combine both species richness and the evenness or equitability of the distribution of individuals among those species.
Potential evapotranspiration (PET)
A crude measure of available environmental energy. Measures the amount of water that, under prevailing conditions, would evaporate or be transpired from a saturated surface.
The evenness with which individuals are distributed among species in a community.
Species richness MAY BE limited by...
available resources, distribution of resources among species, and overlap among species
Species richness may be highest at _______ levels of productivity, predation, or disturbance, but it increases with _______.
Warmer environments might allow species with narrower niches to...
persist and, therefore, support more species in total.
Keystone species can maintain....
The number of species in a community.
Richness ignores the fact that...
some species are rare and others common.
Diversity indices are designed to...
combine species richness and the evenness of the distribution of individuals among those species.
Attempts to describe a complex community structure by one single attribute such as richness or diversity can still be criticized, because so much valuable information is lot. A more complete picture is therefore sometimes provided in a __________.
A simple model can help us understand the determinants of species richness. Within it, a community will contain more species under what three circumstances?
The greater the range of resources, if a species overlap to a greater extent in their use of resources, or if the community is more fully saturated.
If higher productivity is correlated with a wider range of available resources, then this is likely to lead to increase in species richness, but more of the same might lead to __________.
more individuals per species rather than more species.
In general, species richness often increases with the richness of available resources and productivity, although in some cases the reverse is true. This is known as what? Others have found species richness to be highest at ________ levels of productivity.
The paradox of enrichment, intermediate
Predation can exclude certain prey species and _______ richness or permit more niche overlap and, thus, _______ richness.
When predation permit more niche overlap and, thus, greater richness.
Overall, there may be a humpedrelationship between predation intensity and species richness in a community, with greatest richness at _______ intensities.
Environments that are more spatially heterogeneous often accommodate extra species because....
they provide a greater variety of microhabitats, a greater range of microclimates, more types of places to hide from predators and so on - the resource spectrum is increased.
Environments dominated by an extreme abiotic factor - often called ________ - are more difficult to ________ than might be immediately apparent.
harsh environments, recognize
In a predictable, seasonally changing environment, different species may be suited to conditions at different times of the year. More species might, therefore, be expected to....
coexist than in a completely constant environment.
Opportunities for ________ exist in a non-seasonal environment that are not available in a seasonal environment.
Unpredictable climatic variation (__________) could decrease richness by _________ or increase richness by _________.
climatic instability, denying the species a chance to specialize, preventing competitive exclusion.
There is ___________ relationship between climatic instability and species richness.
The intermediate disturbance hypothesis suggests that...
very frequent disturbances keep most patches at an early stage of succession (where there are few species), but very rare disturbances allow most patches to become dominated by the best competitors (where there are also few species).
Islands need not be islands of _________.
land in a sea of water. They can be lakes in a sea of land or mountaintops in a sea of low altitude.
The number of species ________ as island area decreases. Why?
decreases, because larger areas typically encompass more different types of habitat.
MacArthur and Wilson's equilibrium theory of island biogeography argues for a separate island effect based on...
a balance between immigration and extinction.
On isolated islands especially, the rate at which new species evolve may be comparable to even faster than the rate sat which they...
arrive as new colonists.
Richness increases from the ______ to the tropics.
What has been put forward as partial explanations for high species richness at the tropics?
Predation, productivity, climatic variation, and the greater evolutionary age.
In terrestrial environments, richness often (but not always) _________ with altitude.
In aquatic environments, richness usually ________ with depth.
In successions, richness first ________ (because of _______), but eventually ________ (because of _________). There may also be a _________: one process that increases richness kick-starts a second, which feeds into a third, and so on.
The Cambrian explosion of taxa may have been an example of ________.
The Permian decline may reflect a _________, when the Earth's continents coalesced into Pangaea. Te changing pattern of plant taxa may reflect the....
competetive displacement of older, less specialized taxa by newer, more specialized ones.
The extinctions of the many large animals in the Pleistocene may reflect...
the hand of human predation.
a.ka. predator-mediated coexistence. Occurs when one species 'exploits', as food, a number of species in the community, reducing the dominance of the most competitively superior species and allowing less competetive species to maintain a foothold.
Perhaps the most reasonable definition of an extreme condition is one that requires, of any organism tolerating it,....
a morphological structure or biochemical mechanism that is not found in most related species and is costly, either in energetic terms or in terms of compensatory changes in the biological processes of the organism that are needed to accommodate it.
Examples of harsh environments associated with low species richness include...
areas with low pH, hot springs, caves and highly saline water bodies, such as the Dead Sea.
areas such as hot springs, caves, areas with low pH, and highly saline water bodies, such as the Dead Sea may also attribute their low species richness to....
low productivity and spatial heterogeneity, small areas or areas that are rare compared to other types of habitat. Hence extreme environments can often be seen as small and isolated islands.
Describe the features of a stable environment, as it relates to species richness.
They may be able to support specialized species that would be unlikely to persist where conditions or resources fluctuated dramatically. They are more likely to be saturated with species. Theory suggests that a higher degree of niche overlap will be found.
All the above factors could lead to increased species richness.
On the other hand, populations are more likely to reach their carrying capacities.The community is more likely to be dominated by competition and species are, therefore, more likely to be excluded by competition.
intermediate disturbance hypothesis
suggests that communities are expected to contain most species when the frequency of disturbance is neither too high nor too low.
Many have argued that the tropics are richer in species than temperate regions at least in part because the tropics have existed over long and uninterrupted period of evolutionary time, whereas the temperate regions are still recovering from the __________.
Pleistocene glaciations when temperate biotic zones shifted in the direction of the tropic.
In contrast to those who argue that the tropics are species rich because of uninterrupted period of evolutionary time, it now seem that tropical areas were also ________ during the ice ages, not directly by ice, but by....
associated climatic changes that saw tropical forest contracting to a limited number of small refuges surrounded by grassland.
An alternative explanation for lower species richness in temperate than tropical areas invokes the idea that species evolve faster in the tropics because of higher rates of _______ in these warmer climes. In Wright's study, he found that evolution, as assessed by the rate of __________, turns out to be more than twice as fast in the tropical species.
nucleotide substitution in a particular region of DNA
Describe the four predictions of MacArthur and Wilson's theory of island biogeography.
1. The number of species on an island should eventually become roughly constant through time. 2. This should be a result of a continual turnover of species, with some becoming extinct and other immigrating. 3. large islands should support more species than small islands. 4. Species number should decline with the increasing remoteness of an island.
A more transient bu nonetheless important reason for the species impoverishment of islands, especially remote islands, is the fact that many lack species that they could potentially support, simply because there has been...
insufficient time for the species to colonize.
Evolution rates on islands may be _______ than colonization rates.
The earliest successional species starts a cascade, because they provide ______ and introduce ______ that were not previously present.
During the Cambrian explosion, the opening up of space by grazing on the algal monoculture, coupled with the availability of recently evolved _________ cells, may have caused the biggest burst of evolutionary _________ in the planet's history.
Although there are exceptions, what are the generalizations that can be made about patterns of species richness?
1. Species richness may peak at intermediate level of available environmental energy or of disturbance frequency. 2. Richness declines with a reduction in island area or an increase in island remoteness. 3. Richness decreases with increasing latitude and declines or shows a hump-backed relationship with altitude or depth in the ocean. 4. It increases with an increase in spatial heterogeneity, buy may decrease with an increase in temporal heterogeneity (increased climatic variation). 5. It increases, at least initially, during the course of succession and with the passage of evolutionary time.