On the centrality and uniqueness of species from the network perspective

Identifying important species for maintaining ecosystem functions is a challenge in ecology. Since species are components of food webs, one way to conceptualize and quantify species importance is from a network perspective. The importance of a species can be quantified by measuring the centrality of its position in a food web, because a central node may have greater influence on others in the network. A species may also be important because it has a unique network position, such that its loss cannot be easily compensated. Therefore, for a food web to be robust, we hypothesize that central species must be functionally redundant in terms of their network position. In this paper, we test our hypothesis by analysing the Prince William Sound ecosystem. We found that species centrality and uniqueness are negatively correlated, and such an observation is also carried over to other food webs.     

Ecology of nutrition and differentiation of the trophic niches of bats (Chiroptera: Vespertilionidae) in floodplain ecosystems of the Samara Bend

A complex analysis of the food range of 15 bat species inhabiting floodplain ecosystems of the Samara Bend has been performed. It is shown that, in bats, an important component of the structuring of their communities is the division of food resources. The guild structure and position of species in the trophic space are described. Seven food guilds consisting of nonspecialized and specialized species are distinguished. It is noted that most species are characterized by a wide overlapping of their trophic niches, which may be a consequence of their weak competition in an environment that is rich in food resources.     

Feeding of the stone sculpin Paracottus knerii (Cottidae) in the littoral of southern Baikal

The food spectrum and seasonal dynamics of feeding of the stone sculpin Paracottus knerii in the littoral of southern Baikal is investigated. Altogether, 29 species of invertebrates are recorded. All year round, the bulk of food, similarly to other areas of the lake, consisted of amphipods; other groups of zoobenthos were represented insignificantly. The significance of particular amphipod species in the food of fish depended on their ecological and morphofunctional traits. The seasonal changes in the species composition and the ratio of food items in the food depended on life cycles of fish and on the migrations of zoobenthic organisms.     

Modulatory complexity of the feeding repertoire in scincid lizards

The kinematics of jaws and tongue, and jaw muscle activity patterns were investigated in the omnivorous lizard Tiliqua rugosa, and the herbivorous Corucia zebrata (Scincidae) during feeding. Small metal markers were inserted into different parts of the skull, the jaws, and the tongue. Video and cineradiographic images were digitized and displacements of the head, jaws, and tongue were quantified. Additionally, muscle activity patterns were recorded, digitized and several variables were determined quantitatively. The effect of food type on the jaw and hyolingual movement patterns and the jaw muscle activity patterns was investigated for both species. The kinematic data indicate that distinct aspects of gape and tongue cycles are modulated in response to the food characteristics. Similarly, in both species, muscle activity patterns are altered in response to the type of food eaten. A comparison of kinematic and electromyographic patterns during intraoral transport cycles for both species shows that these can be related to food characteristics such as toughness and mobility. Differences between both species in the response to changes in food characteristics are minor. Clearly both species are able to fine tune the activation of the jaw muscles, resulting in the appropriate movement patterns for the type of food eaten. Accepted: 30 January 1999     

The relationship between the duration of food web evolution and the vulnerability to biological invasion

I conducted computer simulations of food web evolution and investigated the relationship between the duration of food web evolution and the vulnerability to biological invasion. Model food webs without evolution consisted of animal species with a limited number of prey species and producer species with small intrinsic growth rates. Because these species were not resistant to predation pressure, model food webs without evolution were vulnerable to invasion of powerful omnivores, which had a wide range of feeding preference and a high ecological efficiency. In model food webs without evolution, the number of animal species depending on producer species was small. Therefore, if a producer species invaded and disturbed the base of such food webs, few animal species became extinct. However, model food webs with a long time evolution had a structure that a small number of producer species supported a large number of animal species. When a producer species invaded and disturbed the base of such food webs in this state, many species became extinct by an indirect effect. The mean number of prey species of animal species and the mean intrinsic growth rate of producer species increased rapidly in the early stage of evolution. Therefore, in the early stage of food web evolution, food webs were temporarily resistant to invasion of powerful omnivores. However, this resistibility was not maintained for a long time. The result of this study strongly suggests that food webs change with time, and consequently the vulnerability to invasion changes with time.     

Breeding seasons of sea-birds in the Galapagos Islands

The sea-birds breeding in the Galapagos Islands show a diversity of breeding cycles. Some species have rigidly fixed annual breeding while others breed throughout the year but have peaks of breeding at less than annual intervals. The eight species which have non-annual breeding are probably breeding as often as possible with the interval between the end of a breeding attempt and the start of the next being the time needed to moult the wing and tail feathers. Only one species is definitely known to breed and moult at the same time.
Although there are well marked seasonal fluctuations in the sea temperature, regular sampling failed to demonstrate any regular fluctuations in the surface plankton. The available evidence suggests that food for some sea-birds is erratic and unpredictable. Some non-annual breeding species have their breeding synchronized by severe food shortages which delay breeding, presumably because females cannot find enough food to form eggs, until conditions improve.
Timing of the breeding season in annual breeders is less easily explained but some species may be feeding well away from the islands in areas where there is a regular fluctuation in the food supply. Most of the annual breeders have prolonged breeding seasons and in two species breeding is out of phase on different islands. Perhaps species are influenced by some weak annual variation in food supply which makes it disadvantageous to breed in a few months of the year.     

Food plants and masticants of the batemi of Ngorongoro District,Tanzania1

Based on independent interviews with 25 persons we report 44 edible species of plants used by the Batemi of Sale Division, Ngorongoro District, Tanzania. Thirty-one of these are specifically consumed as food; six species are chewed as thirst quenchers; seven species of gums and resins are chewed; two species are added to food as flavorants; and one species is used in the preparation of honey beer. We report 8 species used as tooth brushes by the Batemi. An additional 35 species of food and beverage plants were observed under cultivation by the Batemi. With these plants the Batemi ingest phytochemicals in a manner that could have positive health effects.     

A note on the food of the Norway lemning

Norway lemming droppings remaining from the high 1963 population in Jämtland (mid-Sweden) were examined microscopically for an evaluation of food species present. The findings from this were compared with the occurrence of these species growing on the mountains. It is tentatively concluded that lemmings are exercising a degree of selectivity in their choice of food species, but the reasons for this choice still remain obscure.     

Food webs robustness to biodiversity loss: the roles of connectance, expansibility and degree distribution

We analyse the robustness of food webs against species loss by considering the influence of several structural factors of the networks, such as connectance, degree distribution and expansibility. The last concept refers to the absence of structural bottlenecks in the food web, whose removal separate the network into large isolate clusters. In theory networks with identical connectance can display different expansibility characteristics. Using the spectral scaling method we studied 17 food networks and classified them as good expansion (GE) and not-GE networks. The combination of GE properties and degree distribution of species permitted the classification of food webs into six different classes. These classes characterize the differences in robustness of food webs to species loss. While the webs having uniform degree distributions and displaying GE properties are the most robust to species loss, the presence of bottlenecks and skewed distribution of the number of links per species make food webs very vulnerable to primary removal of species.     

食物网关键种的判定及其对稳健性的影响

食物网关键种的定量判定方法不仅有助于揭示食物网中的物种关系, 而且有利于对关键物种进行优先保护。本研究以包含49个物种的河流生态系统食物网为对象, 计算网络中表征度性质的指标(点度中心度、中介中心度、紧密中心度), 并根据指标对物种进行聚类, 分析不同类物种影响食物网连接稳健性程度的差异, 探讨不同节点去除方式下节点数量对食物网连接稳健性的影响。结果发现, 聚类将食物网中物种分成三类。I类物种具有相对较高的点度中心度、中介中心度、紧密中心度; II类物种具有相对更低的点度中心度、中介中心度、紧密中心度; III类物种的点度中心度、中介中心度、紧密中心度介于前两者之间。I类物种处于更为重要的地位, 可认为是关键种。本研究为如何判定关键种贡献了新的思路。另外, 升序去除方式下, 连接稳健性呈降低趋势; 降序去除与随机去除方式下, 连接稳健性在持续降低后又有小幅度增加, 这表明去除方式对食物网连接稳健性有影响。三种去除方式下, 食物网连接稳健性发生显著变化对应着几乎相同的去除节点数量, 表明关键种在维持食物网稳健性方面发挥着重要作用。     

A note on the food of the Norway lemning

Norway lemming droppings remaining from the high 1963 population in Jämtland (mid-Sweden) were examined microscopically for an evaluation of food species present. The findings from this were compared with the occurrence of these species growing on the mountains. It is tentatively concluded that lemmings are exercising a degree of selectivity in their choice of food species, but the reasons for this choice still remain obscure.     

The food and feeding habits of Tilapia species in the Lagos Lagoon

The food of Tilapia mariae (Boulenger), Tilapia guineensis * (Bleeker) and Tilapia melano-theron melanotheron * (Rupell) were investigated. It is shown that T. guineensis and T. m. melanotheron which were the more abundant species had similar food habits. The intensity of feeding of both species and the possible competition for food between them are discussed.     

The role of photosynthesis and food uptake for the growth of marine mixotrophic dinoflagellates

Mixotrophy (i.e. combined use of photosynthesis and food uptake for growth) is widespread among marine dinoflagellates. Species with permanent chloroplasts generally display a growth response towards irradiance like an ordinary autotrophic alga. However, some species cannot grow in the light on a standard inorganic nutrient medium, because they require the ingestion of prey for sustained growth. This includes species with various types of chloroplast origin. Only a few species have been shown to be able to grow in the dark if supplied prey. About half of the studied species were primarily phototrophic species, and food uptake marginally increased their growth rates at low irradiances. In the remaining species, food uptake increases to a large degree their growth rate when light is limiting and in some cases even when irradiance is not limiting growth. Some of these species grow relatively fast at high irradiances without food, while other species only grow slowly or cannot even maintain themselves at high irradiances without food. Dinoflagellates, which form symbioses with endo- and ectosymbionts are a very heterogeneous group, which have been studied only sporadically. Some species are clearly primarily phototrophs, while others rely heavily on food uptake for growth.     

Examining the potential effects of species aggregation on the network structure of food webs

One of the key measures that have been used to describe the topological properties of complex networks is the “degree distribution”, which is a measure that describes the frequency distribution of number of links per node. Food webs are complex ecological networks that describe the trophic relationships among species in a community, and the topological properties of empirical food webs, including degree distributions, have been examined previously. Previously, the “niche model” has been shown to accurately predict degree distributions of empirical food webs, however, the niche model-generated food webs were referenced against empirical food webs that had their species grouped together based on their taxonomic and/or trophic relationships (aggregated food webs). Here, we explore the effects of species aggregation on the ability of the niche model to predict the total- (sum of prey and predator links per node), in- (number of predator links per node), and out- (number of prey links per node) degree distributions of empirical food webs by examining two food webs that can be aggregated at different levels of resolution. The results showed that (1) the cumulative total- and out-degree distributions were consistent with the niche model predictions when the species were aggregated, (2) when the species were disaggregated (i.e., higher resolution), there were mixed conclusions with regards to the niche model's ability to predict total- and out-degree distributions, (3) the model's ability to predict the in-degree distributions of the two food webs was generally inadequate. Although it has been argued that universal functional form based on the niche model could describe the degree distribution patterns of empirical food webs, we believe there are some limitations to the model's ability to accurately predict the structural properties of food webs.     

Novel feeding interactions amplify the impact of species redistribution on an Arctic food web

Species are redistributing globally in response to climate warming, impacting ecosystem functions and services. In the Barents Sea, poleward expansion of boreal species and a decreased abundance of Arctic species are causing a rapid borealization of the Arctic communities. This borealization might have profound consequences on the Arctic food web by creating novel feeding interactions between previously non co‐occurring species. An early identification of new feeding links is crucial to predict their ecological impact. However, detection by traditional approaches, including stomach content and isotope analyses, although fundamental, cannot cope with the speed of change observed in the region, nor with the urgency of understanding the consequences of species redistribution for the marine ecosystem. In this study, we used an extensive food web (metaweb) with nearly 2,500 documented feeding links between 239 taxa coupled with a trait data set to predict novel feeding interactions and to quantify their potential impact on Arctic food web structure. We found that feeding interactions are largely determined by the body size of interacting species, although species foraging habitat and metabolic type are also important predictors. Further, we found that all boreal species will have at least one potential resource in the Arctic region should they redistribute therein. During 2014–2017, 11 boreal species were observed in the Arctic region of the Barents Sea. These incoming species, which are all generalists, change the structural properties of the Arctic food web by increasing connectance and decreasing modularity. In addition, these boreal species are predicted to initiate novel feeding interactions with the Arctic residents, which might amplify their impact on Arctic food web structure affecting ecosystem functioning and vulnerability. Under the ongoing species redistribution caused by environmental change, we propose merging a trait‐based approach with ecological network analysis to efficiently predict the impacts of range‐shifting species on food webs.     

Uncoupling the effects of shade and food resources of vegetation on Mediterranean ants: an experimental approach at the community level

Vegetation is one of the main factors affecting the composition and structure of ant communities. The effect of vegetation is both by offering food resources and by modifying the proportion of ground cover exposed to shade conditions. However, it is difficult in field studies to uncouple the effects of food resources and shade on animal communities. The goal of this study was to analyze experimentally the effects of vegetation through food and shade supply on the structure and composition of a Mediterranean ant community. We have crossed these two factors in a factorial design where we have eliminated vegetation in experimental treatments and we have simulated the effects of vegetation by manipulating separately the supply of food resources and shade. The expected decrease in ant abundance and richness in plots without food resources but with shade was only partially confirmed by the results, because there was a decrease in the number of ant species but not a significant decrease in overall ant abundance in plots without food resources. We did not confirm the second hypothesis that the decrease in shade while maintaining food resources resulted in an increase of heat-tolerant, subordinate species and, consequently, ant richness. Species composition differed in the different experimental treatments. Stress-tolerant species were abundant in areas without shade and without food resources. Heat-tolerant species were mainly present in plots without shade but with food resources. Finally, species with high requirements of shade and/or food resources were associated to control plots with both shade and food resources from vegetation. Overall, this study provides an experiment that uncouples experimentally shade and food resources and dissects their effects on the whole ant community, and shows that the effects of the two factors are independent and affect different components of ant community structure and composition.     

The food habits of four small-sized species of Mormyridae from the floodplain pools of the Sokoto-Rima river basin, Nigeria

The diet of four species of the family Mormyridae, commonly found in the floodplain pools of the Sokoto-Rima river basin, Nigeria, is described. All four species are small-sized and show a degree of food overlap. Evidence is presented, however, that resource partitioning occurs enabling co-existence of the species in a habitat where food resources become increasingly limited after the cessation of the rains.     

食物种类对萼花臂尾轮虫种群增长率的影响

在短期慢性观测过程中,食物类型可能是造成萼花臂尾轮虫(Brachionuscalyciflorus)种群繁殖率变化的一种原因。共观测了分别单独投喂10种不同绿藻对轮虫种群增长率的影响。为验证藻青菌是藻类饵料(如绿藻Scenedesmus)有价值的佐剂这一假说,还用蓝细菌单独投喂或与斜生栅藻(Scenedesmusobliquus)混合投喂轮虫进行实验观测。结果发现食物种类对萼花臂尾轮虫种群增长率影响显著。斜生栅藻组获得最大种群增长(1.6/d),而Desmodesmus组增长率最低(0.3/d)。以占斜生栅藻组最大增长的百分率来表示,其它几种绿藻组种群增长由高到低依次为:Desmodesmussubspicatus88%,小球藻(Chlorellavulgaris)83%,单壳缝藻(Monoraphidiumminutum)77%,D.quadricauda74%,S.falcatus71%,S.acuminatus69%,S.pectinatus64%,莱茵衣藻(Chlamydomonasreinhardtii)57%,D.abundans19%。轮虫增长率的差异不能用藻类饵料的大小差异来解释。蓝细菌(Microcystisaeruginosa)和(Synechococcuselongates)不论是单独投喂还是与优良藻类饵料(斜生栅藻)混合投喂都对萼花臂尾轮虫种群增长有抑制作用。这种副作用似乎与微囊藻素无关。该结果不支持无毒蓝细菌可作为与其他绿藻饵料配合使用的优良佐剂这一假说。本研究所观察到的生长变化显示了饵料种类对萼花臂尾轮虫种群增长的影响,也预示了对毒性实验结果的影响