Consequently, we predict that temperature variation across elevation and latitude should shape macrophysiological patterns in thermal limits. High elevation tropical species may have low CT MIN values similar to those found in temperate species. In the warmer lowland tropics, organisms should exhibit higher CT MAX and CT MIN values compared to organisms from colder, temperate regions. For most organisms, this relationship is driven by maximum (CT MAX) and minimum (CT MIN) critical thermal limits that track maximum and minimum environmental temperatures respectively (Angilletta, 2009). Macrophysiological studies of terrestrial species have shown general support for the predicted relationship between thermal tolerance and latitude (Addo-Bediako, Chown, & Gaston, 2000 Brattstrom, 1968 Calosi, Bilton, & Spicer, 2008 Calosi, Bilton, Spicer, & Atfield, 2008 Deutsch et al., 2008 Feder, 1978 Sunday, Bates, & Dulvy, 2011 van Berkum, 1988) and elevation (Addo-Bediako et al., 2000 Bozinovic et al., 2011 Ghalambor et al., 2006 Sheldon & Tewksbury, 2014). Thus, Janzen ( 1967) proposed that mountain passes should be “higher” for lowland tropical species because colder temperatures at higher elevations would be a greater physiological barrier to dispersal compared to temperate lowland species (Chan et al., 2016 Ghalambor, Huey, Martin, Tewksbury, & Wang, 2006 Huey, 1978 McCain, 2009). The narrower thermal breadth of tropical species should lead to reduced dispersal and a reduced elevation distribution.
However, in tropical mountains at any given elevation, a narrow annual range of temperatures should lead to a narrow thermal tolerance. In temperate mountains, seasonal temperature changes cause low and high elevation localities to have similar annual ranges, which should allow species to physiologically tolerate a wide range of temperatures. In an extension of the CVH, Janzen ( 1967) incorporated the influence of elevation on thermal breadth. Thus, compared to the climatically stable tropics, mid-to-high latitude temperate environments should select for organisms with broader thermal breadths because of the seasonal variation between warm summer and cold winter temperatures (Angilletta, 2009).
The “climate variability hypothesis” (CVH) predicts that more variable climates select for organisms with broader thermal tolerances, whereas less variable (stable) climates select for narrower thermal tolerances (Angilletta, 2009 Dobzhansky, 1950 Gaston & Chown, 1999 Janzen, 1967 Stevens, 1989). Furthermore, because lowland tropical aquatic insects have the narrowest thermal breadths of any region, they may be particularly vulnerable to short-term extreme changes in stream temperature.Ī fundamental goal of integrative biology is to understand how variation in climate shapes the thermal physiology, behaviour, and geographic distribution of organisms (Angilletta, 2009 Bozinovic, Calosi, & Spicer, 2011 Spicer & Gaston, 2009). These macrophysiological patterns are consistent with the narrower elevation ranges found in other tropical organisms, and they extend Janzen's CVH to freshwater streams.In tropical insects, by contrast, CT MAX declined less sharply than CT MIN with elevation, causing thermal breadth to increase with elevation. In temperate species, thermal breadth decreased with increasing elevation because CT MAX declined with elevation while CT MIN was similar across elevations. However, elevation had contrasting effects on patterns of thermal breadth. Temperate streams exhibited greater seasonal temperature variation and overlap across elevations than tropical streams, and as predicted, temperate aquatic insects exhibited broader thermal breadths than tropical insects.the difference between the critical thermal maximum and minimum) of 62 aquatic insect species from temperate (Colorado, USA) and tropical (Papallacta, Ecuador) streams spanning an elevation gradient of c. We tested these predictions by measuring stream temperatures and thermal breadths (i.e.Janzen's extension of the climate variability hypothesis (CVH) posits that increased seasonal variation at high latitudes should result in greater temperature overlap across elevations, and favour wider thermal breadths in temperate organisms compared to their tropical counterparts.
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