The island biogeography of human population size

For decades, biogeographers have sought a better understanding of how organisms are distributed among islands. However, the island biogeography of humans remains largely unknown. Here, we investigate how human population size varies among 486 islands at two spatial scales. At a global scale, we tested whether population size increases with island area and declines with island elevation and nearest mainland, as is common in non-human species, or whether humans escape such biogeographic constraints. At a regional scale, we tested whether population sizes vary among islands within archipelagos according to the positioning of different cultural source pools. Results illustrate that on a global scale, human populations increased in size with island area, similar to non-human species, yet they did not decline in size with elevation and distance to nearest mainland. At a regional scale, human population size often varied among islands within archipelagos relative to the location of different cultural source pools. Despite broad-scale similarities in the geographical distribution of human and non-human species among islands, results from this study indicate that the island biogeography of humans may also be influenced by archipelago-specific social, political and historical circumstances.

The psychology of natural history

Natural history observations are an integral part of ecology and evolution. However, they can be underappreciated because they operate independent of the scientific method. Here, we illustrate that the science of natural history has its own methodology based on a well-known psychological paradigm that describes how the human mind learns.

Scientists' warning - The outstanding biodiversity of islands is in peril

Despite islands contributing only 6.7% of land surface area, they harbor ~20% of the Earth’s biodiversity, but unfortunately also ~50% of the threatened species and 75% of the known extinctions since the European expansion around the globe. Due to their geological and geographic history and characteristics, islands act simultaneously as cradles of evolutionary diversity and museums of formerly widespread lineages—elements that permit islands to achieve an outstanding endemicity. Nevertheless, the majority of these endemic species are inherently vulnerable due to genetic and demographic factors linked with the way islands are colonized. Here, we stress the great variation of islands in their physical geography (area, isolation, altitude, latitude) and history (age, human colonization, human density). We provide examples of some of the most species rich and iconic insular radiations. Next, we analyze the natural vulnerability of the insular biota, linked to genetic and demographic factors as a result of founder events as well as the typically small population sizes of many island species. We note that, whereas evolution toward island syndromes (including size shifts, derived insular woodiness, altered dispersal ability, loss of defense traits, reduction in clutch size) might have improved the ability of species to thrive under natural conditions on islands, it has simultaneously made island biota disproportionately vulnerable to anthropogenic pressures such as habitat loss, overexploitation, invasive species, and climate change. This has led to the documented extinction of at least 800 insular species in the past 500 years, in addition to the many that had already gone extinct following the arrival of first human colonists on islands in prehistoric times. Finally, we summarize current scientific knowledge on the ongoing biodiversity loss on islands worldwide and express our serious concern that the current trajectory will continue to decimate the unique and irreplaceable natural heritage of the world’s islands. We conclude that drastic actions are urgently needed to bend the curve of the alarming rates of island biodiversity loss.

On the selective advantage of coloniality in staghorn ferns (Platycerium bifurcatum, Polypodiaceae)

The staghorn fern (Platycerium bifurcatum, Polypodiaceae) is an epiphyte from Australasia that displays many life history characteristics commonly associated with eusocial animals. Here, I hypothesize about the selective advantage of living in cooperative groups by comparing the morphological characteristics of colonies to their solitary congeners.

Minimal models provide maximally parsimonious explanations

We are delighted that Diniz-Filho et al. agree with the main premise of our paper, and we welcome their critique, as constructive debate will help foster a better understanding of size evolution on islands. Our perspective on each of their criticisms is discussed in greater detail below.

Gender dimorphism in the virulence of a dioecious mistletoe

Parasite virulence often differs between male and female hosts. However, less is known about how virulence might differ between male and female parasites. Here, I show that female plants of the dioecious mistletoe Misodendrum quadrifolium (Misodendraceae) grow larger than male plants. Correspondingly, females reduce the photosynthetic capacity of infected host branches more than males. Results indicate that in addition to playing an important role in determining host susceptibility to parasitism, gender can also play an important role in determining the virulence of dioecious parasites.

A simple null model predicts the island rule

The island rule is a putative pattern in island evolution, where small species become larger on islands and large species become smaller. Despite decades of study, a mechanistic explanation for why some taxonomic groups obey the island rule, while others do not, has yet to be identified. Here, we explore whether the island rule might result from evolutionary drift. We derived a simulation model that predicts evolutionary size changes on islands based on random evolutionary trajectories along bounded trait domains. The model consistently predicted the island rule and could account for its occurrence in plants inhabiting islands in the Southwest Pacific. When support for the island rule was not detected, insular gigantism was often observed, suggesting that natural selection was at work. Overall results indicate that evolutionary drift can provide a parsimonious explanation for the island rule, suggesting future work should focus on circumstances where it does not occur.

Parasite-offspring competition for female resources can explain male-biased parasitism in plants

Male-biased susceptibility to parasites is common in dioecious plants. However, why males have higher parasite loads than females is unclear. Unlike males, females must subsidize post-fertilization costs of reproduction (e.g. seed and fruit development). As a result, females may have smaller pools of resources potentially available to parasites, thus leading to lower parasite loads. We tested this prediction in New Zealand's largest native moth ( Aenetus virescens: Lepidoptera), whose larvae parasitize Aristotelia serrata (Elaeocarpaceae), an endemic species of dioecious tree. We measured parasite loads in male and female trees, as well as annual seed set in females. We then derived a technique to equate the energetic cost of seed set in females to an equivalent number of parasitic larvae. Our results showed evidence for male-biased parasitism: male trees harboured more larval parasites than female trees. However, when parasite loads in males were compared with parasite loads in females, plus the energetic cost of seed production calculated as an equivalent number of parasitic larvae, differences in parasitism between the sexes disappeared. We conclude that male-biased parasitism in plants could arise from parasite-offspring (i.e. herbivore-seed) competition for female resources.

An alternative water transport system in land plants

The evolution of vascular tissue is a key innovation enabling plants to inhabit terrestrial environments. Here, we demonstrate extra-vascular water transport in a giant, prop-rooted monocot from Lord Howe Island. Pandanus forsteri (Pandanaceae) produces gutter-like leaves that capture rainwater, which is then couriered along a network of channels to the tips of aerial roots, where it is stored by absorptive tissue. This passive mechanism of water acquisition, transport and storage is critical to the growth of aerial prop roots that cannot yet attain water via vascular conduction. This species therefore sheds light on the elaborate means by which plants have evolved to attain water.

Phenotypic trait matching predicts the topology of an insular plant-bird pollination network

Conceptualizing species interactions as networks has broadened our understanding of ecological communities. However, the factors shaping interaction patterns among species and, therefore, network structure remain unclear. One potentially important factor is the matching of phenotypic traits. Here, we tested for trait matching in a bird-flower visitation network from New Zealand. We first quantified the overall network structure and tested whether flower size could account for differences in the visitation rates of flowering plants. We then explored the relationship between the flower size and bill size. The results showed that the interaction network is nested. Plant species with large flowers received more visits from birds than plant species with small flowers. Moreover, plant species with large flowers were visited more frequently by birds with large bills, while species with smaller flowers were visited more frequently by birds with small bills. Overall, the interaction patterns between birds and flowering plants could be predicted by their morphology, suggesting that phenotypic trait matching is an important predictor of network structure.

Can you teach an old parrot new tricks? Cognitive development in wild kaka (Nestor meridionalis)

Despite recent efforts to characterize innovative individuals within a species, we still know very little about the ontogeny of innovation ability. A number of studies have found that innovation rates are correlated with personality traits, such as neophilia and exploration. Juvenile birds are frequently more neophilic and explorative, yet few studies have found evidence of age-related differences in innovative problem-solving success. Here, we show consistently higher innovation efficiency in juveniles of a wild, omnivorous parrot species across a variety of tasks and contexts. We tested 104 kaka (Nestor meridionalis), ranging in age from four months to 13 years. Twenty-four individuals participated in all three of our problem-solving tasks, two of which involved a familiar feeder and one an entirely novel apparatus. Juveniles were the most efficient problem-solvers in all three tasks. By contrast, the adults' success was context dependent and limited to the novel apparatus, which did not require modification of a pre-learned behavioural response. This suggests greater behavioural flexibility in the juvenile birds, who also showed higher persistence and exploratory diversity than adults. These traits may enable young kaka to discover efficient foraging techniques, which are then maintained throughout adulthood.

Male New Zealand robins (Petroica longipes) cater to their mate's desire when sharing food in the wild

In many species that have bi-parental care, food-sharing males provide vital nutritional resources to their mates during reproduction. However, it is currently unknown whether females can signal specific desires to their mates, or if males can cater to female desire in the wild. Here we investigate whether and how wild male North Island robins (Petroica longipes) respond to changes in their mates' desires and nutritional need when sharing food. We demonstrate that wild female robins' desire for particular foods changes over short time periods; when given the choice between two types of insect larvae, females prefer the type they have not recently eaten. In our experiments, wild male robins preferentially shared the larvae type that their mate was most likely to desire and also increased the quantity of food shared if she had begun incubating. Males catered to their mates' desire when female behaviour was the only cue available to guide their choices. This is the first evidence that females may behaviourally communicate their specific food desires to their mates, enabling males to cater to fine-scale changes in their mates' nutritional requirements in the wild. Such a simple behaviour-reading mechanism has the potential to be widespread among other food-sharing species.

The repeated evolution of large seeds on islands

Several plant traits are known to evolve in predictable ways on islands. For example, herbaceous species often evolve to become woody and species frequently evolve larger leaves, regardless of growth form. However, our understanding of how seed sizes might evolve on islands lags far behind other plant traits. Here, we conduct the first test for macroevolutionary patterns of seed size on islands. We tested for differences in seed size between 40 island-mainland taxonomic pairings from four island groups surrounding New Zealand. Seed size data were collected in the field and then augmented by published seed descriptions to produce a more comprehensive dataset. Seed sizes of insular plants were consistently larger than mainland relatives, even after accounting for differences in growth form, dispersal mode and evolutionary history. Selection may favour seed size increases on islands to reduce dispersibility, as long-distance dispersal may result in propagule mortality at sea. Alternatively, larger seeds tend to generate larger seedlings, which are more likely to establish and outcompete neighbours. Our results indicate there is a general tendency for the evolution of large seeds on islands, but the mechanisms responsible for this evolutionary pathway have yet to be fully resolved.

Evaluating Frugivore-fruit Interactions Using Avian Eye Modelling

Fruit phenotypes are often hypothesised to be affected by selection by frugivores. Here, we tested two hypotheses concerning frugivore-fruit interactions from the perspective of fruit colours. We measured the spectral properties of 26 fruits and the associated leaves of plants from 2 islands in New Zealand. Visual observations were also performed to record the birds that fed on the fruits. First, we tested the fruit-foliage hypothesis, where fruit colours are assumed to be evolutionarily constrained by their own leaf colour to maximise colour contrast and fruit conspicuousness. We ran a null model analysis comparing fruit colour contrast using an avian eye model. Second, we tested the frugivore specificity hypothesis, where specific fruit colours are thought to be connected with a specific bird frugivore. We performed a regression on the number of bird visits against the fruit colour in tetrahedral colour space based on an avian eye calculation using Mantel's test. The results show that fruit colours are not constrained by their own leaf colours. There is also no relationship or pattern suggesting a link between a specific fruit colour and specific bird visitors. We suggest that although fruit colour is one of the most highly discussed components, it is not the most important single deciding factor in frugivore fruit selection.

What causes size coupling in fruit--frugivore interaction webs?

The simplest and arguably the most ubiquitous pattern in seed dispersal mutualisms is size coupling: large frugivores tend to consume larger fruits and small frugivores tend to consume smaller fruits. Despite the simplicity of this pattern, the potential mechanisms responsible for fruit--frugivore size coupling are mechanistically divergent and poorly resolved. Size coupling could arise deterministically, if large frugivores actively seek out larger fruits to maximize their foraging efficiency. Alternatively, size coupling could also arise passively, if frugivores forage randomly, but are able to consume only those fruit species that are smaller than their gape width. I observed birds forage for fruits in a New Zealand forest reserve at approximately five-day intervals for six years to test for fruit--frugivore size coupling. I then derived a suite of network analyses to establish whether fruit--frugivore size coupling was best explained by active or passive foraging by frugivores. Results showed a strikingly strong pattern in size coupling; the average size of fruits consumed by each frugivore species increased with their maximum gape width. Simulation analyses revealed that over 70% of variation in interaction frequencies in the observed fruit-frugivore web could be explained by a size-constrained, passive, foraging model. Foraging models in which birds foraged actively for different-sized fruits to improve their foraging efficiency performed more poorly. Results were therefore consistent with the hypothesis that apparently nonrandom patterns in seed dispersal mutualisms can sometimes arise from simple stochastic processes.

The ability of North Island Robins to discriminate between humans is related to their behavioural type

Animals are able to learn to identify persistent threats to themselves and their offspring. For example, birds are able to quickly learn to discriminate between humans that have previously threatened their nests from humans with whom they have had no prior experience. However, no study has yet examined whether a bird's ability to discriminate between humans is related to the bird's underlying behavioural type. In this study, we examined whether there were differences among North Island (NI) robins (Petroica longipes), based on their underlying behavioural type, in their abilities to discriminate between familiar and novel human observers. Using a simple feeding experiment, we timed how long it took birds to attack a food item placed next to an observer on each of 7 days. On the eighth day, a different observer timed the birds. We found that birds could be split into two behaviour types based on their attack behaviour: fast attackers (latencies <20 sec) and slow attackers (latencies >20 secs). Interestingly, the fast birds did not increase their attack latency in response to the novel observer whereas the slow attackers did. This result, for the first time, demonstrates that a bird's ability to discriminate between humans can vary among birds based on their behavioural type.

Memory for multiple cache locations and prey quantities in a food-hoarding songbird

Most animals can discriminate between pairs of numbers that are each less than four without training. However, North Island robins (Petroica longipes), a food-hoarding songbird endemic to New Zealand, can discriminate between quantities of items as high as eight without training. Here we investigate whether robins are capable of other complex quantity discrimination tasks. We test whether their ability to discriminate between small quantities declines with (1) the number of cache sites containing prey rewards and (2) the length of time separating cache creation and retrieval (retention interval). Results showed that subjects generally performed above-chance expectations. They were equally able to discriminate between different combinations of prey quantities that were hidden from view in 2, 3, and 4 cache sites from between 1, 10, and 60 s. Overall results indicate that North Island robins can process complex quantity information involving more than two discrete quantities of items for up to 1 min long retention intervals without training.

Large quantity discrimination by North Island robins (Petroica longipes)

While numerosity-representation and enumeration of different numbers of objects-and quantity discrimination in particular have been studied in a wide range of species, very little is known about the numerical abilities of animals in the wild. This study examined spontaneous relative quantity judgments (RQJs) by wild North Island robins (Petroica longipes) of New Zealand. In Experiment 1, robins were tested on a range of numerical values of up to 14 versus 16 items, which were sequentially presented and hidden. In Experiment 2, the same numerical contrasts were tested on a different group of subjects but quantities were presented as whole visible sets. Experiment 3 involved whole visible sets that comprised of exceedingly large quantities of up to 56 versus 64 items. While robins shared with other species a ratio-based representation system for representing very large values, they also appeared to have developed an object indexing system with an extended upper limit (well beyond 4) that may be an evolutionary response to ecological challenges faced by scatter-hoarding birds. These results suggest that cognitive mechanism influencing an understanding of physical quantity may be deployed more flexibly in some contexts than previously thought, and are discussed in light of findings across other mammalian and avian species.

Seed dispersal: the blind bomb maker

A recent study shows that a desert shrub uses a 'mustard oil bomb' to regulate the behaviour of seed-predating rodents - transforming these predators into mutualistic seed dispersers.

Allometry of sexual size dimorphism in dioecious plants: do plants obey Rensch's rule?

Rensch's rule refers to a pattern in sexual size dimorphism (SSD) in which SSD decreases with body size when females are the larger sex and increases with body size when males are the larger sex. Many animal taxa conform to Rensch's rule, but it has yet to be investigated in plants. Using herbarium collections from New Zealand, we characterized the size of leaves and stems of 297 individuals from 38 dioecious plant species belonging to three distantly related phylogenetic lineages. Statistical comparisons of leaf sizes between males and females showed evidence for Rensch's rule in two of the three lineages, indicating SSD decreases with leaf size when females produce larger leaves and increases with leaf size when males produce larger leaves. A similar pattern in SSD was observed for stem sizes. However, in this instance, females of small-stemmed species produced much larger stems than did males, but as stem sizes increased, SSD often disappeared. We hypothesize that sexual dimorphism in stem sizes results from selection for larger stems in females, which must provide mechanical support for seeds, fruits, and dispersal vectors, and that scaling relationships in leaf sizes result from correlated evolution with stem sizes. The overall results suggest that selection for larger female stem sizes to support the weight of offspring can give rise to Rensch's rule in dioecious plants.

A hierarchical framework for investigating epiphyte assemblages: networks, meta-communities, and scale

Epiphytes are an important component of many forested ecosystems, yet our understanding of epiphyte communities lags far behind that of terrestrial-based plant communities. This discrepancy is exacerbated by the lack of a theoretical context to assess patterns in epiphyte community structure. We attempt to fill this gap by developing an analytical framework to investigate epiphyte assemblages, which we then apply to a data set on epiphyte distributions in a Panamanian rain forest. On a coarse scale, interactions between epiphyte species and host tree species can be viewed as bipartite networks, similar to pollination and seed dispersal networks. On a finer scale, epiphyte communities on individual host trees can be viewed as meta-communities, or suites of local epiphyte communities connected by dispersal. Similar analytical tools are typically employed to investigate species interaction networks and meta-communities, thus providing a unified analytical framework to investigate coarse-scale (network) and fine-scale (meta-community) patterns in epiphyte distributions. Coarse-scale analysis of the Panamanian data set showed that most epiphyte species interacted with fewer host species than expected by chance. Fine-scale analyses showed that epiphyte species richness on individual trees was lower than null model expectations. Therefore, epiphyte distributions were clumped at both scales, perhaps as a result of dispersal limitations. Scale-dependent patterns in epiphyte species composition were observed. Epiphyte-host networks showed evidence of negative co-occurrence patterns, which could arise from adaptations among epiphyte species to avoid competition for host species, while most epiphyte meta-communities were distributed at random. Application of our "meta-network" analytical framework in other locales may help to identify general patterns in the structure of epiphyte assemblages and their variation in space and time.

Is crypsis a common defensive strategy in plants? Speculation on signal deception in the New Zealand flora

Color is a common feature of animal defense. Herbivorous insects are often colored in shades of green similar to their preferred food plants, making them difficult for predators to locate. Other insects advertise their presence with bright colors after they sequester enough toxins from their food plants to make them unpalatable. Some insects even switch between cryptic and aposomatic coloration during development. Although common in animals, quantitative evidence for color-based defense in plants is rare. After all, the primary function of plant leaves is to absorb light for photosynthesis, rather than reflect light in ways that alter their appearance to herbivores. However, recent research is beginning to challenge the notion that color-based defence is restricted to animals.

Ontogenetic colour changes in an insular tree species: signalling to extinct browsing birds?

* Animals often use colours to hide from predators (crypsis) or advertise defences (aposematism), but there is little evidence for colour-based defence in plants. * Here, we test whether ontogenetic changes in leaf colour of lancewood (Pseudopanax crassifolius) may have been part of a defensive strategy against flightless browsing birds called moa, which were once the only large herbivores in New Zealand. We tested this hypothesis by conducting spectrographic measurements on different-sized plants grown in a common garden. We also compared these results with observations on a closely related, derived species that evolved in the absence of moa on the Chatham Islands. * Spectrographic analyses showed that birds would have difficulty distinguishing seedling leaves against a background of leaf litter. Conversely, brightly coloured tissues flanking spines on sapling leaves are highly conspicuous to birds. Once above the reach of the tallest known moa, adults produce leaves that are typical in appearance to adult leaves. The Chatham Island species lacks ontogenetic colour changes entirely. * Overall, the results indicate that P. crassifolius goes through a remarkable series of colour changes during development, from cryptically coloured seedlings to aposematically coloured saplings, which may have formed a defensive strategy to protect against giant browsing birds.

Competitive assembly of South Pacific invasive ant communities

BACKGROUND

The relative importance of chance and determinism in structuring ecological communities has been debated for nearly a century. Evidence for determinism or assembly rules is often evaluated with null models that randomize the occurrence of species in particular locales. However, analyses of the presence or absence of species ignores the potential influence of species abundances, which have long been considered of major importance on community structure. Here, we test for community assembly rules in ant communities on small islands of the Tokelau archipelago using both presence-absence and abundance data. We conducted three sets of analyses on two spatial scales using three years of sampling data from 39 plots on 11 islands.

RESULTS

First, traditional null model tests showed support for negative species co-occurrence patterns among plots within islands, but not among islands. A plausible explanation for this result is that analyses at larger spatial scales merge heterogeneous habitats that have considerable effects on species occurrences. Second, analyses of ant abundances showed that samples with high ant abundances had fewer species than expected by chance, both within and among islands. One ant species, the invasive yellow crazy ant Anoplolepis gracilipes, appeared to have a particularly strong effect on community structure correlated with its abundance. Third, abundances of most ant species were inversely correlated with the abundances of all other ants at both spatial scales. This result is consistent with competition theory, which predicts species distributions are affected by diffuse competition with suites of co-occurring species.

CONCLUSION

Our results support a pluralistic explanation for ant species abundances and assembly. Both stochastic and deterministic processes interact to determine ant community assembly, though abundance patterns clearly drive the deterministic patterns in this community. These deterministic patterns were observed at two spatial scales. Results indicate that abundance-based null models may be more sensitive in detecting non-random patterns in community assembly than species co-occurrences analyses.

Adaptive numerical competency in a food-hoarding songbird

Most animals can distinguish between small quantities (less than four) innately. Many animals can also distinguish between larger quantities after extensive training. However, the adaptive significance of numerical discriminations in wild animals is almost completely unknown. We conducted a series of experiments to test whether a food-hoarding songbird, the New Zealand robin Petroica australis, uses numerical judgements when retrieving and pilfering cached food. Different numbers of mealworms were presented sequentially to wild birds in a pair of artificial cache sites, which were then obscured from view. Robins frequently chose the site containing more prey, and the accuracy of their number discriminations declined linearly with the total number of prey concealed, rising above-chance expectations in trials containing up to 12 prey items. A series of complementary experiments showed that these results could not be explained by time, volume, orientation, order or sensory confounds. Lastly, a violation of expectancy experiment, in which birds were allowed to retrieve a fraction of the prey they were originally offered, showed that birds searched for longer when they expected to retrieve more prey. Overall results indicate that New Zealand robins use a sophisticated numerical sense to retrieve and pilfer stored food, thus providing a critical link in understanding the evolution of numerical competency.

Seed Dispersal by Weta

Weta are giant, flightless grasshoppers that are endemic to New Zealand. In the absence of native mammals, weta are thought to perform similar ecological functions. As such, they might be expected to be important seeds dispersers. However, insects are not known to consume fleshy fruits and to disperse seeds after gut passage. We conducted a series of observations and experiments to test whether weta form mutualistic partnerships with fleshy-fruited plants as seed dispersers, similar to small mammals elsewhere in the world. Results showed that weta are indeed effective seeds dispersers, providing an example of ecological convergence between unrelated organisms.

Motion sickness incidence: distribution of time to first emesis and comparison of some complex motion conditions

A statistical mixture model is used to fit time-to-emesis data. The Weibull probability distribution is shown to provide a good fit for those subjects who either become sick or withdraw from the experiment within 2 h. The second part of the mixture accounts for those subjects who neither quit nor vomit within 2 h. The log-normal probability model is shown to give a poorer fit to the data and figures showing the relative fits of the estimated Weibull and log-normal distributions are provided. A nonparametric test is used to compare the five motion conditions of the Correlation Study. That test shows that there are significant differences in severity among the conditions.