Print PageCommunity structure in aquatic habitats is affected both by local conditions and by the terrestrial landscape in which these habitats are embedded. Throughout large regions of North America the abandonment of agriculture has lead to reforestation and increased forest canopy over ponds. In pond-dwelling larval odonates, increased canopy cover is strongly associated with decreased species richness and declines in abundance. While increasing canopy cover is associated with changes in the pond conditions that affect species’ persistence within a site, preliminary data suggested that canopy cover may also limit recruitment to these sites. We used a combination of approaches to understand the effects of forest on recruitment to aquatic environments by adult odonates. Translocations of males were used to assess how forest cover in the terrestrial matrix environment acts as a movement barrier to adults. To assess whether canopy cover affects arrival and recruitment at ponds independently of other habitat features, we arranged a set of water tanks in either open or closed canopy conditions and followed the arrival of adults and larval colonization in these tanks. Finally, to test whether conditions in the tanks themselves affected recruitment we conducted survivorship assays with caged larvae in open and closed canopy tanks.
Results/Conclusions
Translocations of territorial males indicated that at longer distances (500m) forest cover decreases habitat permeability for adult dragonflies but that these effects did not affect their rates of returning to the ponds at shorter distances (130m). However, individuals displaced 130m from their original pond were not all equivalently affected by the terrestrial matrix conditions. When the matrix conditions through which individuals had to transit to return to their original pond include forested habitats, body size was positively related to the probability of return to these sites. However, body size was not related to return probabilities for individuals that had to transit only through open field environments. Canopy cover also negatively affected the diversity and abundance of odonates that arrived at our artificial pools. Larval recruitment, however, was generally low and did not differ between closed and open canopy tanks. There was no effect of canopy cover on survival rates for larvae caged in open or closed canopy tanks.
Our results indicate that patterns of declining diversity and abundance of odonates in ponds with increasing canopy cover may be driven partly by the role forest habitats play in blocking movement to these sites and limiting arrival at these shaded ponds.
