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Ecology - An Introduction Ecology comes from Greek root words oikos - home and logos - to study. Thus a definition: Ecology is the study of the distribution and abundance of species (or populations). It is also the study of causes of those distributions and abundances. We can study distributions and abundances at many levels: at the population level at the species level - a collection of populations at the community level - where other species interact with the one under study at the ecosystem level - where we actively consider the impact of the physical environment on the species at the biome level - combining many similar ecosystems, e.g. temperate forests, deserts at the biosphere level - combining all the different ecosystems (or biomes) on earth. 1) the individual level - Adaptations in behaviour, physiology & morphology; relations between organisms and abiotic conditions are studied. 2) the population level - factors affecting population size, density, and composition are the subjects. We look at life histories, demography, and interactions among individuals within populations. 3) the community level - a community is the set of populations of different species living in a place. We study interactions among populations and how they affect structure and organization of communities 4) the ecosystem level - here we consider the community in its abiotic context. Energy flow through the set of populations and nutrient cycling both among populations and into/out of non-living components (soil, air,…) are included. 5) the landscape level - may not be completely distinct from the ecosystem level, but it studies different phenomena. At this level we look at exchanges of organisms and materials between habitats. It regards the environment as a mosaic, and has been important in developing ideas about how to conserve species. 6) the biosphere level - the biosphere is the sum of all earth’s ecosystems. If abiotic conditions are important in determining where species live, what conditions are most important? Light, temperature, and precipitation Light: (each sunbeam carries the same total energy) Light intensity (energy/unit area) declines with increasing latitude, thus temperature does as well. The pattern is shown in Fig. 34.6A Both light and temperature also vary seasonally. Reason: the tilt of the earth’s axis with respect to its orbit around the sun. At any given place on earth (latitude) the angle at which sunlight strikes the surface changes with the season. Our map latitude is ~42° N. That’s also ‘solar latitude’ at the equinoxes. But on June 21 our ‘solar latitude’ is ~19° and on December 21 it’s ~65°. See Figure 34.6B Sunlight heating the atmosphere (combined with the rotation of the earth on its axis) explains winds and precipitation patterns. Air is heated most at the equator. It rises and cools. Relative humidity rises. Rain results. In the upper atmosphere this rising air spreads and cools. Cold, dense air settles back toward the surface at around 30º. It warms as it falls. Relative humidity decreases. Rain is unlikely. See Figure 34.6C There are more such cells as you move to higher latitude - 3 pairs in all. These cells explain latitudinal patterns of rainfall, but represent only vertical and north/south movements of air. The earth rotates, and winds are deflected east/west by the rotation. The deflections result from the Coriolus effect. Global wind and water circulation is driven by the combined effect of atmospheric heating and rotation. Local topography also influences precipitation. As air rises over mountains, it cools, relative humidity rises. At some altitude, the air becomes saturated, and it rains. Over the mountain tops the air falls, it warms, relative humidity drops, and rain is unlikely. Mountains cause a rain shadow. This is the pattern at the Sierra Nevada mountains of California. See Figure 34.6F Biomes We divide the earth’s life forms into biomes. Each biome represents a grouping of species at a fairly large scale, but a grouping representing the kinds of life found under a particular set of climatic conditions. We can divide the biomes into aquatic and terrestrial ones. The aquatic biomes can be separated into fresh water and marine types. Combined they cover ~75% of the earth’s surface. The freshwater biomes: 1.Eutrophic lakes - nutrient rich, generally shallow, with high rates of photosynthesis and high rates of decomposition in the benthos (bottom). The decomposition can cause anoxic conditions in deeper parts of the lake. 2.Oligotrophic lakes - typically deeper, nutrient poor, with low rates of photosynthesis, and a low density of phytoplankton. With less decomposition they are rarely anoxic. Crater Lake in Oregon 3. The last type of freshwater biome is flowing water rivers and streams. In them there is a wide range of communities, nutrient levels, and oxygen content. What determines conditions is flow rate, the environment through which the water flows, and the turbulence of flow. There are transition zones between fresh water and marine ecosystems. They are called estuaries. They are typically nutrient rich and among the most productive places on earth. Along Chesapeake Bay and much of the Everglades are large estuaries. There are also transition zones between fresh water and terrestrial ecosystems. They are called wetlands (marshes, bogs, and fens). Part of Point Pelee park and much of the Walpole Island Reservation is wetland. Marine communities are divided by physical criteria depth, light penetration, and distance from shore. Here is a map of the zones: At the shoreline is a region called the intertidal zone. Here organisms may be exposed as the tide goes out and submerged when it comes in. Different kinds of organisms and different species within groups are adapted to the different degrees of exposure of upper, mid-, and lowermost zones of the intertidal. The terrestrial biomes are more diverse. Here is a map of the major terrestrial biomes: Tropical forest – one of the most diverse community types on earth. It occurs only where rainfall exceeds 2500mm per year; temperature is >18ºC and changes <3ºC during the year). Trees do not lose their leaves at any given time of the year, though there may be a 3 month season of lower rainfall. Soils are nutrientpoor, since nutrients are bound up in living tissues. Deserts – have dry climates year-round. Most deserts are found near 30ºN or S latitude. The common image is of the Sahara that is nearly devoid of vegetation. However, deserts can have diverse flora and fauna that are adapted to dry conditions in various ways (e.g. cactuses have needles instead of leaves, desert rodents exude a urine that is concentrated uric acid, enormously reducing water loss). Tundra – a Russian word meaning treeless plain. The community lives above permanently frozen ground (permafrost). It is quite wet. Though rainfall is not high, permafrost prevents rain from percolating down through the soil; it puddles and ponds. The growing season is short, no more than 2 months. Plants and animals adapt to the short season. Boreal forest – in Russian, called taiga. These are the coniferous forests between temperate deciduous forest and tundra. Summer is relatively short, winter long and cold. Evergreen trees have the energetic advantage of not having to develop new leaves each year. There are fewer insects than at lower latitude, and plants are mostly wind-pollinated. Temperate grassland (prairie) – temperate communities where limited precipitation (generally 25 -75cm/yr) limits the growth of trees, which are rare. Typically far removed from large bodies of water, and towards the center of continents. This is the biome altered by human activity to develop land for agriculture. Plants are adapted for herbivory (e.g. bison). Chapparral – occurs where there are winter rains and summer drought (a Mediterranean climate). Typically treeless, with grasses and shrubs. Shrubs are adapted to the low rainfall, with sclerophyllous (leathery) leaves that limit water loss. The community is also adapted for the frequent fires that ignite during the dry summer.