Dennis Hansen

University of Zurich | UZH · Institut für Evolutionsbiologie und Umweltwissenschaften

Background: The Aldabra giant tortoise (Aldabrachelys gigantea) is one of only two giant tortoise species left in the world. The species is endemic to Aldabra Atoll in Seychelles and is listed as Vulnerable on the International Union for Conservation of Nature Red List (v2.3) due to its limited distribution and threats posed by climate change. Gen...

The Aldabra giant tortoise (Aldabrachelys gigantea) is one of only two giant tortoise species left in the world. The species is endemic to Aldabra Atoll in Seychelles and is considered vulnerable due to its limited distribution and threats posed by climate change. Genomic resources for A. gigantea are lacking, hampering conservation efforts focused...

Aldabrachelys gigantea (Aldabra giant tortoise) is one of only two giant tortoise species left in the world and survives as a single wild population of over 100,000 individuals on Aldabra Atoll, Seychelles. Despite this large current population size, the species faces an uncertain future because of its extremely restricted distribution range and hi...

We portray the Palaeontological Museum of the University of Zurich and current research at the institute. The text is in German.

Aldabrachelys gigantea (Aldabra giant tortoise) is one of only two giant tortoise species left in the world and survives as a single wild population of over 100,000 individuals on Aldabra Atoll, Seychelles. Despite this large current population size, the species faces an uncertain future because of its extremely restricted distribution range and hi...

The island of Mauritius in the Mascarenes has already suffered high levels of extinction, and habitat destruction and invasive alien species have pushed many of the remaining endemic species to the brink of extinction. Round Island, a small island northeast of Mauritius, is home to remnant populations of several reptile species that were once distr...

The Aldabra giant tortoise ( Aldabrachelys gigantea ) is one of only two remaining giant tortoise species worldwide. Captive-bred A. gigantea are being used in rewilding projects in the Western Indian Ocean to functionally replace the extinct endemic giant tortoise species and restore degraded island ecosystems. Furthermore, large-scale translocati...

Seed dispersal through endozoochory is a critical ecosystem function worldwide. Seed gut retention time (GRT; the duration that seeds are retained in the digestive tract) is an important part of the qualitative component of the seed dispersal effectiveness framework. GRT is a major determinant of when and how far away seeds are dispersed, aiding se...

The Aldabra giant tortoise ( Aldabrachelys gigantea ) is one of only two remaining giant tortoise species worldwide. Captive-bred A. gigantea are being used in rewilding projects in the Western Indian Ocean to functionally replace the extinct endemic giant tortoise species and restore degraded island ecosystems. Furthermore, large-scale translocati...

Aldabra giant tortoises (Aldabrachelys gigantea) are currently listed as Vulnerable on the IUCN Red List of Threatened Species. However, negative impacts of sea-level rise are predicted to result in an overall population decline of 40-65 % over the next 100 years, rendering the species Endangered. Captive propagation is an important tool for in- an...

Despite being among the largest turtles that ever lived, the biology and systematics of Stupendemys geographicus remain largely unknown because of scant, fragmentary finds. We describe exceptional specimens and new localities of S. geographicus from the Miocene of Venezuela and Colombia. We document the largest shell reported for any extant or exti...

With habitat loss and fragmentation among the greatest threats to biodiversity, a better understanding of the habitat use of keystone species is critical in any conservation management strategy. Aldabra Atoll, in the Seychelles archipelago, has the largest population worldwide of giant tortoises. This endemic species (Aldabrachelys gigantea) could...

In recent years, it has become clear that frugivory and seed dispersal (FSD) by turtles and tortoises is much more common than previously thought. We here review published and unpublished records of chelonian FSD, and assess the role of chelonians as seed dispersers, from individual species to the community level. We first discuss the distribution...

Preliminary results of my MSc thesis presented at the Island Biology conference in Reunion Island in 2019.

We recorded the thermoregulatory patterns of five Aldabra giant tortoises (Aldabrachelys gigantea) (25–193 kg) during spring (ZRH spring trial), after the installation of a heating area (concrete heat pad and basking lamp) at Zurich Zoo, Switzerland. The measurements were compared to published results on the same tortoises observed prior to the ins...

Replacing recently extinct endemic giant tortoises with extant, functional analogues provide the perhaps best examples of island rewilding to date. Yet, an efficient future application of this conservation action is challenging in an era of climate change. We here present and discuss a conceptual frame- work that can serve as a roadmap for the stud...

Global change and human expansion have resulted in many species extinctions worldwide, but the geographic variation and determinants of extinction risk in particular guilds still remain little explored. Here, we quantified insular extinctions of frugivorous vertebrates (including birds, mammals and reptiles) across 74 tropical and subtropical ocean...

In recent years, it has become clear that frugivory and seed dispersal (FSD) by turtles and tortoises is much more common than previously thought. Yet, a review and synthesis is lacking. We here review published and unpublished records of chelonian FSD, and assess the role of chelonians as seed dispersers, from individual species to the community l...

1. Habitat loss and modification are hallmarks of anthropogenic ecosystems, but the consequences for ecosystem functions and services often remain unclear. Understanding these links in cities is complicated by strong but fine‐scale differences in habitat structure among green space patches, and a high variance in habitat amount across urban landsca...

We studied the temperature relations of wild and zoo Aldabra giant tortoises (Aldabrachelys gigantea) focusing on: 1) the relationship between environmental temperature and tortoise activity patterns (n=8 wild individuals), and 2) on tortoise body temperature fluctuations, including how their core and external body temperatures vary in relation to...

Today, the UNESCO World Heritage Site of Aldabra Atoll is home to about 100 000 giant tortoises, Aldabrachelys gigantea, whose fossil record goes back to the Late Pleistocene. New Late Pleistocene fossils (age ca. 90–125 000 years) from the atoll revealed some appendicular bones and numerous shell fragments of giant tortoises and cranial and postcr...

For hundreds of millions of years, large vertebrates (megafauna) have inhabited most of the ecosystems on our planet. During the late Quaternary, notably during the Late Pleistocene and the early Holocene, Earth experienced a rapid extinction of large, terrestrial vertebrates. While much attention has been paid to understanding the causes of this m...

Aldabra Atoll has the largest population of giant tortoises (Aldabrachelys gigantea) in the world. As such an important biological resource, it is necessary to understand how the effects of climate change will impact this keystone species; in particular the frequency of drought, which is likely to affect tortoise habitat. To assess whether drought...

The extinction of frugivores has been considered one of the main drivers of the disruption of important ecological processes, such as seed dispersal. Many defaunated forests are too small to restore function by reintroducing large frugivores, such as tapirs or Ateline monkeys, and the long-term fate of large-seeded plants in these areas is uncertai...

How do organisms arrive on isolated islands, and how do insular evolutionary radiations arise? In a recent paper, Wilmé et al. (2016a) argue that early Austronesians that colonized Madagascar from Southeast Asia translocated giant tortoises to islands in the western Indian Ocean. In the Mascarene Islands, moreover, the human-translocated tortoises...

ABSTRACT: Climate change is changing the shape of landscapes across the globe. How this impacts terrestrial animals, especially ectotherms, will depend on their physiological sensitivity. For animals that act as ecosystem engineers, their response to climate change will have wide-ranging direct and indirect effects on both habitat and ecosystem str...

Table S1. Transect details.

Figure S1. Seasonal variation in rainfall at the research station on Picard, Aldabra. Figure S2. Recapture and survival rates of marked animals on Grand Terre East (gte), Grand Terre West (gtw) and Malabar (mal). Figure S3. Size‐dependence of survival.

Table S2. Mark‐recapture analysis of apparent survival rates.

Trophic rewilding is an ecological restoration strategy that uses species introductions to restore top-down trophic interactions and associated trophic cascades to promote self-regulating biodiverse ecosystems. Given the importance of large animals in trophic cascades and their widespread losses and resulting trophic downgrading, it often focuses o...

Understanding the extent of morphological variation in the wild population of Aldabra giant tortoises is important for conservation, as morphological variation in captive populations has been interpreted as evidence for lingering genes from extinct tortoise lineages. If true, this could impact reintroduction programmes in the region. The population...

There is a heated debate about whether all non-native species are 'guilty until proven innocent', or whether some should be accepted or even welcomed. Further fanning the flames, I here present a case where introductions of carefully vetted, non-native species could provide a net conservation benefit. On many islands, native megaherbivores (flightl...

Note: Please download to access the link to the video-presentation Animal-mediated seed dispersal is an important ecological function in many ecosystems worldwide. Obtaining a detailed understanding of this process at a community level in species-rich ecosystems, based on experimental evaluations of all pairwise interactions, is a Herculean task....

1. Ciliate assemblages play a significant role in the microbial food web. The effects of environmental temperature on assemblage composition may be influenced by abiotic factors such as seasonality and disturbance, but the effects of temperature on ciliate assemblages found on different substrata have not been explored. Sandy bottoms and submerged...

Aldabra Atoll, a UNESCO World Heritage Site since 1982, hosts the world's largest population of giant tortoises. In view of recent rainfall declines in the East African region, it is important to assess the implications of local rainfall trends on the atoll's ecosystem and evaluate potential threats to the food resources of the giant tortoises. How...

Floral morphology often directly influences interactions with pollinators, but less is known about the role of extrafloral structures. We studied the relationship between bract motility, floral structural specialization and pollination in Dalechampia aff. bernieri, an endemic Madagascan species with floral structures indicating specialized buzz-pol...

Giant tortoises as drivers of the seed dispersal network of Aldabra Summary: Animal-mediated seed dispersal is an important ecological function in many ecosystems worldwide, but our understanding of this process is still limited. A major gaps is being able to predict where seeds end up. We use the vertebrate frugivore fauna and the fleshy-fruited p...

Evolutionary shifts between pollination systems are often accompanied by modifications of floral traits, including olfactory cues. We investigated the implications of a shift from passerine bird to beetle pollination in Protea for floral scent chemistry, and also explored the functional significance of Protea scent for pollinator attraction. Using...

Rewilding falls within the general framework of restoration ecology, but differs from a traditional view of habitat restoration and species reintroduction. Four initial steps are required to instigate a rewilding project: identification of the issue of conservation concern; identification of the missing ecological processes; identification of the f...

Aim To test whether ingestion by endemic frugivores differentially affects the seed germination time, germination percentage and seedling survival of endemic, native and exotic fleshy fruited plant species, and to identify the principal processes and attributes driving such effects. Location Round Island, Mauritius. Methods We conducted a germinati...

Nectar guides, contrasting patterns on flowers that supposedly direct pollinators towards a concealed nectar reward, are taxonomically widespread. However, there have been few studies of their functional significance and effects on plant fitness. Most previous studies focused on pollinator behaviour and used artificial flowers in laboratory setting...

There is increasing evidence that restoration ecologists should be most concerned with restoring species interactions rather than species diversity per se [1]. Rewilding with taxon substitutes, the intentional introduction of exotic species to replace the ecosystem functions of recently extinct species, is one way to reverse ecosystem dysfunction f...

Background and Aims Although pollination of plants that attract flies by resembling their carrion brood and food sites has been reported in several angiosperm families, there has been very little work done on the level of specificity in carrion mimicry systems and the importance of plant cues in mediating such specialization. Specificity may be exp...

Starting in the late 1970s, ecologists began unraveling the role of recently extinct large vertebrates in evolutionary ecology and ecosystem dynamics. Three decades later, practitioners are now considering the role of ecological history in conservation practice, and some have called for restoring missing ecological functions and evolutionary potent...

Key Words: introduced species, invasional meltdown, oceanic island, plant–animal interaction, positive interaction, seed dispersal Disruption of ecosystems is one of the biggest threats posed by invasive species (Mack et al. 2000). Thus, one of the most important challenges is to understand the impact of exotic species on native species and habitat...

a b s t r a c t Islands harbour much of the world's threatened biodiversity. Recent work has highlighted how it is not species diversity per se but rather the interactions between organisms that breathes life into ecosystems. Thus, the real challenge to preserving and restoring biodiversity on islands is not to only focus on species themselves, but...

We argue that the introduction of non-native extant tortoises as ecological replacements for extinct giant tortoises is a realistic restoration management scheme, which is easy to implement. We discuss how the recent extinctions of endemic giant Cylindraspis tortoises on the Mascarene Islands have left a legacy of ecosystem dysfunction threatening...

In Mauritius, many of the worst invasive plant species have fleshy fruits and rely on animals for dispersal. The introduced red-whiskered bulbul (Pycnonotus jocosus) feeds on many fleshy-fruited species, and often moves from invaded and degraded habitats into higher quality native forests, thus potentially acting as a mediator of continued plant in...

Roussea simplex is the sole member of the endemic family Rousseaceae from Mauritius. Once widespread and locally common on Mauritius, today R. simplex is critically endangered, with 85–90 known remaining individuals in a few scattered populations. We documented the unusual flowering and fruiting phenology and studied the pollination and seed disper...

In Mauritius, the endemic blue-tailed day-gecko Phelsuma cepediana is currently the sole pollinator and seed disperser of the critically endangered endemic plant Roussea simplex (Rousseaceae). The flowers and fruits are often visited by the invasive ant Technomyrmex albipes, which forages on the nectar and fruit pulp, and tends honeydew-producing m...

Experimental design of seed germination experiment, with ‘patches’ around the 20 focal maternal Syzygium mamillatum trees (not to scale). ‘S’ and ‘F’ denotes patches with seeds and whole fruits, respectively. Shading represents cages, that were used during the first few months of germination and seedling growth. ‘Away’ patches were set up 20–25 m a...

Aldabran giant tortoise dispersing ebony seeds. (A) In the nature reserve on the offshore Mauritian island Ile aux Aigrettes, released free-roaming giant Aldabran tortoises Aldabrachelys gigantea eat fruits of the endangered endemic ebony Diospyros egrettarum. (B, C) In the fruiting season, one tortoise turd can contain up to several hundred seeds,...

Research on crop systems and biodiversity conservation in the tropics has mainly been concerned with how low to mid intensity agricultural systems can benefit from adjacent natural habitats by receiving ecosystem services from natural biodiversity. One intensively studied crop in this framework is coffee. Positive effects are relatively easy to qua...

Background: The Janzen-Connell model states that plant-specific natural enemies may have a disproportionately large negative effect on progeny close to maternal trees. The majority of experimental and theoretical studies addressing the Janzen-Connell model have explored how it can explain existing patterns of species diversity in tropical mainland...

Invasive alien plants affect the functioning of ecosystems by altering plant–animal interactions, such as pollination, which may impede natural regeneration of native plant species. In Mauritius, we studied the reproductive traits and pollination ecology of the rare endemic cauliflorous tree Syzygium mamillatum in a restored forest (all alien plant...

Research on crop systems and biodiversity conservation in the tropics has mainly been concerned with how low to mid intensity agricultural systems can benefit from adjacent natural habitats by receiving ecosystem services from natural biodiversity. One in-tensively studied crop in this framework is coffee. Positive effects are relatively easy to qu...

Research on crop systems and biodiversity conservation in the tropics has mainly been concerned with how low to mid intensity agricultural systems can benefit from adjacent natural habitats by receiving ecosystem services from natural biodiversity. One intensively studied crop in this framework is coffee. Positive effects are relatively easy to qua...

Using a sample of 1,403 flowering plant species, we tested the hypothesis that flower openness and flowervisitor generalization level of a plant species correlate positively. The “flower-visitor generalization level” Ln of a flowering plant species n, here defined as number of flower-visiting animal species attracted to the flowers of n in a given...

In natural communities, species are embedded in networks of direct and indirect interactions. Most studies on indirect interactions have focused on how they affect predator-prey or competitive relationships. However, it is equally likely that indirect interactions play an important structuring role in mutualistic relationships in a natural communit...

While coloured nectar has been known to science at least since 1785, it has only recently received focused scientific attention. However, information about this rare floral trait is scattered and hard to find. Here, we document coloured nectar in 67 taxa worldwide, with a wide taxonomical and geographical distribution. We summarise what is currentl...

Most floral nectars are clear as water, and the enigmatic coloured nectar in three endemic plant species in Mauritius has puzzled scientists studying it. One hypothesis about the possible ecological function of coloured nectar is that it serves as a visual signal for pollinators. Recent studies have shown that at least two of the three Mauritian pl...

In my thesis I studied aspects of ecology, evolution, and conservation of plant-animal interactions on islands. My main study site was the island of Mauritius in the Indian Ocean. Mauritius is a biodiversity hotspot, with many endemic plant and animal species and high rates of extinction. Chapter 1 reviewed the global distribution, ecology and evol...

The bird‐flower element of the Canary Islands is a group of endemic plants having traits characteristic of bird pollination, and some are visited by opportunistically nectar‐feeding passerine birds. We investigated evolutionary changes in nectar sugar composition in seven Canarian lineages of ornithophilous plant species and their entomophilous rel...