ECOSYSTEMS AND COMMUNITIES,HIT Environmental Science Wang Xiuheng 2008,OUTLINE,Succession（演替） Primary, Secondary（原生，次生） Terrestrial, Aquatic（陆地，水域） Climax Communities Biomes 生物群系 Aquatic Ecosystems,HIT Environmental Science Wang Xiuheng 2008,SUCCESSION 演替,Ecosystems are dynamic. Succession: A series of regular, predictable changes in the structure of a community over time. Occurs because perturbations（干扰） to an area often make the environment less suitable for current organisms and more suitable for others.,HIT Environmental Science Wang Xiuheng 2008,Primary Succession - Begins with bare mineral surfaces or water. Secondary Succession - Begins with disturbance of an existing ecosystem.,HIT Environmental Science Wang Xiuheng 2008,Terrestrial Primary Succession,Pioneer Community（先驱群落）: Collection of organisms able to colonize bare rock. ? e.g. Lichens help break down rock and accumulate debris, helping to form a thin soil layer. Soil layer begins to support small life forms.,HIT Environmental Science Wang Xiuheng 2008,Eventually a climax community is reached,HIT Environmental Science Wang Xiuheng 2008,In general, climax communities are more stable and have larger, more diverse populations of species than earlier stages of succession. Successional (seral) stage (演替阶段) - each step in the process.,HIT Environmental Science Wang Xiuheng 2008,Aquatic Primary Succession,Except for oceans, most aquatic systems are considered temporary. All aquatic systems receive inputs of soil particles and organic matter from land. - results in gradual filling of shallow bodies of water. Plants can begin to take root.,HIT Environmental Science Wang Xiuheng 2008,Secondary Succession,Occurs when an existing community is disturbed or destroyed. flood, fire, plow,HIT Environmental Science Wang Xiuheng 2008,With most disturbances, most of the soil remains, and many nutrients necessary for plant growth may be available for reestablishment of the previous ecosystem. more rapid.,HIT Environmental Science Wang Xiuheng 2008,Secondary Succession from a beaver bond,HIT Environmental Science Wang Xiuheng 2008,Question Break,How does primary succession differ from secondary succession?,HIT Environmental Science Wang Xiuheng 2008,Climax Communities Biomes,Terrestrial climax communities with wide geographic distributions. Usually defined by undisturbed natural plant communities. Two main non-biological factors determining biomes: Temperature Precipitation,HIT Environmental Science Wang Xiuheng 2008,HIT Environmental Science Wang Xiuheng 2008,HIT Environmental Science Wang Xiuheng 2008,Elevation Effects on Climate and Vegetation,As altitude increases, average temperature decreases. Moving from sea level to mountain tops, it is possible to pass through a series of biomes similar to what would be encountered moving from the equator to the north pole.,HIT Environmental Science Wang Xiuheng 2008,HIT Environmental Science Wang Xiuheng 2008,Desert Grassland Savanna Shrublands Tropical Rainforest Northern Coniferous Forests Temperate Deciduous Forest,Major types of Biomes,HIT Environmental Science Wang Xiuheng 2008,Less than 25cm annual precipitation unevenly distributed throughout the year. Hot? No life? Large daily temp. fluctuations. No Cloud. Most species exhibit specialized adaptations to climate.,Deserts,HIT Environmental Science Wang Xiuheng 2008,HIT Environmental Science Wang Xiuheng 2008,Some definitions, Rangelands, Grassland: the non-forest areas Savannas: mainly grasses and shrubs with scattered trees Woodlands: trees more common; trees widely spaced (park-like) with canopy cover 30%, Mostly 10 m tall, Occupy around 5% continent,HIT Environmental Science Wang Xiuheng 2008,Grasslands,Receive 25-75cm of annual precip. Rainfall sporadic enough to cause draughts Fire regime usually present. Historically evolved with large herds of migratory grazing mammals. Convert to agriculture or stock raising. Why?,HIT Environmental Science Wang Xiuheng 2008,HIT Environmental Science Wang Xiuheng 2008,Savanna 热带稀树草原 50-150cm of annual precip. Not evenly.,HIT Environmental Science Wang Xiuheng 2008,Mediterranean Shrubland,HIT Environmental Science Wang Xiuheng 2008,Tropical Dry Forest,HIT Environmental Science Wang Xiuheng 2008,TropicalRainforests,Located near equator, temp. is relatively warm and constant. Over 200cm annual rainfall. Closed canopy (70% FPC), Often H30 m,HIT Environmental Science Wang Xiuheng 2008,Laterite soil allows high levels of leaching, thus most nutrients are tied-up in biomass. Buttress roots,