Attractive sinks, or how individual behavioural decisions determine source–sink dynamics

Gundersen et al . (2001 , Source-sink dynamics: how sinks affect demography of sources. Ecol. Lett. , 4, 14–21.) suggested that sinks can severely affect the demography of populations in source habitats. We propose this is a common result when animals lack cues associated with reduced fitness inside sinks and consequently select habitat inappropriately. These attractive sinks can result either from undetected risks of mortality (as in the experiment of Gundersen et al . 2001 ) or from undetected poor breeding probabilities (due to bioaccumulation of pesticides, for instance). Thus, individual habitat choice is a key process underlying source–sink dynamics.     

Diurnal osmotic cycling in cotton leaves as an indicator of source–sink balance

Abstract Diurnal cycling of osmotic potential was studied in leaves of cotton plants (Gossypium hirsutum L.) grown in the field. Osmotic potential was determined by a pressure-volume procedure as the value coinciding with zero turgor. In plants grown under favourable conditions (no water stress or N stress), osmotic potential at zero-turgor measured at midday was initially about 0.3 MPa lower than before dawn, but this cycling disappeared during the season as the number of fruits per plant increased. In water-stressed or N-deficient plants, osmotic cycling was decreased or even eliminated. Across treatments, cycling of osmotic potential occurred only when plants carried at least 560 cm² of leaf area per fruit. The results are interpreted to mean that diurnal cycling of osmotic potential reveals a ‘sink-limited’ condition within the plant.     

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.     

Identification of source‐sink dynamics in mountain lions of the Great Basin

Natural and anthropogenic boundaries have been shown to affect population dynamics and population structure for many species with movement patterns at the landscape level. Understanding population boundaries and movement rates in the field for species that are cryptic and occur at low densities is often extremely difficult and logistically prohibitive; however genetic techniques may offer insights that have previously been unattainable. We analysed thirteen microsatellite loci for 739 mountain lions (Puma concolor) using muscle tissue samples from individuals in the Great Basin throughout Nevada and the Sierra Nevada mountain range to test the hypothesis that heterogeneous hunting pressure results in source‐sink dynamics at the landscape scale. We used a combination of non‐spatial and spatial model‐based Bayesian clustering methods to identify genetic populations. We then used a recently developed Bayesian multilocus genotyping method to estimate asymmetrical rates of contemporary movement between those subpopulations and to identify source and sink populations. We identified two populations at the highest level of genetic structuring with a total of five subpopulations in the Great Basin of Nevada and the Sierra Nevada range. Our results suggest that source‐sink dynamics occur at landscape scales for wide‐ranging species, such as mountain lions, and that source populations may be those that are under relatively less hunting pressure and that occupy refugia.     

Consequences of varying regional heterogeneity in source–sink metacommunities

Although the influence of dispersal on coexistence mechanisms in metacommunities has received great emphasis, few studies have addressed how such influence is affected varying regional heterogeneity. We present a mechanistic model of resource competition in a metacommunity based on classical models of plant competition for limiting resources. We defined regional heterogeneity as the differences in resource supply rates (or resource availabilities) across local communities. As suggested by previous work, the highest diversify occurred at intermediate levels of dispersal among local communities. However our model shows how the effects of dispersal depend on the amount of heterogeneity among local communities and vice versa. Both regional and local species richness were the highest when heterogeneity was intermediate. We suggest that empirical studies that found no evidence for source–sink or mass effects at the community level may have examined communities with limited ranges of dispersal and regional heterogeneity. This model of species coexistence contributes to a broader understanding of patterns in real communities.     

Effect of source–sink ratio on seed set and filling in sunflower (Helianthus annuus L.)

Poor seed development in sunflower may result from insufficient assimilate supply (source limitation). To test this hypothesis, the effects of changed source–sink ratio on seed set (measured as percentage of empty achenes) and seed filling (measured as dry mass per filled achene) in individual plants were investigated. Source–sink ratio, defined as leaf area per floret (LAF), was experimentally altered using invasive (floret removal, defoliation) and non‐invasive (pulse of chilling, short days or shading during leaf or floret initiation) treatments. Shading at floret initiation proved the most effective non‐invasive method. Generally, an increase, or decrease, in LAF improved, or impaired, both seed set and filling. Increasing LAF by 2.0 cm²[95% confidence interval (1.5, 2.5)] decreased the percentage of empty achenes by 36.9%‐points (?41.9, ?30.9) and increased dry mass per filled achene by 20.1 mg (13.6, 26.7) in the capitulum centre. The effect of source–sink ratio on seed set was always strongest in the centre, whereas peripheral whorls were not affected. Achene mass was affected in all parts of the capitulum. It is concluded that source limitation is a major cause for empty achenes in sunflower plants grown under non‐stress conditions.     

Asymmetrical gene flow of the recently delisted passerine black‐capped vireo (Vireo atricapilla) indicates source‐sink dynamics in central Texas

Habitat fragmentation can produce metapopulations or source‐sink systems in which dispersal in crucial for population maintenance. Our objective was to investigate connectivity among black‐capped vireo (Vireo atricapilla) populations in tandem with a demographic study (Biological Conservation, 2016, 203, 108–118) to elucidate if central Texas populations act as a source‐sink system. We genotyped 343 individuals at 12 microsatellite loci to elucidate the movement ecology of the black‐capped vireo in central Texas surrounding Fort Hood; the largest and most stable breeding population of black‐capped vireos inhabit Fort Hood. To gain insight into gene flow among populations, we analyzed genetic differentiation, migration rates, number of migrants, and parentage. We found statistically significant, but low levels of genetic differentiation among several populations, suggesting some limited restriction to gene flow. Across approaches to estimate migration, we found consistent evidence for asymmetrical movement from Fort Hood to the other central Texas sites consistent with source‐sink dynamics. Our results are complementary to black‐capped vireo demographic studies done in tandem showing that portions of Fort Hood are acting as a source population to smaller central Texas populations.     

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.     

Source–sink dynamics: how sinks affect demography of sources

Models of source–sink population dynamics have to make assumptions about whether, and eventually how, demographic parameters in source habitats are dependent on the demography in sink habitats. However, the empirical basis for making such assumptions has been weak. Here we report a study on experimental root vole populations, where estimates of demographic parameters were contrasted between source patches in source–sink (treatment) and source–source systems (control). In the presence of a sink patch (simulated by a pulsed removal of immigrants), source‐patch populations failed to increase over the breeding season, mainly due to a high spatially density‐dependent dispersal rate from source to sink patches. The per capita recruitment rate was almost two times higher in source–sink than in the source–source systems, but this did not compensate for the loss rate due to dispersal from source to sink patches. Sex ratio in the source–sink systems became less female biased, probably as a result of an enhanced frequency of dispersal movements in females. Good knowledge of the degree of density‐and habitat‐dependent dispersal is critical for predicting the dynamics of source–sink populations.     

Scent‐marking behavior by subordinate Eurasian beavers

Chemical communication by scent‐marking is widespread among mammals and can serve different purposes, such as territory demarcation, mate attraction and self‐advertisement. In this study, we examined scent‐marking behavior by subordinate Eurasian beavers (Castor fiber) in a population in southeast Norway. We conducted scent experiments in 18 family groups, simulating a territory intruder using experimental scent mounds (ESM). In total, we recorded 196 territorial reactions (destroying and/or overmarking of scent mounds). Subordinates contributed 14% of all territorial reactions and first overmarked 12% of the ESM, and the number of subordinates in the family group did not increase the total number of territorial reactions. This suggests that the contribution by subordinates plays a minor role in territorial defense. The number of territorial reactions by subordinates was positively related to the age of their parents, suggesting that subordinates might take over territorial duties with increasing parental senescence, likely because they have increased chances of inheriting the territory. Increased experience in territorial activities possibly helps subordinates to successfully gain and defend a territory of their own.     

Experimental demonstration of accelerated extinction in source‐sink metapopulations

Population extinction is a fundamental ecological process which may be aggravated by the exchange of organisms between productive (source) and unproductive (sink) habitat patches. The extent to which such source‐sink exchange affects extinction rates is unknown. We conducted an experiment in which metapopulation effects could be distinguished from source‐sink effects in laboratory populations of Daphnia magna. Time‐to‐extinction in this experiment was maximized at intermediate levels of habitat fragmentation, which is consistent with a minority of theoretical models. These results provided a baseline for comparison with experimental treatments designed to detect effects of concentrating resources in source patches. These treatments showed that source‐sink configurations increased population variability (the coefficient of variation in abundance) and extinction hazard compared with homogeneous environments. These results suggest that where environments are spatially heterogeneous, accurate assessments of extinction risk will require understanding the exchange of organisms among population sources and sinks. Such heterogeneity may be the norm rather than the exception because of both the intrinsic heterogeneity naturally exhibited by ecosystems and increasing habitat fragmentation by human activity.     

Simultaneous boosting of source and sink capacities doubles tuber starch yield of potato plants

An important goal in biotechnological research is to improve the yield of crop plants. Here, we genetically modified simultaneously source and sink capacities in potato (Solanum tuberosum cv. Desirée) plants to improve starch yield. Source capacity was increased by mesophyll‐specific overexpression of a pyrophosphatase or, alternatively, by antisense expression of the ADP‐glucose pyrophosphorylase in leaves. Both approaches make use of re‐routing photoassimilates to sink organs at the expense of leaf starch accumulation. Simultaneous increase in sink capacity was accomplished by overexpression of two plastidic metabolite translocators, that is, a glucose 6‐phosphate/phosphate translocator and an adenylate translocator in tubers. Employing such a ‘pull’ approach, we have previously shown that potato starch content and yield can be increased when sink strength is elevated. In the current biotechnological approach, we successfully enhanced source and sink capacities by a combination of ‘pull’ and ‘push’ approaches using two different attempts. A doubling in tuber starch yield was achieved. This successful approach might be transferable to other crop plants in the future.     

陆地生态系统碳源与碳汇及其影响机制研究进展

全球碳循环研究中发现,目前已知碳源与碳汇不能达到平衡。存在一个很大的碳失汇。大气、海洋和陆地生态系统是人工源CO2的3个可能的容纳汇,其中陆地生态系统最复杂、最具不确定性,因此陆地生态系统碳源与碳汇研究是全球碳循环研究的核心问题之一。大气成分监测、CO2通量测定、森林资源清查以及模型模拟等方面的研究都表明,CO2施肥效应、氮沉降增加、污染、全球气候变化以及土地利用变化,是影响陆地生态系统碳储量的主要生态机制,但不确定在过去的10～100年以及未来哪一种机制起最主要的作用。     

江西省森林碳蓄积过程及碳源/汇的时空格局

森林碳蓄积是研究森林与大气碳交换以及估算森林吸收或排放含碳气体的关键参数,不同年龄森林的碳源/汇功能差异则体现出森林生态系统碳蓄积过程的时间特征。以森林资源清查的样方数据作为数据源,通过刻画主要树种的林分蓄积生长曲线、林龄与净初级生产力(NPP)之间的关系,驱动区域碳收支模型(InTEC)模拟江西省1950—2008年的森林碳蓄积过程,了解山江湖工程实施以来的森林碳源/汇状况。结果表明,20世纪80年代以前,江西省森林年平均NPP波动于450—813 gCm-2a-1之间,年净增生物量碳26.55—36.23 TgC/a,年净增木质林产品碳0.01—0.3 TgC/a;80年代初,NPP和年净增生物量碳分别降至307.39 gC m-2a-1和17.31 TgC/a,而年净增木质林产品碳却高达0.6 TgC/a,说明森林被大量砍伐进入林产品碳库;1985年山江湖工程实施后,大面积造林使得年净增碳蓄积呈现急剧上升趋势,生物量和木质林产品碳蓄积分别上升至目前的42.37 TgC/a和0.79 TgC/a,而平均NPP值增加缓慢、碳汇功能降低,说明林分质量有待提高;90年代后碳汇功能开始稳步增强,说明造林面积的迅速增加是引起江西省森林碳增汇的主要驱动因素,但未来森林增汇潜力应源于森林生长和有效的经营管理。     

放牧对新疆草地生态系统碳源/汇的影响模拟研究

<正>确评估新疆草地生态系统碳源/汇效应,对区域尺度碳循环研究具有重要意义。放牧是新疆草地生态系统中主要的人类活动,但放牧对草地碳平衡与碳动态的影响还具有很大的不确定性。利用生态系统放牧模型Biome-BGC grazing,通过情景模拟综合评价新疆草地生态系统碳源/汇的动态。结果表明:1)1979—2007年新疆草地生态系统的碳源总量为0.38Pg C,其中由放牧导致的碳释放为0.37Pg C;2)当平均放牧率小于0.24头标准羊/hm~2时,放牧能够促进草地碳固定。研究实现了BiomeBGC grazing模型在区域尺度的应用,研究结果将有助于理解气候变化及放牧对干旱区草地生态系统碳动态变化的驱动机理,对探明干旱区草原生态系统的源/汇特征具有重要意义。     

Genetic differences in fruit-set patterns are determined by differences in fruit sink strength and a source : sink threshold for fruit set

Background and Aims

Fruit set in indeterminate plant species largely depends on the balance between source and sink strength. Plants of these species show fluctuations in fruit set during the growing season. It was tested whether differences in fruit sink strength among the cultivars explained the differences in fruit-set patterns.

Methods

Capsicum was chosen as a model plant. Six cultivars with differences in fruit set, fruit size and plant growth were evaluated in a greenhouse experiment. Fruit-set patterns, generative and vegetative sink strength, source strength and the source : sink ratio at fruit set were determined. Sink strength was quantified as potential growth rate. Fruit set was related to total fruit sink strength and the source : sink ratio. The effect of differences observed in above-mentioned parameters on fruit-set patterns was examined using a simple simulation model.

Key Results

Sink strengths of individual fruits differed greatly among cultivars. Week-to-week fruit set in large-fruited cultivars fluctuated due to large fluctuations in total fruit sink strength, but in small-fruited cultivars, total fruit sink strength and fruit set were relatively constant. Large variations in week-to-week fruit set were correlated with a low fruit-set percentage. The source : sink threshold for fruit set was higher in large-fruited cultivars. Simulations showed that within the range of parameter values found in the experiment, fruit sink strength and source : sink threshold for fruit set had the largest impact on fruit set: an increase in these parameters decreased the average percentage fruit set and increased variation in weekly fruit set. Both were needed to explain the fruit-set patterns observed. The differences observed in the other parameters (e.g. source strength) had a lower effect on fruit set.

Conclusions

Both individual fruit sink strength and the source : sink threshold for fruit set were needed to explain the differences observed between fruit-set patterns of the six cultivars.     

Implications of a large global root biomass for carbon sink estimates and for soil carbon dynamics

Recent evidence suggests that significantly more plant carbon (C) is stored below ground than existing estimates indicate. This study explores the implications for biome C pool sizes and global C fluxes. It predicts a root C pool of at least 268 Pg, 68% larger than previously thought. Although still a low-precision estimate (owing to the uncertainties of biome-scale measurements), a global root C pool this large implies stronger land C sinks, particularly in tropical and temperate forests, shrubland and savanna. The land sink predicted from revised C inventories is 2.7 Pg yr(-1). This is 0.1 Pg yr(-1) larger than current estimates, within the uncertainties associated with global C fluxes, but conflicting with a smaller sink (2.4 Pg yr(-1)) estimated from C balance. Sink estimates derived from C inventories and C balance match, however, if global soil C is assumed to be declining by 0.4-0.7% yr(-1), rates that agree with long-term regional rates of soil C loss. Either possibility, a stronger land C sink or widespread soil C loss, argues that these features of the global C cycle should be reassessed to improve the accuracy and precision of C flux and pool estimates at both global and biome scales.