Chapter 3 Flashcards
ecological succession
Ecological succession is the gradual replacement of one assemblage of species by another as conditions change over time
two basic types of succession
- Primary succession
- Secondary succession
primary succession
Primary succession is the colonization of a previously unvegetated surface, where little or no soil exists
e.g., when a glacier retreats or a landslide removes all traces of the vegetation of the previous ecosystem
The next stage in successional advance is usually invasion by herbaceous plants such as grasses and ‘weed’ species
The seeds that lie ‘in wait’ in the soil are considered to be part of the soil seed bank
The next stage is hardy shrubs and light tolerant trees, with trees dominating the final stage
Each stage along the way is known as a seral stage
primary colonizers
Primary colonizers are the first species to occupy the area, and must be able to withstand harsh conditions
e.g., lichens (and later, mosses) (grasses in dune systems)
climax community
The last stage of succession
A relatively stable, long-lasting, complex, and interrelated community of organisms
Influenced by climate (climate climax) and can be determined by soil factors (edapic climax)
In the first half of the twentieth century, it was believed that vegetation would reach a well-defined, stable stage called the climax community (final stage of succession).This stage was believed to be in equilibrium with the environment
In the second part of the twentieth century it became clear that equilibrium conditions are rare
Disturbances are so common that most systems never reach a stable climax stage
fires, insect infestations, flooding, ice storms
disturbance
Process that alters ecosystem structure and function
Mountain Pine Beetle invasion in Western Canada
Many disturbances are natural and integral parts of healthy ecosystem functioning
Recovery patterns following disturbance depend on many factors
Ecosystems and landscapes are dynamic, interacting in complex ways, often unpredictably, and over large spatial and time scales.
cyclic succession
Succession is not always linear.
e.g., hardy shrubs and trees can be first colonizers
Cyclic succession may occur when a community has progressed through several seral stage but is then returned to earlier stage by disturbance
Seral stages may blend into each other rather than being discrete
These blended areas have high species diversity and are known as ecotones; richer zones between communities
mature community
Communities do not always reach a stable climax community, however the species assemblage that is more constant over time is characteristic of a mature community.
‘Climax’ vegetation is strongly influenced by climate (climax climate)
Soil can be more important than climate in determining community composition (edaphic climaxes)
secondary succession
Secondary succession is the sequential development of biotic communities on previously vegetated surfaces that have soil cover, and that have been disturbed, e.g., abandoned farm fields
Faster than primary succession, and initiated by invading species such as annual ‘weeds’
Similar processes also occur in aquatic environments; the natural aging process is called eutrophication
–Can be a challenge for farmers and resource managers
as succession occurs..
- NPP declines
- biodiversity increases
intermediate diversity hypothesis
suggests that diversity will not increase indefinitely; and that moderately disturbed ecosystems have higher biodiversity than those that experience either high or low disturbance
intermediate disturbance hypothesis
A hypothesis suggesting that ecosystems subject to moderate disturbance maintain higher level of diversity compared to low or high levels of disturbances
effects of human activities
Humans influence ecological succession
We often keep ecosystems in an early stage
Agriculture, forestry
Increased productivity
Faster nutrient and water cycling, with greater losses
Reduced biodiversity, especially at higher trophic levels
Increase in pioneer species
changing ecosystems
In the early 1970s, lakes in the Okanagan (BC) were invaded by Eurasian Water Milfoil, which later spread to many other lakes in southern BC and other provinces
inertia
is the ability of an ecosystem to withstand change, while resilience refers to the ability to recover to the original state following disturbance
invasive alien species
Organisms found in an area outside their normal range are considered alien species
e.g., Purple Loosestrife and Eurasian Water Milfoil
Species that multiply quickly, out-compete native species, and change native habitats are considered to be invasive alien species
Invasive alien species are often fast-growing generalists that can alter growth form, reproduce quickly both sexually and asexually, disperse readily, and associate with humans
invasive species
Invasive species are second only to habitat destruction as a leading cause of biodiversity loss.
Canada has been affected by over a thousand alien species invasions, many with terrible effects
Dutch Elm Disease, Zebra Mussels, Knapweed
zebra mussel
Native to the lakes of southeast Russia
Invasive to many countries North America, Britain, Spain and Sweden
Colonized the Great lakes
Detected in Lake Winnipeg in 2013.
Threat to native mussels
Cover the undersides of docks, boats, spread into streams and rivers nationwide.
Block pipelines, clogging water intakes of municipal water supplies.
rusty crayfish
Native to Ohio River basin
Have rusty looking spots in each side of their abdomen.
A threat to the aquatic plant bed and the native species
Almost like humans clear cutting a forest.
A relatively new invader of Manitoba being initially spotted in 2007 in the Falcon Lake area
hyper abundance
Native species populations increase to undesirable numbers
Where natural habitats have been disturbed
When predatory species are removed
Species culls are often used to control these population explosions
species removal
Removal of species from food webs can disturb the ecosystem
The reduction of keystone species may be particularly disruptive
–For example: sea otter on the Pacific coast are overhunted sea urchin population explodes kelp (seaweed) habitat disappears biodiversity is lost
feedback
Feedback is an important aspect of maintaining stability in ecosystems, whereby information is returned into a system as a result of change
Feedback initiates responses that may exacerbate (positive feedback) or moderate (negative feedback) the change.
Positive feedback loop: the effect of increased temperatures in the North (polar amplification)
Negative feedback loop: the possible role of phytoplankton in global warming
ecological restoration
Restoration ecology developed as a field of study, and practice, to help repair environmental damage
Examples:
Remediation of Sydney Tar Ponds
Reclamation of treeless areas around Sudbury
Efforts to reintroduce endangered species into national parks
Ecological restoration is very challenging and very costly, and there is widespread agreement that it is better to avoid ruining ecosystems in the first place rather than trying to restore them afterwards
population
the number of individuals in a species
