Topic 25 Flashcards
Linkage density
average number of actual links (arrows) per species in a food web
Connectance
number of links divided by the number of possible links
Food chain length
number of trophic levels
Most food webs have how many trophic levels
4 levels
Food web complexity
defined by a combination of food chain length, species richness, linkage density, connectance
General pattern
higher food web complexity (species richness) –> higher community ‘stability’ (resistance to change)
More complex food webs have ..
lower resilience; if the structure of a complex food web changes, it will take longer and be harder to return to original state
Typical patterns in communities
many rare moderately abundant species (weak interactors) few common species (strong interactors)
Evidence shows that the removal of a strong interactor results in..
changes to community structure
2 conceptual weak interactors models
- redundancy model
- rivet model
Redundancy model
species = passengers on a plane. few key passengers (species) required to fly plane. loss of any one passenger = no effect. lost of key passengers (pilot) = doesn’t fly. DOMINANCE EFFECT
Redundancy model implies
species within functional groups are redundant
Rivet model
species = rivets on airplane. each rivet (species) plays a small but significant role. lots of a rivet weakens plane, loss of many rivets –> plane falls apart. COMPLEMENTARY EFFECT
Rivet model implies
all species within functional groups are important
Evidence (small scale)
higher stability in species rich (vs species poor) communities under changing environmental conditions. high number of weak interactors results in higher stability
Observational (large scale)
phase shifts (=instability) often follow gradual loss of species richness
Suggests high species richness is essential for..
stable community structure under changing environmental conditions.
Species within a functional group (perform the same roles in the food web) are not…
redundant
Diversity stability (insurance) hypothesis
species richness provides insurance and buffers or minimizes chances of changes in structure under environmental change
Response diversity
variation in responses to environmental change among species within a functional group
Response diversity is critical because species with ..
respond differently to environmental change, these different responses will maintain community structure in its original state
Coral/algal phase shift
free living algal species will readily outcompete coral species if left unchecked. in coral dominated communities several herbivores exist that consume algae. as herbivores re removed from community response diversity eroded. some point community becomes particularly prone to irregular changes in environmental conditions
