Irruptive Population Dynamics in Yellowstone Pronghorn

Abstract

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.

Adaptive management of sika deer populations in Hokkaido, Japan: theory and practice

“…Forsyth and Caley (2006) developed mathematical models to better describe irruptive dynamics of large-herbivore populations and evaluated the dynamics of seven ungulate populations either introduced to new ranges or released from harvesting. A recent study on the population dynamics of the pronghorn (Antilocapra americana) in Yellowstone National Park of the western US also supported the paradigm that irruption is a fundamental pattern in large herbivores with high fecundity and delayed density-dependent effects on recruitments (White et al 2007).…”

Decoupling of component and ensemble density feedbacks in birds and mammals

“…Furthermore, the Gompertz-logistic model has performed robustly in describing the general dynamics of populations of birds and mammals over a wide range of body sizes (e.g., Saitoh et al 1997, 2008, Wang et al 2002, White et al 2007, Seavy et al 2009, Pasinelli et al 2011, is present in multi-model inference scenarios where competing models are contrasted (Saitoh et al 1997, Zeng et al 1998, Fryxell et al 2005, Chamaille´-Jammes et al 2008, McMahon et al 2009), is the top-ranked model in meta-analyses of hundreds of species in which various alternatives have also been evaluated (e.g., Brook and Bradshaw 2006), and has been a model used in theoretical development about density feedback (e.g., Dennis et al 2006). We avoided fitting the fully parameterized h-logistic model, due to recent caveats of application to analyses of time series (Clark et al 2010), or other highly parameterized analogues (e.g., hyperbolic growth).…”

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

“…Thus, the increase in known plus unreported harvest combined with strengthening density-dependence presents a more likely set of conditions that helped slow the second irruption. High levels of harvest can replace densitydependent processes regulating population growth (White et al 2007), but our high H model that culled 15% of the population resulted in continued growth, so additional densitydependent mechanisms were needed to slow the irruption. Notably, our modeled densitydependence was relatively moderate when compared to the extreme lower end of vital rates measured in other predator-free Rangifer herds (e.g., calf survival ¼ 50%, pregnancy rate ¼ 7% (Bergerud 2000)).…”

“…Third, most introduced Rangifer are only hunted for a short time before human settlements are abandoned (Leader-Williams 1988), yet populations subjected to multiple and discontinuous pulses of harvest intensity may exhibit growth patterns that diverge from the typical four-stage shape. While long-established ungulate populations released from hunting mortality can undergo weaker irruptions compared to newly introduced populations (McCullough 1997, Forsyth and Caley 2006, White et al 2007, Tyler 2010, empirical studies of populations subjected to multiple periods of harvest are sparse. Fourth, the potential for prolonged delays in initial population growth and spatial expansion (i.e., latency or lag times) because of life history traits of the introduced species or novel environmental conditions is a fundamental concept of invasive species ecology (Crooks et al 1999) that may influence the timing of irruptive dynamics (Forsyth and Caley 2006).…”