PARENT SESSION
Oral Session # 3: Fish Ecology.
Presiding: M Ford
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 102.
Condition-specific asymmetric competition between threatened native and aggressive non-native minnows may mitigate an ongoing invasion.
Wagner, C. Michael*,1, Grossman, Gary1, 1 Warnell School of Forest Resources, Athens, GA, USA
ABSTRACT- Approximately 13 years ago the yellowfin shiner (Notropis lutipinnis) was introduced into the headwaters of the Little Tennessee River of western North Carolina, USA where it exhibits microhabitat preferences that overlap significantly with those of a threatened native minnow, the smokey dace (Clinostomus funduloides ssp.). Previous field observations have established that each of these drift-feeding minnows routinely forage in both single- and mixed-species groups, and employ aggression to maintain the forward positions in profitable habitat patches. Laboratory experiments have further established that the introduced shiner is: 1) approximately twice as aggressive as the native dace in intraspecific foraging groups; and, 2) a less efficient forager at higher stream velocities than the native dace. We conducted a response-surface competition experiment to examine the effects of water velocity (10 or 20 cm s-1) and group size (4 or 8 individuals) on intra- and interspecific aggression and foraging performance in these species (six treatments: 4 dace, 4 shiner, 8 dace, 8 shiner, 2 dace + 2 shiner, 4 dace + 4 shiner). Each group was tested at both velocities on subsequent days. When competing with native dace, the more aggressive shiner occupied significantly higher ranked positions at both velocities and group sizes. However, shiner only successfully monopolized the available prey at low velocities. At high velocity clear competitive release occurred for the native dace as: 1) in small groups dace acquired the largest proportion of the available prey (i.e., competitive reversal); and, 2) in large groups dace received an equal proportion of the available prey. Such asymmetric condition-specific competition implies maintenance of habitat diversity (high and low velocity patches) may provide competitive refugia for the smokey dace.
Key words: aggression, invasion biology, condition-specific competition, stream fishes