Lecture 9, 10 & 11 Disturbance and community structure

Question 1

What does interspecific competition seem more important and less important in?

Answer 1

more important:
vertebrates (especially in stable communities)
sessile organisms e.g. mussels
less important:
phytophagous insects 
disturbed communities

Question 2

define a vacant niche

Answer 2

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)

Question 3

in what circumstance may niche differentiation not be necessary for coexistence? why?

Answer 3

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.

Question 4

Describe how competition is an equilibrium theory

Answer 4

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?

Question 5

What do non-equilibrium theories focus on?

Answer 5

• 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.

Question 6

what does disturbance results in?

Answer 6

temporal and spatial heterogeneity. Predation and disease are special types of disturbance.

Question 7

Define disturbance:

Answer 7

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)
simply:
unusual event in what is normality

Question 8

What can categories can disturbance vary in and what are the categories described as? (6)

Answer 8

Severity
Frequency 
Type
Size
Timing
Intensity
The 'fire regime'

Question 9

Severity (fire regime)

Answer 9

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

Question 10

frequency (fire regime)

Answer 10

every few years? 20 - 200 -2000yrs?

predation event = frequent

Question 11

Type (fire regime)

Answer 11

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

Question 12

Size (fire regime)

Answer 12

the geographical scale

Question 13

Timing (fire regime)

Answer 13

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.

Question 14

Intensity (fire regime)

Answer 14

Bio-intensity
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

Question 15

What is temporal heterogeneity? give an example:

Answer 15

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!

Question 16

Intermediate disturbance theory (temporal heterogeneity)

Answer 16

Huston (1979) (Connell, 1978)looked at communities where disturbances were:
- absent
- 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.

Question 17

Why does IDH occur?

Answer 17

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”

Question 18

What is spatial heterogeneity?

Answer 18

Size
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!

Question 19

Explain the patch/gap dynamics theory (spatial heterogeneity) (Paine & Levin, 1974)

Answer 19

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.

Question 20

What is an open system?

Answer 20

Organisms can move about & repopulate other areas if a disturbance happens

Question 21

what is a closed system?

Answer 21

If a disturbance happens cant recolonise with similar species.
For example tropical islands.

Question 22

Give a spatial approach to reproduction

Answer 22

mobile life stages:

- seeds and larvae, spreading everywhere through wind or insects etc

Question 23

Give a temporal approach to reproduction

Answer 23

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.

Question 24

give examples of disturbance/gaps in grasslands

Answer 24

• 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.

Question 25

Give an example of IDH

Answer 25

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)

Question 26

What is a founder-controlled community?

Answer 26

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.

Question 27

what is a dominance controlled community?

Answer 27

when one dies the gap is filled by the species next in line.