arXiv subjects

Alanna Hoyer-Leitzel

Publications and source records attributed to Alanna Hoyer-Leitzel.

8 recordsLinked to original sources

arXiv2024

Continuation of fixed points and bifurcations from ODE to flow-kick disturbance models

Some ODE models treat ecological disturbance as a continuous process, even disturbances such as fire that occur almost instantaneously on the timescale of system recovery. Alternatively, flow-kick models resolve disturbances as discrete impulses that change an ODE system's state periodically in time. Here we compare the dynamics of continuously disturbed ODE models to those of flow-kick models with the same average disturbance rate. In the case that kicks are small and high-frequency, we find multiple similarities between continuous and analogous discrete disturbance models. First, we prove that flow-kick maps generate an analogous vector field in the limit as the period between kicks approaches zero. Second, we present conditions under which equilibria, saddle-node bifurcations, and transcritical bifurcations continue from ODE to flow-kick systems. On the other hand, we also provide numerical evidence that similarities between continuous and discrete disturbance models can break down as the period between kick grows. We illustrate implications of these differences for climate change in a nonspatial Klausmeier model of vegetation and precipitation dynamics. We conclude that although ODEs may suffice to model high-frequency disturbances, resolving lower-frequency disturbances in time may be essential to effectively predicting their effects.

math.DS↗

arXiv2024

Symmetric relative equilibria with one dominant and four infinitesimal point vortices

We investigate the symmetry of point vortices with one dominant vortex and four vortices with infinitesimal circulations in the (1+4)-vortex problem, a subcase of the five-vortex problem. The four infinitesimal vortices inscribe quadrilaterals in the unit circle with the dominant vortex at the origin. We consider symmetric configurations which have one degree of spacial freedom, namely the (1+N)-gon, kites, rectangles, and trapezoids with three equal sides. We show there is only one possible rectangular configuration (up to rotation and ordering of the vortices) and one possible trapezoid with three equal sides (up to rotation and ordering), while there are parametrically defined families of kites. Additionally we consider the (1+4)-gon and show that the infinitesimal vortices must have equal circulations on opposite corners of the square. The proofs are heavily dependent on techniques from algebraic geometry and require the use of a computer to calculate Grobner bases.

math.DS↗

arXiv2021

Impulsive fire disturbance in a savanna model: Tree-grass coexistence states, multiple stable system states, and resilience

Savanna ecosystems are shaped by the frequency and intensity of regular fires. We model savannas via an ordinary differential equation (ODE) encoding a one-sided inhibitory Lotka-Volterra interaction between trees and grass. By applying fire as a discrete disturbance, we create an impulsive dynamical system that allows us to identify the impact of variation in fire frequency and intensity. The model exhibits three different bistability regimes: between savanna and grassland; two savanna states; and savanna and woodland. The impulsive model reveals rich bifurcation structures in response to changes in fire intensity and frequency -- structures that are largely invisible to analogous ODE models with continuous fire. In addition, by using the amount of grass as an example of a socially-valued function of the system state, we examine the resilience of the social value to different disturbance regimes. We find that large transitions ("tipping") in the valued quantity can be triggered by small changes in disturbance regime.

q-bio.PE↗

arXiv2021

Rethinking the Definition of Rate-Induced Tipping

The current definition of rate-induced tipping is tied to the idea of a pullback attractor limiting in forward and backward time to a stable quasi-static equilibrium. Here we propose a new definition that encompasses the standard definition in the literature for certain scalar systems and includes previously excluded $N$-dimensional systems that exhibit rate-dependent critical transitions.

math.DS↗

arXiv2021

Examining the Modeling Framework of Crime Hotspot Models in Predictive Policing

Predictive policing has its roots in crime hotspot modeling. In this paper we give an example of what goes into mathematical crime hot spot modeling and show that the modeling assumptions perpetuate systemic racism in policing. The goal of this paper is to raise objections to this field of research, not on its mathematical merit, but on the scope of the problem formation.

math.HO↗

arXiv2018

Quantifying resilience to recurrent ecosystem disturbances using flow-kick dynamics

Shifting ecosystem disturbance patterns due to climate change (e.g. storms, droughts, wildfires) or direct human interference (e.g. harvests, nutrient loading) highlight the importance of quantifying and strengthening the resilience of desired ecological regimes. Although existing metrics capture resilience to isolated shocks, gradual parameter changes, and continuous noise, quantifying resilience to repeated, discrete disturbances requires novel analytical tools. Here we introduce a flow-kick framework that quantifies resilience to disturbances explicitly in terms of their magnitude and frequency. We present a resilience boundary between disturbances that cause either escape from a basin of attraction or stabilization within it, and use the resilience boundary to build resilience metrics tailored to repeated, discrete disturbances. The flow-kick model suggests that the distance-to-threshold resilience metric overestimates resilience in the context of repeated disturbances. It also reveals counterintuitive triggers for regime shifts, such as increasing recovery times between disturbances, or increasing disturbance magnitude and recovery times proportionately.

q-bio.PE↗

arXiv2018

Detecting transient rate-tipping using Steklov averages and Lyapunov vectors

A wide variety of physical systems ranging from the firing of neurons to eutrophication of lakes to the presence of Arctic summer sea ice exhibit a phenomenon known as tipping. In mathematical models, tipping can be caused by bifurcations, noise, and the rate at which parameters are changing in time [2]. Because traditional methods in dynamical systems are usually concerned with the long-term behavior of the system, these methods are not always able to detect the transient dynamics characteristic of rate-tipping. In this paper, we consider one- and two-dimensional dynamical systems with nonautonomous parameters that exhibit rate-tipping, as defined as not tracking the evolution of stable equilibria (QSEs) in the corresponding autonomous systems. We find that nonautonomous stability spectra in the form of Steklov averages and their derivatives appear to be correlated with transient rate-tipping in systems with unique QSEs or with parameters that change at a constant rate. Furthermore, for systems in two dimensions and higher, comparison of the angle between leading Lyapunov vectors of different trajectories admits a possible criterion for detecting rate-tipping. Our heuristic results add to the body of work dedicated to studying and understanding the phenomenon of rate-tipping.

math.DS↗

arXiv2017

Existence, stability, and symmetry of relative equilibria with a dominant vortex

We analyze existence, stability, and symmetry of point vortex relative equilibria with one dominant vortex and N vortices with infinitesimal circulation. The dimension of the problem can be reduced by taking an infinitesimal circulation limit, resulting in the so-called (1+N)-vortex problem. In this work, we first generalize the reduction to allow for circulations of varying signs and weights. We then prove that symmetric configurations require equality of two circulation parameters in the (1+3)-vortex problem and show that there are stable asymmetric relative equilibria. In a number of examples, we use rigorous methods from algebraic geometry to count all relative equilibria.

math.DS↗