Forest conversion to oil palm compresses food chain length in tropical streams

Abstract

In Southeast Asia, biodiversity-rich forests are being extensively logged and converted to oil palm monocultures. Although the impacts of these changes on biodiversity are largely well documented, we know little about how these large-scale impacts affect freshwater trophic ecology. We used stable isotope analyses (SIA) to determine the impacts of land-use changes on the relative contribution of allochthonous and autochthonous basal resources in 19 stream food webs. We also applied compound-specific SIA and bulk-SIA to determine the trophic position of fish apex predators and meso-predators (invertivores and omnivores). There was no difference in the contribution of autochthonous resources in either consumer group (70-82%) among streams with different land-use type. There was no change in trophic position for meso-predators, but trophic position decreased significantly for apex predators in oil palm plantation streams compared to forest streams. This change in maximum food chain length was due to turnover in identity of the apex predator among land-use types. Disruption of aquatic trophic ecology, through reduction in food chain length and shift in basal resources, may cause significant changes in biodiversity as well as ecosystem functions and services. Understanding this change can help develop more focused priorities for mediating the negative impacts of human activities on freshwater ecosystems.

Land‐use change erodes trophic redundancy in tropical forest streams: Evidence from amino acid stable isotope analysis

“…In fact, differences in trophic redundancy may similarly explain inter‐drainage differences within our study. While we did not find impacts on food chain length across our study areas as a whole, declines in food chain length were recorded at land use‐impacted streams within the SAFE‐Danum area (Wilkinson et al., 2020). This is consistent with the lower species richness (Wilkinson & Tan, 2018), and by extension lower trophic redundancy (median = 3.21 species per consumer trophic level), of the hill streams that characterise the SAFE‐Danum area, compared to the more species‐rich lowland streams in the other study areas (median trophic redundancy = 5.01 species per consumer trophic level).…”

“…towards dominance of tolerant species) in highly threatened ecosystems such as tropical freshwaters (Chua et al., 2020; Giam et al., 2015). The resulting changes in feeding interactions are therefore expected to have profound impacts on food web properties (Hamer et al., 2015; Price et al., 2019; Wilkinson et al., 2020).…”

Changes in trophic ecology of mobile predators in response to rainforest degradation

“…In general, the food chain length of top aquatic predators is positively associated with ecosystem size and productivity (Post & Takimoto, 2007). Habitat degradation has been associated with shorter aquatic food chains, lower trophic position of top predators (Wilkinson et al, 2021) and reduced trophic redundancy (Price et al, 2019). However, a recent meta‐analysis highlights the interactive effects of these key environmental drivers on food chain length (Takimoto & Post, 2013), and theoretical work identifies omnivory as a key mediator of these relationships (Ward & McCann, 2017).…”

“…Habitat degradation has been associated with shorter aquatic food chains, lower trophic position of top predators (Wilkinson et al, 2021) and reduced trophic redundancy (Price et al, 2019).…”

Tropical land use alters functional diversity of soil food webs and leads to monopolization of the detrital energy channel

“…It is projected that tropical ecosystems will face even greater pressures due to land-use change in the future (Dobrovolski et al, 2011). Decreases in biodiversity and changes in trophic interactions in animal communities (Newbold et al, 2015; Tsiafouli et al, 2015; Wilkinson et al, 2021) are associated with changes in nutrient dynamics and energy fluxes (de Vries et al, 2012; McGrath et al, 2001; Potapov et al, 2020), which ultimately influence ecosystem functioning and stability (Rooney et al, 2006; Rooney and McCann, 2012). However, interrelationships between the loss of diversity and changes in energy pathways in food webs are poorly studied and this applies in particular to tropical ecosystems.…”