it's humbling to stand in the belly of an ancient redwood, to be enveloped by one of the largest living things on Earth. The thick trunk surrounding me on this particular day was heavily charred, its base hollowed out by some long-ago fire, leaving behind an expansive interior cavity. Fires don't ravage this forest often, at least not in the human sense. But measured in redwood lifetimes, sometimes well in excess of 1,000 years, fire is as inevitable as death. Fortunately, evolution has endowed these majestic trees with flame-retardant bark and other survival strategies, even allowing them to benefit from the periodic cataclysms.
Exiting the tree, I shifted my gaze skyward, feeling vertigo as my eyes followed the trunk to dizzying heights. Although the uppermost branches were obscured by a sea of green foliage, I knew that the topmost needles were processing sunlight about 90 meters (300 feet) above me. This was a coast redwood, Sequoia sempevirens, a species that includes the tallest organisms on Earth. The largest known examples achieve heights of about 115 meters (375 feet), taller than a 35-story building. Today, only three kinds of sequoia remain, but their legacy, like their trunks and life spans, is long. Sequoias not so very different from these graced the world of dinosaurs, so a visit to a redwood forest is like stepping back into deep time. As such, these forests provide another excellent place to contemplate the dinosaur odyssey.
This particular hollowed-out tree lives in Muir Woods National Monument, a stand of old-growth redwoods named after the famous tree lover and environmentalist John Muir. It is also the closest patch of old-growth forest to the city of San Francisco. As a result, the Monument receives almost a million visitors each year, a fact that would have pleased Muir greatly. Although I had visited Muir Woods many times, on this day I had come not to dwell on the majestic trees but to contemplate some of the less obvious, hidden aspects of the forest.
Muir Woods is home to several varieties of nonredwood trees. Meandering farther down the loamy path, my attention was drawn to a large overhanging leaf from a California bay laurel. Several holes were visible within it, and a few chunks were missing from the leaf margin. Still closer inspection revealed that these imperfections were edged by characteristic patterns left by the jaws of feeding insects. Though I could not see or hear them, I knew that the world around me was full of tiny herbivores.
A butterfly flitted by chaotically, looking for a meal of flower nectar, totally unaware that its activities were part of the plant's sexual dance. My next encounter was with a massive fallen redwood paralleling the path. After inhaling carbon dioxide and exhaling oxygen for more than a millennium, and after providing sustenance, shade, and safe haven to countless shrubs, microbes, fungi, and animals, this monstrous plant fell to earth, hammering several smaller trees on the way out. Yet the shattering event was not so much an end as a beginning.
When a tree dies, it is occupied by a slow parade of decomposer organisms, all of which, in one way or another, break down and consume its carbon, returning its nutrients from whence they came. The process often begins with beetles, which tunnel into the bark in order to generate larval nurseries. The battery of holes in the tree bark before me revealed that beetle activity was already well underway, providing access points for water and a host of other decomposer organisms—bacteria, mites and worms, among others. Beetles carry fungi on their bodies, which transfer to the wood and begin to consume and further decompose the tree. When the beetle larvae emerge in the tunnels, they feed on the newly arrived fungi before boring still deeper into the fallen tree. About the same time, more kinds of beetles arrive, as well as ants; together these insects penetrate the heart of the dead log. Wasps enter many of the tunnels, locate the beetle larvae, and lay eggs nearby, so that the newly hatched wasps have a plentiful food supply. After a year or so, different kinds of fungi capable of processing the highly fibrous redwood interior follow the insect highways, spreading throughout the log's interior. They are followed closely by termites and their gut bacteria, which together process the extremely coarse cellulose through a fermentation process. Bacteria in the termite guts also capture nitrogen and return it to the environment as their hosts die.
This fallen giant may take three or four centuries to fully decompose. Throughout the lengthy process, the redwood will provide a constant and vital source of nutrients to the environment. In fact, the tree will become a complex microecosystem unto itself, home to a succession of tiny life-forms. Without this complex organic cavalcade, the tree would take much longer to decompose and the valuable chemicals locked within its body would be unavailable for use by the larger system. Paradoxically, this redwood will become more alive in death than it was in life. During its lifetime, a tree is composed of about
5 percent living tissue; other than a thin layer near the surface that transmits water and nutrients up and down, the massive trunk is made up of dead matter. (This is why a tree with a fire-hollowed trunk can still thrive for many years.) A fallen tree in an advanced state of decay may be 20 percent living tissue by weight, thanks largely to occupation by decomposers. In a sense, then, trees have two lives, the first dominated by the capture of solar energy and the second by breakdown of that energy into a bounty of nutrients. The rotting log also underlines an important ecological truism: every species has a unique role in maintaining the larger ecosystem of which it is a part.
Stooping down, I picked up a handful of loose earth. There in my hand was the heart and soul of decomposition, capable of transforming any fallen leaf, tree, or animal into the chemical building blocks of future generations. Running the rich, soft substance through my fingers revealed a moisture-laden mixture of plant detritus, fungal strands, and tiny organisms. I reminded myself for the umpteenth time that microbes make the living world go round, as they always have. A random fistful of forest soil like this one contains billions of microscopic life-forms, mostly bacteria, exceeding the number of humans currently living on the planet—a staggering thought.
Of course, microbes are not restricted to the soil. They exist virtually everywhere in every ecosystem, so small and so firmly embedded in all aspects of life that we barely notice them. To give an example close to home, the human mouth contains more than seven hundred different kinds of symbiotic bacteria, each of which has carved out a unique existence amid a complex topography of tongue, teeth, and gums. Before you run off and brush every square millimeter of your mouth raw, remember that, by combating disease-causing bacteria, this phalanx of microbes is vital to your health. The same is true for many of the bacteria in your gut, on your eyelashes, in your nose, and indeed throughout your body. What you regard as your physical self includes on the order of 10 trillion cells, yet nine out of ten of these are not human cells. Put another way, your body is home to many more life-forms than there are people on Earth or stars in the Milky Way galaxy. Like it or not, you are a walking colony—or, better still, an ecosystem—living in unwitting harmony with these smidgens of life. This phenomenon held true for dinosaurs as well; it is awe inspiring to contemplate the long-necked sauropods, largest animals ever to walk the Earth, as gargantuan, four-legged bacterial colonies.
The previous pair of chapters followed the flow of energy from plants to herbivores to carnivores. Yet the changing web of life includes additional paths of energy flow involving the microscopic extremes of life's spectrum. Although human attention is naturally diverted toward dinosaurs and other creatures at the macro end of the spectrum, this bias should in no way lead us to conclude that absolute size is somehow proportional to ecological importance. Indeed, most ecosystems would likely persist in some form if the large vertebrate contingent were suddenly removed. But so critical are the strands of energy flow maintained by insects and microbes that expulsion of either would lead to immediate systemic collapse. Although Tyrannosaurus never stalked a fly or a bacterium, the complex interlinking of ecosystem subparts means that these Lilliputians of the
Mesozoic were every bit as important to the Tyrant King's survival as was Triceratops. Therefore, any discussion of the dinosaurian web of life would be grossly incomplete without some consideration of the Mesozoic microworld.
Unfortunately, the fossil record of Mesozoic insects, fungi, and bacteria is grossly incomplete, not surprising given their small sizes and lack of hard parts. By necessity, then, much of the following discussion is derived from the living realm. Yet even our understanding of modern microbes is woefully inadequate. Despite sending manned and unmanned vehicles to other worlds, we have the barest acquaintance with the surface of our own planet. Our gross ignorance of Earth's biota is revealed by the fact that we can only make the roughest of guesses as to the total number of species currently living on Earth. Estimates vary wildly, ranging from about 4 million to greater than 100 million, with 15 million an oft-cited ballpark figure. Of that astounding total, less than 2 million kinds of life-forms have been recognized as distinct and given Latinized genus and species names. Of those formally named species, only a tiny fraction is known much beyond this label. For most of nature's burgeoning diversity, then, we lack even basic information regarding life span, diet, and reproduction, let alone details of interactions within and between species.
On the plus side, the fossil record is far from silent with regard to these hidden strands. The past two decades have witnessed an abundance of insights relating to Mesozoic plants and insects in particular, and several exciting discoveries even point to a key role for bacteria in the process of fossilization. The scientific strategies used to find and unravel these fossil strands closely resemble those portrayed nightly in the highly successful crime scene investigation (CSI) genre of television dramas. A diverse range of forensic-style tools and techniques tease out bits of evidence preserved at prehistoric death scenes. In the current media-driven lingo, these investigations into ancient processes amount to the ultimate in "cold cases."
Our record of Mesozoic plants comes from fossilized tree parts—particularly trunks, branches, leaves, and flowers—as well as from microscopic pollen grains. Unlike dinosaurs and other vertebrates, for which mostly complete skeletons are not infrequently recovered, complete trees are virtually absent from the fossil record. Like several blind people touching different parts of an elephant, paleobotanists do their best to connect the pieces and describe entire plant species. Despite this uncertainty, we can be sure that Mesozoic plants faced the same kinds of challenges as their modern counterparts. And, as discussed in chapter 7, they also developed various kinds of mechanical, chemical, and biological defenses to ward off herbivores. Many defenses observed in living plants appear to have arisen during the Mesozoic, when both insects and dinosaurs were proliferating. Like the herbivorous dinosaurs, insects undoubtedly responded by evolving specialized jaw mechanisms and digestive strategies.
Given that insects represent a major, even dominant herbivorous impact on plant communities, it would be very useful to know about the insects that coexisted and coevolved with dinosaurs. Despite their relative lack of hard parts, we have an extensive fossil record of insects. Much of this evidence comes in the form of insects trapped in tree sap and subsequently turned to amber. The vast majority of the insect groups alive today lived alongside dinosaurs from the Jurassic onward. Insect species tend to be long-lived, persisting on average about 10 million years, and many Mesozoic insects appear closely related to forms alive today. This remarkable longevity stands in stark contrast to mammals and other vertebrates, whose species typically persist for about one million years. Most mammal groups, not to mention the species within them, evolved well after the major dinosaur extinction 65.5 million years ago.
Although Mesozoic insects are plentiful at a growing number of amber localities around the world, much more abundant are fossil leaves that preserve distinctive insect-feeding traces. Go outside to the nearest tree, and chances are that many leaves will show insect feeding traces akin to those that I saw in Muir Woods. Spend some time examining
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