Usually, when we see the signs of human settlement on the land we assume the surrounding landscape is degraded. But in some cases, the presence of humans actually improves the local ecology, according to research published last week in Nature Communications. People may even have a hand in creating the ecosystems that sustain them.

Researchers studied temperate rainforest on Calvert and Hecate islands, off the central coast of British Columbia, Canada. This forest is very wet, receiving an average of 4 meters of rainfall a year, and has acidic, nutrient-poor soils. The dominant tree species is western redcedar (Thuja plicata).

The coastline is also dotted with semi-permanent settlements where First Nations groups, as indigenous people are known in Canada, lived seasonally or year-round. Especially over the past 6,000 years, people intensively harvested shellfish from intertidal areas and built up large shell middens—some up to 5 meters deep—near their settlements.

Some of these middens are simple garbage heaps, but people also deliberately placed shells to create terracing or improve soil drainage. The shells cover thousands of square meters of coastal rainforest.

The researchers used a combination of remote sensing and fieldwork to document the effect of shell middens on forest growth, comparing vegetation growing at settlement sites to that of forest more distant from human habitation.

Trees grow taller and greener within about 200 meters of a shell midden than they do elsewhere, the researchers found. Western redcedar trees growing on middens are taller, have higher wood calcium and wider annual growth rings, and exhibit less top die-back, a phenomenon that has been linked to calcium deficiency. Moreover, deeper shell middens have a larger effect on tree growth.

Soils at settlement sites are higher in calcium and phosphorus, nutrients that often limit tree growth in coastal temperate rainforests. They are also less acidic than soils elsewhere. Essentially, shell middens are a slow-release fertilizer bringing marine nutrients into the terrestrial ecosystem.

The soils near settlements are also influenced by fire, which in this soggy environment are mostly human-caused and don’t occur away from habitation sites. Together, these two human influences—fires and shell middens—improve forest productivity by increasing soil pH, adding nutrients, improving drainage, and leveling the ground, the researchers say.

The findings become even more intriguing when you consider the timeline of events along the British Columbia coast. Western redcedar did not appear in the area until 7,000 or 8,000 years ago. But First Nations peoples have lived there for 13,000 years. The current mix of vegetation only arose 2,000 to 4,000 years ago, long after humans were present. In other words, the seemingly “primeval” forest isn’t free from human influence but in fact depends on it.

Still, a skeptic might wonder whether people were simply extracting resources from one ecosystem and moving them elsewhere—a pretty common human pattern, after all, in which we improve one environment only at the expense of another.

Yet indigenous groups along the British Columbia coast also created “clam gardens,” building rock walls in the intertidal zone that altered the slope of beaches and increased the abundance and density of native littleneck and butter clams.

“It is clear that coastal First Nations people have developed practices that enhanced nutrient-limited ecosystems, making the environment that supported them even more productive,” the researchers write.

Shell middens are found throughout the world, and the researchers suspect that these, too, may enhance local ecosystems. “Coastal British Columbia is the first known example of long-term intertidal resource use enhancing forest productivity and we expect this pattern to occur at archaeological sites along coastlines globally,” they write.

In fact, that broader view is key to the significance of the findings. Too often, talk of sustainable land management practices developed by indigenous groups takes on the flavor of a curiosity—a quaint exception that may not be very relevant to the modern world. (To be clear, this isn’t a critique of the current paper; I’m speaking more broadly.) The challenge is to honor the particular cultures that developed these techniques, while also seeing them as part of our common human heritage. That is one approach that could help turn the exception into the rule. — Sarah DeWeerdt | 6 September 2016

Source: Trant AJ et al. “Intertidal resource use over millennia enhances forest productivity.” Nature Communications. 2016.

Header image credit: Lindsey B via Flickr.