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Plant Ecology Text

Chaper 3. Biogeography & Landscape Ecology

If we expect to detect, and understand, the correlations between the Vegetation and the physical/chemical environment within Ecosystems, it is imperative that each aspect must be approached solely in terms of that aspect of the Ecosystem; the Vegetation must be approached in terms of Vegetation without reference to the physical or chemical aspects of the environment, and the environment must be approached in terms of physical and chemical factors without reference to the Vegetation. It is in this spirit that modern Biogeography and Landscape ecology seek to understand the distribution of communities and Ecosystems. However a third, independent factor which may influence the Vegetation has been identified - that being the role of Humans in altering the natural landscape, whether inadvertently as a disturbing force, or with careful management to mitigate disturbances and to create sustainable land use practices. For all practical purposes, wildlife management and land use planning have been absorbed into the domain of Landscape ecology along with the more traditional focus on determining the natural environmental factors which drive the observed distributions of communities. As such, Biogeography and Landscape ecology are highly relevant to the 21st Century. Two major challenges for these disciplines are (1) understanding how natural environmental factors affect the Landscape which has been fragmented into relatively small stands by Human activities, and (2) predicting how the Landscape will change in response to Global Climate Change [not the mythologic climate change debated by politicians, but the well documented climate change currently occurring].
    Biogeography, on the other hand, deals mostly with the factors controlling the distributions of individual species, including changes in distribution patterns, and the implications for the distribution of communities. “Prior to the publication of The Theory of Island Biogeography by Robert MacArthur and E.O. Wilson in 1967 the field of biogeography was seen as a primarily historical one and as such the field was seen as a purely descriptive one. MacArthur and Wilson changed this perception, and showed that the species richness of an area could be predicted in terms of such factors as habitat area, immigration rate and extinction rate. This gave rise to an interest in island biogeography. The application of island biogeography theory to habitat fragments spurred the development of the fields of conservation biology and landscape ecology.(downloaded 24 Feb 2011 from en.wikipedia.org/wiki/Biogeography)” Many ecologists now (late 20th to early 21st Centuries) consider that the factors which control the distribution of individual species provide the basis for the distribution of communities, and therefore should “inform” the theory upon which vegetation analysis is designed. At least some ecologists tend to concern themselves only with “range extensions,” particularly when invasive species (species which are not historically members of a local flora or fauna, and which rapidly increase in population density at the expense of the native flora or fauna) are involved. Flora refers to a complete listing of the plant species known to occur in a defined geographic area, and fauna refers to a complete listing of the animal species known to occur in a defined geographic area.

Landscape: mosaic of patches

We recognize that the Landscape consists of a mosaic of patches. Since the Human species developed agriculture, many of the patches are parcels of agricultural land use and the resulting fragmented patches of the former natural Landscape. In the earlier literature, these patches were sometimes called “stands,” defined by Dansereau as “area occupied by floristically and structurally homeogeneous vegetation. It is the [community],” although I believe that the term was ‘borrowed’ from the surveyors and Naturalists of the 18th and 19th Centuries. The patchwork of agricultural fields and stands of quasi-natural vegetation can be seen clearly in this Google™ Earth image of Erie County (and parts of northern Crawford Co, & northwestern Warren Co), Pennslyvania (and parts of western Chautauqua Co, NY, & eastern Ashtabula Co, Ohio) as seen from an ‘Eye altitude’ = 86.64km (about 54 miles):

The red oval incloses ‘BM 1620’ and is a reference point for the closer images to follow. When we zoom in to Eye alt. = 10.64 (about 6.6 miles), it is more obvious that the light colored patches are agriculture fields. The parallel lines visible on and to the west of BM 1620 (red oval) are “ parallel moraines [which are] closer together than in the Midwest (downloaded 21 Feb 2011 from www.dcnr.state.pa.us/topogeo/field/glacial.aspx)” deposited by the retreating Erie lobe (Wisconsinan glacier) [based on Sevon, W. D., Fleeger, G. M., and Shepps, V. C., 1999, “Pennsylvania and the Ice Age (2nd ed.)”: Pennsylvania Geological Survey, 4th ser., Educational Series 6, 30 p.]. The remaining natural vegetation (darker greens) is largely restricted to the parallel morainal hills. The small black areas are ponds.

Before leaving Erie County, Pennsylvania, I want to zoom in again to Eye Alt. = 3.18 km (about 10,400 feet). At this altitude [high flying, single engine general aviation airplanes & commercial flights as they begin their final approach for landing], the distinct boundaries around the agricultural fields become a bit less distinct. Between State route 86 (the road where BM 1620 is located) and Martin Rd (the next road to the west [left on the image]) at the top of the image, there is a field (with plowed furrows running north-south). On the west side of this field, the boundary between the field and the natural vegetation is about 30 m (about 100 feet) wide, with grassy vegetation & isolated trees. Similar width boundaries can be seen between communities within the remaining natural vegetation. If you look immediately to the left [west] of BM 1620, you will see a home construction site. On the morainal hill west of this house site, there are two communities, a darker green on the northeast-facing side of the hill, and a lighter, mottled green on the southwest-facing side of the hill - separated by another 30 m (100 ft) boundary. [measurements based on the scale at the bottom left of the image]. This is plot material to which we will return in a few moments. [As an aside, the discussion above, with rather detailed descriptions, is an example of “aerial photo interpretation,” a powerful tool for environmental impact studies; however, without ‘ground truth᾿ to confirm the interpretation, I can not identify the communities in the natural vegetation].

    However, where there is data on pre-settlement Landscapes (of lands colonized by Humans [Western Civilization] since the 17th Century), it appears that there was a pre-existing mosaic of natural communities. North America, west of the Appalachian Mountains, is very unusual; between the period of exploration (and trading with the Native American tribes) and the eventual immigration of settlers whose ancestors were European, teams of surveyors crossed the landscape, placing benchmarks (such as our old friend BM 1620), and making maps. Most of these surveyors were also Naturalists, so they recorded information about the biota, the flora and fauna, which they saw. These data, while intriguing for the picture they present of pre-settlement America, are almost entirely qualitative and can not provide the details necessary to interpret the correlations between the flora & fauna and the physical environment. By the time Ecologists began to study these correlations, the pre-settlement period had ended over a century earlier [if for no other reason, Ecology as a Science is, indeed, that young]. Yet not all of the Landscape of our continent has been fragmented to extent we saw in Erie Co, PA. There still remain bits of largely natural areas, notwithstanding the claims of some of the more extreme “environmentalists.” The environmentalists, among those I have met, tend to be more amateur Naturalist than Ecological Scientist. Many of them are very good Naturalists, capable of accurate field observations and of intuitive understanding of the implications of their observations. Few of them are knowledgeable concerning the underlying principles of Ecology. Nearly all Ecologists of my generation have had the luxury of observing natural communities in at least one of the remaining largely natural areas. Based on these observations, we are confident that all Biomes are assemblages of communities, each in its own patch. In some cases, there are patches created by ‘disasters’ such as wildfires, violent storms, landslides, volcanism, etc. In other cases the patches appear, at first glance, to follow roughly the topography of the land. There are well-drained uplands, lowlands with moist soils, wetlands with little to no drainage, river valleys, and canyons with warm, dry south-facing slopes and cool, moist north-facing slopes. In any case, we can draw lines around the communities on aerial photos from 3.0 to 6.0 km (10,000 - 20,000 ft) if we know how to do aerial photo interpretation, which I do. For the morainal hill near BM 1620 in Erie Co, PA, previously discussed [reminder: this is a first draft, and would require ground truth before it can be considered to be the final version];

    What we didn't agree about as the end of the 20th Century approached was the nature of the boundary around the community. We are accustomed in the Biological Sciences to expecting our fundamental units to have discrete boundaries (the cell with its membrane, the individual with its epithelium [or skin], …). At the Genetic & Ecological level, the boundaries become less clear. A population is all individuals of the same species, occupying the same geographic area and interacting & reproducing with each other. So what happens if an individual from a different population happens by and reproduces with a member of the local population? The boundary becomes less distinct. At the community level, we have observed (above) boundaries which can be 30 m (100 ft) wide. One group (the ‘super-organism’ community advocates) argued that the wide boundary was part of neither community, but was an “ecotone” between the two communities. [I once “earned” a “D” on a Lab Report showing that the classic example of a “stable, mature woodland” was no longer stable, because I “must have sampled in the ecotone, rather than the community;” but 5 years later, I read a paper in Ecology which documented that the same woodland was no longer stable due to changing climatic conditions.] The other group (the ‘individualistic’ community advocates) contended that the fuzzy boundary was expected by the hypothesis.

Theory of Island Biogeography:

An Island “is simpler than a continent or an ocean, a visibly discreet object that can be labelled and its resident populations identified thereby… the first unit that the mind can pick out and begin to comprend” (MacArthur, R. H. & Wilson, E. O., 1967. The Theory of Island Biogeography Princeton University Press, Prineston, NJ. p. 3). The island has a clear boundary, the shore line (at low tide, mean tide, or high tide), without an ecotone between it and any adjacent islands, or mainland. MacArtur & Wilson also recognized that it is a model of habitat patches in more complex continental ecosystems where the confusing ecotones restrict our ability to understand the nature of the communities which make up the Vegetation or Landscape (p. 3f). With the intent to provide the basis for a general theory of Biogeography (pp. 5-7), they focused on the biodiversity of the island systems, followed by comments on the implications for resource partitioning by resident and immigrant populations, and on the implications for evolutionary theory. [In the present work, we shall ignore the implications for evolution so we can concentrate on contributimg toward a general theory of Vegetation.] The diversity of islands can be described as the logarithm of the number of species is proportional to the logarithm of the area of the island (the Species-Area curve), although “area seldom exerts a direct effect on a species' presence. More often area allows a large enough sample of habitats, which in turn control species occurence.”
    When a new island appears (such as “Surtsey, a volcanic island approximately 32 km from the south coast of Iceland, is a new island formed by volcanic eruptions that took place from 1963 to 1967.” [downloaded 25 Feb 2011 from http://whc.unesco.org/en/list/1267/) it is soon colonized by Life. Obviously, for any species to immigrate to the island, it must have emigrated from somewhere. This ‘somewhere’ is referred to as the “source region.” (MacArthur & Wilson, p. 23) Because the emigrant is not headed for a specific area, but for any suitable area into which it can immigrate, the chances (probability) that an emigrant individual from the source region will arrive at the island declines as the distance from the source region to the island increases. Not only must the immigrant arrive on the island, it must colonize the island. Colonization implies establishing a population, so the immigrant must find a suitable habitat, including the resources needed for survial, and must reproduce [for animals this generally means there must be at least two individuals, or a gravid female; for plants a single seed or other dispersal agent can suffice to establish a population]. The arrival of the first immigrant does not change the probability than another immigrant will arrive, nor the probability that subsequent immigrants will be of a different species. Following initial colonization by the immigrants, the initial population will become a source region for immigrants to other parts of the island (with higher probability of arriving due to greater proximity to the unoccupied parts of the island). However, the landmasses of Earth include more than a single island plus a single source region, but many islands and continents. Source regions can be other islands as well as continental mainlands. As a result immigrants may arrive from multiple source regions.
    We can anticipate that the early colonizing species should increase their population densities, and spread rather quickly to all suitable habitats on the island. New immigrants (of the same and of other species) will continue to arrive at roughly the same rate as the initial immigrants, and colonize at a similar rate as the initial immigrants, so the island biodiversity should continue to increase. At some time in this process, the island will become saturated with all of the species populations which can be supported on the island, so population growth should slow down, but immigration will still continue at the same rate as before. At this point, the populations established on the islands will begin competing for the resources available, with some populations growing at the expense of their competitors. Similarly, some new immigrants will still be able to colonize, and spread, at the expense of previously established populations. Eventually, the competition for resources will inevitably lead to the local extinction of previously established species. MacArthur & Wilson proposed that these local extinctions should occur quicker on smaller islands (less diverse habitats) than on larger islands, resulting in a dynamic equilibrium of biodiversity. This prediction has been confirmed by Lotze, et al. 2006. "Depletion, Degradation, and Recovery Potential of Estuaries and Coastal Seas." Science 312(23 Jun 2006):1806-1809, who showed that there is a compensatory rise in invasions during depletion [local extinction] events, and that there is an approximately constant sum (declining spp, invading spp).

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