Delineating the effect of temperature
In this study, streams of similar size and same order were chosen, and in addition, we eliminated or minimized the effect of the predatory fish, bedrock geology and ground water influence to limit the local environmental variables that may obscure the effect of elevation. Therefore, due to close geographical proximity (i.e., < 100 km) and similarity in habitats (i.e., first-order streams in mixed-deciduous forest), much of the community differences can be inferred to be caused by elevation, with any differences in the hydrological properties of the streams (e.g., order: substratum, size) having minimal effect on benthic communities’ composition. This argument is further supported by the fact that the variation in the different stream communities was mainly attributed to differences in temperature between streams. Parameters such as water chemistry and catchment area did not explain the observed community variations. We also found that changes in air temperature along the elevation gradient accounted for variations in stream temperature. Given that precipitation (i.e., another climatic factor) did not vary with elevation, we argue that any changes in air temperature due to elevation directly influence stream temperature.
From this, we infer that temperature associated with elevation can account for differences in abundance, diversity and FFG of communities. This is supported by the results of the 2010–2011 pairwise ANOSIM and SIMPER analyses, which indicate that the main differences in benthic communities were between high and low elevation streams, with the greatest difference accounted by cold-stenothermic species, e.g., insects such as Plecoptera species Sweltsa naica, Nemoura trispinosa and Isoperla richardsoni, Ephemeroptera species Siphlonurus marshalli, Trichoptera species Parapsyche apicalis, and Diptera species Prosimulium sp., Simulium sp., Parametriocnemus lundbeckii and Micropsectra polita. Haidekker and Hering (2008) have previously shown that in small streams with differences of up to 3 °C, cold-stenothermic insects can account for majority of the variation in benthic communities. Given that mean April–July 2010 and May–July 2011, temperature difference between high and low elevation streams (i.e., E1, D1, G1, and H1 and H2) was 2.1 and 3.1 °C, respectively, it appears that only the higher elevations had streams close with optimal temperature condition (i.e., cooler) for high benthic insect abundance and diversity.
Distinguishing the effect of water temperature on communities of streams benthos from other environmental variables (i.e., physical and chemical) is a challenging task. This is especially true if streams are of different size, depth, flow, and substrate types (Arai et al. 2015; Duggan et al. 2007; Phillips et al. 2015; Quinn and Hickey 1990; Statzner and Higler 1986). Headwater streams naturally control for many confounding factors as they have significantly lower or no anthropogenic impact and lower variation in flow regimes and water quality year around (Arai et al. 2015; Cummins 1974; Karr and Dudley 1981; Moore and Wondzell 2005; Richardson and Danehy 2007). Therefore, temperature plays a significant role for benthic invertebrates of headwater streams (Arai et al. 2015; Chang et al. 2012). For example, few eurythermic species are found in headwater streams in general (Haidekker and Hering 2008). Increasing a stream’s temperature can have a significant impact on physiology, ecology, and life history of organisms living in streams (Angilletta 2009; Sheldon and Tewksbury 2014a, b). We are still at the preliminary stage of understanding of how temperature change shifts the communities in freshwaters as a whole and in long term. Moreover, consensus on how this change occurs varies perhaps due to differences in geographical regions, scale of the study and type of the freshwaters. In this study, data are only suggestive that temperature accounts for variation among community of benthos (i.e., based on the CA analysis species scores correlation with stream temperature, RDA and CCA analyses, DD, and temperature data).
Intermittent nature of the streams, flow regime, and drawbacks
The intermittent natures of the streams in addition to their inaccessibility were major drawbacks in our study. This means that many routine or daily measurements such as water chemistry, DO, and flow regime were not obtainable throughout the sampling season. The year around flow of the streams has influence on abundance and function of benthic communities (Boulton and Lake 1988; Hose et al. 2005), yet cannot be determined with certainty in this study. In the 2010, freshet started in April and stream flow lasted until late July with all streams at higher elevation (i.e., D1, G1, and E1) and those at mid elevation (i.e., E2, HP3A, and F1) having flow during this period. However, in 2011, freshet started in mid-May and stream flow lasted until mid-July with many streams experiencing (i.e., at all elevations) drought after this period. These combined with the 2 year accumulated DD indicate that streams at mid-to-high elevation dry up much later in summer compared to those of lower elevation which further could indicate a prolonged period of drought for the latter. Given that precipitation did not significantly varied between streams, we can suggest that difference of air temperature driven by difference in elevation may accounts for considerable variation in flow regimes as well. That is if we exclude the influence of the ground water or subsurface flow which were minimized in this study. Therefore, at least for the summer months when the community data are obtained we can expect a lower community measures such as abundance, diversity, and functional differences in low elevation streams compared to those at mid–higher elevation. Results of DD and temperature data suggest that streams water at lower elevations evaporate faster than those in mid–higher elevation. Furthermore, similarity of benthic community of stream F1 located at mid elevation (at lower range for this elevation) to stream at low elevation in 2011 suggests that during prolonged low flow regimes, streams at mid-higher elevation can eventually resemble those of the low elevation. This is because shorter growth periods for benthos can limits their resources such as food, timing of emergence, and synchrony of emergence among competing species (Hynes 1958; Boulton 2003; Cowx et al. 1984) and hence limit space and time for many benthic invertebrates. Therefore, based on the 2010–2011 results of abundance, diversity, and FFG, we can only suggest that higher elevation streams provide a better condition for insect’s growth and colonization during the months that optimal conditions are not obtainable in lower elevations. This also suggests that flow regime change in this region will be more detectable for streams of lower elevation. However, further increase in temperature and drought can eventually impact streams at higher elevations as well.
Elevation and community of benthos
Past studies in their accounting the community differences of invertebrate communities along elevation gradient (e.g., Huey 1978, Jacobsen et al. 1997; Jacobsen 2003, 2004; Lawton et al. 1987; MacArthur 1972; McCoy 1990; Stevens 1992; Scheibler et al. 2014; Vinson and Hawkins 1998) have used large elevations (i.e., ≫ 100 m of difference). These studies in general encountered communities with different taxonomic groups stretched along large elevation. However, in this study, communities are stretched over a short elevation gradient (i.e., 287 m of difference). Therefore, the difference in communities is mainly driven by the change in abundance of benthos along the elevation, and not necessarily driven by different taxonomically structured communities (Hughes et al. 2008). Therefore, for geographical areas such as the Precambrian Shield of Canada which large elevation gradient is not obtainable, task of discriminating communities of benthic invertebrates should rely on this abundance difference and probably lowest possible taxonomic identification of benthos (i.e., genus and species) is also required.
The 2010–2011 result obtained from richness did not produce a support for our objectives of showing difference in communities of benthos along a short elevation gradient. The fact that richness did not significantly vary among the streams is due to the fact that the difference in communities is not based on difference in the number of taxonomically different species but on differences in abundance. An effect on richness per se might be found across a larger elevation gradient (e.g., Jacobsen 2003; McCoy 1990; Monoghan et al. 2000; Ramírez and Pringle 1998; Stout and Vandermeer 1975). We found that only the abundance of different FFG varies between streams of different elevation and not the type of FFG which further indicates that communities are not taxonomically different. Therefore, result of the richness obtained in this study is not comparable with other studies having large elevation gradients (e.g., Jacobsen 2003; McCoy 1990; Monoghan et al. 2000; Ramírez and Pringle 1998; Stout and Vandermeer 1975), so that variation in abundance (i.e., species and FFG) and diversity is better measures for community difference compared to richness using low elevation gradients. Richness, however, varied monthly within and between streams. Therefore, we can assume that certain taxa could locally go extinct during the months that streams dry up or harsh stream conditions (e.g. high temperature) sets in. This also means that populations of these species can thrive in other elevations with suitable environmental conditions (i.e., via migration). Many benthic invertebrates in this study streams (i.e., insects) have high natural dispersal capability and, therefore, capable of rapid colonization of other waters.
In this study, we showed that difference between benthic invertebrate communities of several physico-chemically comparable head water streams stretched along a short elevation gradient can be detected. The variation among benthic communities was mainly based on their abundance, FFG and diversity and not richness. Furthermore, among several obtainable and measured physico-chemical variables, stream temperature associated with altitudinal difference accounted significantly for the variation among benthos. Therefore, effects of change in environmental variables (e.g., temperature) on freshwater communities, associated with short elevational difference, can be inferred and used as a proxy for anticipated environmental change (e.g., climate) over time.