Death of the Dodo

The extinction of the famous flightless bird of Mauritius, the Dodo, tells the sad tale of the effect humans can have when they to colonise another country! 

The Dodo, Raphus cacullatus, used to be found on the isolated, volcanic island of Mauritius, in the middle of the Indian Ocean, as shown by the map to the right. This tiny island was first discovered by the Portuguese in 1507 but did not become popular until the Dutch arrived in 1598, as explored by Gold et al. 

What did the Dutch do?

In 2006, Julian P Hulme created a full account of the history of the Dodo and aimed to solve any of the myths created due to the lack of evidence associated with this fluffy friend. From this he noted that before 1620, there were many Dutch accounts of the Dodo but after this is drastically reduced. Initially, people thought that the hungry sailors that arrived on the island after weeks at sea were to blame, as they'd go for the biggest, easiest to catch food that they could find. As well as Dodo, there was also a rich supply of pigeons and parrots that supposedly tasted nicer and therefore the predation pressure from humans was slightly reduced, meaning that that was not the sole cause of their extinction. 

Unfortunately, however, when the Dutch sailors arrived they didn't just bring themselves, they also brought lots of exotic invasive species that would continue to cause absolute havoc. Hulme explains that in 1600 monkeys were brought over, in 1606 goats, cattle and pigs and finally in 1639, deer made their way on shore. All of these invasions were very successful, being the Dodo's first predator and eventually lead to their extinction in 1662. The pigs were thought to have been the most detrimental due to the habitat destruction they caused and them feeding on the eggs or chicks of the Dodo. 

But that's not all! The colonisation of humans lead to the destruction of the Dodo's home, and feeding on small vertebrates, nuts and fruits, they were very reliant on a large area of land to live off as Gold et al. found through reading Dutch sailors diary entries. This dependence on the forest and the fact they could not fly to escape the invasion lead to their demise. 

It wasn't even just Mauritius!

Sadly, the colonisation of Rodrigues, an island 560km East of Mauritius, saw the extinction of their very own flightless Rodrigues solitare bird due to human activity as explored by Gold et al. As noted in previous posts, islands are at the most risk to extinction from the introduction of Invasive Alien Species and this a classic example to prove this. Their isolation and unique ecosystems leave them very vulnerable to outside influence. So was it the human's fault? Or was it just an unfortunate circumstance? 

Mauritian Identity

Despite the sadness generated from the loss of this bizarre bird, Mauritian people still continue to love and cherish it as part of their culture and identity. Meet Sharvanee Mauree, my best friend and house mate at university who was born and lived in Mauritius until moving to London to study here at UCL. I asked what the Dodo means to her,  and she replied...

"I think it's a symbol of being Mauritian. It's on our passports and all official documents and it's funny because none of us have seen it in real life. I think in a way it's a sign of what we have lost and whether we can preserve the rest of our wildlife now, that's for us to decide!"

Beastly Bullfrogs!

American Bullfrogs around Western North America are being labelled as one of the major causes of amphibian decline! 

The American Bullfrog, Rana catesbeiana, is becoming of great scientific and conservational interest as studies shine light on its impact on native populations of amphibians, especially frogs. With this new information, scientists are hoping to be able to manage this species and potentially eliminate the negative effects it has all together. 

Figure showing the worldwide distribution of the American Bullfrog 

So where have they managed to get to?

Over the years, this Invasive Alien Species has been introduced to over 40 different countries. The world map to the left shows the native distribution in grey and the non-native, introduced areas in black. As you can see, these famous frogs originate in Eastern North America and have managed to spread to areas of Western North America, South America, Europe and Asia due to human transportation and habitat modification.  

Why are they so destructive? 

American Bullfrogs are causing mayhem amongst native frog populations and Blaustein and Kiesecker repeatedly found that at sites where Red-Legged Frogs were absent, American Bull Frogs would be found close by. So what were the Bullfrogs doing that was so detrimental? 

Their studies showed that the American Bullfrog was basically eating the Red-Legged species. At both the tadpole and adult stage they recorded the Bullfrogs feeding on other amphibian species, especially the Red-Legged ones. The middle image to the left shows the scale of the American Bullfrog tadpole compared to the Red-Legged one on the image below that. 

However, this wasn't the only thing that was going on! Sometimes, as a response to the arrival of the American Bullfrog species, the Red-Legged ones would change their microhabitat, possibly by moving somewhere else in the water or changing their feeding habits. Unfortunately, in some cases this pushed the native frogs into an area where they were at increased risk of predation by fish, which in turn reduced the population numbers. 

Similar to this, Kiesecker and Blaustein found in another study that the time took for Red-Legged tadpoles to reach the metamorphosis stage increased and the body mass reached by the end of metamorphosis decreased when in the presence of American Bullfrogs. As a result the Red-Legged frogs that did survive were smaller and less competitive. 

But wait, there's still more!! American Bullfrogs are also causing damage through transmitting the fungus Batrachochytrium dendrobatidis which causes the infectious disease Chytridiomycosis. Greenspan et al explored how this disease does not always lead to death in all species. Unfortunately for the Red-Legged frogs, the disease is lethal and American Bullfrogs are unaffected by it and therefore they act as vectors that spread the disease to new populations they invade.   

   

Have humans had an impact?

Unsurprisingly, yes we have! Blaustein and Kiesecker found that habitat modifications made in Western North America actually led to more optimal conditions for the growth and success of the Invasive Alien American Bullfrogs. Converting large ephemeral wetlands into smaller permanent ponds, possibly though the development of infrastructure, creates prime breeding grounds of the Bullfrogs whilst removing the preferred spaces for other amphibian species. 

Also, American Bullfrogs only made it to places like Europe and Asia through the pet trade and accidental transport, a common way Invasive Alien Species are spread, as mentioned is last weeks post. 

To see The Simpsons experience with Invasive Alien Bullfrog species, click the link 

https://www.youtube.com/watch?v=g3V0wJ3q7l0

Transport and Trade of Invasive Alien Species!

Humans have managed to spread to nearly every corner of the globe, but how have they effected the distribution of Invasive Alien Species? 

As the world becomes more accessible for humans, it also does for other organisms. From species hitching a ride, disturbance of environments and the exchange of non-native species between countries, increased trade and transport has led to more invasions.  

At what point did it start to change? 

Philip E Hulme defines 3 key points in time that have caused a notable change to the spread of Invasive Alien Species: the end of the Middle Ages, the start of the Industrial Revolution and the Era of Globalisation. The Era of Globalisation has been marked with both increased income growth and  transport efficiency with both of these factors having an impact on the spread of non-native species. 

Graph showing the relationship between Alien Plant Richness
and Gross Domestic Product 

Increase income growth

Hulme explores the link between Invasive Alien Species richness and the Gross Domestic Product (GDP) of both continents and islands as displayed by the graph to the left. As GDP increases, so does amount of non-native species. The relationship is also stronger for islands, probably because of increased quantities of merchant imports which transport greater amounts of non-native species. 

Increase transport efficiency

The Industrial Revolution and advances in technology have seen a massive increase in the amount of travel that occurs worldwide. Increases in both personal transportation and global trade has had a knock on affect on invasive plants, animals and microorganisms.
Since the 1970's there has been a four fold rise in the volume of global imports that occur and 46% of this involves dry cargo. The dry cargo, mainly transported by containers, pose threat with Invasive Alien Species hitching a ride both inside and outside the containers and ships that transport them.

Also, between 1984 and 2000, 725,000 pests were recorded entering into US ports and 73% of this was via airports rather than seaports.

Disturbance due to transport!

Increasing demand for transport leads to the creation of more roads, railways and a rising use of waterways. As a result habitats get destroyed to make way for this infrastructure and the surrounding areas are continually disturbed and it allows organisms to travel to areas they may have not previously have been able to reach. Hansen and Clevenger compared the number of non-native species occurring in Canada along some railway lines and highways that were next to either grassland or forest. From this study they found:

• Grasslands had the higher frequencies of non-native species and there were more non-native compared to native species for up to 150m from the corridor edge 
• Forested areas had the lower frequencies and more non-native than native species for only up to 10m from the road or railway edge 
• The frequency of Invasive Alien Species decreased with distance from the corridor edge 
• The frequencies of non-native species along highways and railways was very similar 

These results therefore suggest that the transport corridors are having an impact on the distribution of Invasive Alien Species. Grasslands have shown to be more prone to disturbance than forested areas and non-native species are more able to successfully invade and stay there - which coincides with the idea I explored with Himalayan Basalm in my previous post! 

Graph showing the invasion success of
Wild-Caughtand Captive-Bred bird species 

Global exchange of exotic organisms! 

Interestingly, humans bring a lot of non-native species into other countries on purpose through global trade of plants and pets.  For example, exotic animals that are kept as pets and then escape and non-native plants that are introduced into garden centres, have more of an effect than you might expect. 

Carrete and Tella found that it is not the common caged birds that cause issues if they escape but the wild, exotic birds that are brought to countries through the pet market. The graph to the right shows how successful captive bred and wild-caught pet species are in Spain, where 0 = never seen in the wild and 3 =  breeding regularly in the wild. As it shows, captive bred species do not successfully manage to breed and get to level 3 and are therefore not a threat to the environment they enter. 

An example of a wild caught species that has successfully managed to invade places such as the US, Italy, Spain and Czech Republic is the Monk Parakeet, a bird imported from Argentina as shown to the left!

The ideas explored here are all raising questions when it comes to conservation. It is expected that the amount of transportation and disturbance to the environment is only going to increase in the coming years, so how do we make sure that the numbers of native species do not drop too low?

Click the link to find out more http://www.actionbioscience.org/biodiversity/simberloff.html

Floridian Frog Acoustic Battles!

Invasive Alien frog populations are causing commotion with competitive exclusion across Southern Florida! 

The forests of Florida are densely populated with a vast variety of treefrog species. Being able to produce uninterrupted and unaltered calls to other members of the same species is very important for mating. The sound a frog makes is very specialised and allows both potential mates to know what type of frog they are and where they are situated. Therefore, if an Invasive Alien Species comes along with a call of the same frequency and tends to produce it at the same time of night, this could lead to confusion and missed mating opportunities as they battle it out.

Pinewood Treefrog 

Cuban Treefrog 

Green Treefrog

Tennessen et al explore the arrival of Invasive Alien Species such as Cuban Treefrogs and their impact on native ones like Green and Pinewood Treefrogs. In particular they focussed on the acoustic calls they both make looking for any changes in the patterns of their sounds, relating it to competitive exclusion. 

Competitive Exclusion

As defined by Gause, competitive exclusion is when one group of organisms can potentially exclude another group from their habitat or particular resources. This can produce two responses;

1. the other group becomes entirely excluded and has to migrate or becomes extinct 

2. the other group modifies their lifestyle in order to share the resources and coexist

Either way, the species with the competitive advantage, such as an Invasive Alien Species, will be able to impact the life of other species. 

In this case the Cuban Treefrogs invaded the South Florida forests and starting interfering the acoustic resources of other native treefrogs. Tennessen et al conducted a study involving Cuban, Pinewood and Green Treefrogs where they played the calls of each species to the other and recorded their responses. When the Cuban Treefrog's acoustics were played to the Pinewood Treefrog, no change or modification to their call was recorded. This was because they both produce their calls at different frequencies and tended to make sounds at different times. This therefore meant there was no overlap in the communication between members of the same species and therefore no competition. However, when the Cuban call was played to the Green Treefrogs it was found the native species shortened the duration of their cals and increased the call amplitude.

 Tennessen et al  display this result in the figure above. The top graph (a) shows the Green Treefrog vocal behaviour before the Cuban Treefrogs call was played to them and the bottom on (b) shows the frequencies of their calls afterwards. This shows that the Green Treefrog has infact modified its call in response to the invasive species being there. This adaptive behaviour shows the second response mentioned in the competitive exclusion explanation above and is a key example of how Invasive Alien Species do not always lead to extinction.

Alien Hybrids: friend or foe?

When Invasive Alien Species enter a new environment they  impact others in many different ways, one of them being Hybridisation!

What is Hybridisation?

Hybridisation as defined by Todesco et al, is when two populations that have distinguishably different genes mate together to produce a "hybrid" offspring. This offspring will then have combined genes of both of the adults and will likely show a mix of morphological features too. 

How frequently does it occur?

Interestingly, in 1992 Abbott states how out of 2834 plant species listed in the New Flora of the British Isles, 1264 of these are Invasive Alien Species (IAS). Out of these 1264 species, 70 are recognised as being hybrids due to native-IAS interactions, 21 are due to IAS-IAS and 4 are even due to hybrid-native species interactions.  

It isn't even just retrained to plants, Mooney and Cleland explored other examples of species known for undergoing hybridisation as shown below.

A hybrid of the invasive Mallard Duck when it was introduced to the native New Zealand Grey Duck.

 A Japanese Sitka Deer stag running with a UK Red Deer group which commonly produces hybrid offspring despite  major differences in body size.

Are there any negative impacts?

The hybridisation between two or more species has sadly been known to cause extinctions. It can do this in a number of ways as explained by Todesco et al;

Being better than their parents: the hybrid that is produced can become very successful due to the creation of a mosaic of morphological features that allow them to occupy new environments and exploit extra resources. But why is this negative? Well, this can then lead to the decrease of either one or both of the parent species due to the hybrid out competing them. If one of them is already under threat or has low population numbers, then this can be detrimental.
Being worse than their parents: commonly, the hybrid offspring that are produced are infertile due to an imbalance in the number of genes. Also each parent may has physical adaptations to their local environment and when these are combined it may make the hybrid offspring no longer able to survive in the area they are born. As a result, these can both reduce the reproductive success of the parent species and the offspring and could eventually lead to extinction.

Meilink et al also explored the idea of "Genetic Pollution" where by the genes of the invasive species "pollute" the native gene pool during the hybridisation. This can lead to a reduction in genetic diversity and the eventual disappearance of all the native genes. 

It's not all doom and gloom though!

As conservation becomes an increasingly important area for exploration, more people are discovering that good things can come from this blend. Richards and Hobbs explored this and concluded that hybridisation can lead to an increased amount of genetic diversity as new genes come into a habitat from a elsewhere which some believe in turn makes an ecosystem function more efficiently. Also, as mentioned previously, the mosaic of physical features that hybrids can obtain can actually lead to that organism being able to occupy a new habitat and with this isolation leads to an entire new species being created. This is particularly important due to the overall global decline in species numbers seen today.  

Interestingly, Chris D Thomas believes that the hybridisation of plants in the UK has led new species being produced (due to hybridisation) similar to that of the extinction rate of mammals in the past 100 years. From this he seems to justify that the extinction of mammals is okay, because of the new development of plant species but this is something I do not think is comparable, what do you think?

To find out about a cool hybrid between wolves, dogs and coyotes, click the link below!
  https://www.youtube.com/watch?v=D3MgXRJ_fys