ALTERNATIVE MECHANISMS FOR MAINTAINING DIVERSITY | Equilibrium versus non-equilibrium hypotheses

Although climatic changes in the Pleistocene are clearly relevant to the problem of the origin of tropical diversity, one is still left with the problem of the ‘maintenance’ of diversity: that is, how do so many species manage to coexist, and why do not one or a few species out-compete the others?

Many hypotheses for the maintenance of tropical diversity have been proposed, although researchers have yet to reach a unified conclusion. There are two main lines of thought regarding mechanisms: namely, equilibrium and non-equilibrium hypotheses. According to the equilibrium viewpoint, forests are relatively stable in their species composition such that species are highly adapted to suit the particular environment in which they have evolved. Forest species composition is considered a facet of past and present competition among species.


Equilibrium hypotheses also assume that after some perturbation the forest will return to this pre-defined species composition given adequate time. Non-equilibrium hypotheses, on the other hand, view species as having more generalist tendencies and niche requirements. Non-equilibrium hypotheses assume that species diversity is maintained through disturbance and chance events, and that species composition is in constant flux. According to the non-equilibrium view, species exist in guilds that share similar resource requirements and are more or less interchangeable within the guild.

PLEISTOCENE REFUGIA | another important factor favouring high tropical diversity

Another important factor favouring high tropical diversity is the relative stability of these areas over geological time.

Large expanses of the temperate and boreal zones were covered in ice during the latest glacial maximum, only 10 500 years ago.

Glaciation contributed directly to the extinction of many species at high latitudes, and there has been a much shorter time period over which speciation could potentially occur following glacial retreat.

Tropical forests also experienced climatic changes during the Pleistocene, mainly involving decreased precipitation and changes in sea level. In many tropical regions forests were reduced to small island-like regions called refugia.

These areas now often show particularly high levels of both species diversity and endemism. Examples of important glacial refugia include the foothills and eastern slopes of the Andes, the Choco region of Colombia, and the Mt Cameroon region in Cameroon, West Africa.

NICHE DIFFERENTIATION IN TROPICAL FOREST

Equilibrium hypotheses for the maintenance of tropical diversity generally invoke some form of niche differentiation. This hypothesis is based upon the idea that ecologically similar species are unable to coexist unless they have developed different patterns of habitat distribution and/or resource use. Within this framework, the more specialized the resource requirements of each species are, the more species can be packed into a given habitat. Niche differences among tropical animals are generally related to the type of food resources utilized, or spatial or temporal differences in habitat use.

For example, otherwise ecologically similar animals can differ in terms of height of activity in the canopy, or the time of day they are active. In contrast, all plant species utilize essentially the same set of basic resources, namely: light, water, carbon dioxide, physical space, and nutrients such as nitrogen, phosphorus and potassium.

However, plant species can differ in terms of more subtle ecological characteristics, such as the efficiency of resource use, tolerance of physiological stress, or dependence on specific pollinators, seed-dispersers or root symbionts. Many studies of tropical forest trees have emphasized differences in the "regeneration niche", or the resources and conditions required by seedlings and saplings to successfully establish in the forest. An important distinction is made between ‘pioneer’ tree species that can grow rapidly in large canopy openings or cleared areas, and "late-successional" or "primary forest" tree species that can establish under low light conditions in the understorey. Other kinds of niche differences among tropical trees include ‘structural niche’ differences related to the size reached by adult trees, and differences in ‘habitat preference’ related to soil characteristics and hydrology.

CLIMATE AND BIOLOGICAL PRODUCTIVITY OF THE TROPICS

The warm, wet, and relatively aseasonal climate of the tropics is apparently more favourable for maintaining higher diversity than anywhere else in the world. But why is this the case? One simple idea is that the high solar energy inputs and productivity of tropical regions result in greater numbers of species that can be supported energetically.


However, it is not entirely clear why a small number, or even one species, could not monopolize most or all of the incoming solar energy. Another idea is that climate stability is the main factor promoting species diversification and coexistence. In the harsher temperate and polar regions, species must be able to tolerate drastic fluctuations in seasonal temperatures. Species occurring in habitats nearer the poles are therefore adapted to a wider range of local environments in order to survive the winter months.

As a consequence, one expects a narrower range of adaptation to environmental conditions and narrower latitudinal and altitudinal distributions in the tropics, a hypothesis sometimes referred to as ‘Rapoport’s rule’. The more limited ranges of species in the tropics may allow for greater ‘species packing’ compared to temperate or boreal regions.

Although this idea has received much research attention, recent analyses give only equivocal support, at best, for Rapoport’s rule. On the other hand, it is clear that the relatively aseasonal nature of tropical rain forests allows for the evolution of highly varied and complex species interactions. This complexity itself contributes to the overall species diversity found in the tropics. For example, "dependent" ecological forms, such as specialist herbivores or predators, only persist in the community if their host species is present.

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SPECIES DIVERSITY | Tropical Rain Forests Are More Biologically Diverse

Tropical rain forests are more biologically diverse than any other biome, lying at the extreme of a latitudinal diversity gradient that extends from the poles to the tropics. High species diversity in tropical forests is perhaps most impressively illustrated by the results of surveys of insects obtained using canopy fogging with broad-spectrum insecticides. 

A landmark study by Terry Erwin sampled the canopy of a single Luehea seemannii tree in Panama, finding 163 species of beetles (Coleoptera) co-occurring in this one tree. Through a series of extrapolations (based on estimates of the total number of world tree species and the proportion of insect species comprised of beetles), Erwin estimated that there may be 30 million species of insects occurring in the tropical forests of the world. This figure implies that 495% of the earth’s species remain to be described. 

While more recent sampling efforts have tended to yield lower estimates of tropical insect diversity, there are conservatively between 5 and 10 million species. This quantity is 5-to 10-fold greater than all species described to date. Thus, while tropical forests occupy only 7% of the earth’s land surface they are thought to contain over half of all of the species on the planet. The idea that species diversity of tropical animals (which are mostly insects) is a simple function of the number of plant species also implies that any effort to explain tropical diversity in general must, first and foremost, address the problem of the origin and maintenance of plant diversity.

HISTORICAL IMPORTANCE OF TROPICAL FORESTS IN BIOLOGY

Tropical forests have played a central role in the conceptual development of biology from the time of the major biological expeditions that began at the close of the eighteenth century.

Alexander Von Humbolt initiated the study of plant ecology on his voyages through South America in the early nineteenth century. While climbing Mount Chimborazo in the Andes, Von Humbolt characterized the vegetation changes with climate as he ascended. These early observations on plant distributions provided the foundation of the field of biogeography. The most significant development in biological thinking inspired by tropical forests was the theory of evolution by natural selection. Charles Darwin and Alfred Russel Wallace independently derived this theory as a result of their scientific voyages in the tropics during the mid nineteenth century. Darwin’s inspiration was his exploration of various parts of South America as the naturalist aboard the Beagle beginning in 1831.

Wallace conducted expeditions in both South America (1848–1852) and the Malay Archipelago (1854–1862), where he characterized two sets of fauna distinct to the different parts of the archipelago, a division now known as Wallace’s Line. For both Darwin and Wallace, the high diversity of species in the tropics, and particularly patterns of diversification associated with island groups, allowed an appreciation of evolutionary relationships not apparent in the poorer faunas of the temperate zone. The high diversity of the tropics continues to be an important inspiration and testing ground for ideas in biology, particularly in the fields of ecology and evolutionary biology.

LOWLAND EVERGREEN RAIN FORESTS

The tall, lush evergreen forests envisioned by most when referring to tropical forests are lowland evergreen rain forests. These forests are characterized by canopies with multiple layers of vegetation and the presence of large canopy emergent trees. Lowland evergreen rain forests generally have very high species diversity, with over 1000 tree species per square kilometre found in the richest forests of Amazonia and southeast Asia. Canopy and emergent trees in lowland evergreen forests often have large spreading crowns with a radius of >20m at maturity, can grow to more than 1min girth, and commonly possess plank-like buttresses important in physical support.

 

Beneath the upper canopy layer are smaller understorey trees, treelets, and a layer of herbaceous ground vegetation. Cauliflory and ramiflory are especially common among understorey trees in lowland tropical rain forests. One also generally finds abundant lianas; woody climbers that germinate in the understorey, but possess climbing mechanisms (such as tendrils or hooks) that allow them to use free-standing trees as support structures. Lianas that reach the canopy thus remain anchored to the forest floor.

Also common are epiphytes; plants that live on other plants (most often trees), but which at no point in their life history are rooted in the ground. Orchids, ferns and bromeliads provide many examples of tropical epiphytes, which enhance tropical diversity immensely (epiphytes are thought to comprise 10% of all vascular plants). Another group of plants characteristic of tropical forests are hemi-epiphytes, which germinate in the canopy, as do epiphytes, but produce roots that grow down the trunk of the host tree to become rooted in the ground. The most important group of tropical hemi-epiphytes are figs (many species of Ficus), the fruits of which are an important resource for many vertebrate species.

TROPICAL FOREST FORMATIONS

The main types of tropical forest are distinguished by differences in the distribution of rainfall throughout the year, by elevation, and by soil type. Tropical forests that experience ever-wet conditions with no month receiving less than 100mm of precipitation are generally referred to as ‘tropical rain forests’, although a distinction is also sometimes made between tropical ‘moist forests’ and ‘rain forests’ in a strict sense that receive annual rainfall in excess of 4000 mm.

The two other main tropical forest types, ‘tropical dry forests’ and ‘semi-evergreen rain forests’, experience an annual dry period. In tropical dry forests (also called ‘monsoon forests’) the dry period is severe, and during this most trees drop their leaves in order to reduce water loss. In semi-evergreen rain forests the seasonal drought is less extreme, and a leafless period does not occur to the same extent.

Within these broad moisture regimes, tropical forests are subdivided on the basis of elevation and soil type, and corresponding differences in forest structure. The distinguishing structural characteristics include canopy height, crown layering and the presence (or absence) of different climbers and epiphytes. Tree buttressing, crown shape, leaf structure, and position of flower/fruit formation are other important physiognomic descriptors of tropical forests.

On a global basis the most important types of tropical moist forest include lowland evergreen rain forests, upper and lower montane rain forests, heath forests, peat swamp forests, freshwater swamp forests and mangroves.

DISTRIBUTION PATTERNS TROPICAL RAINFOREST

The Tropical forests exist with some essential facts of geography and climatic systems. By definition, ‘the tropics’ lie between 23o N and 23o S latitude, the area within which the sun lies directly overhead at some point in its seasonal progression.

The flux of solar energy within this region is high, due to the fact that incoming sunlight is projected at a 90o angle to the surface of the earth. This high solar energy flux results in a high rate of water evaporation over the tropical oceans and of evapotranspiration over tropical land surfaces.



The result is a rising column of warm, moist air at tropical latitudes. As this air rises it cools as a result of the gradient in air pressure through the atmosphere (adiabatic cooling). Water condenses out from this air mass, generating rainfall. The air mass ultimately dries out, and is carried poleward from the tropics as a part of wind circulation patterns known as Hadley cells.

At subtropical latitudes near 23–30o N and S, the now-dry air mass descends, creating a region of high pressure that corresponds closely to the world distribution of deserts. As a consequence of these climatic circulation patterns, the earth’s equatorial zone is warm and wet, corresponding to the broad band of tropical forests found along the earth’s equatorial axis.

PEOPLE USE SEEDS | Seeds serve as a major source of food

Seeds serve as a major source of food for millions of people throughout the world. The seeds of cereal grains, including corn, oats, rice, and wheat, are used in making many food products, such as bread, breakfast cereals, and flour. The seeds of plants called legumes, which include beans, peas, and peanuts, are also important sources of nourishment.

Vegetable oils used in cooking are obtained from the seeds of such plants as corn, peanuts, soybeans, and sunflowers. In addition, manufacturers use these oils in making margarine, salad oil, and shortening. Such flavorings and spices as dill, mustard, and pepper are obtained from seeds, and seeds are used in producing beer, coffee, cocoa, and other beverages.

Seeds are also used in the manufacture of many nonfood products. Seed oils are a major ingredient in deter-

gents, soaps, paints, and varnishes. Cornstarch from the endosperm of the corn seed is used in making adhesives, explosives, and other products. Most livestock feed includes the seeds of corn, oats, and other grains. Some seeds, including those of the belladonna and castor-oil plants, provide substances used in medicines.

SEEDS SPROUT | Ripe seeds sprout through a process called germination.

Ripe seeds sprout through a process called germination. After being dispersed, most seeds remain dormant instead of germinating immediately. Dormancy prevents seeds from sprouting when conditions are not favorable for growth. For example, many seeds remain dormant during the cold winter months and germinate after temperatures start to rise in spring.

Seeds can remain dormant for varying periods and still be viable--that is, able to germinate under proper conditions. In general, the period of viability ranges from a few weeks to 50 years. In one case, however, scientists found that dormant 10,000-year-old lotus seeds could germinate when conditions were favorable for growth.

Conditions required for seed germination include abundant water, an adequate supply of oxygen, and proper temperatures. When a seed begins to germinate, it absorbs large amounts of water. The water causes many chemical changes inside the seed. It also causes the seed's internal tissues to swell and break through the seed coat. Water also softens the seed coat so that it breaks apart more easily.

Germinating seeds require an adequate amount of oxygen to support their high rate of respiration. Respiration is the taking in of oxygen and the giving off of carbon dioxide. This process enables a germinating seed to burn food and thus produce energy for growth.

Temperature requirements for germination vary. Species that germinate in summer require higher temperatures than those that germinate in spring. Many seeds require a cold period before they can germinate.





Some kinds of seeds need a certain amount of daylight to germinate. Many seeds will sprout only during the spring, when the number of daylight hours increases. Others sprout only in late summer or early fall, as days shorten.

After the seed coat breaks and germination starts, the part of the embryo below the cotyledon begins to grow down into the soil. This part, called the hypocotyl, develops into the primary root. The developing roots anchor the seedling and absorb minerals and water that the embryo needs for further growth. The upper part of the embryo, called the epicotyl, has a bud called a plumule at its tip. The epicotyl grows longer and pushes the plumule upward above the ground. The plumule then produces the first leaves.

In the seeds of gymnosperms and certain dicots, the cotyledons are also carried above the ground. They remain on the plant until it has formed new leaves that can manufacture food. The cotyledons of monocots and some other dicots remain below the ground as the plumule emerges.

HOW SEEDS ARE SPREAD

After seeds have matured, they go through a state of reduced activity called dormancy, when they do not sprout. During dormancy, seeds are dispersed (scattered) from the parent plant. Seed dispersal increases the chances that some of the seeds will fall in areas suitable for growth.

Some kinds of angiosperm seeds are dispersed while still inside the fruit. The fruit later splits apart or disintegrates and releases the seeds. In other flowering plants, the seeds are released from the fruit before dispersal.

Seeds are dispersed in various ways. In some cases, the fruit drops to the ground and the seeds sprout near the parent plant. However, most seeds have features that enable them to be carried long distances by the wind, animals, water, or people.

Many kinds of seeds are especially suited for dispersal by the wind. For example, some fruits and seeds have winglike structures that keep them aloft. They include the fruits of maple trees and the seeds of ash and elm trees. The wind also carries fruits and seeds that have fluffy coverings, such as dandelion fruits and cottonwood and willow seeds.

Animals also play an important role in seed dispersal. Birds and other animals eat brightly colored fruits. However, the seeds are not digested. They are deposited as part of the animal's body waste--sometimes many miles from the parent plant. Animals also disperse seeds by carrying fruits and seeds on their body. The fruits of the beggarstick, needlegrass, and some other plants have spines and barbs that stick to the fur of an animal. Seeds with sticky coats are also transported by sticking to an animal's body.

The seeds of most water plants are dispersed by floating on rivers, streams, and oceans. Coconut trees and some other land plants have seeds that can float. This type of seed is often transported by water.

Some kinds of seeds are dispersed by an explosive action that occurs when the fruit dries and splits apart. The splitting of a fruit can scatter seeds for several feet. The seeds of geraniums, milkweeds, and touch-me-nots are dispersed in this way.

People have brought along supplies of the seeds of various crop plants and ornamental plants when migrating to many parts of the world. People also aid seed dispersal unintentionally by carrying seeds on their shoes and other articles of clothing.

SEEDS GROWTH AND DEVELOP

Seed formation results from sexual reproduction, in which a sperm (male sex cell) unites with an egg (female sex cell). The production of sperm and egg cells in seed plants involves a number of complicated steps. First, the male and female reproductive organs of the plant produce microscopic cells called spores. The spores grow into gametophytes, which are actually tiny plants that live within the reproductive organs of the parent plant. The gametophytes produce the sperm and egg cells.

Seed development in angiosperms. The reproductive organs of an angiosperm are in its flowers. The female reproductive organ is called the pistil. The ovary, which contains one or more ovules, forms the round base of the pistil. A tube called the style extends up from the ovary and ends in a flat tip called a stigma. The male reproductive organ is called the stamen. The stamen has an enlarged tip called the anther.

The development of a seed begins with cell divisions in the ovule and in the anther. These cell divisions result in the production of spores. In most plants, one spore in each ovule grows into a microscopic female plant, the megagametophyte. This tiny female plant produces one egg cell. In the anther, the spores grow into microscopic male plants called microgametophytes, or pollen grains. Each pollen grain produces two sperm cells.

For fertilization to occur, a pollen grain must be transferred from the anther to the pistil. This transfer is called pollination. Pollen grains are carried from the anther to the stigma by insects or other animals or by the wind. After the pollen reaches the stigma, the grain produces a long, slender pollen tube. This tube grows down through the style and into the ovule. The two sperm cells travel down the tube to the ovule. There, one sperm cell fertilizes the egg cell, and an embryo starts to form. The other sperm cell joins with two bodies called the polar nuclei, and the endosperm begins to develop. After fertilization, the outer layers of the ovule develop into the seed coat.


 
Seed development in gymnosperms. The reproductive organs of gymnosperms take many forms. In conifers, these organs are in cones. A conifer has two kinds of cones, female seed cones and male pollen cones. Each scale of a seed cone has two ovules on its upper surface. Cell divisions occur in the ovules, and each ovule produces a spore that grows into a megagametophyte. This tiny female plant produces egg cells. The scales of a pollen cone have structures that undergo cell divisions and produce spores. These spores develop into pollen grains.

The wind carries pollen grains from the pollen cones to the seed cones. The pollen gets stuck to a sticky substance near the ovules and begins to grow pollen tubes. Each pollen grain has two sperm cells. After the pollen reaches an ovule, one of the sperm cells fertilizes the egg cell, forming the embryo. The other sperm cell disintegrates. The megagametophyte becomes the food storage tissue of the seed. The seed coat develops from the outer layers of the ovule.

THE PARTS OF A SEED

Seeds consist of three parts: (1) the embryo, (2) the food storage tissue, and (3) the seed coat.

The embryo is the part of the seed from which the mature plant develops. It contains the parts that develop into the primary root, the first root to grow; the stem; and the first leaves of the new plant. The embryo also has one or more specialized leaflike structures called cotyledons. Angiosperms have either one or two cotyledons. Those with one cotyledon are called monocotyledons or monocots. Angiosperms with two cotyledons are called dicotyledons or dicots. Gymnosperms have from two to eight cotyledons.

The cotyledons absorb and digest food from the food storage tissue of the seed. In angiosperm seeds, this tissue is called the endosperm. The cotyledons of some dicotyledon seeds quickly absorb all the food in the endosperm. The cotyledons then store the food that the embryo needs for growth. In gymnosperm seeds, food is stored in tissue called the megagametophyte.

The seed coat covers the embryo and food storage tissue and protects them from injury, insects, and loss of water. Seed coats range from thin, delicate layers of tissue to thick, tough coverings.

KINDS OF SEEDS

Seeds develop from structures called ovules, which are in the flowers or on the cones of a plant. Botanists divide seeds into two main groups, enclosed seeds and naked seeds.

Enclosed seeds are produced by angiosperms. Their ovules are enclosed by an ovary, a structure within the flower. As the seed ripens, the ovary enlarges and forms a fruit, which provides some protection for the developing seed. In some plants, the ovaries develop into fleshy fruits, such as apples and peaches. Other plants, such as peas and poppies, have dry fruits that form pods or capsules. In grain plants, such as corn and wheat, the ovary and ovule join together, forming a hard kernel.

Naked seeds are produced by gymnosperms. The most common type of gymnosperm are the conifers. Conifers produce ovules on the upper surface of the scales that form their cones. Gymnosperms have no ovaries, and so their seeds are not enclosed during development. However, the scales of conifer cones close up together when the seeds are ripening and provide some protection for the seeds.