Alien invaders on Planet Earth

It may surprise you to learn that invasive species are regarded by conservationists as THE greatest cause of species extinctions in recent history. Some say there is limited evidence of this claim [1], however it is agreed that the threat these exotic intruders pose is often catastrophic to local species, and hard to mitigate against with the increasing interconnectedness of the world we live in. Some of the earliest invasive species extinctions in human history were probably those caused by ourselves as early Homo sapiens moved around the world, especially into the Americas where the fossil record shows that the megafauna was systematically wiped out in a few thousand years.

Similar megafauna extinctions occurred in Australia (where many giant marsupials disappeared) and New Zealand (where giant moa birds were hunted to extinction). More recently, mankind has enabled the spread of other invasive species, with rats and cats carried to previously isolated islands, where they have decimated many species of flightless birds that evolved without the presence of such predators. Invasive species not only predate vulnerable local species that they encounter, they can also outcompete or displace species through being superior competitors for food, territory or prey – for example, lionfish in the Caribbean are practically invulnerable and have ferocious appetites. Invasive species can also bring novel pathogens with them that cause diseases that local flora and fauna have no resistance to. An example in our own species would be the transfer of smallpox by Europeans to the Native Americans, whilst another would be the devastating squirrelpox virus that has ravaged the red squirrel population of England and Wales, spread by American grey squirrels. As climate change and human globalisation facilitate the spread of species poleward and to all corners of the world, the future does not look bright for conservationists trying to avert the 6^th mass extinction, with a problem like this wildly out of control.

On a brighter note, conservationists are grappling to prevent the spread of some of the deadliest invasive species in the world today, and protect ecosystems from their impacts. There are several examples of animal and plant invasive species that we will all be familiar with, such as American mink (Neovison vison) and Himalayan Bindweed (Impatiens glandulifera) in the United Kingdom, but I will focus on an often forgotten group of organisms that are a major group of invasive species – the fungi.

Many people will have heard about the nightmarish chytrid fungus (although there are around 1000 species, just one gives these fungi a bad name – Batrachochytrium dendrobatidis or Bd.) The fungus infects amphibians, devouring their skin and leading to heart failure and eventually death. It has so far spread to at least 50% of all amphibians and shows no sign of stopping as world trade and amphibian and other wildlife trading threatens to worsen the situation [2].

Dead southern mountain yellow-legged frogs (Rana muscosa) killed by the chytrid fungus. Sixty Lake Basin, Kings Canyon National Park, California USA. Credit: Vance T. Vredenburg

A further cause for concern is ironically the fact that some species, such as the American bullfrog and the African clawed frog, which show resistance to Bd instead become carriers of the disease. The fact that these two species are also invasives species themselves is a double whammy for endemic amphibians in areas they invade, which unfortunately is just about most freshwater environments across the world today. Both have voracious appetites and their populations are booming due to a lack of natural predators in their new homes from home.

Recent work to find a vaccination for Bd have suffered a set-back too, as frogs immunized against the fungus actually suffered significantly higher mortality rates. While this puzzled researchers at first, they quickly realized that this is because the fungus focuses on killing off white blood cells that attack the fungus specifically, meaning immunization just meant more of these are produced and killed off in a highly ineffective attempt to fight off the infection. As the frogs invest a great deal of energy in this specific response, their general health deteriorates and they succumb more easily to the fungus. Researchers are actually suggesting the immune system should be suppressed and are running experiments to check this hypothesis [3].

However, there is also a lesser known invasive species of fungus called Pseudogymnoascus destructans that causes something called white nose syndrome in bats. As the species name suggests, this is a devastating infection responsible for almost wiping out many native North American bat populations, manifesting itself as a white encrusting skin condition on the bat’s nose, ears and wings. The bats succumb to the fungus during hibernation as WNS seems to cause odd behaviour in bats, making them fly out in daylight in the winter and between caves which uses up their fat reserves and ultimately brings on their downfall. An 80% decline in many insectivorous North American species has been documented in recent years since its introduction from Eurasia [4].

However, there is hope yet for the bat. Interestingly, another fungus (a yeast called Candida albicans) has been found to produce high concentrations of tt-farnesol, a chemical that can inhibit P.destructans growth. In Europe, where bats rarely suffer from WNS, populations seem to co-occur with this type of yeast, with some bats even hosting this yeast on their body when they hibernate to protect them during the winter. It is thought C.albicans and additional microbes confer resistance to WNS in European bats, compared to in the US where P.destructans was able to dominate the microbial community in cave systems. However, work still remains to find the right dosage and concentration of this chemical to protect the American bats, as well as how to administer the tt-farnesol [5]. Introducing C.albicans itself is a possibility, but could just unsettle the delicate ecosystem of caves even further and become another invasive species – as we said, one invasive species is far too much!

A White-nosed bat infected with the deadly fungus. Credit: Greg Turner

An interesting thing to note is that many invasive species are not actually a negative influence, seemingly slotting into ecosystems and having neutral or sometimes even positive effects [6]. An example is the Chinese Water deer, which seems to have no negative effects on the native biota. Unfortunately, as it only takes one invasive species to cause chaos in an ecosystem, with species today being transferred across the world freely, widely and at an accelerating rate, ecosystems face waves of potentially destructive invaders every single day.

It’s up to us humans to do our bit to help prevent the spread of these organisms since we are the major cause of this threat. We can do this by making biocontrol procedures at ports, airports and borders stricter (such as in Australia and New Zealand) and cleaning up the legal wildlife trade markets and clamping down on the illegal ones. You can do your bit by cleaning your shoes before travelling abroad and checking your clothes and bags for any seeds, fungi or soil that could help spread invasive species.