Sep 19, area relationship may be consistent with the equilibrium hypothesis view of assumed to be dependent upon the distance of the area in question from the species a critical population size above which extinction becomes. tial dependence of species richness at fine scales (!10, fine-scale data or considered whether species-area relationships We hypothesize that fine-scale. In terms of the graph of a species-area relationship, z determines the 'shape' of . area determines the total population size of the collection of species living there. . The actual (or theoretical) curve for any island is dependent on its isolation. . over long enough periods to test the hypothesis of equilibrium with turnover.
Phrased explicitly, it is the number of species immigrating per unit time onto an island already occupied by S species. Also Es, the extinction rate, measured in species lost per unit time from an island occupied by S species.
Finally, we need to know the size of the pool of species in the source area available to colonize the island. The immigration rate Is must certainly decrease monotonically on average as the number of species on the island increases, since as S increases there are fewer and fewer species remaining to immigrate from the pool P of potential immigrants at the source.
If all species were equally likely to immigrate successfully i. There are, however, considerable differences in the dispersal abilities of species in source areas.
Those with the highest dispersal capacities are likely to colonize an island rapidly have a higher immigration rateand later, on average, those with lower dispersal capacities will follow.
They will not only immigrate later, but the rate at which they immigrate will be lower because they have lower dispersal capacities. The rate at which species accumulate on islands is, therefore, initially rapid and then slower. Also, among those species with lower dispersal capacities the successful immigration of any one species should have less effect on the immigration rates of species remaining in the source pool we have not removed a likely immigrant from the pool than would the earlier immigration of a highly dispersible species.
Therefore, this part of the curve should be 'flatter'; the rate of immigration should be little affected by the arrival of one of these poor dispersers. The result is an observed immigration rate curve which is concave. The actual or theoretical curve for any island is dependent on its isolation. For any source pool, the observed rate, while similar in shape, will be lower for more distant islands than for closer ones.
Immigration rates are graphed from the left hand edge of figure 1, declining from the y axis with an increasing number of species already present. Figure - The basic graphical model of equilibrium in the MacArthur-Wilson model. Figure from Brown and Gibson -Biogeography. The extinction rate Es should be, from parallel reasoning, a monotonically increasing function of S. If area, for example, acts only through its effects on population sizes, and extinctions are the chance result of small population sizes and demographic stochasticity, then as the number of species increases, the number of species with small populations and subject to chance extinctions increases in proportion, i.
The Theory of Island Biogeography
However, if we consider a more realistic biological scenario, then as the number of species increases, depressant interactions within and between species competition, predation, parasitism are more likely to occur, and extinctions are more likely as a result.
Remember that these are not species that have evolved adaptations to interactions. Effects are direct and unmoderated. Since any extinctions resulting from interaction are in addition to those resulting from demographic stochasticity, the more realistic shape for the extinction curve is concave upwards.
The extinction rate begins at 0 when there are no species on the island, then increases as species accumulate. At least for purposes of simplicity in looking at the basic implications of the model, the extinction curve can be thought of as a mirror image of the immigration curve.
We now have all the information to produce the basic graphical model. At that diversity on the island species are immigrating at a rate equal to disappearances due to extinction.
The result is constant change in the species list on the island; that change in names occurs at a rate called x, the turnover rate. The length of the species list, however, should remain constant.
This is a stable equilibrium since, should something happen, and the number of species on the island be perturbed, the imbalance between immigration and extinction rates at the new S would tend to return island diversity toward its equilibrium value.
Tests of the Model To test the model, an important piece of evidence is a carefully designed manipulative experiment studying the fauna which colonize 'islands'. One of Wilson's students, Dan Simberloff, tested the model using islands which consist of mangrove mangles in the Florida Keys. In a second series of studies Simberloffthe manipulations were equally inventive. After the islands had been censused, and an equilibrium number of species determined for each island a 'control' diversitycrews moved in with chain saws, handsaws and hatchets, and each island was split into 2 or more smaller parts, with water gaps of 1m between.
To the insects, apparently this 1m gap was sufficient to make crossing from one sub-island to another a jump dispersal. The smaller, sub-islands were then censused repeatedly over a time interval sufficient to permit re-equilibration to find out how species numbers changed with island area.
Remember, the area censused had been part of a previous island, and should contain all habitats plant parts, vertical structures in the same proportions as before i.
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. 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. 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. The network structure, i. In the study area, the exotic plant species Alliaria petiolata, which has invaded relatively small forest patches surrounded by agricultural fields, may have supported more native pollinator species than initially expected. Therefore, this invasive plant may have altered the original relationships between forest area and plant—pollinator networks.
Conclusions Our results demonstrate scale-dependent effects of forest area on the size and structure of plant—pollinator networks. We also suggest that a single exotic plant species can impact plant—pollinator networks, even in temperate continental habitats. Alliaria petiolata, Biological invasion, Forest area, Mutualistic networks, Plant—pollinator interactions, Species—area relationships Background The relationship between species number and island area, namely, that species number increases with increasing island area, is a fundamental rule in ecology [ 1 - 4 ].
MacArthur and Wilson [ 1 ] hypothesized that this relationship results from a dynamic equilibrium between opposing immigration and extinction rates, which depend on island isolation and size, respectively. Because the same trend occurs in continental environments [ 1 - 3 ], the theory of island biogeography has been applied to the conservation of continental habitats [ 5 - 8 ].
Species—area relationships suggest that the number of interaction links among species, such as prey—predator and plant—pollinator species, increases with island or habitat area [ 910 ]. However, surprisingly, only a few studies have tested this prediction, including one that examined plant—ant interactions on oceanic islands [ 9 ], and another that studied plant—pollinator interactions on continental habitats [ 11 ].
Additionally, the network structure of species interactions is empirically known to relate to the total number of species interacting [ 12 - 14 ], which raises the expectation that there may be a close relationship between area and network structure [ 9 ]; however, only one study has tested this prediction at the landscape scale [ 15 ].
Species—area relationships differ among spatial scales, with the shapes and slopes of the relationships differing among local, regional, and global scales [ 316 - 18 ].
In addition, the effects of spatial scale on species—area relationships may depend on taxonomic group; for example, responses to habitat area differ between animals and plants [ 319 ]. Given that networks of species interactions usually involve different taxonomic groups [ 2021 ], different spatial scales may affect the relationships between these networks and habitat area.
However, how habitat area influences interaction networks at different spatial scales at the landscape level has remained unexplored. Biological invasions impact the interactions among native species [ 2223 ], and island communities are more likely to be invaded and affected by exotic species than continental ones [ 24 - 26 ]. However, exotic species may invade continental communities disturbed by human activities [ 1424 ]. Therefore, exotic species may impact the original relationship between habitat area and interaction networks even in continental environments.
Here we analysed plant—pollinator networks, both including and excluding exotic species, in temperate continental forests at different spatial scales to clarify scale-variable effects of habitat area on interaction networks.
Plant—pollinator interactions provide excellent model systems for investigating the structure of species interactions [ 1227 - 29 ]. Plant—pollinator interactions have recently been targeted for studying the network structure of plant—animal mutualistic interactions [ 28 - 30 ].
For example, network metrics such as connectance and nestedness are often used to clarify the structure of mutualisitic interaction networks [ 12 - 143132 ]. As network metrics are correlated with the total number of interacting species [ 12 - 143233 ], and the number of species is related to habitat area [ 1 - 3 ], we expect that network structure is also related to habitat area.