Experimental Zoogeography of Islands: A Two-Year Record of Colonization
Introduction
This study, conducted in 1966-1967 by Daniel S. Simberloff and Edward O. Wilson, investigates the recolonization of arthropod faunas on six small mangrove islands in the Florida Keys after complete removal via methyl bromide fumigation. The initial phases of recolonization, up to the first year, were previously documented, observing species numbers reaching apparent equilibria before a slight decline to interactive equilibria. Island E1 was an exception due to its distance, lagging in reaching predefaunation equilibrium after one year.
The research aimed to assess species equilibrium theory, particularly regarding immigration and extinction processes, as outlined by MacArthur and Wilson (1967), Simberloff (1969), and Wilson (1969). The initial findings indicated that most islands recovered species numbers similar to pre-fumigation levels within a year, with the distant island E1 showing the lowest recovery.
Methods
Near the end of the second year, the researchers revisited the islands, focusing on four islands (E1, E2, E3, ST2) in the Sugarloaf area of the lower Keys. The same census procedures were followed as in the earlier reports to maintain consistency.
Results and Findings
Species Numbers and Equilibrium
Data from the second year indicate that species numbers remained relatively stable compared to the end of the first year, suggesting an equilibrium state had been reached, mirroring pre-fumigation conditions. The number of species was inversely related to the distance from the nearest mangrove islands and swamps, which act as sources for new colonizers. Island E1 was an exception, not fully regaining its original species count. The authors suggest that, according to basic equilibrium theory, distant islands approach equilibrium slower, and E1 might still be in the process of reaching it. This delay might also be due to unusual population growth patterns among the early colonists.
Species Turnover and Composition
Despite the stability in species numbers, there was a notable turnover in species composition during the second year. Comparing species lists from the first and second year censuses revealed that only 35-52% of species on individual islands were present in both censuses, highlighting significant species replacement.
Convergence Towards Original Composition
The study also considered whether the species compositions would revert to their pre-defaunation states. E.O. Wilson proposed that species turnover generates different species combinations over time and longer-lived species, better adapted to local environments or capable of coexisting with existing species, would eventually dominate.
Wilson termed the shift an "assortative equilibrium" succeeding the "interactive equilibrium". This state pop should exhibit higher species numbers due to increased survival times among resident populations, assuming consistent overall propagule invasion rates. Furthermore, the compositions of species should converge under similar conditions due to fewer potential assortative equilibria.
The data suggested a drift towards original compositions, as three islands showed compositions closer to predefaunation levels at the end of the second year compared to the first. Island E3, however, showed no change.
Tables and Figures
Table 1: Lists the colonists of the four experimental islands two years after defaunation, categorized by island and species, including Coleoptera, Orthoptera, Thysanoptera, Corrodentia, Hemiptera, Lepidoptera, Diptera, Hymenoptera, Araneae, Acarina, and Diplopoda. The number of total species seen and the estimated present were included for each island.
Table 2: Shows the percentages of species present at both given censuses on the four experimental islands. It includes comparisons between censuses just before defaunation and one year later (Section A), just before defaunation and two years later (Section B), and one and two years after defaunation (Section C). The table provides the number of species in each census, the total number of species in common, and the percentage in common.
Figure 1: Illustrates the colonization curves of the four small mangrove islands, showing the estimated numbers of species present over time. The graph demonstrates that the number of species is an inverse function of the distance of the island from the nearest source of immigrants.
Equations
There were no specific equations cited in this text.
Discussion
The results provide evidence for the dynamic nature of island ecosystems, where species numbers may reach an equilibrium, but species composition continues to change due to ongoing immigration and extinction processes. The study supports the theory that distance from a source area affects the rate of colonization and species richness on islands.
The concept of assortative equilibrium suggests that species interactions and environmental factors play a crucial role in shaping long-term community structure. The observed trend towards pre-defaunation species compositions, though not conclusive, indicates that historical factors and species adaptations may influence the reassembly of ecological communities after disturbance.
Acknowledgements
The authors acknowledged the assistance of entomologists who identified most of the arthropod colonists: J. A. Beatty, G. W. Dekle, R. C. Froeschner, D. G. Kissinger, E. L. Mockford, E. S. Ross, L. M. Russell, R. L. Smiley, L. J. Stannard, T. J. Walker, S. L. Wood.
References
MacArthur, R. H., and E. O. Wilson. 1967. The theory of island biogeography. Princeton University Press, xi + 203 p.
Simberloff, D. S. 1969. Experimental zoogeography of islands. A model for insular colonization. Ecology 50: 296-314.
Simberloff, D. S., and E. O. Wilson. 1969. Experimental zoogeography of islands. The colonization of empty islands. Ecology 50: 278–296.
Wilson, E. O. 1969. The species equilibrium. Brookhaven symposia in biology 22: 38-47.
Wilson, E. O., and D. S. Simberloff. 1969. Experimental zoogeography of islands. Defaunation and monitoring techniques. Ecology 50: 267-278.