Tree–grass coexistence in savannas revisited – insights from an examination of assumptions and mechanisms invoked in existing models

Several explanations for the persistence of tree–grass mixtures in savannas have been advanced thus far. In general, these either concentrate on competition‐based mechanisms, where niche separation with respect to limiting resources such as water lead to tree–grass coexistence, or demographic mechanisms, where factors such as fire, herbivory and rainfall variability promote tree–grass persistence through their dissimilar effects on different life‐history stages of trees. Tests of these models have been largely site‐specific, and although different models find support in empirical data from some savanna sites, enough dissenting evidence exists from others to question their validity as general mechanisms of tree–grass coexistence. This lack of consensus on determinants of savanna structure and function arises because different models: (i) focus on different demographic stages of trees, (ii) focus on different limiting factors of tree establishment, and (iii) emphasize different subsets of the potential interactions between trees and grasses. Furthermore, models differ in terms of the most basic assumptions as to whether trees or grasses are the better competitors. We believe an integration of competition‐based and demographic approaches is required if a comprehensive model that explains both coexistence and the relative productivity of the tree and grass components across the diverse savannas of the world is to emerge. As a first step towards this end, we outline a conceptual framework that integrates existing approaches and applies them explicitly to different life‐history stage of trees.     

A test of the niche dimension hypothesis in an arid annual grassland

The niche dimension hypothesis predicts that greater numbers of limiting factors can allow greater numbers of species to coexist through species' tradeoffs for different limiting factors. A prediction that follows is that addition of multiple limiting resources to plant communities will increase productivity and simultaneously decrease diversity. Species loss due to limiting resource enrichment might occur through reducing the number of resources that species compete for or by changing the identity of limiting factors. We tested these predictions of the niche dimension hypothesis in an arid annual grassland by adding combinations of nutrients: nitrogen (N), phosphorus (P), and potassium with other elements (O). We found that species number decreased while biomass increased with greater numbers of added resources. In particular, N in combinations with P or O resulted in the greatest species loss, while biomass increased super-additively with N and P together. The addition of greater numbers of added nutrients decreased the availability of light and soil moisture, consistent with a potential shift in the identity of limiting resources. Species also differed in their responses to different combinations of N, P, and O, supporting predictions of resource-ratio tradeoffs. These results are particularly notable because this experiment was conducted during a drought year in an arid grassland (226 mm annual rainfall), which might have been expected to be water-rather than nutrient-limited. Our results support the hypothesis that plant diversity may be maintained by high-dimensional tradeoffs among species in their abilities to compete for multiple limiting factors.     

Effects of multiple timescales of resource supply on the maintenance of species and functional diversity

It is well known that variable resource supply can allow competitors to coexist on a single limiting resource, and this is one mechanism that may explain the maintenance of diversity in paradoxically speciose communities. Ecosystems experience fluctuations in resource supply on a range of timescales, but we have a poor understanding of how multiple frequencies of resource supply affect the maintenance of diversity and community structure. Here we explore this question using a model of phytoplankton competition for a limiting nutrient, parameterized using empirical tradeoffs between rapid growth, nutrient storage capacity and nutrient uptake affinity. Compared to a single frequency of nutrient supply, we find that multiple frequencies of nutrient supply increase functional diversity, by permitting the coexistence of strategies adapted to different frequencies of supply. Species richness is also promoted by multiple modes of nutrient supply, but not as consistently as functional diversity. Although this model is parameterized for phytoplankton, the fundamental dynamics and tradeoffs likely occur in a variety of ecosystems. Our results suggest that the spectrum of temporal variation driving communities should be further investigated in the context of the maintenance of diversity and the functional composition of communities under different environmental regimes.     

科尔沁沙地差巴嘎蒿种群生态位适宜度分析

以生长在科尔沁沙地的差巴嘎蒿(Artemisia halodendron)为研究对象,对其在不同类型沙地上的综合活力指数和生态位适宜度进行计算,主要结论如下:1)在生长旺季,差巴嘎蒿种群综合活力指数随着沙地的固定显著下降。2)在同一生长季内,综合生态位适宜度表现为半固定沙地>固定沙地和流动沙地;水分生态位适宜度表现为流动沙地>半固定沙地>固定沙地。随着土壤含水量的变化,土壤水分的限制土层深度也有所不同:7月各类型沙地均为15~30 cm土层;8月随着雨水的下渗,限制土层也有所加深,为45~60 cm土层;9月除流动沙地为15~30 cm外,半固定沙地和固定沙地均为30~45 cm。3)在同一生长季内的不同生境上种群的限制因子(NF_min)不同:在流动沙地上为土壤有机质,而在半固定沙地和固定沙地上为土壤含水量,且土壤有机质和土壤水分的配置关系直接影响综合生态位适宜度值的高低。在一定范围内,二者的比值可直接反映有机质对植物细根的有效性,同时种群通过调节细根生长状况来适应限制因子间的配置关系。     

Experimental separation of resource quantity from temporal variability: seedling responses to water pulses

We tested the hypothesis that higher temporal variability in water supply will promote higher species richness of germinating and surviving seedlings using assemblages of 70 species of herbaceous plants from limestone pavement habitats. In a two-factor greenhouse experiment, doubling the total volume of water added led to greater germination (measured as number of germinated seeds and species) and establishment (survival and biomass) but the effects of temporal variability depended on the response variable considered. Low pulse frequencies of water addition with total volume added held constant resulted in greater temporal variability in soil moisture concentration that in turn promoted higher density and richness of germinated seedlings. Low pulse frequencies caused an eight-fold greater mortality in the low total volume treatment and biomass production to decline by one-third in the high total volume treatment. The effects of increasing temporal variability in water supply during recruitment stages can thus be opposite on different components of plant fitness and may also depend on total resource quantity. While greater species richness in more temporally variable soil moisture conditions was attributable to sampling effects rather than species-specific responses to the water treatments, species relative abundances did vary significantly with temporal variability. Changes in the amplitude or frequency of resource fluctuations may alter recruitment patterns, and could have severe and relatively rapid effects on community structure in unproductive ecosystems.Electronic Supplementary Material The French version of this article is available in the form of electronic supplementary material at.     

GENETIC COVARIANCE OF FITNESS CORRELATES: WHAT GENETIC CORRELATIONS ARE MADE OF AND WHY IT MATTERS

The genetic variance-covariance matrix, G, is determined in part by functional architecture, the pathways by which variation in genotype influences phenotype. I develop a simple architectural model for G for two traits under directional selection constrained by their dependence on a common limiting resource. I assume that genetic variance is maintained by mutation-selection balance. The relative numbers of loci that play a role in acquiring versus allocating a limiting resource play a crucial role in determining genetic covariance. If many loci are involved in acquiring a resource, genetic covariance may be either negative or positive at equilibrium, depending on the fitness function and the input of mutational variance. The form of G does not necessarily reveal the constraint on resource acquisition inherent in the system, and therefore studies estimating G do not test for the existence of life-history tradeoffs. Characters may evolve in patterns that are unpredictable from G. Experiments are suggested that would indicate if this model could explain observations of positive genetic covariance.     

Positive interactions among competitors can produce species-rich communities

Although positive interactions between species are well documented, most ecological theory for investigating multispecies coexistence remains rooted in antagonistic interactions such as competition and predation. Standard resource-competition models from this theory predict that the number of coexisting species should not exceed the number of factors that limit population growth. Here I show that positive interactions among resource competitors can produce species-rich model communities supported by a single limiting resource. Simulations show that when resource competitors reduce each others' per capita mortality rate (e.g. by ameliorating an abiotic stress), stable multispecies coexistence with a single resource may be common, even while the net interspecific interaction remains negative. These results demonstrate that positive interactions may provide an important mechanism for generating species-rich communities in nature. They also show that focusing on the net interaction between species may conceal important coexistence mechanisms when species simultaneously engage in both antagonistic and positive interactions.     

Effects of resource availability on seedling recruitment in a fire-maintained savanna

The herbaceous ground cover of the longleaf pine ecosystem harbors the highest plant species richness in North America, with up to 50 species per square meter, but the mechanisms that regulate this diversity are not well understood. In this system, variability in seedling recruitment events may best explain the extremely high small-scale species richness and its relationship to soil moisture and system net primary productivity. To understand the potential mechanistic controls on species richness, we used a long-term resource manipulation study across a natural soil moisture gradient to assess environmental controls on seedling recruitment. We considered the availability of resources to be an indicator of seedling safe-site supply, and also manipulated seed availability to examine the relative importance of recruitment limitations on seedling diversity. We found that water availability regulated the number of species in the seedling community regardless of the underlying natural moisture gradient, and that this effect may result from differential responses of seedling guilds to resource availability. Water supply was more important than seed supply in determining seedling establishment, suggesting that appropriate sites for regeneration are a factor limiting seedling success. This is the first study that shows that the episodic supply of microsites for recruitment could influence species richness in the highly threatened and biodiverse longleaf pine savanna.     

Soil moisture and sex ratio in a plant with nuclear-cytoplasmic sex inheritance

I investigated whether soil moisture affects relative fitness of females and hermaphrodites and sex ratio in a gynodioecious plant with nuclear-cytoplasmic sex inheritance. I contrast these results with those from species with strictly nuclear sex inheritance. I performed a manipulative watering experiment on seed fitness of the two sexes, and field studies measuring seed fitness and sex ratio as a function of soil moisture. In the dry site, watered hermaphrodites produced approximately twice as many seeds as unwatered hermaphrodites, with little treatment effect on female seed production. Over a natural soil moisture gradient, the ratio of female to hermaphrodite seed production was higher in dry than in wet sites. These data show that the seed fitness advantage of females is a function of soil moisture. Despite this, regression of soil moisture on the sex ratio of 23 populations was not significant. These results indicate a sex-dependent effect of soil moisture on resource allocation to seeds that does not translate into a strong effect on sex ratio. This is consistent with theory based on genomic conflict in which sex ratios are predicted to be only partly determined by fitness differences of the sexes.     

A minimum stochastic model evaluating the interplay between population density and drift for species coexistence

Despite the general acknowledgment of the role of niche and stochastic process in community dynamics, the role of species relative abundances according to both perspectives may have different effects regarding coexistence patterns. In this study, we explore a minimum probabilistic stochastic model to determine the relationship of populations relative and total abundances with species chances to outcompete each other and their persistence in time (i.e., unstable coexistence). Our model is focused on the effects drift (i.e., random sampling of recruitment) under different scenarios of selection (i.e., fitness differences between species). Our results show that taking into account the stochasticity in demographic properties and conservation of individuals in closed communities (zero-sum assumption), initial population abundance can strongly influence species chances to outcompete each other, despite fitness inequalities between populations, and also, influence the period of coexistence of these species in a particular time interval. Systems carrying capacity can have an important role in species coexistence by exacerbating fitness inequalities and affecting the size of the period of coexistence. Overall, the simple stochastic formulation used in this study demonstrated that populations initial abundances could act as an equalizing mechanism, reducing fitness inequalities, which can favor species coexistence and even make less fitted species to be more likely to outcompete better-fitted species, and thus to dominate ecological communities in the absence of niche mechanisms. Although our model is restricted to a pair of interacting species, and overall conclusions are already predicted by the Neutral Theory of Biodiversity, our main objective was to derive a model that can explicitly show the functional relationship between population densities and community mono-dominance odds. Overall, our study provides a straightforward understanding of how a stochastic process (i.e., drift) may affect the expected outcome based on species selection (i.e., fitness inequalities among species) and the resulting outcome regarding unstable coexistence among species.     

Gene frequencies of DRB3.2 locus of Argentine Creole cattle

Gene and genotype frequencies of BoLA-DRB3 were studied in seven herds of Argentine Creole cattle. Twenty-one out of thirty previously identified alleles were detected in this breed. The F statistics showed high degree of variability among the studied subpopulations suggesting that subdivision and genetic drift, rather than inbreeding, have been the major factors acting on the observed interpopulation variability. The observed high degree of genetic variation in Argentine Creole cattle could be crucial for the long-term survival of this population. Maintenance of such polymorphism, as a genetic resource, could be an important issue that will demand attention in future breeding programmes in species under high selective pressures.     

Coexistence of competitors in metacommunities due to spatial variation in resource growth rates; does R * predict the outcome of competition?

Simple mathematical models are used to investigate the coexistence of two consumers using a single limiting resource that is distributed over distinct patches, and that has unequal growth rates in the different patches. Relatively low movement rates or high demographic rates of an inefficient resource exploiter allow it to coexist at a stable equilibrium with a more efficient species whose ratio of movement to demographic rates is lower. The range of conditions allowing coexistence depends on the between‐patch heterogeneity in resource growth rates, but this range can be quite broad. The between‐patch movement of the more efficient consumer turns patches with high resource growth rates into sources, while low‐growth‐rate patches effectively become sinks. A less efficient species can coexist with or even exclude the more efficient species from the global environment if it is better able to bias its spatial distribution towards the source patches. This can be accomplished with density independent dispersal if the less efficient species has a lower ratio of per capita between‐patch movement rate to demographic rates. Conditions that maximize the range of efficiencies allowing coexistence of two species are: a relatively high level of heterogeneity in resource growth conditions; high dispersal (or low demographic rates) of the superior competitor; and low dispersal (or high demographic rates) of the inferior competitor. Global exclusion of the more efficient competitor requires that the inferior competitor have sufficient movement to also produce a source‐sink environment.     

Plant feedbacks increase the temporal heterogeneity of soil moisture

Plant feedbacks on resource levels are well-known, but feedbacks on resource variability have received little attention. Semi-arid grasslands have greater temporal heterogeneity of rainfall than mesic forests, leading to the possibility that grasses further enhance this variability as a mechanism for excluding woody plants originating in habitats with less heterogeneity. Here we test the hypothesis that grasses create greater levels of temporal heterogeneity of soil resources than do woody plants. We used monocultures of five replicate species of both growth forms. Daily soil moisture measurements taken 10 and 30 cm beneath monocultures over a growing season showed that temporal heterogeneity was significantly greater under grasses than under woody plants. This occurred during a dry period when plants are most likely to compete for moisture. Differences in temporal heterogeneity between growth forms were related to differences in their abilities to reduce soil moisture: during the dry period, the net effect of vegetation on moisture 10 cm deep was greatest under grasses. Although the rate of change of soil moisture was higher under grasses, the growth forms exploited different depths of soil moisture: soils 10 cm deep were driest under grasses, but soils 30 cm deep were driest under woody species. In summary, grasses increased within-season resource variability in a habitat already characterized by high among-year variability.     

Variation in moisture duration as a driver of coexistence by the storage effect in desert annual plants

Temporal environmental variation is a leading hypothesis for the coexistence of desert annual plants. Environmental variation is hypothesized to cause species-specific patterns of variation in germination, which then generates the storage effect coexistence mechanism. However, it has never been shown how sufficient species differences in germination patterns for multispecies coexistence can arise from a shared fluctuating environment. Here we show that nonlinear germination responses to a single fluctuating physical environmental factor can lead to sufficient differences between species in germination pattern for the storage effect to yield coexistence of multiple species. We derive these nonlinear germination responses from experimental data on the effects of varying soil moisture duration. Although these nonlinearities lead to strong species asymmetries in germination patterns, the relative nonlinearity coexistence mechanism is minor compared with the storage effect. However, these asymmetries mean that the storage effect can be negative for some species, which then only persist in the face of interspecific competition through average fitness advantages. This work shows how a low dimensional physical environment can nevertheless stabilize multispecies coexistence when the species have different nonlinear responses to common conditions, as supported by our experimental data.     

Genetic variability in a population of arbuscular mycorrhizal fungi causes variation in plant growth

Different species of arbuscular mycorrhizal fungi (AMF) alter plant growth and affect plant coexistence and diversity. Effects of within-AMF species or within-population variation on plant growth have received less attention. High genetic variation exists within AMF populations. However, it is unknown whether genetic variation contributes to differences in plant growth. In our study, a population of AMF was cultivated under identical conditions for several generations prior to the experiments thus avoiding environmental maternal effects. We show that genetically different Glomus intraradices isolates from one AMF population significantly alter plant growth in an axenic system and in greenhouse experiments. Isolates increased or reduced plant growth meaning that plants potentially receive benefits or are subject to costs by forming associations with different individuals in the AMF population. This shows that genetic variability in AMF populations could affect host-plant fitness and should be considered in future research to understand these important soil organisms.     

The relationship between species richness and ecosystem variability is shaped by the mechanism of coexistence

Theory relating species richness to ecosystem variability typically ignores the potential for environmental variability to promote species coexistence. Failure to account for fluctuation‐dependent coexistence may explain deviations from the expected negative diversity–ecosystem variability relationship, and limits our ability to predict the consequences of increases in environmental variability. We use a consumer‐resource model to explore how coexistence via the temporal storage effect and relative nonlinearity affects ecosystem variability. We show that a positive, rather than negative, diversity–ecosystem variability relationship is possible when ecosystem function is sampled across a natural gradient in environmental variability and diversity. We also show how fluctuation‐dependent coexistence can buffer ecosystem functioning against increasing environmental variability by promoting species richness and portfolio effects. Our work provides a general explanation for variation in observed diversity–ecosystem variability relationships and highlights the importance of conserving regional species pools to help buffer ecosystems against predicted increases in environmental variability.     

Coexistence and community structure in a consumer resource model with implicit stoichiometry

We combine stoichiometry theory and optimal foraging theory into the MacArthur consumer-resource model. This generates predictions for diet choice, coexistence, and community structure of heterotroph communities. Tradeoffs in consumer resource-garnering traits influence community outcomes. With scarce resources, consumers forage opportunistically for complementary resources and may coexist via tradeoffs in resource encounter rates. In contrast to single currency models, stoichiometry permits multiple equilibria. These alternative stable states occur when tradeoffs in resource encounter rates are stronger than tradeoffs in elemental conversion efficiencies. With abundant resources consumers exhibit partially selective diets for essential resources and may coexist via tradeoffs in elemental conversion efficiencies. These results differ from single currency models, where adaptive diet selection is either opportunistic or selective. Interestingly, communities composed of efficient consumers share many of the same properties as communities based on substitutable resources. However, communities composed of relatively inefficient consumers behave similarly to plant communities as characterized by Tilman’s consumer resource theory. The results of our model indicate that the effects of stoichiometry theory on community ecology are dependent upon both consumer foraging behavior and the nature of resource garnering tradeoffs.     

Notes on Shetland lichens: I

ABSTRACT

Background: According to modern coexistence theory, ecologically similar species can coexist if fitness differences between them are small, or niche differences between them are large. However, these predictions have not been tested extensively in real systems and are difficult to examine in traits-based studies.

Aims: The aim of our study was, by using the carnivorous pitcher plant genus Nepenthes as a model system, to examine (1) species growth ranks as a proxy for fitness; (2) modern coexistence theory at a geographical scale; (3) evidence for pitcher trait-mediated resource partitioning between sympatric Nepenthes species.

Methods: We used growth ranks, obtained from a survey of experienced Nepenthes nursery owners, as a proxy for fitness. Multivariate pitcher-, vegetative- or combined-trait distances, computed from morphometric-trait data from the literature were used as proxies for niche differences. Data on global Nepenthes species-pair sympatry was modelled against these fitness and niche differences.

Results: Niche and growth rate differences were positively and negatively correlated with sympatry, respectively, as expected from theory.

Conclusion: Our results agree with theory and suggest that fitness differences can be approximated from average species growth. Prey- and microhabitat-resource partitioning, operating through divergent pitcher and vegetative traits respectively, are likely mechanisms that stabilise the coexistence of sympatric Nepenthes species.     

Identifying mechanisms that structure ecological communities by snapping model parameters to empirically observed tradeoffs

Theory predicts that interspecific tradeoffs are primary determinants of coexistence and community composition. Using information from empirically observed tradeoffs to augment the parametrisation of mechanism‐based models should therefore improve model predictions, provided that tradeoffs and mechanisms are chosen correctly. We developed and tested such a model for 35 grassland plant species using monoculture measurements of three species characteristics related to nitrogen uptake and retention, which previous experiments indicate as important at our site. Matching classical theoretical expectations, these characteristics defined a distinct tradeoff surface, and models parameterised with these characteristics closely matched observations from experimental multi‐species mixtures. Importantly, predictions improved significantly when we incorporated information from tradeoffs by ‘snapping’ characteristics to the nearest location on the tradeoff surface, suggesting that the tradeoffs and mechanisms we identify are important determinants of local community structure. This ‘snapping’ method could therefore constitute a broadly applicable test for identifying influential tradeoffs and mechanisms.     

Resolving the biodiversity paradox

The paradox of biodiversity involves three elements, (i) mathematical models predict that species must differ in specific ways in order to coexist as stable ecological communities, (ii) such differences are difficult to identify, yet (iii) there is widespread evidence of stability in natural communities. Debate has centred on two views. The first explanation involves tradeoffs along a small number of axes, including 'colonization-competition', resource competition (light, water, nitrogen for plants, including the 'successional niche'), and life history (e.g. high-light growth vs. low-light survival and few large vs. many small seeds). The second view is neutrality, which assumes that species differences do not contribute to dynamics. Clark et al. (2004) presented a third explanation, that coexistence is inherently high dimensional, but still depends on species differences. We demonstrate that neither traditional low-dimensional tradeoffs nor neutrality can resolve the biodiversity paradox, in part by showing that they do not properly interpret stochasticity in statistical and in theoretical models. Unless sample sizes are small, traditional data modelling assures that species will appear different in a few dimensions, but those differences will rarely predict coexistence when parameter estimates are plugged into theoretical models. Contrary to standard interpretations, neutral models do not imply functional equivalence, but rather subsume species differences in stochastic terms. New hierarchical modelling techniques for inference reveal high-dimensional differences among species that can be quantified with random individual and temporal effects (RITES), i.e. process-level variation that results from many causes. We show that this variation is large, and that it stands in for species differences along unobserved dimensions that do contribute to diversity. High dimensional coexistence contrasts with the classical notions of tradeoffs along a few axes, which are often not found in data, and with 'neutral models', which mask, rather than eliminate, tradeoffs in stochastic terms. This mechanism can explain coexistence of species that would not occur with simple, low-dimensional tradeoff scenarios.