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Chapter 26: Community Ecology (Interconnectedness of Species: Food…
Chapter 26: Community Ecology
Diversity
Diversity & Scale
scale matters when considering the diversity of an area
relationship between area & species richness called
species-area relationship
S=cA^z
S: number of species
A: area
c&z: constants that must be discovered by studying individual communities
levels of spatial scale
local:
small area less than a few square kilometer
region:
larger than local, smaller than a continent
biome:
large region, characterized by plants/climate
biogeographical region:
LARGE region, usually coincides with continent, bordered by geographical barriers (ocean, mountains, etc)
Measuring Diversity:
Alpha diversity: # of species/growth forms in small local site
Beta diversity: compares diff b/t several small sites within larger region
Gamma diversity: # of species in a region
species abundance distribution:
number of species in area
Diversity & Latitude
higher latitude = lower diversity
theories for this include more extreme conditions (freezing temp, lack of rainfall)
Predator-Prey Interactions
1 Predator 1 Prey
simplest system of pred-prey interactions
functional response:
feeding rate & handling time
aka how quickly predator finds prey/how quickly predator consumes prey
response is
prey-dependent
dN/dt=rN-aNP
dN/dt
= rate of change w/ time of prey pop
r
= intrinsic rate of increase for prey species
N
= # of individuals of prey species in community
a
= predator’s per capita attack rate
number of prey eaten per prey per predator per unit time
prey-dependent
#both prey-dependent
P
= number of predator individuals present
prey pop stays stable when
dN/dt
= 0
net rate of change with time of predator pop
dP/dt=faNP-qP
dP/dt
= rate of change with time of predator pop
f
= a constant indicating predator’s efficiency at converting eaten prey into new predators
q
= predators per capita mortality rate
independent of pop density
prey pop stable when density of predator equals
r/a
predator pop stable when density of prey equals
q/fa
line indicating population stability:
zero growth isocline
Predator Selection Among Multiple Prey
optimal foraging theory:
study of interactions between 3 factors to understand why certain plants get eaten
3 factors
probability that particular prey will be encountered
decision by predator to attack (worth it?)
probability attacked item will be successfully eaten
optimal diet model:
4 predictions
predators should prefer whichever prey yields most energy per unit of handling time
if high-yield prey become scarce, predator should broaden diet
some prey items will always be eaten if encountered, others never eaten even if easy to obtain
probability that a particular plant will be eaten depends partially on abundance of other plants that are easier to handle/higher value
Competition Between Species
#competition can be examined in web
several species often compete for same resources
exploitation competition:
resource comp. occurs when organisms actually consume a shared resource
interference competition:
one organism restricts another’s access to resources, even if not using it
apparent competition:
one plant species increases, predator increases, other plant species does not increase & declines from predation
Beneficial Interactions Between Species
mutualism:
both organisms benefit from interaction
facilitation:
one organism helps another, doesn’t benefit
Nurse Plants:
alter small area beneath themselves that promotes survival of seedlings of other plants
Interconnectedness of Species: Food Chains/Webs
simplest communities have at least 3 trophic levels
primary producers
ex. plants, algae, cyanobacteria
primary consumers
herbivores
secondary consumers
carnivores
food chain:
direct line of consumption from predator->prey->supporting plant
food web:
all top carnivores, all food sources, connected through web
energy flow web:
tracking flow of energy through community
keystone species:
presence/absence of species dramatically impacts structure of community
Metapopulations in Patchy Environments
metapopulation:
several local populations interconnected by migration & gene flow
#metapop. consists of many local pops
model makes 4 assumptions
region of environment is composed of many discrete patches in which the species can live
some patches occupied by the species while other suitable ones are not
empty patches colonized by migration from occupied patches
populations within individual patches have probability of going extinct within that patch
source habitat:
high-quality patch the species can thrive in
sink habitat:
low-quality patch
fugitive species:
survives by colonizing new patches, flourishing temporarily, then colonizing another patch before dying out in old patch
Concepts
community: group of species that occur together at same time & place
succession: process of change in species structure over time
climax ecology: succession stabilizes with a main species (ex: spruce-fir forest)
community restoration: restoring damaged/degraded/destroyed environments
habitat loss: natural habitat becomes incapable of supporting native species
