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Apocalypse Cow Fossils reveal a marine ecosystem collapse caused by 19th Century livestock in Los Angeles

By Charlotte Stevenson, Science Writer, University of Southern California Sea Grant Program

Published October 18, 2018

OK. We cannot claim credit for this title: Apocalypse Cow. It already has a cult following as a craft beer and a zombie-thriller book. But, assuming there is a little room left for associations, we would like to slap this label on a time period in Los Angeles’ history. When was this cow-mageddon, you ask? And why haven’t we heard about this before? To answer those questions, we are going to have to dive deep…literally.

Unlike the well-preserved and easily carbon-dated fossils paleontologists have stumbled across (sometimes quite literally) on land, ocean fossil beds are not as easily accessed, and, in many cases, the fossils are not as well preserved. As you can imagine, the quintessential image of a paleontologist—delicately brushing away the sediment from a gleaming fossil for weeks under her wide-brimmed hat—is not possible at the bottom of the ocean. Sediment has to be hauled up from the seafloor in a myriad of gallon-sized “grab-samples” before any searching can begin. And then the hard work starts, piecing together a history from the jumbled-multi-generational pile of shells, which burrowing organisms have mixed together.

Dead shells tell their secrets. Researchers carefully pieced together a detailed fossil record of the seafloor off Los Angeles stretching back over several thousand years, and discovered a traumatic and utterly unsuspected chapter of Southern California’s past.

Dead shells tell their secrets

Despite these challenges, Professor Susan Kidwell, a geologist at University of Chicago, and Dr. Adam Tomašových, a geologist at the Earth Science Institute of the Slovak Academy of Sciences, took on the challenge of analyzing marine fossils off the coast of Los Angeles in a University of Southern California (USC) Sea Grant funded study. The original goal of the study was to use fossils to determine what the seafloor and its inhabitants were like before the start of regular scientific monitoring in the mid-20th century. With a longer-term perspective on ocean conditions, Kidwell and Tomašových hoped to improve scientists’ ability to judge how effective anti-pollution efforts over recent decades have been, in response to improved wastewater treatment required by the federal Clean Water Act.

Using the archived seafloor samples collected by local sanitation districts (LA County, City of LA, Orange County, City of San Diego) and other research organizations (Southern California Coastal Water Research Project, LA County Natural History Museum) over the last 40 years, Kidwell and Tomašových have carefully pieced together a detailed record stretching back over several thousand years, and discovered a traumatic and utterly unsuspected chapter of Southern California’s past.

The most common types of fossils found in the ocean are the shells of animals such as clams, scallops, mussels, and the less well-known brachiopods. Off the coast of Los Angeles, Kidwell and Tomašových discovered that the dead shells of the epi-faunal (non-burrowing) brachiopods date back to 9,000 years, using a technique called amino-acid racemization that permits them to accurately date each shell to within a decade. For those who only know scallops from their fine dining menus, scallops, mussels and brachiopods are all shelled, sedentary animals living on the bottom of the ocean on gravel patches, filtering and eating plankton out of the water around them. Clams also mostly filter out plankton, but they burrow into muddy or sandy seafloors.

The neighborhood had changed. After thriving for thousands of years, seafloor-dwelling, filter-feeding communities of scallops and brachiopods were functionally extinct on the mainland California continental shelf by 1910. They were replaced by a new community of burrowing animals, like polychaete worms and clams, which thrive in muddy, murky conditions.

Kidwell and Tomašových found abundant brachiopod shells from every century for thousands of years up until the early 19th century, when they started to become scarce – and there are none younger than 1910. “Despite the 20th century being an extremely recent increment of time, we have fewer shells representing it than any other 100-year increment of time in the last 9000 years,” said Kidwell. “This doesn’t make sense when you consider the fact that the shells of brachiopods that lived (and died) on the sea-floor 500 years ago are more likely to have been destroyed by now than the shells of animals that died only 100 years ago.”

This glaring absence of fossil shells starting in the early 20th century – and evidence that they began to dwindle in the early 1800s -- was the first clue to Kidwell and Tomašových that they had discovered something extraordinary. Something drastic had happened to these gravel ecosystems of filter feeding mollusks off the coast of Los Angeles. Moreover, the existence of extensive shell-gravel communities of any age was unknown on the LA seafloor, much less that they had existed up until so recently and had gone extinct within historical time. In the terrestrial world of paleontologists, this was Kidwell and Tomašových’s version of tripping over a pile of dinosaur bones where none were expected.

Ultimately, the researchers showed that after thriving for thousands of years, seafloor-dwelling, filter-feeding communities of scallops and brachiopods suddenly started to decline in the 1820s and were functionally extinct on the mainland California continental shelf by 1910 (some small communities remain today on the isolated islands off the Southern California coast.) They were replaced by a new community of burrowing animals, like polychaete worms and clams, which thrive in muddy, murky conditions, feeding on microbes and organic debris. The neighborhood had changed.

The rise of the cow. 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.

However, huge numbers of unmanaged, free-ranging cattle transformed LA’s bucolic prairie into an over-grazed desert by the 1820’s. Cattle would have also trampled and compacted the soil severely, so that rainfall would run off to the sea rather than penetrate and remain in the soil.

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 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.

You cannot forget your past. This story makes us wonder about other baselines that shifted under our feet before we were really paying attention. What else might have disappeared before the start of scientific monitoring in the 20th century? It reminds us that uncovering the past may be helpful to make sure we do not make the same mistakes in the future.

You cannot forget your past

There is no question that Kidwell and Tomašových’s research makes a great story. But why do we care about the disappearance of these shelled creatures more than a hundred years ago? And, to take that question one step further for the budget-conscious, why should we fund studies like this that focus on the past when we have so many scary issues like climate change barreling down on us now?

Populations of brachiopods and the other diverse filter feeders that lived with them before the 19th-century—scallops, barnacles, and mussels—represent a fundamentally different kind of seafloor ecosystem than exists along the LA coast today. The seafloor had a greater variety of habitats, and thus supported greater biodiversity. This pre-cow ecosystem is unlikely to recover given the legacy of thick mud on the LA coastal shelf. This makes the tiny isolated populations of filter-feeders on the offshore Channel Islands even more important to study and protect—they’re not just a set of species, but the most visible members of an otherwise-vanished ecosystem.

This story also makes us wonder about other baselines that shifted under our feet before we were really paying attention. What else might have disappeared before the start of scientific monitoring in the 20th century? And where else might similar events have occurred with dramatic land use changes next to the coastal ocean? As coastal communities consider decisions on marine protected areas or restoration efforts, consideration must be paid as to what is actually “natural.” It does remind us that uncovering the past may be immeasurably helpful to make sure we do not make the same mistakes in the future.

Sources for this article:

Carey, John. Putting fossils to work in hopes of restoration. 2017. PNAS: 14 (33), 8663-8666.

Isenberg, Andrew C. “Environment and the 19th Century West.” A Companion to the American West. Edited by William Deverell. John Wiley & Sons, 2008.

Larson-Praplan, Stephanie. “History of Rangeland Management.” UC Rangelands Research and Extension Archive. University of California Davis, Division of Agriculture and Natural Resources. Web. January 31, 2018. <http://rangelandarchive.ucdavis.edu/Annual_Rangeland_Handbook/History_of_Range_Management/>

Tomašových, A. and Kidwell, Susan M. Nineteenth-century collapse of a benthic marine ecosystem on the open continental shelf. 2017. Proc. R. Soc. B, Proc Biol Sci.: 284 (1856).

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Created with images by Patrick Perkins - "Sea the color of lead"

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