Chapter 15MutualismWord下载.docx

1、Meanwhile, back aboveground a deer enters the forest glade and wanders over to the plant recently visited by the hummingbird. The deer systematically grazes it to the ground, lightly chews the plant material, and then swallows it. As the plant material enters the deers stomach, it is attacked by a v

2、ariety of protozoans and bacteria. These microorganisms break down and release energy from compounds such as cellulose, which the deers own enzymatic machinery cannot handle. In return, the protozoans and bacteria receive a steady food supply from the feeding activities of the deer as well as a warm

3、, moist place in which to live.FIGURE 15.1 Hummingbirds feeding on nectar transfer pollen from flower to flower.These are examples of mutualism, that is, interactions between individuals of different species that benefit both partners. Some species can live without their mutualistic partners and so

4、the relationship is called facultative mutualism. Other species are so dependent upon the mutualistic relationship that they cannot live in its absence. Such a relationship is an obligate mutualism. It is a curious fact that though observers of nature as early as Aristotle recognized such mutualisms

5、, mutualistic interactions have received much less attention from ecologists than have either competition or exploitation. Does this lack of attention reflect the rarity of mutualism in nature? As you will see in the following pages, mutualism is virtually everywhere.Mutualism may be common, but is

6、it important? Does it contribute substantially to the ecological integrity of the biosphere? The answer to both these questions is yes. Without mutualism the biosphere would be entirely different. Lets remove some of the more prominent mutualisms from the biosphere and consider the consequences. An

7、earth without mutualism would lack reef-building corals as we know them. So we can erase the Great Barrier Reef, the largest biological structure on earth, from our hypothetical world. We can also eliminate all the coral atolls that dot the tropical oceans as well as all the fringing reefs. The deep

8、 sea would have no bioluminescent fishes or invertebrates. In addition, the deep-sea oases of life associated with ocean floor hot-water vents, discovered just two decades ago （see chapter 6）, would be reduced to nonmutualistic microbial species. On land, there would be no animal-pollinated plants:

9、no orchids, no sunflowers, and no apples. The pollinators themselves would also be gone: no bumblebees, no hummingbirds, and no monarch butterflies. Gone too would be all the herbivores that depend on animal-pollinated plants. Without plant-animal mutualisms tropical rain forests, the most diverse t

10、errestrial biome on the planet, would be all but gone. Many wind-pollinated plants would remain. However, many of these species would also be significantly affected since approximately 90% of all plants form mycorrhizae. Those plants capable of surviving without mycorrhizal fungi would likely be res

11、tricted to the most fertile soils.Even if wind-pollinated, nonmycorrhizal plants remained on our hypothetical world there would be no vast herds of African hoofed mammals, no horses, and no elephants, camels, or even rabbits or caterpillars. There would be few herbivores to feed on the remaining pla

12、nts since herbivores and detritivores depend upon microorganisms to gain access to the energy and nutrients contained in plant tissues. The carnivores would disappear along with the herbivores. And so it would go. A biosphere without mutualism would be biologically impoverished.The impoverishment th

13、at would follow the elimination of mumalism, however, would go deeperthan we might expect. Lynn Margulis and others （Margulis and Fester 1991） have amassed convincing evidence that all eukaryotes, both heterotrophic and autotrophic, originated as mutualistic associa, tions between different organism

14、s. Eukaryotes are apparently the product of mutualistic relationships so ancient that the mutualistic partners have become cellular organelles （e.g., mitochondria and chloroplasts） whose mumalistic origins long went unrecognized. Consequently, without mutualism all the eukaryotes, from Homo sapiens

15、to the protozoans, would be gone and the history of life on earth and biological richness would be set back about 1.4 billion years.But back here in the present, lets accept that mutualism is an integral part of nature and review what is known of the ecology of mutualism. The first part of this brie

16、f review emphasizes experimental studies. Then, in the last part of the chapter, we examine some theoretical approaches to the study of mutualism.CONCEPTS Plants benefit from mutualistic partnerships with a wide variety of bacteria, fungi, and animals. Beef-building corals depend upon mutualistic re

17、lationships with algae and animals. Theory predicts that mutualism will evolve where the benefits of mutualism exceed the costs.CASE HISTORIES: plant mutualismsPlants benefit from mutualistic partnerships with a wide variety of bacteria, fungi, and animals.Plants are the center of mumalistic relatio

18、nships that provide benefits ranging from nitrogen fixation and nutrient absorption to pollination and seed dispersal. It is no exaggeration to say that the integrity of the terrestrial portion of the biosphere depends upon plant-centered mutualism. However, to understand the extent to which ecologi

19、cal integrity may depend upon these relationships we need careful observational studies and experiments. Here are some drawn from studies of mycorrhizae.Plant Performance and Mycorrhizal FungiThe fossil record shows that mycorrhizae arose early in the evolution of land plants, perhaps as long as 400

20、 million years ago. Over evolutionary time, a mutualistic relationship between plants and fungi evolved in which mycorrhizal fungi provide plants with greater access to inorganic nutrients while feeding off the root exudates of plants. The two most common types of mycorrhizae are （1） arbuscular myco

21、rrhizal fungi （AMF）, in which the mycorrhizal fungus produces arbuscules, sites of exchange between plant and fungus, hyphae, fungal filaments, and vesicles, fungal energy storage organs within root cortex cells, and （2） ectomycorrhizae （ECM）, in which the fungus forms a mantle around roots and a ne

22、tlike structure around root cells （fig. 15.2）. Mycorrhizae are especially important in increasing plant access to phosphorus and other immobile nutrients （nutrients that do not move freely through soil） such as copper andzinc, as well as to nitrogen and water. FIGURE 15.2 Mutuatistic associations be

23、tween fungi and plant roots: （a） arbuscular mycorrhizal fungus stained so that fungal structures appear blue; and （b） ectomycorrhizae, which give a white fuzzy appearance to these roots.Mycorrhizae and the Water Balance of PlantsMycorrhizal fungi appear to improve the ability of many plants to extra

24、ct soil water. Edie Allen and Michael Allen （1986） studied how mycorrhizae affect the water relations of the grass Agropyron smithii by comparing the leaf water potentials of plants with and without mycorrhizae. Figure 15.3 shows that Agropyron with mycorrhizae maintained higher leaf water potential

25、s than those without mycorrhizae. This means that when growing under similar conditions of soil moisture, the presence of mycorrhizae helped the grass maintain a higher water potential. Does this comparison show that mycorrhizae are directly responsible for the higher leaf water potential observed i

26、n the mycorrhizal grass? No, they do not. These higher water potentials may be an indirect effect of greater root growth resulting from the greater access to phosphorus provided by mycorrhizae. FIGURE 15.3 Influence of mycorrhizae on leaf water potential of the grass Agropyron smithii （data from All

27、en and Allen 1986）.Plants with greater access to phosphorus may develop roots that are more efficient at extracting and conducting water; mycorrhizal fungi may not be directly involved in the extraction of water from soils. Kay Hardie （1985） tested this hypothesis directly with an ingenious experime

28、ntal manipulation of plant growth form and mycorrhizae. First, she grew mycorrhizal and nonmycorrhizal red clover, Trifoliurn pratense, in conditions in which their growth was not limited by nutrient availability. These conditions produced plants with similar leaf areas and root:shoot ratios. Under

29、these carefully controlled conditions, mycorrhizal red clover showed higher rates of transpiration than nonmycorrhizal plants. Hardie took her study one step further by removing the hyphae of mycorrhizal fungi from half of the red clover with mycorrhizae. She controlled for possible side effects of

30、this manipulation by using a tracer dye to check for root damage and by handling and transplanting all study plants, including those in her control group. Removing hyphae significantly reduced rates of transpiration （fig. 15.4）, indicating a direct role of mycorrhizal fungi in the water relations of plants. Hardy sugges