Sacramento Valley field notes, October.
California’s wetlands run on a three-tier food web that’s tighter and more visible than most ecosystems: aquatic invertebrates feed waterfowl and amphibians, waterfowl and amphibians feed mid-sized raptors and mammals, and a small number of top predators (bobcat, coyote, golden eagle, Northern harrier) sit above all of it. Remove any one tier and the whole structure shifts within a season.
The base layer: invertebrates and plants
A California wetland starts at the bottom with:
- Aquatic invertebrates - midge larvae, freshwater shrimp, dragonfly nymphs, snails. The energy currency of the whole system.
- Algae and submerged plants - the primary producers. Pondweed, duckweed, sago, smartweed.
This bottom tier feeds nearly everything above it. Without it, the food web collapses immediately.
The middle layer: dabblers, divers, and small vertebrates
The middle tier is the visible bird life people come to wetlands to see:
- Dabbling ducks (Mallard, Northern Pintail, Cinnamon Teal, Northern Shoveler) - feed on invertebrates and plant seeds.
- Diving ducks (Canvasback, Ruddy Duck) - feed underwater on invertebrates and tubers.
- Wading birds (Great Egret, Great Blue Heron, Snowy Egret, White-faced Ibis) - feed on small fish, amphibians, invertebrates.
- Amphibians (Pacific tree frog, California red-legged frog) - feed on invertebrates, are themselves prey.
- Small mammals (voles, deer mice, marsh rabbits) - prey base for raptors.
This layer is where most of the energy moves through the system.
The top layer: California's wetland predators
The top predators that hunt in or near California wetlands:
- Bobcat (Lynx rufus) - the wildcat. Hunts ducks, marsh rabbits, voles at dawn and dusk along reed edges.
- Coyote (Canis latrans) - the generalist. Hunts at night, takes whatever it can catch.
- Red fox (introduced; Vulpes vulpes) - active especially around Suisun Marsh. Pressure on ground-nesting birds.
- Northern harrier (Circus hudsonius) - the low, slow-flying marsh hawk. Takes small mammals and small birds.
- Northern goshawk and Cooper’s hawk - take medium-sized birds.
- Golden eagle (Aquila chrysaetos) - winters in California; will take adult ducks and even young deer.
- Great horned owl - the night equivalent; takes ducks, marsh rabbits, even bobcats occasionally.
The aquatic predators
Below the surface there’s a parallel food web:
- River otter - takes fish, amphibians, occasional ducklings.
- Largemouth bass and pike - take small fish, frogs, ducklings.
- Bullfrog (introduced) - eats anything it can fit in its mouth, including small fish and ducklings.
- Gartersnake species - eat amphibians, small fish.
What disrupts the system
Three things that have measurably shifted California wetland food webs:
- Drought. Reducing water means concentrating prey, which helps predators short-term but collapses the system mid-term.
- Introduced species. Bullfrog and red fox in particular shift the balance against native species - fox takes ground-nesting bird eggs and chicks; bullfrog eats native amphibians and young birds.
- Habitat loss. California has lost over 90% of its historical wetlands. The remaining 10% holds the entire migratory waterfowl population that uses the Pacific Flyway - meaning concentrations are unnaturally high.
What you can actually see in the field
Best places to watch this food web in action:
- Sacramento NWR Complex - wintering waterfowl and the raptors that hunt them.
- Merced NWR - winter Sandhill Cranes, Snow Geese, Bald Eagles.
- Suisun Marsh - the tidal marsh, river otters and bobcats most accessible here.
- Salton Sea (south end) - large concentrations of wading birds and the predators that follow.
Dawn and dusk give you the best chance of seeing actual predation. Bobcats and coyotes especially work the marsh edge in low light.
Sibley Field Guide to Birds of Western North America
The book for every California wetland visit.
David Allen Sibley's Western North America guide covers everything you'll see at a California wetland - all the waterfowl, all the raptors, and the small birds that feed the system. Hand-painted plates, range maps, behavioural notes. The standard reference for western birding.
- Every Western bird species, hand-painted
- Range maps and behaviour notes
- Compact, fits a coat pocket or rucksack
Sibley · Western 2nd Ed.
As an Amazon Associate we earn from qualifying purchases.
The bottom line
A California wetland is a working food web you can watch unfold in real time. The base is invertebrates, the middle is the waterfowl, the top is bobcat, coyote, eagle, owl. Drought and habitat loss compress everything; introduced species rebalance against the natives. Visit at dawn in winter; bring binoculars.
