I assessed effects of the widest range of temperatures ever examined on maximal food-chain length (L) and macroinvertebrate species richness (S) by a worldwide comparison of spring ecosystems with mean water temperatures (T) ranging from 4.5 to 93°C. Eukaryotic L averaged ∼3.2 and varied independently of T between 4.5 and 31°C. However, over the relatively narrow T range of 35 to 50°C, L dropped abruptly to 0 and remained so up to 91°C. The negatively nonlinear relationships of L vs T and macroinvertebrate S vs T both deviated from predictions based upon metabolic theory, and the negative effect of T on S contrasted with positive relationships observed at larger regional scales. Thermal tolerance limits apparently play a major role in causing these relationships, but other factors also may be involved (e.g., availability of colonists adapted to different temperatures and temperature-dependent rates of resource use and species interactions that affect population establishment and persistence).
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The length of food chains in ecosystems is limited, usually reaching 3 to 4 levels, and rarely >6 levels. This limit has been attributed to the low efficiency of energy transfer between trophic levels resulting in insufficient energy supplies to support viable populations of high-level predators (Lindemann 1942, Hutchinson 1959). However, although energy availability must set an upper limit to food-chain length, the length that is actually observed in nature may be affected by several other factors. These factors may include environmental stability, ecosystem size or dimensions, predator–prey body-size ratios, colonization history, metacommunity dynamics, and various morphological and behavioral constraints (reviewed in Pimm 1982, Post 2002, Arim et al. 2007b, Sabo et al. 2009, Calcagno et al. 2011). During the last 3 decades, recognition of these potential effects has led to numerous insightful comparative and experimental analyses of foodweb structure (e.g., Pimm 1982, Briand and Cohen 1987, Schoener 1989, Spencer and Warren 1996, Post 2002, Jennings and Warr 2003, Thompson and Townsend 2005, Arim et al. 2007b, Post and Takimoto 2007, Vander Zanden and Fetzer 2007, Sabo et al. 2010). However, a synthetic theory of food-chain length has yet to be achieved.
Variation in food-chain length among ecosystems is not always directly related to energy limitation (cf. Pimm 1982, Yodzis 1984, Spencer and Warren 1996, Thompson and Townsend 2005, Arim et al. 2007b, Sabo et al. 2009), but Arim et al. (2007a) have reformulated the energy limitation hypothesis to show how body size and ambient temperature might affect food-chain length in quantifiable ways. Their model is based on the metabolic theory of ecology (MTE; Brown et al. 2004). It predicts that the trophic position of species in a community should show a humped relationship with body size and that food-chain length should be negatively correlated with environmental temperature.
The purpose of my study is to test the metabolic model of Arim et al. (2007a), concerning how temperature should affect food-chain length, by using literature data on spring ecosystems. Springs are especially useful for making such a test because their physicochemical stability, typically small size, and relatively discrete boundaries make them highly amenable to studies of foodweb structure and energy flow at the ecosystem level. As a result, some of these studies have become well known classics that are often featured in ecology and biology textbooks or anthologies (e.g., Odum 1957, Teal 1957, Tilly 1968). In addition, springs exhibit both low variation in temperature within sites and high variation in temperature among sites. The near thermal constancy of springs allows the biota to become precisely adapted to specific temperature levels, and the extreme variation in temperature among springs (from near freezing to boiling temperatures) provides an excellent opportunity to study the effects of a wide range of temperatures on biological systems at many hierarchical levels (Glazier 2009). My study is the first attempt to examine the effects of a large range of temperatures (nearly 90°C) on foodweb structure in natural ecosystems. Authors of previous related studies have examined a relatively narrow range of temperatures (ranges of only 7–12°C) in artificial micro- or mesocosms (Beisner et al. 1997, Petchey et al. 1999).
The metabolic hypothesis of Arim et al. (2007a) predicts that food-chain length should show a negatively linear relationship with temperature (Fig. 1A) because higher temperatures increase metabolic costs, thereby increasing the amount of energy that populations need from lower trophic levels to maintain a minimal viable size, while also reducing the amount of energy that species populations can transfer to higher trophic levels. Consequently, as temperature increases, the ratio of energy supply to demand decreases successively at each higher trophic level, and thus, increases the relative extinction probability of species populations at the upper vs lower ends of a food chain.
This hypothesis can be contrasted with an alternative hypothesis, the thermal tolerance hypothesis, which predicts that food-chain length should show a negative, stepped relationship with temperature (Fig. 1B). This hypothesis is based on the assumption that the biochemical similarity of all organisms in a given ecological community causes them to have similar thermal tolerance limits. Therefore, below a certain temperature limit, food-chain length should be independent of temperature, but near and beyond this limit, food-chain length should decline abruptly to 0. This hypothesis was inspired by Brock's (1985) review of thermal tolerance limits in various organisms (also see Discussion).
A 3rd possibility is that increased temperature causes trophic levels to drop out abruptly in a successive, stepwise pattern (Fig. 1C). This pattern may be explained by either of the above hypotheses (mechanisms) or both in combination. The metabolic hypothesis may explain this multiple stepped pattern if one relaxes its assumption that consumers are more or less omnivorous (i.e., feed at >1 trophic level), and thus diffe