Abstract
Warming can lead to increased growth of plants or algae at the base of the food web, which may increase the overall complexity of habitat available for other organisms. Temperature and habitat complexity have both been shown to alter the structure and functioning of communities, but they may also have interactive effects, for example, if the shade provided by additional habitat negates the positive effect of temperature on understory plant or algal growth. This study explored the interactive effects of these two major environmental factors in a manipulative field experiment, by assessing changes in ecosystem functioning (primary production and decomposition) and community structure in the presence and absence of artificial plants along a natural stream temperature gradient of 5–18°C. There was no effect of temperature or habitat complexity on benthic primary production, but epiphytic production increased with temperature in the more complex habitat. Cellulose decomposition rate increased with temperature, but was unaffected by habitat complexity. Macroinvertebrate communities were less similar to each other as temperature increased, while habitat complexity only altered community composition in the coldest streams. There was also an overall increase in macroinvertebrate abundance, body mass, and biomass in the warmest streams, driven by increasing dominance of snails and blackfly larvae. Presence of habitat complexity, however, dampened the strength of this temperature effect on the abundance of macroinvertebrates in the benthos. The interactive effects that were observed suggest that habitat complexity can modify the effects of temperature on important ecosystem functions and community structure, which may alter energy flow through the food web. Given that warming is likely to increase habitat complexity, particularly at higher latitudes, more studies should investigate these two major environmental factors in combination to improve our ability to predict the impacts of future global change.
Highlights
Accelerated planetary warming is well established and is predicted to continue over the coming century, with the Arctic region expected to undergo some of the highest rates of warming (Pachauri et al, 2014)
There was an interactive effect of temperature and habitat complexity at the end of the experiment, with the greatest dissimilarity in macroinvertebrate community composition occurring as a result of the habitat complexity treatment in the coldest streams (Table 3, Figure 4b)
There were interactive effects, such that primary production only increased with temperature in the presence of more complex habitat and the greatest effects of habitat complexity on macroinvertebrate community composition occurred in the coldest streams
Summary
Accelerated planetary warming is well established and is predicted to continue over the coming century, with the Arctic region expected to undergo some of the highest rates of warming (Pachauri et al, 2014). Increasing temperature is expected to lead to (H1a) enhanced primary production through the increased rate of photosynthesis observed over the temperature range 0–30°C (Allen et al, 2005); (H2a) increased decomposition rates due to enhanced breakdown by detritivores (Morán et al, 2015; Widden et al, 1989); (H3a) a reduction in macroinvertebrate community similarity, as warm-tolerant species replace cold-tolerant ones (Hillebrand et al, 2010; Woodward et al, 2010); and (H4-6a) a reduction in the abundance, mean body size, and biomass of invertebrates, due to their higher metabolic demands and a general trend toward smaller body size in warmer environments (Brown, Gillooly, Allen, Savage, & West, 2004; Daufresne et al, 2009). Greater habitat complexity is predicted to lead to (H1b) increased growth of epiphytes (Newman, 1991; Pettit et al, 2016), but a reduction in benthic algae due to shading (Charlene et al, 1976; Glasby, 1999; Robinson & Rushforth, 1987); (H2b) increased decomposition rate, due to harboring of detritivorous bacteria and invertebrates (Newman, 1991; Sagrario et al, 2009); (H3b) a reduction in macroinvertebrate community similarity, as different species utilize the novel microhabitat (Gregg & Rose, 1985; Taniguchi & Tokeshi, 2004); and (H4-6b) increased abundance, mean body size, and biomass of invertebrates, due to additional three-dimensional space (Heck & Wetstone, 1977), novel habitat niches (Gregg & Rose, 1985; Sagnes et al, 2008; Taniguchi & Tokeshi, 2004), and resource provisioning (Bakker et al, 2016; Pettit et al, 2016)
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