Effects of beaver on amphibian spatial distributions

Background/Question/Methods

Ecosystem engineers are key to the maintenance of ecological function in natural systems. Some of these species control stream dynamics through changes in abiotic and biotic factors, such as hydrology and biodiversity. For example, by converting free-flowing streams to impounded ponds, beaver (Castor canadensis) stabilize system hydrology by storing water for longer than in unmodified streams. Such changes may be critical in maintaining amphibian diversity. One declining amphibian species, the northern leopard frog (Lithobates pipiens), requires wetlands with warm, semi-permanent water such as those provided by beaver ponds. Beaver, quintessential ecosystem engineers, have ecological effects that often exceed the spatial and temporal scales of their architectural efforts. We test the effects of habitat modification by beaver on leopard frog presence across spatial and temporal scales in the Medicine Bow National Forest in southeastern Wyoming.

We conducted occupancy surveys in a hierarchical sample design to examine presence at three spatial scales: individual ponds, stream reaches, and drainages with and without beaver. Sites (n=60, in 20 reaches and 10 drainages) were surveyed at the beginning (snowmelt) and end (post-metamorphosis) of the breeding season (2 visits per sampling period = 240 total sampling occasions).

Results/Conclusions

While occupancy surveys such as these are extremely valuable, they are time-intensive and may have high pseudo-absence rates.  Molecular detection methods using environmental DNA (eDNA) from water may increase the accuracy and efficiency of occupancy surveys for rare, secretive, or invasive species. Concurrent with the aforementioned surveys, we developed a species-specific mtDNA PCR test for detection of leopard frogs from water samples collected at each site. With the combined presence-absence data obtained from both direct (occupancy) and indirect (eDNA) detection methods, we determined that northern leopard frogs prefer beaver ponds (83% occupied) and that this effect also holds at larger spatial scales. We also found that the eDNA results are concordant with standard occupancy methods.

This research examines how beaver control stream community dynamics by quantifying their effects on amphibian occupancy across a range of scales. The occupancy models incorporating traditional survey methods and eDNA help to elucidate the spatial and temporal scales at which beaver influence frog presence. This project will also inform management activities by determining if beaver are an effective surrogate species for the conservation of native amphibians. Finally, this research illuminates the role of ecosystem engineers in biotic communities.