Using stable isotopes to quantify seed dispersal along habitat corridors

Background/Question/Methods

Negative effects of habitat fragmentation have led to widespread use of habitat corridors in conservation. Yet not all species respond to corridors in the same way.  Predicting these differences is difficult due to lack of a robust, empirically validated framework that applies to the diverse taxa that characterize ecological communities.  In plants, dispersal ability is largely determined by morphology (e.g., pappus) and the mode of dispersal (e.g., animal-mediated or wind). Due to their small size and the rarity of long-distance dispersal, however, it is difficult to trace seeds from a given point across a landscape. In 12 experimental landscapes designed to test corridor effects on dispersal, we used a stable ¹⁵N -isotope to label six wind- and two bird-dispersed plant species. Four thousand seed traps were deployed in these landscapes to trace labeled, long-distance dispersing seeds. Here, we evaluate the efficacy of our spraying technique for labeling seeds. Seeds collected from sprayed plants were analyzed for ¹⁵N-enrichment and compared to ¹⁵N-levels of unsprayed plants. Application frequency and ¹⁵N-concentrations were varied for selected species to retrieve the most efficient enrichment scheme.

Results/Conclusions

Our results showed that the application of ¹⁵N-labeld urea is sufficient to achieve significant enrichment of ¹⁵N in seeds of all sprayed plant species, compared to seeds of unsprayed plants. Further, we could not find a significant difference in ¹⁵N-enrichment between plants sprayed three times with a 0.5g/l ¹⁵N-concentration and plants sprayed six times with a 0.25g/l ¹⁵N-concentration. Our study provides evidence that ¹⁵N-labeling can be used to study patterns and processes of seed dispersal by marking entire plant communities in relatively small areas. To decrease work effort, a lower spray frequency with higher concentrations is preferred to a higher spray frequency with lower concentrations.