Chapter 7: Niche and habitat
How strange it is that a bird, under the form of a woodpecker, should have been created to prey on insects on the ground; that upland geese, which never or rarely swim, should have been created with webbed feet; that a thrush should have been created to dive and feed on sub-aquatic insects; and that a petrel should have been created with habits and structure fitting it for the life of an auk or grebe! and so on in endless other cases. But on the view of each species constantly trying to increase in number, with natural selection always ready to adapt the slowly varying descendants of each to any unoccupied or ill-occupied place in nature, these facts cease to be strange, or perhaps might even have been anticipated. - Charles Darwin (1859), On the Origin of Species
Every organism has a place to live in nature, a functional role in that place, and a complex set of adaptations for reproducing its kind. On the surface, this observation might seem to be obvious, even trivial. However, in order to understand our biological world—the biosphere, how it operates and ultimately how to protect it—we need to understand at a deep level how organisms interact with each other and with their physical environment.
In this chapter we will examine further some of the concepts that ecologists use to organize their thoughts about the ways in which organisms use their environment, relate to each other, and assemble into communities or ecosystems. The most fundamental and perhaps most difficult of these concepts is that of the ecological niche. A niche refers to the way in which an organism fits into an ecological community or ecosystem. Through the process of natural selection, a niche is the evolutionary result of a species’ morphological (morphology refers to an organism’s physical structure), physiological, and behavioral adaptations to its surroundings. A habitat is the actual location in the environment where an organism lives and consists of all the physical and biological resources available to a species. The collection of all the habitat areas of a species constitutes its geographic range. We will examine each of these concepts in turn using a historical approach where appropriate and discuss how these ideas can help us to understand the issues of modern conservation biology.
One of the first things that people noticed as they started to study and write about the world was that when they visited different parts of the earth they found different species. In fact, most species have limited distributions. Giraffes, for example, are found only in Africa, koalas only in Australia, and lemurs only in Madagascar. If we choose a particular species, plot a point on a map of the world for every place in which it is found, and then draw a line surrounding all of the points, we will have delimited its geographical range. Figure 7.1 is a range map for three species found in the United States. Some species, such as the Devil’s Hole pupfish that only lives in one particular desert spring, have a geographical range of just a few square meters; others (e.g., human beings) range over most of the land area of the earth. Most species, though, have intermediate-sized distributions, as does the bighorn sheep.
Factors Limiting the Distributions of Organisms
One factor that limits the distribution of a species is its dispersal ability, i.e., how well individuals or their offspring can move from place to place. Obviously in the case of the Devils Hole pupfish, the only route to other waters is overland through the Mojave Desert, so unless they sprout legs and other adaptations they cannot disperse without human aid. Humans, on the other hand, are the ultimate dispersers, being able to transport themselves quickly to virtually any spot on (and even off) the planet. Desert bighorn sheep fall somewhere in between these extremes. Desert bighorn prefer rugged desert terrain and thus inhabit many of the hundreds of small, isolated mountain ranges of the southwestern United States. Although bighorn are known to move across the flats between close mountain ranges, they don’t really like flat open areas and thus are less likely to move between mountain ranges that are far apart. Thus the availability of suitable habitats within a potentially traversable distance affects the distribution of a species.
Another factor that limits distributions is a species’ tolerance to different environmental conditions. Factors such as temperature, moisture, and light have profound effects on species’ distributions. Mammals living in hot climates tend to have smaller bodies and longer ears and limbs than mammals in colder climates. Small body size and long limbs are adaptations that increase the body’s surface area in relation to its volume and thus function to help the body lose excess heat. Such mammals would be at an obvious disadvantage if they tried to disperse into an area of cooler climate where it is more important to conserve body heat than to lose it. Species adapted to the tropical rainforests of Brazil where water is abundant tend to lack water-conserving adaptations and so would have difficulty surviving in the arid deserts of the southwestern United States. Freshwater fish placed in the ocean (with the exception of a few anadromous species like salmon or steelhead) have difficulty maintaining water balance due to the ocean’s salinity and actually die of dehydration. Some plants require days of a certain length in order to flower and reproduce. If seeds of these plants were carried by wind or birds to an area suitable for germination and growth but lacking sufficient day-length for reproduction, the species would never become permanently established.
Interactions with other species can also limit a species’ distribution. A species invading a new environment will encounter other species with which it has never had contact. If one of these is a predator that uses unfamiliar tactics, the invading species is likely to be eaten. Similarly, if one of the species in the new environment uses the same kinds of resources as the invading species and if it is better able to compete for those resources, then the invading species will have trouble gathering enough resources to meet the needs of survival and reproduction. Obviously in both of these cases the invading species will not be very successful in extending its range.
The flip side of this is that native species may be exceptionally vulnerable to an unfamiliar predator or competitor (see Chapter 9). An example is the decline of most species of large native fish in the Great Lakes due to the introduction of the sea lamprey. The sea lamprey is an eel-like fish that attaches itself to other fishes and sucks out their body fluids, often with fatal results. When sea lampreys found their way into the Great Lakes, they found an abundance of prey that had no adaptations for avoiding their style of predation. The result was that at least one species of whitefish was driven to extinction and other species were severely depleted in numbers.
Implications for Conservation
The concept of geographic ranges has implications for conservation biology. As we have seen, geographic ranges are a function of a species’ various morphological and physiological adaptations—characteristics acquired through the process of natural selection. Sometimes these adaptations will also work well in a location different from that in which the species originally evolved. For a very long time now humans have been moving species around the globe, introducing them into previously unfamiliar habitats. Most often these introductions fail. But as we just saw in the case of the sea lamprey, every so often a species is well-suited to the new habitat and the species proliferates. This is a major problem for those charged with managing and preserving biodiversity. For example, in the southwestern United States prospectors and miners of the 1800s abandoned their burros (donkeys) in the desert when they no longer needed them. Burros originally evolved in the arid desert regions of Somalia in eastern Africa and so were already well-adapted to the deserts of the American west. In places like the Grand Canyon and Death Valley they multiplied rapidly, caused changes in the vegetation, and competed with the native desert bighorn sheep for food and water. The burros in Death Valley and the Grand Canyon were removed at great effort and expense and put up for adoption, giving the bighorn a chance to recover. There are many burros in other ecosystems of the Southwest, however, and these have been implicated as important factors in the decline of other bighorn populations.
HABITAT: THE ORGANISM’S HOME
If we examine the geographic distribution of a widely-ranging species, we find that the distributional range consists of both occupied and unoccupied areas. Those areas actually occupied that meet the requirements for a species’ survival and reproduction are its habitat. We mentioned earlier that the desert bighorn prefers desert mountain ranges—these mountain ranges, along with all the plants and other animals found there, constitute the bighorn’s habitat. If we were to alter the map in Figure 7.1 to show only the habitat we would see a kind of polka-dot pattern as opposed to the solid area depicted.
Kinds of habitats
Any place where organisms live is by definition a habitat. Your backyard, an empty lot, an agricultural field, a pristine mountain wilderness—all these are habitats for some group of organisms. Ecologists often find it useful to talk about kinds of habitats or to classify them into more or less general groupings. Habitats are typically classified on the basis of more or less obvious visual characteristics. Alpine meadows, conifer forests, marshes, lakes, desert scrub, and riparian zones along stream banks are all different habitats that you may be able to visualize from your own experience.
Sometimes habitat classifications have more to do with how the habitat functions than with its visual aspect. Although freshwater and saltwater marshes may appear similar superficially, each supports a different constellation of species. Along the California coastline at the ocean’s edge we can distinguish several zones that are functionally different. Above the high tide line there is a splash zone that only the highest ocean waves can reach. Below this is the intertidal zone that is alternately exposed and submerged with the ebb and flow of the tide. Deeper still is the subtidal zone, which is always submerged. Although all of these zones may occur within a few meters of each other, they are fundamentally different habitats for the organisms that live there and they support distinct assemblages of species.
Habitats have many features or factors that are important to the organisms living there. Conveniently, we can divide habitat factors into two major groupings, physical factors and biotic factors.
In terrestrial habitats some important physical factors are elevation, steepness, slope direction, soil type, and water availability. Elevation affects air temperature and rainfall—higher elevations are cooler and moister than lower ones. The steepness of a slope will affect the kind of soil that can form there and the amount of water that can soak into the ground after it rains. Slope aspect (the direction a slope faces) is particularly important. In the northern hemisphere, south-facing slopes get more sun, are warmer and dryer, and thus support different vegetation than north-facing slopes. In aquatic habitats such characteristics as pH, salinity, dissolved oxygen concentration, temperature, and flow rate are important physical factors.
Biotic factors include all the other species that occur in the habitat. For an herbivore such as the desert bighorn sheep many of the grass, shrub, and herb species of the desert mountains constitutes its food source. Trees and shrubs form both hiding cover from predators and thermal the dead trees in which their holes are found but also the other birds (e.g., Pileated woodpeckers) that make the holes in the first place.
Finally, physical and biotic factors may interact to determine the quality of the habitat for a given organism. For example, the nutritional quality of plants available as food for herbivores, such as deer, is determined in large part by the quality of the soils present.
Habitats and Conservation
Obviously a species cannot survive without its natural habitat, except perhaps in a zoo. It follows then that the fundamental unit in the conservation of biodiversity is not the species but the habitat.
An organism may have more than one habitat. During the summer many of our migratory waterfowl have breeding habitats in the arctic tundra, but they winter on the waterways and marshes of the southern United States. Mule deer of the Great Basin spend the summer in mountain forests, but winter snows drive them to lower elevation sagebrush zones where they can forage more easily. To ensure that there will always be migratory species such as these we need to protect not only the two habitats at the extremes of their seasonal movements but also the migration routes in between that serve as resting and feeding habitats.
Habitats don’t exist in isolation. Many habitats have inputs and outputs. Take Mono Lake, for instance, a spectacular lake on the east side of the Sierra Nevada in California. Its water source is streams fed by winter rains and melting snow in the mountains. In its natural state, water leaves the lake only by evaporation. The balance between the inflowing streams and evaporation created a saline lake with many unique features, including a species of brine shrimp found only in Mono Lake. As a large, food-rich body of water in a desert area, the lake is a major fueling stop for migratory waterbirds and a major nesting area for other species, such as California gulls. When water from the lake’s inflowing streams was diverted to quench the ever-growing thirst of southern California, the lake level dropped drastically. Islands in the lake became connected to the mainland, giving coyotes and other predators access to an easy source of food: nesting California gulls. With adequate inflowing water, the islands were good nesting habitat; without the water they were unsuitable as nesting habitat. Without adequate inflowing water, the lake also would become too saline to be habitat for the Mono brine shrimp and to be a feeding habitat for migratory waterbirds. Recognition of this fundamental relationship between inflow and habitat for many species was the partial basis of a successful court action that reduced the diversion of water from the inflowing streams.
NICHE: THE ORGANISM’S ROLE
Perhaps the simplest, most general definition of the ecological niche is an organism’s "ecological position in the world" (Vandermeer 1972). Even though this may seem straightforward, determining what constitutes an organism’s position in the greater scheme of things is not a trivial pursuit. In fact, the concept of niche is a notoriously difficult one for beginning (and even advanced) students of ecology. Some of the difficulties arise because the word niche has a common meaning to the layperson but a very specific meaning to the ecologist.
History of the Niche Concept
Webster’s Dictionary defines niche as "a place or position suitable or appropriate for a... thing." Biologists and naturalists have long considered each species to have a proper place in nature. Darwin was influenced by this idea, as evidenced by the quote at the beginning of this chapter, while developing his theory of natural selection. It is natural then that the first uses of the word niche in an ecological context had a very strong "place" association. Although not the first ecologist to use the word, Joseph Grinnell, in a series of papers published between 1917 and 1924, is generally credited with being the first to develop the ecological concept of the niche. Grinnell defined niche as the "ultimate [distributional] unit... occupied by just one species or subspecies." To be more specific, Grinnell was interested in determining which factors governed a species’ potential geographical distribution and usually considered these to be physical or climatic factors, as opposed to relationships with other species such as competition or predation. For example, in a 1917 paper entitled "The niche-relationships of the California Thrasher," Grinnell considered the geographical range of thrashers, a common bird of the chaparral, to be limited by temperature since it avoided areas of extreme heat or cold. Thrashers, Grinnell maintained, were further restricted to chaparral areas because, being shy creatures, they needed dense hiding cover. These factors, because they explained the California thrasher’s distribution, constituted the bird’s niche. Grinnell’s concept of the niche also included two important components: first, animals evolved to fill niches and, second, no two species could have exactly the same niche. These two concepts have turned out to be central to the subsequent development of niche theory.
At about the same time, one of the most important ecologists of the early part of this century, Charles Elton, was developing his own concept of niche. Elton’s (1927) niche concept differed from that of Grinnell in several fundamental ways as evidenced by his definition of niche as an organism’s "place in the biotic environment, its relations to food and enemies." When Elton uses the word "place" he really means the organism’s role in its community—what it does, how it makes its living—as opposed to the geographical sense used by Grinnell. In practice, Elton tended to define niches based on an animal’s size and feeding habits. For example, birds of similar size that catch insects on the wing were thought to have a similar niche; similarly, all large mammals that eat only grass were thought to have another. Elton’s view of the niche concept was simply "to give more accurate and detailed definitions of the food habits of animals" than those afforded by words such as carnivore or herbivore.
A few years later the Russian scientist G. F. Gause (1934) combined Elton’s view of the niche with the observation that very similar species cannot co-exist within a community. This is because resources such as food generally are in limited supply. Very similar organisms would have to compete with each other for the resource in question and inevitably one species would prove to be the superior competitor. Although Grinnell and even Darwin had stated much the same thing, Gause based his concept on mathematical reasoning by the Italian mathematician Vito Volterra. This idea, now often referred to as "Gause’s principle," or "the competitive-exclusion principle" can be restated succinctly as "no two species in a community may possess the same niche," and has become a central tenet of modern niche theory.
In order to demonstrate this principle, Gause performed what are now considered to be classic experiments. Gause placed two species of Paramecium, a single-celled protozoan, into flasks containing a bacterial culture that served as food. Thus, in this artificial laboratory system both species of Paramecium were forced to have the same niche. Gause counted the numbers of Paramecium each day and found that after a few days (see Figure 7.2) one species always became extinct because it apparently was unable to compete with the other species for the single food resource. This process of competitive exclusion has since been demonstrated many times in laboratory and field experiments with many species. However, extinction is not the only possible result of two species having the same niche. If two competing species can co-exist for a long period of time, then the possibility exists that they will evolve differences to minimize competition; that is, they can evolve different niches.
Two niche concepts have been discussed so far. The first, propounded by Grinnell, is geographically oriented and we can term it a place niche. The second, championed by Elton and Gause, is defined on behavioral considerations and we might call this the functional niche. Ecologists were relatively happy accepting these views of the niche until the late 1950s when the eminent limnologist and ecologist G. Evelyn Hutchinson devised a rigorous and quantitative concept of niche that, with slight modifications from his original concept, incorporated both place and functional elements and has remained the standard niche model for over thirty years. Prior to Hutchinson the niche was a rather nebulous concept defined only by words; that is, niches could not be measured. Hutchinson’s new idea not only allowed a way to measure niches but also a way to compare niches of two or more species.
An animal that preys upon other animals will be limited in the range of prey sizes that it can kill. Certain prey will simply be too large to kill; others will be too small to bother with because the amount of energy needed to catch them is greater than the amount of energy to be gained by eating them. This situation is illustrated for a hypothetical species in the simple number line graph of Figure 7.3. The upper line depicts the range of prey sizes used by the species compared to the total size range available, shown on the lower line. Such a one-dimensional graph is called a niche axis and represents quantitatively how a species uses one of the factors of its habitat. The range of resources utilized along the axis is referred to as niche breadth. A species with wide niche breadth is referred to as a generalist; a species with narrow niche breadth is called a specialist.
Food is not the only factor that we can graph this way. In Figure 7.4 we add a second axis to indicate the temperature extremes that our hypothetical species can tolerate. Note that this results in a shaded area on the graph indicating the combinations of prey size and temperature that allow the species to exist. It would now be possible to add a third factor such as moisture to our graph making it three-dimensional. The shaded area in Figure 7.4 would become a rectangular solid and would represent the combinations of prey size, temperature, and moisture that allow our species to survive.
Hutchinson’s idea was to continue this line of reasoning indefinitely until we had incorporated all of the biotic and physical factors of the habitat to which a species is adapted. Obviously if we have more than three niche axes we are no longer able to draw a graph of the niche, but mathematicians and computers can deal with such hypervolumes with relative ease. In practice, because it is difficult to determine the responses of a species to all of the factors of the environment, most ecologists limit their investigations of niches to two or three of the most important factors. Such a niche, determined in the absence of relations with other species, is termed the fundamental niche and represents a species’ potential to use available resources. Certain interactions between species can affect the breadth of a species’ niche along one or several niche axes. For example, the risk of predation could shrink the breadth of a species’ food niche axis if searching for certain kinds of food items increased the probability of being eaten. Competition for resources could also reduce the breadth of a species’ niche along the resource axis in question. Thus the fundamental niche is the niche that exists in the absence of predators and competitors (a rare event) and is determined largely by the species morphological and physiological limitations. In the real world, a niche is limited in extent by the presence of interactions with other species and is termed the realized niche. The realized niche of a species may vary from place to place because of the presence of different predators and competitors. In the Eel River of coastal California, for example, rainbow trout live primarily in riffles (shallow, fast-flowing areas) and feed on aquatic insects when predatory pikeminnows are present in the pools; in the absence of the predator, the realized niche of the trout expands to include the pools and more terrestrial insects that fall on the pool surface.
Niches and Conservation
Some of the myriad confusions surrounding the niche concept result from the idea of "empty" or "vacant" niches. Some ecologists have suggested that niches actually exist out in the environment and that organisms evolve to "fill" or "occupy" them. Most ecologists consider this to be nonsense. Hutchinson’s niche concept very clearly expressed the idea that a species’ niche is the sum total of adaptations to the environment possessed by the species in question. The niche is just as much an attribute of a species as its color, size, shape, or physiology.
In addition to causing confusion, the idea of vacant niches has been misused to justify the presence of introduced organisms. A noted paleontologist has suggested that feral burros in the southwest occupy a niche left vacant by a long extinct small Pleistocene horse and should thus be considered a native species. Even though we don’t know exactly what this horse was, it certainly was not the same species as the burro, and it evolved under very different conditions along with other species that the burro never "knew" in its own evolutionary history in Africa. Equally important, this argument implies we need to re-introduce a suite of predators to prey on the burros, such African-type lions and grizzly bears. Even if burros were very similar ecologically to this extinct horse, our native southwestern flora and fauna have been without a native horse for a very long time and may no longer be adapted to its presence. For example, grazing by feral burros in Death Valley altered the species’ composition of plant communities that formed the bighorn sheep’s food source and prevented bighorn from using scarce water supplies. This, in effect, narrowed the bighorn’s niche to the point where reproduction became difficult and populations declined drastically. When we humans modify habitats, whether for economic gain, by accident, or even just for the purpose of survival, we further constrain the realized niches of the organisms living there. Very often the altered niche does not permit survival of the species. There are millions of species on earth and it is probably an impossible task to fully determine the niche of all of them. In fact, it is a difficult task for just a single species. But an understanding of how an organism uses its environment in perhaps two or three important niche axes can help us to predict how a species will react to changes in its habitat.
In this chapter we have discussed in detail some of the concepts that ecologists use to study biodiversity. These concepts are geographic range, habitat, and ecological niche. It should be obvious by now that these are not mutually exclusive topics. Elements of habitat and niche are subsumed into the determination of geographic ranges. It can also be difficult to say exactly where habitat leaves off and niche begins. Nonetheless, an understanding of these three concepts can help us to effectively plan conservation measures and to appreciate the wonderful complexity of our biosphere.
Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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