In the Deep End! Reading: Recurrent insect outbreaks caused by temperature-driven changes in system stability

Recurrent insect outbreaks caused by temperature-driven changes in system stability
Authors: William A. Nelson, Ottar N. Bjørnstad, Takehiko Yamanaka

Introduction:

The effect of temperature on individual life-cycles has been well documented but its effect on population dynamics is less clear. For multivoltine* insects (having multiple broods in a year), the effect of temperature contribute to their outbreak "early in the season when climate has helped synchronize the population stage-structure either through induction of diapause (7,8) or differential winter mortality" (Other words: Synchronize the population due to climate change being a stimuli for the induction of diapause OR changing the mortality of the population during winter times) 

The smaller tea-tortrix is a multivoltine insect with outbreaks being 100-4000 folds higher than their troughs. The summer outbreaks are most consistent with intraspecific mechanisms. However, outbreaks are strongly variable throughout a season with outbreaks occurring in the warmer months (May to September) and relatively rare in winter months.

"The classic explanation for such a pattern is that cool winter temperatures synchronize population stage-structure, which leads to transient generation cycles*. This mechanism generally results in cohort synchrony and outbreak amplitudes that are greatest at the start of the season, and decay through time. The decay in cohort synchrony through time is observed in other structured systems (16), and occurs because developmental plasticity and environmental variability cause development to become increasingly uncorrelated."

However, the smaller tea-tortrix's developmental synchrony is further enhanced than eroded.

Method: 

The model utilized for this study utilized coupled integral delay-differential equations with parameters coming from laboratory, experimental work.

 "The model predicts that the population dynamics should be strongly temperature-dependent with decay to extinction at low temperatures, transient fluctuations around a stable equilibrium at intermediate temperatures and sustained generation cycles at higher temperatures"

To see if temperature drives these dynamics, two testable predictions were made:

"The model provides two testable predictions if the dynamics are driven by temperature-dependent destabilization: (i) cycle amplitude should show a threshold response to temperature, and (ii) above the threshold the cycle amplitude should increase roughly proportionally to temperature"

The study also utilized wavelet analysis*. They concluded that there were two dominant patterns looking at the 51 year time series from Japan: A strong and stational signal which reflect an annual cycle and a more pronounced and punctuated non-signals that reflect 4-6 outbreak cycles.

To test the predictions of the ‘classical’ seasonal hypothesis versus the new temperature induced destabilization hypothesis, we regressed outbreak amplitude against temperature using a mixed-effects linear and thresholded piecewise linear model. 

The paper concludes that:

 "The analysis thus strongly supports the hypothesis that temperature destabilizes population dynamics and sustains recurrent outbreaks in this system"

Jargons Jargons Jargons:

voltinism: Voltinism is the number of broods or generations of an organism within a year. Different types of voltinism exists and describe whether an organism has one, two or many broods a year.SOURCE

induction of diapause: 
Diapause, when referencing animal dormancy, is the delay in development in response to regularly and recurring periods of adverse environmental conditions. (Wiki).

Diapause is induced in advance of the advent of the environmental adversity. Diapause-inducing stimuli (or cues) are perceived during a fixed and specific sensitive period, which is genetically determined, and it ranges from various periods within the parental generation through different stages of embryonal, larval and pupal development to the adult individual. The inducing cues are signalling for the coming deterioration of environmental conditions, and the term token stimuli ( Lees, 1955) is used in the literature to distinguish them from direct effects of other environmental factors on the rate of physiological processes. (Vladimír Koštál) 

transient generation cycles: What is this? Don't know- Google doesn't help.

delay-differential equations: 
The delays or lags can represent gestation times, incubation periods, transport delays, or can simply lump complicated biological processes together, accounting only for the time required for these processes to occur. A great source

Wavelet analysis: A great source