Humans and Our Impact on Biodiversity

For as long as humans have lived on Earth, we have impacted our surrounding environment. We began by chopping down trees to build homes and fires, foraged for plants, and hunted for animals in our surrounding landscapes for food. We then quickly advanced toward manipulating the land to serve our purposes through varying methods of agriculture, travel and increased urbanization and commercial networks.

At this point in Earth’s physical history, our impact on the environment is so substantial that many researchers believe “pristine nature,” or ecosystems untouched by human impact, no longer exist. This era of human dominance has been coined the ‘Anthropocene Era’ by some scientists, who argue that Earth is being overwhelmingly defined by the actions of humans above natural processes. But how can we truly understand in what way humans are changing our environment — or even if humans can impact nature in such a manner that creates irreversible changes, seeing as nature, itself, is a resilient system.

The Importance of Island Ecosystems

To examine just how much humans are impacting global environments, an international team of researchers turned to the world’s most remote oceanic islands. These islands were colonized within the past 3,000 years; therefore, they provide opportunities to analyze what an ecological transition from prehuman to human dominated ecosystems looked like, and the degree in which human impacts can modify an ecological system long-term. Islands are preferred over continents as it’s harder to identify what a prehuman ecosystem may have looked on a continent, not only because humans first settled within these places a much longer time ago, but also because environments on continents can heavily vary by region.

In the study, the researchers utilized fossil pollen extracted from the sediment layers that was then dated and identified to investigate how the vegetation on 27 islands in different regions around the world have developed over the last 5,000 years. They found that almost everywhere they studied, the arrival of humans has triggered an accelerated rate of change in plant species composition. This change was observed on 24 of 27 islands independently of current and past island area, latitude, isolation, and elevation of the sampling site.

El Junco Lagoon, Galápagos Islands. Image by Mikko Koponen.

This dynamic was particularly pronounced on islands colonized within the last 1,500 years. In islands that were settled more recently, like the Poor Knights archipelago in New Zealand (13th century) and the Galápagos Islands (16th century), there was a steeper increase in the rate of change than on islands where humans arrived less than 1500 years ago, such as New Caledonia and Fiji. Researchers suggested that this most likely because introduced species, land use practices, and technology deployed by later settlers was more elaborate or transformative than those of earlier settlers.

Time Can Help Ecosystems Heal

The researchers found that ecological legacies of human arrival on islands may persist for centuries, and are often irreversible. For example, the Tawhiti Rahi island within the Poor Knights archipelago is currently uninhabited. After initial arrival by Polynesians in the 13th century, the island’s forest cover was cleared by fire for human habitation and gardens. In 1820 the islands were declared protected, and humans were no longer permitted to settle there. The island became totally reforested within 150 years, but its current composition of flora and fauna is still completely different from that of the prehuman period.

Poor Knights Islands, New Zealand. Image by Peter Southwood.

These findings indicate that human-affected ecosystems are neither similar to nor likely to return to the dynamic baselines observed before human arrival. Even though ecosystem change can also be driven by a number of natural factors such as earthquakes, volcanic eruptions, extreme weather and changing sea levels, this study shows that disturbances caused by humans surpasses all of these events in their capacity to induce long-lasting changes to these systems. Furthermore, these disturbances produce different effects on the surrounding ecosystem than natural disturbances in the prehuman period, when island ecosystems often recovered rapidly to pre-disturbance states.

For our goal to protect and conserve biodiversity within our ecosystems and mitigate the risks posed by climate change, this study provides great insight on how to move forward. The researchers advise that conservation strategies must account for the long-term impact of humans and the degree to which ecological changes today differ from prehuman times. Our ecosystems are resilient, but with the current increased pressure we are putting on these systems, more ambitious changes to ensure the protection of ecosystem services and biodiversity is needed. These goals are still within reach, but only if we can act on them before more irreversible harm is done.