Global distribution of a key trophic guild contrasts with common latitudinal diversity patterns

Abstract

Most hypotheses explaining the general gradient of higher diversity toward the equator are implicit or explicit about greater species packing in the tropics. However, global patterns of diversity within guilds, including trophic guilds (i.e., groups of organisms that use similar food resources), are poorly known. We explored global diversity patterns of a key trophic guild in stream ecosystems, the detritivore shredders. This was motivated by the fundamental ecological role of shredders as decomposers of leaf litter and by some records pointing to low shredder diversity and abundance in the tropics, which contrasts with diversity patterns of most major taxa for which broad-scale latitudinal patterns haven been examined. Given this evidence, we hypothesized that shredders are more abundant and diverse in temperate than in tropical streams, and that this pattern is related to the higher temperatures and lower availability of high-quality leaf litter in the tropics. Our comprehensive global survey (129 stream sites from 14 regions on six continents) corroborated the expected latitudinal pattern and showed that shredder distribution (abundance, diversity and assemblage composition) was explained by a combination of factors, including water temperature (some taxa were restricted to cool waters) and biogeography (some taxa were more diverse in particular biogeographic realms). In contrast to our hypothesis, shredder diversity was unrelated to leaf toughness, but it was inversely related to litter diversity. Our findings markedly contrast with global trends of diversity for most taxa, and with the general rule of higher consumer diversity at higher levels of resource diversity. Moreover, they highlight the emerging role of temperature in understanding global patterns of diversity, which is of great relevance in the face of projected global warming.

Biotic and abiotic variables influencing plant litter breakdown in streams: a global study

“…Here, we found the relationship to hold with long-term air temperature data. Although we did not detect any variation with temperature for detritivore-mediated breakdown when it was examined separately, total breakdown of alder increased towards cooler streams, and this pattern was most likely related to the higher abundance and diversity of litter-consuming detritivores at higher latitudes [38,57]. Accordingly, our comparison of coarse-mesh and fine-mesh bags revealed a greater role of detritivores on litter breakdown in temperate than in tropical streams, although this difference was most obvious for litter mixtures.…”

“…The effect of pH on alder breakdown was, however, modulated by climate: while breakdown was faster in more alkaline waters at higher temperatures, the opposite was true in cooler streams (figure 1f). This interaction may reflect the facts that (i) microbial decomposition is most important in the tropics and is boosted by higher calcium concentration [66,67], and (ii) major litter consumers such as stoneflies are more important in circumneutral and acidic streams at higher latitudes [68,69], while they are rare in the tropics [57]. Consistent with this explanation, caddiflies, which dominate the guild of litter-consuming detritivores in tropical streams [57], tend to be more sensitive to low pH [70].…”

Effect of experimental and seasonal warming on litter decomposition in a temperate stream

“…The maximum temperature observed in this study (16°C) was lower than that observed in the three correlative studies mentioned above (23.5-26°C), and was well within the tolerance limits for most invertebrates (Gaufin and Hern 1971). The low abundance/biomass of detritivores at low latitudes (Irons et al 1994;Boyero et al 2011a) might also be due to low litter quality (e.g., high concentration of structural and secondary compounds, low concentration of nutrients, thick cuticle), since resident plants may be better defended against herbivory than at higher latitudes. These defenses may persist after senescence (Graça and Cressa 2010;Marquis et al 2012).…”

A Comparison of the Temperature Regime of Short Stream Segments under Forested and Non‐Forested Riparian Zones at Eleven Sites Across North America

“…One recent study by Steel et al (2012) demonstrated that emergence time of salmon eggs was shorter under diurnal temperature fluctuations than at a steady temperature. Even minor alterations to the stream temperature regime can have ecosystem-wide impacts on ecosystem functions such as primary production and decomposition (Dang et al, 2009;Riley and Dodds, 2013), carbon balance of macrophytes (Santamaria and van Vierssen, 1997), fish distribution among microhabitats (Baltz et al, 1987;Nielsen et al, 1994) and distribution of trophic guilds and species at the continental scale (Eaton and Scheller, 1996;Boyero et al, 2011).…”