What Would The Earth Be Like Without Plate Tectonics Essay

Posted on by Gomi

Plate Tectonics and the Diversity of Life

Life has not evolved on its own; it has evolved on a planet that has experienced changing geology, geography, and climate. Life did not evolve in random patterns, either; in order to understand how life has been affected by its global environment over geological time, we have to look at the relationships between the physical and biological world on which it lives.

The Global Diversity Gradient

The diversity of life over the globe can be measured by the number of species that occupy a given habitat or area. Many factors affect diversity, including the available energy supply in the area, the number and variety of the different microenvironments that can be occupied by different species, and the severity of the physical environment. But some factors seem to affect diversity levels more strongly than others.

On the Earth today there is a strong diversity gradient from pole to equator among communities of organisms living in the shallow seas and on land. Many more species live in tropical communities than in high-latitude communities, and the average number of species in any community falls off toward the poles even over a few degrees of latitude. This trend has been documented for many groups of organisms, from birds to bivalves and from foraminiferans to orchids.

Many explanations have been suggested for the diversity gradient, but generally they relate to the fact that the environment is much more seasonal toward the poles. At the equator, the day length never changes much from a twelve-hour day-night cycle; at each pole there is for practical purposes only one day and one night per year, with three months of full daylight, three months of complete darkness, and two three-month periods of twilight in between. Environmental conditions fluctuate greatly in polar regions, especially the supply of food that comes directly from photosynthetic plant production on land and in the sea. Only relatively few organisms have adapted to live successfully in such an extreme set of conditions. What is the connection between seasonal light intensity and low diversity?

Light and temperature are not in themselves important controls on diversity. Polar waters are actually very stable in temperature, never varying much from freezing all year round. Cold water temperatures do not in themselves foster communities with low diversity, because there is a great diversity of life on the ocean floors at depths of 2-3 km, where it is permanently cold. Nor does lack of light automatically result in low diversity, since the ocean floors are permanently dark.

Perhaps, then, physical extremes of any sort are not important factors. If some species can adapt successfully to difficult environments, why can't many other species do so? Land environments must have seemed incredibly forbidding from the point of view of marine creatures of the Silurian, yet since that time organisms have successfully invaded the land and evolved into myriad forms of life.

To explain low diversity, we need to identify a factor that limits diversity. If a community already has a certain number of species, why can't it include more? The most important of the diversity limiting factors that operate in nature seems to be food supply. If a field will feed only a certain number of animals, the introduction of another individual means that one inhabitant will die of malnutrition, possibly but not in-evitably the invading stranger.

The real world is much more complex, but it operates on the same principle. The tropics have a fairly uniform climate, and food supply is stable, available at about the same level all year round. A species can specialize on one or two particular food sources and can rely on them always being available. As each species comes to depend on a narrow range of food sources, it adapts so well to harvesting them that it cannot easily switch to alternatives. Thus, a great variety of specialized species may evolve, competing only marginally with one another, at least for food. In the Serengeti plains of East Africa, for example, several species of African vultures are all scavengers on carcasses. But one species has a head and beak adapted for tearing through the tough hide of a fresh carcass, another is adapted only for eating the soft insides from an opened carcass, another is adept at cleaning bones, and another eats the scraps. Generally, each tropical species lives in a world of stable food resources, and the great diversity of tropical communities is a reliable reflection of this kind of complex ecosystem.

In high latitudes, on the other hand, food supplies may vary greatly from season to season and from year to year. Overall, food supply may be high. Tundra vegetation blooms in spectacular fashion in the spring. There are rich plankton blooms in polar waters during spring and summer, and millions of seabirds and thousands of whales migrate there to share in the abundant food that is produced. Antarctic waters teem with millions of tons of tiny crustaceans (krill) that eat plankton and in turn are fed on by fish, seabirds, penguins, whales, and seals. The Arctic tern migrates almost from pole to pole, timing its stay at each end of the world to coincide with abundant food supply. Yet for organisms that live all year in polar regions, spring abundance contrasts with winter famine, and food variability is the problem, not average food supply.

Many polar species switch from one food to another as the seasons progress, taking advantage of whatever opportunities are available. The Arctic gray wolf eats mammals and small birds in summer but attacks weakened caribou in winter. Where food supplies vary, few species can be specialists on only one food source; they must be versatile generalists. Generalists share some food sources, and probably compete more than specialists do. If so, fewer generalists than specialists can coexist on the same food resources. In seasonal or variable environments, where organisms must be generalists, diversity is lower. This factor probably underlies the global diversity gradient from equator to pole.

The argument about food variability is powerful because it can also explain diversity patterns in other ecosystems. For example, much of the deep sea has a small but steady food supply in the form of debris that drops from the surface waters, and flows down from the continental shelves. In response, the fauna of the deep sea floor is very diverse, even though it is made up largely of tiny arthropods digging and scavenging from seafloor mud.

There is yet another factor. When many food sources vary with the seasons, animals tend to concentrate on supplies that are reliable. The most reliable food source in polar regions is seafloor mud, even though it is not particularly nutritious. Clams, worms, and arthropods living on soft seafloors are the most diverse of all groups of polar organisms. Mud grubbing is practiced by many marine animals in high latitudes, and they simply do not perceive any period when food supply fluctuates. In a different strategy, some Antarctic marine animals stop feeding altogether in winter: krill actually shrink!

In slightly lower latitudes, food may be more reliable. Plankton living in the water can be an alternative food source, and suspension feeding can become a viable way of life. Here, a new ecological level of marine animals can be added to a community, given reliable food supplies.

Both warm and cold deserts are land environments where food supply is routinely low, but where periods of great abundance occur after a brief rainy season. One would predict that diversity should be comparatively low in such environments, and it is. The most diverse groups of desert organisms are those that are able in one way or another to buffer themselves from variations in their food supply so that they don't perceive periods of famine. In cold deserts, many animals hibernate when food is low. In hot deserts, many animals estivate. Desert plants have an analogous lifestyle, which is to spend long periods as seeds, with plant growth and reproduction fitted within brief damp periods. This buffering strategy is the ecological equivalent of mud grubbing on polar seafloors.

Islands and Continents

The gradient from equator to pole dominates the pattern of diversity on the Earth's surface, but another factor is significant for our understanding of the fossil record. Island groups tend to have more equable climates (maritime or oceanic climates) compared with nearby continents, no matter what their latitude. Thus the British Isles and Japan have milder climates than Siberia; the West Indies have milder climates than Mexico; and Indonesia has a milder climate than Indochina.

Large continental areas have comparatively severe climates. Asia, for example, is so large that extreme heat builds up in its interior in the northern summer, forming an intense low-pressure area. Eventually the low pressure draws in a giant inflow of air from the ocean, the summer monsoon, that brings a wet season to areas all along the south and east edges of the continent, from China to Pakistan. In winter the interior of Asia becomes very cold, a high-pressure system is set up, and an outflow of air, the winter monsoon, brings very chilly weather to India, China, and Korea. Land organisms respond to the great seasonality of the monsoon climate, and organisms in the shallow coastal waters are affected strongly too. As nutrient-poor water is blown in from the surface of the open ocean, food becomes scarce; as water is blown offshore, deeper water upwells and brings nutrients and high food levels. As a result, the diversity of marine creatures along the coasts of India is far less than it is in the Philippines and Indonesia, which are far enough away from the Asian mainland that they feel the effects of the monsoons much less strongly.

The effects of continental geography as opposed to oceanic geography thus have an important effect on the general pattern of diversity in major regions of the Earth. Their effects, however, are still directly linked to the seasonality of food supply.

Provinces

Organisms often occur in characteristic sets of species called communities, living together in certain types of habitat: rocky shore communities, mudflat communities, and so on. The northwest coast of North America, bathed by a southward flow of cool water, has a characteristic rocky-shore community of plants and animals that occurs from British Columbia to Central California. The coastal communities of the world can be arranged into geographically separate provinces, with each province containing its own set of communities, such as the Oregonian and Californian Provinces of western North America.

Provinces are real phenomena, not artifacts of the human tendency to classify things. There are natural ecological breaks on the Earth's surface, usually at places where geographic or climatic gradients are sharp, so that one may pass from one environmental regime to another in a short distance.

A classic example is at Point Conception on the California coast. Here, the ocean circulation patterns cause a sharp gradient in water temperature (in human terms, Point Conception marks the northern limit of west coast beaches where one can surf without a wet suit). The communities on each side of Point Conception are very different, so a provincial boundary is drawn here, with the Oregonian province grading very sharply into the Californian province.

As provinces are identified around the coasts of the world, it seems that the number of species in common between neighboring provinces is usually 20% or less. About 30 provinces have been defined along the world's coasts, mostly on the basis of molluscs, which are obvious, abundant, and easily identified members of coastal communities. Some provinces are very large because they inhabit long coastlines that lie in the same climatic belt (the Indo-Pacific, Antarctic, and Arctic Provinces); some are small, like the Zealandian Province, which includes only the communities around the coasts of New Zealand.

Each province contains its own communities and therefore carries unique sets of animals that fill various ecological niches. For example, the intertidal rocky-shore community in New Zealand has its ecological equivalent in British Columbia, even though the families and genera of animals are quite different in the two communities.

Multiplying the number of provinces automatically multiplies the diversity of animals around the coasts of the world. The total diversity of the world's shallow marine fauna directly reflects the number of provinces, which in turn reflects climate and geography. Today, for example, there is a steep temperature gradient from equator to poles, so there is a strong climatic zonation of the oceans and continents. Each north-south continental coast has several provinces along it, each different enough in climatic conditions to hold a unique fauna. If Earth's heat were distributed more uniformly, so that the ice caps melted, there would be a lower temperature gradient, fewer climatic zones between equator and pole, and fewer provinces. Perhaps there would be no polar communities as we know them, and the world would have a lower diversity of life. Other things being equal, climatic diversity increases biological diversity.

The more oceanic the general climate of the world is, the greater its diversity. In an oceanic world, each community in a province tends to have more stable food supplies and greater diversity within it. Therefore, the more the continents are fragmented into smaller units, the more oceanic the world's climate becomes, and the more diverse its total biota.

Furthermore, there are severe barriers to free migration by organisms when continents are greatly fragmented and widely separated. Land organisms and shallow marine organisms find it difficult to cross large stretches of ocean. Therefore, unique provinces of land plants and animals and unique provinces of marine organisms evolve along the coastline of each continent.

Climatic barriers limit the distribution of organisms in a north-south direction, so migration is easier in an east-west direction, and the generally expected geographic distributions of organisms occur in belts that run east-west. Furthermore, many of the wind and ocean current patterns on Earth have strong east-west components. An Earth with the maximum diversity of life would have a large number of barriers to prevent east-west distribution of organisms, such as north-south mountain chains and deep oceans. Such an arrangement would also produce a good deal of climatic variation, setting up geographic situations that would encourage many small provinces, each with its own set of communities. Our present global geography conforms well to that recipe, and the diversity of life on Earth is probably higher now than it has ever been, particularly among shallow-water molluscs.

Once we understand the reasons behind present diversity patterns, as well as the effect of continental movement on geography and climate, we can make some sense of the fossil record. We can suggest how continental movements affect the reliability of food supply, and therefore the diversity of life. When continents are clumped into large masses, they experience great monsoons, so their coastal regions have severe seasons and low biological diversity. When continents are split, the small pieces have oceanic climates, food supply is more even, and communities and provinces are diverse. When there are many continents, especially north-south continents, there are many geographic barriers, and many provinces of animals can evolve, each contributing to a very high total diversity of life on Earth.

Diversity Patterns in the Fossil Record

Jack Sepkoski estimated the global diversity of fossils through time, plotting the number of families of marine fossils. The data show clear and reasonably simple trends.

Few families of animals existed before Cambrian times, but the beginning of the Cambrian saw a dramatic increase that followed a steep curve to a Late Cambrian level of at least 150 families. A new, dramatic rise at the beginning of the Ordovician raised the total over 400 families, a number that remained comparatively stable through the Paleozoic. In the Late Permian there was a dramatic drop down to 200 families, but a steady rise began in the Triassic that has continued to the present, with a small and short-lived reversal only at the end of the Cretaceous Period, which marks the end of the Mesozoic Era. This general pattern has been known ever since John Phillips defined the Paleozoic, Mesozoic, and Cenozoic Eras in 1860, but Sepkoski put the pattern in quantitative terms that can be further analyzed.

It's easy to think of possible problems with Sepkoski's approach. For example, only some parts of the world have been thoroughly searched for fossils; some parts of the geological record have been searched more carefully than others; older rocks have been preferentially destroyed or covered over by normal processes such as erosion and deposition. Can we believe Sepkoski's numbers, even if we agree that the general trends he sees are real? His data may in fact be more reliable than one would think, especially as a more recent compilation by Michael Benton has shown much the same patterns of diversity change.

Paleontologists have been searching the world for fossils for 200 years. The best-sampled fossil communities are shelly faunas that lived on shallow marine shelves, and our estimate of their diversity through time is likely to be a fair sample of the diversity of all life through time. Larger groups of animals are harder to miss than smaller groups, so we have probably discovered all the phyla of shallow marine animals with hard skeletons. Perhaps we have only found a few percent of the species in the fossil record, but we've probably discovered many of the families. In any case, if the search for fossils has been approximately random (and there's no reason to doubt it) the shallow marine fossil record as we now know it is a fair sample of the fossil record as a whole. And since we now know the biases of the record, we can make intelligent inferences not just about the fossil record but about the living world too.

Global Tectonics and Global Diversity

We have known for over 30 years that the Earth's crust is made up of great rigid plates that move about under the influence of the convection of the Earth's hot interior. As they move, the plates affect one another along their edges, with results that alter the geography of the Earth's surface in major ways. Two plates can separate to split continents apart, to form new oceans, or to enlarge existing oceans by forming new crust in giant rifts in the ocean floor. Two plates can slide past one another, forming long transform faults such as the San Andreas Fault of California. Plates can converge and collide, forming chains of volcanic islands and deep trenches in the ocean, volcanic mountain belts along coasts, or giant belts of folded mountains between continental masses. These movements and their physical consequences are studied in the branch of geology called plate tectonics.

Jim Valentine and Eldridge Moores suggested in 1970 that because plate tectonic movements affected geography, they could in turn affect food supply, climate, and the diversity of life. Over the past 250 m.y., they argued, changes in world geography have encouraged the diversification of the world's fauna because continents separated, making the world more oceanic. At the same time, the number of provinces increased, as climatic zonation became stronger and continents separated to create north-south land and sea barriers such as the Americas and the Atlantic Ocean. Does the idea apply to earlier times?

Sepkoski documented three dramatic expansions of diversity: around the beginning of the Cambrian; at the beginning of the Ordovician; and after the Permian extinction. If Valentine and Moores are correct, the first two episodes of expansion would fit the idea that the continents were clumped together in the Late Proterozoic, that they began to split by the Early Cambrian, and that they became well separated in the Ordovician. This theory explains the early radiations of animals and their diversity patterns very nicely. Is it true?

It's difficult to make reliable reconstructions of Late Proterozoic and Early Paleozoic geography. The limited evidence suggests, however, that continental movement really was the major control on diversity. Apparently, a Late Proterozoic supercontinent Rodinia split progressively during the Cambrian and Ordovician, to form a set of small continents that were generally distributed in lower latitudes.

Can we explain decreases in diversity (extinctions), using the same idea? Continental collisions should decrease diversity, just as continental splitting increases it. There were several continental collisions from the Middle Paleozoic through the Permian, and large land masses were formed. The great extinction at the end of the Permian took place shortly after the continents finally merged into a giant supercontinent, Pangea, composed of a large northern land mass, Laurasia, and a southern land mass, Gondwanaland. Pangea was largely complete in the Early Permian, and the Late Permian extinction coincided with a severe global drop in sea level.

The Permian extinction did not occur gradually over the 150 m.y. of the later Paleozoic, as the continents collided and assembled piece by piece. It's not clear whether the dramatic nature of the Late Permian extinction is a problem for the theory. Most likely, the continental assembly set up the world for extinction, then an "extinction trigger" was pulled.

The rise in diversity that began in the Triassic and continued into the Cenozoic coincides very well with the progressive break-up of Pangea. The break-up was under way by the Jurassic, and reached a climax in the Cretaceous. The continental fragments have continued to drift, and today the continents are perhaps as well separated as one could ever expect, even in a random world.

The overall pattern of diversity through time data can receive a first-order explanation from the suggestions of Valentine and Moores. The timing and direction of the changes in diversity correlate well with plate tectonic events. But that cannot be the whole story, for several reasons.

1. Changing faunas through time. If plate tectonics were the only control on diversity, much the same groups of animals should rise and fall with plate geography. Instead, we see dramatic changes in the different animal groups that succeed one another in time.

2. Increase in Global Diversity. An overall increase in global diversity from Vendian to Recent times is not predicted on plate tectonic grounds.

3. Mass Extinctions. Major extinctions are much more dramatic than radiations. There are too many sudden "mass extinctions" in the fossil record for a plate tectonic argument to be completely satisfactory. Even if plate tectonic factors set the world up for an extinction, we seem to need some separate theory to explain the extinctions themselves.

Changing Diversity Through Time

Three Great Faunas

Jack Sepkoski sorted his data on marine families through time to see if there were subgroups that shared similar patterns of diversity. His computer analysis helped him to distinguish three great subdivisions of marine life through time, which accommodate about 90% of the data. He called them the Cambrian Fauna, the Paleozoic Fauna, and the Modern Fauna. The faunas overlap in time, and the names are only for convenience. But they do reflect the fact that different sets of organisms have had very different histories.

The Cambrian Fauna contains the groups of organisms, particularly trilobites, that were largely responsible for the Cambrian increase in diversity. But after a Late Cambrian diversity peak, these organisms declined in diversity in the Ordovician and afterward, even though other marine groups increased dramatically at that time. In the same way, the Paleozoic Fauna was almost entirely responsible for the great rise in diversity in the Ordovician, and slowly declined afterward. The Paleozoic Fauna suffered severely in the Late Permian extinction, and its recovery afterward was insignificant compared with the dramatic diversification of the Modern Fauna.

The three faunas are defined approximately, because Sepkoski's families are grouped at the level of classes or subphyla. There is no zoological affinity between the members of the three faunas, so we might ask whether there are ecological factors at work.

The Cambrian Fauna clearly represents a major addition of animals to the world fauna that existed in the Precambrian. But the Cambrian Fauna is dominated (77%) by trilobites, which were primarily surface-digging deposit feeders, and many of the others are also surface diggers, including many of the other arthropods and soft-bodied worms of the Burgess fauna.

The Paleozoic Fauna is much more diverse, dominated (80%) by suspension-feeding animals, especially articulate brachiopods, crinoids, and bryozoans, plus corals and cephalopods, which are mainly stationary or slow-moving "lie-in-wait" predators. The Paleozoic Fauna did not simply replace the Cambrian Fauna but was richer and added new ecological components to seafloor ecology.

The Modern Fauna is even more diverse. It is rich in molluscs and is dominated (83%) by swimming predators such as fishes and cephalopods, strongly burrowing groups including most bivalves, crustaceans, and echinoids, and versatile opportunists such as crustaceans and gastropods. The Modern Fauna again is not just a replacement for the Paleozoic Fauna but another great addition of ecological novelty to the marine world.

Cambrian communities were dominated by surface deposit feeders and by suspension feeders that either fed low in the water or lived in the sediment. The Paleozoic Fauna lived in more tightly defined communities, with a more complex trophic structure. Filter feeders reached higher in the water and fed at different levels, and there was more burrowing in the sediment. The Modern Fauna has many more infaunal, burrowing animals in its communities, and many more predators.

Altogether, marine animals seem to have subdivided their ways of life more finely through time. The overall trend has been to add new ways of life, or guilds, to marine faunas through time, a generalization that describes most of the diversity increase, even if it doesn't explain it.

The diversity patterns imply that ecological opportunities in the world's oceans somehow changed through time to favor one particular ecological mixture and then to allow the diversification of others. Obviously there can be many different explanations of the facts, and I have room to discuss only a few suggestions. The diversity patterns have been known in outline for some time, so some of the explanations predate Sepkoski's analysis.

Faunas and Food Supply

Valentine, for example, pointed to the different ways of life that are encouraged under different types of food supply. In the Cambrian, he argued, the continents were not widely separated, food supplies were variable, and the most favored way of life would have been deposit feeding: there is always some nutrition in seafloor mud. Thus Cambrian animals are, as Valentine said, "plain, even grubby." The Burgess fossils may not be plain, but many of them were certainly mud grubbers. Even among soft-bodied animals, arthropods dominate Cambrian faunas in numbers and diversity, and most of them were deposit feeders.

In contrast, if the continents were more widely separated in the Ordovician, one might expect much more reliable food supply in the plankton, which would have favored the addition of suspension feeders to marine communities. One would also expect that a larger food supply in the form of stationary benthic filter feeders would have allowed slow-moving carnivores to become more diverse. Geerat Vermeij and Philip Signor documented the Ordovician rise of these ecological groups, which went on to dominate later Paleozoic seafloor communities.

If the Permian extinction was induced by continental collisions, one would predict that Paleozoic suspension feeders and the predators that depended on them would have suffered a greater crisis than did other groups, because the food supply in the world's oceans would have become much more variable. In general, this prediction is correct: corals, brachiopods, cephalopods, bryozoans, and crinoids felt the Permian extinction most acutely.

But it is more difficult to explain the rise of the Modern Fauna. Other things being equal, one would predict that as continents split again in the Mesozoic, Paleozoic-style predators and suspension feeders would again have been favored. They had not become completely extinct, and could surely have been expected to rediversify. In fact, they did, but in a very subdued fashion. Most of the Mesozoic diversification was achieved by other groups that stand out in Sepkoski's analysis as the Modern Fauna. Swimming and burrowing animals were added to marine ecosystems in great numbers.

The Rise of the Modern Fauna

Many people favor explanations that suggest a competitive advantage for the animals of the Modern Fauna, compared with their Paleozoic and Cambrian counterparts, though it is difficult to identify any compelling advantages. Sepkoski and his colleagues suggested some kind of ecological displacement. Each great fauna seems to have displaced its predecessor in shallow-water communities, "pushing" it toward the edge of the continental shelf. In this model, the Cambrian Fauna and then the Paleozoic Fauna declined as they were pushed offshore. It's not clear that the patterns really imply competitive displacement, or, if they do, why shallow-water habitats in particular should have favored more modern kinds of ecological communities. The answers are not likely to be simple, and in fact the patterns are not as clear-cut as Sepkoski argued.

Steven Stanley and Geerat Vermeij suggested predation as a major factor in the rise of the Modern Fauna, in what Vermeij has called the Mesozoic Marine Revolution. The new predators that appeared in the Middle Cretaceous seem to have been more effective than their predecessors at attacking animals on the seafloor. Modern gastropods evolved, capable of attacking shells with drilling radulae backed with acid secretions and poisons. Advanced shell-crushing crustaceans became abundant, as did bony fishes with effective shell-crushing teeth. Perhaps around this time the suspension feeders of the Paleozoic Fauna, which were largely fixed to the open surface of the seafloor, became too vulnerable to predation. They were replaced by animals that can filter food from seawater that they pump down into their burrows. Burrowing bivalves with siphons and burrowing echinoids make up very important components of the Modern Fauna, together with effective, wide-roaming predators such as gastropods and fishes.

Another variant is the bulldozer hypothesis, mostly the work of Charles Thayer. Thayer pointed out a major difference between the Modern Fauna and the Paleozoic Fauna: the relative abundance and diversity of strongly burrowing forms (especially worms, echinoids, crustaceans, and bivalves) in modern soft-sediment communities. Their continual churning of sediment means, among other things, that fixed suspension feeders find it difficult to attach as larvae, continually run the risk of being overturned as adults, and are at least occasionally subjected to clouds of disturbed sediment that tend to clog their filters. Most suspension feeders that are successful on soft sediments today are mobile forms, and fixed suspension feeders are confined to the relatively restricted habitat of hard substrates. Most Paleozoic suspension feeders were immobile, and they have not been able to compete successfully with the Modern Fauna even though the modern world is relatively oceanic and encourages suspension feeders.

Thayer's work has been criticized on the grounds that there were powerful burrowing animals in the Paleozoic, but this criticism is not valid. Many elements of the Modern Fauna were present in the Paleozoic, but they were not dominant, and their bulldozing was limited. We need to explain that fact, of course, but the bulldozer hypothesis is not affected. The predation hypothesis has run into similar difficulties. For example, occasional shell boring by gastropods has now been recorded as far back as the Devonian, but that does not affect the general validity of the hypothesis.

The predation effect and the bulldozer effect could both have operated. Bulldozing and predation are both reasonable explanations of the failure of the Paleozoic Fauna to recover significantly in the Mesozoic, and both could help to explain the diversification of the Modern Fauna in the later Mesozoic.

Energy, or Nutrients?

If one were to seek some kind of overall change in world ecology that favored both bulldozing and predation in the later Mesozoic and not before, one might turn to a suggestion by Richard Bambach. In 1993 he proposed that the additional energy pumped into marine ecosystems by runoff from the land, as it was covered first by advanced gymnosperms and then by angiosperm floras, made it possible to support more complex animals and ecosystems in such diversity.

Geerat Vermeij favors a more general version of this line of reasoning. Most of the world's primary productivity (photosynthesis) occurs in the surface waters of the ocean, and it is likely that variation in ocean productivity through time has been just as significant as the additional run-off from the land. Vermeij argues that variations in the nutrient content of the ocean may have been very important. Nutrients could be high during periods of increased volcanic activity, for example, those that occur as ocean-floor spreading increases as continents separate. In Vermeij's view, this may be the connection between continental splitting and evolutionary radiation, and may be just as important as adding provinces or making environments more equable.

Are There Limits on Diversity? The Logistic Equation

Jack Sepkoski used his data to analyze how fast radiations and extinctions took place. Extinctions were often extremely rapid as well as extremely large. Radiations showed a timetable of their own, in a pattern that has recently been confirmed on an independent data base.

If a few fortunate flour beetles find their way into a very large jar of flour, their food seems unlimited at first, and they grow and breed rapidly. Eventually, however, the population rises to the point where there is some competition for food. The rate of increase slows down, and at some point the population must stop growing when the jar holds as many flour beetles as it can. The population growth in this example follows a mathematically predictable curve called the logistic equation.

It turns out that the curve of diversity of life follows a logistic path for most of the time. This may mean there is some limiting factor (presumably ecological), which places some sort of control on the diversity of life at any given time. This is surprising, because one can envisage a control on the amount of life more easily than one can explain a control on the diversity of life.

What's more, that control seems to change from time to time. Vincent Courtillot and Y. Gaudemer suggested that diversity rose and then levelled out along a logistic path from the beginning of the Cambrian until there was a dramatic extinction at the end of the Permian. After the Triassic period (in which there may also have been logistic growth of diversity followed by a smaller extinction), diversity again followed logistic growth from the end of the Triassic to the present.

If this is real, it implies that there was some sort of global control keeping diversity at a limit of around 460 families for most of the Paleozoic. The rules then changed, to impose a limit of perhaps 360 families during Triassic times, and then changed again to permit a rise to the present diversity of about 1100 families.

Of course, one wonders what that limiting factor might be. If it exists, it is global and ecological. Since we identify families of animals largely by their similar anatomy, we separate out a group of organisms that probably does much the same thing ecologically. There may be only so many ways of making a living in the oceans at any time, which might limit the number of families that can exist: if a new one evolves, chances are that an existing one goes extinct.

Sepkoski, and Courtillot and Gaudemer, agree that extinction events separate periods over which the logistic curves of diversity are different. At various times, Sepkoski has identified such extinction events at the end of the Ordovician, Permian, Triassic, and Cretaceous. Courtillot and Gaudemer think that the Ordovician extinction can be ignored. Either way, these analyses imply that extinction events change the ecological rules of the world ocean.

 

Most of the metallic minerals such as gold, silver, copper, lead and zinc areassociated with magmas found deep within the roots of extinct volcanoes. Without platetectonics would we obtain minerals? There would not have been gold or silver in theworld. The jewellery industry would come to a standstill, as there will be no more gold or silver to mine. Industries using minerals such as copper, lead, zinc would be affected to.Therefore, I feel that plate tectonics is fundamental in obtaining in such minerals.Living in a world without tectonic forces also means losing another source of energy; geothermal power from volcanoes. Geothermal heat from volcanoes is used torun turbines to generate electricity. Eventually, more fossil fuels would have been used upto produce electricity and there would be a greater demand for fossil fuels such as coal, petroleum and hydrocarbons. Thus, the prices of fossil fuels will escalade.Many major populated cities are located near very active fault zones, such as theSan Andreas Fault; millions of people have suffered from personal and economic lossesas a result of very huge earthquakes. The government of these places spend large sums of money rebuilding homes, offices and re-establishing damaged infrastructure. A worldwithout tectonic forces would not have this problem of economic losses from earthquakesand other natural disasters.However, politically if the world did not have any tectonic forces acting on itwould be joined together as Pangaea, the supercontinent that existed 250 million yearsago. I feel that if the world still were Pangaea our life would have been much simpler.There might have world peace (e.g. No World Wars) there would be less or even no political boundaries in the world. In my opinion, the world would be a better and safer  place to live in. Even one single hierarchy might have ruled the world and travellingwould also invariably be easier as you might not need passports to cross boundaries andyou need not cross oceans to travel from a place to another. Singapore would have been joined with countries around us such as Indonesia, Malaysia and Philippines.With mountainous terrains formed by plate movement we can also expect climaticchanges. There are two basic effects on precipitation caused by mountains. On thewindward side, there is a greater amount of rainfall, and on the leeward side, there is asignificantly reduced amount of it. This is why Kentfield, in Marin Country can receive1300 millimetres of rainfall annually and San Francisco can only receive 500 millimetresof rainfall annually. Without mountains, we can expect uniform distribution of rainfall.Aesthetically, over time, plate movements, together with other geological processes, have created some of Nature’s most spectacular scenery. The Himalayas, theSwiss Alps and the Andes are some magnificent examples. Without tectonic forces, theworld would not have any mountains, volcanoes and ridges to which greatly beautifiesthe world in many ways.As a conclusion, I feel humans can lead a better life without tectonic forces.Although, agricultural economy, electricity production, lack of metallic minerals and probably humans may have not come up with great inventions such as aeroplanes as the

Categories: 1

0 Replies to “What Would The Earth Be Like Without Plate Tectonics Essay”

Leave a comment

L'indirizzo email non verrà pubblicato. I campi obbligatori sono contrassegnati *