Some wild species are more tolerant to humans than others and researchers want to know why so conservationists can determine which animals may need our help in the face of inevitably increasing human expansion encounters.

Ecologically speaking, humans are the ultimate invasive species, and the larger our global population grows the more we impede on the lives of the animals living with or around us. Habitat loss is a real and constant threat to many species, but the very presence of people can impact animals too, explains Diogo Samia, a Post-doctoral ecologist from Brazil’s University of São Paulo.

‘We know that the simple presence of humans can change animal’s behavior and physiology, which can have cascading effects on their populations or communities,’ says Samia. An animal that is easily disturbed by humans, fleeing at the very sight of people, is likely wasting a lot of energy avoiding these potential attacks—energy that could be used for more important tasks.

One way to explore the tolerance of animals to human encounters, particularly birds, is to look at their flight initiation distance, or FID, measured as the human-animal distance at which an animal initiates flight. ‘The FID is like an estimate of risk taking, in which the more an animal perceives humans as non-threatening, the shorter their FID,’ says Samia.

Recent studies have shown that populations of European and Australian birds with shorter FIDs are increasing, while less tolerant species are often declining. Samia explains this conclusion raised a few questions, chiefly, if human-tolerance has potential benefits for some species, what about those animals unwilling to adjust or becoming more fearful of human run-ins? What factors encourage shorter FIDs?

Reviewing 75 studies that included data on 180 species of birds, 16 types of lizards, and 16 mammals, Samia, alongside researchers from Australia, New Zealand, and the United States, found that the bigger animal and more frequent its rate of neutral human interaction, the shorter its FID. Because birds made up the bulk of the study species, their most supported conclusions were that larger birds were typically more tolerant than their smaller counterparts, and that birds in more heavily populated urban areas were generally more tolerant than rural birds.

There were other factors taken into consideration that could potentially sway tolerance, like diet, clutch size, and the openness of the species’ original habitat, but in this analysis the impact of each was negligible.

Samia says they achieved what the team set out to, providing the first evidence-based model identifying features that determine tolerance, information that could help conservationists predict which species may be more vulnerable to human expansion.

‘While there may be benefits for those species tolerant to humans, there are costs to those more intolerant,’ says Samia. ‘Hopefully, our results will help to develop conservation actions to minimise such impacts.’

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