The rise of the cow
Declines and extinctions of populations are not uncommon, but what struck Kidwell and Tomašových was the timing of this decline. This mass death, which took nearly a century to play out, happened before the usual suspects—sewage, warming sea temperatures, and destructive fishing gear—were in the picture. The first sewage outfall pipes were extended into Santa Monica Bay in 1925, onto the San Pedro Shelf in 1927, and onto the Palos Verdes shelf in 1937; sewage discharge peaked in the 1970s and then declined rapidly in the 1990s thanks to environmental protections. Sea temperature did not start to rise in Southern California waters until the early 20th century. Southern California fishermen never used bottom-destructive gear like otter-trawls except for a brief one-year trial for shrimp in 1920. As Dr. Kidwell says, “the decline of filter-feeding gravel-dwellers began and was complete too early to be explained by 20th century stressors.”
So when climate change, overfishing, and sewage did not make the line-up of suspects, Kidwell and Tomašových decided they had to become historians and dig deeper into Southern California’s past to determine what potentially catastrophic stressors existed in the 19th century. With advice from Don Cadien at the Los Angeles County Sanitation District and the help of Richard Krause at the Los Angeles Public Library, Kidwell and Tomašových began to focus on the livestock industry, which apparently was a very big deal in the 19th century.
Knowing they needed quantitative data – numbers that they could test statistically -- the scientists pieced together a puzzle of hundreds of disparate reports from Spanish missions, the LA Pueblo, large ranchos, early US Census data, and other 19th-century commentary about the cattle industry. The main products of this industry were cattle hides and tallow (fat) used to make candles and soap. The hides were traded, along with the tallow, for merchandise and goods on Boston-based ships traveling 15,000 miles around Cape Horn to coastal California ports, including San Pedro on the LA coast. After the beginning of the gold rush in 1848, the industry turned more to raising cattle for meat to supply the exploding population in central and northern California. US laws requiring the fencing of cattle soon changed the economics of the industry, however, so that by 1900 the coastal rangeland had been converted entirely to cropland.
It is not the presence of the livestock industry that was a surprise to Kidwell and Tomašových; it was the sheer number of cattle in the Los Angles region that was shocking, and how rapidly they multiplied. Prior to the arrival of the Spanish missionaries in 1769, there were no cows in California. Spanish missionaries brought 200 cattle, along with horses and sheep, into southern California in the early 1770s, and these populations easily grew on the open rangeland of perennial, native, California grasses. Kidwell and Tomašových tabulated that by 1800 there were at least 23,000 cattle in the LA coastal plain alone, a 100-fold increase in population in only 30 years. Feral horses were so abundant around the LA Pueblo by then that thousands were apparently culled to preserve grass for cattle, which were more valuable.
After weeks of scouring libraries for data, Kidwell and Tomašových realized that “livestock in the Los Angeles region had almost certainly reached the carrying-capacity of the land by 1830, with approximately 100,000 grazing animals. It really opened our eyes to just how many animals were in the watershed by then—huge numbers.” For example, Kidwell described being shocked as she read in the LA Almanac that San Gabriel Mission padres had ordered the slaughter of many tens of thousands of cattle before the Mexican government tried to take possession of them in 1834. Despite this, livestock numbers quickly rebounded under Mexican and then American control.
What do huge numbers of unmanaged, free-ranging cattle do to the landscape? Based on reports from visitors, they transformed LA’s bucolic prairie into an over-grazed desert by the 1820’s. The pre-European landscape of perennial shrubs and grasses, thickets, and large live-oak forests was transformed into a monotonous sea of foreign, annual plants. Some sources estimate that, statewide, 50-90% of California’s native grasses were gone by 1900. And when the land in Southern California could no longer deal with this, the land passed the problem over to the ocean.
More important than the massive vegetation changes, the cattle would have trampled and compacted the soil severely, so that rainfall would run off to the sea rather than penetrate and remain in the soil. Kidwell and Tomašových estimate that sediment erosion and delivery to the ocean increased 10-fold from pre-European conditions, “producing many megatons of sediment shed from the land per year for about 100 years, starting in the early 1800s.” During the 20th century, sediment run-off into the coastal ocean actually declined substantially thanks to: improved cattle-range management; soil conservation measures for cultivated land; and residential and industrial development. But between 1800 and 1900, Kidwell and Tomašových think the bottom of the coastal ocean quietly changed forever under a steady, smothering rain of dirt. The ocean communities of shell-gravel animals (who require clear water to filter and eat) were literally being suffocated by an avalanche of dirt pouring into the coastal ocean.
Is it possible that cows and sediment run-off are just a smoking gun for the ecological collapse on the nearby seafloor? Further studies are needed to confirm that the flood of muddy sediments can be dated and linked directly to the cows, likely using pollen and analysis of fungal spores known to depend on animal dung. But for now, all the evidence points to the not-so-holy cows of the City of Angels.