Ronald F. Billings

IMPACT OF MINIMUM WINTER TEMPERATURES ON THE POPULATION DYNAMICS OF DENDROCTONUS FRONTALIS

Predicting population dynamics is a fundamental problem in applied ecology. Temperature is a potential driver of short-term population dynamics, and temperature data are widely available, but we generally lack validated models to predict dynamics based upon temperatures. A generalized approach involves estimating the temperatures experienced by a population, characterizing the demographic consequences of physiological responses to temperature, and testing for predicted effects on abundance. We employed this approach to test whether minimum winter temperatures are a meaningful driver of pestilence from Dendroctonus frontalis (the southern pine beetle) across the southeastern United States. A distance-weighted interpolation model provided good, spatially explicit, predictions of minimum winter air temperatures (a putative driver of beetle survival). A Newtonian heat transfer model with empirical cooling constants indicated that beetles within host trees are buffered from the lowest air temperatures by approximately 1-4 degrees C (depending on tree diameter and duration of cold bout). The life stage structure of beetles in the most northerly outbreak in recent times (New Jersey) were dominated by prepupae, which were more cold tolerant (by >3 degrees C) than other life stages. Analyses of beetle abundance data from 1987 to 2005 showed that minimum winter air temperature only explained 1.5% of the variance in interannual growth rates of beetle populations, indicating that it is but a weak driver of population dynamics in the southeastern United States as a whole. However, average population growth rate matched theoretical predictions of a process-based model of winter mortality from low temperatures; apparently our knowledge of population effects from winter temperatures is satisfactory, and may help to predict dynamics of northern populations, even while adding little to population predictions in southern forests. Recent episodes of D. frontalis outbreaks in northern forests may have been allowed by a warming trend from 1960 to 2004 of 3.3 degrees C in minimum winter air temperatures in the southeastern United States. Studies that combine climatic analyses, physiological experiments, and spatially replicated time series of population abundance can improve population predictions, contribute to a synthesis of population and physiological ecology, and aid in assessing the ecological consequences of climatic trends.

Temperature Extremes, Density Dependence, and Southern Pine Beetle (Coleoptera: Curculionidae) Population Dynamics in East Texas

Abstract Previous studies of the southern pine beetle, Dendroctonus frontalis Zimm., established that its population in east Texas responds to a delayed density-dependent process, whereas no clear role of climate has been determined. We tested two biological hypotheses for the influence of extreme temperatures on annual southern pine beetle population growth in the context of four alternative hypotheses for density-dependent population regulation. The significance of climate variables and their interaction with population regulation depended on the model of density dependence. The best model included both direct and delayed density dependence of a cubic rather than linear form. Population growth declined with the number of days exceeding 32°C, temperatures previously reported to reduce brood survival. Density dependence also changed with the number of hot days. Growth was highest in years with average minimum winter temperatures. Severely cold winters may reduce survival, whereas warm winters may reduce the efficiency of spring infestation formation. Whereas most previous studies have incorporated climate as an additive effect on growth, we found that the form of delayed density dependence changed with the number of days >32°C. The interaction between temperature and regulation, a potentially common phenomenon in ecology, may explain why southern pine beetle outbreaks do not occur at perfectly regular intervals. Factors other than climate, such as forest management and direct suppression, may have contributed significantly to the timing, severity, and eventual cessation of outbreaks since the mid-1950s.

Alternate attractors in the population dynamics of a tree-killing bark beetle

Among the most striking changes in ecosystems are those that happen abruptly and resist return to the original condition (i.e., regime shifts). This frequently involves conspicuous changes in the abundance of one species (e.g., an oubreaking pest or keystone species). Alternate attractors in population dynamics could explain switches between low and high levels of abundance, and could underlie some cases of regime shifts in ecosystems; this longstanding theoretical possibility has been difficult to test in nature. We compared the ability of an alternate attractors model versus two competing models to explain population fluctuations in the tree-killing bark beetle, Dendroctonus frontalis. Frequency distributions of abundance were distinctly bimodal, a prediction of the alternate attractors model, strongly indicating the lack of a single, noisy equilibrium. Time series abundance data refuted the existence of strong delayed density-dependence or nonlinearities, as required by the endogenous cycles model. The model of alternate attractors was further supported by the existence of positive density-dependence at intermediate beetle abundances. Experimental manipulations show that interactions with competitors and shared enemies could create a locally stable equilibrium in small populations of D. frontalis. High variation among regions and years in the abundance of predators and competitors could permit switches between alternate states. Dendroctonus frontalis now provides the strongest case that we know of for alternate attractors in natural population dynamics. The accompanying demographic instability appears to underlie spatially extensive outbreaks that have lasting impacts on forest ecosystems. Understanding feedbacks in populations with alternate attractors can help to identify thresholds underlying regime shifts, and potentially manage them to avoid undesirable impacts.

Spatio‐temporal dynamics of a tree‐killing beetle and its predator

&NA; Resolving linkages between local‐scale processes and regional‐scale patterns in abundance of interacting species is important for understanding long‐term population stability across spatial scales. Landscape patterning in consumer population dynamics may be largely the result of interactions between consumers and their predators, or driven by spatial variation in basal resources. Empirical testing of these alternatives has been limited by the lack of suitable data. In this study, we analyzed an extensive network of spatially replicated time series to characterize the local and regional processes affecting spatio‐temporal dynamics of a tree‐killing bark beetle (Dendroctonus frontalis or SPB) and its key predator (Thanasimus dubius) across the southeastern United States. We first used a mechanistic model to evaluate factors affecting the stability of 95 predator–prey time series and then conducted spatial analyses to evaluate scale dependence in the factors affecting the geographical patterning of this system. Across the region, population fluctuations of both species were correlated in space beyond 400 km but there was notable spatial variation in the deterministic and stochastic processes influencing forest‐scale (local) fluctuations. Time series analyses indicated that local dynamics of SPB and T. dubius are not cyclical. Instead, the abundance of T. dubius responded almost instantaneously to changes in SPB abundance. Spatial variation in long‐term forest‐scale abundance of both species was linked most strongly to the abundance of pine habitat indicating a stronger role for resource availability in SPB population dynamics than top‐down effects. Our results are consistent with other studies indicating that animal populations tend to be synchronized in space via spatially correlated processes such as weather; yet local dynamics tend to be linked to smaller‐scale host patterns. Our study provides a rare empirical assessment of how local processes scale up to produce landscape patterns that influence forest ecology and forest management.

The pine caterpillar Dendrolimus punctatus in Viet Nam; Recommendations for integrated pest management

Abstract In Viet Nam, frequent outbreaks of the native pine caterpillar, Dendrolimus punctatus Walker (lepidoptera: Lasiocampidae), are beginning to occur in young plantations of Pinus massoniana Lamb. and P. merkussi Jungh and de Vriese. Severe defoliation reduces volume growth and resin production; repeated defoliation may cause tree mortality. Current control programs rely heavily on manual methods (hand-removal and destruction of life-stages), use of light traps to catch adult moths, and some insecticide applications. A more integrated approach to prevention and control is suggested in pine forestry programs to reduce defoliator problems. Short-term approaches should focus on biological control, including mass production and application of microbial agents and parasitic insects, rather than total reliance on chemical pesticides. Long-term solutions will need to involve the establishment of mixed stands of different pine species or pines and hardwoods, replacement of pines with non-host species on high-hazard sites, increased fire prevention, and further professional training of Vietnamese protection personnel in all phases of integrated pest management.