2022 Spring semester
Sonia Kefi (Montpellier): The multiplexity of ecological communities
In natural communities, species form complex networks of interdependencies that mediate their response to perturbations. So far, ecological network studies have typically focused on one (or a few) interaction types at a time, but data and models of webs including different interaction types simultaneously have recently become available. How and when does the diversity of interactions matter for the dynamics and resilience of ecological systems? I will present recent efforts in analyzing and understanding ecological networks including different types of interactions. I will argue that moving beyond unidimensional analyses of ecological networks may contribute to improving our understanding and predictive capacity of the way ecological systems respond to disturbances.
Adam Clark (Graz): Measuring ecological stability in systems without static equilibria
Ecological systems are typically characterized by complex dynamics, with central tendencies that vary greatly over time. However, most empirically tractable metrics used to measure stability in ecological systems assume the opposite – i.e. that dynamics are simple, and centred around a single static equilibrium. Here, I suggest a heuristic approach for analysing complex ecological dynamics by separately quantifying effects of observation error, process noise, and deterministic dynamics.
Chris Klausmeier (Michigan State, w/ Thomas Koffel, Brian Lerch & Akshata Rudrapatna): Towards a Unified Framework for Metacommunity Ecology
17 May, 2022 Video
Metacommunity ecology extends the metapopulation concept to provide a theoretical framework for understanding multi-species interactions in spatially subdivided landscapes. Despite the widespread interest in metacommunity ecology, the theory is currently loosely organized into disjunct paradigms such as species sorting, patch dynamics, mass effects, and neutral theory. Reconciling these diverse models in a unified framework requires inclusion of three fundamental ecological processes: selection (niche-based processes), ecological drift (stochasticity), and dispersal. In this talk I will present a competitive Lotka-Volterra metacommunity model that includes all of these processes. First we look at open systems, where immigrants come from a mainland source population. Then we look at true metacommunities, where immigrants come from other patches in the landscape. Using efficient numerical techniques to calculate equilibria and invasion criteria, we determine how the regional outcome of competition depends on local interactions, dispersal, and local population size. Finally, we conclude with prospects for future theoretical development.
Lauren Hallett (Oregon): Integrating population and community synchrony across scales
Fluctuations in the abundance of populations – either of a single species across spatially disjunct locations, or of multiple species within a single location – are often correlated through time. Despite conceptual similarities, these two types of synchrony— population and community synchrony, respectively— have been studied as separate processes. In this talk I link theory and methods from population and community synchrony to explore community dynamics and stability across spatial and temporal scales.
Lauren Shoemaker (Wyoming): Persistence and coexistence in variable environments: Leveraging abiotic and biotic variation to maintain ecological diversity
Environmental variability and species interactions are two cornerstones in our longstanding effort to understand species coexistence and the mechanisms that maintain biodiversity. Recently, it has become increasingly evident that these two factors are not independent, but rather that environmental variability mediates species interactions. In this talk I examine: how do direct effects of environmental variability versus indirect effects mediated by species interactions (i.e., competition) alter species persistence and coexistence? How can we use modern coexistence theory to inform restoration efforts and possible mitigation strategies?
Frederik De Laender (Namur): Simple rules predict effects of environmental change on coexistence
22 March, 2022 PDF
Finding generality in environmental change effects is an important scientific objective. Yet, a main obstacle is specificity: different species respond differently to different environmental factors, and different ecology would then scale up these direct responses to the community level in a different way. Here, I present theory and simulations showing that impacts on coexistence are predictable from simple summary statistics of direct species responses and of the system’s ecology prior to environmental change. My analyses focus on bi- and tritrophic communities and consider the case where feasibility (all species achieve a positive equilibrium) implies coexistence. I illustrate these results with two relevant environmental factors (temperature and pollution) and discuss consequences for predicting effects of regional environmental change on site occupancy.
Bruno Travassos de Britto (São Paulo): Towards a pragmatic view of theories in ecology
Invited commenter: Liz Pásztor (Eötvös University, Budapest)
From a pragmatic view of theories, knowledge is generated as researchers exchange, combine and use models of their choice to learn more about phenomena, even without a prior conceptual unification of these models. Such a context-dependent use of models leads to specific characteristics in pragmatic theories, which are easily identified in ecology. For this reason, we argue that ecology is a science that develops pragmatically, which brings interesting consequences to how we conceive theoretical synthesis.
John DeLong (Nebraska): Evolution of functional responses
Functional responses describe the process of energy flow through food webs by way of consumption. The parameters of a functional response reflect a wide range of predator and prey traits (arising from past evolution) but also set the stage for future evolution by influencing predator energy gain and prey mortality. But beyond a simple “get better at predation or avoiding predation” expectation, how should functional responses change in dynamic and diverse food webs? Here I try to 1) evaluate whether adding more prey types can change selection on predator functional response traits and 2) assess whether patterns of co-evolution follow different paradigms such as an arms race, Red Queen, or tug-of-war pattern given variation in population dynamics and prey generation time.
Axel Rossberg (Queen Mary): Evolution of prudent predation in complex food webs
8 February, 2022 Video
Everybody who modelled consumer-resource population dynamics since A. J. Nicholson (1933, https://doi.org/10.2307/954) will have spent a moment wondered why predators in nature appear to be “prudent” in the sense of having attack rates high enough to sustain their populations but not as high as to undermine their populations’ survival by resource overexploitation. After considering and dismissing some historical alternative explanations, I will describe a process leading to evolution of prudent predation in complex model food webs (doi: 10.1111/ELE.13979, in press). This process does not require vigorous boom-bust dynamics (which are rarely seen). Instead, it has two other unique population-dynamical signatures, and these are often observed. So it appears that indeed there is a route for the evolution of prudent predation consistent with observations.
Brian J. McGill (Maine): Can probabilistic sampling from a regional pool explain community ecology patterns?
A common null model for community assembly is a random sample from the regional list of species. It usually doesn’t work very well and is easy to reject. How much biology do we have to add back in to produce realistic community structure? I will present a framework based on a probabilistic occurrence function as a central tool, show that most community ecology patterns can be written as a function of the occurrence function, and and identify the critical parameters needed in the occurrence function to match empirical data, and hence explore how much biology we need to include.
2021 Fall semester
Ehud Meron (Ben-Gurion): Linking spatial self-organization to community assembly in dryland plant communities
Increasing water stress can result in community shifts towards stress-tolerant plant species but may also trigger spatial self-organization in vegetation patterns, which relaxes this stress. Using a mathematical model to study the interplay between these two response forms, we find that spatial self-organization acts to buffer community-structure changes along environmental gradients.
Jeremy W. Fox (University of Calgary): Higher order interactions don’t matter in species-rich communities
Higher order interactions arise when the per-capita effect of one species on another depends on the density of a third species. For instance, the presence of a predator might cause an herbivore species to forage less, thereby altering the herbivore’s effect on the plants it consumes. Many ecologists have worried that higher order interactions make it impossible to predict community dynamics. Here I show they needn’t have worried. Here I show that, in sufficiently species-rich communities, ubiquitous higher order interactions cancel one another out. Species achieve equilibrium densities similar to those they would’ve achieved in the absence of any higher order interactions. Like several other macroecological patterns, cancellation of higher order interactions in species-rich communities is a “statistical attractor”.
Robert D. Holt (University of Florida): COVID-19 meets the Hutchinsonian niche
9 November, 2021 Video
I will discuss some updates of ideas I sketched in ‘Bringing the Hutchinsonian niche into the 21st century: Ecological and evolutionary perspectives’ (Holt 2009, PNAS), and apply them to aspects of the current pandemic, as illustrated in work from my lab on how the interplay of movement and spatiotemporal variation in transmission degrades pandemic control.
Rachel Germain (UBC): Theory in service of empirical research: examples from experiments on the ecology and evolution of species coexistence
Science operates through a healthy feedback between theory and experiments. As an empiricist who uses theory for different purposes, I will begin by describing some barriers to more fully integrating theory into empirical research in ecology and evolution (and how to overcome those barriers). I will then describe two applications of theory in my own work: 1. how empirical findings have inspired new questions to be addressed theoretically, and, 2. how ecological theories guide the questions I ask and the experiments I design (and some challenges to doing so). Within this overarching framework, I will focus on several projects, including: how spatial scale has counter intuitive effects on the evolution of life history strategies, how species coexistence evolves on microevolutionary and macroevolutionary timescales, and how coexistence theory can provide insights for speciation.
Laura Dee (Colorado Boulder): Putting ecological theory to work for conservation
12 October, 2021 Video
Two grand challenges of our time are climate change and biodiversity loss. This talk focuses on the fundamental question of: to what extent can ecological theory help us understand the consequences of these global changes to inform conservation? I will present two examples investigating this larger question: 1) the consequences of extreme events in boreal forests for ecological stability versus ecosystem services outcomes’ and the 2) vulnerability of food webs versus ecosystem services to species losses in coastal food webs. In the first case, commonly used ecological measures also fall short of predicting disturbance impacts on forest ecosystem services– exhibiting low correlations or systematic biases. In the other case, food web robustness is strongly related to ecosystem service robustness in aggregate, but different types of ecosystem services vary dramatically in their robustness to species’ losses. Together, this work highlights that the complex nature of socioeconomic and ecological systems create challenges for extrapolating ecological theory to understand threats from global change for conservation goals . Thus, I will highlight exciting research avenues for discussion on how ecological theory to contribute to conservation and interdisciplinary research in our inherently socio-environmental systems.
Mathew Leibold (University of Florida): Linking process to pattern in community assembly in diverse metacommunities
28 September, 2021 Video
I’m interested in exploring the degree to which theory on ‘disordered systems’ to community assembly can be linked to statistical methods of pattern analysis. Here, I hope to describe the problem and outline some possible approaches and answers. By doing so, I hope to generate interest and discussion on possible solutions.
Jacob D. O’Sullivan (Queen Mary): The emergent macroecology of Lotka-Volterra metacommunities
For decades Lotka-Volterra community models have been used to try to understand how ecological interactions may drive community-scale properties such as species richness, network structure, and species abundance distributions; in short the various dimensions of biodiversity. Here I show how extending the basic community models into spatially and environmentally heterogeneous landscapes can help us understand how local scale ecological processes – abiotic and biotic filtering, and dispersal – can propagate up the organisational hierarchy to determine regional scale patterns in biodiversity.
2021 Spring semester
Rafael D’Andrea (Stony Brook): Counting niches: Can spatial patterns reveal niche partitioning in tropical forests?
29 June, 2021, Video
We investigate the idea that tropical biodiversity is maintained by a combination of niche segregation and niche sharing among species by asking whether tree species in Barro Colorado Island, Panama, fall into groups differing by the local conditions where they typically occur. We first group together species often found near each other, then show that the resulting three groups are statistically associated with distinct local nutrient levels. Finally, we find clear distinctions among those groups in traits associated with life history strategies.
Neo Martinez (Indiana University): Predicting Ecosystem Metaphenome from Community Metagenome: A Grand Challenge for Environmental Biology
15 June, 2021, Video
Theoretical ecology has investigated a series of concepts from stability and complexity through biodiversity and ecosystem function to coexistence and tipping points for which empirical data typically plays a less-than-satisfying role. To coax theoretical ecology towards increased empirical relevance and broader scientific synthesis, I propose that environmental biologists focus on predicting key characteristics of an ecosystem from the genotypes within its constituent communities. Such metagenomes effectively identify the organisms and their interactions within ecosystems. Following the first of several “virtual cells” built by systems biologists that predicts a human pathogen’s phenotype from its genotype, powerful social (e.g., structured collaborations), scientific (e.g., networks of networks) and technical (e.g., computer and data science) concepts for accomplishing the proposed task will be described along with a plausible workflow based on allometric trophic network theory. Anticipated benefits include more integrated, mechanistic, and predictive theory of how ecosystem structure and function emerge from organisms interacting within a habitat.
Chuliang Song (McGill University): An environment-dependent framework to study ecological networks
1 June, 2021, Video
Ecological networks—how species interactions are organized within ecological communities—are highly structured, which has motivated generations of ecologists to elucidate how these structures affect species coexistence. Unfortunately, we still do not have a clear and consistent answer about the link between network structure and species coexistence. A possible explanation is that most of the studies do not take into account that the environment affects both network structure and species coexistence due to the multidimensional and changing nature of environmental factors. In this context, the structural stability approach provides a theoretical framework grounded on biological realism to quantitatively link network structure, species coexistence, and environmental factors. In this talk, I will introduce the theoretical framework and computational tools of the structural stability approach, and present the empirical validation using field and experimental observations.
Cinzia Soresina (University of Gratz): The influence of cross-diffusion in pattern formation: multistability and Hopf bifurcations
18 May, 2021, Video
The Shigesada-Kawasaki-Teramoto model (SKT) was proposed to account for stable inhomogeneous steady states exhibiting spatial segregation, which describes a situation of coexistence of two competing species. Even though the reaction part does not present the activator-inhibitor structure, the cross-diffusion terms are the key ingredient for the appearance of spatial patterns. We provide a deeper understanding of the conditions required on both the cross-diffusion and the reaction coefficients for non-homogeneous steady states to exist, by combining a detailed linearised and weakly non-linear analysis with advanced numerical bifurcation methods via the continuation software pde2path. In particular, we study the role of the additional cross-diffusion term in pattern formation, showing that the bifurcation diagram undergoes major deformations leading to multistability regions. The presence of time-periodic spatial pattern appearing via Hopf bifurcation points is also investigated.
Masato Yamamichi (Brisbane): How does rapid evolution promote species coexistence?
4 May, 2021, Video
Previous studies have revealed that microevolution (i.e., temporal changes in allele frequencies) is pervasive in the wild and may be an important factor for understanding various ecological dynamics. Here, I show how rapid evolution can promote species coexistence via density-dependent sexual and social selection (intraspecific adaptation load: Yamamichi et al. 2020 Trends Ecol. Evol.) and resource-dependent adaptive foraging in limit cycles (relative nonlinearity: Yamamichi & Letten 2021 Ecol. Lett.). Then I will discuss the potential synthesis of theories on species coexistence and eco-evolutionary dynamics.
Priyanga Amarasekare (UCLA): Predicting the effects of climate warming: from chemistry to evolution
20 April, 2021, Video
I want to make the argument that understanding life on earth requires developing theory that integrates across levels of information, from chemistry to evolution. I am going to focus on temperature variation and phenotypic plasticity, not least because temperature is integral to all life processes, and climate warming poses one of the greatest threats to life on earth. Predicting the effects of warming requires knowing whether organisms have sufficient plasticity to respond to current levels of warming, and whether plasticity can evolve fast enough to keep pace with warming. But, existing plasticity is the result of past evolution, while future plasticity is the result future evolution. My thesis is that chemistry is integral to both understanding past evolution and predicting future evolution. I will present theory and data to support this idea and discuss future research directions.
György Barabás (Linköping University): Coexistence and parameter sensitivity in stationary aperiodic environments
6 April, 2021 Video
First, I present a method for calculating how average population densities respond to parameter perturbations when the dynamics are periodic, and show that this practical problem holds a strong connection with basic questions of coexistence. I then generalize this result to stationary nonperiodic density fluctuations. I finish by discussing the connection with existing formalisms for understanding coexistence in variable environments.
Theresa Ong (Dartmouth): Complex hysteretic patterns: hidden loops and ecological traps
23 March, 2021, Video
Critical transitions whereby small changes in conditions can cause large and irreversible changes in ecosystem states are a cause of increasing concern in ecology. Here, I focus on the irreversibility of these transitions, formally known as hysteresis. I will discuss a variety of complicated hysteretic patterns derived from theory and experiments, which include “unattainable” stable states that once lost may never be recovered.
Géza Meszéna (Eötvös University): Coexistence, niche, adaptation and all that…
9 March, 2021, Video
Why are there so many animals? According an old idea, it is because there are so many different possibilities for life to adapt to. I’ll argue that it is still true, and truer than the suggested alternatives. The challenge is to establish the precise mathematical treatment at this level of generality without losing the reach to the phenomenal complexity of ecosystems.
James O’Dwyer (University of Illinois): Cooperation, Resource Exchange, and Stability
23 February, 2021, Video
Models of microbial interactions have been developed in recent years, drawing from taxonomic abundances via amplicon sequencing. Many of these models assume that dynamics through time are primarily driven by pairwise interactions between taxa, but with the drawback that how these interaction strengths may change with environmental context is less than clear. Here we model the consumption and exchange of resources explicitly, and show that these processes imply new results and principles for the stability of ecological communities.
Thomas Koffel (Michigan State): A niche theory of positive interactions
9 February, 2021, Video
Niche Theory has traditionally focused on competitive interactions. In this talk, we propose a general framework that expands the theory to positive interactions, such as facilitation and mutualism, using angular metrics of niche difference. We develop novel niche concepts such as the Allee niche and niche expansion, and illustrate them using a diverse set of theoretical examples.
Stephen Ellner (Cornell): An invitation to spatial coexistence theory
26 January, 2021, Video
Previously in this series Sebastian Schreiber reviewed stochastic coexistence theory for infinite population models, based on long-term population growth rates of (infinitesimally) rare invaders. Nadav Shnerb presented progress on the challenges posed by demographic stochasticity, in finite populations of discrete individuals. Today we add one more complication: spatial structure and local movement, so that invader populations are clumped, and locally common even when globally rare.
Camille Carpentier (University of Namur): A new link-species relationship connects ecosystem structure and stability
12 January, 2021, Video
How does an ecosystem’s structure determine its capacity to cope with species removal and perturbations of species densities? To answer this question, we develop a network-specific approach to the link-species relationship, and demonstrate that it formally predicts a robustness-resilience trade-off, both theoretically and in empirical networks.
2020 Fall semester
Nadav Shnerb (Bar Ilan University): Quantifying coexistence
8 December, 2020, Video
Modern coexistence theory employs mutual invasibility as a coexistence criterion and mean growth rate when rare as an invasibility criterion. When implemented as quantitative metrics, both criteria have shortcomings: persistence time may decline when the chance of invasion grows, and invasibility may decrease as the mean growth rate increases in magnitude. I will discuss an alternative framework and introduce a new metric for invasibility under both demographic and environmental stochasticity.
Stefano Allesina (University of Chicago): A metapopulation model in which patches have memory
24 November, 2020, Video
Levins’ metapopulation model has been extended in numerous ways. Here we analyze a model in which species have distinct colonization rates that depend on which species previously occupied the patch. We connect this model to the Janzen-Connell hypothesis and show some surprising behavior for a simplified version of the model.
Sebastian Schreiber (UC Davis): General theorems for coexistence and extinction in stochastic models
9 November, 2020, Video
In the 1980s, Josef Hofbauer introduced a criterion for mathematically verifying coexistence using per-capita growth rates of species when rare i.e. Lyapunov exponents. This criterion ensures coexistence is robust to large perturbations of the community state (i.e. permanence) and small structural perturbations of the governing equations (i.e. robust permanence). Originally developed for deterministic models without population structure, Hofbauer’s criterion has been extended in the past decade to models allowing for auxiliary variables and environmental stochasticity. In this talk, I review the key ingredients of this theory and illustrate its use in multi-species models with intransitivities, eco-evolutionary feedbacks, sexual dimorphisms, autocorrelated environmental fluctuations, or spatial structure.
André M. de Roos (University of Amsterdam): Towards a general theory of coexistence: Lyapunov exponents, auxiliary variables, and Hofbauer’s criterion
27 October, 2020, Video
Dynamic models of ecological communities that neglect within-population structure predict that stability depends on substantial self-limitation of species. Using a stage-structured food web model I show that differences between juveniles and adults result in diverse ecological communities that are stable or exhibit limited-amplitude fluctuations, despite that only a single basal species is self-regulated. Eigenvalue analysis reveals that community stability results from dynamic changes in within-population stage-structure that override destabilising effects induced by the species interaction network.
Vadim Karatayev (University of Guleph), with Vadim Karatayev, Marissa Baskett, Egbert van Nes, Marten Scheffer: Species heterogeneity can reduce the potential for alternative stable states in food webs
13 October, 2020, Video
Can alternative stable states arise when food webs dissipate feedbacks across many species in diverse systems like coral reefs? Although consumer loss often characterizes degraded ecological states, food web resilience theory predominantly focuses on specific systems and few-species models. After developing a generalized model of consumer collapse, we show that alternative stable states dominated by either consumer or resource guilds can arise when consumer species improve conditions in the same way, for instance when different herbivore species promote habitat-forming corals by limiting the total cover of macroalgae. Conversely, specialized feedbacks where size refugia or group defense make individual resource species inedible are less likely to drive many-species alternative stable states because losing the most vulnerable consumers cascades into a guild-wide collapse.
Éva Kisdi (Helsinki): The evolution of habitat choice facilitates niche expansion
29 September, 2020, Video
Matching habitat choice and local adaptation are two key factors that control the distribution and diversification of species. We study their joint evolution in a structured metapopulation model with a continuous distribution of habitats. Habitat choice follows from dispersal with non-random immigration, a process always acknowledged yet rarely incorporated into theoretical models. For fixed local adaptation, we find the evolutionarily stable habitat choice as a function linking the probability of settlement to the local environment. When the local adaptation trait co-evolves, the metapopulation can become polymorphic. Our main result shows that coexisting strains with only slightly different local adaptation traits evolve substantially different habitat choice. In turn, different habitat use selects for divergent local adaptations. We thus propose that under wide conditions, the joint evolution of habitat choice and local adaptation can facilitate niche expansion via diversification.
Guy Bunin (Technion, Haifa): Phase-transitions as signatures of complex communities
15 September, 2020, Video
What characterizes species-rich communities with complex species interactions–for example, competing over many niches? Models predict sharp transitions, as conditions are changed, between qualitatively different dynamical behaviors. These include the abrupt appearance of a vast number of alternative steady-states; and the onset of persistent abundance fluctuations, nearly uncorrelated between species. We discuss these phenomena, and how they differ from related low-dimensional behavior.