Irruptive Dynamics Research Articles

Do community changes persist after irruptive population dynamics? A case study from an invasive species boom and bust.

Irruptive or boom-and-bust population dynamics, also known as 'outbreaks', are an important phenomenon that has been noted in biological invasions at least since Charles Elton's classic book was published in 1958. Community-level consequences of irruptive dynamics are poorly documented and invasive species provide excellent systems for their study. African Jewelfish (Rubricatochromis letourneuxi, "jewelfish") are omnivores that demonstrate opportunistic carnivory, first reported in Florida in the 1960s and in Everglades National Park (ENP) in 2000. Twelve years after invasion in ENP, jewelfish underwent a 25-fold increase in density in one year. By 2016, jewelfish represented 25-50% of fish biomass. Using a 43-year fish community dataset at two sites (1978-2021), and a 25-year dataset of fish and invertebrate communities from the same drainage (1996-2021), with additional spatial coverage, we quantified differences in fish and invertebrate communities during different phases of invasion. During jewelfish boom, abundant, native cyprinodontiform fishes decreased in density and drove changes in community structure as measured by similarity of relativized abundance. Density of two species declined by > 70%, while four declined by 50-62%. Following the jewelfish bust, some species recovered to pre-boom densities while others did not. Diversity of recovery times produced altered community structure that lagged for at least four years after the jewelfish population declined. Community structure is an index of ecological functions such as resilience, productivity, and species interaction webs; therefore, these results demonstrate that irruptive population dynamics can alter ecological functions of ecosystems mediated by community structure for years following that population's decline.

Defoliator outbreaks track with warming across the Pacific coastal temperate rainforest of North America

The biogeography of irruptive insect herbivores is determined by host availability and climate conditions. As such, outbreak distributions are sensitive to climatic change, especially across large latitudinal gradients. Here, we investigate the outbreak distributions of two understudied defoliators, hemlock sawfly Neodiprion tsugae (Hymenoptera) and western blackheaded budworm Acleris gloverana (Lepidoptera), that have both recently impacted the greatest land area recorded across the Pacific coastal temperate rainforest since the establishment of aerial survey programs. We compiled polygon‐based estimates of insect damage collected by aerial observers, forest inventory, and downscaled climatic data to develop gridded estimates of bioclimatic conditions across the extent of the Pacific coastal temperate rainforest, including the continental United States, British Columbia and Alaska. We leveraged these data to develop ensemble machine learning models with the goal of predicting the outbreak distribution of each insect. In this manuscript we: 1) describe the historical patterns of defoliator outbreaks, 2) identify and describe climatic conditions associated with outbreaks in both species and 3) assess whether historic outbreaks have tracked geographic shifts in climate conditions across the region. We demonstrate that outbreaks of hemlock sawfly and western blackheaded budworm have been observed across the Pacific coastal temperature rainforests of North America in each decade since the establishment of the Canadian and United States aerial survey programs. The distribution of outbreaks by both insects were best explained by host availability, a limited range of spring, summer, and winter temperatures, and minimum precipitation. Finally, we demonstrate that outbreaks have tracked the poleward shift in suitable climate over the last century. This study establishes a baseline understanding of the climatic constraints and biogeographic patterns of historic sawfly and budworm outbreaks across the Pacific coastal temperate rainforest and emphasizes the overarching importance of climate in driving the irruptive dynamics of these defoliator species.Keywords: climate envelope, defoliators, hemlock sawfly, Pacific coastal temperate rainforest, population dynamics, western blackheaded budworm

On the deduction and quantification of irruptive dynamics in mountain pine beetle population and proxy data

AbstractIn attempting to develop a coherent description of the irruptive dynamics of mountain pine beetle (MPB) in North America, we examined a range of cases where authors described the dependency of intrinsic population growth rates on population attack levels. In some cases, detailed population data were used, but in most cases, investigators relied on operational aerial detection survey data. We found that study conclusions varied significantly depending on the type of data analysed and the spatial and temporal dimensions of the monitoring program. The ability to detect an intrinsic Allee effect (i.e., positive density‐dependent growth for low and rising population densities) depended on the type and the intensity of the sampling effort and the extent of sampling in space and time. Consequently, not all studies were able to quantify the irruption threshold (i.e., the population density at which endemic populations may transition towards the epidemic state). Notably, in every study where an Allee effect was demonstrated, investigators also identified at least one extrinsic environmental factor (e.g., winter weather, summer drought, microclimatic effect) that was regulating its strength. Our results suggest that population surveys conducted from the ground are a necessary complement to aerial survey data if the goal is to make inferences about the irruptive potential for MPB populations and the role of environmental factors in shaping that irruptive potential.

A dynamic outbreak distribution model (DODM) for an irruptive folivore: The western spruce budworm

Forest disturbances caused by irruptive insect outbreaks have become more frequent due to the non-stationary and warming climate that favours insect development and survival, as well as forest management practices that increase host tree susceptibility. However, predicting and managing the impacts of these highly mobile pests is a challenge due to their uncertain distributions. Dynamic species distribution models (DSDM) acknowledge the non-stationary environment and species’ ecological niches and facilitate niche modeling by matching species observational data relative to environmental data, leading to better outcomes and reducing over-prediction in conservation efforts. The western spruce budworm (WSB; Choristoneura freemani Freeman) is a native forest pest with irruptive dynamics characterized by periodic landscape-scale outbreaks. WSB feeds primarily on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) foliage and can cause extensive defoliation throughout western North American forests. Using decades-long presence and absence records of WSB outbreaks detected via aerial surveys and environmental variables, we developed dynamic outbreak distribution models (DODMs) to predict the distribution of outbreaks over time. Our results showed that DODMs provided robust temporal trends of WSB distributions while generating predictions that support documented northward shifts in epidemic populations. We found that in addition to the environmental variables known to affect WSB survival, phenological synchrony between bud burst and the emergence of larvae in spring was the most important predictor of outbreak distributions. We also identified high-risk areas for future WSB outbreaks and predicted range expansions and contractions under different climate change scenarios. Our study highlights the importance of using presence-absence data and incorporating temporal trends in environmental variables for better assessment of forest pest potential outbreak distributions. These findings have implications for proactive management strategies to prevent outbreaks and mitigate the impacts of WSB on forest ecosystems.

Irruptive dynamics of invasive carnivores and prey populations, and predator control, affect kea survivorship across the Southern Alps

ABSTRACT Invasive carnivores are threatening the indigenous fauna of Aotearoa, but impacts vary with time, space and species. Conservation strategies require knowledge of predator-prey dynamics specific to different ecosystems and allowing for long-term guild shifts. To build this knowledge, we model the survivorship of the kea, a long-lived, ground-nesting parrot with an expansive species-range, using a dataset spanning two decades (1998–2021) and spanning several forest-dominated South Island montane ecosystems. Kea survivorship was lower in eastern ecosystems compared to western ones, depressed during carnivore irruptions, and elevated after predator control. Kea of all ages and both sexes fell prey to stoats and feral cats whilst foraging and roosting, not whilst nesting. Stoat depredation happened in all ecosystems, whereas feral cats featured more in eastern ones. Annual survivorship was high in most contexts (adults > 90%, juveniles > 70%), but a peak in stoat and feral cat predation in eastern ecosystems during 2020–2021 reduced adult survival to <60%. This predation peak followed a rodent population crash, the primary prey of carnivores in eastern beech forests. We conclude that predator impacts on kea are concentrated into post-mast years. Recommendations for management are provided.

Landscape predictions of western balsam bark beetle activity implicate warm temperatures, a longer growing season, and drought in widespread irruptions across British Columbia

Warming temperatures and drought are leading to unprecedented irruptions of forest insects worldwide. Altered climatic regimes have the potential to allow native forest insects to express novel behaviors and expand in latitude and elevation beyond their hypothesized thermal tolerances. Yet, disentangling the drivers of novel behaviors and range expansions is difficult due to cross-scalar interactions that dictate forest insect population dynamics. We investigated the irruptive dynamics of western balsam bark beetle (Dryocoetes confusus Swaine), which undergoes intermittent, long-lasting, but low-severity outbreaks in subalpine fir (Abies lasiocarpa Hook) forests. We compiled data from 1998 to 2019 across British Columbia, Canada, at 1 and 16 km2 resolutions to describe the spatiotemporal structure of beetle outbreaks, predict the landscape susceptibility to outbreaks, explain when outbreaks occur based on bioclimatic drivers, explore whether beetle populations exhibit self-driving growth, and determine whether the extent of outbreaks has increased. Overall, we found that outbreaks occur in discrete clusters and are synchronous at distances of 10–25 km. Further, we found that landscape susceptibility to beetle outbreaks was best explained by latitude*elevation (i.e., the product of latitude [m] and elevation [m] Z-standardized) and volume of fir (m3/ha; >17.5 cm size class; >100 yrs.) both within 8 km and each pixel. Meanwhile, a longer growing season, increased growing degree day accumulation, and drought best explained when outbreaks occurred. Further, we did not find evidence to suggest that beetle populations exhibit self-driving population growth. Finally, we found that the extent of beetle outbreaks has increased over the last 20 years and that increases in landscape susceptibility primarily occurred at higher latitude*elevations and were best explained by an increase in the mean growing season and degree day accumulation. In other words, abnormally warm temperatures have facilitated a northward and upward expansion of western balsam bark beetle. Collectively, these results serve as the first quantification of the spatiotemporal dynamics of this understudied insect and support previous modeling efforts to investigate the multi-scale drivers of western balsam bark beetle outbreaks. We propose that future work should identify specific supercooling points, the relationship between temperature and development rate, and how the relatively high within-stand variability in age and size class of subalpine fir affect the irruptive dynamics of this understudied bark beetle.

Irruptive dynamics of the brush-tailed bettong (

Context The fluctuating fortunes of the brush-tailed bettong have seen this species classified as ‘Rare or Likely to Become Extinct’ in the 1970s, delisted and hailed as a conservation success in the 1990s, and re-listed as ‘Critically Endangered’ in 2008. Key actions to recover this species, broad-scale fox control and reintroduction to fox-free habitat, have had variable success. Aims To monitor the reintroduction of bettong to fox-free habitat of Wadderin Sanctuary in the eastern wheatbelt of Western Australia. Methods Growth of the population was monitored over a 12-year period by regular trapping throughout the sanctuary. Key results The population increased monotonically to peak at 305 individuals (0.71 ha−1) at 7.5 years following release, before subsequent decline. Population growth was accompanied by a significant decline in mean condition and a reduction in the proportion of females with pouch young or lactating, presumably owing to resources per head gradually declining. The proportion of large pouch young relative to total young carried by females declined as woylie numbers increased, suggesting that, increasingly, pouch young did not go to full term. Declines in population number, condition and reproduction were exacerbated by dry seasons. Bettongs established successfully, despite a succession of single feral cats within the 427-ha sanctuary. The removal of the last cat, 8 years following the establishment of bettongs, had no obvious impact on bettong numbers, as, at this point, their population was stabilising because of declining body condition and reduced reproductive output. Conclusions The observed pattern of population growth of bettongs was consistent with an herbivore irruption consequent of a release of a species to new habitat. Decline following peak numbers appeared as a result of density-dependent resource limitation (declining resources per head with increased abundance) interacting with years of low rainfall. Implications The dynamics of irruption and decline of herbivores are relevant to management of reintroduced populations to fenced predator-free sites and, potentially, to unfenced populations following release from predation.

Demographic analysis of the irruptive dynamics of an introduced sika deer population

AbstractIrruptions of large herbivores, with a rapid population increase to peak abundance, are widely observed. Occasionally, there is a population crash following such peaks in abundance, after which the population recovers to form another peak typically lower than the initial. There are mathematical models describing this full cycle of irruptive dynamics. The insight for further improvement of such mathematical models will be obtained from demographic analyses of irruptive dynamics incorporating density‐dependent and density‐independent resource limitations. Using a 35‐yr dataset on the irruptive dynamics of an introduced sika deer (Cervus nippon) population on Nakanoshima Island, Japan, we evaluated the factors and stages determining irruptive dynamics (phase 1: the initial irruption and population crash; phase 2: subsequent dynamics) through key‐factor/key‐stage analysis of the population structure, estimated using age‐at‐death data of naturally dead deer. We set two factors (i.e., deer density and snow accumulation) and three life stages (i.e., immature individuals, adult female, and adult male). The estimated population structure showed two population crashes and a density peak higher than the initial peak during population regrowth subsequent to the initial population crash. The most influential factor and stage for determining the population dynamics differed between phases 1 and 2. The contribution of deer density to the variability in population change was the largest in phase 1 (62.30%) and decreased to 24.10% in phase 2. The contribution of snow accumulation was small in both phase 1 (11.74%) and phase 2 (0.64%). The male stage had the largest contribution in phase 1 (39.23%), while the immature stage had the largest contribution in phase 2 (63.31%). The female stage had the smallest contribution in both phase 1 (24.19%) and phase 2 (17.67%). Population growth rate decreased, while carrying capacity increased, in phase 2 compared to phase 1. We suggest that the small contributions of density‐dependent and density‐independent resource limitations on population dynamics in phase 2 were related to the characteristics of alternative foods (fallen leaves and woody plants) that were newly utilized by deer in phase 2. We conclude that alternative resources potentially generate various irruptive dynamics, including unstable dynamics not expected in the classic paradigm.

Climatic and density influences on recruitment in an irruptive population of Roosevelt elk

Current paradigms of ungulate population ecology recognize that density-dependent and independent mechanisms are not always mutually exclusive. Long-term data sets are necessary to assess the relative strength of each mechanism, especially when populations display irruptive dynamics. Using an 18-year time series of population abundances of Roosevelt elk ( Cervus elaphus roosevelti ) inhabiting Redwood National Park in northwestern California we assessed the influence of population size and climatic variation on elk recruitment and whether irruptive dynamics occurred. An information-theoretic model selection analysis indicated that abundance lagged 2 years and neither climatic factors nor a mix of abundance and climatic factors influenced elk recruitment. However, density-dependent recruitment differed between when the population was declining and when the population increased and then stabilized at an abundance lower than at the start of the decline. The population displayed irruptive dynamics.

Irruptive dynamics of introduced caribou on Adak Island, Alaska: an evaluation of Riney‐Caughley model predictions

Large mammalian herbivores introduced to islands without predators are predicted to undergo irruptive population and spatial dynamics, but only a few well‐documented case studies support this paradigm. We used the Riney‐Caughley model as a framework to test predictions of irruptive population growth and spatial expansion of caribou (Rangifer tarandus granti) introduced to Adak Island in the Aleutian archipelago of Alaska in 1958 and 1959. We utilized a time series of spatially explicit counts conducted on this population intermittently over a 54‐year period. Population size increased from 23 released animals to approximately 2900 animals in 2012. Population dynamics were characterized by two distinct periods of irruptive growth separated by a long time period of relative stability, and the catalyst for the initial irruption was more likely related to annual variation in hunting pressure than weather conditions. An unexpected pattern resembling logistic population growth occurred between the peak of the second irruption in 2005 and the next survey conducted seven years later in 2012. Model simulations indicated that an increase in reported harvest alone could not explain the deceleration in population growth, yet high levels of unreported harvest combined with increasing density‐dependent feedbacks on fecundity and survival were the most plausible explanation for the observed population trend. No studies of introduced island Rangifer have measured a time series of spatial use to the extent described in this study. Spatial use patterns during the post‐calving season strongly supported Riney‐Caughley model predictions, whereby high‐density core areas expanded outwardly as population size increased. During the calving season, caribou displayed marked site fidelity across the full range of population densities despite availability of other suitable habitats for calving. Finally, dispersal and reproduction on neighboring Kagalaska Island represented a new dispersal front for irruptive dynamics and a new challenge for resource managers. The future demography of caribou on both islands is far from certain, yet sustained and significant hunting pressure should be a vital management tool.

Dynamics of an introduced population of mouflon Ovis aries on the sub‐Antarctic archipelago of Kerguelen

A commonly reported pattern in large herbivores is their propensity to irrupt and crash when colonizing new areas. However, the relative role of density‐dependence, climate, and cohort effects on demographic rates in accounting for the irruptive dynamics of large herbivores remains unclear. Using a 37‐yr time series of abundance in a mouflon Ovis aries population located on Haute Island, a sub‐Antarctic island of Kerguelen, 1) we investigated if irruptive dynamics occurred and 2) we quantified the relative effects of density and climate on mouflon population dynamics. Being released in a new environment, we expected mouflon to show rapid growth and marked over‐compensation. In support of this prediction, we found a two‐phase dynamics, the first phase being characterised by an irruptive pattern best described by the θ‐Caughley model. Parameter estimates were rm=0.29±0.005(maximum growth rate), K=473±45 (carrying capacity) and S=2903±396 (surplus) mouflon. With a θ=3.18±0.69 our model also supported the hypothesis that density dependence is strongest at high density in large herbivores. The second phase was characterised by an unstable dynamics where growth rate was negatively affected by population abundance and winter precipitation. Climate, however, did not trigger population crashes and our model suggested that lagged density‐dependence and over‐grazing were the probable causes of mouflon irruptive dynamics. We compare our results with those of Soay sheep and discuss the possibility of a reversible alteration of the island carrying capacity after the initial over‐grazing period.

IRRUPTIVE POPULATION DYNAMICS IN YELLOWSTONE PRONGHORN

Irruptive population dynamics appear to be widespread in large herbivore populations, but there are few empirical examples from long time series with small measurement error and minimal harvests. We analyzed an 89-year time series of counts and known removals for pronghorn (Antilocapra americana) in Yellowstone National Park of the western United States during 1918-2006 using a suite of density-dependent, density-independent, and irruptive models to determine if the population exhibited irruptive dynamics. Information-theoretic model comparison techniques strongly supported irruptive population dynamics (Leopold model) and density dependence during 1918-1946, with the growth rate slowing after counts exceeded 600 animals. Concerns about sagebrush (Artemisia spp.) degradation led to removals of >1100 pronghorn during 1947-1966, and counts decreased from approximately 700 to 150. The best models for this period (Gompertz, Ricker) suggested that culls replaced intrinsic density-dependent mechanisms. Contrary to expectations, the population did not exhibit enhanced demographic vigor soon after the termination of the harvest program, with counts remaining between 100 and 190 animals during 1967 1981. However, the population irrupted (Caughley model with a one-year lag) to a peak abundance of approximately 600 pronghorn during 1982-1991, with a slowing in growth rate as counts exceeded 500. Numbers crashed to 235 pronghorn during 1992-1995, perhaps because important food resources (e.g., sagebrush) on the winter range were severely diminished by high densities of browsing elk, mule deer, and pronghorn. Pronghorn numbers remained relatively constant during 1996-2006, at a level (196-235) lower than peak abundance, but higher than numbers following the release from culling. The dynamics of this population supported the paradigm that irruption is a fundamental pattern of growth in many populations of large herbivores with high fecundity and delayed density-dependent effects on recruitment when forage and weather conditions become favorable after range expansion or release from harvesting. Incorporating known removals into population models that can describe a wide range of dynamics can greatly improve our interpretation of observed dynamics in intensively managed populations.

TESTING THE IRRUPTIVE PARADIGM OF LARGE-HERBIVORE DYNAMICS

A dominant paradigm in understanding and managing large herbivores is that, after introduction to new range or release from harvesting, the herbivore population increases to peak abundance, crashes to a lower abundance, and then increases to a carrying capacity lower than peak abundance. However, support for the paradigm has been largely anecdotal. We first developed two mathematical models to better describe irruptive dynamics. The models differed in the form of the postcrash growth toward carrying capacity: the "Caughley model" included a time lag that generated dampening oscillations, and the "Leopold model" did not. We then evaluated which of four models (theta-logistic, delayed-logistic, Leopold, and Caughley) best described the dynamics of seven ungulate populations either introduced to new range (n = 5 populations) or released from harvesting (n = 2). The dynamics of six of the populations were best described by irruptive models (two by the Leopold, one by the Caughley, and three by the delayed-logistic), and one of the populations did not display irruptive dynamics (theta-logistic model). The limited data thus support the widespread existence of irruptive dynamics, and we encourage the consideration of irruptive models in studies of large-herbivore dynamics.

The influence of clear-cut logging and residual leave material on small mammal populations in aspen-dominated boreal mixedwoods

We investigated responses of populations of common small mammals to variation in the amount of standing trees and residual woody material retained during logging in aspen-dominated, boreal mixedwood forest in north-central Alberta. Experimentally manipulated levels of residual material, with two replicates per level, were (i) "zero residual": clear-cut and no woody debris (tree tops and limbs); (ii) "low residual": clear-cut and woody debris; (iii) "moderate residual": 10% basal area standing live tree retention, and woody debris; and (iv) "uncut": 100% uncut. Patterns of small mammal abundance were similar throughout the study on uncut and moderate-residual grids, where southern red-backed voles (Clethrionomys gapperi Vigors) and deer mice (Peromyscus maniculatus Wagner) were numerically dominant. In contrast, C. gapperi were virtually absent from the low- and zero-residual grids during the breeding season following logging and occurred at low numbers in the following 2 years. Peromyscus maniculatus were common on low- and zero-residual grids in all years. We found no evidence that rates of reproductive activity and (or) survival in C. gapperi and P. maniculatus were affected by logging treatments. Meadow voles (Microtus pennsylvanicus Ord) were uncommon on uncut and moderate-residual grids throughout the study but were abundant on both low-residual grids and one zero-residual grid, particularly in the second year following logging, when they exhibited irruptive dynamics in midsummer. In sum, relative abundances of common small mammals were most affected by intensive (clear-cut) logging, and our results suggest that the experimental retention of at least 10% basal area on cutovers may reduce some of these effects.