An island biogeography model for beta diversity and endemism: The roles of speciation, extinction and dispersal

Speciation and endemism under the model of island biogeography

DYNAMICS IN SPECIES COMPOSITION OF STREAM FISH ASSEMBLAGES: ENVIRONMENTAL VARIABILITY AND NESTED SUBSETS

Dynamics in Species Composition of Stream Fish Assemblages: Environmental Variability and Nested Subsets

Relationships between local annual immigration and extinction rates of plant species and total species richness were determined from long-term data in permanent plots in tallgrass and shortgrass prairies in Kansas. Combining plots resulted in higher equilibrium numbers of species as predicted from immigration and extinction rates. Immigration and extinction rates also increased with scale. Extinction rates are higher because the regional scale supports more rare species which, in turn, have high probabilities of extinction. We also tested the hypotheses that extinction rates would be higher on burned versus unburned grasslands, and that immigration rates would be higher on grazed versus ungrazed grasslands. Extinction rates were positively correlated with the number of species at a site, and this relationship was not altered by burning or grazing. Immigration rates were variable, but were sometimes positively correlated with growing season precipitation. Immigration rates decreased in years sites were burned. Therefore, after fire, the number of species going locally extinct was still dependent on earlier species richness, but the number of species added to the site was reduced. Variances in immigration and extinction rates were high, therefore, confident predictions regarding the effects of burning or grazing regimes on species richness could not be made. Variance in rates of immigration and extinction results in a range of values within which the equlibrium number of species fluctuates randomly.

Dictated by limited resource availability for land acquisition, a central question in conservation biology is the ability of areas of different size to maintain species diversity. The selected reserves should not only be species rich at the moment, but should also maintain species diversity in the long run. We used two sets of data on vascular plant species in boreal lakes collected in 1933/34 and 1996 to test the relationships between lake area and the extinction, immigration and turnover rates of the species. Moreover, we investigated, whether the number of species in 1933/34 or water connection between lakes was related to extinction, immigration and turnover rates of species. We found that lake area or shoreline length was not correlated with immigration or turnover rate. But extinction rate was slightly negatively correlated with shoreline length. The original number of species was positively related to the number of species extinctions and to the absolute turnover rate in the lakes, which indicates that species richness does not create stability in these communities. Species number was not correlated with immigration rate. Upstream water connections in the lakes did not affect immigration, extinction or turnover rates. We conclude that length of the shoreline is a better measure of suitable area for water plants than the lake area, and that because the correlation between shoreline length and extinction rate was slight, also small lakes can be valuable for conservation.

Dispersal processes, such as immigration and extinction rates, and habitat properties play a crucial role in determining species composition and nestedness patterns within communities. In the current global scenario of changing environmental conditions and habitat fragmentation, information on the role of natural dispersal mechanisms and disturbance factors are especially important for understanding dynamics of species composition changes in stream ecosystems. We investigated spatial and temporal patterns of nestedness of fish communities in tropical stream systems of central India and the relationship of various dispersal factors (immigration–extinction rates and their variability) with habitat properties (habitat size and heterogeneity) and anthropogenic disturbances. We tested predictions of the classical model by Schlosser on immigration–extinction dynamics along longitudinal gradients in stream community composition for these streams. The results revealed significant patterns of nestedness for all study sites. Sites exposed to varying degrees of disturbance (induced by human activities) showed lower nestedness than undisturbed ones. Immigration rates did not show strong relations with nestedness but extinction rates were significantly related (negatively) to nestedness. In addition, disturbance played an important role in determining immigration rates and variability in immigration rates. Stream characteristics, such as habitat-size and habitat-variability gradients, were not statistically significant predictors of immigration and extinction rates. Our results demonstrate the influence of local anthropogenic disturbances on dispersal dynamics of species. A reduction in availability of suitable habitats could lower immigrations.

AimMacArthur and Wilson's theory of island biogeography was revolutionary, and also inspired the more recent unified neutral theory of biodiversity and biogeography. The unified neutral theory has the potential to make predictions about island biogeography that are not well studied. Here we aim to unify the two theories by using an ecological neutral model to study immigration and extinction rates on islands – the cornerstone of MacArthur and Wilson's theory.MethodsWe conduct simulations of a spatially implicit neutral model and measure species abundances, immigration rates and extinction rates. We study the behaviour of the model at dynamic equilibrium and on approach to dynamic equilibrium both from volcanic origin (low initial diversity) and from land bridge origin (high initial diversity). We extend the model to study the effects of clustered immigration and to explicitly account for the distinction between immigration and colonization.ResultsOur model, in accord with the simplest version of MacArthur and Wilson's theory, predicts linear immigration and extinction rates as functions of species richness at dynamic equilibrium. In contrast, the approach to dynamic equilibrium produces rich and unexpected behaviour where immigration and extinction rates are non‐monotonic functions of species richness, at odds with other theory. Once examined, however, this behaviour makes biological sense and results from the influence of the species abundance distribution over immigration and extinction rates. The turnover predicted by our first model appears high, but can be lowered to realistic levels with an alternative model of clustered immigration or by accounting for the difference between the immigration of a new species and its true colonization of the island.Main conclusionsMacArthur and Wilson's theory of island biogeography and ecological neutral theory are different, but there are strong similarities in their assumptions and predictions that should not be overlooked when evaluating them. Our results highlight the importance of species abundances as indicators of immigration and extinction rates; species richness alone is insufficient. In particular, extinction rate and species abundances are unavoidably linked, as rarity usually precedes extinction.

The evolution of biodiversity among the Southwest European Neogene rodent (Mammalia, Rodentia) communities: pattern and process of diversification and extinction

One of central problems in ecology is to explain species diversity in communities, and a considerable number of hypotheses with this purpose have been formulated during last three decades. One very influential group of hypotheses, largely emanating from Hutchinson (1959, 1961), seek explanations in terms of mechanisms of coexistence. The question addressed by these hypotheses is: why are there so many coexisting species? (implicitly assuming that most potential inhabitants of a certain community are present, or have at least had opportunity to be present, but may have been subsequently outcompeted). These hypotheses usually predict that species, in order to coexist, in some way must avoid negative effects of competition. A common theme for wide array of mechanisms suggested to have this, or alike, effects (e.g. Hutchinson 1961, MacArthur 1972, Grubb 1977, Grime 1979, Huston 1979, Tilman 1982, Fagerstrom 1988) is that they are confined to, in a strict sense, ecological phenomena, such as spatial or temporal separation of resource use, herbivore-mediated suppression of dominants, or chance effects during recruitment. An alternative (or complementary) view of diversity in communities stresses importance of historic, or phylogenetic, aspects of species diversification. Present-day diversity is a result of speciation and extinction processes that have occurred over long time. In his overview of diversity in land communities, Whittaker (1977) concluded that there is not much evidence, either theoretical or empirical, for that species diversity is in equilibrium. Hence, in Whittakers view, to understand patterns of diversity, large scale aspects of speciation, extinction and biogeography must be accounted for. This line of reasoning can be exemplified by some studies of diversity in tropical rain forests. These communities are well known for their exceptional plant species richness (Gentry 1982), and one general explanation for their high diversity is that tropical forests have accumulated species under stable conditions experienced during long time (Stebbins 1974). In a study of tropical tree species diversity, Hubbell and Foster (1986) suggested that tree communities are in a state of non-equilibrium; new species' (either immigrants or newly evolved ones) are capable of invading communites due to a permanent occurrence of empty sites. Since risk of species exclusion from a plant community was found to be very small, diversity would be determined by regional species richness and immigration rates. Thus, variation in species diversity among communities, or among guilds within communities, reflect rate-determining mechanisms behind speciation and extinction. High species diversity is expected when existing speciespool contains many species, and comparatively low species diversity will be found when species-pool is small. Similar suggestions have been made by several authors, in various contexts, (e.g. Grime 1973, 1979, Cracraft 1985, Hodgson 1986, 1987, Connell and Lowman 1989, Hart 1990, Brooks and McLennan 1991, Cornell and Lawton 1992, Zobel 1992), and Taylor et al. (1990) coined term the species-pool hypothesis for explanations of local diversity by reference to size of regional or global pool of species: All else being equal, larger local and/or global area of a habitat type and older its geological age, greater past opportunity for speciation and hence, greater number of available species adapted to a particular habitat type. However, if different phylogenetic lineages do not diversify at same average rate, age (or size) of a habitat type may be only weakly related to richness of species-pool. Furthermore, Zobel (1992) stressed importance of evolutionary factors (i.e. speciation rate) in explanations of species diversity in plant communities, but did not specify what might determine speciation rate per se. Cracraft (1985) approached diversity problem from a different angle and suggested that community diversity is explicable by basic processes speciation and extinction; speciation rates are mainly determined by large scale changes in lithospheric complexity, and extinction rates are dependent on envi-

A common pattern in time-calibrated molecular phylogenies is a signal of rapid diversification early in the history of a radiation. Because the net rate of diversification is the difference between speciation and extinction rates, such "explosive-early" diversification could result either from temporally declining speciation rates or from increasing extinction rates through time. Distinguishing between these alternatives is challenging but important, because these processes likely result from different ecological drivers of diversification. Here we develop a method for estimating speciation and extinction rates that vary continuously through time. By applying this approach to real phylogenies with explosive-early diversification and by modeling features of lineage-accumulation curves under both declining speciation and increasing extinction scenarios, we show that a signal of explosive-early diversification in phylogenies of extant taxa cannot result from increasing extinction and can only be explained by temporally declining speciation rates. Moreover, whenever extinction rates are high, "explosive early" patterns become unobservable, because high extinction quickly erases the signature of even large declines in speciation rates. Although extinction may obscure patterns of evolutionary diversification, these results show that decreasing speciation is often distinguishable from increasing extinction in the numerous molecular phylogenies of radiations that retain a preponderance of early lineages.

1. Methods that assess patterns of phylogenetic relatedness, as well as character distribution and evolution, allow one to infer the ecological processes involved in community assembly. Assuming niche conservatism, assemblages should shift from phylogenetic clustering to evenness with decreasing geographic scale because the relative importance of mechanisms that shape assemblages is hypothesized to be scale-dependent. Whereas habitat filtering is more likely to act at regional scales because of increased habitat heterogeneity that allows sorting of ecologically similar species in contrasting environments, competition is more likely to act at local scales because low habitat heterogeneity provides few opportunities for niche partitioning. 2. We used species lists to assess assemblage composition, data on ecologically-relevant traits, and a molecular phylogeny, to examine the phylogenetic structure of antbird (Thamnophilidae) assemblages at three different geographical scales: regional (ecoregions), intermediate (100-ha plots) and local (mixed-flocks). In addition, we used patterns of phylogenetic beta diversity and beta diversity to separate the factors that structure antbird assemblages at regional scales. 3. Contrary to previous findings, we found a shift from phylogenetic evenness to clustering with decreasing geographical scale. We argue that this does not reject the hypothesis that habitat filtering is the predominant force in regional community assembly, because analyses of trait evolution and structure indicated a lack of niche conservatism in antbirds. 4. In some cases, phylogenetic evenness at regional scales can be an effect of historical biogeographic processes instead of niche-based processes. However, regional patterns of beta diversity and phylogenetic beta diversity suggested that phylogenetic structure in our study cannot be explained by the history of speciation and dispersal of antbirds, further supporting the habitat-filtering hypothesis. 5. Our analyses suggested that competitive interactions might not play an important role locally, which would provide a plausible explanation for the high alpha diversity of antbirds in Amazonia. 6. Finally, we emphasize the importance of including trait information in studies of phylogenetic community structure to adequately assess the mechanisms that determine species co-existence.

GLOBAL VARIATION IN THE DIVERSIFICATION RATE OF PASSERINE BIRDS

The stratigraphic record of the Frasnian-Famennian mass extinction in the eastern United States indicates that a reversal in the magnitude of extinction and speciation rates was the driving mechanism forcing the decline in species diversity in this region. Extinction rates increased sharply during the deposition of the West Falls Group in New York State, and remained high for the remainder of the Frasnian, a period of approximately 4.5 Myr. Sharp reductions in species diversity, however, only occurred during deposition of the Java Group at the very end of the Frasnian. The period of diversity loss was much more abrupt and short term than the period elevated extinction rates, and was more a function of a drop in speciation rate than a direct function of an increase in extinction rate.

Are polyploids really evolutionary dead-ends (again)? A critical reappraisal of Mayrose etal. ().

Summary1. Species richness in a habitat patch is determined by immigration (regional) and extinction (local) processes, and understanding their relative importance is crucial for conservation of biodiversity. In this study, we applied the Island Biogeography concept to spring ponds connected to a river in southwestern Japan to examine how immigration and extinction processes interact to determine fish species richness in temporally variable environments.2. Fish censuses were conducted 15 times in 13 study ponds at 1–4 month intervals from August 1998 through October 2000. Effects of habitat size (pond area), isolation (distance from the river) and temporal environmental variability (water level fluctuation) on (i) species richness, (ii) immigration and extinction rates and (iii) population size and persistence of each fish species were assessed.3. The results revealed predominant effects of distance on species richness, immigration/extinction rates and population size and persistence. Species richness decreased with increasing distance but was not related to either pond area or water level fluctuation. A negative effect of distance on immigration rate was detected, while neither pond area nor water level fluctuation had significant effects on extinction rate. Further, population size and persistence of four species increased with decreasing distance, suggesting that, in ponds close to the river, immigrants from the river reduce the probability of extinction (i.e. provide a rescue effect), contributing to the maintenance of high species richness.4. Overall results emphasise the importance of immigration processes, rather than extinction, in shaping patterns of species richness in our system. The predominant importance of immigration was probably because of (i) high temporal variability that negates habitat‐size effects and (ii) continuous immigration that easily compensates for local extinctions. Our results suggest that consideration of regional factors (e.g. connectivity, locations of source populations and barriers to colonisation) is crucial for conservation and restoration of local habitats.