An experimental study of foraging aggression in two southeastern minnows
Wagner, Clifford Michael
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Competitive aggression is a trait often assumed important in the establishment of invasive species. I examined the effects of competitive aggression on the group foraging dynamics of two southeastern minnows, the native rosyside dace (Clinostomus funduloides), and the introduced yellowfin shiner (Notropis lutipinnis). Specifically, my research addressed three primary objectives: (1) to examine the functional value of aggression to non-territorial, socially foraging stream fishes; (2) to identify mechanisms related to competitive aggression that may have promoted the establishment yellowfin shiner in Coweeta Creek, N.C.; and, (3) to determine if interspecific competition between rosyside dace and yellowfin shiner is mediated by flow. Both yellowfin shiner and rosyside dace readily developed social hierarchies in the presence of food where behaviorally dominant (i.e., aggressive) individuals gained a significant feeding advantage by defending positions upstream of conspecifics. Increasing food abundance within the range of natural variation did not affect this relationship. Increasing food abundance did result in an increase in per capita aggression rates for yellowfin shiner, but not rosyside dace, partially supporting the predictions of resource defense theory. Overall, the yellowfin shiner was both more aggressive and more likely to exhibit aggression than the rosyside dace. A patch-choice experiment also revealed yellowfin shiner were more capable than rosyside dace of meeting a key prediction of the Ideal Free Distribution (input-matching) when food was abundant. In concert, these findings suggest the invasive yellowfin shiner may be better at detecting high quality patches, and more able to acquire the best positions in those patches, when competing with the native rosyside dace. However, the rosyside dace is a more efficient forager in high velocity patches. In a test of foraging success in mixed-species groups, the heightened aggression of the invasive species did allow it to monopolize the forward positions at two velocities (10 and 20 cm s ) and two group sizes (four -1 and eight fish). However, the greater foraging efficiency of the rosyside dace allowed it to feed at a higher rate than the invasive at high velocities despite occupying subordinate positions in the hierarchy. This ability was reduced at the larger group size.