Lecture 7 Flashcards
Ecological Communities
community
- interacting populations of different species found in the same place at the same time
- interrelationships govern the flow of energy and nutrient cycling within community
mun
community structure
a set of characteristics that shape communities
two important descriptors of community
- species diversity (i.e. how many species?)
- species composition (i.e. what species?)
direct vs. indirect effects
- direct effects: the influence of one species on another due to them interacting (e.g. predator/prey)
- indirect effects: the influence of one species on another through intermediaries (e.g. predators impact each other because they feed on the same prey
- direct effects are easier to measure, but indirect effects are more numerous (lead to unpredictable food webs)
trophic cascade
an indirect effect in which consumption at one trophic level results in a change of species abundance/composition at lower trophic levels
e.g. Wolves eat deer. A loss of wolves leads to overpopulation of deer. An increase in deer numbers leads to overeating of trees, and less trees results in a damaged ecosystem.
trophic facilitation
an indirect effect in which a consumer is indirectly helped by a positive interaction between its prey and another species
e.g. Juncus plants aerate the soil for Iva plants, increasing their numbers. Iva plants are eaten by aphids; Juncus hence indirectly helps aphids by increasing the population size of its prey.
the indirect effect of domesticated cats
- domesticated cats are invasive everywhere
- they kill billions of birds and millions of reptiles annually (they are the top human-related cause of bird death)
- however, efforts to eradicate domesticated cats from islands often lead to dysfunctional ecosystems as they control invasive rate populations
dominant species
species that have a large effect on a community, because of their high numbers within that community
EXAMPLE:
deer have a large impact when in high numbers; grazing on young trees and being important seed dispersers for many plants
keystone species
species that have a large effect on a community, despite their low numbers within that community
EXAMPLES:
- sea otters keep urchin numbers in check
- hippos feed on vegetation and defecate in rivers, moving nutrients from the terrestrial ecosystem to the aquatic one
- beavers build dams that create ponds
ecosystem engineers
species that actively shape their physical environment in ways that create a habitat for other species
EXAMPLES
- hippos create grazing lands
- beavers build dams that create ponds
foundational species
species that provide a structural habitat for other species
EXAMPLE:
many plants (trees, coral reefs, sea kelp, mangrove trees, etc.) provide physical structure that other species can live in/on
biological diversity (biodiversity)
all life on earth
three types of biodiversity
- genetic diversity: variation within a species’ genes (among individuals, between populations)
- species diversity: all life on earth, often measured by species richness; varies locally, regionally, and across larger scales
- ecosystem diversity: the variation of species across ecosystems, and the variation of ecosystems within an area
species richness
the total number of species in an area
the impact of habitat heterogeneity on biodiversity
- too little habitat heterogeneity leads to less microhabitats and support for less species
- too much habitat heterogeneity can lead to the death of animals that need large areas to survive (e.g. bears)
the impact of climate variation on biodiversity
- stable climates have more biodiversity than climates with variation
- predictable changes in climate (e.g. seasonality) can increase biodiversity through temporal niche partitioning
- harsh environments are often dominated by an extreme abiotic factor (e.g. temperature)
Global biodiversity is highest in the…
tropics
More than ____ of all life is found in the tropics
67%
four hypotheses to explain high biodiversity in the tropics
- historical hypotheses: the tropics are older, and hence have fewer empty niches
- evolutionary hypotheses: the tropics have higher speciation rates (gain more species faster) and/or lower extinction rates (lose species quicker)
- ecological hypotheses: the tropics have higher primary production rates, stable climates, and longer growing seasons
- null model hypotheses: the tropics are at the middle of the earth, and is where the most species overlap is found
the impact of disturbances on biodiversity
intermediate disturbance hypothesis: communities that experience intermediate levels of disturbance will have a higher species richness because more coexistence is possible
- too rare disturbances favour good competitors
- *too frequent disturbances favour good colonizers
community succession
following the appearance of a new habitat, or a disturbance that removes species, a community undergoes a predictable sequence of changes
pioneer species
early colonizers that persist in a community based on their ability to colonize/recolonize after disturbances (opportunistic species) (e.g. seeds)
climax community
final stage of succession; mature assemply of best competitors (equilibrial species) that persists until a disturbance leads to local extinction (forming gaps)
Out of phase gaps within a community lead to…
high biodiversity (different patches at different successional stages)
types of community succession
- primary succession: colonization following a disturbance that eliminates all the species from an area (e.g. glacial retreats, volcanic eruptions, emergence of new islands, creation of new sand dunes)
- secondary succession: the recolonization following a disturbance that only partially eliminates species from an area; this is faster than primary succession as some species persist as adults or juveniles (e.g. seedbank, eggs in diapause)
the trajectory of a succession’s trajectory
- A community has no life.
- The community undergoes primary succession.
- The community is now at its pioneer stage.
- The community undergoes secondary succession.
- The community is now at its intermediate stage.
- The community reaches its climax stage.
- Disturbances can revert the community to any previous stage.
the impact of historical events on biodiversity
- environments that have lasted longer have higher species richness, due to a lack of large-scale disturbances and longer evolutionary periods
- e.g. Canadian communities are still recovering from glaciation; tundra communities are much longer (less speciation) than tropical ones
the equilibrium theory of island biogeography
S = zA + c
- S = number of species
- z = slope
- A = area
- c = y-intercept
two factors that affect biodiversity, according to the equilibrium theory of island biogeography
- the size of the island: the larger the island, the more species can be supported, and the higher the colonization rates
- the distance to the nearest source of colonists: the closer the island, the easier for new arrivals of the same species (this doesn’t matter if the “rescue effect” is being ignored)
rescue effect
immigration of mainland individuals of a species onto an island can reduce the risk of local extinction of said species
