Communities: reality or chance?

SQ/Reflection questions:
1. After going over the notes, carefully consider whether the serpentine site we've worked on these last few weeks is a true community or as Gleason would describe, just a collection of species. After giving a brief explanation of the characteristics of a community, explain why, using these same characteristics, you would describe Soldiers Delight as a community or not.
2. Does it make more sense to save endangered species or endangered communities?. Explain in very tight economic times, why saving the latter makes more sense ultimately?
3.What sense can you make of Simpson's, Shannon-weavers and similarity indices. What does each really tell us? how can they be used? Could we use them with our forest analysis ( yet to come)? Be able to calculate them if data were given.What are their assumptions?
4. A trophic structure is a lot more important that just knowing what eats what or how webby it is. It very often in real life defines community species diversity, stability, selection and coevolution, and so on. Give an example where changes in feeding trophic chain could break a community.
5.Would you describe the collection of microbes in your mouth as a community?

Above is an old shot I took in the Okeefenokee Swamp down in Georgia many moons ago.The community although distinct in it large size and perhaps in its old age, is much like any other cypress swamp community.

Others ecologists would argue however, that there is in fact no such thing as a community or 'typical' collection of speices which may be replicated in nature; just historical collections of species, coming together by chance.
This question today of what is a community is not trivial. With the loss of so many species out there, little of nature can be protected. So should we protect endangered species as singular entities? or do we protect whole communities?
Before we can deal with this question, we need to understand what constitutes a community and get some feel why some ecologists feel the concept of community holds no water.

A community is an assemblage of populations of living organisms in a prescribed area or habitat. (Unlike an ecosystem which ties together both biotic and abiotic components) I. Characteristics of communities: 1. Species diversity - a species list which reflects its richness or diversity For example a TRF would have high diversity and the tundra low- this would simply reflect the total number of species present, not their relative abundance's. (It doesn't matter if you just have a handful of each,, only the total species list length counts) The reasons for this high TRF diversity is still under debate: it could be a factor of time ( how long these systems have survived undisturbed, energetic's ( incoming radiation annually), types of relationships, resources available and so on. We will go in detail on this topic in a later this week. 2. Growth form and structure: communities can be described readily on the basis of the dominant growth forms, generally the plant forms. There is no way you would mix up a grassland with a forest. TRF- tall canopies of trees Swamp as the Okeefenokee above.. grasses with hammocks of trees Tundra: small perennial herbs 3. Dominance - what are the dominant species? Generally few species dominant any community; the majority of species are rare. It is the properties of the 'dominants' that control the conditions for the rest of the species. If most of the biomass is in the form of wood, then you would find different herbivores and decomposers than in a system dominated by grasses or algae.

4. Relative abundance - measures the relative proportions of different species in the community Individual counts Community A Community B hardwoods 85 10 conifers 10 85 shrubs 5 5 How would you characterize community A? community B? To more clearly differentiate between communities, a more numerically generated descriptor would be better: One diversity index widely used is Simpson's Index: D = 1 / sum of the pi2 where pi is the proportion of individuals of species A that make up that community- if you found in a sample of 100 individuals that you counted 20 maples, 20 red oaks, 20 pine trees, 20 birch and 20 dogwood, then the proportion of each ( pi) is 20/100 or 0.20 An example is: A (maples) B (red oaks) C (pines) D (birch) E (dogwood) Community A .2 .2 .2 .2 .2 Community B .1 .3 .5 .07 .03 The Simpson's index for community A is [ 1 / sum of ( .22+ .22 + .22 + .22 + .22) = 1/.2 = 5 meaning that no one species dominates, it is a more complex environment for herbivores and others Do the calculations for community B? ( answer on bottom) what does the value imply? Another index used in the ecological literature is the Shannon-Weaver index ( also used in communications theory and other fields): H = - sum of pi log pi see text for further explanation The problem with type of analysis comes from sampling error itself. If you sample few individuals as we did in the field, you definitely will find few species dominate, and the rest to be found in smaller numbers. However if you expanded your sample size, you would more likely find a normal distribution of species, because you would find increasing numbers of very rare species

5. Trophic structure - feeding relationships How complex are the feeding relationships? they vary in simple communities and can get quite complicated in complex communities such as the TRF Simple trophic structure: species a eats --> species b eats --> species c a = fox b = rabbit c = grass More complex trophic structure: species a eats b + c and b eats d,e,f while c eats g, h, i   species b -----> species d, e, f species a ----->       species c -----> species g, h, i We no longer have a linear but an emerging web: species a eats b + c and d and g also For a while it was thought that the more complex the chain, the more stable it was; after all if species a can no longer eat b, it can still eat d and g. Again, later we'll go into why complex feeding relationships are not always necessarily more stable. II. Communities are dynamic, not stable: Communities can and do change: If over time the community changes--> and if this is an orderly process, it is called succession ( developmental stage). On the east coast farmlands succeed from weedy herbaceous fields to shrubs and eventually pine to a climax of some form of deciduous woods. We see this pattern repeatedly. Over space --> with environmental gradients in salinity, pH, moisture, solar input and so on. ( fringe edge or in the center, can be a mosaic or gradient). Over the salt marsh we see patterns of tall to short cordgrasses grading into deciduous borders.

III. Do communities actually exist? or are they a figment of the ecologists imagination ( reductionists who believe they can order or simplify nature into reproducible constructs. The great debate earlier this century: Clement's school Gleason's school a plant ecologist an animal ecologist Super organism: the total is greater than the parts, closed community Continuum school: haphazard collection of populations with minimal integration, basically an open community; who is there is due to migration & chance evolution: clearly defined assemblages, very discrete evolution: all have same environmental requirements           The debate lasted for decades: how to prove which one was correct? A. Similarity or dissimilarity of stands: ( based on species listings) Index of similarity: I = [ 2 * C ( species in common) ] / A + B where A is number of species in community A and B is number of species in community B present in community A? present in community B? sp 1 *yes * yes sp 2 * yes sp 3 * yes sp 4 * yes * yes   I = [ (2 * 2 species in common)/ 3 species is A + 3 species in B] = 4/6 = .66 If communities were discrete than you would come up with values moving to 1 Reality: data shows there are very few truly discontinuous communities; if you compare 2 bogs, some have primarily red maple & sedges while in others you would find Vaccinum ( blueberry family) dominating.   B. Continuity vs. discontinuous communities: Where boundaries are sharp, you can find discrete communities ; due to topography or discrete rainfall patterns you might find clear boundaries which would lead you delineate discrete communities. Data: finds most communities are along gradients and thus diffuse- you find grasslands integrating into forests gradually etc. so a lot of commonalties dependent on where you sample. So really a question of how you sample- compare 2 middles, and the 2 communities will be discrete; if you sample the edges you may find much in common.   C. Dynamic relationships between species: If truly a superorganism, species should be bound together in a network of obligate relationships. If just a random mixture, then these types of relationships should be rare, and in fact there should be few discrete relationships These types of relationships would include parasite/ host that have coevolved mutualisms ( see notes on mutualisms for examples) mimicry Answer: Well, it depends on the community you look at: TRF- there are a lot of the above types of relationships - thus could justify superorganism Deciduous forests don't have as many - more typical of Gleason's assessment. Where do we stand currently? both make sense under different circumstances, so neither to be rejected. Answer for community B = 2.81