climate Global and ecological communities (Chapters 7 & 20)
Some definitions
Community--an ecological unit consisting of an assemblage of many populations living in the same place and same time. Biome--a major terrestrial climax plant community (e.g., tropical forest, tundra). Emergent property--a feature of a system that is not deducible from the features of the components of the system or lower order processes.
Emergent properties of communities
Early scientists equated the community to a superorganism
Frederic Clements (1874-1945) suggested that ecology was to the study of communities as physiology was to the study of individual organisms. Clements's ideas are still shared by some today (e.g., the Gaia Hypothesis).
Henry Allen Gleason (1882-1975) proposed an "individualistic" concept of plant association, in place of Clements's hypothesis
Modern view of communities fall in between these two hypotheses. Some communities are clearly organized with clear boundaries while other communities are individualistic. The goal of modern community ecology is to identify the underlying rules that give rise to the emergent properties of communities.
Suggested that distinct ecological communities did not exist. Instead, individual species of plants were viewed as being distributed independently along gradients. Thus, communities could not be assigned boundaries.
Scale and properties of ecological communities
Communities can occur on a wide range of scales and can be nested within one another. Once a community has been identified we can determine its properties: 1) the number of species present (biodiversity), 2) growth form and structure (stratification), 3) relative abundance and/or biomass of the individual species, and 4) the trophic structure (who eats whom).
Climate & communities
There are substantial temperature differentials over the Earth. This variation is largely due to variation in incoming solar radiation. At higher latitudes the sun's rays hit the Earth obliquelyless solar energy strikes the Earth's surface per m2 the closer you are to the poles. More radiation is also dispersed in the higher latitudes, because the sun's rays travel a greater distance through the atmosphere. Result: 40% less total annual insolation at the poles compared to the equator. The distribution of major biomes are set by temperature differentials and the wind patterns that they generate.
Global community classification systems
There are 7 major terrestrial plant communities (biomes)
Biomes can be further divided into floristic provinces (e.g., North American provinces)
I. Tundra II. Northern conifer III. Eastern deciduous forest IV. Coastal plain V. West Indian VI. Grassland VII. Cordilleran forest VIII.Great Basin IX. California X. Sonoran
Holdridge's Life Zone Classification of communities
Scheme depends on three environmental factors. Biotemperature is the mean annual temperature above 0oC. Potential evapotranspiration is the potential sum of water lost from the land from evaporation and plant transpiration if water were unlimited. Annual precipitation is the total amount of precipitation in the form of rain, snow, fog etc.
Not all plant communities fit neatly into the 7 major biomes
There are characteristic regions where one type of biome grades into another.
Some coniferous forests occur in temperate lowlands where you would expect deciduous trees would dominate. For example, most of New Jersey's coastal plain is sandy nutrient-poor soil that cannot support deciduous forest. Instead it supports the pine barrens.
Chapparal, another distinct biome, is a Mediterranean scrub habitat that is adapted to fire. Mountain ranges need to be treated differently.
Rainfall may be sufficient to support trees, but the frequency of fires prevents the establishment of trees. Temperature decreases with altitude. Precipitation also changes with altitude. Mountains can also cause rain shadows. Approaching clouds usually dump all their moisture on the windward side of the mountain. A biome may change from temperate forest through taiga and into tundra on an elevation gradient in the Rocky Mountains.
Aquatic habitats
Marine habitats are divided into 3 major zones.
Intertidal zone. Organisms are alternatively exposed and submerged by the daily cycle of tides. Resident organisms are subject to a great daily variation in the variability of seawater and temperature. Pelagic zone (open ocean). Nutrient concentration is typically low, although upwellings may periodically enrich the surface waters. Phytoplankton activity in the pelagic zone accounts nearly for half of the photosynthetic activity on earth. Increasing phytoplankton could help slow global climate change. Coral reefs. Exist in warm tropical waters. Reefs are one of the most richest biomes on earth. 30-40% of all fish species on earth are found in the coral reefs.
Freshwater habitats are divided into 2 zones.
Lentic or standing water habitats. Lentic habitats are governed by the unusual properties of water (e.g., ice floats, temperature stratification by depth). Accumulation of organic matter causes eutrophication, which greatly is speeded up by addition of sewage and fertilizers. Lotic or running water habitats. Flora and fauna are completely different from those in lentic waters.
Distribution of threatened species among biomes
Data is only available for birds of the world and mammals of Australia and the Americas. Of the top 25 "hot spots" for endemic species, 15 are in tropical forests and 5 are in Mediterranean scrub habitats. Threatened birds tend to occupy the same habitats as threatened mammals. Oceanic islands have many species of threatened birds and plants. Tropical forests and oceanic islands appear to be good habitats for maximizing the protection of endangered vertebrates.
Global Warming
Scientific debate on global warming centers around two questions:
How fast is global warming occurring? How much does human activity contribute to global warming?
Greenhouse effect--the heating effect of
the atmosphere upon the Earth caused by the differential permeability of the atmosphere to short-wave and long-wave radiation.
Energy balance of the Earth
Radiate energy from the sun is short-wave and easily passes through the atmosphere. Radiate energy from the Earth is long-wave (infrared) and is mostly absorbed by the atmosphere. Energy absorbed in the atmosphere caused the temperature of the atmosphere and the surface of the Earth to increase. Energy inputs and energy outputs must balance on Earth or global average temperature will change. Global average temperature is +15oC with atmosphere. Global average temperature would be -17oC without atmosphere.
Greenhouse gases
Greenhouse gases reduce the permeability of the Earth's atmosphere to long-wave radiation. There are approximately 20 greenhouse gases that comprise 1% of the total volume of the atmosphere. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide and chlorofluorocarbons. Greenhouse gases originate from both natural and anthropogenic sources.
Characteristics of important greenhouse gases
Greenhouse gases have increased in the atmosphere in the last 250 years
CO2 concentration 320 300 280 260 1750 1800 1850 1900 1950 2000 Year 310 300 290 280 1750 1800 1850 1900 1950 2000 Ye...