Geography of Communities Often, groups of unrelated species will show similar distribution patterns. Spruces, kinglets, and red-backed voles are all found in coniferous forests that spread across the northern part of North America and extend to the south at high elevations. Species occur together in assocations known as ecological communities. The nature of these associations was a subject debate for much of the 20th Century. Many of the earliest community ecologists studied plant associations. F.E. Clements compared the community to a “superorganism”, with its own life and structure. Community – an assemblage of different species living together in the same place, typically interacting with one another. Ecosystem – an assemblage of organisms considered together with the physical factors acting upon them. Ecologists study two primary aspects about communities: Community structure – includng composition, biomass, diversity, etc. Community function – movement of energy through the system, changes over time, etc. Two characteristics of species that impact their effect on community organization are their: Body mass Trophic status The larger an organism, the more energy it takes to maintain it. Basal metabolic rate (m) varies with body mass (M) according to the relationship: m cM 0 . 75 with the constant c varying somewhat among taxonomic groups. Since the exponent is less than 1, this means that the mass-specific metabolic needs (per unit of mass) are greater for small organisms than for larger ones. A mouse needs about 25 times more energy per gram of body mass than an elephant. Body size is significant in that it influences the scale at which organisms interact with the environment. Since small organisms use fewer resources, they can utilize smaller areas and specialize on more specific resources, yet still maintain population densities high enough to avoid extinction. They can subdivide the environment more finely. An examination of frequency distribution of the number of species relative to body size for different animal groups reveals the following: There is a general pattern of small species being more numerous than larger ones (note the logarithmic scale). There are many more species of smaller organisms (like insects) than larger organisms (like birds and mammals). Large animals must have broad geographic ranges. They require more space, and a larger area of the environment is required to provide the resources they need. Mountain lion range in California A depiction of the geographic range and body mass among North American terrestrial mammals. In general, larger mammals require a larger home range. The areas of the smallest rangest ranges are smaller for herbivores. Trophic status refers to the manner by which they acquire energy. This also influences the role they play in community structure. For virtually all organisms, the ultimate source of energy is the sun. Food chains are made up of trophic levels. Quarternary consumer (third-level carnivore) Tertiary consumer (second-level carnivore) Secondary consumer (first-level carnivore) Primary consumer (herbivore) Primary producer Most communities have more complex trophic relationships that are illustrated as a “food web”. The laws of thermodynamics come into play in the consideration of food chains. First law of thermodynamics – energy is neither created nor destroyed, but may be converted from one form into another. Second law of thermodynamics – as energy is converted from one form to another, its capacity to do work is diminished and entropy increases (in other words, the process is imperfect – there is a loss of energy at each step). Most animals are able to incorporate a very small percentage of the energy they ingest. The efficiency of transfer is often less than 10%. This means that less energy is available at successively higher trophic levels. This can be illustrated in the form of ecological pyramids (next slide). Since the carrying capacity of an area is lower for successively higher trophic levels, predictions can be made about their ecological roles and geographic distributions. There are fewer species of carnivores than of herbivores and plants. Carnivores tend to be larger and more generalized than herbivores. Carnivores typically have to be large enough to overpower their prey. Carnivores tend to be generalists in their prey selection. Carnivores tend to have broad geographic ranges. Distribution of Communities in Space and Time The distribution of species along environmental gradients can differ greatly depending on the nature of the species and the nature of the gradient. The study of such patterns, known as coenoclines, has resulted in a number of hypotheses about how communities transition from one to another. A. Groups of species exhibit similar ranges along the gradient and are distributed as discrete communities. B. Individual species abruptly exclude one another along sharp boundaries, but most species are not associated in discrete communities. C. Species form discrete communities, but replacement of these communities along a gradient is gradual. D. Individual species gradually appear and disappear, independent of other species. Species replacement along a gradient is random. There are no discrete communities. E. Ranges of most species are nested within the ranges of a few dominant species. Five Hypothetical Coenoclines Actual coenoclines showing the distributions of tree species along two moisture gradients. In these cases, species replacement is gradual and seemingly independent of one another. This corresponds to pattern D in the previous figure. However, there are some who believe this may be an artifact resulting from the manner in which the data was collected. Elevational distribution of three-needled (A) and fiveneedled (B) pines on the western slopes of the Sierra Nevada. Similar species show little overlap in elevation. There also seems to be some degree of community organization. Ecological succession is the progressive change in community structure and function over ecological time. In short, one assemblage is replaced over time by another assemblage. Primary Succession Succession is a normal process in any system in which disturbance eliminates entire communities. If the substrate is removed and the community must start “from scratch”, we call it primary succession. If the soil remains, it is known as secondary succession. Secondary Succession We can also examine paleoecological records. Pollen analysis lets us look at the historical distribution of tree species. We can see that distribution of beech and hemlock were considerably further south after the Pleistocene glaciation (~10,000 years ago). World Distribution of Major Terrestrial Biomes A climograph is a way of examining the relationship of terrestrial biomes to two critical environmental factors, mean annual temperature and mean annual precipitation. Climate diagrams provide similar information. These diagrams were developed by Heinrich Walter to help examine the relationship between climate and terrestrial vegetation. They summarize much climatic information. With some practice, a glance at a climate diagram can give an overview of a location’s climate. Climate diagram for a rain forest Climate diagram for a hot desert Climate diagram for a cold desert Global Distribution of Tropical and Temperate Rain Forests Tropical Rain Forests Tropical rain forests are found near the equator in three primary regions: Southeast Asia, West Africa, and South and Central America. Most rain forest occurs within 10º of latitude north or south of the equator. The distribution of rain forest corresponds to areas where conditions are warm and wet yearround. Temperatures vary little from month to month. They are not extremely hot. Average temperatures are typically from 25 º to 27 º C. Annual rainfall ranges from 2000 to 4000 mm Tropical rain forest distribution and climate Tropical rain forest soils are typically poor, as heavy rains leach nutrients and rapid decomposition keeps soil organics low. Rain forest plants are adapted to conserving nutrients. They often make use of mutualistic relationships with fungi associated with their roots. These relationships are known as mycorrhizae. Epiphyte mat in rain forest canopy Life in tropical rain forests is very much three-dimensional Tropical Dry Forest These forests are typically found between 10º and 25º latitude. The climate here is more seasonal than in rain forests. The dry season lasts 6-7 months, followed by a wet season of heavy rains. There is also more seasonal variation in temperature than is seen in the tropical rain forest.Soils in tropical dry forests are usually less acidic and more nutrient-rich than those in rain forests. Highly vulnerable to erosion during the wet season. Tropical dry forest distribution and climate Plant and animal life in the dry forest is tightly linked to cycle of dry and wet seasons. Trees in wet regions tend to be taller than those in dry regions. In addition, trees in wet regions may be evergreen, while those in dry regions typically lose their leaves during the dry season. Many animal species are shared with the rain forest. These include monkeys, parrots, and jaguars in South America. Many animals migrate to wetter habitats during the dry season. Tropical Savanna Found between 10º and 20º latitude. Found in Africa, South America, Australia, and parts of Asia. Alternating wet and dry seasons, but drier than tropical dry forest. Seasonal droughts combine with another important ecological feature….fire. Fires kill young trees while grasses survive and resprout. Soil layers have low permeability. Retain water near surface. Maintains enough water in surface soils to support grasses in dry areas. In wetter areas, trees cannot move onto savannas because the surface soil is waterlogged in the wet season. Tropical savanna distribution and climate Savannas landscapes are somewhat 2dimensional. Trees are present, but much of the biological activity takes place at ground level. Desert Occupy about 20% of the earth’s surface. Most are found are 30º latitude. Correspond to regions of descending, dry, subtropical air. Other deserts are found in the interior of continents or in the rain shadow of mountains. Environmental conditions vary. Some deserts actually receive significant rainfall. However, water loss through evaporation and transpiration exceeds precipitation during most of the year. Soils are low in organic material, and are classified as lithisols. May have high salt concentrations. Desert distribution and climate Plant cover is absent from many places. Where present, it is sparse. Often gray-green in color, because of protective covering. Desert plants show adaptations to prevent water loss. Animal abundance tends to be low, but diversity can be high. Many animals are adapted to deal with environmental extremes. Temperate Shrubland and Woodland Occur on all continents except Antarctica. Most extensive around the Mediterranean and in western North America. Most are found between 30º and 40º degrees latitude. Cool and moist during the fall, winter, and spring. Summers hot and dry. Frost danger varies, but frosts are not often severe. Fire is frequent and intense. Soils are of relatively low fertility and are fragile. Erosion threat can be increased by fire. Temperate woodland/shrubland distribution and climate Organisms are adapted for drought. Trees and shrubs typically evergreen with small, waxy leaves. Fireselected plants dominate. Animals are highly diverse. Includes many migratory birds and insects. Native browsers include deer, wild sheep, antelope, and kangaroos. Temperate Grassland Largest biome in North America (Great Plains). Even more extensive in Eurasia. In southern hemisphere, found in South American and New Zealand. Receive between 300 and 1000 mm of precipitation annually. Most in the summer. Shorter growing season than more tropical climates. Best grassland soils are deep, nonacidic, and rich in organics. Temperate grassland distribution and climate Grassland vegetation is dominated by….grasses. Drought and high summer temps encourage fires, which help exclude woody vegetation. Can be a very high diversity of other vegetation as well. Temperate grasslands once supported large populations of large grazing herbivores, which often formed huge herds. Smaller grassland animals are often either burrowers, or fast-moving predators. Temperate Forest Typically found between 30º and 55º degrees latitude. May be either coniferous or deciduous. Found where temperatures are not extreme and where annual precipitation averages between 650 and 3000 mm. Generally receive more winter precipitation than temperate grasslands. Winters relatively mild. Soils usually moist and fertile, usually neutral or slightly acidic. Temperate forest geography and climate Diversity is typically lower than in tropical forests, but biomass may be higher. Vertically stratified. Canopy height may vary from 40 to 100 m. Birds, mammals and insects make use of all layers from forest floor to the canopy. Most important consumers are fungi and bacteria, which recycle organic material. Much temperate forest habitat has been lost. About 1 to 2% of North America’s old-growth forests remain. Boreal Forest (Taiga) Extends around the world in the northern hemisphere. Pattern of woods and water. Found between 50ºN and 65ºN latitude. Bounded by temperate forests and grasslands to the south and tundra to the north. Long winters and short summers. Include some of the most variable climates on earth. Verkhoyank, Russia, has an annual temperature range of over 100º C. Precipitation is moderate (200 mm to 600 mm). Low evaporation rates make drought rare. Soils of thin and acidic, with low fertility. Nutrients largely tied up in plant litter. Boreal forest geography and climate Generally dominated by evergreen conifers. Low diversity. Some deciduous trees like aspen and birch, and willows are common around bodies of water. Little herbaceous vegetation. Small shrubs like blueberry common. Winter home of caribou. Moose and woodland bison found yearround. Major predator is the wolf. Also contains bears, wolverines, snowshoe hare, porcupine, and many smaller mammals. Nesting habitat for crossbills and spruce grouse. Although humans have lived in boreal forests since the ice ages, impact has been minor until recent times. Now, hunting, trapping, and forestry are beginning to severely impact taiga animals and plants. Tundra Open landscape of mosses, lichens, and low shrubs. Covers most of the lands north of the Arctic Circle. Cold and dry. Temperatures not usually as extreme as in the boreal forest. Precipitation ranges from a little less than 200 mm to a little over 600 mm. Average precipitation exceeds evaporation. Organic matter accumulates in peat and humus deposits. Surface soils thaw in the summer; underlain by permafrost. Tundra geography and climate Many grasses, sedges, mosses, and lichens. Woody vegetation includes dwarf willows and birches. Large numbers of large mammals, including caribou, musk ox, bear, and wolves. Many smaller mammals also, including lemmings, weasels, and foxes. Many migratory bird species in the summer. Insects not diverse, but very abundant. As with taiga, the tundra was long free from serious human impact. Now, much attention as the result of oil exploration. Also, airborne pesticides and radionuclides have been deposited there. Radioactive Cs137 from Chernobyl has become so concentrated in Norway tundra that the reindeer were considered unfit for consumption. Marine and freshwater ecologists and biogeographers do not classify aquatic communities into categories analagous to those used for terrestrial biomes. Oceanographers recognize biogeographic and climatic regions: 1 – Arctic 2 – Subarctic 3 – Northern temperate 4 – Northern subtropical 5 – Tropical 6 – Southern subtropical 7 – Southern temperate 8 – Subantarctic 9 - Antarctic Climatic regions; A – Arctic B – northern boreal SB – Southern boreal T – tropical E – equatorial N – northern notal SN – southern notal ANT - antarctic Marine communities are also classified based on bathymetry (the depth and configuration of the location at which they are found). A similar classification system has been developed for freshwater systems. Another factor controlling the distribution of ecosystems is net primary productivity. Productivity is larger a factor of the available solar energy, although available moisture and nutrients also play a role. Ecosystem geography is the branch of science that examines how ecological processes have influenced the distribution of ecosystems. Practitioners have developed a hierarchical scheme of ecoregions. This ecoregion approach allows us to look at processes on a scale that may range from local sites to landscape mosaics to ecoregions. A hypothetical map of the ecoregion scheme at three spatial scales Ecoregions of the terrestrial realms based on Bailey’s scheme. Check out this web site. …and for aquatic realms.