Poster: Variation Over Time in Population Size and Reproductive Characteristics of Two Invasive Isopod Species
Katarina Juhaszova, Katalin Szlavecz

Many invasive species of terrestrial isopods have become established in mid-Atlantic ecosystems since the colonization of North America by Europeans. Life history and reproductive characteristics, two key elements of successful invasion, are presented in this report for two species- Trachelipus rathkei and Philoscia muscorum- for the 2004 reproductive season. The isopods were collected from a rural forest in eastern Maryland and an urban forest in Baltimore City. This data is also compared to data gathered for the same locations and same species in 1998 (for the rural plot) and 1999 (for the urban plot).
Two trends stand out between the cohorts. First, the population size of the species varies from year to year. Although T. rathkei was dominant during the 1998 reproductive season at the rural plot, the ratio of T. rathkei to P. muscorum was only about 3 to 2, and P. muscorum surpassed T. rathkei in numbers during the March and April collections. During the 2004 season by contrast, almost 100% of the animals caught during most of the collections at that plot were T. rathkei, with the exception of the late May sampling. Sampling on the Baltimore City plots showed an even more extreme variation. During the 1999 reproductive season hundreds of isopods were caught per month, while the 2004 collections produced only about 20 animals per month and were canceled because such low numbers do not permit derivation of reliable population statistics.
An outstanding trend was also observed in the difference in time of reproduction for T. rathkei and P. muscorum. P. muscorum tended to concentrate its reproduction during the spring with a sharp peak in May and almost no reproduction during the summer months. T. rathkei on the other hand began its reproduction in late spring, showed a moderate peak in May, and then continues to reproduce throughout the summer. These differences in reproductive timing may help reduce competition between the young for food, or may indicate preference for leaves at different levels of decomposition by the two species.


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