Investigating Carnivore Guild Structure: Spatial and Temporal Relationships amongst Threatened Felids in Myanmar

The co-occurrence of felid species in Southeast Asia provides an unusual opportunity to investigate guild structure and the factors controlling it. Using camera-trap data, we quantified the space use, temporal activity, and multi-dimensional niche overlap of the tiger, clouded leopard, Asiatic golden cat, marbled cat, and leopard cat in the Htamanthi Wildlife Sanctuary, Myanmar. We hypothesised that the spatio-temporal behaviour of smaller cats would reflect the avoidance of the larger cats, and similar-sized guild members would partition their niches in space or time to reduce resource competition. Our approach involved modelling single-species occupancy, pairwise spatial overlap using Bayesian inference, activity overlap with kernel density estimation, and multivariate analyses. The felid assembly appeared to be partitioned mainly on a spatial rather than temporal dimension, and no significant evidence of mesopredator release was observed. Nonetheless, the temporal association between the three mesopredators was inversely related to the similarity in their body sizes. The largest niche differences in the use of space and time occurred between the three smallest species. This study offers new insight into carnivore guild assembly and adds substantially to knowledge of five of the least known felids of conservation concern.

Niche difference determines coexistence and similar underlying processes in four ecological groups

Understanding how species interactions affect community composition is an important objective in ecology. Yet, the multitude of methods to study coexistence has hampered cross-community comparisons. Here, we standardized niche and fitness differences across 1018 species pairs to compare the processes driving composition and outcomes, among four community types (annual plant, perennial plant, phytoplankton, and bacteria/yeast). First, we show that niche differences are more important drivers of coexistence than fitness differences. Second, in all community types negative frequency dependence is the most frequent process. Finally, the outcome of species interactions differs among community types. Coexistence was the most frequent outcome for perennial plants and phytoplankton, while competitive exclusion was the most prevalent outcome in annual plants and bacteria/yeasts. Overall, our results show that niche and fitness differences can be used as a common currency that allow cross community comparisons to understand species coexistence.

A quantitative synthesis of soil microbial effects on plant species coexistence

Soil microorganisms play a major role in shaping plant diversity, not only through their direct effects as pathogens, mutualists, and decomposers, but also by altering interactions between plants. In particular, previous research has shown that the soil community often generates frequency-dependent feedback loops among plants that can either destabilize species interactions, or generate stabilizing niche differences that promote species coexistence. However, recent insights from modern coexistence theory have shown that microbial effects on plant coexistence depend not only on these stabilizing or destabilizing effects, but also on the degree to which they generate competitive fitness differences. While many previous experiments have generated the data necessary for evaluating microbially mediated fitness differences, these effects have rarely been quantified in the literature. Here we present a meta-analysis of data from 50 studies, which we used to quantify the microbially mediated (de)stabilization and fitness differences derived from a classic plant-soil feedback model. Across 518 pairwise comparisons, we found that soil microbes generated both stabilization (or destabilization) and fitness differences, but also that the microbially mediated fitness differences dominated. As a consequence, if plants are otherwise equivalent competitors, the balance of soil microbe-generated (de)stabilization and fitness differences drives species exclusion much more frequently than coexistence or priority effects. Our work shows that microbially mediated fitness differences are an important but overlooked effect of soil microbes on plant coexistence. This finding paves the way for a more complete understanding of the processes that maintain plant biodiversity.

Order of arrival promotes coexistence via spatial niche preemption by the weak competitor

Historical contingency, such as the order of species arrival, can modify competitive outcomes via niche modification or preemption. However how these mechanisms ultimately modify stabilising niche and average fitness differences remains largely unknown. By experimentally assembling two congeneric spider mite species feeding on tomato plants during two generations, we show that order of arrival interacts with species’ competitive ability to determine competitive outcomes. Contrary to expectations, we did not observe that order of arrival cause priority effects. In fact, coexistence was predicted when the inferior competitor (Tetranychus urticae) arrived first. In that case, T. urticae colonized the preferred feeding stratum (leaves) of T. evansi leading to spatial niche preemption, which equalized fitness but also increased niche differences, driving community assembly to a close-to-neutrality scenario. Our study demonstrates how the spatial context of competitive interactions interact with species competitive ability to influence the effect of order of arrival on species coexistence.

Order of arrival promotes coexistence via spatial niche preemption by the weak competitor

Historical contingency, such as the order of species arrival, can modify competitive outcomes via niche modification or preemption. However, how these mechanisms ultimately modify stabilising niche and average fitness differences remains largely unknown. By experimentally assembling two congeneric spider mite species feeding on tomato plants during two generations, we show that order of arrival interacts with species' competitive ability to determine competitive outcomes. Contrary to expectations, we did not observe that order of arrival cause priority effects. In fact, coexistence was predicted when the inferior competitor (Tetranychus urticae) arrived first. In that case, T. urticae colonized the preferred feeding stratum (leaves) of T. evansi leading to spatial niche preemption, which equalized fitness but also increased niche differences, driving community assembly to a close-to-neutrality scenario. Our study demonstrates how the spatial context of competitive interactions interact with species competitive ability to influence the effect of order of arrival on species coexistence.

Different methods for niche and fitness differences computation offer contrasting explanations of species coexistence

In modern coexistence theory, species coexistence can either arise via stabilizing mechanisms that increase niche differences or equalizing mechanisms that reduce fitness differences.Having a common currency for interpreting these mechanisms is essential for synthesizing knowledge across different studies and systems.Several methods for quantifying niche and fitness differences exist, but it remains unknown to what extent these methods agree on the reasons why species coexist. Here, we apply four common methods to quantify niche and fitness differences to one simulated and two empirical data sets. We ask if different methods result in different insights into what drives species coexistence. We find that different methods disagree on the effects of resource supply rates (simulated data), and of plant traits or phylogenetic distance (empirical data), on niche and fitness differences. More specifically, these methods often do not agree better than expected by chance. We argue for (1) a better understanding of what connects and sets apart different methods, and (2) the simultaneous application of multiple methods to enhance a more complete insight into why species coexist.

Climatic niche pre-adaptation in mainland Europe facilitated the colonization of Madeira by ivies (Hedera L., Araliaceae)

Background and aims: The way plants cope with biotic and abiotic selective pressures determines their success in the colonization of remote oceanic islands, which ultimately depends on the phylogenetic constrains and ecological response of the lineage. In this study we aim to evaluate the relative role of geographical and ecological forces in the origin and evolution of the Madeiran ivy (H. maderensis). Methods: To determine the phylogenetic placement of H. maderensis within the western polyploid clade of Hedera (three species), we analysed 40 populations (92 individuals) using genotyping-by-sequencing and including H. helix as outgroup. Climatic niche differences among the four study species were evaluated using a database with 706 records representing the entire species ranges. To test species responses to climate, a set of 19 vegetative and regenerative functional traits were examined for 70 populations (335 individuals). Key results: Phylogenomic results revealed a nested pattern with H. maderensis embedded within H. iberica. Gradual niche differentiation from the coldest and most continental populations of H. iberica to the warm and stable coastal population sister to H. maderensis parallels the geographical pattern observed in the phylogeny. Similarity in adaptive traits is observed for H. maderensis and H. iberica. The two species show leaves with higher SLA, lower LDMC and thickness and smaller fruits than those of H. hibernica. Conclusions: Acquisition of the Macaronesian climatic niche and the associated functional syndrome in mainland European ivies (small fruits, leaves high SLA, and low LDMD and thickness) was a key step in the colonization of Madeira 1 by the H. iberica/H. maderensis lineage, which points to climatic pre-adaptation as a driver of island colonization (dispersal and establishment). Once in Madeira, speciation was driven by geographical isolation, while ecological processes are regarded as secondary forces with a putative impact in the lack of further in situ diversification.

Pulling on multiple threads of evidence to reveal tree species coexistence

Understanding drivers of species coexistence is a central challenge in ecology. Coexistence cannot be observed directly, and while species co-occurrence in time and space is necessary for coexistence, it is not sufficient to prove coexistence. Species exclusion from a region is potentially observable, but can take decades to occur, and still might occur stochastically. Thus, ecologists generally use theory to identify indirect observations that are indicative of mechanisms driving coexistence or exclusion. Various methods have been developed to indirectly infer coexistence, each of which requires different data, and none of which are usually conclusive on their own. Here, we demonstrate agreement using three different approaches examining coexistence of multiple hardwood species. First, in an experimental planting of three mature tree species we found no relationship between productivity and species diversity, which could be due to a lack of niche differences among species. Second, we used modern coexistence theory to calculate niche and fitness differences for each pair of species, which confirmed the lack of niche differences among species, and showed high fitness differences that could create a neutral distribution of species in nature. Third, we used the United States Department of Agriculture Forest Inventory and Analysis data to examine co-occurrence patterns of our species across thousands of natural forest stands and found that indeed, these three species were distributed randomly throughout the USA. Given that these independent methods agree, we take this as strong evidence about a lack of coexistence.