encyclopedia of ecology || tropical ecology

3616 General Ecology | Tropical Ecology

See also: Abundance; Antipredation Behavior; Carrying

Capacity; Classical and Augmentative Biological Control;

Coexistence; Community; Competition and Competition

Models; Ecological Efficiency; Ecological Niche;

Ecosystem Patterns and Processes; Food Chains and

Food Webs; Herbivore-Predator Cycles; Matter and

Matter Flows in the Biosphere; Predation; Prey–Predator

Models.

Further Reading

Abrams P (1993) Effects of increased productivity on the abundance oftrophic levels. American Naturalist 141: 351–371.

Carpenter S and Kitchell J (eds.) (1993) The Trophic Cascade in Lakes.Cambridge: Cambridge University Press.

Hairston N, Jr. and Hairston N (1997) Does foodweb complexityeliminate trophic level dynamics? American Naturalist149: 1001–1007.

Hairston N, Smith F, and Slobodkin L (1960) Community structure,population control, and competition. American Naturalist94: 421–425.

Hunter M and Price P (1992) Playing chutes and ladders: Bottom-upand top-down forces in natural communities. Ecology 73: 724–732.

Jiang L and Morin P (2005) Predator diet breadth influences the relativeimportance of bottom-up and top-down control of prey biomass anddiversity. American Naturalist 165: 350–363.

Leibold M (1996) A graphical model of keystone predators in foodwebs:Trophic regulation of abundance, incidence, and diversity patterns incommunities. American Naturalist 147: 784–812.

Leibold M, Chase J, Shurin J, and Downing A (1997) Species turnoverand the regulation of trophic structure. Annual Review of Ecology andSystematics 28: 467–494.

Menge B and Sutherland J (1976) Species diversity gradients: Synthesisof the roles of predation, competition, and temporal heterogeneity.American Naturalist 110: 351–369.

Morin P (1999) Community Ecology. Oxford: Blackwell Science.Oksanen L, Fretwell S, Arruda J, and Niemela P (1981) Exploitation

ecosystems in gradients of primary productivity. American Naturalist118: 240–261.

Polis G (1999) Why are parts of the world green? Multiple factors controlproductivity and the distribution of biomass. Oikos 86: 3–15.

Polis G and Strong D (1996) Foodweb complexity and communitydynamics. American Naturalist 147: 813–846.

Schmitz O, Krivan V, and Ovadia O (2004) Trophic cascades: Theprimacy of trait-mediated indirect interactions. Ecology Letters7: 153–163.

Shurin J, Gruner D, and Hillebrand H (2006) All wet or dried up? Realdifferences between aquatic and terrestrial foodwebs. Proceedingsof the Royal Society of London, Series B: Biological Sciences273: 1–9.

Tropical EcologyH Beck, Towson University, Towson, MD, USA

ª 2008 Elsevier B.V. All rights reserved.

Geography of the Tropics

Tropical Climates

Seasonality Drives Many Ecological Processes

Biogeography of Tropical Organisms

Tropical Species Richness: Anyone’s Guess

Tree Plots: A Wealth of Knowledge for Plant Ecology

Interactions and Interdependencies of Tropical Species

Anthropogenic Impacts on Tropical Ecosystems

Conclusions

Further Reading

Geography of the Tropics

The tropics include all geographic regions of the Earth

that extend from the equator toward the Northern

Hemisphere up to the Tropic of Cancer (23�309 latitude),

and in the Southern Hemisphere up to the Tropic of

Capricorn (23�309 latitude, Figure 1). Tropical regions

cover only about 7% of the Earth’s biosphere but harbor

more than 50% of the world’s species. Different types of

forests dominate the plant community within tropical

latitudes; around 58% of rainforests occurs in the

Neotropics, which encompasses southern Mexico,

Central America, and most of South America. Some

32% of the world’s rainforests are located in Brazil, the

remaining 42% occur in the Paleotropics, a region includ-

ing Africa, Madagascar, Southeast Asia, New Guinea, and

parts of Australia.

Tropical Climates

Most people imagine the tropics as steamy lush evergreenforests with high humidity and hot temperature through-out the year. However, a wide range of climates occurwithin tropical latitudes, ranging from snow peakedmountains (i.e., Andes in South America and MountKilimanjaro in Africa) to deserts (i.e., central Australia,Kalahari Desert in Africa).

Temperature

Tropical regions receive perpendicular sun radiation atnoon almost year-round; thus, the mean annual tempera-ture is higher and seasonal changes are less pronouncedthan in areas at higher latitudes. The intensive sun radia-tion also increases evapotranspiration.

Equator

Tropic of Cancer

Arctic Circle

North America

Africa

South America

Europe

Asia

Tropic of Capricorn

Antartic Circle60°

40°

20°

20°

40°

60°

80°160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160°

80°

N

S

W E

Australia

Tropical rainforests of the worldfrom mongabay.com

Figure 1 Approximate distribution of tropical rainforests of the world. The tropical latitudes are centered at the equator and extend northwards up to the Tropic of Cancer(23�309 latitude) and southwards to the Tropic of Capricorn (23�309 latitude). Over half of the original rainforests have been destroyed by human activities and currently only

around 6% remain; most of these are in different stages of degradation. Adapted and reprinted from Mongabay.com.


Typically, low-altitude tropical areas have an annualmean temperature of 26 �C (range 23–35 �C). However,cloud cover, particularly during the rainy season,can reduce sunlight by absorbing photosyntheticallyactive radiation. A fundamental question is the degree towhich sunlight is a limiting factor for plants. Graham andcolleagues increased light availability by installinghigh-intensive lamps above the canopy of a commontree species in Central America. They found that branchgrowth, number of flower buds, and fruit productionincreased significantly compared to control trees.Because the researchers quantified additional variables,they suggested that light rather than water or temperaturewas the main limiting factor for that tree species.

Precipitation

The amount and onset of precipitation is primarilydetermined by the intertropical convergence zone, anequatorial low-pressure system that follows the Sun’szenith. Because the evaporative power of the equatorialsun is at its maximum, areas within tropical latitudesreceive the highest annual rainfall, except in somelocations (i.e., within Africa, Australia). Similar to tem-perature, rainfall is tied to geography and can range from0 mm (Chilenian Atacama desert) to over 11 900 mm(Hawaii) annually. Apart from some ever-wet regions(i.e., Sundaland in southeastern Asia, New Guinea),most tropical regions have a predictable annual season-ality with one or two rainy seasons (monsoon) alternatingwith a dry season. During the dry season, which mayrange from 1 to 6 months, water loss due to evapotran-spiration is greater than the amount of rainfall.

To what extent precipitation is a limiting factor forplants is an essential question and a challenging one toanswer experimentally. Researchers have carried out aremarkable large-scale rainfall exclusion experiment inthe Amazon over several years. Results have shown thatmost trees had reduced transpiration and photosynthesisrates, resulting in lower leaf production, reduced trunkgrowth, and stunted sampling growth. Large canopy treesthat were fully exposed to sunlight had an increase inannual mortality, from 1% (the typical background mor-tality) to 9%.

Other factors that contribute to the variations in tro-pical climates include cold ocean currents (i.e., HumboldtCurrent), warm ocean currents (i.e., El Nino SouthernOscillation), distance from oceans, and prevailing windconditions (i.e., trade winds).

El Nino Southern Oscillation

Only recently, scientists have begun to understand thedirect and indirect effects of El Nino events. El Ninooccurs every 2–8 years with varying intensity, resulting

in interannual climate variation on a global scale. El Ninoepisodes can lead to below-average rainfall and above-average temperature in some areas (i.e., in Indonesia, NewGuinea, West Africa, and Amazonia), whereas in otherareas it can lead to abnormally high rainfall, sometimesresulting in floods (i.e., in South America). Studies foundthat El Nino events can affect plant and animal speciesacross the tropics. Above-average solar radiation duringEl Nino events in some areas in Central and SouthAmerica have resulted in drier and sunnier climates,favoring higher fruit production. However, during thesubsequent milder dry season the fruit production wasunusually low, leading to famine and high mortality ofnumerous frugivorous and granivorous species.Conversely, in Southeast Asia, after an El Nino event,mass flowering and fruiting of many species occurred,triggering migration of numerous animal species, andincreased reproduction.

Future studies monitoring the climate, Earth surfacetemperature, fruit production, and animal densities acrossnumerous geographic locations are needed to betterunderstand the ecological impacts of this phenomenon.

Seasonality Drives Many EcologicalProcesses

Scientists found that annual seasonality directly andindirectly affects the ecology of most organisms. Forexample, many tree species flower during the dry seasonto optimize cross-pollination by insects. The vast majorityof plants that rely on wind for seed dispersal fruit duringthe dry season. In areas with a prolonged dry season,many tree species are deciduous and produce new leavesat the onset of the subsequent rainy season. Seasonalitycan also affect recycling pathways of organic matter,nutrient availability, and energy flow.

Many studies further demonstrated that plants andanimals that experience pronounced seasonality evolvedunique adaptations. For instance, most plants that rely onanimals as seed dispersers fruit at the onset or during therainy season, whereas many mammals (i.e., rodents) cacheseeds for later consumption. Another strategy to copewith seasonal food shortage is migration. One of themost spectacular animal migrations can be observed ineast Africa, where millions of herbivores including wild-ebeest, zebras, antelopes, and gazelles migrate to greenerpastures. Migration also occurs in other tropical regionssuch as in Sumatra and Malaysia, where bearded pigsmigrate to track mast fruiting dipterocarp trees.

Long-term satellite climate monitoring, improvedsatellite animal tracking (GPS) technology, and GIS soft-ware will certainly provide new exciting details on theinfluence of seasonality on migration routes at large andsmall spatial scale.

General Ecology | Tropical Ecology 3619

Biogeography of Tropical Organisms

In the nineteenth century, while working in the Malayan

Archipelago, the British naturalist Alfred Wallace was one

of the first to observe and describe an underlying pattern

in the distribution of species. Bird species with Oriental

origin occur west of the border line, whereas bird species

with Australian origin occur east of it. In recognition for

his pioneering work, this line is called the Wallace’s Line.

The fundamental question as to why species are distrib-

uted the way they are remains to this day an area of

intense research and debate among scientists. To address

it, scientists may use a combination of geological (i.e.,

plate tectonics, volcanism), historical (i.e., glaciation, dis-

persal), geographical (i.e., altitude, stream routes), and

climatic (rainfall, wind direction) factors. For example,

taxa with large geographic distributions (i.e., ants, ferns)

might have originated before certain continental plates

separated. A recent molecular study found that Old

World driver ants and New World army ants had a

common ancestor before the southern supercontinent

Gondwana (over 105 mya) broke apart. Since then, they

evolved into thousands of different species on both con-

tinents. One would, however, not expect to find army ants

on tropical islands (i.e., Galapagos), unless they were able

to disperse over water. In fact, many plant and animal taxa

dispersed from mainland to distant islands. For example,

with over 3900 km to the nearest mainland, the Hawaii

archipelago is the most isolated area in the world.

Nevertheless, over 23 680 species including 8427 insects

(5462 endemic), 294 birds (63 endemic), and 44 mammals

(2 endemic) occur on the Hawaiian Islands. Dispersal to

isolated islands has been repeatedly possible for numer-

ous volant (flying) and nonvolant species. The latter ones

required driftwood for their long journey. On the other

hand, many typical tropical plant (i.e., Annonaceae) or

animal taxa (i.e., Ursidae) are absent, demonstrating that

long-distance dispersal is limited to taxa with certain

intrinsic characteristics that allow for long journeys.Successful colonizing species may evolve into new

species, a process called adaptive radiation to occupy

not-yet-filled ecological niches. For instance, ancestral

finch species that arrived on Hawaii evolved different

bill shapes to explore new resources, including flowers,

resulting in over 40 Hawaiian honeycreepers. Other

examples of adaptive radiation include Galapagos finches,

marsupials in Australia and the Neotropics, and lemurs in

Madagascar. Adaptive radiation can lead to high levels of

endemism, and increases topical gamma diversity (the

regional diversity of all habitats).Active volcanoes can form land bridges between sepa-

rated areas, thereby allowing species exchange. The most

dramatic example of faunal exchange occurred (some

3 mya) after volcanic activity lifted the Isthmus of

Panama out of the sea, connecting North and SouthAmerica. Many animal species migrated in both direc-tions, a phenomenon known as The Great AmericanInterchange.

Prior to the land bridge formation, South America wasisolated for almost 80 million years, since breakingand drifting away from Africa. Therefore, more archaicmammals (including anteaters, armadillos, marsupials,sloths, and now extinct species such as the giant groundsloth and the saber-toothed marsupials) dominated itsfauna. North America, on the other hand, had beenrepeatedly connected to Eurasia and harbored more mod-ern mammal species, including bears, camels, cats, dogs,elephants, horses, peccaries, rodents, and tapirs. Most ofthe original North American species underwent explosiveadaptive radiation and today comprise over 50% of SouthAmerica’s mammal species. The South American species,however, were less successful; only one armadillo, opos-sum, and porcupine species survived in North America.

New fossil records and a better understanding of phy-logenetics, paleoclimatology, and paleogenetics, amongother disciplines, will further improve our understandingof past and extant species distribution.

Tropical Species Richness: Anyone’sGuess

How many species are out there? This is one of the mostfundamental questions in biology, yet we do not know theanswer. Until the early 1980s, biologists estimated thataround 2 million species occur worldwide. In 1982, how-ever, Terry Erwin fumigated the canopies of tropicaltrees with insecticides. After a downpour of invertebrates,mostly unknown species, he estimated that in the tropicsalone there might be as many as 30 million species. Morerecent estimates of global species richness suggestbetween 8 and 50 million species, of which only 1.7million are known to science. Every year new speciesare found either in museum collections or in the field,including the spectacular discoveries in 2005 of three newprimate species from India, Africa, and South America.

Most groups of organisms exhibit a tendency forincrease in species richness or biodiversity from the polestoward the tropical equatorial region. This phenomenonoften referred to as the Latitudinal Gradient in SpeciesDiversity is one of the most widely recognized patterns inbiogeography. Scientists have argued for over a centuryabout its underlining mechanism. In 1808, the Germannaturalist Alexander von Humboldt was the first to suggestthat energy (sun radiation) is the mechanism drivingthis relationship. Since then over 100 hypotheses havebeen proposed to explain increased biodiversity in thetropics but we still lack a satisfactory answer. Most hypoth-eses focused on historical events, energy availability,


productivity (or both combined), species–area relationship,

stability, disturbance, spatial heterogeneity, patchiness,

habitat complexity, evolutionary rate, and direct interac-

tions (i.e., predation, competition, mutualisms).A synopsis of some of the main hypotheses and empiri-

cal evidences are discussed in the following.

Historical events. Continental glaciation during the LatePleistocene in northern latitudes may have accelerated

the extinction of many species, thus preventing species to

reach higher diversity. Given sufficient time (i.e., millions

of years), species will reach equilibrium and the latitudi-

nal gradient in species diversity might vanish.Energy availability and productivity. Because the tropics

receive more solar energy and rainfall, there should be an

increase in net primary productivity compared to the

capricious seasonality of higher latitudes.Intermediate disturbances hypothesis. Species richness

should be the highest in communities with intermediate

levels (i.e., temporal and spatial) of disturbance (i.e., fires,

hurricanes, and treefalls), because no single species can

attain dominance, no equilibrium is reached. At low dis-

turbance levels, however, competitively dominant species

would exclude subordinate species, whereas at high levels

of disturbance, selection would favor only few fast-grow-

ing species.Evolutionary rate hypothesis (or climate-speciation hypoth-

esis). Higher ambient temperature in tropical regions may

result in higher mutation rates and shorter generation

times, which may result in higher speciation rates com-

pared to higher latitudes. Thus, tropical organisms would

evolve at a faster rate than temperate organisms.

Joseph Connell used treefall gaps to support his inter-mediate disturbance hypothesis. Treefalls are the most

frequently occurring disturbance in tropical forests.

These light gaps in the canopy allow more sunlight to

reach the ground and can trigger the germination of many

heliophilic (pioneer) species. Studies have found that gaps

increased plant and animal species richness. For instance,

researchers found higher species richness of insectivorous

birds, and a distinct gap community of lizards, frugivorous

birds, insectivorous bats, and small mammals. Other

research indicated higher small mammal species richness

in gaps than in the undisturbed understory.To test whether the rate of speciation is faster for

tropical organisms than their temperate counterparts,

John Wiens and his collaborator combined three pre-

viously independent ideas and proposed the tropical

conservatism hypothesis. Later, John Wiens et al. used

Neotropical treefrogs as model organisms to test this

hypothesis. Their results supported all predictions and

the authors argued convincingly that the tropical envi-

ronment was not responsible for an increased speciation

rate, but temperate regions were colonized more recently;

thus when given sufficient time, more species will evolvein temperate regions.

Contradictory results were found in another recentstudy. Shane Wright and her colleagues tested the cli-mate–speciation hypothesis (whether warmer tropicalclimate leads to higher metabolic and mutation rates,resulting in higher speciation rates). This hypothesis isvery similar to that one John Wiens et al. tested. To date,the Wright et al. study is one of the most comprehensiveones because they compared the rate of plant evolutionacross a wide geographic distribution (including Borneo,New Guinea, Australia, and South America) with closelyrelated temperate plant species (including NorthAmerica, Australia, Eurasia, and New Zealand). Theyfound that tropical plant species had more than twicethe rate of molecular evolution (nucleotide substitution)compared to temperate plants.

The mixed results from these studies demonstrate thatunderstanding tropical diversity remains a complex andchallenging endeavor. Considering geologically diversesettings, the evolutionary, and biogeographic history oftaxa, it seems more likely that multiple factors rather thana single ‘holy grail’ hypothesis will explain the underlyingmechanisms responsible for high species diversity foundin the tropics.

Tree Plots: A Wealth of Knowledge forPlant Ecology

Tree plots have been a powerful approach to quantify andcompare structural differences between forests across tro-pical areas. Plots range between 0.1 and 50 hectares (ha) inwhich all trees of a given diameter and height are labeled,identified, and various measurements are taken annually.Depending on the specific question, scale is importantbecause the larger a plot, the more likely it will includeindividuals of rare species. However, because of the hightree density and species richness of tropical forests, it maytake years and many dedicated people to establish a single50 ha plot. For example, the initial census of a 50 ha plotin Malaysia rendered over 335 000 individuals from 814species. Therefore, few 50 ha tree plots are established(i.e., in Borneo, Ecuador, India, Panama, Malaysia, SriLanka, and Thailand). But many smaller plots have beenset up, for example, over 450 have been established in theAmazon. Combining data from smaller plots within aregion can be a powerful approach. For example, in arecent study ter Steege et al. pooled data from 275 treeplots (ranging from 0.4 to 4 ha) scattered throughout theentire Amazon Basin. They tested the rainfall–densityhypothesis, which predicts a positive relationshipbetween rainfall and species diversity. The authorsfound that the length of the dry season negatively corre-lated with tree density and maximum �-diversity


(diversity within a particular area). This is one of the fewstudies that confirmed, over a very large scale, that rain-fall is a major factor for local tree diversity.

The 50 ha tree plots have been extremely valuable totest other major ecological hypotheses, that is, testing thespatial distribution of tree species. Results indicate thatthe vast majority of tree species are spatially clumped (inthe neighborhood of a given species there is a higher thanaverage density of conspecifics), rather than randomlydistributed, and rare species are even more clumpedthan common species.

Other results relate to recruitment pattern of juvenileplants, impact of disturbance (i.e., gaps, drought), density-dependent effects, community ecology, management- andconservation-related question, genetic structure amongindividuals, and seed rain. Furthermore, thanks to long-term data records, scientists can test the effects of El Ninoevents, and anthropogenically induced changes (seebelow).

Tree plot studies have stimulated a wealth of hypoth-eses and have increased our understanding of spatialdynamics and plant diversity. Despite this, many fundingagencies are not keen on supporting long-term projects.More projects like this across different tropical ecosys-tems are needed to address fundamental ecologicalquestions.

Interactions and Interdependencies ofTropical Species

To illustrate the manifold and complex interactions andinterdependencies of tropical species, a synthesis ofnumerous studies focusing on one plant and one animalgenus is provided in the following section.

The Ficus Genus: Master of Many Trades

Ficus, the genus to which fig trees belong, has over 1000species that occur throughout the pantropics. Figs have awider variety of growth forms than any other tropicalplant genus, including shrubs, woody lianas, hemiepi-phytes, epiphytes, and trees. Some figs (strangler trees)start their life cycle as epiphytes on other trees andeventually become majestic free standing trees. Soonafter a bird or monkey deposit a fig seed on a large tree,it starts germinating (similar to mistletoes). The youngplant grows and sends aerial roots downward. Once theroots reach the soil, they engage in mutualistic interac-tions with mycorrhizal fungi. The fig providescarbohydrates while the fungi facilitates the uptake ofwater, minerals (i.e., phosphorus), and other nutrients.The plant continues to grow, eventually overtaking thehost’s canopy. Meanwhile, the fig roots expand, forming a

tight network that starts to constrict the trunk. Eventually,

the host tree dies and slowly decomposes within the

woody network of the now free standing fig tree

(Figure 2).Compared to the smooth trunk of most trees, the trunk

of strangler figs started as network of roots and therefore

contains many crevices and holes. The higher structural

heterogeneity of strangler trunks provides a variety of

microhabitats such as den, nest, and foraging habitats for

invertebrates (i.e., ants, bees, spiders) and vertebrates

(geckos, lizards, rodents, marsupials, birds). Species that

create habitat for other species are called ecosystem engi-

neers (sensu Jones) or niche constructors (sensu Odling,

Smee, Laland, and Feldman). Strangler figs are undoubt-

edly ecological engineers.The pollination of fig flowers is a textbook example of

coevolution. In general, each fig species has its own highly

specialized wasp species that pollinates its flowers.

Hundreds of little flowers are enclosed within a small

(0.5–6 cm) fruitlike globular structure called synconium.

Inside the sealed synconium are also eggs laid by fig wasps

before they die. After development, male wasps hatch first

and inseminate the unhatched females. Later these pre-

born impregnated females hatch, picking up pollen as

they chew exit holes through the synconium. The freed

wasps visit other flowering figs, chew entrance holes into

the synconium, pollinate its flowers, lay their eggs, and

die. There are also parasitic fig wasp species that utilize

the synconium and consume fig tissues without providing

any pollination service.Fruiting fig trees are magnets for a myriad of species

such as pigeons, hornbills, toucans, parrots, macaws, bats,

flying foxes, and monkeys. As animals move through the

fruit-loaded canopy, they create a fruit rain, and terres-

trial species like duikers, peccaries, and rodents can

thrive on them. Most of the tiny seeds (0.5–3 mm) sur-

vive digestion and are dispersed at different scales,

depending on retention time and movement pattern of

the animal species. Some seeds will end up in crowns

of other trees, presenting an opportunity for strangler

figs (see above), while others will be deposited on the

forest floor.Because fig species fruit asynchronously year-round,

including the dry season when the vast majority of other

plants do not fruit, figs are critical for many animal

species survival; it is for this reason that figs are called

‘keystone species’ for frugivorous vertebrates. Studies

showed that fig fruits can constitute over 50% of chim-

panzees’ diet (Africa), up to 70% for some primate

species (Peru), and almost the entire diet of some

Neotropical bat species. It is safe to say that without fig

fruits during the dry season, the density of many verte-

brate species would dramatically decline, if not crash

(Figure 2).

Ficus spp.

Figs and mycorrhizae formsymbiotic relationships(facultative mutualism)

Coevolution: reciprocalevolutionary change in interacting spp.

(obligate mutualism)– Figs provide carbohydrates– Fungi facilitate mineral, water, and nutrients uptake

– Fig fruits provide nesting sites– Fig wasps provide pollination

Strangler figs start as epiphytes in a tree crownand eventually kill the host tree

– Outshading of host tree, reducing photosynthesis rate– Root system strangles host tree, preventing sap flow

Tree trunkCrevices in trunk of strangler

figs provide habitat forinvertebrates and vertebrates

– Creates nesting and den sites– Creates foraging habitat

FruitingFigs fruit year-round

providing crucial resources

– Decrease famine among frugivores– Maintain frugivorous populations

Parasitic interactions

Mutualistic interactions

Ecosystemengineers

Keystonespecies

Parasitic wasps use fig fruits as nesting sitesand food resource, without providing pollination service

– Increase energy loss– Decrease seed production

Seed dispersal by frugivoresincreases germination rate

(facultative mutualism)– Figs provide energy– Some frugivores facilitate germination

Figure 2 Summary of some interactions and resulting interdependencies of Pantropical figs with other species and their ecological

ramifications.


Neotropical Peccaries

Three peccary species (a pig-like mammal) occur in theNeotropics. All of them are gregarious species, for exam-ple, white-lipped peccaries can occur in groups of severalhundred individuals and represent the largest terrestrialbiomass (230 kg km�2) for mammals in Neotropical for-ests. Peccaries are omnivores and utilize seeds, roots,fungi, invertebrates, and vertebrates. They consume fruitsfrom over 207 species and destroy the seeds of over 79%of those species. Peccaries are primarily seed predatorsand only small seeds such as those of figs escape theirmastication and digestive system and are dispersed (endo-zoochory) over long distances. Trees that drop their fruitsunderneath the canopy attract herds of peccaries, whichferociously bulldoze through the soil and leaf litter andtrample juvenile plants while searching for fruits.Peccaries prefer seeds infested with nutritional insectlarvae such as bruchid beetles; thereby they may alsocontrol insect populations in a top-down fashion, andindirectly enhance future seed survival. Some seeds aretoo hard to be cracked by peccaries; in those cases pecc-aries chew off the fruit pulp and expectorate (spit out) theseeds in close vicinity of the parent tree. Some of those

seeds are accidentally trampled deep into the soil and are

thereby protected from insect predation. This shot-dis-

tance dispersal can lead to clumped distribution of plants

(Figure 3).Numerous plants have hooks on their seed coat that

allow them to attach to the hair of animals and fall off

later. This dispersal mode is called epizoochory. Some of

those seeds have been found in the fur of peccaries, an

indication that they may facilitate the dispersal of epizoo-

chorous species.Studies have shown that because peccaries destroy a

large number of seeds and seedlings of many plant

species, they play a fundamental role in regulating

recruitment, demography, and the spatial distribution of

plants, thereby reducing competitive exclusion among

plants and promoting plant species diversity.Considering the high biomass of peccaries particularly

of white-lipped and their consumption of such a large

diversity of fruits, they can (out)-compete many other

frugivorous species (including the collared peccaries),

thus affecting their population dynamics.Peccaries can be considered ecosystem engineers,

because their rooting and bulldozing behavior leads to

Peccary spp.

Endozoochory and epizoochoryBoth mechanisms lead to long-distance,

across-habitats seed dispersal

ExpectorationSpitting out seeds and trampling some into

the ground decreases insect seed predationand leads to short-distance dispersal

Peccaries aremajor seed predators

RootingInfrequent soil disturbances create

germination microhabitatsBetween peccaries

Peccaries have great dietary overlap.Exploitative and interference competition affects:

WallowingPeccaries create and

maintain wallows, which holdwater year-round including the dry season

Peccaries preferseeds infested with insect larvae

– Increase spatial heterogeneity of plant distribution– Increase plant diversity– Promotes clumped plant distributions

– Population dynamics– Habitat selection– Activity patterns

Among frugivoresExploitative competition

– Affects population dynamics of terrestrial frugivores

– Decrease populations of invertebrate seed predators– Increase survival of seeds near parent trees

– Increase seed mortality– Affect spatial distribution, recruitment, demography– Decrease competitive exclusion among plants– Increase plant diversity

– Critical breeding habitat for amphibians and insects– Maintains animal diversity

– Increases mortality of established plant individuals– Promotes the establishment of litter-gap dependent species– Increases plant diversity

Seed predation Insect predation

Ecosystemengineers

Seed dispersal

Competitiveinteractions

Figure 3 Summary of some interactions and interdependencies of Neotropical peccaries with other species and their ecological

ramifications. Adapted from Beck H (2005) Seed predation and dispersal by peccaries throughout the Neotropics and itsconsequences: A review and synthesis. In: Forget P-M, Lambert JE, Hulme PE, and Vander Wall SB (eds.) Seed Fate: Predation,

Dispersal and Seedling Establishment, pp. 77–115. Wallingfort: CABI Publishing.


the removal of leaf litter and soil. Leaf litter can act as

physical or chemical barrier to the establishment of litter-

gap-dependent species. Thus, peccaries create new habitats

which may permit the establishment of litter-gap-depen-

dent species.Peccaries also create and maintain wallows. Research

indicates that most of these wallows contain water year-

round, including the dry season when most other terres-

trial water bodies dry up. Studies found that wallows are

critical breeding habitats for several amphibian species

which go locally extinct shortly after peccaries are extir-

pated (Figure 3).Because of habitat fragmentation and hunting, peccary

populations, particularly white-lipped, are continuously

declining, and they are one of the most endangered

mammal species throughout the Neotropics. Aside

from a few isolated white-lipped peccary populations

this species is extirpated throughout Central America.

Considering their manifold interactions with other spe-

cies, local extinction of peccaries may result in changes in

the distribution, community composition, and species

diversity of plant and animal species.

Anthropogenic Impacts on TropicalEcosystems

A vital ecological service tropical forests provide is stor-

ing vast quantities of carbon (in their plant tissue) while

producing over 40% of the world’s oxygen. Habitat

destruction – including deforestation, fragmentation, for-

est fires, and selective logging – has already reduced

global rainforests to half of their original size. If the

current rate of deforestation continues, then the world’s

rainforests and most of their animal species will be gone

within 100 years.Other major threats include increased CO2, aerosol,

and particle release into the atmosphere. These molecules

drive global warming and other climate changes (i.e.,

rainfall) that affect not only tropical systems but the

whole biosphere as well. Consequences include higher

mortality, extinction, and overall decline of plant and

animal species diversity.Legal and illegal hunting and wildlife trade (problems

that also occur in many established preserved areas

and parks) have driven many species to the brink


of extinction, including gorillas, chimpanzees, rhinoceros,macaws, parrots, tigers, amphibians, mahogany, cacti, andorchids, to name a few. Overhunting, primarily of largemammals (i.e., peccaries, ungulates), for commercial rea-sons has led to the so-called Bushmeat Crisis. Theoriginal densities of most large mammals in tropical for-ests have been reduced to around 10%. Many are locallyextinct, causing a phenomenon called the ‘empty forest’(sensu Redford).

Exotic species are another threat. For example, sincehumans arrived to Hawaii with luggage full of exoticanimals (i.e., malaria, house cats, pigs, goats, and rats) towhich the native wildlife was not adapted, extensiveareas of native vegetation were destroyed (i.e., by wildpigs and goats), and a number of native species wereeither easily preyed upon or outcompeted for resources,resulting in the extinction of over 70% of the nativebird species.

The complex abiotic factors, biotic interactions andinterdependencies of tropical species that are interruptedby human activities, will result in an unprecedented eco-logical meltdown we have yet to comprehend. Onlyintensive international collaboration, comprehensivescience-based agreements, true commitment to an ecolo-gically balanced world from individual governments andtheir citizens, and stronger support and efforts by watch-dog and conservation organizations may be able to slowdown this ecological crisis.

Conclusions

Tropical ecology advanced impressively since the lastcentury. A mosaic of individual studies has slowlyrevealed the larger picture. New insights into fundamen-tal questions such as the impact of climate, thedistribution of species, and the interactions among specieshave helped improve management and conservation ofour natural resources and provided future researchdirections. The field of tropical biology and, in particular,conservation has recruited more enthusiastic local

students from tropical regions than ever before. Theirvoices can now be heard loud and clear in their owncountries.

See also: Tolerance Range; Trophic Structure.

Further Reading

Beck H (2005) Seed predation and dispersal by peccaries throughoutthe Neotropics and its consequences: A review and synthesis.In: Forget P-M, Lambert JE, Hulme PE, and Vander Wall SB (eds.)Seed Fate: Predation, Dispersal and Seedling Establishment,pp. 77–115. Wallingfort: CABI Publishing.

Beck H (2006) A review of peccary–palm interactions and theirecological ramifications across the Neotropics. Journal ofMammalogy 87: 519–530.

Chazdon RL and Whitmore TC (2002) Foundations of Tropical ForestBiology. Chicago: University of Chicago Press.

Connell JH (1978) Diversity in tropical rain forests and coral reefs.Science 199: 1302–1310.

Forget P-M, Lambert JE, Hulme PE, and Vander Wall SB (2005) SeedFate: Predation, Dispersal and Seedling Establishment. Wallingfort:CABI Publishing.

Jones CG, Lawton JH, and Shachak M (1994) Organisms as ecosystemengineers. Oikos 69: 373–386.

Kricher J (1997) A Neotropical Companion. Princeton, NJ: PrincetonUniversity Press.

Mittermeier RA, Mittermeier CG, Gil PR, et al. (2002) Wilderness. Earth’sLast Wild Places. Mexico City: CEMEX.

Newmark WD (2002) Conserving Biodiversity in East African Forests. AStudy of the Eastern Arc Mountain. Berlin: Springer.

Primack R and Corlett R (2005) Tropical Rainforests. AnEcological and Biogeographical Comparison. New York:Blackwell Publishing.

Redford KH (1992) The empty forest. Bioscience 42: 412–422.Struhsaker TT (1997) Ecology of an African Rain Forest. Gainesville, FL:

University Press of Florida.ter Steege H, Pitman N, Sabatier D, et al. (2003), A spatial model of tree

�-diversity and -density for the Amazon region. Biodiversity andConservation 12: 2255–2276.

Wiens JJ, Graham CH, Moen D, Smith SA, and Reeder TW (2006)Evolutionary and ecological causes of the latitudinal diversity gradientin hylid frogs: Treefrog trees unearth the roots of high tropicaldiversity. American Naturalist 168: 579–596.

Wright SJ and Calderon O (2006) Seasonal, El Nino and longer termchanges in flower and seed production in a moist tropical forest.Ecological Letters 9: 35–44.

Wright S, Keeling J, and Gillman L (2006) The road from Santa Rosalia:A faster tempo of evolution in tropical climates. Proceedings of theNational Academy of Sciences ot the United States of America103: 7718–7722.

encyclopedia of ecology || tropical ecology

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