In the Light of Evolution Volume II: Biodiversity and Extinction (2008) / Chapter Skim
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5 Species Invasions and Extinction: The Future of Native Biodiversity on Islands--DOV F. SAX and STEVEN D. GAINES
5 Species Invasions and Extinction: The Future of Native Biodiversity on Islands--DOV F. SAX and STEVEN D. GAINES
Pages 85-106
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From page 85... ...
and STEVEN D GAINES†‡ Predation by exotic species has caused the extinction of many native animal species on islands, whereas competition from exotic plants has caused few native plant extinctions.
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From page 86... ...
In particular, should we expect few additional plant species to go extinct -- even on individual islands where hundreds or thousands of exotic plant species have invaded? Should ongoing and future invasions eventually lead to mass extinction events among island plant taxa?
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From page 87... ...
Doing so will help us to advance our basic understanding of ecological and evolutionary theory while simultaneously advancing our understanding of a pressing applied issue, namely the future of plant diversity on islands worldwide. Here, we evaluate the role of species invasions on the extinction of native species.
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From page 88... ...
. Over the past 500 years, humans have reduced the amount of natural habitat worldwide, directly exploited species, introduced exotic species and exotic pathogens, and created many other conditions conducive to species extinction.
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From page 89... ...
An analysis of the International Union for Conservation of Nature database on species extinctions reveals several emerging patterns for terrestrial vertebrate and plant species. First, most extinctions have been on islands as opposed to mainlands (Fig.
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From page 90... ...
is cited as the sole factor responsible for species extinctions in 69 (33.8% of) extinctions, predation together with other contributing factors is cited for 85 (41.7% of)
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From page 91... ...
. In contrast to birds, mammal richness has increased dramatically, particularly on oceanic islands, which have few native mammal species (Blackburn et al., 2004)
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From page 92... ...
. Other island characteristics indi 3.5 Log number of naturalized species 3.0 2.5 2.0 1.5 1.0 1.0 1.5 2.0 2.5 3.0 3.5 Log number of extant native species FIGURE 5.2 Native and naturalized plant richness are highly correlated across oceanic islands.
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From page 93... ...
Indeed, even a multiple regression analysis with these and other island characteristics generates a model that accounts for less of the variation in naturalized species richness than is explained by native richness alone (see Methods)
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From page 94... ...
We evaluate the possibility that plants have reached a colonizationbased saturation point on oceanic islands by examining time series of exotic species additions to individual islands through time (see Methods)
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From page 95... ...
None of these islands show evidence of an asymptote in cumulative richness of 95 naturalized species over time.
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From page 96... ...
. The consistency in the slope but change in intercept implies that the proportion of naturalized-to-native species has generally been consistent across islands within any given time interval but different A 3.5 B 3.5 Log number of naturalized species Log number of naturalized species 3.0 2000 3.0 2000 1920 2.5 2.5 1900 1980 2.0 1880 2.0 1880 1.5 1.5 1.0 1.0 0.5 0.5 0.0 1.0 1.5 2.0 2.5 3.0 3.5 1.0 1.5 2.0 2.5 3.0 3.5 Log number of extant native species Log number of extant native species FIGURE 5.4 Across 20-year time intervals from 1880 to the present, the slope of zpq9990837780004.g.eps log-log regressions between native and naturalized richness has been relatively constant, whereas the intercepts have changed; this implies that the 1:1 relationship currently observed between native and naturalized richness is a recent phenomenon but that, at repeated points during the past 120 years, there has been a consistent ratio of native-to-naturalized species across islands.
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From page 97... ...
At the current rate of change, the average ratio between naturalized-to-native species on oceanic islands should be ≈3:2 by the year 2060. There is no evidence of a decrease in the rate of exotic addition of plants to individual islands or across islands over the past 200 years.
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From page 98... ...
The ultimate consequence of these exotic species additions for native diversity is still difficult to determine with certainty. We see three primary alternatives with respect to exotic plant invasions and their impact on native species.
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From page 99... ...
. In each case, we predict an increase in naturalized plant richness but with different magnitudes and vastly different outcomes for native species extinctions.
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From page 100... ...
a large fraction of native plant species on islands are already on a path to extinction. Hopefully, future work on the interplay between species invasions and extinctions can more fully resolve these issues and, in the process, help us to develop a more comprehensive theory of species extinctions.
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From page 101... ...
Native and naturalized richness were tallied from the literature after applying a standardized set of criteria to published work. These criteria defined native and naturalized plant species as those believed to have self-supporting populations, such that species believed to be ephemeral were not included.
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From page 102... ...
Gaines Multiple Regression Analysis A multiple regression analysis was performed to explain variation in the log number of naturalized plant species on islands. Predictor variables included history of occupation, time of European settlement/trade, latitude, log of island elevation, log of island area, and log of human population size (Table 5A.1)
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From page 103... ...
This artifact lowers the number of naturalized species not just for individual islands, but also for mean estimates of naturalized species across islands. This procedural artifact occurs because exotic species are only counted as ‘‘naturalized'' if they are seen (i)
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From page 104... ...
APPENDIX: NATURALIZED VASCULAR PLANT SPECIES ON ISLANDS OVER TIME Years listed below refer to the most recent survey or plant collection date and not to the date of publication of a manuscript, even though, in many cases, the collection and publication date are the same. Following each year, the number of naturalized species is indicated in parentheses.
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From page 105... ...
creates greater uncertainty about whether particular exotics are naturalized or not in individual time steps. Consequently, data values were taken directly from the literature and range-through values have not been calculated (see Methods in the main text)
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From page 106... ...
hNew Zealand includes North, South, and Stewart Islands, as well as near-shore islands associated with these three islands. iNorthern Line Islands include Kiritimati, Tabuaeran, Teraina, and Palmyra.
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