The Effects of Climate Modes on Growing-Season Length and Timing of Reproduction in the Pacific Nordwest as Revealed by Biophysical Modeling of Lizards

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ABSTRACT.-

The effects of climate change on environments are increasingly obvious and are leading to stronger and more varied biological responses as organisms compensate. To understand better the responses of terrestrial ectotherms to climate oscillations we created a biophysical model based on the degree-days concept to predict relevant life-history events. By studying side-blotched lizards, Uta stansburiana, we were able to calculate the timing and length of the growing season as well as the timing of the breeding season for this species at a site in the Pacific Northwest based primarily on temperature data. We then used historical meteorological data to test for the effects of short- and long-term fluctuations in climate modes on lizards using indices for the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Results indicate that these two climate modes affect different aspects of life history and that their effects, either alone or in combination, have the potential to alter not only phenology (e.g., timing of reproduction) but also population recruitment and even persistence. Yet, using these climate modes to predict biological outcomes of future climate change may be difficult to accomplish due to regional variation of their impacts and their inherent complexity.

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INTRODUCTION

Ongoing climate change is altering natural environments by changing the timing and length of growing seasons (Sparks and Menzel, 2002; Schwartz et al, 2006), the magnitude of daily minimum and maximum temperatures (DeGaetano, 1996; Easterling et al, 1997; DeGaetano and Allen, 2002), the distribution of precipitation (Groisman et al, 1999; Miller and Goodrich, 2007), or a combination of diese (Easterling et al, 2000a, b; Meehl et al, 2000). These changes in heat and moisture distribution have already begun to evoke ecological and evolutionary responses as organisms track the environment (see Parmesan, 2006). Beyond merely documenting past change, one of the major challenges facing biologists is the prediction of life's responses to future change given the myriad of possible interactions not only between organisms and their environments but also among organisms ( see Travis and Futuyma, 1993). By and large, efforts have focused on modeling blanket changes in the environment such as responses to a warmer world (e.g., Buckley, 2008; Crozier et al, 2008; Li et al, 2009). While there is certainly value in this approach, it may overlook subtle variations in atmospheric and oceanic circulations that give rise to natural dynamical modes, such as the El Niño-Southern Oscillation. However, an increasing number of studies have linked these climate modes to effects on organisms in both aquatic (Jonsson and Jonsson, 2004; Lehodey et al, 2006; Brander, 2007) and terrestrial (Post and Stenseth, 1999; Hallett et al, 2004; Woodward et al, 2008) environments (for reviews see Stenseth et al, 2002; Wang and Schimel, 2003). In this paper, we examine the past impacts of climate modes on a terrestrial ectotherm in the Pacific Northwest of North America in an attempt to understand the effects of future environmental variation.

One well-studied climate mode is the El Niño-Southern Oscillation (ENSO). The ENSO cycle, with a periodicity of 2-7 y, consists of a mass of relatively warm seawater that builds in the eastern tropical Pacific Ocean during positive (El Niño) phases of the oscillation, and relatively cold water during negative (La Niña) phases (Rayano et al, 2005). Due to oceanatmospheric teleconnections, the ENSO affects both aquatic and terrestrial ecosystems (for reviews see Glynn, 1988; Stenseth et al, 2002; Wang and Schimel, 2003). During strong El Niño events, the Pacific Northwest experiences relatively warm conditions, while the opposite is true during a La Niña event (Woodward et al, 2008). Due to its seasonal timing, the ENSO should advance or delay the onset of spring depending on its phase and potentially impact phenological responses of organisms. …