Biology subjects

Kraft, N. J. B.

Publications and source records attributed to Kraft, N. J. B..

6 recordsLinked to original sources

bioRxiv2025

Spatial clustering reveals the impact of higher-order interactions in a diverse annual plant community

Spatial patterns are widespread in ecology, but their effects on species interactions remain unresolved, especially in diverse communities. In principle, the degree of spatial clustering could alter the concentration of higher-order interactions, which occur when one (or more) species modifies competition between two others. When species are well mixed, heterospecific neighbors have ample opportunity to modify a competitors interactions with other species. In contrast, species clustering can reduce the concentration of interspecific higher-order interactions. In a field experiment with annual grassland plants in California, we manipulated the spatial arrangement -- but not the number or identity -- of two competitors and measured how they jointly affected a focal individual. We found that focal plants produced more seeds when their competitors were clustered than when they were mixed. These results suggest that interspecific higher-order interactions generally had a stronger competitive (or weaker facilitative) effect than intraspecific ones. However, the effect of clustering varied across species. Larger differences in focal fecundity were correlated with competitors that had greater differences in size and/or functional traits between the spatial arrangements. Additionally, a competitive hierarchy among our study species predicted the effects of clustered versus mixed competitors on focal seed production. Altogether, our work suggests that the spatial arrangement of competitors changes the realized strength of competition in diverse plant communities by modifying the concentration of higher-order interactions. Given the extensive variation in spatial aggregation in plant communities, this mechanism is likely to be a powerful but underappreciated force shaping competition in nature. Significance StatementPlant species coexist in remarkably diverse assemblages throughout the world. Spatial patterns, including aggregation and intermixing, are also widespread in these communities. One potentially underappreciated mechanism that may structure the spatial dynamics of plant communities is interactions that uniquely occur in diverse systems, often called higher-order interactions. Here, we experimentally demonstrated that spatially mediated higher-order interactions operate among annual plants. These higher-order interactions, and their associated changes in competitor size and functional traits, were correlated with the competitive imbalance between competitors. Because both spatial aggregation and competitive hierarchies are widespread in nature, higher-order interactions emerging from their combination may be a more common driver of biodiversity patterns in plant communities than previously thought.

ecology↗

bioRxiv2024

Contrasting effects of competition on plant fitness and distributions along environmental gradients

O_LISpecies distributions have long been understood to depend on the complex interplay between the abiotic environment and biotic interactions. Empirical work in ecological communities has increasingly revealed how competition can mediate species demographic responses to environmental variation, but understanding how the demographic consequences of spatially variable competition manifest in observed distribution patterns remains an important challenge. C_LIO_LIHere, we describe a conceptual framework for characterizing competition-driven mismatches between responses of fitness and occurrence to environmental gradients. We then explore these mismatches for eight annual plant species in an edaphically variable California grassland landscape. C_LIO_LIWe experimentally quantified how species demographic rates (germination rate, fecundity) and fitness respond to spatial variation in the soil environment, either in the presence or absence of naturally occurring neighbors. We also surveyed species occurrence across the study landscape. Combining these demographic and occurrence data, we asked whether observed occurrence patterns are congruent with responses of intrinsic fitness (i.e., fitness in the absence of competitors) to soil gradients. C_LIO_LIWe found that competition altered responses of fitness to the primary soil gradient (soil texture) for many (4/8) species. In turn, occurrence patterns were often poorly or even inversely related to responses of intrinsic fitness to this environmental axis. In contrast, we found that competition had relatively little effect on responses of fitness and occurrence to a secondary soil gradient (soil Ca:Mg). C_LIO_LISynthesis. We demonstrate that spatially variable competition can contribute to mismatched responses of fitness and occurrence to environmental variation. Importantly, these quantitative mismatches depend on the species and environmental gradient in question. Our results caution against assuming that variation in occurrence implies variation in intrinsic fitness (or vice versa) without first disentangling how the abiotic environment and competition impact the demographic processes that underlie species distributions. C_LI

ecology↗

bioRxiv2023

Latitudinal scaling of aggregation with abundance and its consequences for coexistence in species rich forests

The search for simple principles underlying the complex spatial structure and dynamics of plant communities is a long-standing challenge in ecology1-6. In particular, the relationship between the spatial distribution of plants and species coexistence is challenging to resolve in species-rich communities7-9. Analysing the spatial patterns of tree species in 21 large forest plots, we find that rare species tend to be more spatially aggregated than common species, and a latitudinal gradient in the strength of this negative correlations that increases from tropical to temperate forests. Our analysis suggests that latitudinal gradients in animal seed dispersal10 and mycorrhizal associations11,12,13 may jointly generate this intriguing pattern. To assess the consequences of negative aggregation-abundance correlations for species coexistence, we present here a framework to incorporate the observed spatial patterns into population models8 along with an analytical solution for the local extinction risk14 of species invading from low abundances in dependence of spatial structure, demographic parameters, and immigration. For example, the stabilizing effect of the observed spatial patterns reduced the local extinction risk of species when rare almost by a factor of two. Our approach opens up new avenues for integrating observed spatial patterns into mathematical theory, and our findings demonstrate that spatial patterns, such as species aggregation and segregation, can contribute substantially to coexistence in species-rich communities. This underscores the need to understand the interactions between multiple ecological processes and spatial patterns in greater detail.

ecology↗

bioRxiv2020

How many and which species to plant? A multi-trait-based approach to select species to restore ecosystem services

It has been increasingly argued that ecological restoration should focus more on targeting ecosystem services than on species composition of reference ecosystems. In this sense, the role that species play on community assembly and functioning through their functional traits is very relevant, because effect traits mediate ecosystem processes, ultimately resulting in provision of ecosystem services. One major challenge in ecological restoration is to know which species to use that will deliver the target ecosystem services. We developed an algorithm to select the minimum set of species that maximize the functional richness (FRic) and the functional redundancy (FR) of the restored community, a proxy for the provision of multiple ecosystem services and the resilience of the system to environmental changes, respectively. For this, we simulated the restoration of 24 riparian woody communities of the Brazilian Cerrado. Using the species pool of each original local community, we ran restoration simulations for gradually increasing species richness until reaching the total species richness of the original local community. We computed FRic and FR for each simulated restoration community using the traits specific leaf area, maximum plant height and seed mass. Our simulation results indicate that multiple ecosystem services could be restored with an average of 66% of the species of the original community. Moreover, an average of 59% of the species would be needed to restore communities resilient to environmental changes. Our approach contributes to solving one of the major challenges of ecological restoration, which is defining how many and which species should be used to achieve functional targets. We believe this approach can help in projects of restoration by enabling restoration practitioners to select minimum alternative sets of species that optimize the provision of multiple ecosystem services in a resilient restored ecosystem.

ecology↗

bioRxiv2020

Quantifying microbially mediated fitness differences reveals the tendency for plant-soil feedbacks to drive species exclusion among California annual plants

Soil microorganisms influence a variety of processes in plant communities. Many the-oretical and empirical studies have shown that dynamic feedbacks between plants and soil microbes can stabilize plant coexistence by generating negative frequency-dependent plant population dynamics. However, inferring the net effects of soil microbes on plant coexistence requires also quantifying the degree to which they provide one species an average fitness advantage, an effect that has received little empirical attention. We conducted a greenhouse study to quantify microbially mediated stabilization and fitness differences among fifteen pairs of annual plants that co-occur in southern California grasslands. We found that although soil microbes frequently generate negative frequency-dependent dynamics that stabilize plant interactions, they simultaneously mediate large average fitness differences between species. The net result is that if the plant species are otherwise competitively equivalent, the impact of plant-soil feedbacks is often to favor species exclusion over coexistence, a result that only becomes evident by quantifying the microbially mediated fitness difference. Our work highlights that comparing the stabilizing effects of plant-soil feedbacks to the fitness difference they generate is essential for understanding the influence of soil microbes on plant diversity.

ecology↗

bioRxiv2019

Mechanisms underlying higher order interactions: from quantitative definitions to ecological processes

When species simultaneously compete with two or more species of competitor, higher order interactions (HOIs) can lead to emergent properties not present when species interact in isolated pairs. In order to extend ecological theory to multi-competitor communities, ecologists must develop a practical and general definition for HOIs that can be applied to a wide range of competition models. In this paper we propose a definition for HOIs and outline a set of criteria for testing whether a model has or does not have HOIs. These criteria are valuable for empirical ecologists in need of clarity when discussing HOIs in empirical data. We also provide thorough discussion of how our definition compares with previous definitions of HOIs and interaction modification in the literature. In the second part of the paper we demonstrate the steps required for a rigorous test of HOIs in empirical data. To do this we simulate resource competition between three annual plant species which differ in phenology. We then fit phenomenological competition models to the outcome of simulated competition and use these to test for the presence of HOIs. In our simulations, we find the strength of HOIs varies with phenology: species that grow later experience stronger HOIs than earlier growing species. Our simulation shows how HOIs could emerge in ecosystems where resource availability and individual size change rapidly throughout the course of the growing season and where there are differences in the timing of resource acquisition between competitors.

ecology↗