New research reveals humanity’s roles in ecosystems

In two back-to-back symposia at the annual meeting of the American Association for the Advancement of Science in Washington, D.C., on Sunday, Feb. 17 at 1:30 and 3:30 PM respectively, a cross-disciplinary cohort of scientists will present the first comprehensive investigations of how humans interacted with plant and animal species in different cultures worldwide through time. By compiling and comparing detailed data from pre-industrial and modern societies, the researchers are sketching a picture of humans’ roles and impacts in sustainable and unsustainable socio-ecological systems.

New research reveals humanity's roles in ecosystems
In this depiction of the nearshore food web of Sanak Island, Alaska, spheres represent species or groups of species,
and the links between them show feeding relationships. The colors of the spheres indicate types of taxa: green
shows algae; blue shows seagrass, lichen, protozoa, bacteria, and detritus; yellow shows invertebrates such as snails,
 crabs, mussels, and octopus; orange shows fishes; purple shows birds; and red shows mammals such as sea otters.
The red node near the center top of the image represents human hunter-gatherers (Aleut). It is the first
comprehensive food web to include Homo sapiens [Credit: J.A. Dunne]

«Almost all food webs that have been compiled and studied have been put together without including humans,» says Jennifer Dunne (Santa Fe Institute), an ecologist and complex systems scientist who is leading the project with archaeologist Stefani Crabtree (Santa Fe Institute and Center for Research and Interdisciplinarity). «It takes a lot of time and effort to put these kinds of detailed data together. So even though ecologists have been studying food webs for decades, we’re only now in a position where we can start to rigorously compare human roles and impacts across different systems to understand sustainability in new kinds of ways,» says Dunne.

What do we learn when we do include humans?

As part of her presentation during the second symposium, Dunne will reveal initial results from a comparison of food webs that explicitly include humans across several socioecological systems. Three are pre-industrial systems — the Aleutian Islands of Alaska, the Pueblo U.S. Southwest, and the Western Desert of Australia, and one is modern — the Tagus Estuary of Portugal. Given the diversity of cultures, ecologies, climates, and time periods represented in the data, Dunne suggests that we can start to learn «something more general about human roles in, and impacts on, ecosystems» by comparing these systems. For example, humans are often super-generalists compared to other predators — they feed on a huge variety of different species.

In some systems, humans as super-generalist predators can fit into ecosystems without causing extinctions or major environmental degradation. For example, according to Dunne’s pioneering analysis published in Scientific Reports in 2016, the Sanak Island (Alaska) Aleut fed on a whopping 122 of 513 taxa in the nearshore marine ecosystem. However, like other predators, they switched from their favorite prey — sea lions — to shellfish, kelp, or whatever was readily available when the weather did not allow them to hunt in open water. «Prey-switching is very stabilizing for food webs,» Dunne explains, «because it allows prey taxa populations to recover from exploitation, as the predator’s focus shifts to other prey that are easier to forage or hunt given current conditions.» That, plus limited use of hunting technology and other factors helped to minimize potential negative impacts of humans on the Sanak ecosystem — during approximately 7,000 years of human habitation, there is no evidence for any long-term local extinctions.

Jennifer Dunne and Stefani Crabtree describe an ongoing investigation into the roles humans play in sustainable and 

unsustainable socio-ecological systems. Their collaboration with anthropologists, archaeologists, and ecologists is 

the first comprehensive investigation of how humans interacted with plant and animal species in different cultures 

worldwide through time [Credit: Mountain Creative for the Santa Fe Institute]

Humans also stabilized the desert ecosystem of Western Australia, where Crabtree and Rebecca Bliege Bird (Pennsylvania State University) are examining how the Martu Aboriginal foragers are embedded in their surrounding ecosystems. According to Crabtree, Martu Aboriginal foragers stabilized their ecosystem by providing several ecosystem services such as lighting small brush fires to expose the burrows of small prey. The scorched patches left on the landscape served as natural fire breaks against larger, more devastating wildfires. When the Martu were removed from their homeland in the mid-20th century, wildfires increased dramatically in size, and several small mammals, like the Rufous Hare-wallaby, went extinct.
Bird will present a newly published network analysis for the Aboriginal foragers during the first symposium, following Andrew Dugmore (University of Edinburgh) and George Hambrecht’s (University of Maryland) presentation of how Norse people in Iceland and Greenland used governance to mitigate anthropogenic degradation of the ecosystem.

Just as humans can have a stabilizing effect on their ecosystems, they can also play a destructive role. In the first symposium, archaeologist Jennifer Kahn (College of William and Mary) will present her ongoing historical analysis of the French Polynesian islands, including two cases where human interactions with their surroundings led to markedly different outcomes — both for the ecosystems and the societies embedded within them.

Crabtree, in the second symposium, will present her analysis of the 700-year trajectory of the Ancestral Pueblo people in the Southwest U.S., and the extent to which human interactions with the ecosystem eventually led them to depopulate the region.

Iain McKechnie (University of Victoria and Hakai Institute) will then present anthropological and archaeological data that illustrates the resilience of the indigenous peoples of the North American Northwest Coast as they interacted with both marine and terrestrial species.

In the final talk, Dunne will cross-compare these and other systems and synthesize what we know, so far, about humanity’s roles across ecosystems and time periods. In addition to presenting new results about human roles in food webs, she will also discuss new work that moves beyond feeding interactions to consider the myriad ways that humans interact with biodiversity in both simple and complex ways, for example by using species for medicine, shelter, tools, clothing, fuel, ritual purposes, and trade.

«Understanding ecosystems with humans as part of them is essential,» Crabtree says. «We’re not going anywhere. We are here to stay. We are going to keep impacting ecosystems, and we need to understand the ways that our impacts can lead to more sustainable and resilient systems.»

Source: Santa Fe Institute [February 17, 2019]

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Indigenous hunters have positive impacts on food webs in desert Australia

Australia has the highest rate of mammal extinction in the world. Resettlement of indigenous communities resulted in the spread of invasive species, the absence of human-set fires, and a general cascade in the interconnected food web that led to the largest mammalian extinction event ever recorded. In this case, the absence of direct human activity on the landscape may be the cause of the extinctions, according to a Penn State anthropologist.

Indigenous hunters have positive impacts on food webs in desert Australia
Drawing of a Rufous Bettong from ‘Mammals of Australia’, John Gould, 1845-63
[Credit: Public Domain]

«I was motivated by the mystery that has occurred in the last 50 years in Australia,» said Rebecca Bliege Bird, professor of anthropology, Penn State. «The extinction of small-bodied mammals does not follow the same pattern we usually see with people changing the landscape and animals disappearing.»

Australia’s Western Desert, where Bird and her team work, is the homeland of the Martu, the traditional owners of a large region of the Little and Great Sandy Desert. During the mid-20th century, many Martu groups were first contacted in the process of establishing a missile testing range and resettled in missions and pastoral stations beyond their desert home. During their hiatus from the land, many native animals went extinct.

In the 1980s, many families returned to the desert to reestablish their land rights. They returned to livelihoods centered around hunting and gathering. Today, in a hybrid economy of commercial and customary resources, many Martu continue their traditional subsistence and burning practices in support of cultural commitments to their country.

Twenty-eight Australian endemic land mammal species have become extinct since European settlement. Local extinctions of mammals include the burrowing bettong and the banded hare wallaby, both of which were ubiquitous in the desert before the indigenous exodus, Bird told attendees at the 2019 annual meeting of the American Association for the Advancement of Science today (Feb. 17) in Washington, D.C.

«During the pre-1950, pre-contact period, Martu had more generalized diets than any animal species in the region,» said Bird. «When people returned, they were still the most generalized, but many plant and animal species were dropped from the diet.»

Indigenous hunters have positive impacts on food webs in desert Australia
Drawing of Banded-hare wallabies from John Gould Mammals of Australia, 1845-63
[Credit: Public Domain]

She also notes that prior to European settlement, the dingo, a native Australian dog, was part of Martu life. The patchy landscape created by Martu hunting fires may have been important for dingo survival. Without people, the dingo did not flourish and could not exclude populations of smaller invasive predators — cats and foxes— that threatened to consume all the native wildlife.

Bird and her team looked at the food webs — interactions of who eats what and who feeds whom, including humans — for the pre-contact and for the post-evacuation years. Comparisons of these webs show that the absence of indigenous hunters in the web makes it easier for invasive species to infiltrate the area and for some native animals to become endangered or extinct. This is most likely linked to the importance of traditional landscape burning practices, said Bird.

Indigenous Australians in the arid center of the continent often use fire to facilitate their hunting success. Much of Australia’s arid center is dominated by a hummock grass called spinifex.

In areas where Martu hunt more actively, hunting fires increase the patchiness of vegetation at different stages of regrowth, and buffer the spread of wildfires. Spinifex grasslands where Martu do not often hunt, exhibit a fire regime with much larger fires. Under an indigenous fire regime, the patchiness of the landscape boosts populations of native species such as dingo, monitor lizard and kangaroo, even after accounting for mortality due to hunting.

«The absence of humans creates big holes in the network,» said Bird. «Invading becomes easier for invasive species and it becomes easier for them to cause extinctions.» The National Science Foundation and the Max Planck Institute for Evolutionary Anthropology supported this work.

Source: Penn State University [February 17, 2019]

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