Abstract
Top predator loss is a major global problem, with a current trend in biodiversity loss towards high trophic levels that modifies most ecosystems worldwide. Most research in this area is focused on large-bodied predators, despite the high extinction risk of small-bodied freshwater fish that often act as apex consumers. Consequently, it remains unknown if intermittent streams are affected by the consequences of top-predators’ extirpations. The aim of our research was to determine how this global problem affects intermittent streams and, in particular, if the loss of a small-bodied top predator (1) leads to a ‘mesopredator release’, affects primary consumers and changes whole community structures, and (2) triggers a cascade effect modifying the ecosystem function. To address these questions, we studied the top-down effects of a small endangered fish species, Barbus meridionalis (the Mediterranean barbel), conducting an enclosure/exclosure mesocosm experiment in an intermittent stream where B. meridionalis became locally extinct following a wildfire. We found that top predator absence led to ‘mesopredator release’, and also to ‘prey release’ despite intraguild predation, which contrasts with traditional food web theory. In addition, B. meridionalis extirpation changed whole macroinvertebrate community composition and increased total macroinvertebrate density. Regarding ecosystem function, periphyton primary production decreased in apex consumer absence. In this study, the apex consumer was functionally irreplaceable; its local extinction led to the loss of an important functional role that resulted in major changes to the ecosystem’s structure and function. This study evidences that intermittent streams can be affected by the consequences of apex consumers’ extinctions, and that the loss of small-bodied top predators can lead to large ecosystem changes. We recommend the reintroduction of small-bodied apex consumers to systems where they have been extirpated, to restore ecosystem structure and function.
Highlights
Predation is an important species interaction that has implications for biological populations, communities, and ecosystems
From the 32 studies, Brashares et al [10] found that 40% of the interactions were triangular: those in which top predators feed on mesopredators and on prey, resulting in intraguild predation (IGP; characterised by predators that feed on other predators with which they share prey taxa)
We studied the top-down impacts of B. meridionalis to determine if the loss of the top predator (1) leads to a ‘mesopredator release’, affecting primary consumers and changing whole community structure, and (2) triggers a cascade effect modifying ecosystem function
Summary
Predation is an important species interaction that has implications for biological populations, communities, and ecosystems. In that case, ‘mesopredator release’ could lead to an increase on prey top-down control, neutralising apex consumer loss. This would result in a negative or a null net effect on prey taxa, and dampen the trophic cascade on primary production [13,14,15]. According to the predator-mediated coexistence theory [16] and to recent modelling work [17], apex consumer loss can cause secondary extinctions in adjacent and non-adjacent trophic levels [12,18,19], mainly because predators can facilitate coexistence among prey species. Top predator extinctions have been related to an increase in mesopredator abundance and to a decline in biodiversity [9,12]
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