Abstract
Human disturbances, such as overfishing, may disrupt predator–prey interactions and modify food webs. Underwater surveys were carried out at six shallow-water reef barrens in temperate waters of northern-central Chile from October to December 2010 to describe the effects of predation, habitat complexity (low, medium and high) and refuge availability on the abundance and population structure of the rock shrimp Rhynchocinetes typus (Rhynchocinetidae), an important mesoconsumer on subtidal hard substrata. Three sites were within managed (restricted access) areas for fishermen, and three were unmanaged (open-access). Field observations and tethering experiments were conducted to examine the relationship between fish and shrimp abundances, and the relative predation rates on shrimps. Direct effects of predation on R. typus body-size distribution were examined from shrimps collected in the field and fish stomachs. The presence and the abundance of R. typus increased with habitat reef complexity and refuge availability. Shrimp abundance was negatively related to fish abundance in managed areas, but not in open-access areas, where shrimp densities were the highest. Also, predation rates and body-size distribution of shrimps were unrelated, although fish consumed more large shrimps than should be expected from their distribution in the field. R. typus occurred most often in shelters with wide openings, offering limited protection against predators, but providing potential aggregation sites for shrimps. Overall, direct effects of predation on shrimp densities and population structure were weak, but indirect effects on shrimp distribution within reefs appear to have been mediated through behavioural responses. Our study highlights the need to assess both numerical and behavioural responses of prey to determine the effects of predator loss on mesoconsumer populations.Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-012-1994-6) contains supplementary material, which is available to authorized users.
Highlights
The loss of marine top predators due to overfishing may disturb entire food webs through trophic cascades (Pauly et al 1998; Pace et al 1999)
Managed areas may have higher fish abundance compared to open-access sites, as shown for sites elsewhere along the Chilean coast (Gelcich et al 2008) or in other parts of the world (McClanahan et al 2006)
We examined the relationship between fish size (TL) and the number and average size of the shrimps found in each fish stomach using a Spearman correlation
Summary
The loss of marine top predators due to overfishing may disturb entire food webs through trophic cascades (Pauly et al 1998; Pace et al 1999). The effects of such disturbances on intermediate consumers are hard to predict because species of a same trophic level can be affected differently by predators (McPeek 1998). An increasing literature has demonstrated the importance of top-down effects on predator–prey interactions, with decreasing stocks of large predators releasing prey from predation pressure Habitats of high structural complexity may permit prey to maintain higher abundance or species richness by reducing predation intensity (reviewed by Denno et al 2005). An important task for ecologists is to estimate biotic (direct and indirect predation) and environmental (habitat structure) effects on predator–prey interactions (Denno et al 2005) in order to better predict the consequences of predator loss on mesoconsumer prey
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