On the elusiveness of enemy-free space: spatial, temporal, and host-plant-related variation in parasitoid attack rates on three gallmakers of goldenrods

Host shifting by phytophagous insects may play an important role in generating insect diversity by initiating host-race formation and speciation. Models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host in order to balance the maladaptation expected following the shift. Such "enemy-free space" has been documented for some insects, at some times and places, but few studies have assessed the occurrence of enemy-free space across years, among sites, or among insect species. We measured parasitoid attack rates on three insect herbivores of two goldenrods (Solidago altissima L. and Solidago gigantea Ait.), with data from multiple sites and multiple years for each herbivore. For each insect herbivore, there were times and sites at which parasitoid attack rates differed strongly and significantly between host plants (that is, enemy-free space existed on one host plant or the other). However, the extent and even the direction of the attack-rate difference varied strongly among sites and even among years at the same site. There was no evidence of consistent enemy-free space for any herbivore on either host plant. Our data suggest that enemy-free space, like many ecological and evolutionary forces, is likely to operate as a geographic and temporal mosaic, and that conceptual models of host shifting that include enemy-free space as a consequence of host novelty are likely too simple.     

Trophic network structure emerges through antagonistic coevolution in temporally varying environments

Understanding the mechanisms underlying ecological specialization is central to our understanding of community ecology and evolution. Although theoretical work has investigated how variable environments may affect specialization in single species, little is known about how such variation impacts bipartite network structure in antagonistically coevolving systems. Here, we develop and analyse a general model of victim-enemy coevolution that explicitly includes resource and population dynamics. We investigate how temporal environmental heterogeneity affects the evolution of specialization and associated community structure. Environmental productivity influences victim investment in resistance, which will shape patterns of specialization through its regulating effect on enemy investment in infectivity. We also investigate the epidemiological consequences of environmental variability and show that enemy population density is maximized for intermediate lengths of productive seasons, which corresponds to situations where enemies can evolve higher infectivity than victims can evolve defence. We discuss our results in the light of empirical studies, and further highlight ways in which our model applies to a range of natural systems.     

Incidental predation, enemy-free space and the coexistence of incidental prey

Many communities consist of a generalist predator that consumes multiple prey species whose persistence is thereby threatened through the indirect effect of apparent competition. However, uncommon and/or ephemeral prey may be encountered only incidentally through the predator's effort expended to consume primary prey. In such instances, the functional response to incidental prey is driven entirely through the density of primary prey. Moreover, rarity and brevity in the predator's diet precludes a numerical response to incidental prey. Instead, the persistence of incidental prey may be critically linked to gaps in space unexploited by predators, i.e. enemy-free space. I use optimal foraging theory to derive a mechanism by which enemy-free space is created as a result of a predator's forging aptitude and patch-use behavior. In non-competitive environments enemy-free space provides a behavioral refuge for incidental prey that may prevent their extinction. In competitive environments, greater enemy-free space is associated with higher incidental prey densities and concomitantly greater competitive effects. As a result, incidental prey diversity declines with an increase in enemy-free space.     

Self-sustaining populations, population sinks or aggregates of strays: chum (Oncorhynchus keta) and Chinook salmon (Oncorhynchus tshawytscha) in the Wood River system, Alaska

Small populations can provide insights into ecological and evolutionary aspects of species distributions over space and time. In the Wood River system in Alaska, USA, small aggregates of Chinook (Oncorhynchus tshawytscha) and chum salmon (O. keta) spawn in an area dominated by sockeye salmon (O. nerka). Our objective was to determine whether these Chinook and chum salmon are reproductively isolated, self-sustaining populations, population sinks that produce returning adults but receive immigration, or strays from other systems that do not produce returning adults. DNA samples collected from adult chum salmon from 16 streams and Chinook salmon from four streams in the Wood River system over 3 years were compared to samples from large populations in the nearby Nushagak River system, a likely source of strays. For both species, microsatellite markers indicated no significant genetic differentiation between the two systems. Simulations of microsatellite data in a large source and a smaller sink population suggested that considerable immigration would be required to counteract the diverging effects of genetic drift and produce genetic distances as small as those observed, considering the small census sizes of the two species in the Wood River system. Thus, the Wood River system likely receives substantial immigration from neighbouring watersheds, such as the Nushagak River system, which supports highly productive runs. Although no data on population productivity in the Wood River system exist, our results suggest source-sink dynamics for the two species, a finding relevant to other systems where salmonid population sizes are limited by habitat factors.     

The role of natural enemies in the expression and evolution of mixed mating in hermaphroditic plants and animals

Although a large portion of plant and animal species exhibit intermediate levels of outcrossing, the factors that maintain this wealth of variation are not well understood. Natural enemies are one relatively understudied ecological factor that may influence the evolutionary stability of mixed mating. In this paper, we aim for a conceptual unification of the role of enemies in mating system expression and evolution in both hermaphroditic animals and plants. We review current theory and detail the potential effects of enemies on fundamental mating system parameters. In doing so, we identify situations in which consideration of enemies alters expectations about the stability of mixed mating. Generally, we find that inclusion of the enemy dimension may broaden conditions in which mixed mating systems are evolutionarily stable. Finally, we highlight avenues ripe for future theoretical and empirical work that will advance our understanding of enemies in the expression and evolution of mixed mating in their hosts/victims, including examination of feedback cycles between victims and enemies and quantification of mating system-related parameters in victim populations in the presence and absence of enemies.     

The role of habitat selection behavior in population dynamics: source–sink systems and ecological traps

Ecological traps, poor-quality habitat that nonetheless attract individuals, have been observed in both natural and human-altered settings. Until recently, ecological traps were considered a kind of source–sink system, but source–sink theory does not model maladaptive habitat choice, and therefore cannot accurately represent ecological traps or predict their population-level consequences. Although recent models of ecological traps addressed this problem, they used patch-based models containing only two habitats that were very different from one another, but were internally homogeneous. These sorts of patch models may not apply to many real populations, and using them for populations in landscapes with mosaic or gradient habitat structures may be misleading. I developed models that treat source–sink dynamics and ecological traps as special cases of a single process, in which the attractiveness and quality of the habitat are separate variables that can be either positively or negatively related, and in which habitat quality varies continuously throughout the landscape. As expected, sinks are less detrimental to populations than ecological traps, in which preferential use of poor habitat elevates extinction risk. Furthermore, ecological traps may be undetected, and may even appear to be sources, when population sizes are large, but may still prevent recovery in spite of the availability of high-quality habitat when populations drop below threshold levels. Conservation biologists do not routinely consider the possibility that apparent sinks are actually traps, but since traps should be associated with the rapidly changing and novel habitat characteristics primarily produced by human activities, ecological traps should be considered an important and potentially widespread conservation concern.     

Terrestrial carbon sinks in China and around the world and their contribution to carbon neutrality

Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO₂) concentration and to achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (?0.2±0.9) Pg C yr^?1 (1 Pg=10¹⁵ g) in the 1960s to a sink of (1.9±1.1) Pg C yr^?1 in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20–0.25 Pg C yr^?1 in China during the past decades, and predict it to be 0.15–0.52 Pg C yr^?1 by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO₂ concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO₂ concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.

     

具有脉冲投放捕食者阶段结构时滞的捕食-食饵模型

讨论了具有阶段结构脉冲时滞的HollinglI功能反应的捕食模型,其中天敌（益虫）进行人工脉冲周期投放,害虫具有阶段结构及成熟期的时滞现象,并进行了系统的数学及生物方面的研究．利用脉冲及时滞微分方程的基本知识证明了该害虫根除周期解的唯一性和全局吸引性．进一步证明了当天敌的投放量或者投放周期在一定的范围内,能够控制害虫在作物的经济危害水平（EIL）运行的情况下使天敌与害虫可以共存．得出的结论为害虫治理提供了策略基础．     

Reciprocal effects of host plant and natural enemy diversity on herbivore suppression: an empirical study of a model tritrophic system

Many theoretical and empirical studies have shown that species diversity in a trophic level can impact the capture of limited resources in ways that cascade up or down a food web. Only recently, however, have ecologists begun to consider how diversity at multiple trophic levels might act in concert to have opposing or reinforcing effects on resource use. Here, we report the results of an empirical study of a model, tritrophic food web in which we manipulated the diversity of host plant species ( Medicago sativa , Trifolium pratense and Vicia faba ) and natural enemy species ( Harmonia axyridis , Coleomegilla maculata and Nabis sp.) of a widespread herbivorous pest (the pea aphid, Acyrthosiphon pisum ) in laboratory microcosms. We found that increasing natural enemy richness from one to three species increased the proportion of aphids consumed by 0.14. This effect of enemy diversity was due to facilitative interactions and/or a reduction in intraspecific competition in the more diverse assemblages. We also found an independent and additive main effect of host plant richness, with the proportion of aphids consumed by natural enemies decreasing by 0.14 in plant polycultures. A reduction in predator efficiency on a single host plant, Vicia faba , appeared to be responsible for this plant diversity effect. Aphid population sizes were, therefore, simultaneously determined by a top-down effect of natural enemy diversity, and an opposing bottom-up effect of host plant diversity that modified enemy–prey interactions. These results suggest that population sizes in nature, and biotic controls over insect pests, are influenced by species diversity at multiple trophic levels.     

Effects of an attractive sink leading into maladaptive habitat selection

Habitat sinks can attract dispersing animals if high mortality or breeding failure are difficult to detect (e.g., when due to human hunting or pollution). Using a simple deterministic model, we explore the dynamics of such source-sink systems considering three scenarios: an avoided sink, no habitat preference, and an attractive sink. In the second two scenarios, there is a threshold proportion of sink habitat above which the whole population decreases to extinction, but this extinction threshold varies with habitat preference and the relative qualities of the two habitat types. Hence, it would be necessary to know the habitat preferences of any species in a source-sink system to interpret data on population increases and declines. In the attractive sink scenario, small changes in the proportion of sink habitat may have disproportionate effects on the population persistence. Also, small changes in growth rates at the source and the sink severely affect the threshold and the time of extinction. For some combinations of demographic parameters and proportion of habitat sink, the decline affects the source first; thus, during some time, it will be hidden to population monitoring at the sink, where numbers can even increase. The extinction threshold is also very sensitive to the initial population sizes relative to carrying capacity. Attractive sinks represent a novel aspect of source-sink dynamics with important conservation and management implications.     

Ecosystem Management of Desertified Shrublands in Israel

The objectives of this study were to understand the ecological processes and possible management strategies in desertified shrublands. We hypothesized that biological production and diversity in desertified shrublands in the Negev in Israel are low due to water, soil, and nutrient leakage from the ecosystem. We designed a series of field experiments in order to examine (a) whether source–sink relationships exist between the crusted soil and the shrub patches, (b) whether resources (water, soil, and nutrients) leak from the system, and (c) whether management, which changes the landscape mosaic by introducing new sink patches that reduce leakage of resources, may increase productivity and diversity. The results indicate that the low number of shrub patches, which serve as sinks for resources, leads to water, soil, and nutrient leakage from the ecosystem. This leakage reduces ecosystem production and diversity. We found that artificially created pits, which act as sinks for resources, decrease leakage and increase biomass production and annual plant species diversity. Based on the experimental results, we developed conceptual models for shrubland desertification and ecosystem management. The models are based on a source–sink relationship between two patch types characteristic of shrublands. The models relate landscape productivity to the number of sink patches and suggest that, in cases where there are too few sinks, artificially created sink patches should be added. Management methods were developed to reduce resource leakage in the desertified shrubland of the Negev. Methods included construction of man-made pits in the landscape that add resource-enriched patches to the landscape. These patches are used to create parks consisting of clusters of trees integrated into a matrix of shrubs and herbaceous vegetation. The managed parks are used for recreational purposes and for rangeland. Received 8 July 1997; accepted 7 July 1998.     

The role of mobility for the emergence of diversity in victim-exploiter systems

Theoretical and empirical studies indicate that exploitation is a possible driver of exploiter and victim diversification. However, there are many factors which could promote and limit this diversification process. Using a spatially explicit individual-based model, where an exploiter's success depends on matching between its own and a victim's continuous trait, we simulate local communities of victims and exploiters. We investigate how exploiter mobility (searching ability and movement strategies) can influence diversification of victims. We find that if victim traits are under intermediate intensity of stabilizing selection, disruptive selection exerted by exploiters can indeed lead to diversification in victim population and the victim trait distribution can split into two or more groups. Searching ability and movement strategy of exploiters (local vs. global movement) play a role in determining the number of victim trait groups emerging. Moreover, they affect the proportion of infected victims and the formation of spatial patterns in the victim trait distribution. In addition, with a high searching ability, exploiters with global movement drive victims to be more diverse than exploiters with local movement.     

Insularity and adaptation in coupled victim–enemy associations

Employing a mathematical model we show how insularity, genotypic interactions and victim life‐history/demography can influence adaptation in a simple enemy–victim interaction where genotypes migrate between a large source and a smaller, initially unoccupied, isolated habitat. We find that when there are explicit costs to heightened enemy virulence and victim resistance, large/close islands resemble their immigration sources, whereas small and/or distant islands tend to be occupied only by the least defended victims and least virulent enemies. In a model with no explicit cost to genotypic identity, frequencies do not differ on average between source and island. Despite these trends in genotype frequencies, for a range of realistic conditions, both cost and cost‐free genotypic interactions yield an increase in the frequency of resistant encounters as a function of isolation. Moreover, in models with explicit costs, maximal island to island variation in genotypic frequencies is found on islands of intermediate distance from the source. In contrast, the model without explicit costs produces more variable communities, attaining maximum variability in genotypic frequencies at the most isolated islands. We hypothesize that adaptive patterns in mainland–island comparisons may differ substantially from those generated by centre‐periphery comparisons in continental systems.     

Temporal variability of top-down forces and their role in host choice evolution of phytophagous arthropods

The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account.     

Empirical evidence for source–sink populations: a review on occurrence,assessments and implications

Assessing the role of local populations in a landscape context has become increasingly important in the fields of conservation biology and ecology. A growing number of studies attempt to determine the source–sink status of local populations. As the source–sink concept is commonly used for management decisions in nature conservation, accurate assessment approaches are crucial. Based on a systematic literature review of studies published between 2002 and 2013, we evaluated a priori predictions on methodological and biological factors that may influence the occurrence of source or sink populations. The review yielded 90 assessments from 73 publications that included qualitative and quantitative evidence for either source or sink population(s) for one or multiple species. Overall, sink populations tended to occur more often than source populations. Moreover, the occurrence of source or sink populations differed among taxonomic classes. Sinks were more often found than sources in mammals, while there was a non‐significant trend for the opposite to be true for amphibians. Univariate and multivariate analyses showed that the occurrence of sources was positively related to connectivity of local populations. Our review furthermore highlights that more than 25 years after Pulliam's widely cited publication on ‘sources, sinks, and population regulation’, in‐depth assessments of the source–sink status of populations based on combined consideration of demographic parameters such as fecundity, survival, emigration and immigration are still scarce. To increase our understanding of source–sink systems from ecological, evolutionary and conservation‐related perspectives, we recommend that forthcoming studies on source–sink dynamics should pay more attention to the study design (i.e. connectivity of study populations) and that the assessment of the source–sink status of local populations is based on λ values calculated from demographic rates.     

我国主要海域海水养殖碳汇能力评估及其影响效应——基于我国9个沿海省份面板数据

随着海水养殖业的碳汇功能逐渐被认识和肯定,海水养殖不再单是一项经济活动,而是对环境具有正向影响的碳汇生态活动。以我国沿海9个省份为例,选取海水养殖业碳汇主要贡献的贝类和藻类海产品,并按照各自的碳汇方式对我国沿海地区2008—2015年海水养殖碳汇能力测算,进一步将9个沿海省份按照主要海域划分为渤海、黄海、东海、南海,利用LMDI模型从海水养殖的结构效应和规模效应角度分析碳汇能力的区域差异和主要影响因素。研究结果显示,黄海沿岸海水养殖碳汇能力最强,南海沿岸海水养殖的碳汇转化比例最高,规模效应与我国沿海地区海水养殖碳汇能力始终呈正相关,结构效应的作用显著但不稳定。基于上述结论,我国沿海地区碳汇养殖业应首先提升碳汇养殖技术、稳定海水养殖产量,其次注重优化养殖结构,对碳汇潜力巨大的贝类多加关注。     

Temporal autocorrelation can enhance the persistence and abundance of metapopulations comprised of coupled sinks

In spatially heterogeneous landscapes, some habitats may be persistent sources, providing immigrants to sustain populations in unfavorable sink habitats (where extinction is inevitable without immigration). Recent theoretical and empirical studies of source-sink systems demonstrate that temporally variable local growth rates in sinks can substantially increase average abundance of a persisting population, provided that the variation is positively autocorrelated--in effect, temporal variation inflates average abundance. Here we extend these results to a metapopulation in which all habitat patches are sinks. Using numerical studies of a population with discrete generations (buttressed by analytic results), we show that temporal variation and moderate dispersal can jointly permit indefinite persistence of the metapopulation and that positive autocorrelation both lowers the magnitude of variation required for persistence and increases the average abundance of persisting metapopulations. These effects are weakened--but not destroyed--if variation in local growth rates is spatially synchronized and dispersal is localized. We show that the inflationary effect is robust to a number of extensions of the basic model, including demographic stochasticity and density dependence. Because ecological and environmental processes contributing to temporally variable growth rates in natural populations are typically autocorrelated, these observations may have important implications for species persistence.     

The effects of habitat fragmentation on persistence of source-sink metapopulations in systems with predators and prey or apparent competitors.

We consider systems with one predator and one prey, or a common predator and two prey species (apparent competitors) in source and sink habitats. In both models, the predator species is vulnerable to extinction, if productivity in the source is insufficient to rescue demographically deficient sink populations. Conversely, in the model with two prey species, if the source is too rich, one of the prey species may be driven extinct by apparent competition, since the predator can maintain a large population because of the alternative prey. Increasing the rate of predator movement from the source population has opposite effects on prey and predator persistence. High emigration rate exposes the predator population to danger of extinction, reducing the number of individuals that breed and produce offspring in the source habitat. This may promote coexistence of prey by relaxing predation pressure and apparent competition between the two prey species. The number of sinks and spatial arrangement of patches, or connectivity between patches, also influence persistence of the species. More sinks favor the prey and fewer sinks are advantageous to the predator. A linear pattern with the source at one end is profitable for the predator, and a centrifugal pattern in which the source is surrounded by sinks is advantageous to the prey. When the dispersal rate is low, effects of the spatial structure may exceed those of the number of sinks. In brief, productivity in patches and patterns of connectivity between patches differentially influence persistence of populations in different trophic levels.     

Temporal variation can facilitate niche evolution in harsh sink environments

We examine the impact of temporal variation on adaptive evolution in "sink" environments, where a species encounters conditions outside its niche. Sink populations persist because of recurrent immigration from sources. Prior studies have highlighted the importance of demographic constraints on adaptive evolution in sinks and revealed that adaptation is less likely in harsher sinks. We examine two complementary models of population and evolutionary dynamics in sinks: a continuous-state quantitative-genetics model and an individual-based model. In the former, genetic variance is fixed; in the latter, genetic variance varies because of mutation, drift, and sampling. In both models, a population in a constant harsh sink environment can exist in alternative states: local maladaptation (phenotype comparable to immigrants from the source) or adaptation (phenotype near the local optimum). Temporal variation permits transitions between these states. We show that moderate amounts of temporal variation can facilitate adaptive evolution in sinks, permitting niche evolution, particularly for slow or autocorrelated variation. Such patterns of temporal variation may particularly pertain to sinks caused by biotic interactions (e.g., predation). Our results are relevant to the evolutionary dynamics of species' ranges, the fate of exotic invasive species, and the evolutionary emergence of infectious diseases into novel hosts.