| Dept of Biology, Lewis and Clark College | Dr Kenneth Clifton |
|
Biology
141 Lecture Outline |
Defining communities quantitatively and adding "abiotic" into the equation: ecosystems and ecosystem function
Defining communities quantitatively:
Remember, a community is the composite of different species that live together in the same geographical area (but NOT the physical envt.).
Observation: different communities differ greatly in their diversity.
What forces explain these differences in diversity?
Do these differences affect how an ecosystem functions?
What is diversity? Many definitions in our society: diversity has a precise meaning in ecology.
Some example communities: which is most diverse?
Two components of diversity:
Number of species (called "species richness").
Proportional abundance (called "species evenness").
When ecologists discuss "diversity", both components are usually meant unless otherwise specified.
Species richness is easy to measure, is there as a single number for "diversity"?
The use of a "diversity index" (see pg 433 in text)
While the index value has no biological meaning in isolation, it is useful when comparing 2 or more communities.
Some important points to remember given the example:
The Shannon-Wiener diversity index is affected by both the number of species and evenness.
Both a greater number of species AND a more even distribution increase diversity as measured by H'.
The maximum diversity (Hmax) of a sample is found when all species are equally abundant.
Hmax = ln S, where S is the total number of species.
The evenness of the sample is obtained from the formula:
Evenness (E) = H'/Hmax = H'/lnS
By definition, E is constrained between 0 and 1.0. As with H', evenness assumes that all species are represented within the sample.
Trends and patterns involving diversity.
Latitudinal patterns of diversity
Temporal patterns (succession)
In every case, the community found in a particular physical environment defines the ecosystem
The products of the interactions among biotic and abiotic factors within an ecosystem define "ecosystem functions"
Ecosystem function: the attributes of an ecosystem that determine the living conditions of its inhabitants and those of neighboring ecosystems.
Hypotheses about how diversity might affect ecosystem function....
Experiments to test this?
Moving towards ecosystems: A little more on coexistence, biotic vs. abiotic "disturbance" and biodiversity
Predation and abrupt, unpredictable environmental perturbations can both have strong effects on local species diversity... is it okay to think of both as forms of "disturbance"
Keystone Predation.
The competitive exclusion principle says that two species with the same niche cannot coexist This principle presumes that resources are in limited supply. Recall that predation (top-down control) can regulate a population of prey below its K.
Paine's study of predation by seastars in the intertidal zone.
15 different intertidal invertebrates feed by attaching to rocks and filter-feeding tiny organisms and particulates from seawater. They are clearly potential competitors. How can so many spp. coexist?
Paine's hypothesis: predation by seastars reduces competition, allows coexistence.
His experiment: remove seastars from a portion of the intertidal zone, allow them to remain in others (control areas). Both areas had similar species richness at start of study.
result: 8 spp. where seastars were removed, 15 spp. in control areas. Species richness continued to decrease over time where seastars were absent, until these sites were dominated by 1 sp., mussels.
Mussels grew faster than other spp., could overgrow other spp., thus they were competitively dominant.
Why did seastars have this effect on diversity? Feeding trials showed that mussels were seastars' preferred prey.
Conclusion: in the intertidal, seastars are a keystone species, i.e. a species that has an effect on the rest of the community that is stronger than its numbers alone would predict.
Other studies of the effect of predation on prey diversity
Darwin observed that mown turf (mimicing herbivory) contained more plant spp. than unmown turf. Sea otter predation on sea urchins influences kelp forest community structure
Conclusion: predators/consumers tend to increase the species diversity of their prey. But the effect of grazers/predators on the species diversity of their prey depends on the selectivity of the predators.
Disturbance:
Succession
Disturbances create patches of early-successional areas within a larger matrix of later-successional areas, thus increasing species diversity of the overall area. In addition, disturbance that is moderate in its scope can act like predation.
Study by J. Connell on Heron Island, coral reef in Australia.
highest species diversity of corals in in more exposed areas. lower diversity in more protected areas (where competitive exclusion had more opportunity to run its course).
generalization: the intermediate disturbance hypothesis.
Disturbance can create patches of early-successional species in a matrix of later-successional species.
Implications for management of natural areas and the preservation of biodiversity.
So, disturbance may be an important feature of a community.... what does this say about managing "disturbances" such as fire or flood?
This raises the question... are more diverse communities "better" than less diverse communities?
LTER sites (Long-term Ecological Research) funded by NSF (National Science Foundation).
