Basic Concepts. Ecosystem structure Ecosystem processes. ENV 101: Introduction to Environmental Science - Lecture 4

Transcription

1 Ecosystems

2 Basic Concepts Life is sustained by the interactions of many organisms functioning together, interacting through their physical and chemical environments This is referred to as an ecosystem Basic characteristics of ecosystems: Ecosystem structure Ecosystem processes

3 Ecosystem Structure Ecosystem structure An ecosystem has two major parts Living Called the ecological community is the set of species interacting within the ecosystem Non-living Consists of the physical-chemical environment including the local atmosphere, water and mineral soil (on land) or other substrate (in water) Ecosystem processes An ecosystem has two major processes Cycling of elements Flow of energy

4 Ecological Communities Defined in two ways A set of interacting species found in the same place and functioning together, thus enabling life to persist The community exists of all the species found in an area, whether or not they are known to interact (Practical/operational definition) One form of interaction found within communities is the transfer of energy, chemical elements and compounds between different species (known as a food chain)

5 Ecological Communities (terminology) Food Chain The linkage of who feeds on whom Food Web A network of who feeds on whom, consisting of complex linkages Trophic Level All the organisms in a food web that are the same number of feeding levels away from the original energy source In most ecosystems this original source is the sun In others it is the energy in certain inorganic compounds

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7 Ecological Communities (terminology) Photosynthesis The process whereby green plants and algae create sugars from CO 2, water and sunlight: Autotrophs Those organisms who photosynthesise (1 st Trophic Level) organisms that produce their own food

8 Ecological Communities (terminology) Heterotrophs Organisms that feed on other organisms Herbivores: Feed on plants, algae or photosynthetic bacteria (2 nd Trophic Level) Carnivores: Feed directly on the herbivores (3 rd Trophic Level) Feed on third level carnivores (4 th Trophic Level) Decomposers (detritivores) Organisms that feed on dead organic material (occupies the highest trophic level in a given ecosystem)

9 5 th Trophic level 4 th Trophic level 3 rd Trophic level 2 nd Trophic level FLOW OF ENERGY 1 st Trophic level

10 Complexity (A simple food web)

11 Complexity (A more complex food web)

12 Ecosystems as Systems Ecosystems are open systems having both a flow of matter and energy Ecosystems vary greatly in the their structural complexity as well as the clarity of their boundaries What all ecosystems therefore have in common is NOT size or shape, but processes which give them the ability to sustain life

13 Cycling of Elements 21 chemical elements required by at least some form of life Elements must be available at the right time, in the right amounts, and in the right ratio compared to other elements They must also be recycled (food to waste back to food) For complete recycling several species need to interact A simple ecosystem (theoretical) Via photosynthesis green plants/algae produce sugars These sugars are used to build organic compounds (e.g. wood y tissue) HOWEVER, green plants cannot break this material down into its original organic compounds. The presence of other species are required

14 Flow of Energy All life requires energy Energy is the ability to do work Ecosystem Energy Flow: the movement of energy through an ecosystem from the external environment through a series of organisms and back to the external environment Energy enters an ecosystem in one of two ways: Energy fixed by organisms which moves through the food chain Heat energy transferred by air/water currents or through convection through sediments/soils to warm living things

15 Laws of Thermodynamics First law of thermodynamics Aka law of the conservation of energy In any physical or chemical change, energy is neither created nor destroyed but merely changed from one form to another Second law of thermodynamics Whenever some work is done, some energy is inevitably converted to heat (which from our organisms perspective gets lost to the surrounding environment) More specifically this deals with a change in the state of the energy from organised to disorganised (which we refer to as entropy)

16 An impossible ecosystem

17 Energy Flow The net flow of energy through an ecosystem is always a one-way flow For example: Green plants/algae make sugars during photosynthesis using energy derived from the sun Herbivores use the sugars manufactured by plants as their main source of energy (process of respiration) Therefore, ecosystems must exist between a source of usable energy, and a sink for degraded energy

18 Biological Production and Biomass Biomass: the total amount of organic matter in any ecosystem Production: The change in biomass over a given period Biological production: The capture of usable energy from the environment to produce organic matter or organic compounds (also referred to as energy fixation) Gross production: the increase in stored energy before any is used Net production: the amount of newly acquired energy stored after some energy has been used

19 Biological Production and Biomass We burn a fuel in order to obtain energy (respiration) Respiration: The complex series of chemical reactions in organisms that make energy available for use. Output is CO 2, H 2 O and water Primary production: The production carried out by autotrophs Secondary production: The production carried out by all heterotrophs

20 Energy Efficiency How efficiently organisms use energy is important in terms of the conservation of biological resources No system can be 100% efficient (2 nd Law of Thermodynamics) therefore energy is degraded as it flows through the food web Energy efficiency: The ration of output to input as well as the amount of useful work obtained from some amount of available energy

21 Energy Efficiency Can be viewed in tow ways (depending on your perspective) Converts a lot of energy to stored reserves while using little for biological functions (Herbivores) Uses as much energy as possible and stores very little (Carnivores) Trophic-level efficiency: The ration of production of one trophic level with that of the next lower trophic level Never very high More than 90% of energy transferred between trophic levels is lost as heat Less than 10% (typically 1% in natural systems) is fixed as tissue Example: Cattle require ±7.2kg of vegetable matter to produce ±0.45 kg of meat

22 Ecological Stability and Succession Ecosystems are dynamic and constantly changing due to external as well as internal processes When disturbed ecosystems can recover through environmental succession if the damage is not too great. Succession: The process of development and establishment of an ecosystem Primary succession: Establishment and development of an ecosystem where one did not exist previously e.g. vegetation growing on recent lava flows Secondary succession: Reestablishment of an ecosystem after disturbances. There are remnants of a previous biological community including organic matter and seeds

23 Ecological Stability and Succession Early-successional species: also known as pioneers, they have evolved to the environmental conditions in early stages of succession (grow rapidly and spread easily) Late-successional species:these tend to be slower growing, longer lived and can persist under intense competition with other species Middle-successional species: Characteristics in between the other two types

24 How Species Facilitate Succession Facilitation An earlier successional species changes the local environment in ways that make it suitable for other species characteristic of a later successional stage Interference Situations where an earlier successional species changes the local environment so it is unsuitable for a species in a later successional stage Life History Differences Not true succession Occurs because differences in time it takes for species to establish themselves give the appearance of succession Chronic Patchiness: A situation where ecological succession does not occur. One species may replace another, or an individual of the first species may replace it, but no overall general temporal pattern is established Characteristic of highly disturbed environments

25 Nutrient Cycling and Succession One of the most important effects of succession is a change in the storage of chemical elements necessary for life In terrestrial environments the storage of these chemical elements increase: Increased storage Organic matter stores chemical elements. If it it accumulates then the amount of elements accumulate Increased rate of uptake Presence of root nodules that can assimilate atmospheric nitrogen (nitrogen fixation) increasing the uptake by the plant Decreased rate of loss The presence of live and dead organic material decreases rates of erosion which would remove these chemical elements from the ecosystem