Linking Movement Dynamics To Ecosystem Structure: Suckers As Ecosystem Engineers

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There is substantial evidence that animals can engineer ecosystems, but little attention has been given to how movements of engineering taxa may modulate their effects on ecosystem parameters and generate heterogeneity. Spatial and temporal heterogeneity in resource availability plays an essential role in structuring many ecosystems. A wide variety of organisms may contribute to environmental heterogeneity and ecosystem engineers are particularly likely to change the spatial and temporal distribution of resources within ecosystems by creating or altering local habitat structures, through substrate bioturbation, or by transforming the chemical and light environment. Ecosystem engineers can control the spatial and temporal distribution of resources and movement by engineers within an ecosystem may transport resources across boundaries and distribute engineering effects. Movement patterns of fishes may cause physical changes to stream habitat though nesting or feeding, both of which often vary in space and time. To understand how movement can modulate the influence of an ecosystem engineering fish, I investigated 1) movement patterns of the Sonora sucker, Catostomus insignis, and desert sucker, Catostomus clarki, on several spatial and temporal scales, 2) the direct and indirect engineering effects of Sonora suckers on streambed heterogeneity (divot creation) and sediment export, and 3) the effects of fish-generated surface roughness on near-bed flow patterns, retention of coarse particulate organic matter, and invertebrate habitat. I used stationary antennas and PIT (passive integrated transponders) tags to monitor short-term and small-scale individual movement and population turnover within a 150-250 m reach and a portable antenna system to assess large-scale, longer term movement and home range in a > 4 km reach of the West Fork of the Gila River, New Mexico, USA from 2008 to 2010. I found that both species of suckers exhibit distinct diel movement patterns, spending daylight hours in refuge habitats (typically deep pools) while moving into shallow habitats at night to feed. While a large number of tagged fish used the stationary antenna reach, only a subset of these fishes was consistently present. The population size of the focal pools was variable, ranging from 12 to >45 individuals (approximately 0.06 to 0.22 individuals/m2). Population turnover rates were variable from day to day, ranging from 0 to >65%. Although some individuals spent the majority of their time within the focal reach, most made extended departures (one or more days) from their home pools. Many individuals displayed substantial fidelity to a particular daytime refuge habitat despite forays elsewhere, returning to the focal reach for the duration of the study. At the reach scale, fish were generally mobile and typical home ranges exceeded 300 m, but about 25% of individuals were only ever detected in a single habitat segment. I observed increased movement after spates caused by summer monsoon rains, and fish used areas of the stream differently under high and low flow conditions. Individuals predominantly used major refuge habitats, but during high flow periods, fish often used small refuge sites with more limited overhead cover. Fish moved more between years than within years, but a subset of individuals was found in the same locations from year to year. Movement behavior did not appear to be a fixed trait for an individual, and many individuals exhibited both stationary and mobile behavior among years. Feeding by suckers creates disturbance in soft sediments that are patchy in space and time. These disturbances move up to 2.4 x 104 cm3 of sediment per square meter per week in locations that are up to hundreds of meters away from sucker daytime refuges. The diel cycles in feeding activity (i.e., nocturnal digging in benthic substrates) caused pulses in suspended sediment that comprise up to 34% of the daily suspended load of a stream reach. Sediment export due to fish activity made up a substantial proportion of the sediment transport from the stream reach as a whole, particularly during moderate to low flow periods. Fish create a complicated and relatively long-lived honeycomb of depressions (divots) and plateaus as they forage for invertebrates in soft substrates, physically roughening the sediment-water interface. Divots act as traps for coarse particulate organic matter and are rapidly colonized by different invertebrate taxa and at greater biomass than are found in adjacent undisturbed sediment. The surface roughness created by fields of divots, as well as the structure of the divot itself, modifies small scale flow patterns near the streambed. Divots showed increases in turbulent stress associated with a shear layer forming between the outer channel flow and the cavity flow within the divot. Stokes number scalings of coarse particulate organic matter suggest retention in the divot is due to inertial effects caused by streamline curvature, despite increased turbulence in the near bed region. The cavity flow within a divot further concentrates particulates in an upstream corner eddy. In the Gila River sucker system, the timing and location of movement by fish modulates effects on the ecosystem and consequently engineers generate heterogeneity in resources in space and time. My findings suggest that understanding movement patterns can allow us to better assess the modifications engineering organisms make to spatial and temporal distribution of resources within the ecosystem.

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fish movement; ecosystem engineers; southwest streams


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Union Local


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Hairston Jr., Nelson George

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Flecker, Alexander S

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Peckarsky, Barbara Lynn

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Ph. D., Ecology

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Doctor of Philosophy

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dissertation or thesis

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