Biology subjects

Schreiber, S. J.

Publications and source records attributed to Schreiber, S. J..

6 recordsLinked to original sources

bioRxiv2025

Using Modern Coexistence Theory to understand community disassembly

Community disassembly examines how species extinction alters ecological communities. Sometimes, the extinction of one species can trigger the loss of others, known as secondary extinction. These secondary extinctions often result from complex species interactions, complicating the identification of underlying mechanisms. Here, we leverage Modern Coexistence Theory to identify when and why secondary extinctions occur. To identify when secondary extinctions occur, we introduce the community disassembly graph, that uses invasion growth rates to identify transitions between coexisting communities due to extinction. When a secondary extinction is identified, we decompose the invasion growth rates associated with the secondary extinction to understand why it occurs. We demonstrate the utility of this framework by applying it to models in which different species interactions-competition, facilitation, and predation-contribute significantly to secondary extinctions. Our results show that Modern Coexistence Theory offers a flexible and interpretable approach to understanding when and why secondary extinctions occur.

ecology↗

bioRxiv2022

Extinction Risk of Sonoran Desert Annuals Following Potential Changes in Precipitation Regimes

Rapid environmental change can affect both the mean and variability in environmental conditions. Natural selection tends to favour those organisms that best respond to such changes. Here, we consider delayed germination as bet hedging strategies for 10 Sonoran Desert annuals. We use a germination model parameterized with long-term demographic and climate data to explore potential effects of changes in the mean and variance in precipitation on the evolution of germination strategies, as well as the risk of extinction. We then explored the potential for evolutionary rescue in response to these changes. As expected, results indicate that as rainfall declines, or uncertainty in rainfall increases, all species have higher extinction risk (the former being more detrimental). These shifts also increased the benefit of delayed germination. Results also indicate that evolutionary rescue can often occur for small shifts, especially for more variable rainfall regimes, but would not likely save populations experiencing larger environmental changes. Finally, we identified life history traits and functional responses to precipitation that were most strongly correlated to the ability to cope with changes in rainfall and with potential for evolutionary rescue: dormant seed survivorship and, to a smaller degree, chance of reproduction and seed yield sensitivity to precipitation.

ecology↗

bioRxiv2018

The shape of density dependence and the relationship between population growth, intraspecific competition and equilibrium population density

The logistic growth model is one of the most frequently used formalizations of density dependence affecting population growth, persistence and evolution. Ecological and evolutionary theory and applications to understand population change over time often include this model. However, the assumptions and limitations of this popular model are often not well appreciated. Here, we briefly review past use of the logistic growth model and highlight limitations by deriving population growth models from underlying consumer-resource dynamics. We show that the logistic equation likely is not applicable to many biological systems. Rather, density-regulation functions are usually non-linear and may exhibit convex or both concave and convex curvatures depending on the biology of resources and consumers. In simple cases, the dynamics can be fully described by the continuous-time Beverton-Holt model. More complex consumer dynamics show similarities to a Maynard Smith-Slatkin model. Importantly, we show how population-level parameters, such as intrinsic rates of increase and equilibrium population densities are not independent, as often assumed. Rather, they are functions of the same underlying parameters. The commonly assumed positive relationship between equilibrium population density and competitive ability is typically invalid. As a solution, we propose simple and general relationships between intrinsic rates of increase and equilibrium population densities that capture the essence of different consumer-resource systems. Relating population level models to underlying mechanisms allows us to discuss applications to evolutionary outcomes and how these models depend on environmental conditions, like temperature via metabolic scaling. Finally, we use time-series from microbial food chains to fit population growth models and validate theoretical predictions. Our results show that density-regulation functions need to be chosen carefully as their shapes will depend on the study systems biology. Importantly, we provide a mechanistic understanding of relationships between model parameters, which has implications for theory and for formulating biologically sound and empirically testable predictions.

ecology↗

bioRxiv2017

Restoration of Eastern oyster populations with positive density dependence

AO_SCPCAPBSTRACTC_SCPCAPPositive density dependence can create a threshold of population states below which extinction of the population occurs. The existence of this threshold, which can often be a complex, multi-dimensional surface, rather than a single point, is of particular importance in degraded populations for which there is a desire for successful restoration. Here, we incorporated positive density dependence into a closed, size- and age-structured integral projection model parameterized with empirical data from an Eastern oyster, Crassostrea virginica, population in Pamlico Sound, North Carolina. To understand the properties of the threshold surface, and implications for restoration, we introduced a general method based on a linearization of the threshold surface at its unique, unstable equilibrium. We estimated the number of oysters of a particular age (i.e. stock enhancement), or the surface area of hard substrate required (i.e. habitat enhancement), to move a population from an extinction trajectory to a persistent trajectory. The location of the threshold surface was strongly affected by changes in the amount of local larval retention. Traditional stock enhancement with oysters less than a year old (i.e. spat) required three times as many oysters relative to stock enhancement with oysters between ages three and seven, while the success of habitat enhancement depended upon the initial size distribution of the population. The methodology described here demonstrates the importance of considering positive density dependence in oyster populations, and also provides insights into effective management and restoration strategies when dealing with a high dimensional threshold separating extinction and persistence.

ecology↗

bioRxiv2017

Rapid evolution slows extinctions in food webs

Historically, evolutionary changes have been thought to act on much longer time scales than ecological dynamics. However, a recent body of research has demonstrated that evolution that is rapid enough to dramatically affect ecological dynamics can lead to feedbacks between ecological and evolutionary processes. Thus, to understand the stability of ecological communities, we must also consider evolutionary change in the component species. Here, we use individual-based simulations of a quantitative genetic eco-evolutionary model to describe how trait evolution influences the stability of ecological communities. On short time scales, faster evolutionary rates decreased the probability of species extinctions as populations at low densities were rescued via trait evolution. However, on longer time scales, evolutionary had little effect on the number of extinctions. The extent of short-term evolutionary rescue depended on the source of trait variation; populations with variation generated through mutation experienced more rescue events and were less prone to extinction, relative to populations with only standing trait variation. Trait evolution leading to more rescued populations increased the stability of the community on timescales relevant to conservation. Our work highlights the importance of intraspecific trait variation and the evolutionary mechanisms maintaining this variation for community ecology, as well as management of declining populations in a community context.

ecology↗

bioRxiv2017

Partitioning the effects of eco-evolutionary feedbacks on community stability

AO_SCPCAPBSTRACTC_SCPCAPA fundamental challenge in ecology continues to be identifying mechanisms that stabilize community dynamics. By altering the interactions within a community, eco-evolutionary feedbacks may play a role in community stability. Indeed, recent empirical and theoretical studies demonstrate that these feedbacks can stabilize or destabilize communities, and moreover, that this sometimes depends on the relative rate of ecological to evolutionary processes. So far, theory on how eco-evolutionary feedbacks impact stability exists for only for a few special cases. In our work, we develop a general theory for determining the effects of eco-evolutionary feedbacks on stability in communities with an arbitrary number of interacting species and evolving traits for when evolution is slow and fast. We characterize how eco-evolutionary feedbacks lead to stable communities that would otherwise be unstable, and vice versa. We show how this characterization provides a partitioning of the roles of direct and indirect feedbacks between ecological and evolutionary processes on stability, and how this partitioning depends on the rate of evolution relative to the ecological time scales. Applying our methods to models of competing species and food chains, we demonstrate how the functional form of trade offs, genetic correlations between traits, and the rate of evolution determine whether eco-evolutionary feedbacks stabilize or destabilize communities.

ecology↗