Dispersal behaviour of boreal Xestia moths in old-growth forest corridors

Gergely Várkonyi1, Mikko Kuussaari2 & Harri Lappalainen3

1 Kainuu Regional Environment Centre, Research Centre of Friendship Park, Tönölä, FIN-88900 Kuhmo, Finland
2 Finnish Environment Institute, P.O. Box 140, FIN-00251 Helsinki, Finland
3 Department of Biology, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland



Introduction

Destruction and fragmentation of old-growth forests in Finland have rapidly increased during the past few decades. Corridors hardly can compensate habitat loss but may enhance landscape connectivity for a range of species. There is an urgent need for well-designed empirical studies, e.g. for those focusing on animal movements along and out of corridors, in order to assess conservation value of corridors. This study examines the role of old-growth forest strips (Fig. 1) in dispersal of two congeneric moth species with different habitat requirements and physique (Fig. 2).

Methods

The study was conducted in Kuhmo, eastern middle-boreal Finland. We individually marked 1796 and 683 imagines of Xestia speciosa and X. rhaetica, respectively (see e.g. the marked moth in the top left corner). Moths were released in equal numbers among 4 sites within the old-growth forest strips and recaptured with 60 sugar-bait traps in and around the corridors (Fig. 1). We analysed recapture rates of the species and sexes, as well as flight route and flight period data of recaptured individuals. To test the effectiveness of corridors, we compared moth recapture frequencies in traps connected with the site of release by a continuous corridor with respective frequencies in traps without such a direct physical connection.

Results

Recapture traps caught 1.9 and 8.5% of released Xestia speciosa and X. rhaetica individuals, respectively. The low recapture rate in X. speciosa is probably due to its good dispersal ability, resulting in high emigration rate from the study area before searching for food supply.

Female Xestia moths spent longer time in the area prior to recapture and flew shorter distances than males, obviously because males actively search for mates and deplete more energy per unit of time than females. The two species did not significantly differ from each other in flight period or flight route length, but average flight distances were longer in X. speciosa.

Both Xestia rhaetica and females of X. speciosa avoided crossing corridor edges in their short-distance (50-250 m) movements but there was no clear pattern in their mid-distance (251-550 m) dispersal. Intriguingly, X. rhaetica was able to spread over a wide sapling stand (Fig. 3), suggesting that its persistence in the study area depends on the amount of its larval habitat rather than on adult dispersal.



Fig. 1. Aerial photograph of the study area showing the location of releasing sites and recapture traps. Dark, medium and light grey areas are old-growth forest patches, sapling stands and clear-cuts, respectively.





Fig. 2. Differences between study species in their habitat requirements and physique. Xestia rhaetica prefers natural spruce forests while X. speciosa is a more habitat generalist species, occurring even in heavily managed mesic forests.





Fig. 3. Typical dispersal pattern of Xestia spp. in the study area. Panels a and b show recapture figures of X. rhaetica and X. speciosa, respectively. Actual releasing site is marked with B. Size and colour pattern of pies correspond to the local catch.