Extreme homogenization: The past,present and future of mammal assemblages on islands

Aim

We documented how the similarity of mammal assemblages on continental and oceanic islands has changed since initial human colonization, since European arrival and overall. We investigated how levels of similarity might change in the future.

Location

Continental and oceanic islands worldwide.

Time period

Human settlement of islands to the present, as well as projections for the future.

Major taxa studied

Mammals.

Methods

We used mammal occurrence data on islands to calculate the change in similarity using a pairwise approach based on Jaccard's index and a multisite approach based on Jaccard's and Sørensen's measures. We divided the mammal assemblages into two time periods, before and after island colonization or trade began with Europeans. We unpacked the mechanisms driving changes in similarity, exploring how initial similarity interacts with seven types of species turnover events to determine overall change. Finally, we calculated how future similarity levels will change if past trends in introductions and extinctions continue.

Results

Mammals, on both continental and oceanic islands, show one of the most pronounced cases of homogenization ever observed, and on oceanic islands mammals show the largest increase in homogenization ever observed for a terrestrial group. Most of the homogenization observed to date has been driven by recent historical changes, not by changes that occurred before European arrival. If current patterns of species introductions and extinctions continue, then oceanic islands will experience little additional homogenization, whereas continental islands will homogenize greatly beyond current levels.

Main conclusions

Mammal assemblages on oceanic islands show nearly an order of magnitude greater change in similarity than plant and bird assemblages. Projections of future similarity indicate that continental and oceanic islands are on different trajectories of change. These trajectories could be altered by management actions, but in some cases those actions that would be impactful run counter to current conservation norms.     

Industrialization and ethnic change in the modern world

Despite the large recent attention given to ethnicity within the social sciences, the sources of modern ethnic change have remained opaque. Drawing upon social theory from Marx and Gellner, I argue here that industrialization incentivizes ethnic homogenization by lowering the relative value of land. Using carbon emissions per capita as a proxy for industrialization, I show that cross-country changes in ethno-linguistic fractionalization between 1961 and 1985 are negatively correlated with industrialization, and that this result is robust to the use of a variety of control variables, sub-samples and alternative measures of industrialization such as cement production, urbanization and agriculture as a percentage of GDP. In particular, I find no evidence for the direct role of the state in promoting ethnic homogenization, which adds to other recent evidence on how economic incentives may trump political ones as regards identity change, at least in the short- to medium term.     

A new framework for investigating biotic homogenization and exploring future trajectories: oceanic island plant and bird assemblages as a case study

Studies of biotic homogenization have focused primarily on characterizing changes that have occurred between some past baseline and the present day. In order to understand how homogenization may change in the future, it is important to contextualize the processes driving these changes. Here, we examine empirical patterns of change in taxonomic similarity among oceanic island plant and bird assemblages. We use these empirical cases to unpack dynamic properties of biotic homogenization, thereby elucidating two important factors that have received little attention: 1) initial similarity and 2) the influence of six classes of introduction and extinction events. We use Jaccard's Index to explore the interplay among these factors in determining the changes in similarity that have occurred between human settlement and the present. Specifically, we develop general formulas for changes in similarity resulting from each of the six types of introductions and extinctions, so that the effect of each event type is formulated in terms of initial similarity and species richness. We then apply these insights to project how similarity levels would change in the future if the present patterns of introductions and extinctions continue. We show that the six event types, along with initial similarity, can show dramatically different behavior in different systems, leading to widely variable influences on similarity. Plant and bird biotas have homogenized only slightly to date, but their trajectories of change are highly divergent. Although existing patterns of colonization and extinction might not continue unchanged, if they were to do so then plant assemblages would show little additional change, whereas bird assemblages would become much more strongly homogenized. Our results suggest that moderate changes in similarity observed to date mask the potential for more dramatic changes in the future, and that the interaction among initial similarity and differential introduction and extinction regimes drives these dynamics.     

Are plant communities on the Canary Islands resistant to plant invasion?

Oceanic islands are renowned for their unique flora and high levels of endemism. Native island plants, however, are imperilled by non-native species that can become invasive by outcompeting natives. The threat of native island assemblages generally increases with isolation and the number of endemics featured, but also with human-associated disturbance and land use. Based on this, the Canary Island native plant systems should be highly threatened by invasives, similar to other oceanic islands globally. However, Canarian native plant systems are only weakly infiltrated and are rarely directly threatened by invasive plants. Further, highly disturbed areas, usually among the first colonized by invasives on islands, are recolonized here by natives. Based on this, we postulate four hypotheses (climatic filter, well-preservation status, human legacy and permanent colonization) for explaining this unusual behaviour of plant systems on the Canary Islands, providing an opportunity to understand the drivers and processes behind invasion into plant communities on islands.     

Spatial scale and evolutionary history determine the degree of taxonomic homogenization across island bird assemblages

It is widely documented that human activities have elevated the extirpation of natural populations as well as the successful introduction to new areas of non-native species. These dual processes of introduction and extirpation can change the similarity of communities, but the direction and magnitude these changes take are likely to depend on the manner in which introductions and extirpations occur, the spatial scale at which the changes are measured, and the initial similarity of the communities before the human-induced drivers occurred. Here, we explore patterns of extirpation and introduction and their influence on the similarity of global oceanic island bird assemblages from four different Oceans (Atlantic, Caribbean, Indian, Pacific). We show that different historical patterns of introduction and extirpation have produced varying trends in compositional similarity both between islands within archipelagos and between islands across different archipelagos within the same ocean. Patterns of bird assemblage convergence (i.e. taxonomic homogenization) or divergence (i.e. taxonomic differentiation) among islands depended on the scale of examination, the evolutionary associations among species of the region, and the cultural history of human colonization. These factors are all likely to be leading to a series of multiple interacting processes that are shaping the complex compositional changes observed among global island bird faunas over time.     

Ecological and evolutionary consequences of biotic homogenization

Biotic homogenization, the gradual replacement of native biotas by locally expanding non-natives, is a global process that diminishes floral and faunal distinctions among regions. Although patterns of homogenization have been well studied, their specific ecological and evolutionary consequences remain unexplored. We argue that our current perspective on biotic homogenization should be expanded beyond a simple recognition of species diversity loss, towards a synthesis of higher order effects. Here, we explore three distinct forms of homogenization (genetic, taxonomic and functional), and discuss their immediate and future impacts on ecological and evolutionary processes. Our goal is to initiate future research that investigates the broader conservation implications of homogenization and to promote a proactive style of adaptive management that engages the human component of the anthropogenic blender that is currently mixing the biota on Earth.     

Filming sea‐level rise: media encounters and memory work in the Indian Sundarbans

In this article, I discuss social consequences of the visualization of climate change. I focus on encounters between film crews and displaced islanders on rapidly eroding islands at the sea‐facing edge of the Indian Sundarbans. As these islands have become the epitome of global environmental damage and dystopian futures, film crews regularly travel here to produce captivating footage on degradation and displacement. I first situate the ensuing encounters in global regimes of visibility and infrastructural possibilities. Building on that, I argue that these encounters set forth what I call unintentional memory work. By virtue of being a spectacle, by enabling a return to the film subjects’ former homes, and by staging acts of remembering, film teams unintentionally induce a reworking of memories and, at times, the production of new memories. In both instances, particular pasts are prioritized and imbued with a sense of urgency and relevance to foreign film teams and global audiences. I frame these media encounters as interfaces through which global anxieties about Anthropocene futures become implicated in the fashioning of selves on remote shores.     

The Politics of Adaptation: Subsistence Livelihoods and Vulnerability to Climate Change in the Koyukon Athabascan Village of Ruby,Alaska

The concepts of vulnerability and adaptation have contributed to understanding human responses to climate change. However, analysis of the implications of the broader political context on adaptation has largely been absent. Through a case study of the subsistence livelihoods of Koyukon Athabascan people of Ruby Village, this paper examines the implications of adaptation to the social changes precipitated by colonization for the articulation of current responses to climate change. Semi-structured interviews, seasonal rounds, and land-use mapping conducted with 20 community experts indicate that subsistence livelihoods are of continued importance to the people of Ruby in spite of the dramatic social change. While adaptive responses demonstrate resilience, adaptation to one form of change can increase vulnerability to other kinds of perturbations. Research findings illustrate that a historical approach to adaptation can clarify the influence of the present political context on indigenous peoples’ responses to impacts of climate changes.     

Narrow water barriers prevent multiple colonizations and limit gene flow among California Channel Island wild buckwheats (Eriogonum: Polygonaceae)

The relative roles of chance colonization and subsequent gene flow in the development of insular endemic biotas have been extensively studied in remote oceanic archipelagos, but are less well characterized on nearshore island systems. The current study investigated patterns of colonization and divergence between and within two wild buckwheat species (Polygonaceae), Eriogonum arborescens and E. giganteum, endemic to the California Channel Islands to determine whether geographical isolation is driving diversification. Using plastid and nuclear sequence data and microsatellite allele frequencies, we determined that gene flow in these Eriogonum spp. is restricted by isolation. The data suggest that successful colonization of and gene flow among the islands are infrequent. Colonization appears to have followed a stepping‐stone model that is consistent with a north‐to‐south pattern across the islands. This colonization pattern coupled with relatively little post‐colonization inter‐island gene flow, particularly among southern islands, has generated a pattern of more divergent lineages on the isolated southern islands. These results run counter to the general expectation that all islands close to a continental source should receive a high level of gene flow. Finally, management recommendations focused on protecting the lineages from loss of private alleles and the erosion of the remaining genetic diversity are offered.     

Human display and dispersal: A case study from biotidal Britain in the Middle and Upper Pleistocene

Hominin dispersal and human colonization have been hallmark concepts in the last two decades of palaeanthropological research, 1 - 7 even though the terminology in these approaches is loosely defined (Box 1). The number, frequency, and routes of dispersal have been analyzed on a global scale, 8 beginning with the earliest movement of hominins between Africa and Asia, and back again. 9 Investigating dispersals has provided a much‐needed dynamic to account for recent human origins in Africa 10 and the replacement elsewhere of older regional hominin populations that include the Neanderthals. 11 In the last twenty years, phylogeographies based on a wealth of molecular studies have added enormously to our understanding of global population history from the Paleolithic to the Vikings 12 and has, in particular, revitalized the study of farming dispersals. 13 As a result, Quaternary hominins and humans have been on the move as never before. However, not all of these movements are considered within an evolutionary framework. Interest has focused instead on using dispersals to support the claims for either a Neolithic or human revolution as the turning point in human prehistory. Here, I explore an alternative by considering the implications of the major shift that occurred in Paleolithic technology from instruments to containers. 14 I argue that this development can be explained by the selective pressure from population dispersal for novel forms of cultural display that enhanced information exchange among adaptive generalists and which allowed the stretching of social relationships in space and time. 15 The British Paleolithic record provides a case study.     

Mode and tempo in the evolution of socio-political organization: reconciling 'Darwinian' and 'Spencerian' evolutionary approaches in anthropology

Traditional investigations of the evolution of human social and political institutions trace their ancestry back to nineteenth century social scientists such as Herbert Spencer, and have concentrated on the increase in socio-political complexity over time. More recent studies of cultural evolution have been explicitly informed by Darwinian evolutionary theory and focus on the transmission of cultural traits between individuals. These two approaches to investigating cultural change are often seen as incompatible. However, we argue that many of the defining features and assumptions of 'Spencerian' cultural evolutionary theory represent testable hypotheses that can and should be tackled within a broader 'Darwinian' framework. In this paper we apply phylogenetic comparative techniques to data from Austronesian-speaking societies of Island South-East Asia and the Pacific to test hypotheses about the mode and tempo of human socio-political evolution. We find support for three ideas often associated with Spencerian cultural evolutionary theory: (i) political organization has evolved through a regular sequence of forms, (ii) increases in hierarchical political complexity have been more common than decreases, and (iii) political organization has co-evolved with the wider presence of hereditary social stratification.     

Bridging the bonding gap: the transition from primates to humans

Primate societies are characterized by bonded social relationships of a kind that are rare in other mammal taxa. These bonded relationships, which provide the basis for coalitions, are underpinned by an endorphin mechanism mediated by social grooming. However, bonded relationships of this kind impose constraints on the size of social groups that are possible. When ecological pressures have demanded larger groups, primates have had to evolve new mechanisms to facilitate bonding. This has involved increasing the size of vocal and visual communication repertoires, increasing the time devoted to social interaction and developing a capacity to manage two-tier social relationships (strong and weak ties). I consider the implications of these constraints for the evolution of human social communities and argue that laughter was an early evolutionary innovation that helped bridge the bonding gap between the group sizes characteristic of chimpanzees and australopithecines and those in later hominins.     

Cluster randomization and political philosophy

In this paper, I will argue that, while the ethical issues raised by cluster randomization can be challenging, they are not new. My thesis divides neatly into two parts. In the first, easier part I argue that many of the ethical challenges posed by cluster randomized human subjects research are clearly present in other types of human subjects research, and so are not novel. In the second, more difficult part I discuss the thorniest ethical challenge for cluster randomized research--cases where consent is genuinely impractical to obtain. I argue that once again these cases require no new analytic insight; instead, we should look to political philosophy for guidance. In other words, the most serious ethical problem that arises in cluster randomized research also arises in political philosophy.     

Do junior academic bioethicists have an obligation to be activists?

Activism and bioethics have enjoyed a somewhat strained relationship. In this paper, I consider activism specifically from the perspective of junior academics. I will argue that although there may be a prima facie duty for bioethicists to be activists, countervailing considerations for junior academics may mean that they, in particular, should refrain from undertaking activist activities. I will argue this on the basis of two key claims. First, I argue that activism may come at a potential cost to the academics who undertake it, and that these costs are potentially of greatest detriment to junior academics undertaking activism. Second, I argue that junior academics are likely to be less effective activists than established academics. Moreover, undertaking activism as a junior academic may prevent one from becoming an effective activist later. Finally, I will discuss the implications of this argument for activist commitments later in one’s career.     

Unravelling the peculiarities of island life: vicariance, dispersal and the diversification of the extinct and extant giant Galápagos tortoises

In isolated oceanic islands, colonization patterns are often interpreted as resulting from dispersal rather than vicariant events. Such inferences may not be appropriate when island associations change over time and new islands do not form in a simple linear trend. Further complexity in the phylogeography of ocean islands arises when dealing with endangered taxa as extinctions, uncertainty on the number of evolutionary ‘units’, and human activities can obscure the progression of colonization events. Here, we address these issues through a reconstruction of the evolutionary history of giant Galápagos tortoises, integrating DNA data from extinct and extant species with information on recent human activities and newly available geological data. Our results show that only three of the five extinct or nearly extinct species should be considered independent evolutionary units. Dispersal from mainland South America started at approximately 3.2 Ma after the emergence of the two oldest islands of San Cristobal and Española. Dispersal from older to younger islands began approximately 1.74 Ma and was followed by multiple colonizations from different sources within the archipelago. Vicariant events, spurred by island formation, coalescence, and separation, contributed to lineage diversifications on Pinzón and Floreana dating from 1.26 and 0.85 Ma. This work provides an example of how to reconstruct the history of endangered taxa in spite of extinctions and human‐mediated dispersal events and highlights the need to take into account both vicariance and dispersal when dealing with organisms from islands whose associations are not simply explained by a linear emergence model.     

Genetic and phylogenetic consequences of island biogeography

Abstract.— Island biogeography theory predicts that the number of species on an island should increase with island size and decrease with island distance to the mainland. These predictions are generally well supported in comparative and experimental studies. These ecological, equilibrium predictions arise as a result of colonization and extinction processes. Because colonization and extinction are also important processes in evolution, we develop methods to test evolutionary predictions of island biogeography. We derive a population genetic model of island biogeography that incorporates island colonization, migration of individuals from the mainland, and extinction of island populations. The model provides a means of estimating the rates of migration and extinction from population genetic data. This model predicts that within an island population the distribution of genetic divergences with respect to the mainland source population should be bimodal, with much of the divergence dating to the colonization event. Across islands, this model predicts that populations on large islands should be on average more genetically divergent from mainland source populations than those on small islands. Likewise, populations on distant islands should be more divergent than those on close islands. Published observations of a larger proportion of endemic species on large and distant islands support these predictions.     

Breeding birds on small islands: island biogeography or optimal foraging?

1. We test MacArthur and Wilson's theory about the biogeography of communities on isolated habitat patches using bird breeding records from 16 small islands off the coasts of Britain and Ireland. 2. A traditional examination of patterns of species richness on these islands suggests that area and habitat diversity are important predictors, but that isolation and latitude have a negligible impact in this system. 3. Unlike traditional studies, we directly examine the fundamental processes of colonization and local extinction (cessation of breeding), rather than higher-order phenomena such as species richness. 4. We find that many of MacArthur and Wilson's predictions hold: colonization probability is lower on more isolated islands, and extinction probability is lower on larger islands and those with a greater diversity of habitats. 5. We also find an unexpected pattern: extinction probability is much lower on more isolated islands. This is the strongest relationship in these data, and isolation is the best single predictor of colonization and extinction. 6. Our results show that examination of species richness alone is misleading. Isolation has a strong effect on both of the dynamic processes that underlie richness, and in this system, the reductions in both colonization and extinction probability seen on more distant islands have opposing influences on species richness, and largely cancel each other out. 7. We suggest that an appropriate model for this system might be optimal foraging theory, which predicts that organisms will stay longer in a resource patch if the distance to a neighbouring patch is large. If nest sites and food are the resources in this system, then optimal foraging theory predicts the pattern we observe. 8. We advance the hypothesis that there is a class of spatial systems, defined by their scale and by the taxon under consideration, at which decision-making processes are a key driver of the spatiotemporal dynamics. The appropriate theory for such systems will be a hybrid of concepts from biogeography/metapopulation theory and behavioural ecology.     

Transit towards floristic homogenization on oceanic islands in the south-eastern Pacific: comparing pre-European and current floras

Aim  To quantify the occurrence of processes of homogenization or differentiation in the vascular flora of six oceanic islands.
Location  Six islands in the south-eastern Pacific drawn from the Desventuradas Archipelago, Easter Island and the Juan Fernández Archipelago.
Methods  Using published floristic studies, we determined the floristic composition of each island at two different stages: (1) pre-European colonization and (2) current flora. We compared changes in the number of shared plants and the floristic similarity among islands for each stage.
Results  The number of plant species doubled from 263 in pre-European flora to 531 species currently. Only three native species became extinct, four natives were translocated among the islands and 271 plant species were introduced from outside. The frequency of plant species shared by two or more islands is higher in the post-European floras than prior to European contact, and the level of floristic similarity between islands increased slightly.
Main conclusions  Considering the low naturalization rate of alien plants, the small number of extinctions and the meagre increase in floristic similarity, these islands are undergoing a slow process of floristic homogenization.     

The Importance of Species Traits for Species Distribution on Oceanic Islands

Understanding species'' ability to colonize new habitats is a key knowledge allowing us to predict species'' survival in the changing landscapes. However, most studies exploring this topic observe distribution of species in landscapes which are under strong human influence being fragmented only recently and ignore the fact that the species distribution in these landscapes is far from equilibrium. Oceanic islands seem more appropriate systems for studying the relationship between species traits and its distribution as they are fragmented without human contribution and as they remained unchanged for a long evolutionary time. In our study we compared the values of dispersal as well as persistence traits among 18 species pairs from the Canary Islands differing in their distribution within the archipelago. The data were analyzed both with and without phylogenetic correction. The results demonstrate that no dispersal trait alone can explain the distribution of the species in the system. They, however, also suggest that species with better dispersal compared to their close relatives are better colonizers. Similarly, abundance of species in the archipelago seems to be an important predictor of species colonization ability only when comparing closely related species. This implies that analyses including phylogenetic correction may provide different insights than analyses without such a correction and both types of analyses should be combined to understand the importance of various plant traits for species colonization ability.     

Lessons from a kill

During their colonization by Polynesians and later by Europeans, the Hawaiian islands suffered a massive loss of species. All the extinctions are indirectly attributable to human impact. Nonetheless, it has proved extremely difficult to specify which of several possible mechanisms caused each particular extinction. This seems to admit defeat in the battle to understand past extinctions. Such understanding could guide our efforts to protect species that are now threatened with extinction. Will it be easier to understand the causes of future extinctions? Surveys of future extinctions stress habitat destruction as the simple and dominant mechanism. This contrasts to its secondary (and generally confused) role in past extinctions. I argue that this contrast between the complexity of the past and the apparent simplicity of the future arises because extinction mechanisms are inherently synergistic. Once extensive species losses begin, it may be impossible to separate the mechanisms and thus manage an individual species as if its decline had a single cause.