Population impacts of movements of individuals

Previously, we had considered how growth, survival, and behavioral preferences of individuals affected population dynamics, but it became clear in these studies that movement is also an individual response that must be incorporated. Much of our recent and current work continues to focus on understanding population and community consequences of individual behaviors and vital rates, but with an emphasis on individual movements and their implications.

Dr. Elizabeth Marschall

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The cost of dispersal: predation as a function of movement and site familiarity in ruffed grouse]
James M. Yoder, Elizabeth A. Marschall, and David A. Swanson (Ohio Division of Wildlife). 2004. Behavioral Ecology 15: 469-476 [paper].

Ecologists often assume that dispersing individuals experience increased predation risk owing to increased exposure to predators while moving. To test the hypothesis that predation risk is a function of movement distance or rate of movement, we used radio-telemetry data collected from 193 ruffed grouse (Bonasa umbellus) during 1996­1999 in southeastern Ohio. Cox's proportional hazards model was used to examine whether the risk of predation was affected by the rate of movement and site familiarity. We found evidence indicating that increased movement rates may increase the risk of predation for adult birds but not juveniles. We also found juvenile and adult birds inhabiting unfamiliar space were consistently at a much higher risk of predation (three to 7.5 times greater) than those in familiar space. Our results indicate that although movement itself may have some effect on the risk of being preyed upon, moving through unfamiliar space has a much greater effect on risk for ruffed grouse. This supports the hypothesis that increased predation risk may be an important cost of dispersal for birds.

Understanding movement and spatial structure of Lake Erie fish populations
Paris Collingsworth, Jason Van Tassell, Roy Stein, Libby Marschall

Walleye and yellow perch in Lake Erie are known to move great distances over the course of a year. While young fish come from a number of different spawning stocks, we believe they eventually mix into lakewide populations as adults. During the course of their lives, these fish may move back and forth between Lake Erie tributaries, between Lake Erie basins, and even between different lakes. Our longterm goal is to understand these movements and their consequences. Currently, we are focusing on understanding how larvae and juveniles from specific spawning areas contribute to the lakewide populations of walleye and yellow perch.

Movement of reservoir-stocked riverine fish between tailwaters, streams, and rivers
Rick Silk, Jeff Spoelstra, Karen Blocksom, Roy Stein, and Libby Marschall

We first characterized the timing and direction of movement of stocked saugeye (hatchery produced walleye x saugeye hybrid) from Ohio reservoirs so that we could begin to understand the potential for undesirable side effects, such as genetic mixing with downstream parental stocks and ecological impacts on native stream communities. We designed the study to address the conventional wisdom that small fish are susceptible to being flushed from reservoirs during high discharge and that adult saugeye exhibit seasonal directional movement patterns similar to those exhibited by its parent species, walleye and sauger. Contrary to our initial expectations, (1) saugeye tailwater abundance was unrelated to reservoir discharge, (2) first-summer saugeye survival was highest in reservoirs with fast turnover (i.e., high discharges), (3) small saugeye were less likely to move downstream than larger ones, and (4) soon after stocking, YOY saugeye moved both up- and downstream from reservoirs, in some cases moving upstream some 50 km during high discharges. In our view, much of conventional wisdom suggesting that discharge drives saugeye abundance in tailwaters and negatively affects within-reservoir survival through direct losses of YOY can be explained by seasonal hydrographs that happen to coincide with behavioral cues (i.e., temperature, photoperiod, etc.) rather than by reservoir discharge per se.

Given how quickly reservoir-stocked saugeye moved out of the reservoir into the tailwaters, we next asked when and at what rate these fish moved from the tailwaters into the connecting streams and rivers. We used fixed-station, radio telemetry to quantify temporal patterns of movement of 203 reservoir-stocked saugeye hybrids. During October 1999 through July 2000, most (75%) saugeye never left the tailwaters, and those that left returned 75% of the time. Overall, saugeye spent 90% of their time in the tailwaters, 7-8% downstream in small streams, and 2-3% farther downstream in the Scioto River (45 km downstream). The probability of downstream movement generally 1) increased with increasing flow, and 2) increased when dissolved oxygen levels dropped to lethal levels in summer. Movement probability was highest in winter and spring, likely caused by spawning related movement, and low in summer (except for that caused by low dissolved oxygen) and fall. Patterns of saugeye movement seemed to reflect the relative suitability of tailwater over stream habitat. Predominant use of and return to tailwater habitat after downstream movement limited stream and river residence time. Although movement probability on a per-saugeye, per-day basis was low, when we apply these rates to all stocked saugeye in the Ohio River drainage, we conclude that substantial numbers of saugeye move from reservoirs, compromising the genetic integrity of the parental stocks. We recommend that managers refrain from stocking systems in which concerns for native species in connected drainages exist.