Movement in corridors: Enhancement by predation threat, disturbance, and habitat structure

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predation, habitat, marking, water level, recapture, pool, models

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Movement by stream fish is known to be strongly influenced by abiotic factors such asfloods and temperature, but roles of biotic factors, such as predation threat, and interactions ofabiotic and biotic factors are less clear. Predation threat is known to fragment populations ofkillifish, (Rivulus hartii), in Trinidad rivers by rendering habitat inhospitable. We asked whethersuch spatial fragmentation was accompanied by reduced movement by fish in the predatoroccupiedzone of a river, relative to a zone free of the strong piscivore, (Hopliasmalabaricus), that causes the fragmentation. We used a 19-mo marking study in a river with apredator barrier, field experiments in the river, and mesocosms to evaluate four hypotheses: (1)the predator reduces prey movement in the river; (2) for the special case of prey leaving refugia,the predator increases movement; (3) movement positively correlates with water level in thepredator's presence; and (4) complex physical structure in hazardous habitat promotes preymovement. We marked 1467 Rivulus in the natural study areas and had 1015 recaptures.Contrary to Hypothesis 1 but in support of Hypothesis 2, prey showed greater movement alongthe river in the presence of the predator, regardless of whether the fish resided in a refuge at itsprevious capture. An experiment with introduced fish confirmed the findings that movement waselevated in the predator's presence. Effects of an abiotic factor (water level, Hypothesis 3) and aphenotypic trait (body size) depended upon whether the predator was present: movement wasindependent of water level and body size in the absence of the predator, but positively related toboth variables in the predator's presence. Emigration from the river to tributaries was alsoindependent of body size in the predator's absence, but positively size-dependent in thepredator's presence. Complex physical structure (Hypothesis 4), in the form of cobble added toexperimental pools, enhanced the transit of fish through hazardous pools. This study shows thatspatial fragmentation does not necessarily imply that movement between fragments will beimpeded (dynamical fragmentation). Rather, it is possible that movement among spatialfragments may be enhanced by the same factor, predation threat, that produced the spatial fragmentation in the first place. Because of the context-dependent effects of an abiotic factor(water level) and a phenotypic variable (body size) on movement, the study also emphasizes theneed to clarify the exact role of predation as an agent promoting or retarding movement, and itsuggests a need for incorporating such parameters into models of movement and metapopulationdynamics. http://dx.doi.org/10.1890/0012-9658(2001)082[0258:MICEBP]2.0.CO;2







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