Introduction
Forest loss is one of the most critical threats to the biosphere in the Anthropocene (Ruddiman, 2013; Malhi et al., 2014). Impacts of forest loss are extensive and far-reaching (Morris, 2010)—driving species extinctions (Brook et al. 2003; Giam, 2017), climate change (Ramankutty et al., 2006; Lawrence & Vandecar, 2015), and threatening the health of local human communities (Myers et al., 2013). The consequences of forest loss are relatively well-documented (Gibson et al., 2011; Barlow et al., 2016) so our understanding of its impacts on terrestrial ecosystems and biodiversity is fairly robust. However, these repercussions are rarely localised to the immediate (terrestrial) area (Lawton et al., 2001; MacKenzie, 2008) and often affects adjacent ecosystems (Maina et al., 2013). Forest streams, for example, are impacted by a reduction in terrestrial inputs (e.g., leaf litter, woody debris) which are important for the creation and maintenance of microhabitats (Giam et al., 2015; Naman et al., 2018). This results in simplified systems which lack niche diversity, and are hence less speciose (Loke & Todd, 2016). Unsurprisingly, forest loss is generally detrimental to freshwater biodiversity and commonly results in the extirpation of sensitive species (Liew et al., 2018a, Wilkinson et al., 2018).
In addition, allochthonous terrestrial inputs have, until recently, been thought of as constituting an important resource subsidy for low-order forest streams (Lau et al., 2013). This seemed logical given the expected abundance of such inputs (e.g., high standing stocks of leaf litter), and lower internal (aquatic) primary productivity resulting from light attenuation under dense canopy cover. As such, it was generally assumed that forest loss in catchments would result in resource limitations, thus impacting stream food webs from a bottom-up (resource-driven) direction (Liew et al., 2018). Recent data appear to cofound these expectations, however. Stream consumers grow at a significantly lower rate on diets comprising exclusively of leaf litter (Lau et al., 2013; Guo et al., 2016), suggesting that terrestrial resource subsidies may play a more peripheral role in steam food webs than previously supposed (Lau et al., 2009; Brett et al., 2017).
Current uncertainties about the role of allochthonous terrestrial subsidies in stream food webs complicate predictions about the effects of forest loss and its underlying mechanisms. It is perhaps for this reason that studies investigating changes in aquatic food webs in response to anthropogenic impacts sometimes report conflicting findings. For example, Takimoto et al. (2008) observed no significant links between disturbance and food webs—or more specifically, food chain length—while McHugh et al. (2010) found disturbance to be one of the primary predictors of food chain length. The lack of a clear consensus is critical from a conservation perspective. Disrupted food webs are thought to impair ecosystem functions, especially those related to energy flow (Holt & Loreau, 2001). Moreover, communities associated with functionally-impacted ecosystems are also often more vulnerable to further species loss (Chua et al., 2019).
In this paper, we aim to clarify the relationship between forest loss and freshwater food webs using a longitudinal (i.e., before and after) study design to help minimise potential confounders (e.g., baseline differences in the communities surveyed). We do this by measuring changes in forest cover and food web structure at two time points approximately two decades apart in tropical Southeast Asia, a region presently undergoing significant forest loss (Miettinen et al. 2012). We used a combination of state-of-the-art ecological tracers—i.e., amino acid-specific Carbon-13 (Liew et al., 2019) and Nitrogen-15 (Chikaraishi et al., 2009) stable isotopes—and high-resolution remote-sensing, as both methods are currently the most precise tools for measuring food web indices and catchment forest cover in a longitudinal design.
At each time point, we measure two food web indices, namely, the proportion of consumer tissue comprising assimilated terrestrial organic carbon (which we interpret as the extent of allochthony ) and trophic position (TP ) of representative apex predators (Choy et al. 1996). Together, allochthony and TP provide information about the role of terrestrial subsidies as a basal resource (Liew et al., 2019) and the vertical complexity of food webs (Takimoto & Post, 2013; Digel et al., 2014). Conversely, we measured catchment forest cover at both time points by generating raster data of cloud-free USGS Landsat mosaic images using machine-learning algorithms. With this, we asked: (1) how have freshwater food webs changed over time?; and (2) can these changes be predicted by the extent of forest loss in the respective catchments?