Spines and surplus: existing inducible defenses and abundant resources may reduce the impacts of Cercopagis pengoi on a likely prey species.
A lack of co-evolved defensive mechanisms can make organisms especially vulnerable to invasive predators. Bosmina longirostris is an herbivorous zooplankton known to change morphology as an inducible defense mechanism against the native predatory invertebrate Leptodora kindtii. In 1999, Cercopagis pengoi, another predatory zooplankton, invaded six of the eleven New York Finger Lakes in which both L. kindtii and B. longirostris co-occur. This created a natural experiment for studying C. pengoi's (1) influence on the presence of B. longirostris' long morphological form, a known inducible defense against predation by native L. kindtii, (2) consumption rates of B longirostris using mass-balance equations for several prey preference scenarios, and (3) overlap with the native predatory cladoceran L. kindtii. Bosmina longirostris densities were lower in the invaded Finger Lakes and exhibited more long form morphology, with longer percent relative mucro lengths, than in non-invaded lakes (Mann Whitney, p = 0.006). The percent relative mucro length was positively correlated with predator densities, particularly those of C. pengoi (Spearman, r = 0.812, p < 0.001), consistent with C. pengoi predation on B. longirostris as a potential driver of this inducible defense expression. Mass balance calculations estimated predation by C. pengoi could reduce B. longirostris densities by 10-100% during peak C. pengoi densities if the latter only consumed the former or preferentially selected large zooplankton from the available prey assemblage. Additionally, C. pengoi and L. kindtii populations overlapped temporally in the invaded lakes, suggesting that their prey species were abundant enough to support both populations. This study indicates that (1) B. longirostris likely produce an inducible defense protective against native predators in response to invasive C. pengoi, and (2) C. pengoi distribution overlaps with L. kindtii in invaded lakes.