Abstract
Trophic organisation defines the flow of energy through ecosystems and is a key component of community structure. Widespread and intensifying anthropogenic disturbance threatens to disrupt trophic organisation by altering species composition and relative abundances and by driving shifts in the trophic ecology of species that persist in disturbed ecosystems. We examined how intensive disturbance caused by selective logging affects trophic organisation in the biodiversity hotspot of Sabah, Borneo. Using stable nitrogen isotopes, we quantified the positions in the food web of 159 leaf-litter ant species in unlogged and logged rainforest and tested four predictions: (i) there is a negative relationship between the trophic position of a species in unlogged forest and its change in abundance following logging, (ii) the trophic positions of species are altered by logging, (iii) disturbance alters the frequency distribution of trophic positions within the ant assemblage, and (iv) disturbance reduces food chain length. We found that ant abundance was 30% lower in logged forest than in unlogged forest but changes in abundance of individual species were not related to trophic position, providing no support for prediction (i). However, trophic positions of individual species were significantly higher in logged forest, supporting prediction (ii). Consequently, the frequency distribution of trophic positions differed significantly between unlogged and logged forest, supporting prediction (iii), and food chains were 0.2 trophic levels longer in logged forest, the opposite of prediction (iv). Our results demonstrate that disturbance can alter trophic organisation even without trophically-biased changes in community composition. Nonetheless, the absence of any reduction in food chain length in logged forest suggests that species-rich arthropod food webs do not experience trophic downgrading or a related collapse in trophic organisation despite the disturbance caused by logging. These food webs appear able to bend without breaking in the face of some forms of anthropogenic disturbance.
Highlights
Trophic organisation defines the flow of energy through ecosystems [1,2] and can have far-reaching effects on ecosystem properties and processes, and on the conservation of biodiversity [3,4,5,6]
We investigate how selective logging affects the relative abundance of species within a tropical rainforest, and we use the numerical measures of trophic positions provided by stable isotope ratios to quantify any shifts in the trophic positions of species
We found no relationship between the trophic position of a species in unlogged forest and its change in relative or absolute abundance (LME: F1, 44 = 0.1, p.0.5 in both cases)
Summary
Trophic organisation defines the flow of energy through ecosystems [1,2] and can have far-reaching effects on ecosystem properties and processes, and on the conservation of biodiversity [3,4,5,6]. Large-bodied vertebrates with high trophic positions (species at the top of the food chain) tend to have small population sizes and require large areas for foraging, and are influenced by variation in prey populations, making them susceptible to anthropogenic pressure [9,10,11,12] This prediction is supported by analyses on the threat status of mammals [13], but smaller-bodied organisms can attain high trophic positions (e.g. compare [14] with [15]), and a negative relationship between trophic position and susceptibility to disturbance may not occur when these other taxa and different forms of disturbance are considered [16]. Studies comparing the trophic positions of species in disturbed and undisturbed ecosystems have generated ambiguous results, with decreases [17], increases [14,18] and no difference in trophic position found between habitats [19,20]
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