Michael P. Moulton

The Introduced Hawaiian Avifauna: Biogeographic Evidence for Competition

We discuss the patterns of introduction and extinction of the species of land birds (passeriforms and columbiforms) introduced to the Hawaiian Islands over the last century. The data are consistent with the idea that rising extinction rates will eventually match immigration rates leading to a dynamic equilibrium. Turnover in species composition is a prominent feature of the introduced Hawaiian avifauna with extinctions common even among populations that had persisted for decades. We could not detect an effect of island size on extinction rate. However, the per species extinction rates do increase with the number of species on the island. This suggests that the species mutually affect each others chances of extinction. Other explanations for an increasing per-species extinction rate do not seem to be consistent with our data. Thus, interspecific competition seems to have been an important process in determining extinction rate and, by extension, the equilibrium number of species on these islands.

Species Introductions to Hawaii

What determines whether species have the potential to invade new environments? And how should we evaluate community properties to determine whether a biological system is vulnerable to invasion? These questions are fundamental to a basic understanding of the structure of ecological communities. The ability of a species to invade, and of a community to resist invasion, are central features in models designed to look at species diversity, niche overlap, limiting similarity and community change. Looking at invasions—why they succeed or fail—may be the most direct way of testing a number of important theories. Here we address these topics by looking at the patterns of organisms introduced to the Hawaiian Islands.

Sexual selection and the risk of extinction of introduced birds on oceanic islands

We test the hypothesis that response to sexual selection increases the risk of extinction by examining the fate of plumage-monomorphic versus plumage-dimorphic bird species introduced to the tropical islands of Oahu and Tahiti. We assume that plumage dimorphism is a response to sexual selection and we assume that the males of plumage-dimorphic species experience stronger sexual selection pressures than males of monomorphic species. On Oahu, the extinction rate for dimorphic species, 59%, is significantly greater than for monomorphic species, 23%. On Tahiti, only 7% of the introduced dimorphic species have persisted compared to 22% for the introduced monomorphic species. For the combined Oahu and Tahiti data sets, addition of plumage-by-fate interaction significantly improves the fit of the log-linear model, fate+island+plumage+(fate-by-island)+(island-by-plumage). To control for phylogenetic constraint, a logistic regression model is analyzed using a data subset consisting of only the two best represented families, Fringillidae and Passeridae. Here, plumage and the plumage-by-family interaction are significant. Plumage is significantly associated with increased risk of extinction for passerids but insignificantly associated for fringillids. Thus, the hypothesis that response to sexual selection increases the risk of extinction is supported for passerids and for the data set as a whole. The probability of extinction was correlated with the number of species already introduced. Thus, species that have responded to sexual selection may be poorer interspecific competitors when their communities contain many other species.

Ascription of resemblance of newborns by parents and nonrelatives

Abstract It has been hypothesized that human females ascribe the resemblance of their infants to the father or his relatives to promote assurance of paternity. Assurance renders fathers more likely to invest in and less likely to harm mothers and their children. Other studies have documented a bias toward the fathers side of the family when mothers ascribe the resemblance of their infants. We test if this bias extends to newborns (1–3 days old) and if such bias reflects actual resemblances. This was done by noting the responses of mothers when asked whom they thought their newborns resembled and by comparing those responses to resemblances ascribed by unrelated judges. During in-hospital visits, mothers were significantly more likely to ascribe resemblance to the domestic father than to themselves. This bias was exaggerated in the presence versus absence of domestic fathers. Yet, judges matched photographs of these mothers to their newborns significantly more frequently than they matched domestic fathers to newborns. Thus, the bias in how mothers remark resemblance does not reflect actual resemblance and may be an evolved or conditioned response to assure domestic fathers of their paternity. The low rate with which newborns are matched to fathers may be biologically significant. Concealment of paternity may be favored when suspicion of cuckoldry leads fathers to abandon or harm newborns. Genomic imprinting is one mechanism by which resemblance of newborns could be biased toward mothers. We suggest that on average females benefit from concealed paternity even if cuckoldry is rare provided that their verbal ascriptions of resemblance are effective in assuring domestic fathers of their paternity.

Null matrices and the analysis of species co-occurrences

Abstract Patterns in species occurrences on islands have been analyzed by several authors. At issue is the number of non-occurring pairs of species (also known as checkerboards). Previous authors have suggested that if the number of checkerboards differs from what is expected by chance, then island communities might have been structured by competition. Investigators have pursued this problem by first generating random (or null) matrices and then testing a metric derived from the collection of null matrices against the metric calculated from the actual species co-occurrence matrix. The random matrices were constrained by requiring the number of species on each island, and the number of islands on which each species occurred to be equal to their observed values. We show that results from previous studies are generally flawed. We present a fast, efficient algorithm to generate null matrices for any set of fixed row and column sums, and propose a modification of a previously proposed metric as a test statistic. We evaluated the efficacy of our construction method for null creation and our metric using incidence matrices from the avifauna of Vanuatu (formerly New Hebrides).

THE ALL-OR-NONE PATTERN IN INTRODUCED HAWAIIAN PASSERIFORMS: THE ROLE OF COMPETITION SUSTAINED

Certain landbirds (Columbiformes and Passeriformes) introduced on more than one of the Hawaiian Islands exhibit an all-or-none pattern in introduction success. D. Simberloff and W. Boecklen have advanced an interesting hypothesis that does not require competition to explain this pattern. The all-or-none pattern among introduced passeriforms reported by Simberloff and Boecklen, however, is exaggerated by three types of bias (phylogeny, errors in their data matrix, and differences in the number of islands per introduction). Moreover, I argue that, once corrected, the observed all-or-none pattern is consistent with the competition hypothesis. Simberloff and Boecklen have also criticized four analyses that I interpreted as supporting the hypothesis that competition has influenced patterns in introduction success among these birds. I show that in all cases, however, the criticisms do not alter the conclusions.

The Role of Stochastic and Deterministic Processes in Structuring Neotropical Bat Communities

The extent to which deterministic or stochastic processes affect the composition and morphological structure of communities is an intense debate in evolutionary ecology. For many years, competition was considered to be the dominant deterministic factor that affected structure of animal communities. More specifically, interspecific competition can affect morphological patterns within communities by either size assortment or size adjustment. We investigated the extent to which size assortment could have structured bat assemblages from two well-defined bat communities from northeast Brazil by comparing real ecomorphological characteristics (mean and variance of segment lengths of minimum-spanning trees from principal component scores) to the distribution of like statistics obtained from simulations derived from nested-species pools. Regardless of species pool, the ecomorphological patterns of bat assemblages were no more structured than expected by random selection of taxa from the available species pool. This was true for frugivorous, nectarivorous, and foliage-gleaning insectivorous guilds, and for the phyllostomid community in general, from both Caatinga and edaphic Cerrado biomes. The alternative hypothesis, that coevolutionary processes determine ecomorphological patterns in these neotropical communities, similarly is not supported by our results. The characteristic spatial and temporal heterogeneity of northeast Brazil, combined with the mobile nature of bats, could decrease the intensity, pervasiveness, or predominance of deterministic interactions, thereby leading to an ecomorphological structure indistinguishable from that produced by stochastic processes.

Morphological assortment in introduced Hawaiian passerines

SummaryTo evaluate the role of competition in structuring communities, we conducted morphological analyses on the surviving species of passerine birds that were successfully introduced to the Hawaiian islands. Forty-nine species have been introduced a total of 111 times to five of the six main islands. There have been 33 extinctions. Our analyses were done at two separate organizational levels: all species introduced to an island; and all forest-dwelling species. If competition determines which species can coexist, and the intensity of competition is correlated with morphological similarity, then the surviving species should be overdispersed in morphological space. Further, sets of surviving species that coexist should be regularly positioned in morphological space. At the island-wide organizational level, the surviving species were neither overdispersed nor regularly positioned in morphological space. However, at the forest-wide level the surviving species were not only highly overdispersed, they were also regularly positioned when compared to randomly assembled communities.

Ecomorphological pattern in Bermuda birds: The influence of competition and implications for nature preserves

SummaryPrevious ecomorphological studies of introduced land birds on oceanic islands have revealed several patterns consistent with the hypothesis that interspecific competition influenced the assembly process of that community. We extend one of these analyses to Bermuda, which differs from the previous islands in its size, latitude and isolation from the mainland. We tested for the presence of morphological overdispersion in surviving introduced passeriforms. Despite a possible predominance of random events in the assembly process due to Bermudas small size and the presence of migrant species, the surviving passeriforms were morphologically overdispersed. The presence of this pattern supports the hypothesis that the assembly of the Bermuda passeriform community has been influenced by interspecific competition. This marks the third distinct community of island land birds in which this competitive pattern has been identified. This is particularly interesting because certain attributes of Bermuda and its avifauna make it more equivalent to communities in mainland refuges than avian assemblages on islands studied previously. This suggests that interspecific competition may have important effects on the structure of refuge communities.

Patterns of success in passeriform bird introductions on Saint Helena

Ecologists have long attempted to predict the success of species that are introduced into foreign environments. Some have emphasized qualities intrinsic to the species themselves, whereas others have argued that extrinsic forces such as competition may be more important. We test some of the predictions made by both the extrinsic and intrinsic hypotheses using passeriform birds introduced onto the island of Saint Helena. We found direct evidence that extrinsic forces are more important predictors of successful invasion. Species introduced when fewer other species were present were more likely to be successful. In a direct test of the alternative hypothesis that intrinsic forces play a more prominent role in success or failure, we found a tendency for species which successfully established on Saint Helena to be also successful when introduced elsewhere. However, the vast majority of species unsuccessful at establishing on Saint Helena had probabilities of success outside Saint Helena of 50% or greater, making this result somewhat equivocal. Finally, we found no evidence to support the hypothesis that species that are successful early are those that are intrinsically superior invaders. These results are consistent with similar analyses of the introduced avian communities on Oahu, Tahiti, and Bermuda.