Our second example involves time scales in ecology. If we could know the environmental factors essential to determine the location of a species, we would still have the problem of how these factors interact with the species in time. Again, this is a concept that we experience in our daily lives. We can quickly stick our hand into a very hot oven and remove it without harm.
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While these temperature extremes can be tolerated, they cannot be tolerated for very long. Similarly, the factors that interact to control geographic distributions of plants and animals have a time frame in which they cause responses; at other levels they do not. Ian Woodward illustrated this concept when considering the responses of different plant processes to climate factors. Other periodic changes occur in the climate, as well. For example, extreme long-term periodicities cycling over tens of thousands of years will be discussed in the next chapter.
Woodward conjectured that the expansion and contraction of the ranges of different types of plants late successional trees, herbaceous perennials and annuals should be controlled by longer frequencies of 34 The Black-Headed Bird Named Whitehead climatic variation. Because they have a long life and are large enough to exert a degree of local control over their environment, late successional trees would require climatic changes of multiple centuries to induce a contraction in their range. Such a contraction involves the death of established plants, a process that takes time.
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Because they are shorter lived, other types of plants should have a more rapid response than trees. The temperature variation felt when the sun goes behind a cloud, or a cool breeze touches your face, lasts minutes or seconds. Seemingly straightforward, these variations do run somewhat counter to popular, almost mystical, ideas that everything in the ecosystem is tied to everything else. Not much at all. Knowledge of the appropriate scales in space and time for a given question is essential in this determination.
Species abundances would be proportional to the area containing suitable places for them. The species with inappropriate niches over much of the land- The Black-Headed Bird Named Whitehead 35 scape would be rare and might move elsewhere or even perish. This expectation of the outcome is fairly simple, whereas the process could actually be rather complicated.
However, the rules that produce patterns of commonness and abundance could be much more complex.
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Usually, it is purposefully designed to be extremely simple and straightforward. Further investigations are needed to determine just what these effects might be. Null models assume that it is appropriate always to select the simplest most parsimonious explanation of a natural phenomenon. The preference for a simple explanation over a complex one as a criterion for judging competing theories is associated with a thirteenthcentury English monk, William of Occam, or Ockham — Occam was engaged in the medieval religious argument over whether God revealed knowledge to humankind or hid it.
For example, the U. Bureau of Reclamation dredges the Colorado River for sand and silt that have eroded into the river and piles the spoils as sand islands. The islands are devoid of vegetation, but by pumping water from the river through a drip-irrigation system to small planted cottonwood Populus fremontii trees, one can create a rapidly growing forest.
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Since only trees that are planted and drip-irrigated become established in the initial stages of sand-island succession, the 36 The Black-Headed Bird Named Whitehead structure of vegetation can readily be controlled. Earlier, they had developed measurements of vegetation structure as the niches of a long list of Arizona birds.
These data were used to forecast the species found on the sand islands. The presence or absence of birds could be predicted correctly about 90 percent of the time in these experimentally manipulated habitats.
Changes in habitat structure will favor some species and disadvantage others. Today the omnipresence of environmental change and human alteration in natural ecosystems has elevated the need to better understand what controls the abundance of plants and animals. GPS allows rapid location of the exact positions of ground observations. Powerful computers can then interweave layers of complex spatial data on the environment over large areas. New technologies are already augmenting these applications. While Grinnell made it clear that it was vital to consider interactions with other animals both predators and competitors , these were not his paramount factors.
The Grinnellian niche is, in most of its aspects, a concept of individual organisms or species. The niche is an amalgam of the environmental factors involved in understanding why a given organism may occur in a given location. The California thrasher is but one of a set of western thrashers with relatively similar body size and coloration. It is associated with and is adapted to the chaparral habitat. In the eastern United States only one species, the brown thrasher T.
What is the pattern of differences among all these thrashers? Do thrashers play different roles in different shrubby habitats? What conditions could allow the species to be found together? These and an array of similar questions that could be asked about the California thrasher are associated with a different concept of the niche. A British ecologist, Charles Elton, developed this later niche concept with a fundamentally different intent. Elton considered the interactions among species populations that produce regularities in ecological communities. He knew that the penguins of the southern Figure Convergence among African left and South American right rainforest animals.
Pairs from top to bottom: pygmy hippopotamus Hexaprotodon liberiensis and capybara Hydrochaeris hydrochaeris ; African chevrotain Hyemoschus aquaticus and paca Agouti paca ; royal antelope Neotragus pygmeus and agouti Dasyprocta leporina ; yellow-backed duiker Cephalophus silvicultor and gray brocket Mazama gouazoubira ; giant pangolin Manis gigantea and giant armadillo Priodontes maximus. Meggers, E. Ayensu, and W. Duckworth, eds. The Black-Headed Bird Named Whitehead 39 oceans would look and behave like the familiar auks of the northern oceans.
Elton saw convergence as evidence of similar niches in geographically separated communities and reasoned that these similarities implied rules for the patterns of species niches. He emphasized interactions among the different populations of animals and plants.
That focus changed in , when a brilliant experimental biologist, G. Gause, produced a clever set of laboratory experiments to understand competition between species. Gause worked with three species of the microscopic protozoan genus, Paramecium; pairs of these species competed for food. The experiments used test-tube Paramecium populations fed by regular inoculations of yeast cells.
The intent of these now-classic experiments was to test then-current mathematical theories for the interactions between populations of different species. The joining of mathematical descriptions of the growth, death, and competition of species with experimental results held the promise of moving population and community biology to a more rigorous, mathematically formal level—an exciting breakthrough for theoretical population ecology.
Gause observed that both of the non-coexisting paramecia fed in the top portion of test tubes, where each was able to survive in the absence 40 The Black-Headed Bird Named Whitehead of the other. Also, either competitor could coexist in the presence of a third species, which appeared to feed at the bottom of the test tubes. A top-of-the-test-tube feeder could live with a bottom-of-the-test-tube feeder, but two top feeders could not coexist.
Internal factors that structure communities have remained central to the interest of researchers involved in competition studies and with other Eltonian issues such as the limit of similarities among species, or the degree to which different communities can be invaded by alien species owing to their empty niches. However, demonstrability under natural conditions is a topic of debate. A rich array of circumstantial evidence has been used to support the concept that the structure of ecological communities results from competitive interaction. One can conceive of ecological communities of plants and animals being shaped by the set of all the pairs of species having different relations.
Webs of positive and negative interactions theoretically could produce patterns in the abundance of plants and animals at a given location. These patterns include differences among individuals degrees of similarity in shape and size and species patterns of distribution and abundance. While there are exceptions, the modern Eltonian theory strongly emphasizes competition. Theoretical treatments of competition as the structuring force inside ecological communities are topics of considerable debate among ecologists.
Rather, the issue is how competition can be used in truly predictive theories. Some ecologists consider competition to be a concept that has failed to develop into theories worthy of the name. The result is a complex network of concepts that are internally consistent and, while close to the observations data, are often based on circular arguments.
An increased appreciation for the dynamic and interactive nature of ecological systems has developed in the process. A consistent observation in ecological communities is that if two species are considered with enough detail or enough different niche factors , almost invariably ecological differences in the species will be found. There are two interpretations of this observation of separation of species niches in communities. This construction derives from considerations of competition and niche interactions.
The second interpretation is that communities of animals and plants are relatively open. Niches are available for additional species, and species niches do not overlap very much. The concept of highly interactive communities is central to the development of community theory, but evidence suggests that real communities are a mixture of biologically noninteractive and interactive—with the noninteractive cases being more in evidence.
The objectives of the Grinnellian approach are relatively straightforward. They involve determination of the attributes of a species and of the environmental factors controlling the range of the species.
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The geographic scale is large. The Eltonian concepts are much more abstract. They involve community structure: ratios of sizes, numbers, and other patterns. The focus is on the ecological community, a more local assemblage of plants and animals. Thus, the space scale is relatively smaller than that in the Grinnellian applications.
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