Range expansion and population dynamics of co-occurring invasive herbivores

Although a range of studies have suggested that competition plays a critical role in determining herbivore assemblages, there has been little work addressing the nature of interactions between competing invasive herbivores. We report the results of research on the hemlock woolly adelgid Adelges tsugae ('HWA') and elongate hemlock scale Fiorinia externa ('EHS'), invasive herbivores that both feed on eastern hemlock (Tsuga canadensis). HWA has been linked to hemlock mortality throughout the East Coast of the US; the loss of hemlock threatens to permanently alter surrounding ecosystems. We assessed the spread and impact of both species by resurveying 142 hemlock stands across a 7,500 km² latitudinal transect, running from coastal CT to northern MA, for HWA and EHS density as well as hemlock mortality. These stands had been previously surveyed in either 1997-1998 (CT) or 2002-2004 (MA). While the number of HWA-infested stands has increased, per-stand HWA density has substantially decreased. In contrast, EHS distribution and density has increased dramatically since 1997-1998. Hemlock mortality was much more strongly related to HWA density than to EHS density, and many stands remain relatively healthy despite an overall increase in hemlock mortality. There was a positive correlation between HWA and EHS densities in stands with low mean HWA densities, suggesting the potential for host-plant-mediated facilitation of EHS by HWA. Our findings underline the importance of research explicitly addressing interactions between competing invasive species, and of determining the potential consequences of these interactions for the invaded ecosystem.