Morphological traits mediate fish occurrences in oil palm‐impacted tropical streams

Abstract

Land‐use change is a leading driver of biodiversity loss, especially in tropical fresh waters where the conversion of natural forest to monoculture plantations impacts freshwater fish assemblages. The environmental pathways underpinning shifts in fish assemblages, however, are poorly understood, but could potentially be inferred from trait–environment relationships. We addressed this knowledge gap using eco‐morphological traits to explain fish occurrences in oil palm‐impacted streams of the Endau drainage in Peninsular Malaysia. We also investigated how traits relate to differences in environmental conditions associated with land‐use change. We then integrate findings from the above to test how potential pathways of land‐use driven environmental changes can impact species occurrences through effects on life history, feeding habits, and mobility. Mixed‐effects models show that fishes with superior (upward‐facing) mouths and low body mass were more likely to occur in oil palm streams than forest streams, and these traits were associated with grass‐dominated riparian zones and reduced woody debris in oil palm streams, respectively. Structural equation models show that mouth positions statistically mediated the effect of riparian vegetation on fish species occurrences in oil palm streams. Specifically, fishes with superior mouths were more likely to feed on terrestrial invertebrates. Our analysis of easily measurable traits revealed pathways of land‐use impact that are potentially more widely applicable than conventional taxa‐based approaches. Fishes with superior mouths tended to occur in oil palm streams as they were able to more effectively exploit inputs of terrestrial invertebrates that are potentially associated with grass‐dominated riparian vegetation. Moreover, shifts in traits may suggest land‐use driven changes in stream ecosystem functioning (e.g. in terms of the role of terrestrial subsidies), thereby informing targeted management actions in land‐use impacted habitats (e.g. retention/restoration of riparian trees).

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

“…However, there was no reduction in niche breadth of the meso-predators, consistent with the literature (Price et al, 2019). Our findings are likely the result of species turnover, as sensitive, endemic species with narrow trophic niches are either replaced with generalists or lost altogether from the ecosystem (Chua et al, 2020;Giam et al, 2015;Wilkinson et al, 2018).…”

“…Chua et al, unpublished manuscript). Redundancy can promote stability of the food web (and food chain lengths) by buffering changes in species composition and altered species links when redundant species occupy similar trophic positions (Oliver et al 2015, Chua et al 2020. Resilience to climate stressors has been shown to be high for a common meso-predator species (N. everetti; Wilkinson et al 2019).…”

“…In contrast, the lack of change in trophic position of the three meso-predators may be due to high trophic redundancy lower down the food web, and particularly in invertivorous fish that are commonly represented in all streams considered in this study (Chua et al In Review). Redundancy can promote stability of the food web (and food chain lengths) by buffering changes in species composition and altered species links when redundant species occupy similar trophic positions (Chua et al, 2020;Oliver et al, 2015). Resilience to climate stressors has been shown to be high for a common meso-predator species (N. everetti) (Wilkinson et al, 2019).…”

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

“…of fish species/[empirical food chain length − 1], where empirical food chain length is a decimal number estimated using the CSIA‐AA method described earlier; Borrvall et al., 2000; Ebenman et al., 2004). Recognising that land‐use change may affect certain trophic guilds more than others (Chua, Lim, et al., 2020; Purvis et al., 2000), we additionally investigated the potential impacts of intraguild trophic redundancy. To do so, we calculated species richness within each trophic guild (i.e.…”

“…Deforestation and subsequent conversion to agricultural and urban land cover is known to drive declines in species richness and shifts in community structure (e.g. 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).…”