3: Understand the food web

We read labels and pay attention to the food we eat because food is important and it connects us to the world. Who eats who or what also is important for ecosystems and understanding the food web is critical for helping us understand and design a healthy ecosystem.

Small animals like the Sand Lance and other forage fish can be important components of the food web. Understanding the food web is critical for protecting important species, knowing where we need to protect special habitats and understanding how toxins move through the system. Photo: J. Gaydos

Food webs represent complex trophic interactions among species: they can change seasonally and geographically. Although often simplified for communication purposes, food web linkages are complex, subtle and interactive; they play a major role in ecosystem connectivity as well as in ecosystem resiliency and capacity for renewal.

A working food web model is a powerful tool for managing ecosystems. Around the world traditional harvest management tools, such as maximum sustainable yield models, focus on how many individuals can be harvested sustainably by humans.

However, the models fail to take into account the full range of trophic interactions and trophic needs. For example, an acceptable salmon harvest level is designed to ensure that sufficient individuals are left to spawn in order to maintain viability of the salmon run into the future. What it fails to account for are the needs of other species dependent on the same salmon run, i.e. those species that prey on salmon or those species that are salmon prey.

Determining the impact of human-harvested salmon on killer whales, eagles or any of the other 136 vertebrate species that rely on salmon or salmon carcasses has proved elusive. Yet it has important biological and policy consequences. For instance, an important factor in listing Southern Resident killer whales as threatened under the U.S. Endangered Species Act was the decline in its primary prey, salmon.

Food webs can be used to identify priority or key species in biological communities. Measures taken to protect them and their habitats benefit the entire ecosystem. For instance, Pacific sand lance and surf smelt are key forage fish for some Puget Sound birds and mammals. Locating and protecting their intertidal gravel-sand spawning beaches and associated upland riparian habitats assures food supplies for many species. Human alteration of the shoreline can change environmental conditions of these beaches and halve egg survival resulting in “bottom up” impacts on the ecosystem through the food web.

Knowledge of food web dynamics allows managers to monitor movement of contaminants in the ecosystem and the effects of the toxins on species composition, abundance, diversity and ultimately the food web itself. Bioaccumulation of toxins has been shown to impact multiple species in many ways; from the immunologic health of harbor seals to the density and species richness of Phoxocephalid amphipods.