The concept of exotic species is being more present in the press due to some famous cases like apple snails, monk parakeets, American minks, pond sliders and red swamp crayfishes in Spain. Here we will focus on defining the concept of exotic and invasive species and what we can do to avoid their presence.
EXOTIC SPECIES AND INVASIVE SPECIES: DIFFERENT CONCEPTS
An exotic species, known also as introduced, alien, non-native or non-indigenous species, is that foreign species that have been introduced in a zone out of its natural distribution. This introduction usually happens for human causes, either voluntarily or involuntarily. The opposite concept is indigenous species.
It is necessary not confusing the first concept with the concept invasive species. A species is invasive when, being exotic or indigenous, the increase of its population supposes an environmental problem, so put in danger the rest of the species present in the specific zone. Despite most of the invasive species are exotic, there are also some cases in which can be indigenous. To give an example, if in a forest disappears the main predator of a particular species, this can increase the number of individuals, so it can become an invasive species.
It’s advisable to highlight that the establishment of exotic species in a specific zone is not easy, so the ecosystems have some filters that have to be exceeded. The first barrier that have to be exceeded is the geographical separation between the origin and the arrival point. Then, it can just establish if it has the ability of surviving in the new habitat and of reproducing. Finally, the species would be able to spread and, in this way, it is an exotic species that can become invasive.
IMPACT OF EXOTIC SPECIES
The presence of exotic species, by itself, not necessarily represent a problem. Imagine a field of potatoes or corn, which come form America and don’t suppose an environmental problem by themselves. In most of the cases, the problem is when they become invasive species, which represent a worldwide problem, especially in islands and archipelagos, for the impact that they suppose:
Alteration and degradation of habitat.
They can suppose a health problem.
They can suppose a negative impact on economy, for the negative effect on natural resources and on tourism.
NUMBERS IN SPAIN
According to the Spanish Catalogue of Invasive Exotic Species, inside the Spanish territory there are 13 invasive exotic species of algae, 75 of plants, 14 of non-arthropod invertebrates, 26 of arthropods, 19 of fishes, 4 of amphibians, 4 of reptiles, 17 of birds and 15 of mammals.
WHAT CAN I DO?
Acquisition of pets:
To acquire pets in specialised shops to guarantee the legal and healthy security.
Don’t abandon or free exotic species in the nature.
Don’t acquire invasive species.
In the garden:
Plant indigenous species.
Never throw ornamental plants, aquarium plants or pieces of exotic plants in humid zones or rivers.
Don’t transport animals, plants or seeds without declaring them from a country to another.
Clean the soles of your boots and your equipment before doing hiking in a new zone.
Don’t transport water from a place to another.
Don’t use exotic bait.
Notes of the subject Marine Reserve of the Master in Oceanography and Marine Environmental Management (University of Barcelona)
Generally, we tend to think of migration as an event exclusively linked to complex organisms (like mammals or birds). But there are always exceptions: the North American populations of the monarch butterfly (Danaus plexippus) cover a distance of almost 5000km (more than some complex animals!) in order to reach their hibernation areas, where there can be concentrated thousands of specimens during the winter. Unfortunately, the migration phenomenon depend on many factors that are being damaged by anthropogenic pressure nowadays, so that the future of these populations and also their migration are in danger.
Throughout this article, you will learn some of the most curious biology traits of these organisms, the main causes that could be endangering their populations and the consequences that this would entail.
The monarch butterfly (Danaus plexippus) is a butterfly of the Nymphalidae family. It’s also probably one the most well-known butterflies of North America due to its long migration, that their specimens perform from the north of EEUU and Canada to California coast and Mexico, covering a distance of almost 5000km to reach their hibernation areas. It’s, by far, the insect that performs the widest and large migration of all.
Although the North American populations of this species are the most known worldwide due to their migration pattern, there are also monarch butterflies in some Atlantic islands (Canary islands, Azores and Madeira), and sometimes also as eventual migrators that reach the coasts of Western Europe (United Kingdom and Spain). Moreover, they were introduced in New Zealand and Australia during the XIX century.
The life cycle of this species is very unique. First of all, they’re considered specialist butterflies: they lay their eggs exclusively over plants of the Asclepias genus (also known as milkweeds), and their newborn caterpillars (which are black, white and yellow striped) feed only on these plants. This is a very interesting fact, because the plants of this genus contain cardiac glycosides that are progressively assimilated by the caterpillar tissues, which let them to acquire a disgusting taste that prevents them to be predated. This taste will last during their adult phase.
Once completed the larva phase, the metamorphosis take place so that the caterpillars become adult butterflies colored in black, white and orange. Both caterpillar and butterfly color patterns carry out a communicative function: it’s a mechanism to warn other animal of their toxicity, fact which is known as aposematic mimicry (this phenomenon is very frequent in a lot of group of animals, even in mammals).
The adult phase also has some particularities: during the mating season (April-August) some generations of adults are generated, and each of them has a life expectancy of a few weeks, more or less. Then, an awesome event takes place: the butterflies of the generation born at the end of August (when temperatures get low and days became shorter) stops the maturing process of their reproductive organs (phenomenon known as diapause) so they can spend their energy on enlarging their life expectancy to 9 months. This generation is known as “Methuselah generation” due to its longevity.
The increase of their longevity allows this generation to cover the long distance required to reach the hibernation areas during the autumn (Mexico and California coast) and then to come back to the north of America at the end of the winter.
A ROUND TRIP: THE GREAT MIGRATION
Although the monarch butterfly isn’t only located in North America, there is no evidence nowadays showing that the other populations of monarch butterflies do such a long migration. It’s believed that the fact that only these populations of butterflies go on a trip this long is due to the wide spreading of plants of the Asclepias genus over all North America that took place in the past. Scientists suggest this event allowed the monarch butterflies to spread progressively to the south.
WHICH PLACES DO THE BUTTERFLIES VISIT?
A migration is always a complex process. In this case, the migration to the south is divided into two simultaneous migrations:
The east migration: this trip is made by those butterflies that fly from the east of the Rocky Mountains, South of Canada and a big part of USA to the central part of Mexico (90% of all the monarch butterflies located in North America go on this trip).
The west migration: this trip is made by those butterflies that fly from the west of the Rocky Mountains, South of Canada and a little part of USA to the California coast (10% of all the monarch butterflies located in North America go on this trip).
Once in the winter habitats, the butterflies plunge into a lethargic state until the next spring, when they become sexually active and start mating before heading again to the north.
It’s a very surprising event seeing all the butterflies sleeping together and covering all the plants and trees of the winter habitats!
There exists a lot of protected areas all over the places where the butterflies go through.
One of the most important protected areas is the Monarch Butterfly Biosphere Reserve (Mexico City), which is considered a World Heritage Site by the UNESCO since 2008.
And why are these butterflies so protected? Besides the fact that their migration pattern is considered an incredible phenomenon, they are pollinators that contribute to the pollination of the wild flora and also of the crops of North America.
THE ‘QUEEN’ IS IN DANGER!
Although there exists a huge effort to protect them, the migratory phenomenon of the North American monarch butterflies is in danger nowadays due to the anthropic pressure, which could also put their populations at risk in the future.
According to the data generated by the WWF, the surface of the winter habitats occupied by these butterflies has decreased 94% in 10 years, going from 27,48 acres occupied in 2003 to only 1,65 acres in 2013. This is the lower value registered in the last 20 years.
Even though the surface occupied by these organisms has been fluctuating over the years as a part of a natural process, this pronounced decreasing that has taken place in only a few years suggests that butterflies are stopping their annual migrations to the south.
This recession has also been registered in other species of butterflies at different emplacements all over the world, so there must exist some kind of factor in common with the ones affecting the North American monarch butterfly populations.
WHAT COULD BE THE MAIN CAUSES OF THIS RECESSION?
According to the WWF, the main causes that could being putting in danger the migration process of the monarch butterflies are:
The reduction of the surface occupied by plants of the Asclepias genus: as we said above, the caterpillars feed exclusively on these plants. But the use of certain herbicides and the changes on the rain patterns could being limiting their dispersal over a big part of North America.
Deforestation: cutting down trees massively and the subsequent desertization could being reducing their winter habitats.
Extreme climate: the global change, which entails changes in temperature and rain patterns, could being putting at risk the survival of adult butterflies, preventing them to reach the longevity required to carry out complete migrations.
WHICH EFFORTS ARE MADE TO PROTECT THESE POPULATIONS?
As I said above, monarch butterflies are an essential part of the pollination net of North America and also iconic insects, so there exists a big interest on protecting them.
Nowadays, most of the protected areas of North America are making a big effort to improve the quality of their habitats. Among them, the Monarch Butterfly Biosphere Reserve (Mexico) along with the WWF are trying to restore the woods where butterflies hibernate and also promoting a sustainable tourism (enter this link to see more information).
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The case of the monarch butterfly is only one of a huge list of animals in danger. Nowadays, a lot of animals with complex migration patterns and wide spreading areas are suffering similar pressures, mostly of them with an anthropogenic origin. There’s still so much work to do, and it depends on all of us.
Nowadays, concern about the health of inland waters (rivers, lakes, etc.) is growing, mainly due to increased use (and abuse) of these for human consumption. A few years ago, an expansion of the use of biotic indices took place, which allow us to determine the health of aquatic ecosystems; these indices usually use data such as presence, absence or/and abundance of different organisms known as ‘bioindicators’, that is, species that can be used to monitor the health of an environment or ecosystem. Among these organisms, there are a lot of arthropods.
Along this article, I will briefly explain what bioindicators are, the main role of arthropods as bioindicators and also introduce some of the most used bioindication indices to monitor the quality of riverine ecosystems in the Iberian Peninsula.
What is a bioindicator?
The term ‘bioindicator’ is used to refer to those biological processes, species or/and communities of organisms which can be used to assess the quality of an ecosystem and also how this ecosystem evolves over time, which is especially useful when changes take place due to anthropogenic disturbances, such as pollution.
Thus, in accordance with the above, a bioindicator can be:
A particular species, whose presence/absence or abundance rate informs us of the state of health of a studied ecosystem, or
A population or a community composed of various organisms which varies functionally or structurally according to the conditions of environment.
Example: Lecanora conizaeoides lichen is highly resistant to pollution. Its presence on the studied ecosystem, coupled with the disappearance of another lichens, is indicative of high air pollution.
What do we consider a ‘good bioindicator’?
Do all the organisms have the necessary traits to become bioindicator subjects? The answer is no. Even though there is not a bioindicator prototype (because all depends on the studied ecosystem), we can resume here some of the traits that scientists take into account to select good bioindicator organisms:
They have to respond to disturbances that take place on their ecosystem to a greater or lesser degree. This response should be comparable to that emitted by the rest of the organisms of the same species, and this response also has to be well correlated with the studied environment disturbances.
Their response have to be representative of all the community or population.
They must be nativeof the studied ecosystem and also be ubiquitous(that is, to be present in almost all ecosystems of the same or similar characteristics).
They have to be abundant(rare species aren’t optimum subjects).
They must be relatively stable to moderate climate changes (i.e. a storm or a natural temperature change does not affect them more than normal).
They should be easy to detect and, as possible, they have to be sedentary.
They have to be well studied, both from an ecological point of view as taxonomic (to know, therefore, their tolerance to environmental disturbances).
Finally, they should be easy to manipulate and monitor in the laboratory.
The use of bioindicators will be optimized if we use entire communities or populations instead of using a single or a couple species, because this allows us to cover a wide interval of environmental tolerances: from organisms with a narrow tolerance range (that is, stenotopic) and sensitive to pollution, to very tolerant organisms that can survive in very polluted environments.
Thus, we will be able to know if an ecosystem is highly altered if we find only a very tolerant species and none of the considered sensitive species.
Bioindicator animals from inland waters
Nowadays, scientists use a lot of animals as bioindicators: from microorganism and microinvertebrates to terrestrial and aquatic vertebrates (micromammals, birds, fishes, etc.). In inland waters, and especially in the context of studies of riverine water quality, scientists mostly use aquatic macroinvertebrates to assess the quality of these ecosystems. Next, let’s see what a macroinvertebrate is.
What are macroinvertebrates?
The term ‘macroinvertebrate’ does not correspond to any taxonomic classification, but with an artificial concept that includes different aquatic invertebrate organisms.
Generally, is said that macroinvertebrates are organisms that can be trapped by a net with holes about 250μm.
Macroinvertebrates are mainly benthic, that is, animals that inhabit the substrate of aquatic ecosystems at least during some stage of their life cycle (although there are some that swim freely in water column or on its surface).
We can find a lot of macroinvertebrate groups in rivers and lakes, which can be classified in two main groups:
Among these groups, there are both tolerant organisms to environment distrubances (i.e. leeches) and sensitive organisms (i.e. a lot of larvae insects).
Most inland aquatic macroinvertebrates (≃80%) are arthropods(of which I will discuss in the next section), among which there are many insectsand, especially, their larvae (which are generally benthic), whose study and observation play an essential role on calculating indices of water quality.
Importance of insects in bioindication
As I’ve said above, about 80% of macroinvertebrates of inland waters are arthropods and, mostly, different orders of insects in its larval or nymphal form. Let’s see some of the most common groups we can find in rivers and lakes:
Trichoptera (or caddisflies)
They are insects closely related to the Lepidoptera order (butterflies and moths). Their aquatic nymphs can build a shelter around their bodies made of substrate materials. We can distinguish them from other aquatic insect larvae because they have a couple of anal filaments provided with strong hoofs. They usually inhabit clear and clean waters with a lot of currents.
Ephemeroptera (or mayflies)
One of the most ancient orders of flying insects. Their aquatic nymphs, which usually inhabit rivers, are characterized for having three long anal filaments. Adults, which fly over the water surface, are very fragile and have a short life cycle in comparison with nymphs (the name Ephemeroptera is derived from Greek ‘ephemera’ meaning sort-lived, and ‘ptera’ meaning wings).
Plecoptera (or stoneflies)
Flying insects very similar to Ephemeroptera order. Like these, they have anal filaments, but they differentiate from them because they have two apical hooks in each leg. They usually inhabit lakes and streams.
Other groups of insects with aquatic larvae or nymphs
Among the most common insects inhabiting rivers and lakes we can also find species of Odonata order (dragonflies and damselflies), Coleoptera (beetles), Diptera (mosquitoes and flies), etc.
Among all the organisms mentioned above, there are very tolerant species to pollution (i.e. some Diptera larvae; this is the case of some species of Chironomidae family, which are very tolerant to organic and inorganic pollution due to the presence of heavy metals in their environment) and also very sensitive species (i.e. some species of Trichoptera order).
Depending on their tolerance to environment disturbances, scientists group these organisms (plus the rest of macroinvertebrates) into different categories that are assigned a value. This values, at the end, allow us to calculate water quality indices.
Biotic indices for riverine waters
The different pollution tolerance degrees among macroinvertebrates of a community allow us to classify them and to assign them a qualitative value (the bigger the number is, more sensitive are organisms to pollution). Thanks to these values, we can calculate different biotic indices, which are no more than qualitative values assigned to a community in order to classify it according to its quality: the greater the value is, better is the water quality.
One of the most used indices on the assessment of ecological state of rivers from the Iberian Peninsula is the IBMWPindex (Iberian Bio-Monitoring Working Party), an adaptation by Alba Tercedor (1988) of the British index BMWP. In rough outlines, the greater the value is, better is the water quality. On this website you will find more details about this index, and also the pre-established values assigned to each macroinvertebrate (available in Spanish only).
In additions, there is also used the IASPTindex, a complementary index which is the result of divide the IBMWP value by the number of identified taxa. This index give us information about the dominant community in the studied location. You can see more details on this website (available in Spanish only).
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As you probably have seen while reading of this article, macroinvertebrates, and insects especially, play an important role in the study of inland water quality. Furthermore, their presence or absence is extremely important for the rest of the organisms of their ecosystem, because of what we must become aware of the problems deriving from the reduction of their number or diversity.