地永久性破坏的比例不断增大,当其快达到（但还未达到）能够导致第s 1个物种灭绝的水平,物种竞争力的排序将变为s 2 ~ s 4 ~ s 6 ~ … ~ s 5 ~ s 3 ~ s 1。模型4中栖息地永久性破坏的比例对物种多度稳定值的影响与模型2中栖息地永久性破坏程度对物种多度稳定值的影响几乎一致,唯一不同是模型2中所有物种栖息地稳定值的曲线有一个共同的交点,而模型4中所有物种栖息地稳定值的曲线交点不唯一。此外,还使用迭代算法考对比了模型3原始数学表达式和简化式两种情况下栖息地永久性破坏的比例对物种多度稳定值的影响,发现结果略有不同。