The Theory of Island Biogeography
The species–area relationship (SAR) describes theincrease in species area; isolate; island biogeography; community ecology; power law; z‐value; species. Species-area relationships are a common focus of habitat fragmentation research differences in evolutionary history, sample area, habitat heterogeneity , island size, A representation of the island biogeography theory that takes both area. The relationship between species and area is a Island biogeography Species extinction rate decreases Abele & Patton () .. Ecology and evolution of.
Journal of Animal Ecology Gleason HA On the relation between species and area. May RM Patterns of species abundance and diversity.
Preston FW Time and space and the variation of species. Preston FW The canonical distribution of commonness and rarity: Svensk Botanisk Tidsskrift Ecology Letters 16 Suppl: Journal of Ecology Journal of Theoretical Biology Biodiversity and Conservation Encyclopedia of Biodiversity, vol.
Griffin DA Diversity theories. Alterations were only quantitative, in the form of area reduction, no unique feature was removed. The results were clear-cut. Each island reduced in size re-equilibrated at a lower insect diversity. Considering all the experimental islands in developing a model for the pattern in reduction, the diversity change fit a log-log relationship i. Thus, Simberloff's data fit the original species-area relationship.
Area was the key determinant. The process of re-equilibration, however, involved extinction of species from islands supersaturated due to their reduction in size. We have already encountered the underlying biological cause of those extinctions: Such extinctions are an important component of the equilibrium model of island biogeography.
Figure - Effect of island fragmentation on insect diversity in mangrove mangles. There are few islands that have been studied over long enough periods to test the hypothesis of equilibrium with turnover, i. Among those few are the California Channel Islands. The interpretation of these data is a source of continuing controversy. That's important, because the crux of the equilibrium theory is proof or documentation of insular turnover at equilibrium. A paper Gilbert found 25 attempts to document turnover at equilibrium, and found few basically just mangrove island studies by Simberloff acceptable without question.
In Simberloff's original defaunation studies, for example, one island supported 7 species of Hymenoptera prior to fumigation and 8 after equilibrium had been re-established about one year later.
However, only two of these species were present both before and after fumigation. This sort of experimental study is designed to allow for rapid re-equilibration. The Channel Island studies represent an interesting attempt to deal with the problems of scale here time when dealing with most real ecosystems.
Recognizing that there may be difficulties the initial, historical survey of species presences on the island used breeding records collected over many years, rather than a single survey at one timeDiamond's studies of turnover on the Channel islands are still regularly cited Diamond Initial data reported collections and observations indicating the fauna of individual islands in Diamond compared those species lists with a survey he did in The islands had the following characteristics: Numbers remain almost constant while turnover occurs in a significant number of species.
Neither was the case; instead turnover was approximately inversely proportional to the number of species present.
Species–area relationship - Wikipedia
That is not forecast by the model. Figure - The number of species in censuses of 3 of the California Channel Islands. Why should turnover be related to island area or isolation?
Consider first 2 islands at equal distance from the source, but differing in area. Long distance jump dispersal is generally assumed to be a chance event, not directed or goal oriented. In that case, dispersal probabilities and immigration rates onto the 2 islands should be the same. Area, however, does affect the extinction rate of colonists.
The larger island should have 1 higher habitat heterogeneity, 2 decreased intensity of interactions due to reduced niche overlaps resulting from habitat heterogeneity and 3 larger population sizes making chance extinctions less likely.
These factors should be operative, at least in a relative way, independent of the number of species present. Therefore, the extinction curves should have similar shape, but have lower values for the larger island.
Putting this comparison on a graph, but using a linearized version of immigration and extinction curves, we find a larger equilibrium number of species on the larger island, but also a lower turnover rate on that island. To assess the effects of isolation consider 2 islands of equal size, but located at differing distances from the source.
With identical sizes we assume that habitat heterogeneity, population sizes and interactions on the islands are quantitatively identical, and thus they have the same extinction rate curves. Immigration rates onto the more distant island should, however, be lower at any S since the probability of a successful dispersal decreases possibly exponentially with distance. We can go further, and suggest that the decrease should be most noticeable for species which tend to be among the first colonists.
Later immigrants with lower dispersal capacities have only a slim chance anyways, and depend on rare, special conditions like storms for successful immigration. For these species a change in distance should mean less in shifting immigration rates. Once more we turn these suggestions into a comparison on the graph. The more distant island has a lower equilibrium number of species, but also a lower turnover rate at equilibrium than an island closer to the source.
Figure - Multiple immigration and extinction curves indicating effects of differences in size and isolation on equilibria and turnover rates. Brown and Gibson These comparisons can be combined in various interesting and complicated ways. Rather than document the possibilities, it is probably more valuable to attempt to list the assumptions and predictions of the basic MacArthur-Wilson model. Some of the ideas in this list will not be fully examined until later in this section.
There are no gross environmental changes over the time period of colonization 3 Species counted on islands are residents 4 There is a definable mainland species pool What Are the Characteristics of the Equilibrium?
Extinction rates increase with increasing species numbers What Influences the Equilibrium Number of Species? With regard to Diamond's data, no combination of size and isolation leads to the prediction that turnover rate is inversely or in any other sense proportional to the number of species on an island.
Since the data are repeatedly cited and classic, it's worth trying to understand why this anomalous result was reported. There are a number of possible answers, and arguments in the literature could be described by indicating that 'the fur has definitely flown'.
For one thing, the interval between the censuses was very long. That may have had significant effect on the measured turnover. If the time interval is long enough it becomes likely that some of the species which had gone extinct at some time between the censuses also re-immigrated during that interval or the converse. In either case the measured turnover would underestimate actual rates.
To attempt to correct for that possibility, Diamond and his collaborators went back to the Channel Islands annually during the early 's, and also used thorough data gathered for Farnes Island off Great Britain. The result of differences in the interval between censuses is evident in Fig.
The result for the Farne Islands is parallel. In either case the apparent turnover decreases rapidly as the census interval increases.
To show you why, consider what happened to the meadow pipit on Farnes between and May and Diamond The pipit bred for 2 years, went extinct in the 3rd, then went through 5 more cycles of immigration and extinction over the remainder of the period.
From annual census records that indicates 11 turnover events in 29 years, where a census after 30 years would have recognized only a single extinction, as well as a constant diversity of 6 species on the island. The same basic pattern applies to the Channel Islands.
That's not the only corrective surgery which has been suggested for the theory. It is also evident that monotonic rate functions particularly the immigration rate curve may be overly simplistic.
That should be evident by drawing a parallel between accumulation of species on an island and primary succession.Theory of Island Biogeography
When an island is newly formed frequently volcanic it has no organic content in and frequently no mineral soil. The first plants must be special sorts that have no requirement for nutrients from the soil or possibly no requirement for soil at all ; instead they are soil formers, leaving behind their nutrients extracted from the rock as well as their bodies to improve conditions for later arrivals.
Krakatoa, East of Java, was not only a B movie, but a real historical event in the 's. What kind of immigration curve described the relationship between immigration rate and the number of species on Krakatoa after its formation.
All other things being equal habitat diversity and distance to the mainland or source of colonizing speciesif we have two islands with the same immigration rates, we would expect extinction rates on large islands to be lower because of their larger population sizes.
Therefore larger islands, at equilibrium, would have a greater number of species. On the other hand, if we have two islands equal in size and habitat diversity but at different distances from the source, then extinction rates would be expected to be the same, but immigration rates would be higher for the nearer island, and at equilibrium the near island would have more species. The species-area relationship can be approximated by a power function of the form: The constants C and z are fitted from the data on island area and number of species, and so are specific to a data set.
Browne and Peck used long-horned beetles Cerambycidae: Coleoptera to investigate the species-area relationship in the Florida Keys and mainland. Their data are plotted below, using the log10 of the area and species number.