Trampling in a seagrass assemblage: direct effects, response of associated fauna, and the role of substrate characteristics

Abstract

Human wading in shallow coastal waters is a common activity that inherently involves trampling of the substrate. An experiment was conducted in Thalassia testudinum seagrass beds in Puerto Rico to determine how seagrass and associated mobile fauna respond to this anthropogenic disturbance. Three trampling intensities were applied to replicate seagrass beds throughout a 4 mo period. Seagrass biomass was inversely related to trampling intensity and duration. There was moderate recovery in the trampled areas 7 mo after the last trampling event. Intense levels of trampling resulted in decreased shrimp abundances, especially for Thor manningi. Fish abundances and composition of shrimp and fish assemblages did not change significantly after 4 mo of trampling. T. testudinum beds with softer substrates lost more seagrass biomass as a result of trampling than seagrass beds with firm substrates, suggesting that substrate firmness can modify disturbance effects. Educators and resource managers should limit trampling by large groups, or confine it to small areas with firm substrates, and researchers should be mindful of artifacts arising from trampling in and around sampling areas.

Effects of propeller scarring on macrofaunal use of the seagrass Thalassia testudinum

“…Ten net passes were made in each trap, removing ~95% of target fauna (A. Uhrin pers. obs., Holmquist 1997, Eckrich & Holmquist 2000, Bell et al 2002. Buckets were transported to the laboratory where shrimp, fish, crabs, and molluscs were sorted live, enumerated, and identified to species, with the exception of crabs, which were grouped as either Brachyura or Anomura.…”

“…Although terrestrial fragmentation has been widely examined, fragmentation in marine systems has received little attention (McNeill & Fairweather 1993, Strong & Bancroft 1994, Hastings et al 1995, Frost et al 1999, Barberá-Cebrián et al 2002. Because seagrass beds are dominant features in the coastal zone, where human populations continue to rise, there is tremendous potential for fragmentation resulting from disturbances linked to human activities such as wading (Eckrich & Holmquist 2000), dock and boardwalk construction (Burdick & Short 1999), boat anchoring (Walker et al 1989, Creed & Amado Filho 1999, and boat operation in shallow water that often leads to scarring of seagrass beds by propellers (Zieman 1976, Sargent et al 1995, Bell et al 2002. Scarring, a unique process in which narrow trenches are created within a seagrass bed, may be severe in some regions (Gonzalez-Liboy 1979, Eleuterius 1987, Fonseca 1994, Sargent et al 1995, Bell et al 2002.…”

Ecology of freshwater shore zones

“…Trampling damages vegetation; kills or dislodges plants and animals from the rocky intertidal, shifting community structure from branching algae to encrusting or turf species; and kills subtidal seagrasses and bivalves. Damage to subtidal communities may be more severe in soft muds than in firm sands (Eckrich and Holmquist 2000). Hikers, swimmers, and bird-watchers sometimes (but not always) reduce populations of birds in the shore zone (van der Zande and Vos 1984;Cardoni et al 2008;Trulio and Sokale 2008).…”

“…Visitors attracted to popular sites in the shore zone may cause substantial ecological damage (e.g., Povey and Keough 1991;Brosnan and Crumrine 1994;Eckrich and Holmquist 2000;Pinn and Rodgers 2005;Davenport and Davenport 2006;Rossi et al 2007). Trampling damages vegetation; kills or dislodges plants and animals from the rocky intertidal, shifting community structure from branching algae to encrusting or turf species; and kills subtidal seagrasses and bivalves.…”

Effects of sediment hardness on the upper limit of the distribution of the burrowing amphipod Haustorioides japonicus on sandy shores: a field evaluation

“…In another study, the digital force gauge gave a good regression to the ignition loss of muddy sediments (Kajihara et al 2010). Penetration pressure by a rod has also been found to provide a good estimate of human trampling damage on seagrass rhizomes (Eckrich & Holmquist 2000). The most appropriate method of measuring substrate hardness may therefore depend on the particular sedimentary habitat and/or organisms being studied and should be selected carefully according to the particular field conditions and animals under investigation.…”