COMMUNITY ECOLOGY I: BIODIVERSITY
Any assemblage of populations [of plants and/or animals] in a given
area or habitat. Community Biodiversity:
Number of species, relative abundance of each species, kinds of
species present inthat habitat. Why is community biodiversity so
important?
Communities with high biodiversity are moreproductive. Communities
with high biodiversity are more stable. Communities with high
biodiversity are better able towithstand and recover from
environmental stresses. Higher-diversity communities are more
resistant toinvasive species. How can we describe biodiversity
quantitatively?
Species Richness: number of different species present in the
community Species Evenness: relative abundance of the different
species present Species Diversity: number and relative abundance of
each species Which Forest is More Diverse?
Each forest has the same four tree species (same species richness),
but they differ in species evenness (relative abundance of each
species). How can we describe biodiversity quantitatively?
Shannon-Weiner Diversity Index Ranges from 0 (only one species
present) to infinity (though usually less than 3 in temperate
habitats). How can we describe biodiversity qualitatively? Marine
Food Web Bottom-Up and Top-Down Controls
V H: increasing V increases H, but not vice versa (Bottom-Up) V H:
increasing H decreases V, but not vice versa (Top-Down) V H P:
Trophic Cascade Consider a grassland with five trophic levels:
plants, grasshoppers, snakes, raccoons, and bobcats.If you released
additional bobcats into the grassland, how would biomass change if
the Top-Down model applied? Plants would increase Plants would
decrease Raccoons would increase Snakes would decrease Grasshoppers
would increase What Factors Affect Biodiversity?
1) Certain types of species have major influences on community
composition... a) Foundation species have major positive or
negative influences because of their physical effects on the
environment. Beaver Beaver dam b) Dominant species have major
(usually negative) influences because of their high
abundance.
e.g. Ponderosa Pine Effect is positive (enhances
biodiversity)
c) Keystone species influence ecological communities more than
would be expected from their abundances. Effect is positive
(enhances biodiversity) Keystone predator: - a species whose
predatory behavior has regulating
effects on other species in the community maintains higher species
richness by altering competitive relationships Classic
Example:Rocky Intertidal Zone Sessile invertebrates
Acorn barnacle Gooseneck barnacle Mussel Mobile invertebrates Whelk
Starfish Chiton Intense competition for space among sessile
invertebrates; one is dominant competitor One mobile invertebrate
species keeps dominant competitor in check and maintains
biodiversity: Keystone predator Expt: Removed and excluded
different mobile
invertebrate species to see the effect on biodiversity Keystone - #
species dropped from 17 to 2.5 in 3 years Keystone (Exptl) Keystone
Recent Example: Sea Otters
Like the rocky intertidal zone, kelp forests are communities of
extremely high biodiversity. Sea otters feed on sea urchins, which,
in turn, feed on kelp. However, orcas have recently turned to
feeding on sea otters along west coast. As a result, sea urchins
have increased and kelp forests have declined. What Factors Affect
Biodiversity?
2) Disturbance Events that damage communities, remove organisms
from them, and alter resource availability (storms, floods, fire,
glaciers, volcanoes) Succession i) Primary Succession ii) Secondary
Succession
The change in species richness and species composition of a
community over time, usually after a disturbance of some kind. i)
Primary Succession ii) Secondary Succession i) Primary Succession -
succession on newly exposed, soil-free areas
e.g. glacial retreat ( community gradients) Primary Succession at
Glacier Bay e.g. volcanic eruption 1981 1999 Primary succession
around Mount St. Helens ii) Secondary Succession
succession in disturbed areas, where at least soil remains (usually
due to clear-cutting or fire) e.g. old field succession Old growth
Spoil banks Biodiversity can increase with forest age. 3)
Species-Area Relationship What Factors Affect Biodiversity?
Species richness increases with the size of the habitat.