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Cannot See the Forest for the Bees

Written by National Geographic Young Explorer Lauren Ponisio.

Osmia bee visiting a phacelia flower.

Osmia bee visiting a phacelia flower.

Deep in the backcountry of Yosemite National Park, the flora and fauna in the understory form a mosaic of vibrant colors in constant motion. Buckwheat flowers, pinkish-white pom-poms on naked stems, sag with the weight of  visiting bumblebees bees probing for nectar and rummaging for pollen; I like to watch as they spring upright again when the bee leaves in search of another bloom catapult. Glistening green-blue mason bees disappear into a clump of phacelia  flowers. Metallic black sweat bees crawl from the scarlet, tube-like flowers of gilia. Leaf-cutter bees with pollen-covered bellies rummage in wild sunflowers. With a flick of the wrist, I scoop one of these unsuspecting visitors into my net. I coax the struggling creature into a small jar that I keep in an elastic belt across my shoulders.

The Illilouette Basin in Yosemite National Park.
The Illilouette Basin in Yosemite National Park.

This has been my life for two months. Using my net as a walking stick, I begin heading back to the campsite. I make slow progress — continuously walking around or crawling over fallen, charred trees. Once there, I remove today’s bees from their jars and pin them into an insect box to preserve them. Bees in Yosemite are incredibly diverse; around 200 bee species occur in the Illilouette alone. My pinned specimens help me identify the overwhelming variety of species I catch each day.

A days worth of pinned pollinators.
A days worth of pinned pollinators.

Bees are incredibly important. Along with other animals, they pollinate a third of the crops humans depend on—including some of the most important ones, like coffee and cacao (which is used to make chocolate). Pollinators are also critical for maintaining the diversity of plant communities like those in the backcountry of Yosemite. Yet pollinators around the world are suffering from a variety of threats imposed by people, like pesticides and habitat destruction. To maintain the security of our crops and ecosystems, we desperately need to understand how to restore pollinator populations; to enhance populations, we first must understand what sustains their diversity.

California itself is a hotspot of bee diversity. There are around 1,500 species of bees in across the entire state, so it is amazing that we have seen 200 (over 10%) of those species just in the Illilouette basin, a relatively small area of land. Bees need three key ingredients for a successful life: nectar to power flight, pollen to feed the next generation, and a place to live. Yet in most forests the forest floor is barren, without a wildflower or bee in slight. So then why are the bees of the Illilouette Basin so diverse? This is the question that brought me here and, after two years of research, the answer is simple – fire.

Prepping new recruits before a survey day. Photo by Leithen M’Gonigle
Prepping new recruits before a survey day. Photo by Leithen M’Gonigle

Lightning strikes a tree, and the fire spreads to the understory — creeping along the vegetation and debris on the ground, sometimes climbing up another tree into the canopy, then crawling back into the undergrowth when it could not reach another tree. Just a hundred years ago, most of the forests western U.S. would be burning like this in the summer. In the Illilouette, the park has allowed fires to burn for almost 40 years. In that time, the modest 40,000 acres in the basin have experienced more than 150 fires.

Still, historical fires were different from the hot, fast spreading fires we have been experiencing. The severity of a fire is measured by the percent of trees that were killed. In areas where greater than 90 percent of trees are killed — generally because the fire entered the canopy and started jumping from tree to tree — these are called “high severity” fires.

Through the years, continuous suppression of fires has allowed dead material, like branches and dried grass, to pile up, creating the perfect fuel for a spreading fire. In combination with cattle grazing and logging, we have transformed the forests of the western U.S. into dense stands of young trees – much better conductors of fire than a patchwork of trees of different ages.

Climate change has also dried the system, causing later and less frequent summer rains and less snow pack. The end result: an unprecedented number of fast spreading, high severity fires.

High severity fires open the canopy, allowing sunlight to reach the seeds and spores adapted to emerge in these conditions. Fire-following plants and mushrooms, usually seen only rarely because of fire suppression, appear in high abundances. Some patches where no trees survived are quickly replaced by carpets of yellow and blue goldenrod and lupine. More flowers attract more animals like bees. Through my research over the years, I found the higher the fire diversity, the higher the diversity of bees and flowering plants.  Fire diversity is thus important for sustaining biodiversity.

Lupins and fireweed in a high severity fire patch in the Illilouette.
Lupins and fireweed in a high severity fire patch in the Illilouette.

So, fires are neither “good” nor “bad”—it’s the configuration of the severity that matters for wildlife. Fire patterns have shifted so that there is often more of the bad and less of the good. This will hurt the wildlife supported by our forests. It feels like all of the forests of the west are on fire — and they should be, just with more variability in fire severity and other fire characteristics.

The Illilouette is deep in the backcountry of Yosemite, so a fire is unlikely to put people at risk, which is why fires there are allowed to burn freely. Acknowledging and maintaining these frequent fire systems should be a main consideration in decisions that affect the future of the forests of the western U.S.. We need more places like the Ilillouette, where we let fires burn naturally, and also see fires controlled by managers. Without greater awareness of the need for fire, however, this will be impossible and wildlife supported by forests will suffer.

The field crew before a day of surveying pollinators. Photo by Leithen M’Gonigle
The field crew before a day of surveying pollinators. Photo by Leithen M’Gonigle