Research

One of the overarching themes of ecology is studying how biotic interactions change under different environmental factors, and how these changes affect community structure and biodiversity. Using a combination of theoretical models, experiments, and large monitoring datasets, I approach this overarching theme from two aspects:

1. How does the timing of species interactions affect the community structure and biodiversity?

2. How do changes in spatial and temporal overlaps between species affect their interactions?

If you are interested in any of the following projects, especially the ongoing ones, or would like a copy of the paper, feel free to contact me!

Functional Shifts of North American Avian Communities - Theme 2

Climate change has led to consistent intraspecific shifts in functional traits, but we don’t know how these species-level patterns translate into community-level changes. For example, if smaller body size is beneficial, then smaller individuals should be selected within species, and smaller species should have an advantage over larger species, leading to an overall decrease in community-weighted mean of body size. With Adam Smith, Brian Weeks and Kai Zhu, I used long-term monitoring data from the North American Breeding Bird Survey to evaluate the functional shifts of North American bird communities on a fine spatial scale. We found that communities shifted toward larger body mass and smaller appendages over time, the opposite of widely documented within-species changes. These unexpected patterns arise from a small proportion (5%) of hyper-dominant species that respond idiosyncratically to human-induced land use changes.

Illustration in the figure was created by Olivia Stein, undergraduate student at the University of Michigan.

Preprint

Zou, H.-X.*, A.C. Smith, K. Zhu, and V.H.W. Rudolf. 2025. Hyper-dominant species drive unexpected functional shifts in North American birds. bioRXiv. [paper] [code]

Opportunities for Novel Species Interactions from Range Shifts - Theme 2

Shifts in species distributions and abundances not only lead to changes in functional composition in communities but also the spatial overlap between species. These changes may lead to new opportunities or declined potential for species interactions. Using long-term monitoring datasets and a novel dataset that documents biotic interactions between North American birds, I am evaluating how changes in spatial co-occurrences may impact species interactions and evaluate where novel, undocumented interactions may have occurred due to range shifts. This is a collaborative effort with Brian Weeks, Kai Zhu, Phoebe Zarneske, and Chia Hsieh.

Ongoing project – stay tuned!

Collecting Interactions from Community Science Platforms - Theme 2

In order to accurately evaluate potential shifts in species interactions under climate change, we need more timely observations in nature. Community science platforms provide a unique opportunity for crowd-sourced, fast-updating documentation of species interactions. I am currently working with a team of undergraduate students through the University of Michigan’s Undergraduate Research Opportunity Program to collect interactions among birds from records on the community science project eBird. This is a collaborative effort with Xiaohao Yang, Phoebe Zarneske, and Brian Weeks.

Ongoing project – stay tuned!

Time-dependence of Plant-microbe Interactions - Theme 1

Plant-microbe interactions are ubiquitous and fundamental to plant community structure, yet theory and experiments often focus on plants and treat microbes as either fixed environments or static snapshots of time. With Jiaqi Tan, we design experiments using duckweeds (subfamily Lemnoideae) and their naturally associated microbiomes to study how the length of interaction between the resident plant and its microbes affect the microbiome composition and its feedback on the future colonizing plant. This research was supported by the Student Research Award from the American Society of Naturalist, and a Small Research Grant from the British Ecological Society.

Ongoing project – stay tuned!

Time-dependence of Higher-order Interactions - Theme 1

Complex communities in nature often contain interactions beyond pairwise (i.e., higher-order interactions). While common, little is known about how they change with the timing of species during community assembly. In nature, the time-dependence of interaction modifications could arise from plant-soil feedback: the different phenology of plants may affect the composition of soil microbiomes and subsequently their effects on other plants. With Xinyi Yan, we first develop a quantitative framework of time-dependent interaction modification, then apply this framework to data generated by a novel mechanistic plant-soil feedback model. We find strong time-dependent interaction modifications when the soil microbiomes of all plants in the community greatly overlaps, which can arise from plants sharing pathogens or mutualists in nature. Our findings highlight the need to consider time in complex communities.

Publication

Zou, H.-X.*, X. Yan, and V.H.W. Rudolf. 2024. Time-dependent intetaction modification generated from plant-soil feedback. Ecology Letters. [paper] [code]

Bridging Theory and Experiments of Priority Effects - Theme 1

Despite a long history and recent advances, the current body of theory only describes priority effects driven by positive frequency dependence, while many empirical studies identify biological mechanisms beyond this mechanism. To bridge this gap, we categorize priority effects as frequency-dependent or trait-dependent based on their underlying biological mechanisms. The two categories represent different biological mechanisms of priority effects that can have distinctive local and regional consequences that are still largely unexplored. Our framework encourages future studies to explicitly consider the biological mechanisms and the time scales at which they operate when studying and modeling priority effects.

Publication

Zou, H.-X.*, and V.H.W. Rudolf. 2023. Bridging theory and experiments of priority effects. Trends in Ecology and Evolution. [paper] [supplemental information] [code and data]

Priority Effects and Dispersal-Diversity Relationships - Theme 1

Priority effects are ubiquitous in nature and determine local community structure, but we know surprisingly little about how it influences biodiversity across different spatial scales. Here, we use a seasonal metacommunity model to show that biodiversity patterns and the homogenizing effect of high dispersal depend on the specific mechanisms underlying priority effects. When priority effects are driven by a numeric advantage, diversity measurements are highly sensitive to initial conditions and high dispersal homogenizes communities. However, when local priority effects arise from phenological shifts that alter species interaction strength (trait-dependent priority effects), local, regional, and temporal diversity are higher and are surprisingly robust to variation in dispersal and initial conditions, given spatiotemporal variations in phenology. Our results highlight that accounting for the mechanisms underlying priority effects is fundamental to understanding the maintenance of regional biodiversity.

Publication

Zou, H.-X.*, and V.H.W. Rudolf. 2023. Priority effects determine how dispersal affects biodiversity in seasonal metacommunities. The American Naturalist. [paper] [code] [data]

Effect of Arrival Time and Number of Generations per Season on Coexistence - Theme 1

The role of priority effects in shaping the long-term dynamics of seasonal communities is poorly resolved. With Sebastian Schreiber, we use a general stage-structured competition model to determine how different mechanisms of priority effects may influence the long-term competition outcomes in seasonal systems. Specifically, we assume that the strength of interspecific competition depends on the stage of both species. This mechanism generates “stage-mediated priority effects” that strongly affect long-term competition outcomes with fewer generations per growing season, but these priority effects diminish when each season consists of more generations. Our model reveals a novel link between the number of generations in a season and the consequences of priority effects, suggesting that consequences of phenological shifts driven by climate change should depend on specific life histories of organisms.

Our experiments with two competing Tribolium flour beetles in the lab largely supported the conclusion of our model, that trait-dependent priority effects were the strongest with only one season and tapered off with two seasons, but did not further weaken with three seasons. This result indicates the complexity between strengths of priority effects and number of generations within a season in real-world communities.

Preprint and Publication

Zou, H.-X.*, and V.H.W. Rudolf. 2025. Consequences of phenological shifts are determined by the number of generations per season. bioRXiv. [paper] [data and code]

Zou, H.-X.*, S.J. Schreiber, and V.H.W. Rudolf. 2023. Stage-mediated priority effects and species life history shape long-term competition dynamics. Proceedings of Royal Society B: Biological Sciences. [paper] [code]

Primary Succession of Plant Community on Industrial Waste

Slag, waste from the steel-making process, contains large amounts of calcium, magnesium, iron, and other heavy metals. While many sites have undergone active restoration, traditional methods such as topsoil capping or phytoremediation performed poorly. Nevertheless, slag hosts surprisingly diverse spontaneous plant communities, but the assembly and structure of such communities are poorly studied. Collaborating with Cathy Pfister and Alison Anastasio, we characterized plant communities on two slag sites in the Calumet region on the south side of Chicago. Together with controlled experiments, we showed that species composition was fundamentally different between slag and non-slag communities and that slag communities recovered slower from disturbance, suggesting a slower succession process. Interestingly, native plants with high conservation values were also spotted on slag, highlighting the potential of industrial sites serving as native plant refugia through habitat reclamation.

Publication

Zou H.-X., A.E. Anastasio, and C.A. Pfister. 2019. Early succession on slag compared to urban soil: A slower recovery. PLoS ONE 14(12): e0224214. [paper] [data]