Community (Broad definition)
an assemblage of populations interacting in the same place at the same time
Community (Narrow definition)
either functionally or taxonomically. So, we might refer to the "small-mammal community" of the southeastern U. S., which would include predatory mice like the grasshopper mouse, as well as seed-eating mice.
is a functional subgroup of populations in an area that use a common set of resources in similar ways
Community as "superorganism"
sanddunes certain distances from shore
Causal- species that need one another must live together, and collectively they act as one large system - like a big organism.
the "individualistic" concept of community
lived in the eastern deciduous forest, and recognized that although some species are found together, they aren't ALWAYS found together. Species have overlapping distributions, but these distributions are largely independent of one another, dependent on the responses of each species to its environment.
resolved this dilemma by appreciating that there were different types of communities
a community with a large degree of overlap in species composition
a community composed of a specific set of species
the place where environments changed abruptly, and where discrete communities met
Picket and White
Patch dynamic theory
Patch dynamic theory
The environment is not the only determinant of community type; the history of the site must be considered, as well. In reality, even in a very large expanse of habitat, there will be a series of patches of different successional ages, representing communities that are recovering from a disturbance at some time in the past. So, the community is a dynamic mosaic of successional patches of different ages.
4 Types of community concepts
1. Clements- community as superorganism
2. Gleason- individualistic community
3. Whittaker- Gradient Analyses
4. Picket and White- patch dynamic theory
this is simply the number of species in the community
species diversity includes some reference to the relative abundance of the species in the community. If all species are equally abundant, the community is very diverse. If there are species that are very abundant and others that are very very rare, then we would experience a less diverse community. Several indices measure diversity and are affected by changes in both richness and relative abundance (evenness).
predators may regulate the growth of populations uon which they feed. In limiting these populations, they may release lower levels from the limitation previously provided by the intermediate species. So, the structure of a community may be influenced from the top down.
Adding more nutrients to a system may change trophic dynamics and community structure, typically by increasing the biomass in all trophic levels above it.
"the enemy of my enemy is my friend"
are mediated by changes in the per capita competitive effect. These changes can be seen when abundances are kept constant and the effects are non-additive.
resource partitioning and character displacement at the community level. What we might expect to see is a non-random ordering of species along a resource or morphology axis.
can occur if the presence of a third species affects the impact of the second on the first, which is aN2. So, there are two ways a non-additive effect can occur- the addition of a third species can change the abundance of the second (N2), and thus the competitive impact, or it can change the interaction, itself (a).
change the environment in a way that increases the ability of other species to colonize.
mechanisms of succession (3)
the structure of the community is dependent on the order of colonization.
The theory of island biogeography
MacArthur and Wilson (1967) suggested that species diversity in a community might be an equilibrium between colonization (adds species) and extinction (subtracts species). They created a model which considered how these effects might play out on "islands" of isolated habitat, being populated from a source 'mainland' with a particular fixed species pool.
Island Isolation Effects (Distance from Source Populations)
MacArthus and Wilson realized that isolation could have similar effects, in that more distant islands would have lower colonization rates, and higher extinction rates, than most islands. Near islands might even maintain themselves just by migration- continually 'rescued' from extinction by migrants.
-given the reduction and fragmentation of habitat, how should we best maintain biological diversity?
A set of communities is said to be nested if the species found in low-diversity communities are also found within the set of species in high diversity communities.
the energy-diversity hypothesis
the amount of energy/unit are also declines with increasing latitude, and many ecologists have suggested that this correlation with diveresity might be causal.
an assemblage of organisms that live in a region, together with their chemical and physical environments
-biotic and abiotic components of a landscape
studies the flow of energy and matter through and between ecosystems
Gross Primary Productivity
total amount of solar energy fixed by plants for all of their biotic functions: metabolism, growth, and reproduction
Net Primary Productivity
represents the energy stored by plants in new biomass (growth and reproductive tissue)
Variables that affect NPP (4)
compensation point of light E
light intensity where photosynthetic gains= respiration losses
photosynthesizing at a max. rate and additional light cannot increase metabolic rates
low compensation point
low saturation point
high compensation, saturation points
Net secondary productivity
Energy stored in animal biomass- relates to organism's E budget
elements of NSP (know 3)
1) the biomass they remove from the environment
2) the biomass the injest
3) the excreted waste
4) assimilated ingested material
5) assimilated E lost in respiration
A/I- the fraction of ingested E that is assimilated (how easily food is ingested by an animal)
Net Production Efficiency
P/A and describes the fraction of E stored as biomass relative to the E assimilated
High metabolism, low P/A
quantitative ways productivity increases diversity
with more NPP at the base of the trophic pyramid, it can support more levels, thus allowing more species of predator, which has more keystone effects that increase diversity at lower trophic levels
qualitative ways productivity increases diversity
the increase in NPP is not the same material, so it probably means an increase in diversity in plants, increasing diversity at higher trophic levels
Ways diversity can increase productivity
as number of species increases, there is a greater chance of randomly including the most productive species in the plot
reducing the abundance of positive species, and replacing them with another species that has different niche requirements might allow them to grow better and increase Total productivity
a species may exert positive direct and indirect effects on one another
ex. legumes increase N in soil
Types of stability (2)
the ability of a system to withstand disturbance
the ability of a system to return to its initial state after a disturbance
an innate tendency for humans to focus on life and life-like processes