Jungle Read online

  Turning our heads up from the floor, if we looked closely enough at the dense, dark forest subcanopy, we might begin to see how these earliest forests, while relatively quiet, also hosted some of the first major step changes in land-based animal communities. To observe this most clearly, we must travel to Hamstead in Birmingham in the United Kingdom, the “Jewel of the Midlands.” Like Wales, it might seem a peculiar location to pick when discussing the origins of tropical forest ecosystems on Earth. However, the coal layers that attracted industrialists to Birmingham in the eighteenth and nineteenth centuries provide an ideal place to look at the forests of the Carboniferous—a time when Birmingham would actually have been located near our planet’s equator. In 2016, scientists presented an incredibly significant find in the context of early “tetrapod” (four-legged animal) evolution. Their research concerned twenty red sandstone slabs that had been sitting neglected at the Lapworth Museum of Geology since their discovery by a schoolteacher in the early twentieth century. Using state-of-the-art scanning approaches, scientists took around one hundred photographs at a range of heights and orientations (a methodology known as “photogrammetry”) around the specimens to build 3-D models of the marks left behind on the slabs. The lead author of the resulting study, Luke Meade, notes how reexamining these blocks after over a century “opened a window onto Birmingham at the end of the Carboniferous.” Analyzing the 3-D models, the researchers found a series of footprints left by increasingly active animal life on the muddy, forested floodplains during this period of Earth’s history.19

  Most numerous were amphibians or amphibian-like creatures. These were the first four-legged animals to emerge from the oceans and inherit the Earth—making short trips onto land as early as the Late Devonian. The warm, humid swamp forests of the Carboniferous period and their regulation of the planet’s atmosphere enabled amphibians to diversify in terrestrial habitats. They became the top land predators, some reaching several meters in length, consuming the available creepy crawlies on land and fish in the swamps and rivers. Yet, like frogs and newts today, they were always forced to return to the water to lay their eggs, limiting the degree to which they could successfully colonize the land. The Birmingham footprints, however, reveal two other vertebrate groups that were beginning to walk, reproduce, and form permanent land-based communities. Rare tracks of large pelycosaurs, which may have looked a bit like Komodo dragons, represent one of the first members of an ancient lineage that would eventually produce mammals. Meanwhile, smaller tracks show the appearance of the sauropsid reptiles. Initially small and lizard-like, they produced “amniotic” eggs, whose surrounding fluid could resist drying out on land, and initiated a reptilian dominance of the Earth that would include creatures as diverse as dinosaurs, crocodiles, turtles, and, later, birds. Much of what first springs to mind when we think of animal life thus began in these warm, wet equatorial forests.20

  Figure 1.2. Artistic reconstruction of Sphenacodon (an early synapsid, the group from which the ancestor of all mammals would later emerge). wikimediacommons.org

  The earliest forests of the tropics completely redesigned the Earth’s surface, providing nutrients, shelter, stable soils, and atmospheric conditions for lively terrestrial ecosystems that contained insects and various animals living at different trophic (predator-prey) positions and exploring different adaptations. Within these new types of environment appeared all of the main terrestrial groups of animals that we now know today, captured in time by the Birmingham footprints. Tropical forests were thus the first complex land-based ecosystems on the planet. They also had a lasting impact on the Earth’s atmosphere and climate. Absorbing significant amounts of CO2, through a now expanded factory line of photosynthesis, and aiding the cycling of water into the atmosphere and through the ground thanks to evaporation from larger leaves and effective root systems, they made themselves critical to life on Earth. In essence they developed an entirely new planetary order that was to host a diversity of life on land from that point onward. Tropical forests were vital to the air breathed in by new, terrestrial organisms and the climates that they experienced, and, as we will see in Chapter 12, they remain crucial to the functioning of the various systems of our planet today. This is something we should perhaps consider more often as they rapidly disappear from its surface. Yet this does not mean that they stood unmoving. In fact, they had only just started evolving. The 3-D scans of the Birmingham sandstone reveal much more than just footprints. They reveal the raindrops that fell in the humid Carboniferous “coal forests” as well as the mud cracks that formed in river basins about 300 million years ago during drier periods that were becoming increasingly common. Tropical forests were going to need to change if they were to survive, as were the creatures that inhabited them. And change they did, impacting and hosting some of the most dramatic periods in planetary history that were yet to come.21

  Chapter 2

  A TROPICAL WORLD

  Tropical forests have been on Earth for over 1,000 times longer than we have, so it is easy to act as though they’ve just always been here—timeless, primordial guardians of our planet. Conservation efforts even often use their great age, contrasting unchanged and uninhabited forests with the dangerous, destructive “progress” of twenty-first-century industry and development. Even how we commonly present or talk about tropical forests in popular culture works to sweep away any ideas of transformation or diversity. Novelists and film producers are all too happy to describe or capture the same tangles of vines, the same dense undergrowth, and the same dark, humid settings to represent a stock perilous “tropical” environment, while, as we have seen, the colloquial word for tropical forests in English, “jungle,” simply refers to a generalizable wilderness. If these perceptions of static, homogeneous forests were correct, we would be living in a world still decked in the forests dominated by club moss and ferns of the Devonian and Carboniferous that we met in Chapter 1. Today, however, tropical forests, as a whole, are home to over half of the Earth’s plant and animal species. This is not because they are all the same, and it is certainly not because they have remained unaltered for 300 million years. Instead, over the course of their history, they have proven themselves to be just as dynamic, vibrant, and geographically variable as the rest of the Earth’s environments.1

  What are tropical forests? As we have already seen in Chapter 1, there are two broad ways of thinking about them. One focuses on the climatic necessities known to be required for the growth of tropical forest types (e.g., for tropical rainforests, 2,000 millimeters of annual rainfall, minimum temperatures of 18ºC, and no significant dry season). These definitions can be incredibly useful in “deep time” when exploring periods in the Earth’s history, like the Devonian and Carboniferous, when much of the world was what we would conventionally consider “tropical” in a climatic sense, and forests reliant on warm and wet conditions, once present, could extend well beyond the equator. They also provide us with a way of looking at the surroundings of the first plants and trees to have colonized the Earth in Chapter 1. Definitions focusing on necessary climatic parameters are also useful in acknowledging that the forests of the past may not have had exact modern counterparts. For example, we will see in this chapter how terms such as “megathermal moist forest” can be used to explore how even the most unlikely of regions could have hosted hot, wet, frost-free forest ecosystems of a type that we might expect solely to prevail in the tropics today. The movement of the Earth’s surface (and thus the “paleolatitudes” of different continental landmasses at different points in time) and changes in the planet’s climate systems add yet another layer of drama to the winding road of “tropical” forest origins and evolution.2

  The second way of defining “tropical forests,” and the one formally used in this book from now on, is “any forest located between the Tropic of Capricorn and the Tropic of Cancer.” Although the tropical latitudes were also often the warmest, by not focusing on the climatic requirements of a particular forest “type,” t
his definition allows us to explore the vast variety of the tropical forests we coexist with at present: from the wet, lowland evergreen rainforests we know so well from documentaries on the Amazon and Congo River basins, to the seasonally thirsty, dry, and thorny tropical forests in places such as Central America and Madagascar, to the snow-tipped branches of montane tropical forests that disappear into the heavens in regions such as New Guinea and the Andes. Although rainforests receive a lot of attention for the incredibly diverse insect communities they host, as well as for their great apes and forest elephants, more seasonal tropical forests are home to equally rare, though perhaps less conventionally rousing, specialized species of plants and animals, such as lemurs, fruit bats, and the world’s most endangered tortoise. Crucially, this definition also allows us to investigate varied forest ecosystems that inhabited the tropical latitudes at different points in time and how they led to the origins of the tropical forest ecosystems that exist there in the present. We are able to chart a course, from the equatorial Carboniferous “coal forests” we met in Chapter 1 to the first closed-canopy Neotropical rainforests that represent the origins of ecosystems that have made the Amazon Basin one of the most renowned homes to tropical forests today.3

  Long-term tropical forest evolution and its significance for the emergence and diversification of life on Earth is a story of change. The first complex tropical forests, emerging under warm, moist conditions at the equator, had already played a role in the permanent alteration of the Earth’s systems and the appearance of some of the first land-based animal life during the Carboniferous. But this was by no means the final word. To truly understand how—and how much—tropical forests have impacted the various evocative life-forms that have graced our planet in the last half billion years, and to truly determine the degree to which they managed to extend their roots into the functioning of the Earth’s atmosphere, climate, and nutrient cycles, we must now investigate how they themselves went on to change. This chronicle is one of ongoing shifts in global climate and, significantly, the crunching process of continental drift and plate tectonics. It is key to the evolution of all plant life and its increasingly permanent seat at the table of atmospheric, geological, and climate change. So, without further ado, let’s begin our tour of tropical forest evolution and change. While the Devonian and Carboniferous both represented major ecological accomplishments for arboreal life-forms, tropical forests, not for the last time, were about to grow headlong into catastrophe.

  GIVEN THE VARIETY of life that we met beneath their canopy in Chapter 1, the Carboniferous “coal forests” that covered the continental landmasses at the equator could already be considered a highly successful evolutionary experiment by c. 300 million years ago. Nevertheless, the final coalescence of the world’s continents into a single supercontinent, Pangaea, approximately 330 million years ago began a slow shift toward a warmer and, more significantly, drier planet that would completely overhaul the vegetation communities of the tropics and beyond. So severe was the resulting “rainforest collapse,” as it has become known, that it arguably led to one of the two mass extinction events for plants revealed in the fossil record (by comparison, animals have faced five mass extinction events and may soon be heading for their sixth). Tree and treelike species that had begun to dominate the earliest ancient forests, such as club mosses, giant horsetails, and the immense cordaites we met in Chapter 1, gradually disappeared from the fossil record, with the number of tree families slashed in half by the time of the Permian (298.9 million to 251.9 million years ago). Not only did species change, but by the start of the Triassic period (251.9 million years ago), forests in all forms had almost entirely disappeared from the face of the Earth. Amazingly, this was the only point in time, since the evolution of the first tree until today, that forests have been practically absent from our planet’s surface.4

  As these forests had already inserted themselves into the Earth’s systems, knock-on effects were inevitable. Anchoring trees were suddenly missing, causing a thinning of life-giving soils, soil erosion and the movement of once reliable rivers, and a return to a bleak surface with the exception of a few isolated “islands” of vegetation. Furthermore, although the planet was drying anyway, a reduction in forests, greenery, and the evaporation of water from leaf surfaces almost certainly compounded this increasing global aridity. Any water that did make its way onto or into the ground was no longer captured by dense root systems. Instead, it flowed away, carrying valuable nutrients from the Earth’s crust into rivers and, eventually, thriving Permian marine ecosystems. At the end of the Permian, explosive volcanic activity in Siberia spat dust and poisonous gas into the atmosphere for hundreds of thousands of years, choking photosynthesis and the diversity of four-legged creatures that had begun to expand within and beyond forests. Over half the families of animal life on land went extinct. However, while these dramatic eruptions at the end of the Permian rightly get attention in most popular accounts of this period—for nearly wiping out all life on Earth—the forests had actually already encountered their apocalypse. Although plants faced a further reckoning at the end of the Permian and start of the Triassic, with an additional slashing in number of the remaining plant families, much of the damage to the planet’s greenery had been done. Without widespread forests, animal life continued to face frequent flooding and a relatively barren planetary surface, even as it recovered from its brush with volcanic hell. The first real attempts by forests to make their way in the world seemed to have ended in failure.5

  As is so often the case in evolution, however, adversity can lead to innovation. Out of disaster sprouted the beginnings of all plant and forest life as we know it today. Nowadays we divide seed-producing (spermatophyte) plants into two broad categories, the gymnosperms and the angiosperms. The former can actually trace their origins back to the “progymnosperms” that we met in Chapter 1 and were the first beneficiaries of the Carboniferous “rainforest collapse.” Their name comes from the Greek word gymnospermos, meaning “naked seed,” and refers to the fact that the seeds they rely on for reproduction are unenclosed prior to fertilization. While they had already begun to dominate ecosystems during the Carboniferous, they were often found on forest floors—in and among the earliest cast of tree-forms, such as Archaeopteris. However, Pangaean drying and increasing CO2 concentrations caused problems for other plant life, creaking open a window in the canopy for gymnosperms, such as cycads and ginkgoales (made famous by the ginkgo tree, or Ginkgo biloba, which is the only surviving member of this group), to rise to the top. Plants reproducing via seed have the advantage that their seeds can lie dormant, enabling them to outlast dry spells and more seasonal conditions. When forests did finally begin to return to Earth as the Triassic period wore on (251.9 million to 201.4 million years ago), these seed-producing gymnosperms, this time including the familiar conifers, now dominated the forest overstories in the tropics and beyond. The resulting ecosystems are still represented by today’s temperate forests, from northern Siberia to their southern outposts in Chile and New Zealand. Meanwhile, spore-bearing plants were left to pick themselves up on the forest floors, and the ancient “progymnosperms,” such as Archaeopteris, were consigned to the Carboniferous fossil record.6

  Given the cold temperatures they inhabit today, you might be thinking that the boreal forests of Russia are not really what you would consider “tropical,” and you would be entirely entitled to do so. Indeed, while the gymnosperm cycads are still represented widely in the tropics, and these gymnosperm-dominated forests extended across the tropics for much of the Mesozoic era (Triassic to Cretaceous period), to find the true beginnings of the types of forests that inhabit the tropics in the twenty-first century, we must turn to the evolutionary history of the other group of plant seed producers, the angiosperms (or flowering plants). To do so, there is no better guide than Dr. Carlos Jaramillo, appropriately based at the Smithsonian Tropical Research Institute in Panama. With his wide-brimmed hat, geological pick, and mud-covered tro
users, Carlos is one of the finest explorers of ancient flowering plants and tropical forests around. Undertaking long-term fieldwork and excavations in the Neotropics, he and his team are on the lookout for all types of fossil plant evidence, from miniscule pollen grains to huge petrified trees, in order to piece together the deep-time history of tropical ecosystems. While he cannot resist the occasional analysis of giant turtle and crocodile fossils, which the team also often find in their Central and South American excavations, he tells me that his heart truly lies with the enchanting and underappreciated evolution of tropical plants.

  The history of tropical forest vegetation is one of radical and fascinating change. While tropical forests might be ancient, they have never been short of excitement or dynamism, particularly across the last 200 million years of their evolution. As Carlos puts it, “120 million years ago the entire tropics were home to just a single flowering plant; yet today they are almost completely composed of some of the greatest concentrations of flowering plant diversity anywhere in the world.” In fact, the origins, expansion, and diversification of the most diverse group of land plants—the group that yields the plants that today sit in our vases and manicured gardens, dominate celebration and mourning rites, and make up the majority of plants exploited by humans for food, medicine, and fibers—are intimately linked to the evolution and emergence of the varied tropical forest habitats as we know them. So how did these enormous ecological changes happen? To find out we have to look at the planetary drama out of which the angiosperms emerged. During the Jurassic period (201.4 million to 143.1 million years ago), the supercontinent of Pangaea began to experience a tectonic divorce, as subsurface tensions bubbled up into the Earth’s crust to unleash the irresistible forces of plate tectonics. First, Pangaea broke in two, forming the northern Laurasian and the southern Gondwanan landmasses. By 140 million years ago, Gondwana was itself beginning to split into Africa, South America, India, Antarctica, and Australia.