CONCEPTS & SYNTHESIS EMPHASIZING NEW IDEAS TO STIMULATE RESEARCH IN ECOLOGY Ecology, 100(7), 2019, e02734 © 2019 The Authors Ecology published by Wiley Periodicals, Inc. on behalf of Ecological Society of America. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. A theory of pulse dynamics and disturbance in ecology 1 2,3 ANKE JENTSCH AND PETER WHITE 1Disturbance Ecology, Bayreuth Center of Ecology and Environmental Research BayCEER, 95440 Bayreuth University, Bayreuth, Germany 2Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27561 USA Citation: Jentsch, A., and P. White. 2019. A theory of pulse dynamics and disturbance in ecology. Ecology 100(7):e02734. 10.1002/ecy.2734 Abstract. We propose four postulates as the minimum set of logical propositions necessary for a theory of pulse dynamics and disturbance in ecosystems: (1) resource dynamics characterizes the magnitude, rate, and duration of resource change caused by pulse events, including the continuing changes in resources that are the result of abiotic and biotic processes; (2) energy flux characterizes the energy flow that controls the variation in the rates of resource assimilation across ecosystems; (3) patch dynamics characterizes the distribution of resource patches over space and time, and the resulting patterns of biotic diversity, ecosystem structure, and cross-scale feedbacks of pulses pro- cesses; and (4) biotic trait diversity characterizes the evolutionary responses to pulse dynamics and, in turn, the way trait diversity affects ecosystem dynamics during and after pulse events. We apply the four postulates to an important class of pulse events, biomass-altering disturbances, and derive seven generalizations that predict disturbance magnitude, resource trajectory, rate of resource change, disturbance probability, biotic trait diversification at evolutionary scales, biotic diversity at ecological scales, and functional resilience. Ultimately, theory must define the variable combina- tions that result in dynamic stability, comprising resistance, recovery, and adaptation. Key words: biodiversity; biogeography; biological traits; disturbance; ecosystem dynamics; resilience; stability. otherwise affecting responses to climate warming (DeFr- INTRODUCTION enne et al. 2013). Characterizing pulse dynamics is also Pulse events, defined as abrupt changes in ecological important as the basis for determining the degree of nov- parameters, are ubiquitous in ecosystems and include a elty of events (Hallett et al. 2013). Finally, event charac- wide array of phenomena, such as heat waves, marine teristics are central to understanding ecological resilience upwelling, mass reproductive and mortality events, and (Ratajczak et al. 2017), because pulse characteristics biomass-altering disturbances (Yang and Naeem 2008). reflect ecosystem resistance, create the initial conditions Understanding pulse events is important because of this for recovery, and are, by definition, the change to which ubiquity and because the frequencies and magnitudes of the system may or may not be resilient (Carpenter et al. such events as droughts, fires, floods, windstorms, and 2001). In this paper, we use a deductive approach to pest outbreaks, are changing because of human influences derive the minimum set of propositions, here called pos- including, most importantly, climate change, land-use tulates (after Marquet et al. 2014), that create a general change, and species invasions (Franklin et al. 2016, Seidl explanation for pulse dynamics across ecosystems and for et al. 2017, Loehman et al. 2018, McDowell et al. 2018). places with different biogeographic histories. We then In turn, pulse events can affect the responses of ecosys- develop seven generalizations, phrased as predictions, for tems to these influences, e.g., increasing invasion rates an important class of pulse events, biomass-altering dis- (Hobbs and Huenneke 1992) and accelerating or turbances, that emerge from these postulates. Our interest in a theory of pulse dynamics developed Manuscript received 19 February 2019; accepted 11 March from the challenge of generalizing disturbance and 2019. Corresponding Editor: Jeffrey Shima. ecosystems dynamics (White and Jentsch 2001). Distur- 3 E-mail: [email protected] bance ecology has produced a number of conceptual Correction statement: The figure caption in Figure 4 was incorrectly set and has since been corrected. The copyright frameworks over the last several decades (Shugart 1984, statement for this article was also corrected. We apologize to White and Pickett 1985, Pulsford et al. 2016), including the authors and our readers for this error. the intermediate disturbance hypothesis (Connell 1978, Article e02734; page 1 Article e02734; page 2 ANKE JENTSCH AND PETERWHITE Ecology, Vol. 100, No. 7 Fox 2013, Shiel and Burselm 2013), the dynamic equilib- Disturbance has been defined both broadly and nar- rium model (Huston 1979, 2014), the theory of nutrient rowly. In its broad sense, disturbance encompasses all dynamics (Vitousek and Reiners 1975, Vitousek 1984), pulse events (White and Pickett 1985). In its narrow the theory of forest dynamics (Shugart 1984), and the the- sense, disturbance applies to a special class of pulse ory of landscape dynamics (Turner et al. 1993). Impor- events characterized by direct alteration of biomass and tant recent frameworks include disturbance interactions ecosystem structure (Grime 1979), termed here “bio- and cross-scale perspectives (Peters et al. 2004, Raffa mass-altering disturbance” (see below). Under both defi- et al. 2008, Buma and Wessman 2011, Buma 2015, Can- nitions, the pulse perspective focuses on the dimensions non et al. 2017), the concepts of biological legacy and of resource change and the mechanisms of response, ecological memory (Johnstone et al. 2016), generalizable whether a single disturbance event causes one resource biogeochemical responses to ecosystem disturbance pulse or a cascade of pulses over time. (Kranabetter et al. 2016), the network-based view on the We can recognize four types of pulse events (modified role of disturbance in biodiversity and productivity from Yang and Naeem 2008): (1) fluctuation in physical (Gross 2016, Seidl et al. 2017), and patterns of trait diver- environmental conditions such as heat waves or sity shaped by evolutionary trade-offs (Diaz et al. 2016). droughts; (2) abiotic changes in resource supplies such We build on this past work by proposing a general the- as those caused by ocean upwelling or lake turnover; (3) ory of pulse dynamics and disturbance that serves as an changes in biotic resources through sudden demographic overall structure for the insights that have developed over events, such as mass reproductive or mortality events the last several decades. This structure consists of a set of (Holt 2008, Yang and Naeem 2008, Yang et al. 2008); four fundamental postulates and the relationships among and (4) changes through abrupt alteration of biotic them that together circumscribe the common denomina- structure, that is, biomass-altering disturbances. Spatial tors and rules for all pulse events. Although disturbances subsidies, that is, the transfer of resources across space, can initiate successional change, we do not review theo- have been recognized as a fifth category (Yang and ries of successional mechanisms and pathways here (see Naeem 2008); however, these transfers involve forces Glenn-Lewin et al. 1992, Meiners et al. 2015, Peet 1992, such as wind, water flow, and gravity in the movement YNTHESIS Walker and del Moral 2003, Pickett et al. 2011, Walker of resources, organic materials, soils, or geological sub- and Wardle 2014, Pulsford et al. 2016); nor do we treat strates and thus can be considered forms of the second the dynamics among species that occur after pulse initia- (abiotic changes in resource supply) or fourth (biomass- tion, such as trophic interactions (see Holt 2008, Nowlin altering disturbances) pulse types. Indeed, pulse events et al. 2008, Karakocß et al. 2018). almost always result in the spatial movement of &S resources, though the scale of this movement varies from local to global. By including biomass-altering distur- Definition and types of pulse events bance within the larger framework of pulse dynamics, we A pulse event is any abrupt change, positive or nega- tive, in system parameters (Yang et al. 2008), with abrupt- ness defined as the magnitude of the change divided by its duration (White and Jentsch 2001). Pulse events can be characterized by seven continuous variables that describe the dimensions of a particular parameter change: magni- tude, duration, abruptness, initial pulse rate, the rate of ONCEPTS recovery, the degree of recovery, and the area under the pulse curve (Fig. 1). Because magnitude and duration are C continuous variables, pulse events become gradual changes as magnitude decreases and duration increases (sometimes called “presses”). This means that “pulsed- ness” (Yang et al. 2008) is, itself, a continuous variable (Fig. 2A, modified from Yang et al. 2008). However, we posit that resistance to pulse forces, discreteness of indi- vidual organisms, finite life spans, limits to niche breadth, and the interval between pulse events often result in thresholds and, therefore, discontinuities and patchiness. In essence, abruptness and discreteness can develop, at least