Discussion
Overall, our findings suggest that catchment forest loss is associated
with a reduction in the trophic position of apex predators, which we
interpret as a general shortening of food chain lengths (Wolkovich et
al., 2014). Decreases in food chain lengths are commonly driven by
resource limitation because the number of viable successive trophic
steps (i.e., predator-prey interactions) are limited by the total
metabolic energy in a food web (Doi et al., 2009; Takimoto & Post,
2013). While terrestrial inputs into fresh waters have been shown to
diminish with forest loss (Tanentzap et al., 2014), we do not believe
that limitation of terrestrial resource subsidies is the primary driver
of the changes observed. This is because average allochthonylevels (~7%) and its temporal invariance (Supplementary
Fig. 1, 2), both suggest that terrestrial inputs are a relatively
unimportant basal resource in the freshwater food webs that we studied,
reinforcing shifting perceptions about the subject (Takimoto et al.,
2008).
Despite this, there are some plausible mechanisms through which
reductions in terrestrial subsidies can result in resource limitations.
Firstly, the contribution of terrestrial inputs to overall resources may
not necessarily be commensurate with their importance as a direct food
source. For example, Hobbs et al. (2006) found growth rates to be higher
in aquatic invertebrates feeding on a mix of leaf litter and algae when
compared to individuals subsisting exclusively on either. The
experimental data also show that the proportion of terrestrial carbon
assimilated remained low in the mixed-diet treatment, at a value similar
to those recorded in our study (~6%), despite the
apparent positive effect on consumer growth (Hobbs et al., 2006).
Secondly, terrestrial organic matter can also serve functions which are
unrelated to nutrition. For example, terrestrial detritus adds
mechanical structure and refugia to freshwater ecosystems—both
beneficial to aquatic primary (Brothers et al., 2013) and secondary
(Sass et al., 2006) production.
Although we found forest loss to be predictive of changes in food webs
over time, the relative performance of the different forest loss metrics
(Table 2; Fig. 4) suggests intricacies in our generalised conclusion.
Here, we found that food web changes were better predicted by forest
loss in the upstream catchment area than by overall forest loss across
the entire catchment basin. This is unsurprising, as a majority of
terrestrial subsidies are likely to enter stream systems with
groundwater/runoff from the upstream catchment. We also found that the
ratio between forest loss and forest gain (i.e., net change in forest
cover) was a better predictor of food web changes than total forest lost
(Table 2), suggesting that the impacts of deforestation may be
attenuated by re-forestation. However, an increase in TP was
recorded only in the Danum catchment, where upstream forest cover
comprised part of a protected conservation area and was relatively
unchanged over time (Fig. 2; Table 1). While sample size is limiting, it
seems that as with most measures of environmental health (e.g., species
diversity), freshwater food webs benefit more from forest preservation
than from forest restoration (Hobbs et al., 2006), at least within the
time interval (~20 years) of our study (Chazdon, 2008).
We note two possible confounding factors in our study, specimen
body-size and substantial differences in catchment basin areas. The
susceptibility of larger individuals to anthropogenic disturbances
(Walters & Post, 2008) and a strong positive correlation between
trophic position and body size (Brose et al., 2005) means that there may
be a feasible alternative driving mechanism to the one we proposed.
Specifically, forest loss could result in the loss of larger individuals
(Walters & Post, 2008), hence lowering trophic positions of apex
predators overall (Brose et al., 2005). While this may be the case where
loss or similar anthropogenic impacts causes a reduction in trophic
positions (Wilkinson et al., in revision), it is not likely to be
relevant to our study. Here, our data show that fish individuals
collected from both before and after were of comparable
sizes (Supplementary Fig. 3).
Another factor likely to complicate the interpretation of our findings
is the influence of ecosystem (or catchment) size on food web structure
(Takimoto & Post, 2013). This is partially accounted for by our
longitudinal study design because unlike cross-sectional studies, our
primary response variable is less prone to conflation with baseline
variations between survey locations. Thus, we were able to determine if
observed trends in a response variable (e.g., food web structure) were
driven by differences in the intensity of a disturbance regime (e.g.,
forest loss) independently of ecosystem size driven starting conditions
(Takimoto & Post, 2013). We also found that when tested as predictors
of food web change, ecosystem size performed more poorly than forest
loss metrics measured at an equivalent spatial scale (i.e., whole
catchment or immediate upstream area of catchment; Table 2).