Ecology is the scientific study of relationships in the natural world. It includes relationships between organisms and their physical environments (physiological ecology) between organisms of the same species (population ecology) between organisms of different species (community ecology), and between organisms, and the fluxes of matter and energy through biological systems (ecosystem ecology).
1. Unit 4 : Ecosystems Overview Why are there so many living organisms on Earth, and so many different species? How do the characteristics of the nonliving environment, such as soil quality and water salinity, help determine which organisms thrive in particular areas? These questions are central to the study of ecosystems— communities of living organisms in particular places and the chemical and physical factors that influence them. Learn how scientists study ecosystems to predict how they may change over time and respond to human impacts. Elk in Yellowstone National Park. 1. Introduction 2. Major Terrestrial and Aquatic Biomes 3. Energy Flow Through Ecosystems 4. Biogeochemical Cycling in Ecosystems 5. Population Dynamics 6. Regulation of Ecosystem Functions 7. Ecological Niches 8. Evolution and Natural Selection in Ecosystems 9. Natural Ecosystem Change 10. Further Reading Unit 4 : Ecosystems -1- www.learner.org
2. 1. Introduction Ecology is the scientific study of relationships in the natural world. It includes relationships between organisms and their physical environments (physiological ecology); between organisms of the same species (population ecology); between organisms of different species (community ecology); and between organisms and the fluxes of matter and energy through biological systems (ecosystem Ecologists study these interactions in order to understand the abundance and diversity of life within Earth's ecosystems—in other words, why there are so many plants and animals, and why there are so many different types of plants and animals (Fig. 1). To answer these questions they may use field measurements, such as counting and observing the behavior of species in their habitats; laboratory experiments that analyze processes such as predation rates in controlled settings; or field experiments, such as testing how plants grow in their natural setting but with different levels of light, water, and other inputs. Applied ecology uses information about these relationships to address issues such as developing effective vaccination strategies, managing fisheries without over-harvesting, designing land and marine conservation reserves for threatened species, and modeling how natural ecosystems may respond to global climate change. Figure 1. Tropical ecologist Stuart Davies in the field Change is a constant process in ecosystems, driven by natural forces that include climate shifts, species movement, and ecological succession. By learning how ecosystems function, we can improve our ability to predict how they will respond to changes in the environment. But since living Unit 4 : Ecosystems -2- www.learner.org
3. organisms in ecosystems are connected in complex relationships, it is not always easy to anticipate how a step such as introducing a new species will affect the rest of an ecosystem. Human actions are also becoming major drivers of ecosystem change. Important human-induced stresses on ecosystems are treated in later units of this text. Specifically, Unit 7 ("Agriculture") examines how agriculture and forestry create artificial, simplified ecosystems; Unit 9 ("Biodiversity Decline") discusses the effects of habitat loss and the spread of invasive species; and Unit 12 ("Earth's Changing Climate") considers how climate change is affecting natural ecosystems. 2. Major Terrestrial and Aquatic Biomes Geography has a profound impact on ecosystems because global circulation patterns and climate zones set basic physical conditions for the organisms that inhabit a given area. The most important factors are temperature ranges, moisture availability, light, and nutrient availability, which together determine what types of life are most likely to flourish in specific regions and what environmental challenges they will face. As discussed in Unit 2, "Atmosphere," and Unit 3, "Oceans," Earth is divided into distinct climate zones that are created by global circulation patterns. The tropics are the warmest, wettest regions of the globe, while subtropical high-pressure zones create dry zones at about 30° latitude north and south. Temperatures and precipitation are lowest at the poles. These conditions create biomes— broad geographic zones whose plants and animals are adapted to different climate patterns. Since temperature and precipitation vary by latitude, Earth's major terrestrial biomes are broad zones that stretch around the globe (Fig. 2). Each biome contains many ecosystems (smaller communities) made up of organisms adapted for life in their specific settings. Unit 4 : Ecosystems -3- www.learner.org
5. Figure 3. Biome type in relation to temperature and rainfall Land biomes are typically named for their characteristic types of vegetation, which in turn influence what kinds of animals will live there. Soil characteristics also vary from one biome to another, depending on local climate and geology. compares some key characteristics of three of the forest Table 1. Forest biomes. Forest type Temperature Precipitation Soil Flora Tropical 20-25°C >200 cm/yr Acidic, low in Diverse (up to 100 nutrients 2 species/km ) Temperate -30 to 30°C 75-150 cm/yr Fertile, high in 3-4 tree species/ nutrients km 2 Boreal (taiga) Very low 40-100 cm/year, Thin, low in Evergreens mostly snow nutrients, acidic Aquatic biomes (marine and freshwater) cover three-quarters of the Earth's surface and include rivers, lakes, coral reefs, estuaries, and open ocean (Fig. 4). Oceans account for almost all of this area. Large bodies of water (oceans and lakes) are stratified into layers: surface waters are warmest and contain most of the available light, but depend on mixing to bring up nutrients from deeper levels Unit 4 : Ecosystems -5- www.learner.org
7. Species are not uniformly spread among Earth's biomes. Tropical areas generally have more plant and animal biodiversity than high latitudes, measured in species richness (the total number of species present) (footnote 1). This pattern, known as the latitudinal biodiversity gradient, exists in marine, freshwater, and terrestrial ecosystems in both hemispheres. Figure 5 shows the gradient for plant species, but it also holds true for animals. Figure 5. Plant species diversity Barthlott, W., Biedinger, N., Braun, G., Feig, F., Kier, G., and Mutke, J. (1999): Terminology and methodological aspects of the mapping and analysis of global diversity. Acta Botanica Fennica 162, 103–110. Why is biodiversity distributed in this way? Ecologists have proposed a number of explanations: • Higher productivity in the tropics allows for more species; • The tropics were not severely affected by glaciation and thus have had more time for species to develop and adapt; Unit 4 : Ecosystems -7- www.learner.org
8. • Environments are more stable and predictable in the tropics, with fairly constant temperatures and rainfall levels year-round; • More predators and pathogens limit competition in the tropics, which allows more species to coexist; and • Disturbances occur in the tropics at frequencies that promote high successional diversity. Of these hypotheses, evidence is strongest for the proposition that a stable, predictable environment over time tends to produce larger numbers of species. For example, both tropical ecosystems on land and deep sea marine ecosystems—which are subject to much less physical fluctuation than other marine ecosystems, such as estuaries—have high species diversity. Predators that seek out specific target species may also play a role in maintaining species richness in the tropics. 3. Energy Flow Through Ecosystems Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources. At the first trophic level, primary producers (plants, algae, and some bacteria) use solar energy to produce organic plant material through photosynthesis. Herbivores—animals that feed solely on plants—make up the second trophic level. Predators that eat herbivores comprise the third trophic level; if larger predators are present, they represent still higher trophic levels. Organisms that feed at several trophic levels (for example, grizzly bears that eat berries and salmon) are classified at the highest of the trophic levels at which they feed. Decomposers, which include bacteria, fungi, molds, worms, and insects, break down wastes and dead organisms and return nutrients to the soil. On average about 10 percent of net energy production at one trophic level is passed on to the next level. Processes that reduce the energy transferred between trophic levels include respiration, growth and reproduction, defecation, and nonpredatory death (organisms that die but are not eaten by consumers). The nutritional quality of material that is consumed also influences how efficiently energy is transferred, because consumers can convert high-quality food sources into new living tissue more efficiently than low-quality food sources. The low rate of energy transfer between trophic levels makes decomposers generally more important than producers in terms of energy flow. Decomposers process large amounts of organic material and return nutrients to the ecosystem in inorganic form, which are then taken up again by primary producers. Energy is not recycled during decomposition, but rather is released, mostly as heat (this is what makes compost piles and fresh garden mulch warm). Figure 6 shows the flow of energy (dark arrows) and nutrients (light arrows) through ecosystems. Unit 4 : Ecosystems -8- www.learner.org
12. that land plants produce, such as roots, trunks, and branches, cannot be used by herbivores for food, so proportionately less of the energy fixed through primary production travels up the food chain. Growth rates may also be a factor. Phytoplankton are extremely small but grow very rapidly, so they support large populations of herbivores even though there may be fewer algae than herbivores at any given moment. In contrast, land plants may take years to reach maturity, so an average carbon atom spends a longer residence time at the primary producer level on land than it does in a marine ecosystem. In addition, locomotion costs are generally higher for terrestrial organisms compared to those in aquatic environments. The simplest way to describe the flux of energy through ecosystems is as a food chain in which energy passes from one trophic level to the next, without factoring in more complex relationships between individual species. Some very simple ecosystems may consist of a food chain with only a few trophic levels. For example, the ecosystem of the remote wind-swept Taylor Valley in Antarctica consists mainly of bacteria and algae that are eaten by nematode worms (footnote 2). More commonly, however, producers and consumers are connected in intricate food webs with some consumers feeding at several trophic levels (Fig. 9). Figure 9. Lake Michigan food web Courtesy of NOAA Great Lakes Environmental Research Laboratory and the Great Lakes Fishery Commission. Unit 4 : Ecosystems -12- www.learner.org
13. An important consequence of the loss of energy between trophic levels is that contaminants collect in animal tissues—a process called bioaccumulation. As contaminants bioaccumulate up the food web, organisms at higher trophic levels can be threatened even if the pollutant is introduced to the environment in very small quantities. The insecticide DDT, which was widely used in the United States from the 1940s through the 1960s, is a famous case of bioaccumulation. DDT built up in eagles and other raptors to levels high enough to affect their reproduction, causing the birds to lay thin-shelled eggs that broke in their nests. Fortunately, populations have rebounded over several decades since the pesticide was banned in the United States. However, problems persist in some developing countries where toxic bioaccumulating pesticides are still used. Bioaccumulation can threaten humans as well as animals. For example, in the United States many federal and state agencies currently warn consumers to avoid or limit their consumption of large predatory fish that contain high levels of mercury, such as shark, swordfish, tilefish, and king mackerel, to avoid risking neurological damage and birth defects. 4. Biogeochemical Cycling in Ecosystems Along with energy, water and several other chemical elements cycle through ecosystems and influence the rates at which organisms grow and reproduce. About 10 major nutrients and six trace nutrients are essential to all animals and plants, while others play important roles for selected species (footnote 3). The most important biogeochemical cycles affecting ecosystem health are the water, carbon, nitrogen, and phosphorus cycles. As noted earlier, most of the Earth's area that is covered by water is ocean. In terms of volume, the oceans dominate further still: nearly all of Earth's water inventory is contained in the oceans (about 97 percent) or in ice caps and glaciers (about 2 percent), with the rest divided among groundwater, lakes, rivers, streams, soils, and the atmosphere. In addition, water moves very quickly through land ecosystems. These two factors mean that water's residence time in land ecosystems is generally short, on average one or two months as soil moisture, weeks or months in shallow groundwater, or up to six months as snow cover. But land ecosystems process a lot of water: almost two-thirds of the water that falls on land as precipitation annually is transpired back into the atmosphere by plants, with the rest flowing into rivers and then to the oceans. Because cycling of water is central to the functioning of land ecosystems, changes that affect the hydrologic cycle are likely to have significant impacts on land ecosystems. (Global water cycling is discussed in more detail in Unit 8, "Water Resources.") Both land and ocean ecosystems are important sinks for carbon, which is taken up by plants and algae during photosynthesis and fixed as plant tissue. Table 2 compares the quantities of carbon stored in Earth's major reservoirs. Unit 4 : Ecosystems -13- www.learner.org
14. Table 2. Global carbon storage. Location Amount (gigatons carbon) Atmosphere 750 Land plants 610 Soil and detritus 1,500 Surface ocean 1,020 Intermediate and deep ocean 37,890 Sediments 78,000,000 Carbon cycles relatively quickly through land and surface-ocean ecosystems, but may remain locked up in the deep oceans or in sediments for thousands of years. The average residence time that a molecule of carbon spends in a terrestrial ecosystem is about 17.5 years, although this varies widely depending on the type of ecosystem: carbon can be held in old-growth forests for hundreds of years, but its residence time in heavily grazed ecosystems where plants and soils are repeatedly turned over may be as short as a few months. Human activities, particularly fossil fuel combustion, emit significant amounts of carbon each year over and above the natural carbon cycle. Currently, human activities generate about 7 billion tons of carbon per year, of which 3 billion tons remain in the atmosphere. The balance is taken up in roughly equal proportions by oceans and land ecosystems. Identifying which ecosystems are absorbing this extra carbon and why this uptake is occurring are pressing questions for ecologists. Currently, it is not clear what mechanisms are responsible for high absorption of carbon by land ecosystems. One hypothesis suggests that higher atmospheric CO2 concentrations have increased the rates at which plants carry out photosynthesis (so-called CO2 fertilization), but this idea is controversial. Controlled experiments have shown that elevated CO2 levels are only likely to produce short-term increases in plant growth, because plants soon exhaust available supplies of important nutrients such as nitrogen and phosphorus that also are essential for growth. Nitrogen and phosphorus are two of the most essential mineral nutrients for all types of ecosystems and often limit growth if they are not available in sufficient quantities. (This is why the basic ingredients in plant fertilizer are nitrogen, phosphorus, and potassium, commonly abbreviated as NPK.) A slightly expanded version of the basic equation for photosynthesis shows how plants use energy from the sun to turn nutrients and carbon into organic compounds: CO2 + PO4 (phosphate) + NO3 (nitrate) + H2O → CH2O, P, N (organic tissue) + O2 Because atmospheric nitrogen (N2) is inert and cannot be used directly by most organisms, microorganisms that convert it into usable forms of nitrogen play central roles in the nitrogen cycle. So-called nitrogen-fixing bacteria and algae convert ammonia (NH4) in soils and surface waters into Unit 4 : Ecosystems -14- www.learner.org
17. example, an organism that spends much of its energy on reproduction early in life will have lower growth and survival rates, and thus a lower reproductive level later in life. An optimal life history strategy maximizes the organism's contribution to population growth. Understanding how the environment shapes organisms' life histories is a major question in ecology. Compare the conditions for survival in an unstable area, such as a flood plain near a river that frequently overflows its banks, to those in a stable environment, such as a remote old-growth forest. On the flood plain, there is a higher chance of being killed early in life, so the organisms that mature and reproduce earlier will be most likely to survive and add to population growth. Producing many offspring increases the chance that some will survive. Conversely, organisms in the forest will mature later and have lower early reproductive rates. This allows them to put more energy into growth and competition for resources. Ecologists refer to organisms at the first of these two extremes (those adapted to unstable environments) as r-selected. These organisms live in settings where population levels are well below the maximum number that the environment can support—the carrying capacity—so their numbers are growing exponentially at the maximum rate at which that population can increase if resources are not limited (often abbreviated as r). The other extreme, organisms adapted to stable environments, are termed K-selected because they live in environments in which the number of individuals is at or near the environment's carrying capacity (often abbreviated as K). Organisms that are r-selected tend to be small, short-lived, and opportunistic, and to grow through irregular boom-and-bust population cycles. They include many insects, annual plants, bacteria, and larger species such as frogs and rats. Species considered pests typically are r-selected organisms that are capable of rapid growth when environmental conditions are favorable. In contrast, K-selected species are typically larger, grow more slowly, have fewer offspring and spend more time parenting them. Examples include large mammals, birds, and long-lived plants such as redwood trees. K- selected species are more prone to extinction than r-selected species because they mature later in life and have fewer offspring with longer gestation times. Table 3 contrasts the reproductive characteristics of an r-selected mammal, the Norway rat, to those of a K-selected mammal, the African elephant. Table 3. Reproduction in r-selected and K-selected species. Feature Norway rat (r-selected) African elephant (K-selected) Reaches sexual or reproductive 3-4 months 10-12 years Average gestation period 22-24 days 22 months Time to weaning 3-4 weeks 48-108 months Breeding interval (female) Up to 7 times per year Every 4 to 9 years Unit 4 : Ecosystems -17- www.learner.org
18. Feature Norway rat (r-selected) African elephant (K-selected) Offspring per litter 2-14 (average 8) 1 average, 2 high Many organisms fall between these two extremes and have some characteristics of both types. As we will see below, ecosystems tend to be dominated by r-selected species in their early stages with the balance gradually shifting toward K-selected species. In a growing population, survival and reproduction rates will not stay constant over time. Eventually resource limitations will reduce one or both of these variables. Populations grow fastest when they are near zero and the species is uncrowded. A simple mathematical model of population growth implies that the maximum population growth rate occurs when the population size (N) is at one-half of the environment's carrying capacity, K (i.e., at N = K/2). In theory, if a population is harvested at exactly its natural rate of growth, the population will not change in size, and the harvest (yield) can be sustained at that level. In practice, however, it can be very hard to estimate population sizes and growth rates in the wild accurately enough to achieve this maximum sustainable yield. (For more on over-harvesting, see Unit 9, "Biodiversity Decline.") 6. Regulation of Ecosystem Functions A key question for ecologists studying growth and productivity in ecosystems is which factors limit ecosystem activity. Availability of resources, such as light, water, and nutrients, is a key control on growth and reproduction. Some nutrients are used in specific ratios. For example, the ratio of nitrogen to phosphorus in the organic tissues of algae is about 16 to 1, so if the available nitrogen concentration is greater than 16 times the phosphorus concentration, then phosphorus will be the factor that limits growth; if it is less, then nitrogen will be limiting. To understand how a specific ecosystem functions, it thus is important to identify what factors limit ecosystem activity. Resources influence ecosystem activity differently depending on whether they are essential, substitutable, or complementary. Essential resources limit growth independently of other levels: if the minimum quantity needed for growth is not available, then growth does not occur. In contrast, if two resources are substitutable, then population growth is limited by an appropriately weighted sum of the two resources in the environment. For example, glucose and fructose are substitutable food sources for many types of bacteria. Resources may also be complementary, which means that a small amount of one resource can substitute for a relatively large amount of another, or can be complementary over a specific range of conditions. Resource availability serves as a so-called "bottom-up" control on an ecosystem: the supply of energy and nutrients influences ecosystem activities at higher trophic levels by affecting the amount of energy that moves up the food chain. In some cases, ecosystems may be more strongly influenced by so-called "top-down" controls—namely, the abundance of organisms at high trophic levels in the ecosystem (Fig. 12). Both types of effects can be at work in an ecosystem at the same time, Unit 4 : Ecosystems -18- www.learner.org
19. but how far bottom-up effects extend in the food web, and the extent to which the effects of trophic interactions at the top of the food web are felt through lower levels, vary over space and time and with the structure of the ecosystem. Figure 12. Predators impose top-down control on ecosystems Many ecological studies seek to measure whether bottom-up or top-down controls are more important in specific ecosystems because the answers can influence conservation and environmental protection strategies. For example, a study by Benjamin S. Halpern and others of food web controls in kelp forest ecosystems off the coast of Southern California found that variations in predator abundance explained a significant proportion of variations in the abundance of algae and the organisms at higher trophic levels that fed on algae and plankton. In contrast, they found no significant relationship between primary production by algae and species abundance at higher trophic levels. The most influential predators included spiny lobster, Kellet's whelk, rockfish, and sea perch. Based on these findings, the authors concluded that "[e]fforts to control activities that affect higher trophic levels (such as fishing) will have far larger impacts on community dynamics than efforts to control, for example, nutrient input, except when these inputs are so great as to create anoxic (dead) zones" (footnote 4). Drastic changes at the top of the food web can trigger trophic cascades, or domino effects that are felt through many lower trophic levels. The likelihood of a trophic cascade depends on the number of trophic levels in the ecosystem and the extent to which predators reduce the abundance of a trophic level to below their resource-limited carrying capacity. Some species are so important to an entire ecosystem that they are referred to as keystone species, connoting that they occupy an ecological Unit 4 : Ecosystems -19- www.learner.org
20. niche that influences many other species. Removing or seriously impacting a keystone species produces major impacts throughout the ecosystem. Many scientists believe that the reintroduction of wolves into Yellowstone National Park in 1995, after they had been eradicated from the park for decades through hunting, has caused a trophic cascade with results that are generally positive for the ecosystem. Wolves have sharply reduced the population of elk, allowing willows to grow back in many riparian areas where the elk had grazed the willows heavily. Healthier willows are attracting birds and small mammals in large numbers. "Species, like riparian songbirds, insects, and in particular, rodents, have come back into these preferred habitat types, and other species are starting to respond," says biologist Robert Crabtree of the Yellowstone Ecological Research Center. "For example, fox and coyotes are moving into these areas because there's more prey for them. There's been an erupting trophic cascade in some of these lush riparian habitat sites." 7. Ecological Niches Within ecosystems, different species interact in different ways. These interactions can have positive, negative, or neutral impacts on the species involved (Table 4). Table 4. Relationships between individuals of different species. Type of interaction Effect of interaction Examples Competition Both species are harmed Oak trees and maple trees (population growth rates are competing for light in a forest, reduced). wading birds foraging for food in a marsh Predation Parasitism One species benefits, one is Predation: wolf and harmed. rabbitParasitism: flea and wolf Mutualism Both species benefit. Humans and house pets, insect Relationship may not be pollination of flowers essential for either. Commensalism One species benefits, one is not Maggots decomposing a rotting affected. carcass Unit 4 : Ecosystems -20- www.learner.org
21. Type of interaction Effect of interaction Examples Amensalism One species harms another Allelopathy (plants that (typically by releasing a toxic produce substances harmful substance), but is not affected to other plants): rye and wheat itself. suppress weeds when used as cover crops, broccoli residue suppresses growth of other vegetables in the same plant family Each species in an ecosystem occupies a niche, which comprises the sum total of its relationships with the biotic and abiotic elements of its environment—more simply, what it needs to survive. In a 1957 address, zoologist George Evelyn Hutchinson framed the view that most ecologists use today when he defined the niche as the intersection of all of the ranges of tolerance under which an organism can live (footnote 5). This approach makes ecological niches easier to quantify and analyze because they can be described as specific ranges of variables like temperature, latitude, and altitude. For example, the African Fish Eagle occupies a very similar ecological niche to the American Bald Eagle (Fig. 13). In practice it is hard to measure all of the variables that a species needs to survive, so descriptions of an organism's niche tend to focus on the most important limiting factors. Figure 13. African fish eagle Courtesy Wikimedia Commons. Public domain. Unit 4 : Ecosystems -21- www.learner.org
22. The full range of habitat types in which a species can exist and reproduce without any competition from other species is called its fundamental niche. The presence of other species means that few species live in such conditions. A species' realized niche can be thought of as its niche in practice —the range of habitat types from which it is not excluded by competing species. Realized niches are usually smaller than fundamental niches, since competitive interactions exclude species from at least some conditions under which they would otherwise grow. Species may occupy different realized niches in various locations if some constraint, such as a certain predator, is present in one area but not in another. In a classic set of laboratory experiments, Russian biologist G.F. Gause showed the difference between fundamental and realized niches. Gause compared how two strains of Paramecium grew when they were cultured separately in the same type of medium to their growth rates when cultured together. When cultured separately both strains reproduced rapidly, which indicated that they were adapted to living and reproducing under the same conditions. But when they were cultured together, one strain out-competed and eventually eliminated the other. From this work Gause developed a fundamental concept in community ecology: the competitive exclusion principle, which states that if two competitors try to occupy the same realized niche, one species will eliminate the other (footnote Many key questions about how species function in ecosystems can be answered by looking at their niches. Species with narrow niches tend to be specialists, relying on comparatively few food sources. As a result, they are highly sensitive to changes in key environmental conditions, such as water temperature in aquatic ecosystems. For example, pandas, which only eat bamboo, have a highly specialized diet. Many endangered species are threatened because they live or forage in particular habitats that have been lost or converted to other uses. One well-known case, the northern spotted owl lives in cavities of trees in old-growth forests (forests with trees that are more than 200 years old and have not been cut, pruned, or managed), but these forests have been heavily logged, reducing the owl's habitat. In contrast, species with broad niches are generalists that can adapt to wider ranges of environmental conditions within their own lifetimes (i.e., not through evolution over generations, but rather through changes in their behavior or physiologic functioning) and survive on diverse types of prey. Coyotes once were found only on the Great Plains and in the western United States, but have spread through the eastern states in part because of their flexible lifestyle. They can kill and eat large, medium, or small prey, from deer to house cats, as well as other foods such as invertebrates and fruit, and can live in a range of habitats, from forests to open landscapes, farmland, and suburban neighborhoods (footnote 7). Overlap between the niches of two species (more precisely, overlap between their resource use curves) causes the species to compete if resources are limited. One might expect to see species constantly dying off as a result, but in many cases competing species can coexist without either being eliminated. This happens through niche partitioning (also referred to as resource partitioning), in which two species divide a limiting resource such as light, food supply, or habitat. Unit 4 : Ecosystems -22- www.learner.org
23. 8. Evolution and Natural Selection in Ecosystems As species interact, their relationships with competitors, predators, and prey contribute to natural selection and thus influence their evolution over many generations. To illustrate this concept, consider how evolution has influenced the factors that affect the foraging efficiency of predators. This includes the predator's search time (how long it takes to find prey), its handling time (how hard it has to work to catch and kill it), and its prey profitability (the ratio of energy gained to energy spent handling prey). Characteristics that help predators to find, catch, and kill prey will enhance their chances of surviving and reproducing. Similarly, prey will profit from attributes that help avoid detection and make organisms harder to handle or less biologically profitable to eat. These common goals drive natural selection for a wide range of traits and behaviors, including: • Mimicry by either predators or prey. A predator such as a praying mantis that blends in with surrounding plants is better able to surprise its target. However, many prey species also engage in mimicry, developing markings similar to those of unpalatable species so that predators avoid them. For example, harmless viceroy butterflies have similar coloration to monarch butterflies, which store toxins in their tissues, so predators avoid viceroy butterflies. • Optimal foraging strategies enable predators to obtain a maximum amount of net energy per unit of time spent foraging. Predators are more likely to survive and reproduce if they restrict their diets to prey that provide the most energy per unit of handling time and focus on areas that are rich with prey or that are close together. The Ideal Free Distribution model suggests that organisms that are able to move will distribute themselves according to the amount of food available, with higher concentrations of organisms located near higher concentrations of food . Many exceptions have been documented, but this theory is a good general predictor of animal behavior. • Avoidance/escape features help prey elude predators. These attributes may be behavioral patterns, such as animal herding or fish schooling to make individual organisms harder to pick out. Markings can confuse and disorient predators: for example, the automeris moth has false eye spots on its hind wings that misdirect predators (Fig. 14). Unit 4 : Ecosystems -23- www.learner.org
24. • Features that increase handling time help to discourage predators. Spines serve this function for many plants and animals, and shells make crustaceans and mollusks harder to eat. Behaviors can also make prey harder to handle: squid and octopus emit clouds of ink that distract and confuse attackers, while hedgehogs and porcupines increase the effectiveness of their protective spines by rolling up in a ball to conceal their vulnerable underbellies. • Some plants and animals emit noxious chemical substances to make themselves less profitable as prey. These protective substances may be bad-tasting, antimicrobial, or toxic. Many species that use noxious substances as protection have evolved bright coloration that signals their identity to would-be predators—for example, the black and yellow coloration of bees, wasps, and yellowjackets. The substances may be generalist defenses that protect against a range of threats, or specialist compounds developed to ward off one major predator. Sometimes specialized predators are able overcome these noxious substances: for example, ragwort contains toxins that can poison horses and cattle grazing on it, but it is the exclusive food of cinnabar moth caterpillars. Ragwort toxin is stored in the caterpillars' bodies and eventually protects them as moths from being eaten by birds. Unit 4 : Ecosystems -24- www.learner.org
27. between autogenic succession—change driven by the inhabitants of an ecosystem, such as forests regrowing on abandoned agricultural fields—and allogenic succession, or change driven by new external geophysical conditions such as rising average temperatures resulting from global climate As discussed above, ecologists often group species depending on whether they are better adapted for survival at low or high population densities (r-selected versus K-selected). Succession represents a natural transition from r- to K-selected species. Ecosystems that have recently experienced traumatic extinction events such as floods or fires are favorable environments for r-selected species because these organisms, which are generalists and grow rapidly, can increase their populations in the absence of competition immediately after the event. Over time, however, they will be out- competed by K-selected species, which often derive a competitive advantage from the habitat modification that takes place during early stages of primary succession. For example, when an abandoned agricultural field transitions back to forest, as seen in Figure 15, sun-tolerant weeds and herbs appear first, followed by dense shrubs like hawthorn and blackberry. After about a decade, birches and other small fast-growing trees move in, sprouting wherever the wind blows their lightweight seeds. In 30 to 40 years, slower-spreading trees like ash, red maple, and oak take root, followed by shade-tolerant trees such as beech and hemlock. A common observation is that as ecosystems mature through successional stages, they tend to become more diverse and complex. The number of organisms and species increases and niches become narrower as competition for resources increases. Primary production rates and nutrient cycling may slow as energy moves through a longer sequence of trophic levels (Table 5). Table 5. Characteristics of developing and mature ecosystems. Ecosystem attributes Developmental stages Mature stages Energetics: Production/respiration More or less than 1 Approaching 1 Production/biomass High Low Food chains Linear Web-like Community structure: Niches Broad Narrow Species diversity Low High Nutrient conservation Poor; detritus unimportant Good; detritus important Nutrient exchange rates Rapid Slow Stability Low High Unit 4 : Ecosystems -27- www.learner.org
28. Many natural disturbances have interrupted the process of ecosystem succession throughout Earth's history, including natural climate fluctuations, the expansion and retreat of glaciers, and local factors such as fires and storms. An understanding of succession is central for conserving and restoring ecosystems because it identifies conditions that managers must create to bring an ecosystem back into its natural state. The Tallgrass Prairie National Preserve in Kansas, created in 1996 to protect 11,000 acres of prairie habitat, is an example of a conservation project that seeks to approximate natural ecosystem succession. A herd of grazing buffalo tramples on tree seedlings and digs up the ground, creating bare patches where new plants can grow, just as millions of buffalo maintained the grassland prairies that covered North America before European settlement (footnote 10). 10. Further Reading Paul A. Colinvaux, Why Big Fierce Animals Are Rare: An Ecologist's Perspective (Princeton, NJ: Princeton University Press, 1979). A survey of major questions in ecology, including why every species has its own niche. Chris Reiter and Gina C. Gould, "Thirteen Ways of Looking at a Hedgehog," Natural History, July/ August 1998. Hedgehogs' spines are unique adaptations, but they have thrived in many regions for millions of years because they are generalists in terms of climate zones and diet. ReefQuest Centre for Shark Research, "Catch As Catch Can," http://www.elasmo-research.org/ education/topics/b_catch.htm. Contrary to their popular image as mindless eating machines, great white sharks' foraging strategies are selective and efficient. 1. One important exception is microbes, which are more diverse in temperate areas; see Unit 9, "Biodiversity Decline," for details. 2. Cornelia Dean, "In An Antarctic Desert, Signs of Life," New York Times, February 3, 1998, p. F1. 3. U.S. Geological Survey, "Mineral Substances in the Environment," http://geology.er.usgs.gov/ 4. Benjamin S. Halpern, Karl Cottenie, and Bernardo R. Broitman, "Strong Top-Down Control in Southern California Kelp Forest Ecosystems," Science, May 26, 2006, pp. 1230–32. 5. G.E. Hutchinson, "Concluding Remarks," Cold Spring Harbor Symposia on Quantitative Biology 22 (1957), pp. 415–27. 6. G.F. Gause, The Struggle for Existence (Baltimore: Williams and Wilkins, 1934). 7. Matthew E. Gompper, The Ecology of Northeast Coyotes, Working Paper No. 17 (New York, NY: Wildlife Conservation Society, July 2002), http://www.wcs.org/media/file/ Unit 4 : Ecosystems -28- www.learner.org
29. 8. S.D. Fretwell and H.J. Lucas, "Ideal Free Distribution," Acta Biotheory 19 (1970), pp. 16–21. 9. Matthew E. Gompper, The Ecology of Northeast Coyotes, Working Paper No. 17 (New York, NY: Wildlife Conservation Society, July 2002), http://www.wcs.org/media/file/ Ecology_of_NE_Coyotes.pdf, pp. 17–20. 10. For more information, see http://www.nps.gov/tapr/index.htm bioaccumulation : The increase in concentration of a chemical in organisms that reside in environments contaminated with low concentrations of various organic compounds. biomes : Broad regional areas characterized by a distinctive climate, soil type, and biological carbon dioxide fertilization : Increased plant growth due to a higher carbon dioxide concentration. carrying capacity : The number of individuals an environment can support without significant negative impacts to the given organism and its environment. coevolution : Simultaneous evolution of two or more species of organisms that interact in significant competitive exclusion principle : The hypothesis stating that when organisms of different species compete for the same resources in the same habitat, one species will commonly be more successful in this competition and exclude the second from the habitat. denitrification : Process of reducing nitrate and nitrite, highly oxidised forms of nitrogen available for consumption by many groups of organisms, into gaseous nitrogen, which is far less accessible to life forms but makes up the bulk of our atmosphere. fundamental niche : The full range of environmental conditions (biological and physical) under which an organism can exist. gross primary productivity (GPP) : The rate at which an ecosystem accumulates biomass, including the energy it uses for the process of respiration. K-selected : Those species that invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood. keystone species : A single kind of organism or a small collection of different kinds of organisms that occupy a vital ecological niche in a given location. latitudinal biodiversity gradient : The increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal gradient in species diversity. Unit 4 : Ecosystems -29- www.learner.org
30. life history strategy : An organism's allocation of energy throughout its lifetime among three competing goals: growing, surviving, and reproducing. mimicry : Evolving to appear similar to another successful species or to the environment in order to dupe predators into avoiding the mimic, or dupe prey into approaching the mimic. mutualistic : Refers to an interaction between two or more distinct biological species in which members benefit from the association. Describes both symbiotic mutualism (a relationship requiring an intimate association of species in which none can carry out the same functions alone) and nonsymbiotic mutualism (a relationship between organisms that is of benefit but is not obligatory: that is, the organisms are capable of independent existence). net primary productivity (NPP) : The rate at which new biomass accrues in an ecosystem. niche partitioning : The process by which natural selection drives competing species into different patterns of resource use or different niches. Coexistence is obtained through the differentiation of their realized ecological niches. nitrogen fixing : The conversion of nitrogen in the atmosphere (N2) to a reduced form (e.g., amino groups of amino acids) that can be used as a nitrogen source by organisms. primary producers : Organisms that produce organic compounds from atmospheric or aquatic carbon dioxide, principally through the process of photosynthesis. Primary production is distinguished as either net or gross. All life on earth is directly or indirectly reliant on primary production. r-selected : Species with a reproductive strategy to produce many offspring, each of whom is, comparatively, less likely to survive to adulthood. realized niche : The ecological role that an organism plays when constrained by the presence of other competing species in its environment species richness : A type of approach to assessing biodiversity that examines the distribution of all resident terrestrial vertebrates: amphibians, reptiles, birds, and mammals. succession : A fundamental concept in ecology that refers to the more or less predictable and orderly changes in the composition or structure of an ecological community. trophic cascades : Occur when predators in a food chain suppress the abundance of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is an herbivore). Trophic cascades may also be important for understanding the effects of removing top predators from food webs, as humans have done in many places through hunting and fishing activities. trophic level : A feeding level within a food web. Unit 4 : Ecosystems -30- www.learner.org
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