Quantifying the sensory experience of biodiversity

Walking through a forest, we instinctively sense when a place feels alive. The sunlight filtering through a canopy of leaves, a soft green glow surrounding we, and somewhere above, a chorus of birds calling from hidden branches. We can feel the richness of the place. Now, the question is: can that feeling be measured?

A team of researchers from Germany, the UK, Belgium, and Poland recently tried to answer this very question in a study published in People and Nature. The study, titled "Perceived biodiversity: Is what we measure also what we see and hear?", asks whether our perceptions of biodiversity line up with what scientists actually measure in the field.

Their goal was simple: to find out if "what we see and hear" when we walk through nature corresponds to the real, biological diversity around us. Because as it turns out, our eyes and ears might be better ecologists than we think.

To discover this, the researchers used a very interesting method. They brought 96 volunteers into the lab and asked them to sort either photographs or sound recordings of European forests. The photos came from forests in Germany, Belgium, and Poland, and the sounds captured everything from solitary woodpeckers to lively choruses of songbirds.

Participants were asked two things. First, to group the images or sounds however they felt made sense, based on color, structure, mood, or anything else ("open sorts”). Second, to rank them by how biodiverse they seemed ("closed sorts").

Meanwhile, the scientists had already measured "actual" biodiversity in these forests, things like tree species richness, forest structure, and bird species diversity. They also used advanced tools to compute visual and acoustic diversity indices, including one intriguingly named the Acoustic Complexity Index, which analyzes how varied the soundscape is.

In essence, the researchers were comparing three worlds: the objective measurements of biodiversity, the data-driven indices, and the human perception of what feels diverse.

When sorting the forest photos, participants didn't focus on scientific traits. Instead, they paid attention to things like vegetation density (that is, how thick or open the forest looked), light conditions (for instance, dappled sunlight vs. deep shade), color variations in greens and browns, the sStructure, shape and layering of trees, and, surprisingly, emotions such as the sense of calm, mystery, or comfort the scenes evoked.

For the sound recordings, the story was similar. People grouped clips based on birdsong complexity (in terms of melodic variety and rhythm), loudness and distance of sounds, time of day or season suggested by the audio, and the feelings the sounds stirred; say peace, alertness, or joy.

These findings highlight something important: biodiversity is a sensory and emotional concept. We perceive the richness of life not through species counts, but through patterns, colors, and sounds that make a forest feel alive.

So, can we actually tell how diverse a forest is? Surprisingly, yes. When the researchers compared participants' judgments to scientific measures, they found strong correlations between what people perceived and what ecologists measured.

For visual diversity, people's ratings closely matched indices like the Greenness Index, which quantifies how much "green" is in an image. For acoustic diversity, human perception lined up almost perfectly with the acoustic diversity indices that measured the variety and intensity of bird songs.

In other words, when we sense that one forest feels more "alive" than another, we're often right, even without realizing we're making an ecological assessment.

Beyond academic pursuits, understanding how people perceive biodiversity is, therefore, a cultural and psychological question. Biodiversity nurtures human well-being. Previous work had shown that exposure to biodiverse environments can reduce stress, boost mood, and foster a sense of connectedness to nature. But what drives these effects may not be the number of species hidden in the undergrowth, it may be the perceived biodiversity that our senses pick up. So, if perception plays a key role, then the design of parks, urban green spaces, and conservation projects should consider how biodiversity looks and sounds to people, not just how it scores on an ecological index.

Beyond the statistics, the study touches on a deeper issue, what ecologists call the "extinction of experience." As more people move into cities and spend less time in nature, we risk losing our ability to recognize or even notice biodiversity. When birdsong fades from daily life or the sight of layered forests becomes rare, our sense of what's "normal" shifts toward monotony.

Next, the team hopes to explore multisensory biodiversity, how sight, sound, smell, and even touch combine to shape our perception of nature's richness. After all, a forest is not just something you see or hear. It's also the coolness of shade, the scent of pine, the texture of moss. Future studies might use virtual reality or immersive recordings to understand how these senses interact to create our experience of biodiversity.

If you want to learn more, read the original article titled "Perceived biodiversity: is what we measure also what we see and hear?" on People and Nature at http://dx.doi.org/10.1002/pan3.70087.