Evidence for a three-way trade-off between nitrogen and phosphorus competitive abilities and cell size in phytoplankton

Trade-offs among functional traits are essential for explaining community structure and species coexistence. While two-way trade-offs have been investigated in many systems, higher-dimensional trade-offs remain largely hypothetical. Here we demonstrate a three-way trade-off between cell size and competitive abilities for nitrogen and phosphorus in marine and freshwater phytoplankton. At a given cell size, competitive abilities for N and P are negatively correlated, but as cell size increases, competitive ability decreases for both nutrients. The relative importance of the two trade-off axes appears to be environment dependent, suggesting different selective pressures: freshwater phytoplankton separate more along the N vs. P competition axis, and marine phytoplankton separate more along the nutrient competition vs. cell size axis. Our results demonstrate the multidimensional nature of key trade-offs among traits and suggest that such trade-offs may drive species interactions and structure ecological communities.     

Ideal dominance distributions: a test using red-winged blackbirds (Agelaius phoeniceus)

Summary The ideal dominance distribution model predicts that competition between individuals of a species for territories will result in socially dominant individuals acquiring territories in higher quality habitat than their subordinates. Although the dispersion and relative reproductive success of male red-winged blackbirds (Agelaius phoeniceus) across habitats in eastern Ontario appears to conform to the ideal dominance distribution model, data from a study of three captive groups comprised of males from both high (marsh) and low (upland field) quality habitats failed to support the prediction that males from marsh habitat are dominant to those from upland habitat. Contrary to the prediction males from uplands were generally dominant to males from marshes. We found a significant positive correlation between dominance and both increased epaulet size and increased body size. Controlling for these positive effects, upland males remained generally dominant to marsh males. Measurements of independent samples of males from both habitats indicated that the overall distribution of males does not conform to ideal dominance. We suggest that the strong between-year territory fidelity shown by male red-winged blackbirds and chance events when they initially acquire territories may contribute to this lack of conformity.     

Patch use and vigilance by sympatric lemmings in predator and competitor-driven landscapes of fear

Prey living in risky environments can adopt a variety of behavioral tactics to reduce predation risk. In systems where predators regulate prey abundance, it is reasonable to assume that differential patterns of habitat use by prey species represent adaptive responses to spatial variation in predation. However, patterns of habitat use also reflect interspecific competition over habitat. Collared (Dicrostonyx groenlandicus) and brown (Lemmus trimucronatus) lemmings represent such a system and possess distinct upland tundra versus mesic meadow habitat preferences consistent with interspecific competition. Yet, we do not know whether this habitat preference might also reflect differences in predation risk or whether the two species differ in their behavioral tactics used to avoid predation. We performed experiments where we manipulated putative predation risk perceived by lemmings by increasing protective cover in upland and meadow habitats while we recorded lemming activity and behavior. Both lemming species preferentially used cover more than open patches, but Dicrostonyx was more vigilant than Lemmus. Both species also constrained their activity to protective patches in upland and meadow habitats, but during different periods of the day. Use of cover and vigilance were independent of habitat, suggesting that both species live in a fearsome but flattened landscape of fear at Walker Bay (Nunavut, Canada), and that their habitat preference is a consequence of competition rather than predation risk. Future studies aiming to map the contours of fear in multi-prey–predator systems should consider how predation and competition interact to modify prey species’ habitat preference, patch use, and vigilance.     

Managing Aquatic Species of Conservation Concern in the Face of Climate Change and Invasive Species

Abstract:  The difficult task of managing species of conservation concern is likely to become even more challenging due to the interaction of climate change and invasive species. In addition to direct effects on habitat quality, climate change will foster the expansion of invasive species into new areas and magnify the effects of invasive species already present by altering competitive dominance, increasing predation rates, and enhancing the virulence of diseases. In some cases parapatric species may expand into new habitats and have detrimental effects that are similar to those of invading non-native species. The traditional strategy of isolating imperiled species in reserves may not be adequate if habitat conditions change beyond historic ranges or in ways that favor invasive species. The consequences of climate change will require a more active management paradigm that includes implementing habitat improvements that reduce the effects of climate change and creating migration barriers that prevent an influx of invasive species. Other management actions that should be considered include providing dispersal corridors that allow species to track environmental changes, translocating species to newly suitable habitats where migration is not possible, and developing action plans for the early detection and eradication of new invasive species.     

Social dominance in migrant and resident turkey vultures at carcasses: evidence for a despotic distribution?

We placed carcasses in three different vegetation types in the heterogeneous savannas of central Venezuela to investigate the role of social dominance in habitat use by flocking migrant and resident turkey vultures (Cathartes aura meridionalis and C. a. ruficollis). Migrants foraged primarily in savanna habitats while residents foraged almost exclusively in gallery forest. In the gallery forest residents discovered carrion first significantly more often than migrants, despite there being equal densities of residents and migrants foraging over this habitat. Because residents fed in smaller groups than migrants at carcasses they had higher feeding rates. There was also a negative relationship between group sizes of residents and migrants. The feeding rate of residents declined in response to increased group size of migrants, but group size of residents had no effect. Migrant group size also had a greater effect on resident feeding rates than king vulture presence or absence. When the effect of migrant and resident group size on feeding rates in migrants was compared, the most significant factor was migrant group size. A second analysis showed that both resident group size and presence or absence of king vultures had a significant effect on feeding rates in migrants. Rates of agonistic encounters in migrant and resident turkey vultures increased weakly in relation to group size. However, there was an increase in residents' encounter rate with migrants in relation to increased migrant group size; there was no difference in resident encounter rates with other residents in relation to resident group size. Migrants dominated residents in almost all agonistic interactions over carcasses. We suggest that savanna habitats were less attractive to residents for foraging because they held larger groups of migrants.     

An analysis of water level dynamics in Esteros del Ibera wetland

In an apparent violation of Gause’s principle of competitive exclusion, many metapopulation models of interspecific competition make the claim that identical species can coexist in spatially structured habitats. In these models, it is assumed that extinction and colonization parameters are always the same for both species, independent of the relative abundance of the two species in doubly occupied patches. We show that it is this simplifying assumption that gives an unfair advantage to the regionally rarer species. More realistic assumptions in these models would lead to different conclusions, which indicate that two identical species cannot coexist regionally in a network of many habitat patches.     

Experimental evidence for mediation of competition by habitat succession

Habitat succession is thought to influence the importance of competition in assemblages. Competitive interactions are considered of critical importance in structuring ant assemblages, but field experiments show varied effects. I tested how succession in managed boreal forests affects the outcome of competition from dominant red wood ants, Formica aquilonia, through a removal experiment in replicated stands of three different ages (0-4, 30-40, and 80-100 years old). F. aquilonia abundance was reduced by 87%, and procedural controls showed no nontarget effects. The succession gradient revealed the full range of possible responses from ant species: decreases in 1-4-year-old stands, increases in 30-40-year-old stands, and no effects in 80-100-year-old stands, where diversity was lowest. Habitat succession thus regulates competitive interactions in this system. I propose a model for this system, where competitive effects depend on time since disturbance. In this case, soon after disturbance the dominant species facilitates increases in the abundance of other species. At intermediate times, competition reduces the abundance of some species. Finally, in long-undisturbed habitats, competitors may fail to evolve, particularly in high-stress environments. Interactions between competition and habitat succession may explain why structuring effects of ecologically dominant species appear inconsistent.     

林下和撂荒地生境紫茎泽兰幼苗的生长特征及其种内竞争形态反应

比较研究了林下和撂荒地生境有无竞争条件下紫茎泽兰（Ageratina adenophora（Sprengel））幼苗的生长特征、生长速率、生物量和种内竞争的形态反应。结果表明：（1）2类生境条件下紫茎泽兰植株幼苗形态特征基本一致,无竞争条件下紫茎泽兰幼苗根长和叶片数明显高于在竞争条件下对应指标值;（2）同类生境中,在无竞争条件下紫茎泽兰幼苗的生长速率、生物量积累显著比在竞争条件下的高;（3）不同生境中,撂荒地生境紫茎泽兰幼苗生物量显著高于林下生境的值,在无竞争条件下,林下生境紫茎泽兰幼苗生长速率比撂荒地生境的高,而在竞争条件下反之;（4）林下和撂荒地生境紫茎泽兰幼苗植株竞争强度差异不显著,但在林下生境其地上部分的竞争强度显著大于其在撂荒地生境的值,而根的竞争强度相反。可见,紫茎泽兰幼苗在种群建立的过程中具有不同的生态适应性和形态反应,这可能是其能成功入侵不同生境的一种重要生理生态策略。     

Nonequilibrium coexistence in a competition model with nutrient storage

Resource competition theory predicts that, in equilibrium, the number of coexisting species cannot exceed the number of limiting resources. In some competition models, however, competitive interactions may result in nonequilibrium dynamics, allowing the coexistence of many species on few resources. The relevance of these findings is still unclear, since some assumptions of the underlying models are unrealistic. Most importantly, these models assume that individual growth directly reflects the availability of external resources, whereas real organisms can store resources, thereby decoupling their growth from external fluctuations. Here we study the effects of resource storage by extending the well-known Droop model to the context of multiple species and multiple resources. We demonstrate that the extended Droop model shows virtually the same complex dynamics as models without storage. Depending on the model parameters, one may obtain competitive exclusion, stable equilibrium coexistence, periodic and non-periodic oscillations, and chaos. Again, nonequilibrium dynamics allows for the coexistence of many species on few resources. We discuss our findings in the light of earlier work on resource competition, highlighting the role of luxury consumption, trade-offs in competitive abilities, and ecological stoichiometry.     

Experimental tests of the information-center hypothesis with black vultures (Coragypsatratus) and turkey vultures (Cathartesaura)

In field tests of the information-center hypothesis (ICH) in south Texas with black vultures (Coragyps atratus) and turkey vultures (Cathartes aura), large carcasses were provided and kept under continuous observation. The use vultures made of these bait sites and their patterns of arrival were recorded to evaluate predictions derived from the ICH. Turkey vultures discovered most bait sites (30 of 31) first, but frequently were displaced from the food by later-arriving black vultures. This competitive exclusion by black vultures limited subsequent feeding opportunities for turkey vultures sufficiently that few (27%) returned on subsequent days to bait sites they had previously visited. I found no evidence that those turkey vultures that did return to bait sites acted as leaders for groups of naive birds and led them to bait sites – knowledgeable and naive turkey vultures did not arrive at bait sites together, and groups arriving at bait sites were not larger on subsequent days than on the first days carcasses were available. In contrast, a significantly larger percentage (47%) of knowledgeable black vultures returned to bait sites they had visited on previous days, and the first groups of black vultures arriving at bait sites on subsequent days were significantly larger than the equivalent groups on first days. Nine flocks of black vultures that arrived on subsequent days at bait sites before sunrise (which suggests the birds had commuted directly from a roost) contained knowledgeable birds, and two of these flocks contained both knowledgeable and naive individuals. Overall, 10 of 54 naive tagged black vultures (18.5%) arrived at bait sites under circumstances that suggested they had followed conspecifics to the food from a roost. However, most black vultures apparently found carcasses through independent search or by using local enhancement. Therefore, I conclude that while following from roosts to food sites is a strategy used by black vultures, at this study site it is one they use relatively infrequently. Received: 20 February 1997 / Accepted after revision: 28 June 1997     

Avian Species Richness in Different-Aged Stands of Riparian Forest Along the Middle Rio Grande, New Mexico

Riparian forests are important for maintaining vertebrate species richness in the southwestern United States, but they have become restricted in distribution due to both historical and current management practices. In order to counteract continued loss of this habitat, several mitigation programs were developed in the middle Rio Grande Valley of New Mexico. Three areas ranging from 50 to 140 ha were revegetated with native trees using pole planting and cattle exclosures, and changes in vegetation structure were quantified after 2, 3, and 5 years of growth. As expected, the older site contained the most heterogeneous mix of plant species and the greatest structural diversity. We compared year-round avian use of the revegetated sites with a mature cottonwood forest site of approximately 30 years of age. As the revegetated sites matured and salient habitat features changed, the population dynamics of individual avian species and patterns of guild structure varied. The older revegetated sites showed a greater similarity to the mature cottonwood site, suggesting that reclamation efforts established quality riparian habitats for birds in as little as 5 years. The revegetated sites appeared especially important for Neotropical-migrant birds. We suggest that a mosaic of riparian woodlands containing mixtures of native tree and shrub species of different size classes is necessary to maintain avian species richness in the middle Rio Grande drainage, and probably throughout the southwestern United States.     

Interacting effects of habitat destruction and changing disturbance rates on biodiversity: Who is going to survive?

Changes in disturbance rates due to climate change may increase or decrease diversity, whereas permanent loss of habitat is generally believed to decrease diversity. It is, however, very likely that the effects of disturbances and habitat destruction interact. Understanding such combined effects is essential to predict the response of communities to global changes and in particular which functional types of species are most endangered. Using an individual-based spatially explicit community model, we investigate (1) whether diversity-disturbance curves alter when spatially uncorrelated or autocorrelated habitat destruction is added, and (2) which functional types of species are able to survive under these altered conditions. Model communities consisted of four functional types of species trading off between colonisation ability and competition strength. We found that habitat destruction may alter both height and shape of diversity-disturbance curves: maximum diversity at intermediate disturbance rates may shift to other disturbance rates or even split into two peaks giving rise to bimodal diversity-disturbance relationships with different sub-communities persisting at low and high disturbance rates. Diversity responded differentially depending on how the colonisation-competition trade-off was represented. Our results suggest that, for trade-offs in seed production rate, generally the best coloniser will better withstand the interacting effects of habitat destruction and changing disturbance rates; however, for trade-offs in mean dispersal distances, functional types characterized by intermediate abilities will perform best. We conclude that predictions of the impacts of changing disturbance rates on biodiversity depend on community structure and cannot be made without knowledge of concurrent permanent habitat destruction.     

基于景观中性模型的不同竞争力外来种入侵对栖息地毁坏的响应

栖息地毁坏和生物入侵被认为是全球物种多样性的两大威胁,也是当前研究的热点。同时,两者的共同作用将比单独作用所带来的物种多样性丧失更大。本文基于竞争-扩散均衡机制,考虑了外来种的不同竞争力,结合景观中性模型和元胞自动机模型,模拟了外来种入侵对栖息地毁坏（栖息地破碎和面积丧失）的响应。研究结果表明：1）外来种入侵成功将首先威胁竞争力与其相邻的弱物种;2）栖息地毁坏与种间竞争共同决定着外来种入侵的成功与否。当栖息地破碎促进了与外来种竞争力相邻的本地强物种时,将会抑制入侵;当其抑制了与外来种竞争力相邻的本地强物种时,则会促进入侵。当栖息地丧失促进了外来种相邻的本地强物种时,将会抑制入侵;当栖息地丧失抑制了外来种相邻的本地强物种,将一定程度地促进入侵,但随着丧失面积的增加,则转为抑制。因此,为了抑制不同竞争力的外来种入侵,应采取不同的栖息地保护策略,并保护和促进其竞争力相邻的本地强物种。     

Recent Invasions of the Gulf of Maine: Three Contrasting Ecological Histories

Introduced species are common members of estuarine communities where their role as competitors and predators is of concern, This paper examines the invasion of Gulf of Maine benthic habitats by the ecologically similar alien invertebrates Styela clava, Botrylloides diegensis , and Membranipora membranacea .
Styela clava increased slowly in abundance at study sites in Beverly, Massachusetts and Portsmouth, New Hampshire. We found no evidence of competitive dominance by S. clava , even though it is the competitive dominant in similar habitats elsewhere. Botrylloides diegensis rapidly became a dominant species after its arrival in the Great Bay Estuary, but this dominance was short-lived. B. diegensis persists in the estuary as an early colonist of primary space and as an epibiont on secondary substrates in established communities. Membranipora membranacea became the dominant epiphyte on laminarian kelps within two years. Although M. membranacea overgrew the native epiphytes Obelia geniculata and Electra pilosa in the overwhelming majority of encounters these native species are more common on other algal hosts. Therefore, competitive dominance is not likely a factor in the successful invasion of the Gulf of Maine by M. membranacea .
These species provide evidence for opposing views of the role of competition in mediating community invasion. We show that ecological similarity among species is not an accurate criterion to predict either the mechanism of invasion or the means of persistence. In addition, these data indicate that biological invasions must be examined on broad spatial and temporal scales; short-term or narrowly focused studies can lead to incorrect conclusions.     

Functional Equivalency between Rice Fields and Seminatural Wetland Habitats

Abstract: Evaluating the potential for anthropogenic habitats to act as surrogates for the natural habitats they replace is a key issue in conservation biology. In California, flooded rice fields are used by numerous aquatic birds during winter. If this habitat functions similarly to more natural wetlands, increased flooding may help replace the extensive wetlands that occurred in the region prior to agricultural development. I tested whether food abundance, perceived predation threat, foraging performance, and the way in which birds allocate their time to different behaviors differed between flooded rice fields and seminatural wetlands for several species of aquatic bird. When appropriate, I also compared flooded and unflooded fields. Invertebrate densities did not differ among habitats. Seminatural wetlands had less rice grain but more seeds from other plants than the two rice habitats. The frequency with which predators passed over a feeding area was lower in flooded fields than in unflooded fields or seminatural wetlands. Most differences in feeding performance and time allocation among habitats were small and statistically insignificant. For some species, feeding efficiency was greater in seminatural wetlands than in flooded fields. Increasing attack rates and the amount of time spent feeding when in flooded fields, however, may allow birds to compensate for reduced efficiency. Multivariate analyses showed that group size, predation threat, time of day, date, and water depth often were associated with behaviors, but that these variables rarely accounted for habitat differences. Flooded fields apparently provide equivalent foraging habitat to seminatural wetlands and, because of reduced predation threat, may be a safer habitat for waterbirds. Thus, if managed appropriately, one of the world's dominant forms of agriculture can provide valuable waterbird habitat.     

Flexible habitat selection and interactive habitat segregation in the marine congeners <Emphasis Type="Italic">Idotea baltica</Emphasis> and <Emphasis Type="Italic">Idotea emarginata</Emphasis> (Crustacea,Isopoda)

Habitat segregation among competing species is widespread yet very little is know how this is achieved in practice. In a case study, we examined short-term effects of conspecific and congeneric density on habitat selection in two competing marine isopod species, Idotea emarginata and Idotea baltica. Under semi-natural conditions in large outdoor cylindrical tanks (4 m high; volume 5.5 m³), animal groups of different size and composition had the choice between a set of relevant habitat samples (surface-floating seaweed, the water column, seaweed on the bottom). Habitat selection in both I. baltica and I. emarginata proved to be largely independent of conspecific density (level of intraspecific competition). In single-species treatments, both species showed a similar and stable pattern of distribution, with a clear preference for seaweed on the bottom. In mixed-species treatments (MST), however, the species were largely separated by habitat. While the distribution of I. emarginata was completely unaffected by the mere presence of interspecific competitors, habitat selection of I. baltica changed notably when I. emarginata was present. The habitat use patterns observed in MST conformed to those realized in geographical areas where the two species overlap in distribution: I. emarginata is dominant among decaying seaweed on the sea floor, and I. baltica is the dominant species among surface-floating seaweed. Our findings suggest that habitat segregation between the two species is essentially interactive, resulting from rapid decision-making of I. baltica with respect to habitat selection. The underlying mechanism is discussed. I. emarginata is highly superior to I. baltica in interference competition and rapidly eliminates the latter from one-habitat systems which do not allow I. baltica to escape from this interaction. In more natural, heterogeneous environments, however, I. baltica seems to be able to coexist with the superior competitor due to its broader habitat niche, flexibility in habitat selection, and a behavioural disposition to avoid normally preferred habitats when these are occupied by I. emarginata.     

Die Umsetzung der FFH-Richtlinie in der Agrarlandschaft

The EU Habitats Directive (92/43/EEC) does include provisions for setting up the Natura 2000-network of protected areas based on listed species and habitats, and in addition specific regulations on species protection. Three Quarters of all designated sites (SCI’s) do not only include natural habitat types in a strict sense like forests or water-bodies, but also agricultural land. 18?% of the SCI’s even include between 25 and 50?% agricultural land and 24?% above 50?%. 48 species and three habitat types listed under the Habitats Directive have a clear focus in agriculture. Another eleven habitat types are dependant from a nature-friendly low intensity use or management. A large proportion of these habitats and species are actually in an unfavourable conservation status. The paper analyses the impact of EU nature conservation on agriculture based on the species and habitats falling under the Habitats Directive. On the other hand indirect negative influences of agriculture are discussed, that may have considerable impact on the future development of the conservation status of endangered habitats and species.     

The influence of soil community density on plant-soil feedbacks: An important unknown in plant invasion

Plant-soil feedbacks have been implicated in several successful plant invasions. However, simple identification of a feedback alone may not be enough to establish feedbacks as a mechanism behind plant invasion. I suggest that the relationship between soil community density and plant growth is an important unknown that strongly influences the impact of plant-soil feedbacks. I developed a mathematical model of two-plant species competition with plant-soil feedbacks. Each plant species obligately generates its own soil community. Each soil community then influences both plant species’ growth. The model allows for every possible combination of positive and negative effects of the soil community on plant growth. I model the relationship between soil community density and plant growth with non-linear functional responses. I use a range of plant competitive abilities and feedback scenarios from the literature to explore how different functional responses influence the outcome of plant competition. Sensitivity analysis of the model reveals that altering the relationship between feedback strength and soil community development can reverse the outcome of plant competition. Analysis of the model also shows how the importance of different feedback scenarios depends on the strength of plant competition.     

Mechanistic analysis of interspecific competition using foraging trade-offs: implications for duck assemblages

The role of interspecific competition and resource partitioning in determining the composition of species assemblages is often controversial. In many cases data on species co-occurrence or resource use (prey or habitat) have been interpreted without a clear understanding of how, or even whether, phenotypic differences constrain performance to allow resource partitioning or how these constraints and the density of resources and competitors should shape resource selection by each species. Instead, predictions have been based on assumed constraints, possibly leading to conflicting results. One such controversy involves the role of bill morphology in mediating resource partitioning among dabbling ducks (Anas spp.). To determine whether incorrect assumptions may have contributed to this controversy, I constructed mechanistic models that predict filter-feeding performance for seven species of ducks directly from bill morphology and kinetics and compared these predictions to those of earlier studies that tested the bill morphology hypothesis. The models predicted that species should share a preference for their most profitable (primary) prey while partitioning their less profitable (secondary) prey by size. Consequently, ducks should forage in the same habitats and exhibit high overlap in prey size when competitor/resource ratios are either high or low. In contrast, earlier studies expected that resource partitioning should always be evident, which implicitly assumes that species partition their primary resources. The models also predicted that the ecological similarity of species in assemblages should increase as prey abundance and size variability declines, contrary to the expectations of an earlier study. A more consistent understanding of the mechanisms regulating assemblages of dabbling ducks, and other species, might emerge if patterns of resource use and species co-occurrence were predicted directly from a mechanistic understanding of how performance trade-offs affect resource selection in the context of varying resource and competitor densities.     

Spatial and Seasonal Patterns of Bird Communities in Italian Agroecosystems

Abstract:  Despite agricultural landscapes covering almost 60% of the total land area of Italy, knowledge of the effects of agriculture and its intensification on bird communities is still scarce. I analyzed the effects of land uses on bird diversity and community structure in different farmland habitats of lowland northwestern Italy. I surveyed breeding and overwintering birds with a hierarchically nested sampling design and used generalized linear and mixed models to investigate the relationships between the diversity and abundance of birds and habitat or landscapes attributes. The effects of agriculture on α avian diversity varied with season and spatial scale, whereas nonagricultural habitats (long-term fallows or woodlands) had a generally positive effect that was constant throughout time and space. As the amount of woodland habitat increased, spatial turnover (β diversity) of breeding birds decreased. Arable landscapes supported low levels of avian diversity throughout the year but were favored by emblematic farmland birds that have declined severely in Europe and in the study area. Farmland birds (40% of which are experiencing population declines) were more abundant or foraged more frequently in the less-disturbed habitat types such as fallows, grasslands, and winter stubbles and tended to avoid the prevailing cultivations (maize, vineyard, and wheat). Landscape simplification, the expansion of maize cultivation, winter plowing practices, and the conversion of highly diverse grasslands to tilled lands are likely to be responsible for the local decline of most farmland species (Skylark [Alauda arvensis ], Starling [Sturnus vulgaris ], buntings [Emberiza spp.], and wagtails [Motacilla spp.]) and for the increase of birds that are turning into agricultural pests (Hooded Crow [Corvus corone cornix ]).