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Ecology Review What is ecology? Word is derived from the green Oikos, meaning home. Coined by the German biologist Ernst Haeckel in the 19th Century. There are a number of definitions. Most focus on the interactions between organisms and their environment. Study of the "household". More informative definition - Krebs (1972): Ecology is the scientific study of the interactions that determine the distribution and abundance of organisms. Environment occupies central role. Environment includes both biotic and abiotic factors. Ecology deals with three levels of concern: 1. The individual organism How does the environment (biotic and abiotic) affect the individual organism? 2. The population, consisting of individuals of the same species. Deals with the presence or absence of particular species, their relative abundance, and trends or fluctuations in their numbers. 3. The community, consisting of a number of interacting populations Deals with the composition and structure of communities, and with the functioning of communities, and with the movement of energy and mater through them Approaches to Ecology: A number of different approaches have developed: a. Physiological. b. Population. c. Community. d. Behavioral e. Ecosystem. f. Landscape. Effects on Individuals The distribution of black and white spruce in Canada, showing the northern distributional limit of these trees. The northern and eastern boundaries of the range of the saguaro cactus in Arizona. Dots represent locations where there are no records of periods longer than 36 hours without a thaw. Crosses represent locations where such periods have been recorded. Even mammal distributions are controlled by temperature. The northern limit of the distribution of the flying squirrel is related to temperature. In the northern extremes of their range, they use huddling behavior to stay warm. Temperature and Performance of Organisms • Most species perform best within a narrow range of temperatures • Begins at enzyme level – – – – Rigid shape at low temperature Excessively high temperatures destroy their shape Work best at some intermediate temperature Can measure enzyme effectiveness by determining substrate concentration required for enzyme function at a particular temperature. Low substrate concentration = high enzyme affinity Homeostasis – an organism’s physiological mechanisms that work to maintain a constant internal environment in the face of varying external factors. Homeostatic mechanisms typically require the expenditure of energy, and can typically operate only within a narrow range of a factor. Life Histories Life History: refers to any aspect of the developmental pattern and mode of reproduction of an organism. We can consider five fundamental aspects of life history: • • • • • Size Metamorphosis Diapause Senescence Reproductive Patterns Frequency distribution with respect to log body mass for North American mammals, birds, and freshwater fish. The curve of reproductive power as a function of log body mass, as predicted. Closely follows observed distribution. Metamorphosis – Organisms that metamorphose undergo radical changes during development. What sort of fitness advantage could outweigh the complications of such a strategy? 1. Exploitation of habitats with high, but transient, productivity? 2. Dispersal? 3. Reduction of competition? Diapause A stage in the life cycle characterized by a cessation of development and a protein synthesis, and by suppression of the metabolic rate. Red kangaroo reproductive cycle Senescence – the timing of aging and death. Why must organisms die? Could the timing of death have evolved? Why do some organisms live longer than others? Some recent findings suggest that life span has some genetic basis. Many insects have very short life spans. The adult mayfly may live only a few hours. Annual plants have a lifespan of less than a year. Galapagos tortoises live at least 150 years, and perhaps as much as 200 years. If this is true, there may be tortoises in the Galapagos that were youngsters when Darwin was there. Atlantic sturgeon may live 150 years. Methuselah Grove in the Ancient Bristlecone Pine Forest of Inyo National Forest in eastern California. The world’s oldest living thing lives here, a bristlecone pine 4,723 years old. It is not identified for its own protection. How does life span evolve? Currently, there are two main hypotheses to explain the process of senescence 1. Mutation accumulation hypothesis. 2. Evolutionary senescence hypothesis. Reproductive Strategies Organisms have only a certain amount of energy available to them for reproduction. 1. Species must make an evolutionary “decision” on how to apportion that energy. Clutch size, parental care, age at reproduction, etc. There are a series of tradeoffs. 2. A relationship exists between the demography of the species and its reproductive pattern. Reproduction and mortality interact. Each reproductive effort may be expected to increase the mortality rate. Now consider the trade-offs. 1. Clutch size. 2. Present versus future reproduction. 3. Age at sexual maturity. The theory of r and K selection. Suggests that organisms can be placed into two fundamental groups on the basis of their position on the sigmoid growth curve and the resulting life histories. Populations Population: a group of individuals of a single species inhabiting a specific area. Usually implies interaction, and often implies a shared gene pool Spatial structure of populations has three main attributes….. • Distribution • Dispersion • Density Distribution: What controls where organisms are found? The physical environment limits the geographic distribution of species. Plants in the genus Encelia show distributions along a moisturetemperature gradient from the California coast eastward. Dispersion: Where are organisms found in relation to one another? We often recognize three patterns of dispersion, which have different indications for the biology of the species. Distribution patterns of the creosote bush of the American southwest are well studied. Density – the number of individuals per unit area or volume. Estimating population size is critical to the study of population dynamics. It is rarely possible to count all members of a population. Whale population sizes have been estimated using mark-andrecapture techniques for years. Classification of Commonness and Rarity Deborah Rabinowitz devised a system for classifying commonness and rarity based on combinations of three factors: 1. Geographic range – extensive versus restricted. 2. Habitat tolerance – broad versus narrow. 3. Local population size – large versus small. We can use this system of classification to come up with eight possible combinations of these factors, which give us seven forms of rarity, and one of abundance. Population Dynamics Population dynamics involves the “behavior” of a population. What happens to it over time. At the core of population dynamics are life and death. Nnow = Nthen + B – D + I - E Some Definitions • • • • • • Demographic process – a process capable of changing the size or composition of a population Life table – a summary of the age- or stage-related survivorship of individuals in a population Mortality – death rate of individuals in a population. The probability that a representative newly-born individual will die before reaching a certain age. Survivorship – the complement of mortality. The probability of an individual surviving to a given age. Fecundity – the number of eggs, seeds, or offspring produced by an individual. Fecuncity schedule – a data table displaying lifetime birth patterns among individuals of different ages in a population. Life table and survivorship curve Mortality rate among perennial plant population Three types of survivorship curves Age distribution of white oak population Age distribution of Rio Grande cottonwoods population Size and generation time Sex Ratios: another way in which we can examine population structure Defined as the ratio of males to females in the population. We can recognize sex ratios at four different stages: Primary Secondary Tertiary Quaternary Sex ratio can change over the life history of an organism. Population Growth When resources are unlimited, populations may be expected to grow at an exponential rate. In rare circumstances, we may see this in nature. N t N 0e rt Equation describing exponential population growth. In derivative form, may be written as: dN dt rN Typically, however, we expect population growth to slow at increasing population size, resulting in this “sigmoid” growth pattern. Some organisms, like these English herons, show small-magnitude irregular fluctuations. Others, such as the moth Dendrolimus, show large-scale fluctuations on an irregular timescale. Collared lemmings in the arctic show tremendous variation on a very predictable four-year cycle. We may also see regular cycles in abundance. House mice may show “irruptions”, in which they live at low population densities for long periods, then explode. Time now to introduce the concept of competition: an interaction between individuals or species over a limiting resource that negatively affects the fitness of one or both. We divide competition into two classes: Intraspecific competition – between members of the same species. Interspecific competition – between members of different species. Must define a couple of other terms as well: Density-dependent effects: situation occurring when population regulation is related to the density (size) of the population. Density-independent effects: situation occurring when population regulation occurs in a manner that is unrelated to the density of the population. For several decades, ecologists have debated the relative significance of different factors on the regulation of natural populations. The main dichotomy centers on the relative importance of density-dependent versus density-independent factors. In general, we now believe that densitydependent factors have to play a role. In fact, such things as density-dependent effects on fecundity and survival have come to define population regulation. We believe that a number of factors can regulate population size in a density-dependent fashion. We further believe that we can group those factors into extrinsic and intrinsic factors. Extrinsic Factors: Food supply Extrinsic Factors: Predation Densities of wolves and moose on Isle Royale are related in a rather complicated manner. Extrinsic Effects: Parasitism and Disease Prevalence of brucellosis in Yellowstone bison increases with population size. For density-dependent factors, the birth rate and the mortality rate tend to change with population density….. ….. while for densityindependent factors these rates are unrelated to population size. Intrinsic Mechanisms? Is it possible that populations could be “self-regulating”, with intrinsic factors such as stress, territoriality, and dispersal playing a primary role in controlling population size? Several types of possible intrinsic mechanisms have been suggested: Social Stress Hypothesis Territoriality Genetic Polymorphism Dispersal Nonequilibrium Ideas Early in the debate, two Australian entomologists, H.G. Andrewartha and L.C. Birch, were emphasizing the importance of densityindependent factors like weather on population density. There is currently a lot of interests in these ideas, stemming in part from the availability of long-term data sets in stable environments. There seems to be good evidence that many populations do not behave in an equilibrium-type fashion. Abiotic Extrinsic Regulation: Many factors could be involved. Rainfall for instance. Density of Larrea at 11 sites in the Mojave Desert Doesn’t seem to be related to food limitation, because the density doesn’t decline as the density of competing shrubs increases. Rainfall flucuates greatly in the Galapagos. The density of two species of Darwin’s finches seems to respond. Community Structure Community: The entire assemblage of interacting species in a given area. We must determine the extent of the interactions that we want to use to define the community. Must also distinguish communities from taxonomic associations and guilds. Consider two communities: The great lakes of the African Rift Valley are inhabited by a large number of fish belonging to the family Cichlidae. More than 250 species are found in Lake Victoria alone. They have undergone an adaptive radiation that has allowed them to diverge into different niches including at least ten different trophic styles. There is apparently a great deal of niche overlap. A combination of the number of species and their relative abundance defines species diversity A commonly applied measure of species diversity is the Shannon-Wiener index: s H pi log pi i 1 Plots of species number versus sampling effort typically only show part of this normal curve. Much sampling effort is needed to find the rare species. Rank abundance curves show the proportional abudance of a given species plotted versus their rank abundance. This shows the evenness of the community at a glance. Two major groups of ideas regarding community structure have been developed. They are typically categorized as equilibrium and nonequilibrium explanations. Do communities reach equilibrium, with the structure at equilibrium being determined by biological processes occuring within it? Or do external disturbances prevent equilibrium from being established. Equilibrium Approaches to Community Structure The Effect of Interspecific Competition: How are the coexisting species in a community related to one another. How are the niches arranged in highly diverse communities. Two phenomena can be viewed suggesting that interspecific competition plays a real role in community structure. 1. Changes in Niche Dimensions Recall the idea of the realized versus the fundamental niche. Robert MacArthur studied the way in which resources are divided among five similar species of warblers in coniferous trees in northern forests. The birds are very similar in size and in bill shape. All are insectivores. MacArthur showed that the five species are actually foraging at slightly different locations within the tree. They are not sharing the same niche. 2. Patterns of Species Distribution We saw earlier the results of Jared Diamond’s study on the distribution of cuckoo doves in the Bismarck Archipelago. This seems to be strong evidence that the bird communities on these islands are determined at least in part by competition through competitive exclusion. Equilibrium Approaches to Community Structure The Effect of Predation: Predation is also known to play a major role in organizing communities. Interesting, the mechanism by which predation is thought to structure communities includes a major role for competition. One of the classic studies leading toward an understanding of the role of predation in structuring communities was conducted by Robert Paine in the intertidal zone of Neah Bay, Washington. Paine found that when he removed the sea star Pisaster from experimental plots, species diversity was significantly lower. Only 8 of 15 species remained in communities protected from Pisaster predation. Paine believed this was related to competition. In the absence of sea stars, the mussel Mytilus came to dominate the community and eliminated several species. Competitive exclusion by a few dominant species led to decreased community diversity. This phenomenon gave rise to the keystone predator hypothesis. A keystone predator is one whose presence is central to the organization of the community. Nonequilibrium Approaches The Role of Disturbance: Many abiotic factors (fire, eruptions, floods, storms, etc.) disturb communities in ways that affect the coexisting species. These factors have different impacts on the various species and thus affect community organization. Joe Connell introduced the intermediate disturbance hypothesis. He felt that the frequency and intensity of disturbance determined the importance of processes such as competition and predation. At high levels of disturbance, species diversity is low because few species can tolerate the disturbance. At low levels of disturbance, dominant competitors may drive other species to extinction. Diversity is highest at intermediate levels of disturbance.