How climate extremes—not means—define a species' geographic range boundary via a demographic tipping point

Abstract

Abstract. Species' geographic range limits interest biologists and resource managers alike; however, scientists lack strong mechanistic understanding of the factors that set geographic range limits in the field, especially for animals. There exists a clear need for detailed case studies that link mechanisms to spatial dynamics and boundaries because such mechanisms allow us to predict whether climate change is likely to change a species' geographic range and, if so, how abundance in marginal populations compares to the core.The bagworm Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae) is a major native pest of cedars, arborvitae, junipers, and other landscape trees throughout much of North America. Across dozens of bagworm populations spread over six degrees of latitude in the American Midwest we find latitudinal declines in fecundity and egg and pupal survivorship as one proceeds toward the northern range boundary. A spatial gradient of bagworm reproductive success emerges, which is associated with a progressive decline in local abundance and an increase in the risk of local population extinction near the species' geographic range boundary.We developed a mathematical model, completely constrained by empirically estimated parameters, to explore the relative roles of reproductive asynchrony and stage-specific survivorship in generating the range limit for this species. We find that overwinter egg mortality is the biggest constraint on bagworm persistence beyond their northern range limit. Overwinter egg mortality is directly related to winter temperatures that fall below the bagworm eggs' physiological limit. This threshold, in conjunction with latitudinal declines in fecundity and pupal survivorship, creates a nonlinear response to climate extremes that sets the geographic boundary and provides a path for predicting northward range expansion under altered climate conditions. Our mechanistic modeling approach demonstrates how species' sensitivity to climate extremes can create population tipping points not reflected in demographic responses to climate means, a distinction that is critical to successful ecological forecasting.

Projected sea surface temperatures over the 21st century: Changes in the mean, variability and extremes for large marine ecosystem regions of Northern Oceans

“…The shift in the SST distribution results in exceptionally large warm extremes and the disappearance of cold extremes relative to the end of the 20 th century in the RCP8.5 simulations. As the growth and reproduction of many species depends on their thermal tolerances, extreme temperatures could have a substantial impact on fish population dynamics and biodiversity (Pörtner et al, 2001;Pörtner and Peck, 2010;Lynch et al, 2014;Deutsch et al, 2015).…”

“…Extreme conditions, however, also occur in the world oceans (Hobday et al, 2016) and there is a growing appreciation that extremes strongly influence population dynamics and biogeography of many organisms (Portner et al, 2001;Lynch et al, 2014). Recent studies have explored periods with very warm SSTs or "ocean heat waves" in the northwest Atlantic (Mills et al, 2013;Chen et al, 2014Chen et al, , 2015, Northern Hemisphere oceans (Scannell et al, 2016), Mediterranean Sea (Black et al, 2004;Olita et al, 2007), off the coast of western Australia (Pearce and Feng, 2013;Wernberg et al, 2013), and over global coastal regions (Lima and Wethey, 2012).…”

Evolution caused by extreme events

“…Environmental conditions at the margins of geographical distributions are likely to be extreme-that is often why those locations are margins. Diurnal, seasonal or annual fluctuations in limiting factors in such environments put strong evolutionary pressures on organisms that live at the boundaries [23,24]. Those pressures are expected to increase in the future at some boundaries and relax at others.…”

When hot rocks get hotter: behavior and acclimatization mitigate exposure to extreme temperatures in a spider

“…Although persistent changes in mean temperature will have chronic effects on fitness, thermal extremes are acute and often result in death (Welbergen et al 2008, McKechnie and Wolf 2010, Bauerfeind and Fischer 2014. The ability to survive extreme events (thermal or otherwise) will determine whether an individual or population persists in a changing environment (Parmesan et al 2000, Culumber and Monks 2014, Lynch et al 2014 because the probability of extreme weather events, rather than higher mean temperatures, correlate with upper thermal tolerances (critical thermal maximum, CT Max ) (Clusella-Trullas et al 2011).…”