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Diversity 2015, 7(3), 229-241;

The Importance of Scaling for Detecting Community Patterns: Success and Failure in Assemblages of Introduced Species
Geological Survey, Nebraska Cooperative Fish and Wildlife Research Unit, School of Natural Resources, University of Nebraska–Lincoln, Lincoln, NE 68583-0961, USA
Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment, Box 7050, SE-750 07 Uppsala, Sweden
Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL 32611, USA
Resilience Center, Vancouver Island, Nanaimo, BC V9R 5S5, Canada
Author to whom correspondence should be addressed.
Academic Editor: Tom Oliver
Received: 14 May 2015 / Accepted: 17 June 2015 / Published: 26 June 2015


Community saturation can help to explain why biological invasions fail. However, previous research has documented inconsistent relationships between failed invasions (i.e., an invasive species colonizes but goes extinct) and the number of species present in the invaded community. We use data from bird communities of the Hawaiian island of Oahu, which supports a community of 38 successfully established introduced birds and where 37 species were introduced but went extinct (failed invasions). We develop a modified approach to evaluate the effects of community saturation on invasion failure. Our method accounts (1) for the number of species present (NSP) when the species goes extinct rather than during its introduction; and (2) scaling patterns in bird body mass distributions that accounts for the hierarchical organization of ecosystems and the fact that interaction strength amongst species varies with scale. We found that when using NSP at the time of extinction, NSP was higher for failed introductions as compared to successful introductions, supporting the idea that increasing species richness and putative community saturation mediate invasion resistance. Accounting for scale-specific patterns in body size distributions further improved the relationship between NSP and introduction failure. Results show that a better understanding of invasion outcomes can be obtained when scale-specific community structure is accounted for in the analysis.
body size; community assembly; community structure; competition; Hawaii; Introduced; Oahu

1. Introduction

Biological invasions provide an opportunity for testing ecological theory, including assessments of the role of competition in community assembly and structure. Invasion biologists have suggested that invasion resistance increases in strongly interacting communities as community saturation is approached [1,2,3]. Previous analysis used the numbers of invasive species present (NSP) as a surrogate of potential community saturation and compared NSP for failed versus successful introductions [4,5]. Because invasive and remaining native species are strongly segregated by habitat and altitude, the outcome of invasion success or failure in ecosystems is most likely associated with the structural and functional attributes of communities, and interactions between, species [6,7]. Supporting the idea that the structure of the existing community influences the relative success of invasions, Moulton [4] found higher NSP values for failed introductions in the lowland avifauna of the Hawaiian island of Oahu. A higher failure rate for introductions when more introduced species were present suggested that the bird community was approaching saturation. However, NSP values were significantly higher for failed introductions on Oahu only for introductions up to the year 1960, but when the species list was updated through 1981 [4], NSP values were not-significantly (p < 0.235) higher for failed introductions, despite the number of invasive species having increased. This finding is counterintuitive because if competition and invasion resistance increase as communities become saturated, then differences in NSP between failed and successful introductions should be even more pronounced following additional introductions.
The approach based on NSP by Moulton and colleagues [4,8,9] is useful to evaluate the role of community saturation on invasion success or failure. A recent study of successful versus unsuccessful vertebrate introductions in Florida, USA, supported a NSP effect for mammals, herpetofauna and fish but not for birds [5]. Refinements of the method are needed to explore its full potential and for testing current theories about mechanisms conferring resistance to invasions. Here we advance an alternative that allows for improved inference. This is achieved by using NSP at the time a species goes extinct (NSPe) to evaluate unsuccessful establishment instead of NSP at the time a species is introduced (NSPi). NSPe considers that introduction is not equivalent to establishment, meaning that a species can be introduced in a first stage to an ecosystem which then can, but must not necessarily, become established in a second stage [10]. Second, although NSP may serve as a surrogate of relative invasion resistance [11], the quantitative evaluation of competitive interactions is impossible with census data, which is a potentially severe limitation in any analysis of competitive effects, and thus the importance of community saturation. We therefore take an indirect approach to assess competitive interactions qualitatively within the community of introduced birds. We use techniques and theory which account for scale-specific structures and processes in the environment and which is mirrored in the structure of ecological communities [12,13,14], particularly birds [15,16].
Peterson et al. [17] suggested that groups of species operating at the same range of scale may represent strongly competitive units regardless of their taxonomic or functional similarity. Exploiting resources at different spatiotemporal scales in the environment reduces the strength of interactions between differently sized species relative to interactions among animals that operate at similar scales [18]. Consider the hypothetical invasion of a bird community by a raven that weighs 1000 g and occupies a large home range. The addition of the raven to the community increases the NSP by one, but it is unlikely that it interacts with the extant small-bodied birds as intensely as another small-bodied bird would. It may be ecologically more relevant to assess how the raven interacts with other species of similar body mass.
Animal body mass is a useful index for linking animal community structure with scales of resource use (see [19]). The average adult body mass of a species strongly correlates with many ecological attributes, including energy use, movement, home range size, and foraging [20,21]. The ecological scales at which a species operates corresponds with average species body mass, making body mass a useful index of the scale at which an animal perceives and exploits its environment [12,22].
Here we assess the importance of scale-specific patterns and NSPe of introduction outcomes. We test the hypothesis that relationships between introduction success and failures as a function of NSP become better discernable in analyses that account for scale versus approaches that do not.

2. Experimental Section

Study Site and Analyses

The Hawaiian island of Oahu has lost nearly its entire native avifauna in lowland habitats (<1000 m elevation) since European colonization [23]. A diverse community of introduced birds has replaced the native avifauna. The development of this introduced community has been well documented, and the fate of species introductions is relatively well known [4,24,25,26,27,28]. These species represent a community distinct from that of the surviving native species. Native species are mostly restricted to native forests at high elevations [23,24,29]. Non-indigenous species are mostly restricted to lower elevation landscapes with a high degree of anthropogenic transformation [30]. The present introduced bird community includes approximately 38 successfully established species, primarily passeriformes (26 species), but also four Galliformes, three Columbiformes, two Psittacidae, one Tytonidae, one Apodidae, and one Ciconidae. Thirty-seven species from the same families have been introduced but subsequently failed to establish breeding populations or went extinct after a period of establishment in the island.
We used the species list of Moulton [4] for passeriform introductions with the addition of all other families of introduced birds [23,27,31]. This list includes all known avian introductions to Oahu, both successful and unsuccessful, since European colonization through the year 1993. Although introduction effort was not available for most species, limiting an assessment of propagule size on introduction success [32], we highlight that we were interested in the consequences of community assembly processes when invaders have become established for some time in the ecosystem rather than in the factors that potentially mediate their establishment per se.
Introductions were ranked by year of introduction, and the number of other introduced species (NSP) present in the community was calculated for each successful or failed introduction (Table 1). The NSP values for the successfully introduced and unsuccessfully introduced species were then compared using a 1-tailed t-test (if the data were normally distributed and variance homogenous) or using a 1-tailed Mann-Whitney U-test. In our analysis we compared NSP for successful introductions with both the NSP at the time of introduction and NSP at the time of extinction for failed introductions. This comparison is appropriate because if competition is one of the factors driving species success or failure, it is the number of species present in the community at the time of extinction that is most relevant. For species that failed immediately, NSP at the time of introduction and extinction are identical.
Table 1. Order and fate of introductions of birds introduced to Oahu. Numbers in parentheses after the birds’ names indicate membership of the body size group revealed by the discontinuity analysis.
Table 1. Order and fate of introductions of birds introduced to Oahu. Numbers in parentheses after the birds’ names indicate membership of the body size group revealed by the discontinuity analysis.
SpeciesYear of IntroductionYear of ExtinctionNSPiNSPe
Gallus gallus (3)10001935031
Columba livia (2)1850 1
Callipepla californica (3)18551941228
Alauda arvensis (2)1870 3
Numida meleagris (3)18741908415
Phasianus colchicus (3)1875 5
Passer domesticus (2)1879 8
Acridotheres tristis (3)1879 8
Streptopelia chinensis (3)1879 8
Lonchura punctulata (1)1883 10
Carpodacus mexicanus (2)1883 10
Pavo cristatus (3)1896 11
Amandava amandava (1)1900 13
Garrulax canorus (2)1900 13
Colinus virginianus (3)190619261423
Syrmaticus soemmerringii (3)190719201514
Coturnix chinensis (2)192119271422
Geopelia striata (2)1922 19
Grallina cyanoleuca (3)192219361931
Geopelia humeralis (3)192219381927
Geophaps lophotes (3)192219271922
Phaps chalcoptera (3)192219271922
Alectoris chukar (3)192319282023
Chalcophaps indica (3)192419282223
Rollulus rouloul (3)192419302227
Rhipidura leucophrys (2)192619372329
Parus varius (2)192819632330
Leiothrix lutea (2)1928 23
Geopelia cuneata (2)192819312328
Paroaria coronata (2)1928 23
Mimus polyglottos (2)1928 23
Streptopelia decaocto (3)192819442324
Zosterops japonica (1)1929 27
Cettia diphone (1)1929 27
Cyanoptila cyanomelana (2)192919582724
Cardinalis cardinalis (2)1929 27
Paroaria dominicana (2)193119322931
Sturnella neglecta (3)193119372929
Copsychus saularis (2)193219763140
Chrysolophus pictus (3)193219413128
Chrysolophus amherstiae (3)193219413128
Lophura nycthemera (3)1932 3128
Passerina cyanea (1)1934 3131
Lonchura malacca (1)1936 31
Copsychus malabaricus (2)1940 26
Passerina leclancherii (1)1941 2824
Garrulax caerulatus (3)1947 24
Syrmaticus reevesii (3)1957 2439
Francolinus pondicerianus (3)1959 25
Bubulcus ibis (3)1959 25
Gracula religiosa (3)1960 26
Tyto alba (3)1961 27
Vidua macroura (1)1962 3038
Gallus sonneratii (3)1962 3041
Meleagris gallopavo (3)1962 3040
Serinus mozambicus (1)1964 31
Lonchura oryzivora (2)1964 31
Estrilda troglodytes (1)1965 3938
Estrilda melpoda (1)1965 39
Lagonosticta senagala (1)1965 3941
Estrilda caerulescens (1)1965 39
Uraeginthus angolensis (1)1965 3942
Uraeginthus bengalus (1)1965 3938
Sicalis flaveola (2)1965 39
Pycnonotus jocosus (2)1965 39
Pycnonotus cafer (2)1966 39
Francolinus erckelii (3)1967 40
Uraeginthus cycnocephala (1)1969 4042
Myiopsitta monachus (3)1970 4138
Nandayus nenday (3)1971 4140
Tiaris olivacea (1)1974 41
Amazona viridigenalis (3)1975 42
Estrilda astrild (1)1981 38
Psittacula krameri (3)1982 36
Lonchura malabarica (1)1984 37
We also determined NSP values among species operating at the same range of scale to account for strong interactions among those species as compared to interactions among species operating at different scales. We used the methods similar to those described in [33] to objectively determine scaling patterns in the established bird community, based on their body mass distributions, which reflects the hierarchical organization of ecosystems [12,22].
Body mass estimates were obtained from [34] and body mass distributions were analyzed using simulations that compared the observed data with a null distribution established by estimating a continuous unimodal kernal distribution of the log-transformed data [35]. Significance of discontinuities in the data (significantly large gaps between adjacent body masses that represent transitions between scaling regimes [22]) was determined by calculating the probability that the observed discontinuities were randomly generated by comparing observed values with the output of 1000 simulations from the null distribution. Groups of species of similar body mass (body mass aggregations) are defined by the discontinuities detected. Species within a body mass aggregation are assumed to exploit their environment at the same range of scale [12,17,36]. After determining the significant discontinuities in the successfully introduced Oahu bird community, we proceeded to compare NSP values as described above, but comparisons were made within body mass aggregations to take into account the strong interactions among species operating at the same scale as compared to relatively weak interactions among species operating at different scales. Body mass estimates for three failed species (Serinus leucopygius, Luscinia akahige and Erithacus komadori) were unavailable so these species were omitted from our analysis.
Finally, we used correlation analysis to test the hypothesis that the strength of association between the number of extinctions and the NSP at the time of extinction becomes stronger when scale is accounted for in the analyses. We also examined whether extinction rates show non-linear patterns, i.e., whether extinctions are increased upon a threshold in the number of species composing the invaded community. These relationships were examined for the unscaled data and for each body mass aggregation group identified by the discontinuity analysis.

3. Results and Discussion

Of the 75 bird species that have been introduced to Oahu as of 1991, 38 introductions were successful and 37 failed [4,27]. The mean NSP value for the successful introductions was 24.6 and for unsuccessful introductions the mean NSPi was 25.4 (Table 2). There was no difference between these two groups (p = 0.374, 1 tailed t-test). However, when we considered NSPe for unsuccessful introductions median values for successful introductions (26) and for unsuccessful introductions (29) were significantly different (p = 0.026, 1 tailed Mann-Whitney U-test; Table 2).
The body mass distribution of the introduced bird community of Oahu was significantly discontinuous; that is, we identified birds operating in different scaling regimes. Two very distinct breaks in the body mass distribution were detected, leading to the conclusion that there were three distinct body mass aggregations or scales (Figure 1). From an ecological perspective the bird species composing these body mass aggregations presumably exploit the environment at small, meso and large scales, respectively. The first aggregation of species had body masses ranging from 7.5 to 14 g (11 successful and 8 unsuccessful species; Table 1). Body masses of the second aggregation of birds ranged from 20 to 56 g (15 successful and 7 unsuccessful; Table 1). The third aggregation included birds that weighed more than 86 g; contrary to patterns observed in body mass aggregations 1 and 2, we observed almost double the number of unsuccessful vs. successful introductions in this third body mass group (12 successful and 22 unsuccessful; Table 1).
Table 2. Mean or median NSPi or NSPe values for Oahu birds in three different body mass categories, with body mass data pooled, and for unscaled data. Number of observations is given in parentheses.
Table 2. Mean or median NSPi or NSPe values for Oahu birds in three different body mass categories, with body mass data pooled, and for unscaled data. Number of observations is given in parentheses.
Number of Species Present (NSP)
Body mass aggregation 1 NSPi6.27(11)8.88(8)**0.090
Body mass aggregation 1 NSPe6.27(11)10.5(8)**0.020
Body mass aggregation 2 NSPi*11(15)11(7)**0.137
Body mass aggregation 2 NSPe8.33(15)12.57(7)**0.026
Body mass aggregation 3 NSPi*8(12)9.5(22)**0.144
Body mass aggregation 3 NSPe8(12)9.77(22)**0.047
Pooled data NSPi*8(38)11(37)0.051
Pooled data NSPe*8(38)12(37)0.002
Unscaled data NSPi24.58(38)25.43(37)0.374
Unscaled data NSPe*26(38)29(37)0.026
* Comparisons made with the Mann-Whitney Rank Sum Test because either the data were not normally distributed or variances were not equal. Otherwise, comparisons represent t-tests.** Numbers in parentheses indicate the number of observations that are specific to each scale.
Figure 1. Body mass structure of the Oahu introduced bird community. The upper axis shows the location (black circles) of successfully introduced bird species along a log body mass axis. Gray rectangles represent the three identified body mass aggregations. Gap Statistic is a measure of the deviation of the observed body mass distribution from the unimodal null model.
Figure 1. Body mass structure of the Oahu introduced bird community. The upper axis shows the location (black circles) of successfully introduced bird species along a log body mass axis. Gray rectangles represent the three identified body mass aggregations. Gap Statistic is a measure of the deviation of the observed body mass distribution from the unimodal null model.
Diversity 07 00229 g001
Comparing the NSPi values of successful versus unsuccessful species, probability values were marginally significant (p = 0.09) for small-scale species in the first body mass category, and nonsignificant for the other two body mass categories (p > 0.10; Table 2). Comparison of NSPe values for failed species yielded significant results for all three body mass aggregations (p < 0.05; Table 2). When data were pooled across body mass aggregations, the comparison between successful and failed species was either marginal (p = 0.05), using NSPi or highly significant (p < 0.01), using NSPe.
Correlations between NSP at the time of extinction and the number of extinctions were positive and significant when conducted with both unscaled data and for the data segregated into three scales. These correlations were higher for the scaled data in all three body mass categories (r = 0.540, 0.683, and 0.507, respectively) than for the unscaled data (r = 0.467). Comparing the number of failed invasions versus the NSP at the time of extinction suggests that a threshold may be present (Figure 2 upper panel). Only two species were lost from the community when there were fewer than 20 species present, but when more than 20 species were present the number of extinctions dramatically increased. Similar nonlinear patterns were found when examining the same relationships within body mass groups with the number of extinctions increasing upon thresholds of 12 species for body mass aggregations 1 and 2 and 9 species for body mass aggregation 3 (Figure 2 lower panel).
Figure 2. Plots of the number of extinctions versus the number of species present at the time of extinction. Upper panel, all species; Lower panel, species segregated into three body mass categories.
Figure 2. Plots of the number of extinctions versus the number of species present at the time of extinction. Upper panel, all species; Lower panel, species segregated into three body mass categories.
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Our results indicate that calculating NSP at the time of extinction, rather than at the time of introduction, and scaling the community to account for differences in interaction strength between species in different size classes both represent improvements over earlier analyses of introduced bird communities [4,5]. For species that went extinct, calculating NSP at the time of extinction rather than at the time of introduction is less conservative and assays patterns of invasion resistance in the community more realistically. Scaling communities prior to such an analysis also represents an improvement, because of the strong interaction among species operating over the same or similar ranges of scale relative to species operating at grossly different scales.
If competition is one of the forces in the community assembly process [37], we would expect that to be manifested at the time of extinction rather than the time of introduction. For example, Callipepla californica was introduced in 1855, when only one other species was present. However, it went extinct in 1941 when 28 species were present. Clearly the consideration of NSP at the time of introduction is less appropriate than NSP at the time of extinction. For species that went extinct immediately following introduction, NSPe and NSPi are identical.
Correlations between NSP at the time of extinction and the number of extinctions were positive and significant but higher for the scaled data in all three body mass categories than for the unscaled data. We also found that thresholds exist when comparing the number of failed introductions with the NSP at the time of extinction, and also these patterns were consistently found for unscaled data or data scaled into body mass aggregations. This finding of thresholds is consistent with the conclusions derived from computer models [1,2] and suggests that if competitive interactions are indeed the ultimate factor shaping the island bird community, they do not become significant in producing community structure until saturation is approached. Most importantly, and supporting our hypothesis, this relationship is strengthened when the community is segregated into body size categories, accounting for scaling relationships in ecosystems.
Scaling interactions within communities is one of many different approaches that have been taken to compartmentalize biological interactions in communities. However, most efforts are based on phylogeny and thus ignore an often substantial portion of the community. The model of Peterson et al. [17] accounts for both phylogeny and scale. We used that model to guide our analysis based on segregating the community based on species body size. The two breaks in scale (discontinuities) we identified and used to guide our aggregation of the community into three body mass categories are prominent, unmistakable, and based on an objective identification of scaling patterns in ecological communities. Our results support the model of Peterson et al. [17] that suggests the importance of scaling species interactions and conclusions suggesting the prominent role of competition in structuring animal communities [8,24,38,39,40,41]. However, it should be noted that differences in taxonomic and guild diversity exist among the three body size categories. Specifically, all species in the first body mass category are passeriformes, and all except two (Cettia diphone and Zosterops japonica) are primarily seed eaters [42]. There is more taxonomic and guild diversity among species in the second body mass category, which consists of both Passeriformes and Columbiformes. The largest body mass category consists of six different families with a greater diversity of feeding strategies. Additionally, the size range of members of the largest body size category is greater than the body size range of species in the first two body size categories.
Despite this higher variability in the third body mass aggregation, we found that the number of failed invasions is comparatively higher in this aggregation (22 species or 65% of all species found in this aggregation) compared to aggregations 1 (8 species; 42%) and 2 (7 species; 31%). Previous research has shown that establishment success is more likely in large-bodied birds [43], but our analysis suggests that larger-bodied species also fail more often to persist in the bird communities. That extinction risk is higher in birds with higher body masses is consistent with patterns found for large mammals [44] and other bird communities [45]. Single or combined effects related to small population sizes, lower reproductive rates and larger home or geographic ranges have been suggested to increase the extinction risk in large-bodied birds [45]. Although the causes mediating the patterns observed for birds in the largest body mass aggregation on Oahu Island, cannot be ascertained with the data at hand, the pattern itself is consistent with a large body of theory and empirical findings.

4. Conclusions

Our study underscores the usefulness of body mass as a predictor of ecological processes [12,19,22,46,47]. Our results particularly underscore the benefit for elucidating clearer patterns of invasion outcomes when body mass is partitioned into scale-specific patterns. Further research is warranted to assess the generality of pattern found in this study also for other organism groups.


An earlier version of this manuscript was improved by comments from Lance H. Gunderson, Garry D. Peterson, Stuart L. Pimm and anonymous reviewers. The Nebraska Cooperative Fish and Wildlife Research Unit is jointly supported by a cooperative agreement between the U.S. Geological Survey, the Nebraska Game and Parks Commission, the University of Nebraska−Lincoln, the U.S. Fish and Wildlife Service and the Wildlife Management Institute. We acknowledge funding by the August T. Larsson Foundation (NJ Faculty, Swedish University of Agricultural Sciences).

Author Contributions

Craig R. Allen developed and wrote the first draft of the paper. David G. Angeler, Michael P. Moulton, and Crawford S. Holling contributed to writing and idea development.

Conflicts of Interest

The authors declare no conflict of interest.


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