gm tropical rain forest

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CHAPTER4The Tropical Rain Forestas an Ecosystem

Gerald G. Marten

ropical rain forests are the prevailing natural vegetation in the humid'tropics. Rainfall in the humid tropics generally exceeds 2,000 mm peryear and is d i ~ t r i b u t e d throughout the year, at least eight months of the

year having more than 100 mm of rain. This abundance of rainfall allowsforest trees to remain green throughout the year and supports a richness of flora and fauna and a level of biological production that is,greater than occursin any other natural ecosystem in the world.

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The structure of a tropical rain forest is quite distinctive. Many of thetrees have a characteristic smooth bark, pointed "drip-tip" leaves, and aflaired "buttress" base. There is an overwhelming impression of greenness, thetrees tending to occur in three vertical layers, though the layers are notdistinct (Figure 4.1). The main layer is a dense and continuous canopy of trees that are 30 to 60 meters in height. Another layer consists of even tallertrees that protrude above the canopy from place to place. The third layer is adiscontinuous understory of trees below the canopy. The tallest trees areadapted to full sunlight; the understory trees are adapted to shade. Theunderstory is dense only in spots where one of. the taller trees has fallen andleft a gap in the canopy, allowing direct sunlight to come through to the treesbelow. About 1 to 2 percent of the trees fall each year.

A few hectares of tropical rain forest may contain several hundredspecies of trees and thousands of species of smaller plants. There are many


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66 TROPICAL RAIN FOREST AS AN ECOSYSTEMnutrient-depleted soils are able to continue using mineral nutrients f9r theirgrowth although minerals are only slowly entering the soil system byweathering of the soil parent material. The only way the forest can retainenough minerals is by minimizing loss of minerals from the system.

Tropical rain forests on' poor soils have a variety of intricate mechanismsfor holding minerals within the system. Their main strategy is to hold nearlyall the minerals of the entire forest system in the living organisms themselves,so only a small fraction of the minerals are "loose" in the soil, where theyhave the possibility of being washed out of the system. When a leaf falls tothe forest floor, fungi b e g i ~ to grow into it, decomposing it and causing it todisappear Within a few weeks. The roots of forest trees and other plants growinto the leaf as it decomposes, and special fungi (mycorrhizae) form a livingbridge between the decomposing leaf and the roots that are penetrating it .The fungi absorb the leafs minerals as they decompose it , and minerals suchas phosphorus pass directly from the fungi into tree roots. As a consequence,an atom of phosp horus from a decomposi ng leaf is never loose in the soil forvery long before it is takef! up by another plant.Energy Flow

Energy enters the forest as sunlight and undergoes numerous physicalt,ransformations (Figure 4.1). Soil and plant surfaces absorb sunlight and emitinfrared radiation; warm air carries heat energy as it passes to areas of coolerair. Heat passes between the soil and the air above, and between the forestand the atmosphere - in one direction during the day, and the oppositedirection at night.

Biological energy'flow (Figure 4.3) refers to the transfer of energy intoliving organisms by photosynthesis (production) and from one organism toanother through the food web (consumption). The movement of energy isno t cyclic like the movement of minerals. Energy is incorporated into livingtissues by photosynthesis, when sunlight energy is bound into the carbonchains that green plants use for their production (Le., growth). The carbonchains contain potential energy, which the plants can use to drive themetabolic reactions by -which they grow and maintain themselves. Internalconsumption of energy for metabolic purposes is respiratif)n, and some of tl:tisenergy is lost as heat. The net energy that goes into the growth of the plantafter using energy for respiration (Le., its net accumulation of potentialenergy in carbon chains) is net pnmary production.

Animals and microorganisms are consumers, which have a variety ofecological roles (e.g., herbivore, frugivore, predator, parasite, pathogen,scavenger, decomposer), depending upon the kind of food they eat, whetherit is live or dead, and whether or not they eat it whole. Consumers that feedupon plants utilize the carbon chains in their food as building blocks toconstruct their own tissues, and they degrade a certain percentage of thechains in order to release energy for their metabolic needs (i.e., respiration).

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6968 TROPICAL RAIN FOREST AS AN ECOSYSTEM:Minerals that are acquired in surplus of the needs of the animals ormicroorganisms are excreted along with other metabolic products to theenvironment. Fo r example, nitrogen is released as ammonia, urea, or phenolicsubstances such as those passing into the soil from decomposing Imperataleaves, described by Sajise in Chapter 8. The rest 'of the energy and minerals,which are not excreted by animals or microorganisms, are retained for theirgrowth (Figure 4.3).

The same is true for consumers that feed on other consumers. As oneconsumer eats another, there is a "flow" of biological energy (in carbonchains) along the food chain, and there is a loss of energy (to respiration) ateach step in the food chain. The percentage of energy at one step, whichpasses to the succeeding 'step, is the food chain efficiency, typically 10 to 50percent. Because an animal or microorganism may feed upon more than onekind of plant, animal, etc., the chains are interconnected to form a food web.

Figure 4.4 shows the pattern of production and consumption in a typicalundisturbed tropical rain forest. Only a minor fraction of the productionpasses through animals, and only a miniscule fraction passes to humans(assumed to be hunters and gatherers). Most of the plant material falls to theforest floor to become litter, which is decomposed by soil microorganisms.Although the forest contains many kinds of animals, it does not support alarge biomass of animals, in p ~ r t because the plants are effective. at making'themselves inedible. The woody parts of the plants are indigestable to mosta'nimals because of their cellulose and lignin content. Leaves, fruits, and seedscan be more digestable, but they may also contain chemicals that make thempoisonous or interfere with the animals' digestive enzymes. To counter thisstrategy, many animals specialize on particular species of plants, upon whichthey can feed successfully despite the toxicity or indigestability. Many plantscounter with highly synchronized fruiting, which sometimes occurs asinfrequently as once every ten years, so that most of the time there is very,little food to sustain the animal populations that specialize in feeding uponthem. Not all forest trees follow this strategy, however; many bear fruitrandomly throughout the year.

FOREST ECOSYSTEM ORGANIZATIONAn observer of a tropical rain forest is impressed not only by the

diversity of living organisms bu t also by the fact that all


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- 70 TROPICAL RAIN FOREST AS AN ECOSYSTEMare highly adapted to growing under the soil and moisture conditions wherethey are found; and the animals are adapted to obtaining the particularorganisms on which they feed, and they are also adapted to avoiding theirenemies. In the case of natural ecosystems such as for.ests, the coadaptationof the ecosystem's living components to one another is the product of a longprocess of evolution that has generated a highly intricate organization at thelevel of the ecosystem as a whole.

The organization of a forest ecosystem can be clarified by comparing itto another kind of system, a television set. A television set has someproperties in common with a forest ecosystem - properties that are held incommon by all systems. There are additional properties that are unique toecosystems because of - their biological organization. One of the mostconspicuous characteristics of a television set is its design. It consists of manycomponents, but each component (a particular transistor, resistor, orcapacitor) is suited exactly to the components to which it is connected. Ifthere were simply a random connection of components, the television setwould blow up as soon as it was plugged in. An ecosystem has a similar design(coadaptation of components), which contributes to its continued persistence.

The television set and -the forest ecosystem derive their wholesystembehavior from the fact that the behavior of each component in the system is'constrained by the actions of other components. Although every resistor inthe television set could theoretically output a wide range of voltages, theoutput of each resistor depends in fact upon inputs from other components.These inputs are limited to those that generate an orderly television picture.Although all the plants, animals, and microorganisms in a forest- have the.reproductive capacity to multiply to enormous numbers, their populationsare in fact held in check by natural enemies and other natural forces. Becauseuncontrolled populations would exhaust their food supply, possibly destroying themselves and the syste m, the ecosystem ensures its persistence byevolving feedback m ~ c h a n i s m s for regulating the biological populationswithin it. Details of population regulation are discussed by Marten in Chapter6.

There are, however, some important differences between ecosystems andtelevision sets. Ecosystems have a higher level of redundancy than televisionsets, and this g i v e ~ them a greater reliability. Because television sets aredesigned to be constructed as economically as possible, there is only onecomponent for every function. (In this regard, many agroecosystems tend tobe similar to television sets.) If a component is removed, the television setceases to function. In the case of ecosystems, there is a considerable overlapof function between different organIsms. If a single species is removed from atropical rain forest, the forest may continue to function almost as thoughnothing has happened. In addition, the biological components of anecosystem are adaptive. A resistor in the television set has th e same operatingcharacteristics regardless of the rest of the circuit, and once soldered into

I 71TROPICAL RAIN FOREST AS AN ECOSYSTEMplace it cannot change the connection it has to the rest of the components. Incontrast, some predators in the forest may switch from one kind of prey toanother whenever one prey becomes scarce and the other becomes abundant.

We have seen that a tropical forest is organized in space because ofvertical stratification of the trees a ~ because the community of mutuallyadapted plants, animals, and microorganisms can be different according todifferent environmental conditions at different sites. Forest ecosystems arealso organized in time to persist despite periodic environmental disturbancesor pertubations (e.g., fires, typhoons, drought), which might otherwiseeliminate them. Ecosystems persist by means of change, an orderlyprogression in the species composition and structure of the ecosystem (Le., achange in plant and animal communities) known as succession. If a portion ofthe forest is destroyed by fire, or if a patch of forest is opened up by a fallingtree, a different group of plants and animals replace the old forest. Over theyears these plants and animals will in turn be replaced by others in a processthat eventually leads to the same forest as before. The mature forest and thesuccessional stages leading up to it constitute a total system that is adapted tothe fluctuating environmental conditions of the area (Figure 4.5). At anyonemoment an area may .have a patc hwork of communities in varioussuccessional stages.

r---Cullivalio-;;- - - -T -CI;;;ring - - - -1MatureForestSecondaryForest

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Figure 4.5. Stages of community succession in a tropical rain forest ecosystem. 'Solid arrows indicate natural changes; dashed arrowsindicate changes induced by external disturbance.

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732 TROPICAL RAIN FOREST AS AN ECOSYSTEMIt is important to avoid unproven generalities and stereotypes concerninil..tropical ecosystems. A diversity of soil, rainfall, and other environmental

conditions are found in the tropics; and there is a similar variation in theplants,. a n i m ~ s , and resulting ecosystems that have developed in t h o s e ~ environments, as already Jndicated by Hutterer in Chapter 5. Some tropicai1forests have soils that are infertile or. easily ero ded and others do not. S ~ ~ ~ ~ will .support 'permanent agriculture; others will not. Some will quic14y"

.regenerate a forest the same as the one that was cut; others may first'experience a succession to some other kind of v ~ g e t a t i o n (e.g., grassland), ~ ~ ' " described by Sajise in Chapter 8, and take many years before returning to t ~ original forest (Figure 4.5).

MAN-MADE ECOSYSTEMS ON FOREST LANDSThe complexity' ~ u t 1 i n e d in the preceding pages is not unique to' tropical}

rain forests.' Although 'most ecosystems are considerably less complex than ft1rain' f o r e s t ~ 'even the simplest ecosystem i s overwhelmingly complex. All of,.the processes' - the' invisible movement of gases, minerals, and energy; t h e ' ~ c'onsumption of ' o n ~ - ' o r g ~ n i s r i l by 'another, ' etc. - are present in ' anecosystems, inclu'dirlg agricultural ecosystems. , t r : ; ~

. , , ', ,:;,t,JThere are some significant differences between agricultural and forestce c o s y s t e m s ~ however. Compared to a forest, most agricultural ecosystems are

less complex and intricate in design (Le.; coadaptation of components), t h e r ~ ~ is . less redund ancy, arid most agroecosystems are less able to withstandexternal disturbances -l:l:nless sustained by external ',inputs .(e.g.; fertilizers,'pesticides, plowing). ' A n ~ a g r i c u l t u r a i ecosystem, sil-cli as sugarcane, rice p a d d y , ~ o p a ~ t u r e , may be e y ~ n more productive than the natural rain 'forest ~ ~ once'-occupied the same land;--'and it is specifically designed by mOan , ~ t ~ r , I, ' . , . " j ~ ; channel a much greater percentage of its production to, h:uman consumption'. - ( ,... .. .,' '"than does a forest,.but.it .is.,'not" self-sustaining.::The.soil may betoo i r i f e r t i l e ~ , " . , '(.if o r = " ~ c o n t i n u o u s crop ~ , o w t h runless it is fortified with fertilizers, or pest m a y , ~

,_. __ ~ _ ~ ~ ~ ! o y - . - ~ ~ ~ s r o p unless they 3 ! ~ ~ h e I 4 . ~ n , c l } ~ ~ ~ by pesticides." ,, __ , , ~ .._,,_ , . , . , - J ~ Ecologists have. used their knowledge of the structure and 'function 0"(

natural ecosystems to suggest how man-made ecosystems might be d e s i g n e ~ ' ~ for -haf:lllony, rath er than confl ict,' with na'ture. One of the major points o f conflict lies in' human efforts 'to establish and maintain artificial ecosystems!that are different from.the natural ecosystems-theyhave replaced. Un der s u c h ~ conditions t h e ~ e are strong natural forces to revert to the natural ecosystemby means of community succession. An example' is the invasion of tropical':slash-bum fields by weeds, which are the; fust stage in a normal succession:that occurs when a patch of forest is opened'up(Figure 4 ~ . S ) ~ Allowing the'succession to ..take place (Le.; allowing a falloW that eventually leads" toreplacem,ent 9f the "field by .a weedless, mature:'forest) 'is:' i n a c c ~ r d ' withnatural ecological p r o c e s s e ~ -like' those d e ~ r i b ~ d by' Sajise' iri' Chapter 8. Itmay only bepossible to'c'ombat weeds Without 'a fallow by'using external

J"7 TROPICAL RAIN FOREST AS AN ECOSYSTEM,. inputs such as herbicides, which are not only costly but may have undesirable

side effects on the environment. :.,r:. A forest fallow can also rebuild soil that has degraded under agricultural'\ use. It may be difficult to maintain the fertility of the soil in a slash-burn

field withou t applying significant 'quantities of ' fertilizers; and even with fertilizers, changes in soil pH or loss of soil organic matter may ,seriously,

reduce crop yields. During a forest fallow, the trees, draw mineral nutr ient sfrom deep in the soil and use those minerals for the growth of their leaves.When the leaves fall to the forest floor, they replenish the mineral supply inthe topsoil as they decompose. The decomposing leaves also replenish, theorganic matter i.n the topsoil - organic matter that is the basis of the soil'scapacity to hold mineral nutrients and store water, sustaining cr

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