A framework for quantifying the relationship between intensity and severity of impact of disturbance across types of events and species

Abstract

Understanding the impacts of natural disturbances on wildlife populations is a central task for ecologists; in general, the severity of impact of a disturbance (e.g., the resulting degree of population decline) is likely to depend primarily on the disturbance intensity (i.e., strength of forcing), type of disturbance, and species vulnerability. However, differences among disturbance events in the physical units of forcing and interspecific differences in the temporal variability of population size under normal (non-disturbance) conditions hinder comprehensive analysis of disturbance severity. Here, we propose new measures of disturbance intensity and severity, both represented by the return periods. We use a meta-analysis to describe the severity–intensity relationship across various disturbance types and species. The severity and the range of its 95% confidential interval increased exponentially with increasing intensity. This nonlinear relationship suggests that physically intense events may have a catastrophic impact, but their severity cannot be extrapolated from the severity–intensity relationship for weak, frequent disturbance events. The framework we propose may help to clarify the influence of event types and species traits on the severity–intensity relationship, as well as to improve our ability to predict the ecological consequences of various disturbance events of unexperienced intensity.

Drought intensification alters the composition, body size, and trophic structure of invertebrate assemblages in a stream mesocosm experiment

“…crustaceans, leeches, flatworms) were more sensitive to pool fragmentation than in the present study. This discrepancy is consistent with the importance of disturbance duration relative to species’ generation times (Iwasaki & Noda, ), with longer droughts having disproportionately severe impacts on weak dispersers with limited capacity for recolonisation of disconnected habitats.…”

A theory of pulse dynamics and disturbance in ecology

“…In biomass‐altering disturbances resource availability changes as a function of the force acting on biomass (Iwasaki and Noda ). Resource availability after disturbance changes in four ways: through transformation of resources directly by the disturbance event; through changes in biotic uptake; through flows of organic matter, nutrients, water, and atmospheric gases; and through release of resources stored in biomass.…”

“… Prediction 1—Disturbance magnitude (Table ): Disturbance magnitude, expressed as changes in resource availability, resource ratios, limiting resources, and biotic legacy, are the product of the force (or intensity) of the disturbance and ecosystem resistance (Iwasaki and Noda ). Prediction 2—Resource trajectory (Table ): Resource trajectories after disturbance are determined by the stoichiometric requirements of organisms, energetic constraints, physiological capacity, luxury uptake, and loss through spatial transfers (e.g., via water, wind, and gravity). Resource uptake changes the hierarchy of limiting. Prediction 3—Rate of change (Table ): Energy flux, resource availability, and biotic traits determine the rate of resource change after disturbance.…”

“…As essential assumption of Postulate 1 is that there are predictable and generalizable pathways of changes in resource availability, where resource is defined as any aspect of an organism's environment that it uses to increase its own fitness while pre-empting other organisms from using that same resource. The pathways of resource dynamics depend on the magnitude of the pulse event (Iwasaki and Noda 2018), on rates of ecosystem processes based on resource flux (Postulate 2), and on the processes of biotic uptake, determined by the availability of organisms, their stoichiometric requirements, and their physiological states (Vitousek 1984, Sterner and Elser 2002, Rastetter et al 2013, Helton et al 2015, van Huysen et al 2016Fig. 2).…”

“…Disturbance magnitude, expressed as changes in resource availability, resource ratios, limiting resources, and biotic legacy, are the product of the force (or intensity) of the disturbance and ecosystem resistance (Iwasaki and Noda 2018). Prediction 2-Resource trajectory ( Table 2): Resource trajectories after disturbance are determined by the stoichiometric requirements of organisms, energetic constraints, physiological capacity, luxury uptake, and loss through spatial transfers (e.g., via water, wind, and gravity).…”