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- 1. A biological community is merely the assemblage of all of the species living in a given area.
- a. It can be defined as narrowly as the critters that live on campus or in a particular tree.
- b. It can also be defined as broadly as the 'southwest desert community' encompassing much of southern California, Arizona, and northern Mexico.
- c. It can include only one type of organism, or guild.
- i The community of seed-eating birds in the tropics
- ii A community of filter-feeding invertebrates in the intertidal zone
Communities have emergent properties
- a. Species diversity
- b. Food webs
The greater the geographical area that one samples, the greater the number of species one encounters.
Species accumulation curves
- a. As the number of sampling units increases, generally the number of species encountered increases.
- b. This is because of increased sampling effort. The more you look for stuff, the more stuff you find.
- c. As it is usually impossible to know exactly how many species actually live in a particular habitat, we can estimate the number of species by generating species accumulation curves.
- a. Species richness is the number of species in a community.
- b. Species evenness is the relative abundance of species.
- c. Species diversity is a combination of both richness and evenness.
Shannon-Wiener index (H')
- a. There are many indices of species diversity. Perhaps one of the most common is the Shannon-Wiener index (H').
- b. This is a measure of uncertainty. How uncertain are we about the next species we may encounter in a community?
- c. H' = 0 means that the community is composed of only one species and therefore we have 0 uncertainty as to the identity of the next individual encountered.
- However, H' does not have values only between 0 and 1. You can convert values of H' to another index to get an absolute probability.
Alpha vs beta vs gamma diversity
- a. α-diversity is the species diversity found in a given local. It may be as small an area as the micros inhabiting a strawberry or as large an area as Yosemite National Park. The SimUText is loose in its use of the word “diversity.” It defines it as the number of species; this is incorrect. Diversity is a measure of species richness (number of species) and the evenness of species abundances in a given area.
- b. Β-diversity is the change in species composition over relatively short distances. If the habitat changes dramatically over the distance of interest, the species composition will change as well. This is most commonly seen as one goes up in elevation on a mountain.
- c. ϒ-diversity is the total species diversity in a landscape and is determined by the mean species diversity in sites or habitats at a more local scale (α) and the differentiation among those habitats (β).
Wake up Question
Propose two reasons why one community would have greater species diversity than another. Think about the various ways populations interact.
- Less competition through various niches of the present population.
- Preference to environment and ability to survive in the local area.
Relationships between local and regional species richness
- a. If local species richness increases proportionately with regional species richness (slope = 1), all species within a region will be found in all communities of that region. This implies that the only limiting factor for any given community’s species richness is the pool of regional species; interspecific interactions do not result in fewer species than possible.
- b. In reality not all species in a region will likely colonize every community. The slope is less than one, but local richness still increases as regional richness increases.
- c. In some situations, the local species richness reaches an apparent maximum where no increase in the regional species richness results in increases in the local species richness. Here the community species richness is driven in large part by interspecific interactions with some species preventing the colonization of other species in the community. The community is “saturated” at this point.
Environmental complexity or heterogeneity
- a. Vertical complexity in a forest increases bird species diversity. This is due in large part to the relative number of niches in the more complex forest and how birds distribute themselves (MacArthur’s warblers).
- b. Nutrient complexity and relative competitive abilities also influence species diversity (Tilman's diatoms).
- c. Heterogeneity leads to different microclimates and variation in soil moisture and nutrients. This can lead to many dominant species based on microclimate or microhabitat as opposed to a single microclimate or microhabitat with one dominant species (Pyramid Lake, a reserve in Michigan, and soils in the Amazon).
As nutrients become more available, species diversity declines.
Wake up Question
Why does species diversity decline as nutrient availability increases?
When nutrients increase, some species may have an advantage over others. Species that may have been specialized may now become overwhelmed by the invasive or opportunistic species that will reduce their competition by increasing their overall uptake of nutrients and decreasing solar input.
- a. A disturbance is defined as something that removes biomass from a community.
- b. More on this in the next section, Community Dynamics.
Geographical ecology - Islands
- a. Any patch of landscape can be considered an island.
- b. Some patches may be surrounded by water, others by inhospitable climates, etc.
- c. Generally an island is a habitat that is separated from other similar habitats by non-similar habitat in between.
- Some examples include: Oceanic islands, mountain tops, fragmented habitats, lakes
1. Wake up Question
The previous figure shows a correlation. How would you test for causation?
- 1. The number of species increases as area increases for a variety of islands.
- 2. Distance from the mainland can also influence numbers of species.
MacArthur and Wilson's theory of island biogeography
- a. Diamond's data on bird species in the Channel Islands supports the idea of an equilibrium number of species and species turnover.
- b. Simberloff and Wilson played with mangrove islands and insect recolonization and found that a relationship does exist between distance and colonization. They also documented considerable species turnover.
- c. Simberloff returned to the mangroves and manipulated island area.
Species richness and latitude
Both plant and bird species richness varies with latitude.
An exception to this trend is shown for a type of wasp.
Hypotheses to explain the trend
- a. Time
- i Time for dispersal of new species into an area may explain why some habitats are relatively poor in species diversity. These are sometimes referred to as "young habitats." Mountaintops recently uplifted are an example.
- ii The tropics have supposedly experienced the longest length of relatively stable climate throughout the past tens of millions of years. This is in comparison to the temperate region that has experienced periods of glaciation. This would mean that extinction rates should be lower on average in the tropics. Time for speciation is also longer in the tropics due to this stability.
- b. Heterogeneity
- i Physical characteristics of a region play an important role. Specifically topographic heterogeneity leads to different microclimates and variation in soil moisture and nutrients.
- ii Biological diversity may lead to greater biological diversity for many of the same reasons as in "i" above. MacArthur's warblers revisited.
- iii In essence, heterogeneity "creates" more niches, opportunities for specialization.
- c. Favorableness
- i Temperature is negatively correlated with latitude.
- ii Precipitation and evaporation are most favorable at lower latitudes than higher latitudes.
- d. Productivity
- i The areas of highest primary productivity are in the tropics.
- e. Area - there is more land area in the tropics than in other biomes. Larger continents support more species.
Exceptions to the trends
Species richness hot spots
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