Conservation Implications

Representative high, mid and low elevation habitats within the Hubbard Brook Experimental Forest (photo by N. Rodenhouse).

Collecting data from the same place year after year is clearly useful for figuring out how ecological processes are related to one another - for instance, the number of caterpillars and the reproductive success of warblers. It is also useful for making predictions about how changes in the environment may impact different species.

In New Hampshire, scientists are predicting changes in climate that will likely influence the amount, location, and quality of habitat available for Black-throated Blue Warblers. Currently the best habitat is at higher elevations, where food is abundant and the number of nest predators is low. The large number of young produced in those locations is important at a regional level because dispersal to lower elevation sites helps to maintain those populations, some of which don't produce enough young on their own to balance the number of adults that die each year (called "sink habitats").

Higher temperatures in the years to come are expected to alter the elevational distribution of the forest community at Hubbard Brook, pushing habitats of lower quality further up the mountains. What this means is that high-elevation habitats, currently the most productive sites for Black-throated Blue Warblers, will essentially disappear because there is no land at even higher elevations for them to expand into. Even the habitats of moderate quality will become less common because the further up the mountains you go, the less land you have to work with.

The current distribution of low-quality habitat (low food, high predators) and high-quality habitat (high food, low predators) at different elevation zones within the Hubbard Brook Valley, as well as the projected movement of low-quality habitat (red line) with a 1°C and 2°C increase in annual temperature (image by N. Rodenhouse).

The key question then becomes:

Can Black-throated Blue Warblers maintain a stable population size over the long-term given these potential changes to their environment?

Current thinking says no, at least not at the current population size. Decreases in habitat quality, and especially the loss of the best habitat, are expected to reduce the reproductive success of Black-throated Blue Warblers and thus the number of young birds that survive to become breeders themselves. That is the current prediction, based on our knowledge of how the environment affects the reproductive success of these birds.

Only time will tell whether this prediction holds true and the Black-throated Blue Warbler population does decline. Fortunately, researchers at Hubbard Brook continue to monitor warblers across a range of elevation zones, and to measure key environmental variables like food, predation, and climate, so they should quickly detect any changes that occur. So, the long-term data collected at Hubbard Brook have not only been essential to understanding factors that limit the reproductive success of Black-throated Blue Warblers, but will also serve as a valuable "baseline" when monitoring the potential for future impacts of climate change.