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Know Your Biomes IX: Chaparral

Fynbos in the Western Cape, South Africa*

As much as any biome or global ecoregion is a challenge to group, differentiate or otherwise generalize, the chaparral or Mediterranean woodlands (scrubland/heathland/grassland) biome may be the best example such classification difficulties. There’s perhaps more general agreement regarding the features of this biome, even if the name tends to change from author to author. Many texts will not even include this biome in their list of major regions, instead making a small reference to it in the section regarding deserts. However, these areas, considering their combined territory, contain about 20 percent of the world’s species of plants, many of them endemic gems found nowhere else. On the flipside, due to the often environmentally heterogeneous nature of this biome, organisms that are prominent, integral members of other biome classifications are found in the chaparral as well. For the sake of consistency in this post, I’ll continue to refer to this biome as chaparral, as incomplete a descriptive designation as that may be.

Specifically, chaparral biomes exist in five major regions: South Africa, South/Southwest Australia, Southwestern California/Mexico, Central Chile and in patches wrapped around the Mediterranean Sea, including Southern Europe and Northern Africa. These regions are unified by their hot, dry summers and mild winters, referred to as an archetypal Mediterranean climate at 40 degrees north and south approximately.

The vast majority of rainfall usually comes with the cold fronts of winter. Annually, chaparral can experience anywhere from 250 mm of rain all the way up to 3000 mm in isolated subregions like the west portion of Fynbos in South Africa.

Plants in chaparral areas tend to be sclerophyllous (Greek: “hard-leaved”), meaning the leaves are evergreen, tough and waxy. This adaptation allows plants to conserve water in an area where rainfall is discontinuous, but probably evolved to compensate for the low levels of phosphorous in ancient weathered soils, particularly in Australia where there have been relatively few volcanic events to reestablish nutrients over millions of years. Obviously, these plants also happen to do very well during the xeric summers of the chaparral where drought is always a threat.

Because of the aridity and heat, the chaparral plant communities are adapted to and often strategically dependent on fire. Evolutionary succession scenarios constructed by scientists typically point to fire as one of the major factors that created much of chaparral areas in Australia and South Africa from Gondwanaland rainforest. (Fire ecology really deserves at least a post of its own, which I’d like to discuss given the time in the future.)

Some of the regions in the chaparral are exceptional. In South Africa, the area known as the Fynbos constitutes its own floristic region (phytochorion) among phytogeographers, the Cape Floristic Region. While it is the smallest of these floral kingdoms, it contains some 8500 species of vascular plants, 70 percent of which are endemic. The March rose (Oromthamnus zeyheri) is one of the standout specimens of the group as well as the national flower of South Africa, the King protea (Protea cynaroides). P. cynaroides is a “resprouter” in its fire-prone habitat, growing from embedded buds in a subterranean, burl-like structure. Another endemic species, the Cape sugarbird, is shown feeding on a King protea below**.

There is one unique threat to the chaparral: anthropogenic fire. In the past, if nature had not provided a fire to burn back the accumulated brush in these areas, often the native peoples would do so, and generally speaking, the fires seemed to be controlled and effective. But increased frequency of fires due to negligence or downed power lines can potentially cause catastrophic, unrecoverable fire. Only so much tolerance to such a destructive force can be built by evolutionary processes.

*Image by Chris Eason
**Image by Derek Keats

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Know Your Biomes VIII: Boreal Forest

Sep 24 2007 Published by under Basic Concepts, Ecology, Environment, Evolution

Larix_gmelinii0.jpg

A stand of Dahurian Larch beginning to change color in Northeast Siberia.

Between 50 and 65 degrees N latitude lies a globe encircling band of forest dominated by conifers and chilly winds called the boreal forest (boreal is from the Greek word for north) or the taiga (Russian for "marshy pine forest"). From Iceland's wiry birch forests to the larch covered northern areas of Siberia, the boreal forest grows in a climate where extremes are commonplace, and where much of the wilderness remains preserved.

The boreal forest is generally a cold place; it is winter in these areas for over half the year, and most of the precipitation is locked up in snow and ice, about 20 to 60 cm per year. However, in places like Central Siberia for example, the range of temperatures throughout the year can be over 100 degrees C, reaching 30 degrees C in the summer and dropping to a breath taking -70 degrees C in the winter.

Naturally, the plants and animals are well adapted to the climate. Most of the boreal forest is dominated by evergreen conifers - spruce, fir, pine, etc. - but hardy deciduous trees crop up here and there as well, like birch and aspen. Willows grow near the clear, mountain lakes of the northern forest. The needles of the larch, a deciduous conifer, turn bright yellow in autumn before falling in and around the sparse underbrush, in some areas composed of rose, juniper and blueberry, which provide food for small animals like the red squirrel and the porcupine, as well as nesting crossbills and spruce grouse. Moose and bison are wary of the elusive wolf. Lynx and coyote scour the forest floor for a giveaway twitch from the snowshoe hare. Another of the smaller predators, the tenacious wolverine, has been observed in fierce disputes over prey with the huge brown bears of the North.

Such a high concentration of conifers leads to acidic soils in most areas of the boreal forests. The cold and low pH limits the decay of organic material, so unlike most biomes, the most fertile soil horizon is the leaf litter, where mycorrhizal fungi (fungi that lives within the roots of plants, sharing nutrients fixed from the soil) break down dead plant and animal material. Naturally, the roots that tap into this store of nutrients are shallow ones. In the coldest of climes, permafrost turns the subsoil into a near impenetrable mass.

The boreal forest has been the focus of much ecological research due to the general lack of human influence in the past.* Perhaps one of the best studied cases of predator-prey dynamics was done on two inhabitants of the boreal forest, the lynx and the snowshoe hare, which is famously the food of choice for a hungry lynx. Ecologists were able to take 200 years of pelt purchasing records from the Hudson Bay Company and reconstruct the relative abundances of the hare and the lynx. When graphed and superimposed, they found that population spikes in the hare population were followed by lagging spikes in the lynx population. Additionally, plummeting numbers of hare were followed by a proportional decrease in the lynxes.

A long chain of studies have been done since, which has given us a window in to how predation and food abundance affects animal and plant populations. In a nutshell, as more food (underbrush like rose and willow) became available to the hares, they increased in population up to a certain maximum feeding level. At this level, the population is no longer sustainable. An increased population of hares means more food for the lynxes, which undergo a similar transformation, explaining the lagging population spikes followed by proportional drops in population. (Eventually I will find the time to talk about functional and numerical responses in predator-prey/food availability situations - this post could go on forever just about that.)

Next in the biomes series: Tundra.

*The major threats to these forests in recent times is logging, the power industry and a lack of strong legislation preventing abuse.

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Know Your Biomes VII: Temperate Forest

May 24 2007 Published by under Basic Concepts, Conservation, Ecology, Environment, Evolution

Walking through a streamside copse of eastern hemlock in the ancient Appalachians is revealing for several reasons. First, the sheer size and age of these virgin stands can be humbling - at 45+ meters high, one tree may have been alive for more than 600 years. Second, a closer look at the forest's composition can tell ecologists two things: By assessing the pollen contained within pond sediment, you learn that these hemlocks started repopulating the eastern US about 12,000 years ago, following in the "footsteps" of the maple genus (Acer spp.) after the retreat of the massive glaciers covering most of the United States. We also learn that eastern hemlocks tend to hug water sources, giving way to deciduous trees as the incline of the valley steepens. Mixed forests like these are principle in most of the Appalachian mountains.

But the Appalachian mixed forest is only one small ecoregion in a much larger biome, the temperate forests. Named for relatively mild temperatures and moderate annual precipitation, they stretch across the globe between 30 and 40 50 degrees latitude, from the Gondwanaland throwback Valdivian forests of Argentina and Chile (they resemble forests in New Zealand and Australia), to the home of the pandas.

Temperate forests vary greatly in the amount of rain they receive, anywhere from 650 mm 3,000 mm. On the high end of the scale are regions like the Pacific northwest, where redwoods and sequoias live in what is sometimes classified as a temperate rainforest due to the high levels of precipitation, mid range for a tropical rainforest. They're seasonal. Deciduous (and one or two conifers like the larch) drop their leaves during the winter to conserve energy.

The soils of temperate forests are typically fertile, but their specific properties depend on the composition of the forest. In deciduous dominated forests (oak-hickory, beech-maple, etc.), nutrients cycle quickly, creating a substrate rich in organics. Soils in coniferous dominated forests are much more acidic and nutrient cycling tends to be more conservative.

Fire is important to nutrient cycling and population regulation. Many conifers have specially adapted thick bark to ward off the effects of fire and the cones of some species, the "fire-climax" pines like the pond pine or the Monterey pine, often depend on the touch of flame to open.

Like the tropical rainforest, temperates are vertically stratified, with organisms living and growing in the canopy, a shorter layer of mature trees below, the shrub layer and, of course, the understory, where nematodes, fungi and bacteria break down the thick mat of leaf litter into organically rich soil. Light is relatively abundant in the forest understory, allowing ferns and herbaceous plants to thrive. Mosses and lichens cover tree trunks and rock in the more moist portions of the forest.

Vertebrate life is equally diverse. In China, the red panda and the giant panda live in the same general area and subsist on the same food - bamboo - without being in direct competition. They fill very specific niches, however, predominantly eating different parts of the plant and browse slightly different regions. White-tailed deer, grouse, bobcats and black bear dominate the Appalachian forest. In eastern Russia the and leopard, both highly endangered, found refuge in Manchuria, which the last ice age left untouched by glaciers.

Humans have affected temperate forests more than any other biome due to the habitability, fertility and resource richness of these areas. Forest covered most of the eastern US and western Europe until civilizations moved in to urbanize.

Next time: Taiga

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Know Your Biomes V: Deserts

Mar 24 2007 Published by under Basic Concepts, Conservation, Ecology, Environment, Evolution

From a human perspective, deserts, like tundras, seem barren and desolate, inhabited by organismal oddities, pressed into their respective niches by patch of bad luck, or a salt flat, as it were. But thinking beyond our prejudice, seeing through the eyes of a camel or transpiring through the stomata of a saguaro cactus, some conception of deserts as biologically viable and diverse regions of the planet can be gained. Life may not be particularly abundant in most of these areas, but it is varied, unique and beautiful.

For the most part, deserts occur in a consistent band at 30 degrees north and south, with some exceptions on the coasts of North and South America. Dry, subtropical air robs these areas of moisture as it descends, circulating it to more temperate zones. Not all deserts are as parched as the Sahara in Africa or the Atacama-Sechura in Chile and Peru, which receive less than an inch of rain per year - essentially nil. The Sonoran Desert, for example, receives as much rain as the lower threshold of a temperate grassland, about 300 mm per year. The Sonoran remains a desert because of this cardinal rule of being: evaporation exceeds precipitation.

Temperatures are typically hot during the day and freezing at night because of the lack of cloud cover, though in areas of the Gobi Desert in Mongolia, the average temperature is only about 3.6 degrees C annually, with temperatures dropping well below zero C.

Soil has a low concentration of organic matter, so much so that it is often classified as lithosols, or strictly inorganic soils. This is especially evident in aged, undisturbed soils, where a special limestone horizon called a caliche is formed (because of its inherent low level of disturbance, ecologists can use this layer to accurately age a desert). Great salt flats are common in desert areas, where pools of accumulated water from heavy rains evaporate, leaving crusts of salt crystals spanning large areas, making it more difficult for organisms to extract water from their environment.

But extract they do, in various specialized ways. Desert perennials like the prickly pear have evolved a thick, waxy cuticle capable of retaining water more effectively year round. The stems have low surface area exposure, its "thin" parts facing into the sun. Roots extend horizontally below the surface, increasing their area of absorption. Their leaves, which would have been a liability in the extreme sun and moisture sapping aridity, have become reduced to non-photosynthesizing, defensive spines. Even their cycle of photosynthesis is different than most plants, closing their carbon dioxide absorbing stomata during the day, when the potential for water loss is greater (CAM).

Annuals are a different story. Given the extreme rarity of significant rainfall, these plants grow rapidly when water is available, producing seeds that can lie dormant for years, until the rains come again. Some of these plants keep a death grip on their seeds until the touch of water hydrates the cellulose of their seed pods, opening the pod releasing the seed.

Some plants, called halophytes, have even adapted to thrive in the salt flats. Atriplex is an extraordinary example. It can maintain higher levels of salt within its cells in order to extract water from the salt flats. Some of these cells burst, coating Atriplex with a defensive layer of salt, making the plant a dangerous meal for any water conserving herbivore.

One species, however, has found a way around this problem: The red vizcacha rat (in the same family as the chinchilla) has evolved a series of teeth that can remove the outer salty layer from Atriplex, making it just edible for the resourceful animal.

Other animals have similarly adapted to the heat and aridity; in fact, some, like the camel, have independently evolved the same measures as plants for keeping cool and hydrated. The camel faces into sun, keeping a slim profile, reducing its surface area exposure. It maintains a store of fat in its hump in order to produce metabolic water. A thick coat of hair, much like cactus spines, covers its body, reducing heat absorption. As the prickly pear keeps its stomata closed during the day, the camel does not sweat, reducing its own loss of water.

Most desert animals, however, are nowhere near the size of the camel, preferring to hide in burrows during the day, emerging to hunt and/or forage at dusk.

I could go on indefinitely about this biome. The extremity of the climate has produced fascinating characteristics in desert wildlife, and they are so different from region to region that they deserve descriptions of their own (especially areas like the Madagascar succulent woodlands, home of the bizarre Didiereans, pictured at the beginning of this post). Perhaps I will return to these areas of interest at a later date.

Next we'll look at the bread baskets of the world - the temperate grasslands.

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Know Your Biomes IV: Tropical Savanna

Feb 27 2007 Published by under Basic Concepts, Ecology

A cheetah crouches, shoulders hunched, barely visible through golden stems. The antelope on the edge of the herd has stopped chewing, and scans the horizon with a nervous eye. As it takes a step forward to rejoin the safety of the group, the cheetah makes her move, bounding with impossibly huge leaps towards her prey. The entire herd is on the move with her first step, but the stray is dangerously lagging behind. It flies only for a few seconds before the cheetah leaps one final time, clinging to the young antelope's rump with all her strength, pulling the animal to the ground for the coup de grace.

This should be a familiar scene for anyone with a passing interest in wildlife films; the great African savannas are often filmed to illustrate with detail the great theater of competition and predation in the wild. Visibility plays a large factor in this choice; the savannas have very few trees, and are home to some of the largest groups of the largest mammals in the world, not only in Africa, but also in South America, India and Australia. The same drama could not be effectively captured in a rain forest; animals are much less visible generally.

But that does not mean that the creatures of the savannas are immediately apparent. Most are well camouflaged, especially the predators, matching the golds, browns and sage greens of the flora, or appearing as a distant rock, as in the case of rhinos. Huge herds of ungulates pound the hard earth of the savanna, following the summer rains as they sweep across the enormous stretches of land that savannas occupy. As in all biomes, the climate sets the standard of living.

Like the tropical dry forests, the savanna is doused with rain in the summer seasons (often in just a few weeks depending on the year), and suffers from extreme drought for the rest of the year. What keeps dense plant growth at bay? Well, the answer to that question is threefold:

  • Fire: After the five month long dry season, lightning storms swirl violently over the plains, igniting the parched vegetation. These fires can be prolific, clearing huge tracks of grasses and saplings. The grasses grow back quickly, but millennia of fires like these have selected a small number of fire resistant trees and shrubs that are constantly kept in check by the annual fires.
  • Soil: The soils of the tropical grasslands are relatively impermeable, causing the heavy rains to skim along the surface instead of soaking deeply into the ground.
  • Herbivory: Animals also keep the density of non-poacean plants at a relatively low level. The herbivores of the African savanna, for example, have evolved to fill specific niches* (the specific factors that influence a species existence, in this case feeding) in which they consume plant material at different levels of the ecosystem. Giraffes and elephants browse the upper and middle levels of leafy trees, respectively, while wildebeests and zebras graze on different levels of grasses. The large herds of ungulates roaming the plains are also restrictive to growth.

The number and diversity of trees differ from savanna to savanna around the globe, however. In the Miombo Woodlands, trees of the genus Brachystegia can reach heights of 80 feet (25 meters) or more, assisted by a nutrient fixing fungus in its roots (mycorhizae). There are a few dozen species of this tree in a relatively small area.

Insects, as usual, play a large part in recycling nutrients from dead and dying tissue. Termites are perhaps the most visible of these detritivores (detritus eating) in just about every savanna across the world, living in mounds in excess of two meters high, each home to millions of foraging termites. In the Cerrado Savanna of South America, the giant anteater plays Godzilla, wandering through entire cities of termite mounds and dismantling them with its powerful claws to expose tunnels full of workers and lapping them up with its sticky tongue (links to video of anteater feeding).

Modern humans are believed to have originated in the African savanna before trekking out to inhabit every other biome in one way or another. The savanna is used for supporting non-native livestock in most cases, but there is a movement for domesticating antelope and other ungulates due to the naturally higher feed to meat ratio observed in these animals.

Next we'll move out of the tropics and into the desert.

*We will return to niche theory in a later post; Cheetah image courtesy of schani

This post is part of a series of Basic Concepts: Ecology (Intro, Biomes I, II, III). For the entire list of Basic Concepts in Science, visit Evolving Thoughts.

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Know Your Biomes III: Tropical Dry Forest

Feb 20 2007 Published by under Basic Concepts, Ecology

As you move about 10 to 25 degrees north and south of the equator, the unchanging wetness of the rain forest begins to dissipate, giving way to a more seasonal climate. The temperatures are still warm throughout the year, but precipitation in tropical dry forests comes in bursts of only five or six months. The rest of the year is dry and relatively bare; in some areas during the dry season, these forests may resemble savannas or even deserts.

The soils are often relics of the ancient continent Gondwana, especially in directly detached, isolated systems like New Caledonia. The soils are more fertile than that of the rain forests, but erosion is higher, especially in deforested areas.

The dichotomous nature of the climate in the dry forest drives life to cope with the extreme seasonality of precipitation in different ways. The driest areas are dominated by deciduous trees that drop their moisture-exuding leaves once the rains stop, allowing sunlight to pass through the once-thick canopy, to reach the lower levels of the forest. Wetter areas, like Southeastern Indochina, are home to large forests of dipterocarps (pdf) and other evergreens, able to utilize the blessings of the monsoon rain season to keep growing throughout the year. And, unlike the plants of the rain forests, that rely almost entirely on animals to disperse seeds, seeds in the dry forest are swept through open areas by strong winds.

Wildlife is diverse in the dry forests, but necessarily migratory in some areas due to the extreme seasons. During the dry season, many animals will travel to a certain area in the neighboring rain forest to subsist, waiting for the rains to return to their home. This extra level of complexity has given ecologists a new challenge: Tracing the migratory paths of animals reliant on both tropical dry and wet forests. How much do these species rely on each biome for subsistence?

Dry forest systems are not quite as biologically diverse as rain forests, but they are home to many rare species found nowhere else: tigers, leopards and jaguars, sloth bears, a myriad of kingfishers and flycatchers, the Komodo dragon, the maned wolf, the elusive Javan rhino and unique plants like Gouania and Cycas beddomei. The Madagascar dry forests are home to seven species of baobab tree. The whole of Africa can claim only one.

While I'm generally working under the assumption that the reader knows the dangers faced by each biome, the dry forest has been massively altered by human influences. There is a huge difference in human population between tropical wet and dry forests. Murphy and Lugo provided an estimate in 1986: In Central America, only 7 percent of the people lived in wet forest areas, while 79 percent live in dry forest areas. The relatively fertile soil drives deforestation for agricultural purposes, and has whittled dry forests down to just about 2 percent of their former area in places like Central America.

Guanacaste National Park in Costa Rica is usually hailed as the prevailing example of how dry forests can be saved and care of them placed in capable, educated hands of native peoples. Ecologist Dan Janzen began with a simple problem - why does the guanacaste tree produce so much fruit if it just lays around - found a simple answer - it had evolved to rely on herbivores like camel and ground sloths to disperse its seeds; unfortunately, those animals were hunted to extinction 10,000 years ago - and then used this knowledge to set up a system to save the tree - by introducing domesticated herbivores to the park - and encouraging the Costa Rican people to preserve their lands. Janzen called it a "biocultural restoration."

Next in the order of increasing latitude is the savanna, tropical grasslands that actually cross the boundaries with dry forests. More on that later.

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Know Your Biomes II: Tropical Rain Forest

Feb 11 2007 Published by under Basic Concepts, Ecology

As we begin to take a general sweep through the Earth's biomes, the ever-changing hubs of natural history, the idea of biodiversity should be preeminent. In recent years, biodiversity has become an increasingly important term for ecologists, describing not only the entirety of organisms themselves, but also their behaviors, interactions and genetics. Think about each of these levels as we progress, coupled with the different lenses of ecology we discussed earlier. Hopefully, by the end, we will have a more satisfactory perspective on biodiversity and will be able to better define the term.

As with many systems of classification in science, there are different classifications of biomes; you may run across lists shorter or in excess of the one described in this series of posts. I will detail the most expansive terrestrial biomes here, though later we may delve into some of the more obscure classifications.

The tropical rain forest (tropical wet forest, tropical moist broadleaf forest) is probably the most well-known biome to the non-biologist, immediately recognizable for its garish displays of color, shape and texture. Evolution has produced a nearly unlimited array of plants, animals, fungi and microbes, all interwoven in an intricate organic tapestry.

The source of the density and richness of the tropical rain forest is two fold: temperatures are relatively constant throughout the year and these areas are drenched by rain almost daily. Rainfall can exceed 400 cm in a single year in these forests (temperate forests rarely exceed 100 cm of rain), draining the soil of nutrients and leaving it acidic.

Not all rain forests have soils that are poor in nutrients; those along rivers are consistently rejuvenated by flooding, while those rooted in the soil of young volcanoes have yet to be leached by steady rains.

Nutrients are rapidly recycled in a rain forest system. With as many as 300 tree species inhabiting a single hectare and as many as 1,000 species of insects on a single tree, not to mention the high density of other organisms, it is easy to see how the nutrient budget in the forest would be tight, and how organisms rely on one another to perpetuate. Bacteria, mites and other soil creatures quickly break down organic matter from dead organisms which is absorbed by other animals or by the shallow, twisted roots systems of trees and plants. Mutualistic fungi called mycorrhizae often assist these root systems in acquiring elusive elements like nitrogen and phosphorous. Termites break down tough cellulose with the help of microbes in their guts. Leaf cutting ants slice up plant material and cultivate a nutritious fungi for the colony that aids in decomposition. And, since the density of plant life within the forest almost negates the effects of wind, plants are almost completely reliant on insects for fertilization and frugivorous (fruit-eating) animals for the dispersal of seeds.

If you want to find most of the life in a rain forest, it might be best to look up. The canopy is home not only to countless species of birds, monkeys, amphibians and snakes, but also to thick draping vines called lianas. Epiphytes, plants that live on other plants, make homes out of the crooks and crannies of trees that can reach 80 meters in height.

Rain forests make up only about 2% of the Earth's terrestrial surface, but contain about half of the planet's land-loving species. On a macroscopic level, these forests are the taxonomist's last great frontier of discovery, especially in entomology; low estimates predict tens of millions of unclassified insects.

The thick web of life, the behavior guiding (sometimes forcing) interaction and the underlying genetics of it all that constitutes the tropical rain forest system is the quintessential representation of biodiversity. While other biomes on the planet may not by as dense, the biological relationships of each provide a fresh view of the diversity of life on Earth.

Some authors skip the tropical dry forest when listing biomes, but due to its peculiar (and extreme) double life of wet and dry seasons, we'll give it a nod, next time.

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Know Your Biomes I

Feb 07 2007 Published by under Basic Concepts, Ecology

Over the past few billion years, life has persisted through countless geologic, atmospheric and extraterrestrial disturbances through its ability to change with the environment. Ecosystems exist in their present state because they have evolved to be as such. It took trillions of events - biotic and abiotic - for these complex systems to weave their thick web of dependence.

One way for ecologists to define and correlate these varied environments is by categorizing these areas by the types of plants that inhabit them. These categories are called biomes. Categorizing each biome by plant life is not an end in itself; instead, indigenous plant life acts as an indicator of the animal life, soil composition and the climate of an area.

Most of us already know the biomes - desert, rain forest, savannah, tundra, etc. - but how exactly are they defined?

Life within a terrestrial biome is founded by its soil. Soil is created by the weathering of rock (inorganic content) and by the decay of tissue from animal, plant and microbial life (organic content). A mature soil - one that has been undisturbed by geologic or human activity - is characterized by distinct layers called soil horizons that will vary from biome to biome. Deserts have a very different soil stratification than rain forests due to the varying organic and inorganic composition of the biome. The horizons will be immediately apparent and distinct with a deep core sample.

In Western Maryland, there are large swaths of land reclaimed from strip mining operations and repopulated with native or analogous plant life. If you take a core sample in these reclaimed areas, there is no stratification. You will see the same trend in park land that was once used for agriculture. The soil will not become mature again for some centuries, perhaps even millennia.

Soil horizons are typically defined as such:

  • The O horizon, as shown above, is the "organic" layer, a mixed bag of leaf litter and other organic bits found on the biome floor. This layer will vary with particular biomes, obviously, with deserts or tundra having very little litter at all.
  • In the topsoil layer, or A horizon, is usually associated with humus, mostly decayed organics that have been compacted and mixed with fine inorganic particulates such as clay, silt and sand.
  • The B and C horizons are mostly composed of inorganic rock in different sizes; C will have larger "chunks" than B.
  • Below C is the bedrock.

So, plants form the foundation of each biome, and soil provides a suitable anchor and a source of nutrients for the plants (not to mention for the trillions of microbes and fungi feeding and assisting plant growth).

Climate is also an essential factor in understanding the distinctions made between environments. The Earth's "average" climate is determined by an enormous number of factors, but generally speaking, we can zero in on a few.

Heat from the sun is what drives the circulation of air on Earth, and with the circulation of air, the factors of precipitation. Much of the basis of climatic variation seen between biomes is created by uneven heat from the sun due to the Earth's tilt on its axis and its ovoid orbit around the sun. In short, these variations combined with the ever changing geology have created our extant biomes, which lie within more or less predictable latitudes.

Take a look at the map below:

The middle dotted line is the equator, while the outer two both lie at 30 degrees. Notice that the dark green areas (tropical rain forest) are pretty much bound to the equator, while the yellow areas (deserts, generally) skirt along the 30 degrees north and south. This is caused by the constant heating and cooling of air at the equator leading to almost daily rainfall.

With the moisture expelled in the form of rain, this now dry air mass rises, moving north and/or south. It sinks into the areas at 30 degrees N & S, reabsorbing moisture, and in the process, robbing the area of rain. The mass continues its journey, eventually dumping moisture into the temperate zones. This is a generalized model, but it proves the point.

So, biomes are categorized by the plants that live within certain areas, but those plants exist in their present state only because of the atmospheric and geologic pressures placed upon ancestral organisms. With recent evidence confirming that human beings are indeed causing the global climate to rise prematurely, ecologists will rely on plants to continue their role in indicating climatic influences.

In the next few posts, we'll briefly review the different types of biomes on Earth, making distinctions in precipitation levels, geography and indigenous life.

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What Is Ecology?

Feb 06 2007 Published by under Basic Concepts, Ecology

Ecology is a study of interactions or relationships between organisms and the environment; the connectedness between living systems and non-living systems on the Earth. Ecology is, in a sense, a historical field, founded upon the Earth's far reaching and ever evolving natural history.

The term ecology comes from the Greek root words oikos logos, literally “the study of household,” first combined by Ernst Haeckel in 1866. Haeckel was referring to the interactions within the house of nature, and we have used the word ecology (translated from the German Oekologie or Ökologie) to describe complex systems of life both extant and extinct.

The shear range of fields that the term "ecology" encompasses is staggering. Why? Think about how many levels of biological interaction can be described by focusing on one animal, a red panda (Ailurus fulgens).

At the individual level, the red panda itself, an ecologist could look at a particular panda's ability to thermoregulate, or absorb and expel heat within its environment. Within a population of red pandas, the next step up, an ecologist could analyze the gene flow within the population and how this particular group of red pandas is distinct in adaptations from a neighboring one.

Communities of organisms are composed of two or more populations. At this level, an ecologist could take a closer look at the cohabitation of red pandas and giant pandas in a certain area, studying how the animals share food and space.

The distinction between a community and an ecosystem is slight, but essential to understand. While a community describes interaction between organisms in an area, an ecosystem describes the entirety of the area, including chemical and physical factors. Research at this level would concentrate on things like nutrient cycling (i.e. the phosphorus or carbon cycle) or the distribution of energy within the slope forests of the Himalayas.

As we expand, things become more generalized. We are not longer talking specifically about the red panda, but about the living/nonliving system of which the red panda is a part. Landscape ecology looks at a certain heterogeneous conglomerate of ecosystems, their composition ("patches" of forests, plains, etc.) and the interaction between these ecosystems. (I will zero in on this level and be a bit more descriptive in a later post.)

A geographic ecologist (who studies regions of interaction) might take a look at the geologic history of an island or lake and try to explain the distribution of organisms in that area due to the large scale geologic activity or other environmental variables. R.H. MacArthur (an associate of E.O. Wilson) thought that geographic ecology could be described as the "search for patterns of plant and animal life that can be put on a map."

Finally, ecologists can look at biological interaction through the widest scope by analyzing the biosphere, or the entirety of life's systems on the globe. Ecology at this level usually involves major atmospheric phenomena like the long term effects of climate change or El Niño on the Earth's living systems.

At each of these levels of organization, there are near infinite examples of questions to ask about interaction. Those are just a few examples. Additionally, we have not even considered narrowing the focus to the levels of physiological, cellular and molecular interaction.

In the next posts in the Basic Concepts: Ecology series will discuss terrestrial biomes, but in order to appreciate the distinction and formation of these regions across the Earth, we'll delve into some basics about climate distribution and soil composition.

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Why Spiders Aren't Insects IV: Molecular Homology

Photo by: Gerald Yuvailos


This is the fourth part in an ongoing series discussing the distinction and evolution of spiders and other arthropods.
Part I, part II and part III have led up to this point:

In the last post of this series, we established that spiders descended from marine arthropods called the eurypterids, distinct and separate from insects, appearing in the fossil record in the late Silurian/early Devonian, about 425 million years ago.

The cladogram we used to analyze the spider's history was based on the organism's morphological characteristics, that is, visible structures like chelicerae and book lungs that can be tied to other organisms that possess the same structures. Limulus (the extant horseshoe crab) has both of these structures and predates the spiders, placing them further back in the chelicerates' evolutionary history.

Homologous bones from human (I), dog (II), pig (III), cow (IV), tapir (V) and horse (VI):
r — Radius, u — Ulna, a — Scaphoid, b — Lunare, c — Triquetrum, d — Trapezium,
e — Trapezoid, f — Capitatum, g — Hamatum, p — Pisiforme

Paleontologists call this comparison of physical characteristics homology (coined by Richard Owen an anatomist and, ironically an opponent of Darwin). The mouth parts of a spider are homologous to the mouth parts of Limulus because of the cherlicae's exact form and function. This is a different designation than analogy; analogous structures may function in the same way, but they are different in form because of their different lineage. For this reason, scientists call analogy an artificial classification system.

A good example of analogous structures are wings from bat and bird. They perform the same function in varying degrees, but they have evolved very different forms. A bat's wing is basically a modified mammal hand, while the bird wing is a modified tetrapod arm.

Homology is essential in building an organism's phylogeny (evolutionary history). More recently, geneticists have employed this classification technique to analyze and find similarities among the less visible traits in life, RNA (ribonucleic acid) and the building blocks of proteins, amino acids.

Think of these cellular chemicals this way: If DNA is the blueprint of life, RNA is the builder and its materials are amino acids. When these amino acids are placed in the correct sequence by RNA, they become proteins, the framework of our body. And, since the genetic code for protein constructions is nearly universal*, geneticists can compare entire swaths of RNA from one organism to those of another and find homology at the molecular level.

Here's an example (sequences are greatly abbreviated for the sake of our sanity):

Organism 1: ACGC-CCCCC
Organism 2: ACGC-CCCUC
Organism 3: ACGU-CUCUC

Basically, from noting the differences in each RNA sequence, and determining the homologous sequences (such as the ACGU sequence above), a cladogram can be constructed that shows common ancestry without the murky distinctions that sometimes cloud the comparison of bones to bones, or mouth parts to mouth parts.

The problem with this molecular system of analysis is that it often provides vastly different cladograms than the ones crafted through morphological analysis. This is not necessarily the case between the spiders and and Limulus, the molecular evidence supports the fossil record's interpretation of ancestry, but it calls into question the descent of insects from chelicerates like spiders.

In short, the molecular evidence agrees with the morphological evidence: spiders are more closely related to horseshoe crabs than insects. But where and when did the insects arise?

Next time we'll tackle the more recent movements to elucidate the phylogeny of arthropods, including a discussion on the significance Hox genes and evolutionary-developmental biology (evo-devo).

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