Dining outside of the stream

The Endurance of Living Things (The Spring Salamander), Steve Perkins, 1993
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Several summers ago, IES post-doctoral Associate Dr. Winsor Lowe, with colleagues, set out to unravel where spring salamanders (Gyrinophilus porphyriticus) find their food. Do animals forage for their insect prey in aquatic or in nearby woodland habitats? If they venture out of the water to hunt, does the vegetation they encounter influence their success? Dr. Lowe's findings, recently published in a Proceedings of the International Association of Theoretical and Applied Limnology paper, reveal that spring salamanders get a large portion of their diet from terrestrial prey, with dense young forests providing the best dining opportunities. Co-authors included USDA Forest Service Biologist Keith H. Nislow and Institute Director Dr. Gene E. Likens.

For some time, ecologists have understood that terrestrial habitat influences stream food webs. By creating shade, trees alter stream light availability and water temperature. The leaves and branches that fall into streams provide food and shelter for aquatic animals. Water from upland forests drains into streams, enriching them with dissolved nutrients, such as nitrogen and carbon. Dr. Lowe comments, "Past research on how vegetation composition affects aquatic food webs has largely focused on fish. By looking at an animal that is able to move between the stream and the surrounding forest, we sought to gain a broader understanding of how forest change influences stream food webs."

Due to their life history patterns, stream salamanders are well suited to investigating links between aquatic and terrestrial systems. Common to headwater streams in eastern and western North America, salamanders are strictly aquatic as juveniles. At maturity, their range expands to include streamside environments. Adult spring salamanders, which live along the Appalachian Mountains in eastern North America, forage on the land and in the water. The lungless species acquires oxygen through its skin. Because salamanders need to remain moist to respire, forays into terrestrial habitat are likely to be limited by the availability of ground moisture.

During the summer months, Dr. Lowe and colleagues investigated salamander populations in 10 New Hampshire streams. Study sites were limited to streams that had not been exposed to human disturbance, such as logging or road construction, in over 50 years. Terrestrial habitat was surveyed at each site; variables measured included tree diameter, tree type (deciduous or conifer) and canopy coverage. Dr. Lowe explains, "By surveying streamside vegetation, we gained insight into the quality of terrestrial habitat and the influence it might be exerting, through things like shading and leaf litter, on nearby stream environments."

Spring salamanders spend their summer days in refuges in and along streams, where they keep moist and avoid predators such as brook trout and shrews. Surveying the animals involved wading into streams, overturning preferred hiding places, such as rocks and logs, and capturing salamanders with aquarium nets. After their stomach contents were collected through flushing, a non-invasive procedure, captured animals were released. By looking at what the salamanders had eaten, and identifying prey as aquatic or terrestrial, Dr. Lowe and colleagues gained insight into where the animals had foraged for food.

When statistical tests were run to analyze the relationship between salamander diet and forest vegetation structure, Dr. Lowe noticed an interesting trend. Salamanders living in streams near young forests (i.e., with many small trees) were more likely to dine on terrestrial invertebrates than salamanders living near older forests (i.e., with fewer but larger trees).

Dr. Lowe notes, "In our study, forest structure was a good predictor of where salamanders found their food. Salamanders living in streams surrounded by young forests foraged predominantly on terrestrial prey. Animals living near older forests ate more aquatic prey. This relationship was independent of aquatic prey abundance, which suggests that it was either a direct result of forest conditions, or a result of another variable closely related to forest conditions, such as terrestrial prey adundance."

Not all forests are created equal. The young forests where salamanders dined on terrestrial prey are called early successional forests. These forests develop after a disturbance has occurred. While the study forests had been free of human disturbance for 50 years, New Hampshire forests are prone to a suite of natural disturbances, such as ice storms and insect outbreaks. High levels of leaf litter characterize successional forests. Leaves and branches on the ground might provide food and shelter for invertebrates, increasing terrestrial prey abundance. A thick litter layer could also help retain moisture and heat on the forest floor, creating a more hospitable environment for salamanders.

More research is needed to determine the precise reason why salamanders forage so successfully on land in early successional forests. One thing is clear-access to two food pantries may make salamander populations more resilient when confronted with habitat or environmental change. This is especially true for smaller populations of animals living in fragmented landscapes. "By maintaining forested buffers around streams, land managers can preserve the direct connection between stream and forest that favors stream salamander success," Dr. Lowe concludes.

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