Flashcards in Lecture 9, 10 & 11 Disturbance and community structure Deck (27)
What does interspecific competition seem more important and less important in?
vertebrates (especially in stable communities)
sessile organisms e.g. mussels
define a vacant niche
possibility that in ecosystems or habitats more species can exist than are present at a particular point in time, because many possibilities are not used by potentially existing species (Rohde, 2005)
in what circumstance may niche differentiation not be necessary for coexistence? why?
when disturbance events happen. If disturbance is frequent then there's not enough time for competitive exclusion to occur as carrying capacity is not met, therefor both species can coexist in a wavelike pattern.
Describe how competition is an equilibrium theory
Competition is an equilibrium theory
- focuses attention on the properties of a system at an equilibrium point
- time and spatial variation are not central concern
- e.g., niche differentiation & resource utilisation
focusses on the idea of succession?
What do non-equilibrium theories focus on?
- focus attention on temporal and spatial variation
- concerned with transient (lasting for a short time/impermanent) behaviour of a system away from equilibrium point
- any interruption to competitive exclusion may prevent extinction and enhance diversity (disturbances)
- any interruption to competitive exclusion effects diversity
Disturbances that reduce the pop size of certain species that would take over in competition, allow for certain species to coexist.
what does disturbance results in?
temporal and spatial heterogeneity. Predation and disease are special types of disturbance.
a relatively discrete event in time and space that alters the structure (identity and abundance of organisms) of populations, communities and ecosystems and causes changes in resource availability or the physical environment” (Pickett & White, 1985)
unusual event in what is normality
What can categories can disturbance vary in and what are the categories described as? (6)
The 'fire regime'
Severity (fire regime)
Magnitude of change in the resource supply or environment caused by the disturbance e.g.
avalanche or tree falling over = small severity
flood event across the whole of britain = large severity
frequency (fire regime)
every few years? 20 - 200 -2000yrs?
predation event = frequent
Type (fire regime)
How pre-adapted are organisms to this disturbance?
Organisms adapt through evolution to disturbances that are relatively frequent, but not to novel or very infrequent ones. e.g.
a river running more than it should - there will be adaptations for this i.e. invertebrates burying deeper into rocks.
trawling - no organism will be adapted to this as unnatural
Size (fire regime)
the geographical scale
Timing (fire regime)
Predictability often allows organisms to evolve adaptations to minimise effects of disturbance.
Human activities can alter natural timing of some forms of disturbances (e.g., floods, fire, grazing)
When to shed leaves in deciduous trees. Sometimes not normal for the times of year.
Seasonality. Sometimes floods happen seasonally, but are changing times e.g. amazon.
Intensity (fire regime)
The energy released per unit area and time
- Applies to fires, hurricanes, marine storms
Often influences the severity of the disturbance, but not always
- Slow, smouldering fires (not intense) are often more severe than rapidly-moving intense fires
What is temporal heterogeneity? give an example:
Continually changing conditions – frequency of disturbance.
Phytoplankton, there are so many & do the same thing (photosynthesize) how come some have not become the ultimate phytoplankton? their environment is constantly changing (o2 & pH). they have trade-offs & environment doesn't allow them to outcompete eachother.
think intermediate disturbance theory!
Intermediate disturbance theory (temporal heterogeneity)
Huston (1979) (Connell, 1978)looked at communities where disturbances were:
- intermediate frequency
- very frequent
He found low diversity where disturbance was absent, and where it was very frequent. Highest diversity was found at intermediate disturbance frequencies.
Why does IDH occur?
At low frequencies of disturbance, the dominant organisms are allowed outcompete.
Other end there are only species that have adapted to live with these disturbances
Middle, all can coexist and none can outcompete each other.
IDH is about frequency of disturbances not “levels”
What is spatial heterogeneity?
disturbance can occur at various sizes, but this does not necessarily reflect its importance
- Large scale, e.g., glaciation, flood, fire, wind, landslip
- Small Scale, e.g., molehills, cowpats
- Micro scale, e.g., worm casts, hoof prints
Think patch/gap dynamics!
Explain the patch/gap dynamics theory (spatial heterogeneity) (Paine & Levin, 1974)
The concept of patch dynamics is based on the observation that ecosystems are spatially heterogeneous—that is, they contain a diverse and unevenly distributed mixture of organisms and resources—and heterogeneity occurs across scales of time and space. Ecological disturbances, such as floods, fires, and outbreaks of disease, are responsible for much of the dynamics of spatial heterogeneity; they disrupt the continuity of the biological community by reducing populations and eliminating species from small areas in the landscape and creating the conditions for other species to move in. As a result, ecosystems can be thought of as mosaics of patches. The patches differ in a variety of ways, including size, shape, composition, natural history, duration in the landscape, and boundary characteristics. As a result, FLUX, rather than balance, is a major emphasis in patch dynamics research. metapopulations are important here.
What is an open system?
Organisms can move about & repopulate other areas if a disturbance happens
what is a closed system?
If a disturbance happens cant recolonise with similar species.
For example tropical islands.
Give a spatial approach to reproduction
mobile life stages:
- seeds and larvae, spreading everywhere through wind or insects etc
Give a temporal approach to reproduction
seed banks – wait until conditions are favourable, given enough time the place they're in will become suitable.
sapling banks - have evolved mechanisms to stay dormant until ready.
give examples of disturbance/gaps in grasslands
- cow pats - not many plants can grow through except for a few e.g. thistles can grow through. Plants next to the pat get more nutrients/fertiliser & cows won’t eat plants around their own shit and grow more! eeds that can pass through cows guts & get pooed out and grow, but have to be very well adapted as cow guts are built to get through the cellulose.
In any case this changes the structure of the grassland community.
Having cows in a field this enhances diversity, creates space for more diversity
- molehills e.g., Trifolium repens will only germinate by seed in mole hills in North Wales, famous study.
Give an example of IDH
seaweeds on boulders
small pebbles, medium boulders, large boulders. Medium has most seaweeds, medium get disturbed a medium amount of time. Small ones all the time & big ones need a storm to turn. This represents frequency of disturbance. (Sousa, 1979)
What is a founder-controlled community?
after disturbance or death more like a lottery:
reef damselfish occupy a spot and will defend until they die, that gap is open by some sort of disturbance is filled with an EQUALLY FIT COLONIZER. whichever organism is down to chance. high diversity as no one species is likely to colonise than any other.