The Muwekma Ohlone Tribe of the San Francisco Bay Area conducted a cultural burn in oak woodland as part of cultural revitalization efforts. This low-intensity burn only moderately affected soil nematodes that infest acorn-feeding insects. The nematodes were frequently found after the burn, even within burned piles. This finding suggests that cultural burning may help to prevent outbreaks of acorn-feeding insects not only directly by killing them with heat, but also indirectly by having only minor effects on the soil nematodes. Preventing outbreaks of acorn-feeding insects increases the likelihood of more acorns being available for wildlife and cultural use.

Pollination involves plants, pollinators, and microorganisms, challenging the traditional bipartite understanding. Floral nectar is the primary medium where tripartite plant-pollinator-microbe interactions occur. These interactions face anthropogenic disruptions temporally, geographically, and across diverse taxa. Herbarium specimens can provide untapped historical records to understand their long-term response to anthropogenic change.

Populations are regulated by processes operating at both local and regional scales. As a result, species can interact not only within local communities but also through regional-scale processes. However, the potential feedback between local and regional processes in shaping metapopulation dynamics has rarely been tested experimentally. Using nectar microbes as a model system, we examine how a bacterial sink species, sustained by external supply, can interact with a resident yeast species directly by modifying local nectar chemistry and indirectly by altering metapopulation connectivity. We show that the external supply of bacterial sinks increases resident yeast densities, potentially through the provision of additional resources, and that this benefit is further enhanced when bacterial supply biases dispersal away from bacteria-inoculated patches. We supplemented our experimental results by analyzing a simple two-patch metapopulation model and demonstrated that metapopulation productivity is maximized when patch-specific dispersal rate is lower in the more productive patch (e.g., productivity enhanced by bacterial sink supply). Together, these results underscore the role of dispersal in shaping how a metapopulation experiences density-dependence across heterogeneous landscapes, highlight the relevance of material flows across ecosystem boundaries, and suggest the plausible mechanism in which transient invaders may leave lasting impacts on ecological communities. ### Competing Interest Statement The authors have declared no competing interest.

Heatwaves can affect species abundances by changing how species interact with one another in local communities. These effects can be complex and remain poorly understood, especially in cases where the outcome of species interactions is contingent on the history of species arrival. We studied how heatwaves affect interactions between the yeast Metschnikowia reukaufii and the bacterium Acinetobacter nectaris , both commonly found in the floral nectar of Diplacus aurantiacus , a hummingbird-pollinated shrub native to California. The microbes were introduced to artificial nectar in different orders of arrival in the presence or absence of simulated two-day heatwaves. We found that heatwaves made yeast—bacterium interactions more contingent on arrival history, thereby causing large variation in nectar acidity, a factor known to affect hummingbird preference and seed production. In the absence of heatwaves, Acinetobacter always became abundant regardless of arrival history, suppressing Metschnikowia and reducing nectar pH. In contrast, in the presence of heatwaves, Acinetobacter dominance depended on arrival history and heatwave timing. If Acinetobacter arrived after Metschnikowia during a heatwave, Metschnikowia suppressed Acinetobacter substantially enough to keep nectar pH at a high level. These results suggest that heatwaves shift species interactions from determinism to historical contingency, with both taxonomic and functional consequences.

Priority effects, in which species arrival history influences community assembly, are increasingly recognized to affect host-parasite systems. However, priority effects across disparate groups of parasitic organisms are poorly understood despite the wide range of taxonomic groups involved. In California oak woodland, we investigated how priority effects between two insect-parasitic fungi (Metarhizium and Beauveria) influenced emergence of nematodes from insect larvae. Field and laboratory results indicated that both fungi were common, but priority effects prevented them from co-emerging from the same larva. Metarhizium- and Beauveria-infected insects did not differ in the species composition of emerging nematodes, but larvae without fungal emergence had distinct nematode communities, with Oscheius almost always emerging without fungi. Experiments indicated that none of the commonly found nematodes (Acrobeloides, Mesorhabditis, Oscheius, and Rhabditis) were entomopathogenic, but that Oscheius could exclude Beauveria if it arrived early. This time-dependent exclusion was likely caused by a bacterium that Oscheius nematodes carried (Serratia proteamaculans). Together, these findings suggest that fungi enter insects as primary arrivers, while nematodes come as secondary arrivers to exploit fungus-killed insects, with priority effects influencing both groups. We suggest that this system is a promising natural microcosm for understanding priority effects across disparate groups in host-parasite systems.

Dispersal regulates patterns of diversity in metacommunities, but a primary focus on dispersal rates may mischaracterize effects that actually arise from dispersal kernels. Kernels describe probabilistic movements between donor and recipient patches, with steep kernels displaying a rapid decline in dispersal probability with distance, and shallow kernels decaying gradually over longer distances. We used simulations to measure how kernels affect diversity across metacommunity scales and ecological contexts, and developed a novel approach quantifying the effects of environmental filtering, competition, stochasticity, and dispersal on fitness. Metacommunities with shallow kernels followed expectations: emigration increased α-diversity but decreased β-diversity and γ-diversity. Metacommunities with steeper kernels did not follow expectations: steep kernels maintained regional diversity by reducing interspecific competition and stochastic extinctions, while weakening the fitness benefits of low dispersal and costs of high dispersal. Our work suggests dispersal kernels and rates jointly regulate exposure to environmental variation and the balance of assembly mechanisms in metacommunities.

Dispersal rate has long been considered a primary determinant of species diversity in ecological communities. However, this knowledge is mostly built on studies of organisms that disperse actively by themselves or passively via physical forces. For organisms that disperse passively via other organisms, the landscape matrix that affects their vectors should indirectly shape the diversity of the vectored, but this relationship remains poorly understood. We investigated landscape–dispersal–diversity relationships in nectar-inhabiting bacteria that disperse via flower-visiting insects. Field observation and experiments revealed that bacterial diversity and abundance peaked at an intermediate frequency of insect visits, which was in turn determined by the surrounding landscape characteristics observed at the 200-m radius scale. Based on this finding, we discuss the possibility that species diversity tends to be maximized at an intermediate dispersal rate, especially when habitat patches of vectored organisms constitute consumable resources for their vectors. ### Competing Interest Statement The authors have declared no competing interest. JSPS KAKENHI, 23K27246, 24K01777 JST FOREST, JPMJFR2247 the Stanford Doerr School of Sustainability Discovery Grants

Symbiotic microbial communities often appear highly variable in their composition and function in ways that environmental factors alone cannot explain. One potential reason for this variability is priority effects, where historical contingency in arrival order affects how symbionts assemble into communities. Focusing on the luminous bacterium ( Photobacterium mandapamensis ) in the light organ of the sea urchin cardinalfish ( Siphamia tubifer ), we studied how priority effects might influence bacterial symbiont assembly. In in vitro experiments that used three P. mandapamensis strains isolated from the same individual fish, we found that whichever strain arrived first dominated over the other two strains, indicating strong priority effects. We also found that the strains differed in growth and luminosity, and that the bioluminescence of the three-strain community could not be predicted from individual strain performances. These findings suggest that priority effects can be a major process shaping the composition and function of these symbiotic microbial communities. ### Competing Interest Statement The authors have declared no competing interest. National Institutes of Health, DP5-OD026405

Bacteria in the honeybee gut are a well-recognized factor affecting bee health. However, the primary focus of this research has been the hindgut, while the crop, or honey stomach, is assumed to be dominated by environmentally acquired transient taxa that matter little to the bees. To evaluate this assumption, we examined bacterial taxa in the crop and mouth of Apis mellifera and A. cerana japonica foragers and in the nectar of Prunus mume flowers visited by the bees in the Minabe-Tanabe region of Japan. We found that in bacterial composition, the crop was distinct from both the mouth and the nectar, whereas mouth and nectar samples were indistinguishable. Furthermore, the crop remained similar in bacterial composition and diversity, while the mouth showed a sharp drop in alpha diversity and a large increase in beta diversity, from summer to winter. These results refute the conventional assumption, suggesting instead that the crop contains a conserved bacterial community largely distinct from environmental taxa. We also found that strains of a crop-associated species, Apilactobacillus kunkeei, could be season- and host species-specific. Together, these findings suggest that crop-associated bacterial communities should be studied further to better understand the relationship between honeybees and their gut bacteria.

Successional pathways of microbial communities are influenced by the complex interactive dynamics among the resident and immigrating species, along with the interactive feedback loops with their environment. Although studies on microbial communities have described patterns of microbial succession, quantitative evidence of how resident communities respond to immigrating species and how such relationships translate into successional changes remains limited, especially for species-rich communities under natural settings. Here, we carried out a field experiment to investigate how the identity of immigrating species influences the successional pathways of wood-inhabiting fungi. We simulated immigration through inoculations of nine selected wood-inhabiting fungal species and characterized resident fungal communities before and one and two years after the inoculations through DNA metabarcoding. The experiments included 275 naturally fallen and 185 artificially felled fresh logs of Norway spruce, with different log types hosting distinct initial resident communities of fungi and representing different abiotic conditions. While the resident community succession was mostly explained by the log-level abiotic characteristics, the identity of immigrating species also influenced the composition of resident communities, and consequently community succession. The immigrating species influenced resident species mostly negatively, suggesting competitive interactions to be important determinants of community succession. The responses of resident species to the immigrating species were phylogenetically correlated, suggesting that shared traits underlie species interactions in the species-rich wood-inhabiting fungal communities. This study advanced the understanding of community succession in species-rich natural systems by providing experimental evidence that the immigrating species influence community succession through the phylogenetically structured responses of resident species.

Quantification of different processes affecting the assembly of ecological communities remains challenging, especially in species-rich communities. While the role of environmental filtering has generally been well established, fewer studies have experimentally shown how other ecological assembly processes, such as biotic filtering, structure species-rich communities. Here, we studied the relative roles of biotic and environmental filtering in the colonization of wood-inhabiting fungi, a species-rich, highly interactive, and environment-sensitive group of species. We conducted a field experiment where we simulated colonization with inoculations of nine fungal species in habitat patches (i.e., logs) with varying biotic and abiotic conditions. We characterized the local resident communities before the inoculations and the colonization success of the inoculated species after one and two years using DNA metabarcoding. We asked what determined the colonization success of the inoculated species by comparing the predictive performance of alternative models. These models included either only abiotic environmental predictors (i.e., physical log properties) or additionally different aspects of the resident fungal communities (i.e., resident fungal species richness, community composition, and DNA amount) as biotic predictors. While all nine species successfully colonized the logs, the rate of success and the factors explaining their colonization success varied among species. The colonization success of four of the inoculated species was explained mostly by the abiotic environmental variables, while the colonization success of three species was additionally explained by the resident communities. The influential biotic predictors varied from the presence of individual species to the collective presence of multiple species. Finally, for two of the inoculated species, all the models showed poor predictive performance. Our results indicate how environmental and biotic filtering may jointly structure species-rich communities. Overall, the results show that species vary idiosyncratically in their response to biotic and environmental factors, highlighting the need to consider the complexity of species-level responses when predicting community-level changes.

Dispersal plays a central role in shaping patterns of diversity in metacommunities. However, a primary focus on emigration rates may mischaracterize dispersal effects that actually arise from dispersal kernels. Kernels describe probabilistic movements between donor and recipient patches, but the influence of kernel shape on metacommunity diversity remains unclear. We used simulations to measure how kernels affect diversity across metacommunity scales and ecological contexts. We disentangled causes of these patterns using a novel approach quantifying the effects of environmental filtering, competition, stochasticity, and dispersal on fitness. Although metacommunities with shallow kernels followed expectations where emigration increased alpha-but decreased beta- and gamma-diversity, metacommunities with steeper kernels did not. Steeper kernels maintained regional diversity by reducing interspecific competition and stochastic extinctions, with dispersal conferring weaker benefits but less homogenization. Our work suggests dispersal kernels and emigration rates jointly regulate exposure to environmental variation and the balance of assembly mechanisms in metacommunities. ### Competing Interest Statement The authors have declared no competing interest. United States Department of the Treasury, Mississippi Based RESTORE Act Center of Excellence National Science Foundation, https://ror.org/021nxhr62, 2019528, 2033292 Natural Sciences and Engineering Research Council, https://ror.org/01h531d29, 557373-2021

Many cases of animal-mediated dispersal are non-random, with the animals altering their movement pattern in response to the local species composition of the organisms that the vectoring animals disperse. Yet, this dispersal–community feedback has received little attention in metacommunity ecology. We use a mathematical model to show that dispersal–community feedback can promote regional species coexistence. As a well-characterized system, our model focuses on nectar-inhabiting bacteria and yeast that are dispersed by pollinators and affected by priority effects within flowers once dispersed. Model analysis suggests that bacteria and yeast coexist regionally only when their occurrence in flowers influences the frequency of flower visits by pollinators. This microbe–pollinator feedback creates positive density dependence in each plant, causing competitive exclusion at the plant scale, but spatial partitioning across multiple plants, realizing coexistence at this scale. Our finding highlights dispersal–community feedback as an overlooked potential mechanism of species coexistence. ### Competing Interest Statement The authors have declared no competing interest.

An emerging focus of research at the intersection of botany, zoology, and microbiology is the study of floral nectar as a microbial habitat, referred to as the nectar microbiome, which can alter plant-pollinator interactions. Studies on these microbial communities have primarily focused on yeasts, and it was only about a decade ago that bacteria began to be studied as widespread inhabitants of floral nectar. This review aims to give an overview of the current knowledge on nectar bacteria, with emphasis on evolutionary origin, dispersal mode, effects on nectar chemistry and plant-animal interactions, community assembly, agricultural applications, and their use as model systems in ecological research. We further outline gaps in our understanding of the ecological significance of these microorganisms, their response to environmental changes, and the potential cascading effects.

A long-standing question in biology is whether there are common principles that characterize the development of ecological systems (the appearance of a group of taxa), regardless of organismal diversity and environmental context.1,2,3,4,5,6,7,8,9,10,11 Classic ecological theory holds that these systems develop following a sequenced, orderly process that generally proceeds from fast-growing to slow-growing taxa and depends on life-history trade-offs.2,12,13 However, it is also possible that this developmental order is simply the path with the least environmental resistance for survival of the component species and hence favored by probability alone. Here, we use theory and data to show that the order from fast- to slow-growing taxa is the most likely developmental path for diverse systems when local taxon interactions self-organize in light of environmental resistance. First, we demonstrate theoretically that a sequenced development is more likely than a simultaneous one, at least until the number of iterations becomes so large as to be ecologically implausible. We then show that greater diversity of taxa and life histories improves the likelihood of a sequenced order from fast- to slow-growing taxa. Using data from bacterial and metazoan systems,14,15,16,17,18,19 we present empirical evidence that the developmental order of ecological systems moves along the paths of least environmental resistance. The capacity of simple principles to explain the trend in the developmental order of diverse ecological systems paves the way to an enhanced understanding of collective features of life.

Although primarily studied through the lens of community ecology, phenomena consistent with priority effects appear to be widespread across many different scenarios spanning a broad range of spatial, temporal, and biological scales. However, communication between these research fields is inconsistent and has resulted in a fragmented co-citation landscape, likely due to the diversity of terms used to refer to priority effects across these fields. We review these related terms, and the biological contexts in which they are used, to facilitate greater cross-disciplinary cohesion in research on priority effects. In breaking down these semantic barriers, we aim to provide a framework to better understand the conditions and mechanisms of priority effects, and their consequences across spatial and temporal scales.

Priority effects, where the order and timing of species arrival influence the assembly of ecological communities, have been observed in a variety of taxa and habitats. However, the genetic and molecular basis of priority effects remains unclear, hindering a better understanding of when priority effects will be strong. We sought to gain such an understanding for the nectar yeast Metschnikowia reukaufii commonly found in the nectar of our study plant, the hummingbird-pollinated Diplacus (Mimulus) aurantiacus. In this plant, M. reukaufii can experience strong priority effects when it reaches flowers after other nectar yeasts, such as M. rancensis. After inoculation into two contrasting types of synthetic nectar simulating early arrival of M. rancensis, we conducted whole-transcriptome sequencing of 108 strains of M. reukaufii. We found that several genes were differentially expressed in M. reukaufii strains when the nectar had been conditioned by growth of M. rancensis. Many of these genes were associated with amino acid metabolism, suggesting that M. reukaufii strains responded molecularly to the reduction in amino acid availability caused by M. rancensis. Furthermore, investigation of expression quantitative trait loci (eQTLs) revealed that genes involved in amino acid transport and resistance to antifungal compounds were enriched in some genetic variants of M. reukaufii. We also found that gene expression was associated with population growth rate, particularly when amino acids were limited. These results suggest that intraspecific genetic variation in the ability of nectar yeasts to respond to nutrient limitation and direct fungal competition underpins priority effects in this microbial system.

1. Flowering crops are heavily managed during bloom to both promote pollination and prevent disease. Disease management practices can alter the floral microbiome, including pathogens and nontarget microbes. However, whether agrochemical presence or altered microbiome composition affect pollinator foraging and pollination services is unclear. 2. We assessed effects of orchard management tactics and landscape context on the flower microbiome in almond, Prunus dulcis. Fourteen orchards (five conventional, four organic and five conventional with habitat augmentation) were sampled at early and peak bloom to characterize bacterial and fungal communities associated with floral tissues. The surveys were complemented by an artificial flower experiment to assess effects of fungicides and microbes on honey bee foraging. Finally, a field trial was conducted to test effects of fungicides and microbes on pollination. 3. As bloom progressed, bacterial and fungal abundance and diversity increased across all floral tissue types and management strategies. The magnitude by which microbial abundance and diversity were affected varied, with proximity to apiaries and orchard management having notable effects on bacteria and fungi respectively. 4. Experiments revealed that fungicides reduced nectar removal by honey bees; however, neither fungicide nor microbe treatments affected pollination, as measured through pollen tube initiation and growth. 5.Synthesis and applications. Our results reveal that microbiota associated with flowers of a pollinator-dependent crop are temporally dynamic and sensitive to management practices. However, pollination services in almond may be resilient to both agrochemical disturbance and microbial augmentation of flowers, the latter of which may become more prominent as microbial solutions to disease management are embraced in agroecosystems.

Along with bacteria, fungi can represent a significant component of animal- and plant-associated microbial communities. However, we have only begun to describe these fungi, much less examine their effects on most animals and plants. Bacteria associated with the honey bee, Apis mellifera , have been well characterized across different regions of the gut. The mid- and hindgut of foraging bees house a deterministic set of core species that affect host health, whereas the crop, or the honey stomach, harbors a more diverse set of bacteria that is highly variable in composition among individual bees. Whether this contrast between the two regions of the gut also applies to fungi remains unclear despite their potential influence on host health. In honey bees caught foraging at four sites across the San Francisco Peninsula of California, we found that fungi were less distinct in species composition between the crop and the mid- and hindgut than bacteria. Unlike bacteria, fungi varied substantially in species composition throughout the honey bee gut, and much of this variation could be predicted by the location where we collected the bees. These observations suggest that fungi may be transient passengers and unimportant as gut symbionts. However, our findings also indicate that honey bees could be vectors of infectious plant diseases as many of the fungi we found in the honey bee gut are recognized as plant pathogens.