PREDATOR-PREY INTERACTIONS FROM THE TJÖRNES PENINSULA, ICELAND, ACROSS THE TRANS-ARCTIC INVASION

Ecosystem conservation efforts can benefit by using the fossil record to explore how ecosystems reacted to change before anthropogenic impact, because invasion is not a new threat to ecosystems. The impact of invasive predators in an ecosystem can be quantified by analysis of drill holes.

The opening of the Bering Strait ~3.5 Ma allowed for an asymmetrical faunal interchange between the Pacific and Atlantic oceans known as the trans-Arctic invasion (TAI). This interchange could have changed naticid gastropod drilling predation on bivalves due to the migration of Pacific fauna into the Atlantic Ocean. This research expands on that of McCoy (2007) to compare predator-prey interactions before and after the TAI.

The Tjörnes locality of northeast Iceland well characterizes the TAI. This site has preserved genera in three distinct levels: Tapes and Mactra zones are pre-invasion, whereas Serripes is post-invasion. Specimens from the Tjörnes peninsula, Iceland, were analyzed in collections housed at the Icelandic Institute of Natural History. Height and length of bivalve specimens were measured. The occurrence of complete and incomplete (unsuccessful) drill holes and drill hole diameter were recorded for all whole bivalves. Drilling frequency (DF = % mortality) and prey effectiveness (PE = % of attempted drill holes that were incomplete) were calculated. Prey size and drill hole size were correlated.

Pre-invasion samples included 28 bivalve species (857 specimens), whereas 37 bivalve species (1295 specimens) were recorded in post-invasion samples. DF decreased from 0.070 to 0.058 from pre- to post-invasion. However, DF increased from Tapes to Mactra zones in the pre-invasion (0.065 to 0.077). PE increased from pre- to post-invasion (0.115 to 0.167) and from Tapes to Mactra zone (0 to 0.214). The increase in DF between Tapes and Mactra zones can be attributed to the invasion of naticids into the Atlantic Ocean; however, the decrease in DF in post-invasion samples supports escalation of bivalves in response to invasive naticids. The escalation hypothesis is further supported by the increase in PE from pre- to post-invasion. Analysis of the size of prey bivalves (Macoma, Lentidium, and Thracia) compared to drill hole size suggests that invasive naticids transitioned to select smaller-sized prey in the later post-invasion.