1. Community Ecology Definition of community: A community is a group of populations that coexist in space and time and interact with one another directly or indirectly. Note that this definition does not exclusively consider plants; a community includes all the decomposer, plant, herbivore, and carnivore populations that coexist, even if we will concentrate on plants here.

2. Are communities like ‘super-organisms’, the species bound to each other by their interactions, or Are communities associations of species assembled by coincidence, and with species independent of each others’ presence and absence? The first view is sometimes called a closed community. The idea was developed in the 1920s by a plant ecologist named F.E. Clements. He even coined the term ‘superorganism’ to describe it. The second view is called an open community. This was proposed by H.A. Gleason.

3. The implications of these two views of community structure are distinct, and allow us to assess whether one view works better than the other, or whether we need some compromise intermediate between them. The organismal community suggests (due to interdependence) that only particular groups of species should be found together, and that the boundaries of communities should sharply divide one group of species from another. This is almost a common sense view. Foresters talk about particular kinds of forests, defining them by the most common species: oak-hickory forests or oak-maple forests that dominate around here, pinyon pine or ponderosa pine forests in the Rockies and the west. Tall grass prairies are dominated by Andropogon gerardii (big bluestem), the characteristic species of the community.

4. Oak-hickory forest

5. Tall grass prairie Andropogon gerardii

6. Gleason’s view, of an open community comprised of ‘individualistic’ species, suggests that community boundaries should be indistinct, that there may well be species substitutions in what we might describe as an oak-hickory forest just because those species are the most common ones in it. As well, when we look at the boundaries of distributions of species, those boundaries should not be linked. The boundaries are transitions between one habitat/ community and another, called ecotones. They are usually quite sharply evident. Here are some examples:

7. There is a transition evident from the photograph. Consider the picture on the right. It isn’t evident from the picture that the soil beneath the grassland is a serpentine soil, with much different nutrients than beneath the forest.

8. Clements’ organismal community concept suggests clear and relatively sharp community boundaries. Are ecotones always clear and sharp? No! On a continental scale Eastern deciduous forest has relatively sharp transitions to boreal forest to the north (due to cold), to open grassland to the west (due to limits on rainfall), to fire- tolerant pine forests to the south, and to ocean on the east. However, eastern deciduous forest is not ‘monolithic’. There are different species dominant, and different component species in different parts of the eastern deciduous forest. This sounds more like Gleason’s view, not only described as open, but as the basis of the continuum concept for communities. We can draw logical predictions from the two views:

9. Here is what Whittaker found along mountainsides in Oregon (the Siskyou Mts.), in the Santa Catalina Mts. of Arizona/California and, in the figure, in the Great Smoky Mts. Of Tennessee. Are there any signs of organismal interdependence and correlation in distribution limits among species? These two views produce different theoretical distributions of component species of communities:

10. Species’ responses to environmental gradients similarly suggest their individuality. What set the distributional limits that Whittaker observed along the various mountain ranges? Even without detailed environmental measurements, we can surmise that there were differing abiotic conditions at work.

11. The evidence we have points generally to the open, individualistic view of living communities. However, there is another way to learn about communities: study of the fossil record they leave behind. Trees (and other plants) that are wind pollinated leave behind evidence of their presence in the form of pollen grains. By aging segments of sediment cores (usually taken from the bottom of lakes) and identifying the types of pollen present in these segments, the component species in communities at different times in the past can be learned. In northern temperate North America, we can see what happened to communities as the glaciers receded northward over the 15,000 years since the last (Wisconsin) ice age (glaciation).

12. Pollen diagrams tell us that the communities (or at least species coexisting) as the glaciers began to recede are not combinations we would observe today. During recession various other combinations occurred. The diagram below comes from a marsh in Minnesota…

13. In this area of Minnesota spruce (Picea) and ash (Fraxinus) were dominant 20,000 years ago. After that there were waves of dominance by birches (Betula), pines (Pinus), elms (Ulmus) and oaks (Quercus), then they declined and grasses (Poaceae) became dominant. In recent times (the last 2000 years or so) pines and oaks have returned, now sharing the space with sedges (Cyperaceae), whose pollen is common in part because the samples come from a marsh. The dominant plants are different in southern Ontario, but the process is conceptually similar…

14. In southwestern Ontario 10,000 years ago, the forest was dominated by white pine… Pinus strobus eastern white pine There was no hemlock or hickory in those forests. But, by 5,000 years ago, with the gradual recession of the glacial edge northwards, hemlock and hickory had migrated northward from their glacial refuge from the southeastern U.S. White pine remained a component of the community.

15. Tsuga canadensis eastern hemlock Carya glabra pignut hickory These post-glacial migrations tell us two things: 1.The boundaries of communities shift continuously in response to environmental conditions. 2. There was no necessary community integrity involving all, or even most of the species that now comprise a recognized community during post-glacial migration.

16. There is much more information about the northward migration. For example, some species basically moved straight northward. Others moved back into Canada by way of the Atlantic provinces and eastern Quebec. Still others migrated all the way around the Great Lakes, re-entering Canada a little west of Lake Superior. What had been a community before the Wisconsin glaciation fragmented for the duration of the glaciation and migrated separately northward as the glaciers receded. All this does not suggest Clements’ organismal community.