TOPIC 18: ECOSYSTEMS Ecology = THE STUDY OF INTERACTIONS BETWEEN ORGANISMS AND THEIR ENVIRONMENT. I. Ecosystem components Species do not live alone, but live in complex environments with other species. Higher-order systems must be defined: A community = ALL THE POPULATIONS LIVING IN A GIVEN SPACE. (Recall that a population = a group of individuals that belong to the same species, interbreed and live in the same place. A community generally contains populations of a number of different species.) A habitat = THE SPACE (ENVIRONMENT) IN WHICH AN ORGANISM OR SPECIES LIVES. The environment INCLUDES ALL THE FACTORS (physical and chemical factors, predators, prey, etc.) THAT AFFECT AN INDIVIDUAL. AN ecosystem = A COMMUNITY PLUS ITS PHYSICAL ENVIRONMENT. The area can be any size, from a small region to the whole biosphere. An ecological niche = THE ROLE AND THE POSITION OF AN ORGANISM IN AN ECOSYSTEM. It is the use the organism makes of the biological and physical resources in its environment. The habitat is part of the niche, as are food, predators etc. II. Symbiosis Any species is part of the environment of the other species in its ecosystem. Intimate relationships can develop between species: Symbiosis = THE LIVING TOGETHER OF INDIVIDUALS OF TWO DIFFERENT SPECIES IN CLOSE PHYSICAL ASSOCIATION. There are three main types of symbiosis, defined according to the benefits of the relationship to the species involved: (1) Commensalism: ONE INDIVIDUAL BENEFITS AND THE OTHER IS NOT APPRECIABLY AFFECTED. (2) Mutualism: BOTH INDIVIDUALS BENEFIT. (3) Parasitism: ONE INDIVIDUAL BENEFITS AT THE EXPENSE OF THE OTHER. III. Population ecology A. Definition We saw last time that populations reach a maximum size. Population ecology = THE STUDY OF THE FACTORS RESPONSIBLE FOR CONTROLLING THE SIZE AND DISTRIBUTION OF POPULATIONS. THEORETICAL MODELS, LABORATORY EXPERIMENTS AND FIELD STUDIES are all used in an attempt to understand these factors. B. Controlling relationships Recall that some features of an ecosystem that affect populations are density-dependent factors - i.e. they depend on the number of individuals in a given area. There are also density-independent factors -- e.g. climate, weather, soil fertility -- which can also be very important. We will consider two prominent types of density-dependent factor that have been proposed: (1) Topic 18 page 1 COMPETITION and (2) PREDATOR-PREY RELATIONSHIPS. (These concepts were actively developed in the 1950's. Their importance, and the way in which they should be incorporated into general theories about the control of populations, are matters of heated debate.) Competition and predator-prey relationships are both DENSITY-DEPENDENT. 1. Competition Competition = UTILIZATION OF THE SAME RESOURCES BY TWO OR MORE INDIVIDUALS OR POPULATIONS. There are two types of competition: a. Intraspecific competition = COMPETITION BETWEEN INDIVIDUALS OF THE SAME POPULATION. The amount of food, habitat space etc. may limit the number of individuals that can be supported in a given location. Although it appears to be an important factor in regulating the size of plant populations, ITS IMPORTANCE IN REGULATING THE SIZE OF ANIMAL POPULATIONS IS A MATTER OF CURRENT CONTROVERSY. b. Interspecific competition = COMPETITION BETWEEN TWO OR MORE SPECIES. Theoretical models, laboratory experiments and field studies all suggest that GENERALLY, TWO SPECIES THAT COMPETE FOR THE SAME NICHE CANNOT COEXIST INDEFINITELY. This implies that, generally, TWO SPECIES THAT APPEAR TO SHARE THE SAME NICHE ACTUALLY HAVE SLIGHTLY DIFFERENT NICHES. (However, two species can compete for the same resource.) 2. Predator-prey relationships a. Description A Predator-prey relationship = A SITUATION IN WHICH ONE FREE-LIVING SPECIES FEEDS UPON ANOTHER. ("Free-living" distinguishes this from parasitism. The prey can be a plant, fungus, protist or bacterium as well as an animal. The predator can also be a plant, in rare cases.) Theoretical models, laboratory experiments and field studies indicate that, under some conditions, THE POPULATION SIZES OF THE PREDATOR AND THE PREY CAN VARY RHYTHMICALLY: b. Mechanism: changing ideas What causes these cyclic variations in population? Various theories have been proposed. One theory is that the predators and prey limit each other?s population. A commonly-quoted example of cyclic variations that has been explained with this theory involves hares and lynx: Variations in the population sizes of HARES and LYNX, as estimated from the numbers of pelts brought in by hunters (largely from records of the Hudson's Bay Company) IN ARCTIC CANADA, FROM 1845- 1935, SHOW CYCLES OF ABOUT TEN-YEAR PERIODS. The idea is that an abundance of rabbits Topic 18 page 2 allows an excess of lynx to be born. The lynx in turn eat too many rabbits, so the rabbit population falls. Some lynx then starve, allowing an abundant rabbit population -- and the cycle is repeated. However, this simple explanation now appears to be incomplete, at least for this example. More recent work indicates that the cyclic variations in hare populations involve three food levels - not only the hares and their predators, but also the food supply of the hares. The hares probably limit the lynx population, but the hare population appears to be limited by a combination of predation and varying food supply. Many workers now consider such cyclic curves to be due to more complex interactions than simple predator-prey relationships. [See Campbell, pp. 1189-1190.] III. Ecosystem economy Some general statements can be made about the flow of energy and materials through a complete ecosystem. A. Energy flow 1. Food chain (web) Recall that almost ALL ENERGY FOR LIFE COMES FROM SUNLIGHT VIA PHOTOSYNTHESIS. But only ~0.1% of the sunlight falling on earth is used for photosynthesis. THIS ENERGY THEN FLOWS THROUGH A SERIES OF ORGANISMS IN THE COMMUNITY (as food); this series of organisms is called a food chain. Recall that the photosynthetic plants are autotrophs = ORGANISMS THAT MAKE THEIR OWN REDUCED ORGANIC COMPOUNDS. All the other organisms in the food chain are heterotrophs = ORGANISMS THAT NEED AT LEAST SOME ORGANIC COMPOUNDS IN THEIR FOOD. In this context, AUTOTROPHS = PRODUCERS; HETEROTROPHS = CONSUMERS. [See Campbell, p. 1205-1206. Consumers can be a source of secondary production.] A generalized sketch looks like: IN REAL SYSTEMS, THERE ARE SO MANY SPECIES AT EACH LEVEL, AND SO MANY PATHWAYS Topic 18 page 3 THAT FOOD TRAVELS, THAT THE FOOD "CHAIN" IS ACTUALLY A food web. 2. Pyramid of productivity Recall that the second law of thermodynamics implies that EVERY ENERGY TRANSFORMATION INVOLVES A LOSS OF FREE ENERGY. This implies that THERE IS A LOSS OF FREE ENERGY AT EACH LEVEL OF THE FOOD CHAIN. It turns out that THE FREE ENERGY REQUIRED / BIOMASS IS ROUGHLY INDEPENDENT OF THE TYPE OF ORGANISM. These two statements imply that THE TOTAL AMOUNT OF MATERIAL (i.e. the biomass = MASS STORED IN LIVING THINGS, or the weight) AT EACH LEVEL OF THE FOOD CHAIN IS LESS THAN THAT OF THE PRECEDING LEVEL. That is, there is a loss of biomass at each level of the food chain. In practice, organisms do not store energy very efficiently, so that EACH LEVEL OF THE FOOD CHAIN HAS ONLY ~10% OF THE BIOMASS OF THE PRECEDING LEVEL. B. Material cycles Free energy cannot be recycled, but there is a continual input (via sunlight). The situation is reversed for raw materials: They are not continually used up and replaced with new ones, but they are recycled. THERE ARE CYCLES OF MATERIALS IN THE BIOSPHERE. The cycles of a number of elements have been studied. We will consider the CARBON CYCLE. [See Campbell, pp. 1231-1234, for the water, carbon, nitrogen and phosphorus cycles.] There is a steady flow of carbon between CO 2 and organisms: (The solid arrows are short-term turnover; the dashed arrows are long-term turnover.) Some of the carbon has remained in reduced form, deposited as FOSSIL FUELS, and some is oxidized in rocks as LIMESTONE (calcium carbonate). IN THE TWENTIETH CENTURY, WE HAVE BEEN RELEASING LARGE AMOUNTS OF THE STORED FOSSIL-FUEL CARBON AS CO 2 , (AND CUTTING DOWN FORESTS, which decreases the amount of CO 2 converted to O 2 + sugar). As a result, ITS CONCENTRATION IN THE ATMOSPHERE HAS BEEN INCREASING (from 0.029% before 1900 to over 0.034% in 1990 -- an increase of ~17%). This CO 2 absorbs heat (i.e. infrared radiation) that is radiated from the earth, and re-radiates it back to the earth. An increase in CO 2 in the atmosphere can therefore raise the temperature of the earth's surface. Topic 18 page 4 Stuart Goldstein 1009-18.cwk
Want to see the other 4 page(s) in 1009-18 notes.pdf?JOIN TODAY FOR FREE!