PS 36-82
Competition, habitat compression, and hybridization: Evaluating mechanisms of invasive coccinellid impact on neartic native species
The last several decades have seen a major shift in the complex of lady beetles (Coccinellidae) from a diverse assemblage of native species to one of reduced diversity and dominated by introduced species. To elucidate the larger issue of native coccinellid decline we focus on one native species, Coccinella novemnotata, that has undergone a precipitous decline and one invasive species, Coccinella septempunctata, that we suspect is the main reason for the decline of the native. We examine several potential mechanisms through which C. septempunctata may have negatively affected C. novemnotata. We investigate trends in relative abundance and habitat distribution by comparing historical records with recent observations compiled primarily through a citizen science program entitled the Lost Ladybug Project. We combine these field trends with directed laboratory experiments measuring the levels of competition and hybridization. Competition experiments measure conspecific and heterospecific fitness across a range of conditions while comparisons of conspecific and heterospecific mating measure the potential for hybridization through assessment of mating behavior and success.
Results/Conclusions
The results of our competition experiments show that, compared to larvae reared conspecifically, C. novemnotata larvae reared with C. septempunctata larvae were significantly less likely to survive to adulthood (p = 0.0008) and produced significantly smaller adult beetles (p = 0.0003). We predicted that C. septempunctata may be relegating C. novemnotata to suboptimal nonagricultural habitats. Comparing our recent survey results for C. novemnotata with historical records we find evidence for the distribution of C. novemnotata having shifted from primarily agricultural habitats (79% of individuals recorded) before the invasion of C. septempunctata to primarily nonagricultural habitats (98%) following invasion. Accounting for the relative level of sampling in each habitat type, the current recoveries of C. novemnotata are significantly lower than expected in agricultural habitats (p < 0.0001). Through the observation of many adults of these congeneric species it was determined that 1) spotting patterns can overlap, and 2) field-collected individuals from the different species engage in copulatory behavior and these findings suggested the possibility that C. septempunctata may be replacing C. novemnotata through hybridization. Through a series of mating trials we determined that heterospecific mating occurs but is significantly less frequent than conspecific mating and it never produces viable offspring.