D4.2 Stability & Change Flashcards
What is stability?
Stability refers to ability to maintain or support systems & processes continuously over time.
What ensures stability in an ecosystem?
Steady conditions with a continuous supply of rainfall & sunlight, combined with warm temperatures have ensured stability for many ecosystems (at least until advent of humanity).
What ecosystems illustrate stability?
Stability is a property of natural ecosystems. This is notably illustrated by forest and desert ecosystems that have shown continuity i.e. been left undisturbed without management or anthropogenic use over long periods, even millions of years (the rainforests of SE Asia are estimated to be 70 million years old e.g. Danum Valley Malaysia and Namib desert which hasn’t changed in thousands of years).
What does structural & functional stability of an ecosystem depend on?
- Nutrient cycling without leakages (causing nutrients to leave the system)
- Individual species (especially keystone species) must have high genetic diversity so populations can survive selection pressures.
- Climatic variables within tolerance limits
- Absence of disruptions which would interfere with sustainability
- Steady supply of energy
What is genetic diversity? And what is its relation with ecosystems?
Genetic diversity = a range of genetic material present in a gene pool or pop of a species. Usually = diversity within one species, but it can also be used to refer to diversity of genes in all species within an area. Ecosystems that are complex (supported by a large biomass of producers) lead to an increased number of niches, which increases species & genetic diversity, resulting in greater stability. Ecosystems that contain species with small pop sizes can lead to low genetic diversity, which leaves species more prone to disease.
How is biome distribution determined?
Biome distribution is determined by climatic variables (temperature, precipitation & insolation) that need to remain within tolerance levels for the plant species that are found there.
What type of disruptions can interfere with stability & sustainability?
- Erosion = leads to a loss of nutrients
- Harvesting & removal of materials = disrupts nutrient cycles & food webs
- Eutrophication = Nutrient enrichment of water causes pop imbalances
- Poaching & selective removal = This disrupts ecosystem structure, especially if a keystone species is removed
What is the tipping point?
Tipping point = This is a critical threshold when even a small change can have dramatic effects & cause a disproportionately large response in overall system.
It occurs if an ecosystem’s stability is compromised so much disturbance occurs that a critical point is reached where a reversal might be difficult or impossible.
What impact of positive feedback loops on stability?
Positive feedback loops (e.g. water cycle) have tendency to amplify changes & drive a system to a point where a new form of stability or equilibrium is reached.
What changes to stability are caused by human population?
- Resource consumption
- Habitat transformation & fragmentation
- Energy production & consumption
- Climate change.
How do causes an unstable equilibrium to a ecosystem?
- All of these factors exceed, in both rate & magnitude, the changes seen in most recent global-scale shift in equilibrium at end of last ice age.
- Most projected tipping points are linked to climate change.
- Increases in carbon dioxide levels above 450ppm would lead to increased global mean temp, causing melting of ice sheets & permafrost.
Why is Amazon rainforests an example of a tipping point in an ecosystem’s stability?
Amazon rainforests experience increased frequencies of drought.
Main reasons which contribute to this are burning & deforestation to make space for grazing land & housing, & to provide timber.
Mining further leads to loss of forest habitat.
Where is the Amazon rainforest located? And why is it important?
Amazon rainforest covers a very large area & it spans over nine countries in South America (Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Guyana, Suriname and French Guiana). It contains half of planet’s remaining tropic rainforests.
What disturbances does deforestation of the Amazon lead to?
Disturbances to water cycle such as deforestation in this large ecosystem augment effects & further & might lead to the tipping point. The rainforest is needed for generation of atmospheric water vapour - & this cycle is fuelled by rainfall, transpiration in plants, evaporation & air flows.
What is the water cycle? (Amazon rainforest no deforestation)
- Moisture comes into to region on the wind (Input)
- Some of it falls as rain & enters the soil (Rainfall)
- It is absorbed by roots & taken up by plants
- Evapotranspiration moves water back into air (Evapotranspiration)
- A similar amount of moisture leaves rainforest on wind as arrived (Moisture leaves on wind)
- Amount of water in river is not significantly changed
What is the water cycle? (Amazon rainforest with deforestation)
- Moisture comes into to region on the wind (Input)
- Some of it falls as rain & enters soil (Rainfall)
- With fewer plants evapotranspiration is reduced. Less water is returned to atmosphere (evapotranspiration)
- This increases run off (river flow) from area
- The air leaving area is much drier than air coming in (Less moisture on the wind)
This decreases amount of moisture carried away by atmosphere, reducing rainfall in regions downwind. Topical deforestation can reduce 100s-100s km away.
Why is the rainforest important? What is the issue climate change is causing to this importance?
A large area of rainforest is needed for generation of atmospheric water vapour by continuous transpiration, with consequent cooling, ait flows & rainfall. There is scientific uncertainty over minimum area of rainforest that is sufficient to maintain these processes. CC further reinforces these processes by increasing air temp, dry season severity & frequency of extreme weather events.
How can ecosystem stability & sustainability be investigated?
Ecosystem stability & sustainability can be investigated or monitored in mesocosms.
These are small experimental areas to model ecosystems.
Mesocosms can be completely sealed up & closed with only light entering as a source of energy, or open but controlled in some way.
What factors such be considered when building a mesocosm?
- Container should betransparenttoenable sunlight to reach producersinside mesocosm
- Autotrophsshould be included so thatlight energycan be converted intochemical energyinside mesocosm
- Small primary consumers such as zooplankton or other small invertebrates could be included, but it is important to consider whether the mesocosm is likely to be large enough to support them
- Do not include secondary consumers in a mesocosm bc there won’t be enough energy in food chain to sustain them for long, & it could be consideredunethicalto allow primary consumers to be eaten in this way
What are different ways to set of mesocoms and why?
Mesocosms can be set up as open systems, i.e. without a lid, but sealed systems aremore controlled, & therefore more useful for experimental purposes
Sealed systems prevent organisms and substances from entering or leaving
What is purpose of setting up mesocosm?
In lab, a mesocosm can be set up & then aknown factor can be alteredto assess its effect
E.g. difflightlevels, difftempetc.
In order to assess impact of changing one factor, acontrolmesocosm must be set up at same time. A control mesocosm will beexactly the sameas experimental mesocosm,but altered variable won’t be changed. Purpose of this is to demonstrate that any change in mesocosm isdue to altered factor & not another factor.
How do you set up a terrestrial mesocosm?
- Placedrainage materialsuch as gravel in bottom of a clear container
- Add a layer of charcoal on top of drainage layer; this can help to prevent growth of mould
- Place a layer of sphagnum moss or filter paper on top of charcoal toprovide separationbetween base layers & organic matter above
- Add a layer of soil or compost above separation layer; this providesorganic material & micro-organismsto aid with nutrient cycling
- Plantslow-growing producerssuch as healthy mosses & ferns in growth medium
- Water growth medium before sealing container with a lid : mesocosm may need watering while it establishes, butavoid excessive watering; once mesocosm has stabilised plants should release enough water vapour during respiration to maintain moisture levels
- Place container in alight location, & ensure thattemp is stable
How do you set up an aquatic mesocosm?
- Base layer of mesocosm should consist oforganic substratefrom bottom of a lake or pond; this will provide naturally occurring nutrients & microorganisms
- Addlake or pond water; this ensures that it contains required microscopic organisms & avoids chemicals from tap water
- Addhealthy aquatic plantsto produce carbohydrates &oxygenate the water
- Small aquatic organisms such as water fleas or water snails can be added, but not more organisms than the size of mesocosm can support: Only primary consumers should be used
- Place container in alight location, & ensure thattemp is stable
Define mesocosm?
Mesocosm = enclosed environments that allow a small part of a natural environment to be observed under controlled conditions.
What are the advantages & disadvantages?
What is a keystone species?
Keystone species = one whose activity has a disproportionate effect on structure of an ecological community. Could be predator e.g. wolf or
What is an example of a keystone species?
American Beaver (Castor canadensis)= example of a keystone species in North America. Beavers perform unique and crucial roles within an ecosystem - functions that would be substantially different or non-existent without them.
What is sustainability?
Sustainability = responsible maintenance of ecological systems so that there is no reduction of conditions for future generations, ensuring long-term viability of a system. Sustainability depends on rate of harvesting being lower than rate of replacement.
How are the forests managed sustainably in Finland?
Forests have played an important role in Finnish history. Finnish forests have provided food, shelter, employment + income for Finns through centuries.
Forest Act provides a framework for forest management in Finland. Leading principle of forest legislation has been obligation to regenerate after logging.
How is the terrestrial plant black cherry managed sustainably?
Black cherry is a species of hardwood tree, found in N America, that is popular for furniture production. Hardwood tree species grow slowly, so it is especially important that sustainable harvesting methods are used; methods such asclear felling will leave no timberfor many years to come. Sustainable harvesting of black cherry timber involves:
- Selective felling; choosing specific individual trees to harvest, leaving gaps in the forest canopy that will encourage growth of more plants on forest floor
- Leaving enough individuals behind in the forest toflower & produce seeds, ensuring that new black cherry saplings will germinate
- Regularmonitoringto ensure that new growth is keeping up with logging
How are fisheries managed sustainably?
Many fish, such as the Cod (Gadus morhua) of N Atlantic, are on verge of extinction – with most probably cause being overfishing. Rapid decline of many fish species & difficulties associated with international conservation programmes make sustainability goals more & more important.
Why is it important to sustainably fish?
If pop of fish are persistently overfished (as regularly are), stocks will be rapidly depleted to point where they collapse & can no longer support a commercial fishery. Extent to which species are fished should be based on pop dynamics that consider their reproductive & growth rates. Pop size & age structure can be used to inform practices associated with sustainable fishing.
How is the marine fish Alaska pollock managed sustainably?
Pollock are a species of fish found across N Pacific. Alaska pollock fishery is largest sustainably certified fisheryin the world. This certification is awarded by Marine Stewardship Council (MSC). Fishery is considered to be sustainable because Pollock are afast-growing specieswhich can reproduce from the age of 3-4 years and nets haveminimal contact with the sea-bed, so do not damage this habitat. The proportion of fish caught that are not pollock, known asbycatch, is very low; less than 1 % & according to regulation,any bycatch that is caught cannot be sold commercially, so it is in interest of fishermen to avoid catching it. Close monitoringis carried out by science research vessels & by trained individuals on board commercial fishing vessels. Any areas where a higher than normal number of salmon bycatch occurs areclosed off to fishing vessels.
Which factors affect the success of agriculture?
- Precipitation (type, frequency, intensity, amount)
- Temp (growing season (>6°C), ground frozen (0°C), range of temp)
- Soil (fertility, nutrient status, structure, texture, depth)
- Pests (vermin, locusts, disease)
- Location (slope gradient, relief, altitude, aspect – shady or sunny)
What are the side effects of agriculture?
- Soil erosion – this is caused by tillage, which involves plowing, harrowing & results in a reduction of soil quality.
- Fertilisers : Growing crop causes nutrient depletion over time which needs fertilizers (agrochemicals) to compensate. Excess amounts of nitrogen & phosphate containing fertilizers leach into water systems potentially leading to eutrophication.
- Leaching: soil exposure can lead to leaching of nutrients from soil. When it rains, nutrients are washed away & not available anymore for plants.
- Carbon footprint: use of heavy machines for tillage, harvest, transport needs a lot of energy & produces large amounts of carbon emissions (diesel engines). Glasshouses & stables for animals need heating & further contribute to this.
- Monocultures: repeated growth of same crop results in pests & weeks to become increasingly problematic. Use of pesticides & herbicides can cause pollution problems & production of chemicals needs energy.
How is soil erosion caused by agriculture?
In order to grow crops or keep large numbers of grazing animals,land needs to be clearedto make space for crops or grass. Removal of larger trees & shrubs means thatroots that hold soil together are lost, resulting inless stable soilthat can easily be washed or blown away. This is a particular risk for nutrient-rich upper soil layer, known astopsoil. This leads tosoil erosion, & a reduction in availability of soil needed for crop/grass growth. While crops & grass themselves can aid soil stability, they may only provide partial cover, or they may be removed after harvest, or due to overgrazing/poor weather. Farmers sometimes plant ‘cover crops’ to hold topsoil together in between growing seasons.
How is leaching & nutrient run-off caused by agriculture?
Use of synthetic fertilisers in agriculture can lead tonutrient runoffdue toleaching. This occurs when rainfall washes fertilisers out of soil &into nearby bodies of water. Minerals in synthetic fertilisers are highly soluble sodissolve in rainwaterbefore being washed away. Problem of leaching can be reduced by applying fertilisers in small volumes, at times when rain is not forecast, & by using organic rather than synthetic fertilisers
How is fertiliser supply caused by agriculture?
Chemical fertilisers are important for many farmers, but they are not always easy to supply. They areexpensive. They are used by many farmers, sosupply may not meet the large demand. Process of fertiliser production is very energy intensive, so cost of fertilisers is affected by global energy prices. Switching toorganic fertiliserscan help to reduce some of difficulties associated with chemical fertiliser use.
How is pollution caused by agriculture?
Some types of farming rely on use of chemicals, known asagrochemicals. Additional examples of agrochemicals include:
- Pesticides
- Herbicides
- Fungicides
These chemicals are used toimprove yield, which might otherwise be damaged byinsect pests, competition fromweeds, or byfungal disease. These chemicals canenter natural environment& cause problems, e.g. by killing non-target species. Biological pest controlcan reduce pesticide use. This involves release of a pest’s natural predators, e.g. ladybirds that prey on aphids. Scientists hope thatgenetic modificationmay allow introduction of crop varieties that areresistant to pests & disease.
How is carbon footprint caused by agriculture?
Reliance on fossil fuels fortransportation,machinery, & productionofsyntheticfertilisershas significant implications for sustainability of food production
Combustion of fossil fuels releases carbon dioxide into atmosphere, increasing carbon footprintof agriculture & contributing toclimate change.
Transitioning torenewable energy sources& promoting energy-efficient practices can helpreduce carbon footprintof food production.
What are the natural and human causes of eutrophication?
- Natural effects include nutrients being added from decomposing biomass & run-off from areas surrounding body of water. Eutrophication can occur naturally through gradual drying up of pools of water & disproportionally increasing amount of nutrient rich sediment.
- Human causes include run-off of fertilisers or manure from agricultural land, domestic waste water containing phosphates from detergents, & non-treated sewage. Usually eutrophication is due to leakage of excess chemicals used in agriculture (fertilisers) which contain nitrogen and phosphates.
- Leaching of mineral nutrients from agricultural land into rivers can also cause eutrophication.
What is eutrophication?
Eutrophication is referring to the excessive nutrient enrichment of water. Nutrients such as phosphates & nitrates favour growth of algae, which multiply in great numbers & lead to algae bloom. Raised temp in the Summer months add to algal growth rates. Algal blooms form a thick, sunlight-blocking cover on surface of the water. This prevents oxygen from dissolving & also photosynthesising plants on ground from carrying out photosynthesis so they die. Decomposition by bacteria further depletes oxygen supplies.
How can dissolved oxygen be measured?
Amount of dissolved oxygen required to break down organic material in a given volume of water through aerobic biological activity can be measured by BOD (biological oxygen demand).
How does BOD also help measure pollution?
Aerobic organisms use oxygen in respiration. With more organisms & faster respiration, more oxygen will be used thus resulting in a higher BOD. In aquatic systems with organic pollution (e.g. sewage, fertilizers etc) pop of organisms that feed on or use nutrients in pollutants increases. BOD therefore indirectly measures pollution.
How BOD determine water quality? And how is it measured?
BOD is an indirect method to determine water quality & aerobic activity by bacteria. It’s determined by taking a measured volume of water from a source (e.g. pond, stream etc) & measuring the dissolved oxygen content on Day 1 in mg/L using an oxygen probe. Sample is then placed in dark (to prevent photosynthesis) at 20°C for 5 days. After 5 days oxygen level is measured again. BOD is difference between initial & final Oxygen conc.
What is an example f a body of water which has a high BOD?
BOD levels are dependent on body of water being tested. Shallow, slow-moving waters, such as ponds & wetlands, will often have large amounts of organic material in water & high BOD levels.
What happens to pollutant through a food chain?
Polluting chemicals from human actions seep into food chains of many organism. Pollutants might only be released in very low concentrations, but at higher end of a food chain an organism might have accumulated almost lethal dosages.
What is Bioaccumulation?
Increase in conc of a toxin in body tissues during an animal’s lifetime. Usually this happens with fat soluble chemicals which cannot be excreted (e.g. mercury compounds). It occurs over time, only one particular animal life type.
What is Biomagnification?
Increase in conc of a chemical at each successive trophic level in a food chain. Predators tend to accumulate higher chemicals of a toxin than their prey because larger quantities are consumed.
(Along a food chain)
What is the difference between Biomagnification and Bioaccumulation?
- Biomagnification =increase in concentrationof persistent or non-biodegradable pollutantswith ascending trophic levelthrough a food chain
- Not to be confused withbioaccumulation, which=build-up of pollutantswithin an organism, or within a single trophic level
How does Biomagnification work?
Aspollutants are passed up food chainfrom one trophic level to the next, they can becomemore concentrateddue to decrease in total biomass of organisms at higher trophic levels. I.e. smaller organisms at bottom of food chain willeach consume a small volume of pollutant, & then organisms at top of food chain will consume many smaller organisms andreceive a much larger doseof pollutant.
Why does nearly all fish and shellfish contain traces of mercury? (explain using biomagnification)
- Mercury is released into environment through activities such as coal-fired power plants & gold mining.
- Once in environment, mercury can be converted by microorganisms into a highly toxic form calledmethyl mercury, which can accumulate in bodies of fish.
- As larger fish eat smaller fish, conc of methyl mercury within tissues of these fish increases, leading to potential harm forhumanswho eat large predatory fish e.g. swordfish, shark, mackerel, cod, bluefin tuna (especially during pregnancy).
What is Dichlorodiphenyltricholorethane (DDT) ?
Toxins such asDDT(dichlorodiphenyltrichloroethane) are persistent pollutants that can enter food chains
DDT was awidely used insecticidein mid-20th century that was found to haveharmful effects on top predatorssuch as birds of prey.
What are the negative effects of DDT?
When DDT was sprayed on crops, it would leach into waterways & eventually enter freshwater & marine ecosystems. DDT would thenenter food chains via plankton&accumulate in bodies of fish. These fish would then be eaten by birds, which wouldaccumulate higher concentrations of DDT. Bc DDT is persistent & doesn’t break down easily, it cancontinue to accumulate in bodies of animals at higher trophic levels, leading to harmful effects such as thinning of eggshells & reduced reproductive success. Thin eggshells couldn’t withstand weight of parent bird during incubation, so eggs would break & fewer young birds would hatch. Over time this resulted in reduced bird pop. DDT has now been banned worldwide, with exception of its use in areas where it is essential in dealing with mosquitos that transmit malaria.
What is the problem with plastic?
- Plastics describe a range of polymers which are mostly non-biodegradable.
- Plastics are used in an increasing number of consumer products.
- Plastic waste in oceans is a big problem.
- Marine animals mistake plastics for food & often suffocate or become entangled in floating materials.
What are two main groups of plastic?
- Macroplastics (large, visible, more than 5mm in size e.g. bottles, plastic bags which can be confused with food by marine predators)
- Microplastics (extremely small pieces of plastic debris, less than 5mm resulting from breakdown of consumer products & industrial waste)
What is problems with microplastics?
- Microplastics & their non-biodegradable contaminants can bioaccumulate in food chains.
- Predators in higher trophic levels are often at extra risk because of biomagnification effects of some chemicals.
- Abundance of macro & microplastics has been steadily increasing.
How does plastic enter the food chains?
- Animals often try toeat plastic, e.g. Turtles may attempt to eat a plastic bag that resembles a jellyfish. Plastic garbage harms & kills or Albatrosses may accidentally consume plastic when they fish, giving it to their chicks when they regurgitate food; chicks may later starve due to their stomachs being full of plastic
- Animalsbecome caught in plastic, such as fishing lines, leading to injuries and death
- As plastic breaks down it canrelease toxinsthat can lead tobiomagnificationin food chain
- Once it has broken down intovery small particles, it’s commonly ingested by animals &enters food chain
What are the different origins of microplastics (MP)?
- Laundering synthetic textiles
- Manufacturing using polymers
- MPs in treated wastewater
- MPs in biosoilds
-MPs in stormwater runoff - Trash disposal/littering
- Erosion
- Dust from amended soil
- Plastic trash degradation
- Dust settling/stormwater runoff
-Plastic degradation/stormwater runoff - MPs in salt spray
- Point degradation
- Plastic fishing gear
- Maritime accidents
What has the degradation to ecosystems by human actions caused?
Ecosystem degradation by human actions has caused severe consequences such as loss of biodiversity, species extinction & loss of ecosystem services such as flood protection, carbon sequestration & climate regulation.
What can be done to return ecosystem to original state?
Efforts are being made to encourage natural ecosystems to return. One approach is rewilding.
What is rewilding?
Rewilding aims to restore ecosystems and reverse declines in biodiversity by allowing wildlife & natural processes to reclaim areas no longer under human management. Rewilding is all about returning land and oceans to a more natural state. This is often done by doing nothing.
What are methods to rewilding an ecosystem?
- Rewilding reintroduces lost animal species to natural environments, such as top (apex) predators that have a significant effect on food web & trophic levels in an ecosystem.
- Keystone species are also reintroduced, which are essential for functioning of the ecosystem.
- Establishment of connectivity of habitats over large areas, using corridors of habitat that connect different areas, allows for movement of animals between fragments & gives larger animals area they need to feed and breed.
- Agriculture & other resource harvesting are no longer allowed & natural ecosystems are allowed to recover through ecological management.
What is the restoration project of Hinewai Reserve, NZ?
- Hinewai Reserve is an ecological restoration project on Banks Peninsula, New Zealand.
- It was once farmland, but is now privately owned, with aim ofrestoring natural ecosystemof area
- Some initial human intervention was involved, withremoval of non-native species (alien species such as exotic trees and vines), but area is now managed withminimal human interventionto allow nativecommunitiesto be restored bysuccession
- Human activities are limited in area, though public can enjoy walking in Reserve
What are some examples of interventions which can speed up rewilding?
- Distributing seeds of plants (endemic species)
- Reintroduction of apex predators (e.g. wolves or beavers) & other keystone species
- Re-establishing connectivity between fragmented parts of an ecosystem.
- Control of invasive alien species
- Removal of livestock (cattle, goat, horses etc)
What is ecological succession?
All ecosystems develop over time from point of colonisation of a pristine area to maturation of a stable final community. This is caused by ecological succession. Succession is orderly process of change over time in a community.
What does ecological succession start with?
Succession usually starts with a pioneer community (1st community), goes through a range of intermediate stages & finishes with a climax community (final stage).
What changes in abiotic factors triggers succession?
Succession can be triggered through changes in abiotic factors such as:
- changes in water availibility (e.g. bc of changed weather patterns),
- temp
- soil composition,
- natural disturbances such as floods, fires, vulcanic eruptions etc.
What changes in biotic factors triggers succession?
Biotic factors such as changing relationships within food chains can also trigger succession.
Changes in community of organisms frequently cause changes in physical environment, allowing another community to become established & eventually replace former through competition. Often, but not inevitably, the later communities in such a sequence (or sere) are more complex than those that appear earlier.
What are two example of ecological succession caused by changed to abiotic factors?
- Succession in freshwater:
aquatic plants -> reeds -> low woodland species - Succession in an abandoned quarry:
mosses and lichens -> grasses & herbs -> shrubs -> woodland
What is a pioneer community?
A pioneer community is 1st stage of an ecological succession that contains hardy species able to withstand difficult conditions with little nutrient availability. E.g. mosses, fungi, lichen, bacteria.
What is a climax community?
A climax community is a community of organisms that is more or less stable an equilibrium with natural environmental conditions such as climate. It is end point of ecological succession. It includes large trees & dense forests & complex interspecific relationships.
What are the transition between communities in primary succession?
Primary succession start with a pioneer community, then goes through stages of intermediate communities, & finally succession ends with a climax community.
What are two types of succession?
There are two types of succession – primary & secondary.
What is primary succession?
Primary succession begins with a completely barren environment with very early colonizers generating small amount of growth on which more & more communities develop.
What is secondary succession?
Secondary succession occurs where a disturbance has cleared away an existing community but leaves the soil intact.
Where does primary succession begin?
Primary succession begins in an environment were living organisms are largely or completely absent. Mostly, this is bare rock or gravel, deposits of silt or clay. Nutrient rich soil is usually not present.
What is a xerosere?
A xerosere = succession under dry, exposed conditions where water supply is an abiotic factor limiting growth of plants (at least initially).
What does the arrival of pioneer species allow for?
With the arrival of pioneer species small amounts of soil can form allowing herbs with roots to start to grow. With deeper soil species diversity increases & more species join than are eliminated. Primary production increases & complex foodwebs can form with nutrient cycling ensuring sustainability.
What are the stages of primary succession?
- Pioneer species arrive (e.g. lichens, mosses & bacteria)
- As pioneers die, soil is created
- New species of plants arrive that need soil to survive – these displace pioneer species
- Growth of plants causes changes in environment (e.g. light, wind & moisture)
- Growth of roots enables soil to be retained, nutrients & water in soil increases
- Nitrogen-fixing plants arrive, adding nitrates to soil
- Soil depth increases further, allowing shrubs & other taller plants to arrive
- Animal species arrive as species of plants they rely on become established
- A climax community is established.
How does productivity change through succession?
- In pioneer communities, gross productivity is low due to initial conditions & low density of producers. Proportion of energy lost through community respiration is also relatively low. Net primary productivity (NPP) is therefore high: system grows and biomass accumulates.
- Gross primary productivity increases due to increase in biomass of community. Sunlight is available for all plants as there is little shading.
- In later stages there is an increased consumer community, with increased levels of respiration, leading to a reduction of NPP. Rate of accumulation of biomass begins to slow.
- In climax communities gross productivity may be high, but this is balanced by community respiration & so overall NPP may be lower than earlier in succession.
How does size of plants, nutrients, complexity change through succession?
- Size of plants increases through a succession, & complexity of food webs also increases.
- Nutrients initially come from outside the succession in pioneer stages, but through succession, with increases in biomass, soil depth & complexity, nutrient cycling within ecosystem becomes increasingly important.
- With increased complexity of ecosystem during succession, there are increased opportunities for a variety of diff niches & so species diversity increases.
- Nutrient availability changes from poor initial stores through to gradual accumulation of nutrients as biomass develops & soil fertility increases.
What is cyclical succession?
- Sometimes ecosystems experience a cycle of change rather than a stable climax.
- Species replace each other over time repeatedly without very large disturbances.
- Dominating climax communities are regularly replaced by other ones.
How is grazed woodlands an example of cyclical succession?
An example of cyclical succession can be seen ingrazed woodlands, which can contain achanging mosaic of grassland, woodland, & scrub This habitat can be described aswood pasture as farm animals or deer are systematically grazed on tree land. In a grazed wood pasture, following cycle of events can be observed :
- Grazers, such as cattle, graze on pockets of open grassland within a woodland, where theyconsume tree seedlingsand prevent growth of trees
- Fast-growing thorny species, such as brambles, may arise in some areas within the grassland, creating amosaic of thorny scrub
- Tree seedlings can survivewithin thorny vegetation, whichdeters grazers
- Trees growup through thorny scrub, which will eventually fail to get enough light through tree canopy & die off
- At end of its lifetree dies & falls, creating an open area where grass can grow, & restarting cycle
How is natural wildfires an example of cyclical succession?
- Regular fire cycles in Australian bush lead to continous replacement of climax communities.
- Bushfires occur every 10 – 15 years on a regular pattern.
- Some of 1st plants to regrow are grasstrees (Xanthorrhoea australis).
- These plants often flower as a direct result of fire, quickly bringing an essential food source to surrounding birds, insects & other wildlife.
- It’s often 1st spurt of colour in an otherwise blackened environment.
How is Honey barnacles an example of cyclical succession?
Honeycomb barnacles (Chlamaesipho columna) growing on the rocky shore of the North Island of New Zealand follows a cyclic pattern of succession.
1. Honeycomb barnacles attach to bare rock
2. Crustose algae (Ralfsia confusa) colonizes
3. Black mussels (Xenostrobus pulex) settle on top
4. This smothers the barnacles and algae underneath
5. Mussels detach, bare rock becomes available again.
What does ecological succession tend to lead to?
Given any specific environmental conditions (climatic and edaphic – soil), ecological succession tends to lead to a particular type of climax community.
Why are climax communities are more stable than earlier seral stage?
- They contain more complex food webs (this provides more stability because, if 1 organism becomes extinct, it can be replaced by another)
- Negative feedback mechanisms lead to steady-state equilibrium
- Each stage in succession helps to create a deeper & more nutrient-rich soil, so allowing larger plants to grow
- A climax community is more productive, providing more energy to support consumers & decomposers
- Increased biomass leads to an increased number of niches, which increases species & genetic diversity, resulting in greater stability.
What happens after succession reaches climax community?
With the progression of succession, the pace of change slows down and eventually a climax community exists until a disturbance causes another change. Sometimes human activity alters the natural process.
What is interrupted succession?
Interrupted succession is known as arrested succession or plagioclimax.
How does human activity effect climax communities?
- Reduction in productivity through removal of primary producers
- Reduction in producers, leading to reduced habitat diversity & fewer niches, which threatens more specialised species
- Deterioration in abiotic factors, leading to harsher conditions that fewer species can adapt to
- Species extinction, leading to shorter food webs
- A less complex community, leading to decreased stability.
These activities divert progression of succession to an alternative stable state so that original climax community is not reached.
What are two examples of plagioclimax?
Two examples of this so-called plagioclimax are pastoral grazing & wetland drainage.
How does pastoral grazing prevent ecosystems from reaching a climax community?
- Pastoral farming involves cattle, sheep or other animals on pasture, which are protected from predation & which continuously feed on grasses.
- Small plants of trees & seeds of shrubs are regularly removed that way, never being able to develop into trees & forests of a climax community.
How does wetland drainage prevent ecosystems from reaching a climax community?
- Swamps & wetlands are specialised ecosystems with many plants & animals adapted to anaerobic conditions in soil.
- Wetlands often accumulate peat.
- Drainage is often performed to make these areas habitable for crops, housing or for extraction of peat.
- Soil dries out changing ecosystem considerably.
