Weaving the Web of Life

Do you recall the grade school diagrams illustrating a simple food chain? This predator eats that prey and that prey eats plants, and plants obtain their energy from the sun and nutrients from soil. Arrows represent one organism eating another. Food chain diagrams help us visualize the flow of energy through an ecosystem. While the reality is much more complex than the simplified models we find in textbooks, the fundamental idea remains: it’s an organism eat organism world!

Every living thing is part of multiple food chains, which are like descriptions of energy flows and nutrient transfers between organisms. Different chains represent different possible flows throughout the system based on who eats who—or who eats what. Together these chains link up to form the food web, a complex series of interactions between all the species in an ecosystem. A fundamental concept in ecology, the food web model impresses upon us the myriad interconnections and interdependences of nature. Appreciating the food web where you live and getting to know its major players is like getting to know your neighbors, with all of their accompanying dramas and conflicts!

The cycles of growth and decay, life and death, nourishment and starvation create an ongoing balancing act of energy in the ecosystem. This vital flow of energy is always in flux in response to environmental conditions, such as too much or too little rain, or the introduction of invasive species. On a larger scale, climate change and widespread species extinctions fundamentally alter the environmental conditions of ecosystems around the world, which changes the availability and flow of energy in the food web.

The food web consists of several different categories that describe an organism’s role in energy transfer, the primary ones being producers, consumers, and decomposers.

Producers are plants. They kick off the food chain by converting sunlight into biomass (a process known as photosynthesis). In the Sonoran Desert, producers include cacti, grasses, shrubs, and trees. Common species like the saguaro, prickly pear, palo verde, mesquite, and brittlebush form the bulk of vegetable mass in the region, but other less abundant species can be extremely important for some animals. For example, yucca seeds are required for the growth of yucca moth larvae, and yucca moths are the only pollinators of these plants.

Saguaro cactus fruiting. These fruits provide hydration and nourishment to countless animals at a critical time. Photo by Jay Pierstorff.

Without the ability to photosynthesize, animals must obtain their energy by eating plants or other animals. Primary consumers like packrats, desert tortoise, and butterflies are herbivores, and the transfer of energy and nutrients from tasty plant to hungry herbivore forms the first link in the food chain.

Secondary consumers may be carnivores (animal-eaters) or omnivores (plant and animal eaters) that eat the primary consumers for dinner. Creatures like the gray fox, kit fox, elf owl, red-tailed hawk, scorpion, roadrunner, rattlesnake, and spiders form this link in the chain. Many food chains have tertiary and even quaternary consumers, and some animals like the coyote will fill multiple roles depending on what’s available and on the menu. At the very top of the energy chain is the apex predator. While these top predators don’t usually face predation from others (exceptions include the young, old, and injured, hunting by humans, as well as intra-species predation, such as among great horned owls), they do compete with other predators, including humans, for access to food sources and territory. Bobcats, mountain lions, and great horned owls are some of the Sonoran Desert’s apex predators.

The mountain lion is an apex predator of the Sonoran Desert region.

Animals and plants that don’t become food die in other ways such as injury, disease, or old age, and it is the role of the detritivores, like beetles and vultures, and decomposers, like fungi and bacteria, to recycle the nutrients stored in animal flesh and decaying plant matter and return them to the soil for the producers to use, thus renewing the cycle of energy flow.

Some species have an outsized effect on their environment, and we call these keystone species. They are so vital to the web of life that when you take them out, the whole ecosystem collapses or fundamentally changes, just as an arch collapses when the keystone is removed. For example, trees are keystone species in forest habitats. Without enough trees, countless other animals and plants are left without the architecture and resources to survive. Likewise, the saguaro cactus is a keystone species in the Sonoran Desert. Because so many animals depend on the saguaro for shelter and nourishment, especially during the hot and dry foresummer months (May and June) when saguaro fruits bloom (and not much else), threats to the saguaro cactus should be regarded as a threat to the entire region where saguaros grow.

The food web model illustrates several key lessons from which everyone, not just ecologists, can learn. Visualizing all the possible connections in an ecosystem instills appreciation for the complexity and interconnectedness of our world. In addition to the food web, plants, animals and people depend on each other for a host of other services, like shelter, dispersing seeds, pollination, stabilizing soil, releasing oxygen, etc. Observing how the quickly the extirpation (removal from an area) or extinction of one species can collapse or degrade an ecosystem shows just how dependent we all are on each other. Direct and indirect interactions in the food web cause cascading effects throughout the web of life within which we are all ensnared.  

Humans play a central, outsized role in this complicated, enmeshed web of life, and have the power to protect the biodiversity that supports healthy ecosystems. Even if we are not involved in local food chains in the same immediate ways as the black bear or the coyote, our actions have a fundamental impact on the food web. By destroying habitat, competing with top predators, introducing invasive species, and changing the climate, our actions directly and indirectly degrade the productivity and abundance of the food webs of which we’re a part.

This is why, last year, the Desert Museum signed the Campaign for Nature’s 30×30 Petition, calling on world leaders to protect 30% of the world’s land and ocean by 2030. The need to protect functioning ecosystems and take action to prevent the worst effects of climate change becomes more urgent with every passing year. This effort recognizes the critical role of rural and indigenous communities in stewarding sustainable landscapes, and respects private and sovereign property rights. As a conservation non-profit, we are dedicated to inspiring action through appreciation and education. Learn more here and sign the petition as an individual.

Written by Elena Makansi. Featured image by Jay Pierstorff.

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