The island is sharply divided into a northern limestone plateau with a general elevation of about 500 feet (150 metres) and a higher area of volcanic hills to the south. The plateau is covered with a thick growth of jungle, while the volcanic hills support mainly sword grass. The hills rise to an elevation of more than 1,000 feet (300 metres); their lower slopes to the east (and also, in part, to the west) are covered with younger limestones, generally similar to those of the northern limestone plateau. The island rises to 1,332 feet (406 metres) at Mount Lamlam, in the southwest. To the southeast of Mount Lamlam is another major hill, Mount Bolanos (1,240 feet [378 metres])


Natural surroundings shape culture

In addition to analyzing artifacts, archeologists often study ancient environments to understand how prehistoric societies both shaped, and were shaped by, their natural surroundings. For example, an archeologist might be interested in learning how an ancient population managed to live in a particular environment, such as a desert. They might ask, how did people use available natural resources? What strategies were humans using to face the challenges of their natural environment, such as protection from intense heat, or finding access to fresh water? How did they modify the environment to suit their needs for growing or acquiring food?

By analyzing recovered artifacts as well as the environmental landscape, archeologists can determine specific ways in which culture and environment relate to each other, especially regarding change. When an environment changes, the ways in which a population interacts with that environment may change as well. Activities may be altered; new tools and technology may develop; or different rituals and customs may be established.

In the Marianas, archeologists have tried to examine how environmental changes may have impacted the early ancestral Chamorros, who began occupying these islands 4,000 years ago. These ancient people adapted to a tropical environment while facing frequent typhoons, periods of drought and occasional earthquakes and tsunamis. Based on research of the climate and geology of the region, the Marianas archipelago also experienced significant changes in sea level and landscape topography that affected not only the kinds of plants and animals that thrived there, but also human settlement patterns.

Tropical forest

Guam’s position on planet Earth near the equator ensures that there is no winter period and a consistent length of day. Guam’s tropical climate is almost uniformly warm and humid throughout the year.  The seasonality of rainfall is influenced by Guam’s isolation from continental masses, seasonal shifts of the Inter-tropical Convergence Zone from north to south, and sea-currents.

In general the Marianas experiences two primary seasons and two secondary seasons. Between the months of January through April Guam’s climate is relatively dry, though it receives about 35 cm (nearly 14 inches) of rainfall during this four month period. From mid-July through mid-October Guam’s climate is wet with approximately 120 cm (just over 47 inches) of rainfall during this four month rainy season. The other months of the year are transition periods, which may be either wet or dry. The equatorial  western Pacific is also subject to the periodic effects of El Niño and La Niña (ENSO) weather patterns. Changing sea surface water temperature, in addition, greatly influences the amount and intensity of rainfall.  Tropical typhoons are generally more frequent and intense when sea surface water temperatures are warmer.

Mariana soil

This tropical pattern greatly affects the weathering of the soil, decomposition of organic material, and leaching of nutrients.  Prior to the arrival of humans, the dominant vegetation on Guam was  tropical forest, with a strong affinity with the Malaysian tropics. Floristics, or the distribution of plant species, of Guam’s tropical forest was influenced by the parent material of the soil.  Soils derived from volcanic parent materials are generally acid and are found largely in southern Guam, while soils derived from limestone are generally found in northern Guam. Central Guam has soils in varying mixtures of the two types of parent material.

Limestone soils are well drained and not very deep. The northern elevated hard limestone soils supported a more diverse community of  the forest species, including Artocarpus mariannensis (breadfruit), Pandanus fragrans, Elaeocarpus joga (yoga or blue marble tree), Intsia bijuga (ifit or ifil), Ficus prolix (banyan) with Neisosperma oppositifolia (fagot), Guamia mariannae (paipai) dominated along with other species as understory.

Less is known about the original composition of plant communities on the upland volcanic soils in southern Guam since the remaining forested areas are limited to lower valleys. These forests are often referred to as ravine forests. The ravine forests are generally lower in stature and are dominated by Hibiscus tiliaceus, Pandanus species, and Ficus prolix. Barringtonia racemosa (langasat) is a common native species found in riparian (or riverbank) forests along rivers and streams.

Along the coastal strands and sandy beaches, where salt spray affects the species composition Mammea odorata (chopak), pago, and Casuarina can be found, among others, and in marshy or brackish water mangroves and wetland shrubs, sedges, and grasses such as phragmites are found.

Climate change effects

Climate change throughout the last several thousand years has played a role in changing vegetation in the Marianas, but it has also had other impacts.  Sea level was about 375 feet below the present sea level 22,000 years ago during the last full glacial climate cycle.  Over the next 16,000 years the sea level gradually rose and then rapidly climbed until about 5,500 years ago when it topped out about 2.0 meters (6.6 feet) higher than it is now.

Some climatologists argue that about 8,000 years ago climate kicked into a warming cycle caused by the emergence of rice paddy agriculture and forest clearing in Southeast Asia and China.  These practices led to increased methane and carbon dioxide, respectively, in the atmosphere.  Otherwise the Earth was in a cooling trend that by this time would have been pronounced and distinctly arctic.  The warming trend peaked again about AD 1000 to 1200 during a period known globally as the Little Climatic Optimum (LCO), but then cooled dramatically over the next 500-600 years during a period called the Little Ice Age (LIA).  Since 1850, however, carbon dioxide from industrial pollution has contributed dramatically once again to global warming and increasing average temperature.

The sea level remained 2.0 meters above modern shorelines until about 3,000 years ago, then dropped one meter.  By 2,000 years ago the sea level dropped again dramatically by another meter to present levels.

Scientists know this on Guam by measuring the elevation of wave-cut notches that formed about 2.0 meters above the present beaches.  Other, higher notches were formed more than 130,000 years ago about 6.0 meters above today when climate was warmer by more than 1.0 degree centigrade as compared to today; lower notches would have formed during periods of shoreline stability that are presently underwater.

Archeologists can also document these changes in the landscape from examining sandy beach terrain and the types of sand that are found.  Coarse, unbroken coralline and foraminiferal sands would have formed in calm lagoons within ancient reefs.  If covered over and undisturbed they provide carbonate tests from which radiocarbon ages can be obtained that can date the period of formation and burial.

Climate change is a very complex and interactive process that is poorly understood, but recent data from ice cores, lakebed cores, isotopic studies of ancient coral reefs, and other proxy data sources have contributed major new data toward modeling climate change over the last 160,000 years.  Cycles of orbital wobble, for example, are on cycles of 22,000 years with the size of the cycles varying every 100,000 years.  These account for glacial and interglacial periods on a grand scale.  Smaller cycles may be caused by changes in solar flares or other changes in solar radiation; they may also be occasioned by earth process like major episodes of volcanism such as the eruption of Krakatoa in Indonesia that led to the “year without a summer” following the eruption in 1883.  Global temperature dropped by 1.2 degrees Celsius and remained cooler until 1888.  This range of warming is expected within the next few decades.

Decadal scale shifts in climate like the ENSO patterns, where warm water pools in the eastern Pacific and farther northward toward North America, influence Pacific Ocean climate and the distribution of marine life.  Conditions in the Pacific may be more arid and droughty.  La Nina conditions emerge as water is pushed to the west by prevailing winds, leading to a warm pool in the western Pacific.  Warmer areas of the Pacific spawn typhoons and wetter than normal conditions.

These climatic patterns are interactive and synergistic, and can lead to rapid climate change once threshold states are crossed.  For example, the melting of glacial ice in Greenland floods the North Sea with fresh water that eventually disrupts the conveyor belt of warm salty water from the Gulf Stream and the North Atlantic current.  Climate theorists think that this could have actually induced rapid cooling and glaciation.  Current trends toward increasing CO2 and other greenhouse gases appear to be driving climate increasingly warmer, and it is feared that this could lead to another sea level rise of as much as 2.0 meters by the year 2100.  Furthermore, increasing aridity in the northern hemisphere in areas with continental climate patterns could be massively disruptive to agricultural production.  The rise in sea level would impact as much as one-third of the world population that currently lives near coasts.

Contrary to these calamitous predictions, oceanic islands may be refuges from extreme climate change.  Small islands are buffered by oceanic weather, perhaps promoting sustained rainfall and less extreme temperatures.  Islands in the northern hemisphere like Taiwan and in the far south like New Zealand could become climate havens.

The scars in the landscape like wave-cut notches and the paleo-evidence of previous and changing vegetation communities demonstrate that climate is dynamic and that islands in the western Pacific are constantly adjusting and adapting.  The human impacts of new vegetation, forest clearing, habitation and farming, along with later effects of urban development interleave with these natural cycles, and ultimately introduce human-induced climate change from increasing greenhouse gases.  The earth has buffered many of these cyclical adjustments for millennia and longer periods.  Whether humans can adjust as well remains to be seen.

Plant communities

Since the arrival of humans, native plant communities have been greatly altered by introduced species (both plant and animals), forest clearing and logging, burning, grazing, and loss of native wildlife.  Forests in southern Guam are limited to lower valleys where higher humidity prevents the encroachment of wildland fire. Much of the volcanic soils in southern Guam supports only grasses on sloped hillsides.

Now that most of the natural environment of Guam has been degraded from centuries of grazing; bombed and burned by warfare; and graded and developed for both military and civilian expansion, there are very few areas of pristine forest on Guam.  In fact, much of the south of Guam is now covered with swordgrass savannah with casuarina in protected areas.  This was not the natural vegetation, however.  Pollen and phytolith identification from coring studies show that before people came to Guam around 3,500 years ago the island was uniformly forested, with possibly small savannah clearings caused by natural disturbances.

Human settlement brought new plants that now appear to be natural.  Coconut palms, seedless breadfruit, mango, bananas, yams, and sweet potatoes required human agency to invade Guam.  In the earliest periods of settlement these were useful plants that were part of a “transported landscape” that ensured productive year-round food resources.  Some inadvertent species hitched a ride, and even some creatures like rats and mosquitoes were probably introduced by people to Guam.

In recent years notable invasive species have become pests, including the brown treesnake, many savannah grasses, false rattan, limonchina, and other species that are often mistakenly considered native.

By John A. Peterson, PhD and Robert W. Wescom

For further reading


Athens, J. Stephen and Jerome V. Ward. “Holocene Vegetation, Savanna Origins and Human Settlement of Guam.” Records of the Australian Museum, Supplement 29, 2004.

Hunter-Anderson, Rosalind. “Savanna anthropogenesis in the Mariana Islands, Micronesia: Re-interpreting the Palaeoenvironmental Data.”  Archaeol. Oceania 44, 2009.

Peterson, John A. and Mike T. Carson.  “Mid- to Late Holocene Climate Change and Shoreline Evolution in Tumon Bay, Guam.”  Proceedings, Pacific Science Intersession, Tahiti, June 2009. Accessed 24 July 2012.

Coral Reef Fish of Guam

Marine fishes fulfill several important functions for Guam. Traditionally, fishing on coral reefs has been an important part of local Chamorro culture, and fish were valued as an important food source. While dependence on the fishery has decreased, these resources remain culturally important today. The bright colors and interesting behaviors of fishes provide attractions for snorkelers and scuba divers, and help support Guam’s tourism industry, a major part of Guam’s modern-day economy. Fish play important roles in the ecology of coral reefs, and indeed are critical for the health of coral reefs.

Nearly 100 families of fishes, and more than 880 fish species, occur on the coral reefs of Guam. Compare this to Hawaii, which “only” has around 600 species. About half of Guam’s species are contained within ten species-rich families. These include the gobies, wrasses, damselfish, snappers, blennies, cardinalfish, butterflyfish, surgeonfish, moray eels, and pipefish.

Fishes inhabiting the reefs of Guam can be divided into two large taxonomic groups, the Chondrichthyes (the cartilaginous sharks and rays) and the Osteichthyes (the bony fishes). This article will focus on the bony fishes.

Guam is in part of the world known by bio-geographers as the Indo-West Pacific, which includes the western Pacific Ocean as well as the Indian Ocean. This large area is the most species-rich marine faunal region in the world and the species composition of the reef fish fauna of Guam is similar to that of other islands and reefs in the western Pacific and Indian Oceans.

It is presumed that, with the exception of a few endemic species, the fish species found on Guam originally arrived from other places. New fish species could travel to Guam by drifting on the currents while eggs and larvae, by associating with floating debris drifting in the ocean or, in more recent times, by traveling in ballast water in ships and barges. There have been no successful intentional introductions of marine fishes to Guam’s waters.

Some species of reef fish are endemic to (only found in) the Mariana Islands. Among these are the Guam damsel, Pomachromis guamensis, and the Yellow-crowned butterflyfish, Chaetodon flavocoronatus.

Coral reef growth is limited by how much sunlight the coral animals can receive. Generally, coral reef growth is limited to about two hundred feet in depth. Thus, most coral reef fiash fishes are found within this depth range. Coral reefs can grow in a variety of forms, depending upon several variables, including current, amount of sunlight, and proximity to the nearest land mass.

Reefs around Guam fall into one of two types; barrier reefs and fringing reefs. There are two barrier reef lagoons on Guam, Cocos Lagoon to the south and Apra Harbor on the west. Most of the rest of the island is edged by fringing reefs. These reefs and lagoons provide a variety of habitats for reef fishes.

In general, the number of fish species is greatest in areas with more rugged and variable topography than in flat areas of sand or reef rock. Also fish diversity is usually greatest in shallow reef areas and declines with increasing depths. Many coral reef fishes are rarely found below two hundred feet deep.

Activity and feeding habits of reef fishes

There is a great diversity of food habits among reef fishes, which helps to account for the coexistence of so many species on Guam reefs. On reefs, you can find fish that eat other fish, algae, corals, crabs, sea urchins, and many other items as well.

The majority of reef fish species are active during the daylight hours, and they retreat into sheltered locations at night. Several of these species exhibit nighttime color patterns that differ from their patterns during the day. A few fish groups, including squirrelfishes and cardinalfishes, are more active at night, and several predatory fishes, such as snappers and groupers, are more active during the twilight periods at dawn and dusk.

Color patterns

One of the most conspicuous features of fishes living on coral reefs is their bright color patterns. While some fishes, especially small blennies and gobies, have rather drab color patterns, which camouflage them, others, such as butterflyfishes, angelfishes, damselfishes, and parrotfishes, have bright colors and bold patterns that make them very visible on the reef.

Several theories have been proposed to explain why such color patterns, which would seem to expose fish to greater risk of detection by predators, should be common. Reef fish color patterns are largely species-specific, and so it seems likely that a fish can use color patterns to recognize other members of its own species, and, it is quite likely that small, individual variations in color patterns allow a fish to recognize other fish as individuals.

In some species, males and females differ in their color patterns, and so sexual recognition also plays a role in conspicuous reef fish coloration.

Reef fish have been, and continue to be, an important marine resource for the people of Guam.

By Steven S. Amesbury, PhD

For further reading

Amesbury, Steven S., and Robert F. Myers. Guide to the Coastal Resources of Guam. Vol. 1, The Fishes Mangilao, GU: University of Guam Press, 1982.

Myers, Robert F. Micronesian Reef Fishes: A Comprehensive Guide to the Coral Reef Fishes of Micronesia. 3rd ed. Barrigada, GU: Coral Graphics, 1999.