Lost Frogs of Britain

Today there are three native species of tailless amphibian in the UK, the Common Frog, Common Toad and Natterjack Toad. Even 20 years ago however there were four, and in Roman times there may have been as many as seven. This is the story of those lost species.

The Pool Frog Pelophylax lessonae

This is the most recent extinction, and particularly unfortunate as it could have been saved. However, these large, green, water loving frogs were believed to have been an introduction in the 19th century, and so no special measures were taken to protect them in their last site in the Norfolk Broads until it was too late.In the last few years a reintroduction project has been undertaken using frogs from Sweden, which are the most closely related to the extinct British form. It is not endangered on the continent, and with the closely related Marsh Frog P.ridibunda (which was introduced in the early 20th Century to Romney Marsh where it still survives) it hybridises to produce the fertile hybrid known as the Edible Frog, which was the main supplier of the famous French delicacy

Green Tree Frog Hyla arborea

There is a possibility that this species may be, like the Pool frog, a neglected former resident. Several times (before it was made illegal) there were attempts to introduce this species to the UK, but one very old colony (now extinct) was known from the New Forest. Green Tree frogs are assumed to be a warm climate animal, but they reach the coast of France. The introduction attempts mostly failed or did not persist for long, probably because most of the individuals released were males, which are easier to locate by their calls, and also because the wrong habitat was chosen – the northern green tree frogs prefer shrubby or herbaceous vegetation and warm, fish-free pools to spawn in.

Agile Frog Rana dalmatina

Unlike the previous two species, this does survive in the UK, with a single, highly threatened population on Jersey which Jersey Zoo is working to preserve. However, remains from 7th Century deposits suggest this long legged (it can jump 2 metres) relative of our common frog once also could be found on the UK mainland. As its centre of distribution is mainly France, with the Jersey individuals on the extreme north of the range, its loss from the UK was probably due mainly to the cooling of the climate during the post-Roman period. Its preferred habitat of wet grassland, woodland close to water and pools to spawn in are however classic Beaver habitat, and the loss of this from the UK may have had a knock-on effect on the remaining population

Moor Frog Rana arvalis

Known only from the remains of a single individual, this close relative of the Common Frog was probably never common in the UK. A primarily Eastern European species, with a preference for extensive shallow wetlands to breed in, the gradual post glacial transformation of the Norfolk Broads into a reedbed probably removed its habitat, and its preference for a more continental climate resulted in it dying out in the UK

Others

These are the species known or reasonably believed to have been resident in the UK. However, it is becoming clear that the relatively small numbers of species of reptile and amphibian in the UK compared to mainland Europe are not simply a result of the formation of the Channel preventing them colonising, but also extinctions resulting from climate change, in particular a cooling of the climate in the last few thousand years culminating in the Little Ice Age. Amphibians have delicate bones, and small animals with restricted ranges or habitats may disappear without a trace. For example, Midwife Toads are in Germany mainly associated with Beaver dams, and the Yellow Bellied Toad prefers warm, shallow ponds or ditches. Small colonies of these species have bred on several occasions in the UK, and the Midwife Toad has survived in a colony in Bedfordshire for at least a hundred years. Possibly these were once also native in the UK?

An interesting evening

This week I went to the monthly Severn Counties Bird Society meeting (see link opposite). This is an avicultural society I have belonged to for several years. The society has monthly lectures or slide shows on (mostly) bird related topics, and a couple of shows each year. We also have various events during the year and have a stand at the Bath & West show in May.

This months’ talk was from a guy in Wales who specialises in breeding colour varieties of the Ring-Necked parakeet. This is a fairly popular aviary bird, which has a wide range across Africa and India. The talk covered all aspects of their care, including housing, feeding and genetics. The members were especially interested in two items of kit – a pretty good database system which enables tracking of family trees of the birds and the genetic makeup, medical history etc, and a useful black box which measures the heart rate of a chick still in the shell, which lets the keeper monitor whether a chick is alive, ready to hatch, or tiring during hatching and may be in need of help.

The Ring neck is an interesting bird from the point of view of this blog. First, it is the ancestor of the Mauritius Echo parakeet, which I will cover later, and it has established numerous feral populations worldwide, including Florida, California, and London. The London population is currently estimated at close to or above 10,000 birds. The largest roost is at Esher Rugby Club in Surrey (see photo)

Although urbanites do not have many objections to them, Ring necks are a significant agricultural pest in India, as their diet includes grain, fruit, and anything vegetable they can find. They also are potential competitors with native hole-nesting birds, although I am not aware of any impacts being noted so far.

The Ring Neck is only one of a successful and widespread genus of parrots, with a center of distribution in India but spreading to South East Asia. Many are popular aviary birds, although like all parrots they can be quite noisy, and are probably not the best bird to keep in built up areas. For more information look at http://home.wanadoo.nl/psittaculaworld/PsittaculaWorld.htm

Land of the Dodo 3:Invertebrates

As might be expected of islands covered with tropical rainforest, the Mascarenes have a very extensive invertebrate fauna. As you might also expect from the habitat destruction and introduced predators on the islands, much of this is either extinct or endangered. The problem is compounded by the lack of any major surveys of the islands for invertebrates prior to the 19th Century, by which time a high proportion of the endemics were probably already extinct.

The main introduced predators are Black Rat, House Shrew, and Common Tenrec. (see above) These have been either introduced deliberately (like the Tenrec) or arrived with ships as stowaways. These novel threats have resulted in widespread losses, particularly of the terrestrial invertebrate fauna.

From other islands which have been studied, it is likely that a keystone invertebrate on the forest floor would have been land crabs, similar to those that are still prominent on Christmas Island. These would have scavenged fruit, dead animals, and anything else they could get. In the same line of work would have been a variety of land hermit crabs, which are still prominent today along the shores.

There was a significant diversity of endemic snails on the islands, a high percentage of which are extinct. Several species have also been introduced, notably Giant African snails, Achatina, and the just as ubiquitous Euglandina, which is a predatory species widely used in the mid 20th century for biological control of Achatina, and which has caused mass extinctions of native snails throughout the world tropical islands. It is possible that the effect of this on Mauritius is less than on , for example, Tahiti, as there were already endemic predatory snails on the islands before Euglandina arrived, so the native species would have been prepared to face that type of pressure.

There are still just about surviving the native Scolopendra centipedes. These are part of a widespread genus of often large and highly predatory invertebrates, and would have been among the top invertebrate predators. Recently rediscovered on Serpent Island is a large tarantula, which seems to feed extensively on lizards.

There is a large radiation of cockroaches, several of which are flightless, and an even bigger radiation of endemic beetles. Reported from Round Island was the flightless beetle Pulsopipes herculeanus, which we have at Bristol Zoo and is now found today known only from Fregate in the Seychelles, although it is known to have a distribution throughout the other islands in the group in the

past. (see image left)

There are also several species of endemic Phasmid on the islands – Reunion for example has four out of five know species as endemic – and an equally large number of endemic Lepidoptera. At least one species of butterfly on Mauritius, Salamis augustina, died out by 1957 because its food plant, the nettle tree, became extinct as a result of Achatina snails eating the seedlings and preventing regeneration. The species still survives on Reunion.

There is at present little conservation action directed towards invertebrates on Mauritius, Some of the native snails have been captive bred, but until the introduced predators, particularly the shrew, can be eliminated there is little habitat left to reintroduce. Part of the problem is that the introduced species interact – when on Isle Aux Aigrettes the rats were eliminated, the result was a plague of shrews, which were not even known to be on the island before then,

At least in theory, captive breeding programmes could be set up for the tarantula and a reintroduction attempted. Fregate beetles are already being captive bred and are a potential reintroduction for Round island. As the grubs feed on rotten wood however, this will either have to be imported or the reintroduction wait until the habitat recovery has advanced enough for there to be a sufficient supply of native dead wood top sustain the population.

Land of the Dodo Part 2

Part 2: Plants

The Mascarene islands have a huge proportion of endemic plants, all of which are threatened in various ways. Mauritius for example has 670 native flowering plants, of which 45% are endemic, plus ferns and fungi, many of which have restricted habitat. Many of these had important ecological relationships with the now extinct or highly threatened fauna, and introduced species have affected these in many ways. 150 of the native Mauritian species are listed as threatened as they are known from less than 50 individuals in the wild.

So how did this state of affairs come about?

Plants reach newly emerged islands in various ways, by wind, floating in the sea (for such species such as coconuts and probably screw pines Pandanus sp, of which there are several endemic species), or carried by birds or fruit bats.

However they arrived, unique local forms developed on each of the islands, which became covered, depending on local rainfall, in palm savannah or rainforest for the most part, with the higher parts of Reunion in heather moorland vegetation.

The longest distance that any Mascarene plant may have travelled is, improbably, a species of Acacia that lives at high altitudes on Reunion. Its nearest relative is found 17,000 km away, in Hawai’i. How it got there is made more clear by the fact that an endemic Reunion seabird, Barau’s petrel, seems to have evolved from the Hawaian Petrel, which breeds near or sometimes under the roots of the Hawaian acacia. Sea birds sometimes colonise islands surprising far from their birth place,

The most important trees in Mauritius are probably the various species of ebony, Diospyros, which can grow to 20m or more. These trees, related to the cultivated Persimmon which is in the same genus, were a major factor in human colonisation of the islands, as the wood was extremely valuable. Unfortunately the trees are dioecious, which means that trees come as either male or female plants, and several of the surviving forms survive mainly in single-sex groups, which obviously restricts regeneration.

One of the more famous (or infamous) trees is the Tambalacoque, Sideroxylon grandiflorum. Some years ago it was noticed that regeneration of this species was not happening effectively, and the story was dreamed up that this was because its seeds were designed to be spread by the Dodo, and without it the species was doomed. Actually, this is not true, and the true cause was that the fruits of the tree were being eaten by introduced monkeys who destroyed the seeds.

As well as these and many other forest trees, there was an extensive understory of shrubs, including several endemic Hibiscus species (some of which are in cultivation). Many of these flowering shrubs produce red nectar, and this is probably because it is an extra attractant to the plants pollinators, which are not as one might expect birds but are various species of day gecko, Phelsuma. However, small lizards are themselves prey for many other creatures, so the Phelsuma prefer to spend as much time as they can in the ferociously spiky Pandanus trees and bushes, which provide protection.

Introduced plants are unfortunately a major threat to the natives, Guava, Blackberry, and Privet are the most important weeds, which choke off other plants and prevent regeneration. Hybridisation with introduced ornamentals is also a threat to the native Hibiscus for example. A problem with control is that many of the fruits are eaten not just by local people, but by many birds and animals, including the threatened endemics.

The other main threat is introduced mammals. Rats feed on the seeds of many of the natives, severely restricting regeneration, and introduced deer especially browse heavily on native plants, which do not have the toxins in their leaves that plants on the mainland have to reduce browsing. The only threat the native plants formerly faced was browsing by tortoises, and many have vividly coloured juvenile foliage or spiny trunks to prevent this.

There are efforts to restore the native vegetation, but these are extremely expensive and labour intensive. Fencing will keep out deer, but hand weeding is required to remove guava, privet, and other introduced plants to allow the native trees and shrubs to grow. However, on small plots this has proved effective.

Another method is to subcontract the problem. On Isle Aux Aigrettes, an islet off Mauritius, Aldabra Giant Tortoises (which we have at Bristol Zoo) have been used to graze the native grasses and plants. This has the useful side effect that introduced plants top not have much defence against tortoise grazing and browsing, and as a result the balance is tipped in favour of the local plants. This has proved so successful that it is planned to introduce the tortoises to Round Island, where non-native plants are competing with the local natives.

(Picture at head of post - Hibiscus genevii in habitat)

Land of the Dodo

Part 1: Geography and colonisation

In order to understand how Mauritius came to be the home of the Dodo and many other unique animals, it is first necessary to consider how these animals arrived.

Mauritius is of volcanic origin, and lies at the end of a chain of islands and submerged plateau extending more or less due north towards the Deccan Traps on continental India. This is significant, as it explains the islands origin. As with the Hawaian island chain, the heat source is a volcanic hot spot sourced from deep under the earth, and as the movement of the Indian Ocean tectonic plate has carried India northwards over the last 66 million years first the Deccan, and then a series of oceanic islands, have erupted and then eroded away. South west of Mauritius is the current center of volcanic activity, the island of Reunion, and situated some distance east over a volcanic ridge is the small island of Rodrigues. Scattered around these main islands are various small outcrops, the most famous of which is Round Island near Mauritius. The entire group is known as the Mascarenes.

Situated to the north of Mauritius, and not part of this chain, are the Seychelles. These are not volcanic, but are instead a tiny fragment of India that broke off as it moved northwards. As a part of a continent, they have retained some animals that cannot cross salt water, such as unique endemic amphibians, and have been a major source of colonisations of other Indian ocean islands.

The oldest lavas on Mauritius are about 10 million years old, the youngest about 25,000, but Reunion erupts almost every year. The last major eruption was about 180,000 years ago, which must have devastated the island and probably wiped out many species. It is noteworthy for example that Reunion had no flightless birds, although the Reunion Ibis apparently was very reluctant to fly. The age of Rodrigues is unclear, but given the number of endemic species it is probably fairly old.

The problem for any creature getting to the Mascarenes is that both wind and sea currents blow from the east, and east of the Mascarenes there is basically nothing but open sea for many thousands of kilometres, until you reach Australia or Indonesia. For anything incapable of flying very long distances, such as seabirds, or which can survive long periods of travel on floating rafts of vegetation, such as some reptiles, arrival can only be from the north. Flying from Madagascar is a shorter trip, but involves flying into the prevailing wind, and does not seem to be the major source of Mauritian endemics.

The eroded remnants of older islands however would have been above sea level on numerous occasions during the ice ages, and species such as the fruit bats, parakeets, pigeons and several other groups probably reached Mauritius in these periods. When Mauritius first appeared above the sea the nearest reef, St Brandon shoals, would have been a high island, and it is probably from there that the ancestral dodo flew to the island.

Death of the Dodo

In 1662 some survivors of the shipwrecked Arnhem were exploring the coast of Mauritius. They waded over at low tide to an islet (probable Isle d’Ambre off the north-west cost), where they found doddaersan “larger than geese but not able to fly, instead of wings they had small flaps, but they could run very fast”. Starving, they killed them all. These may have been among the last of their kind – a few probably survived in remote areas for a decade or two more, but most later accounts probably refer to another flightless bird, a large kind of rail, itself now extinct.

The conventional story of the Dodo’s demise is that, living on an island without predators, it took to nesting on the ground and lost the power of flight, and was as a result helpless against people killing it for food. Despite the story above, this, like most conventional stories, is not the whole truth. For one thing, except when desperate people avoided eating Dodos, as they apparently did not taste good, much preferring to eat the tortoises which abounded on the island. In addition, the Dodo had by all accounts a nasty bite.

Nor were the islands devoid of predators. There were owls, a harrier (similar to the Marsh harrier that breeds in the UK), three species of snake, at least one of which was over 1.5 m long, and several large skinks, one over 60cm long. In addition, the forest floor swarmed with land crabs, as numerous as those that can be found on Christmas Island today. None of these were large enough to tackle an adult Dodo, with the possible exception of the harrier, but any could have taken eggs or young. In fact, it is a bit unclear why the Dodo took to ground nesting at all.

The true cause of the extinction of the Dodo probably lies in the usual suspects – introduced species and habitat destruction. The reason Mauritius was colonised was because it made a convenient way station to the East Indies (now Indonesia). It was also discovered that the lowland forests were rich in ebony trees, a highly prized commodity. The deforestation that resulted probably destroyed both the Dodo’s habitat and its food source, which was probably the fruits of forest trees and terrestrial invertebrates. The arrival of black rats – which arrived from shipwrecks even before the islands were permanently settled, would have added competition for food, but the Dodo would have been used to defending its nest against crabs and other predators, and would have been able to drive off raids on its eggs.

The main cause of the Dodo’s demise however, was probably the introduction of the pig. It was common for sailors to leave livestock behind to breed on islands they discovered, with the aim of having food when they next passed that way, and they would certainly have devoured any eggs they came across.

The Dodo died out so early that many naturalists did not even believe that it ever existed., but was only a travellers tale. Only in the 19th century was the Dodo “rediscovered”, and the phrase “As dead as the Dodo” entered the language.
There was a good deal of debate about what a Dodo actually was, but it is now known to have been a kind of fruit pigeon, whose nearest relatives are to be found in south east Asia.Here at Bristol we are fortunate to have two of the Dodo’s closest living relatives. Probably the closest of all is the Nicobar Pigeon Caloenas nicobarica, now found only in Indonesia. However, it migrates between islands even today and although it nests in trees it spends all its time feeding on the ground. Also mainly a ground feeder is the Victoria Crowned Pigeon (Goura victoriae), which is the largest living pigeon, and comes from New Guinea. Both of these species have been bred here at Bristol and are well represented in collections worldwide.

I will be following this post with a series on the natutral history of Mauritius and its related islands. To see what is happening there today, and what is being done about it, have a look at the Mauritius Wildlife Foundation website on the links section

Alan