lecture 40 - communities and succession

Question 1

What is a community?

Answer 1

“the set of all populations

found in a given place.”

Question 2

How do communities begin and become stable?

Answer 2

succession- pioneer species

Bare rock –> lichens –> small annual plants and lichens –> grasses and perennials

Question 3

what is secondary succession?

Answer 3

when a disturbance occurs and destroys life and succession has to take place again
Sources of disturbance
Natural, unusual, short term events:
ABIOTIC (droughts, floods, hurricanes, fire)
BIOTIC (outbreaks, invasions, extinctions)
Anthropogenic (man-made) - Pollution, invasions,
exploitation, global warming

Question 4

What is community stability?

Answer 4

• Resistance of a community to disturbance
• Resilience of a community to disturbance
– Elasticity (speed) of recovery
– Amplitude of change recovered from
• Global resilience (recovery from large changes)
• Local resilience (recovery from small changes)
What to measure?
• Resistance
– How much does population
abundance/community biomass change?
– How often are invasions successful?
– How often do extinctions occur?
– How much does community biomass vary?
• Resilience
– How quickly does population
abundance/community biomass return to normal?

Question 5

What are biological invasions?

Answer 5

Populations entering a novel community.
• Biological invasions are natural – the
processes of speciation and extinction are
adding and subtracting species.
• Successful species tend to expand in range, which also means invasion.
• Invasions may have no measurable effect on the invaded community, or they may have a profound effect.

Question 6

What are the consequences of invasions?

Answer 6

1. Invader dies out before establishing, original community is unaffected
2. Invader establishes,
   and augments the community,
   with no effect on the rest of the community
   Ecesis: The successful establishment of a
   plant or animal in a new habitat.
3. Invader establishes,
   with one or more extinctions in the
   resident community
4. Invader establishes, with one or more extinctions in the resident community, but eventually the invader dies out

Question 7

What are human induced invasions?

Answer 7

• Most invasions are currently the result of
human activity
• Especially as a result of trade and colonisation
• Ships provide many opportunities in cargo
holds and ballast tanks
• Leading to a homogenization of the world’s biota
• Human-dominated ecosystems are growing,
dominated by a few crop species
and a handful of ubiquitous commensal
species

Question 8

rusty crayfish in the USA?

Answer 8

Spread by non-resident anglers
who bring them to use as bait
• Displace native crayfish
• Reduce abundance and diversity of aquatic plants
• Decrease density and variety of invertebrates
• Reduce some fish populations
No environmentally sound methods for removal have yet been developed, so prevention and slowing down spread is the best method of control at the moment.

Question 9

What are keystone species?

Answer 9

• A species with strong interactions with a
disproportionate effect on communities.
• A species whose removal would lead to
significant changes spreading through the
food web
• A keystone species may be important not
because of its trophic role.
• Ecosystem engineers: actively shaping the
physical environment, creating habitat that
otherwise wouldn’t exist.

Question 10

What are ecosystem engineers?

Answer 10

actively shaping the physical environment, creating habitat that otherwise wouldn’t exist.

Question 11

What are keystone mutualists?

Answer 11

pollinators of ecologically dominant plants, or nitrogen fixing bacteria supporting legumes and hence a whole plant community and its reliant animals.