Month: August 2018

It takes (at least) two: coevoultion and species interactions

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The environmental context of adaptation

We’ve talked many times before about how species evolve in response to some kind of environmental pressure, which favours (or disfavours) certain traits within that species. Over time, this drives changes in the frequencies of species traits and alters the overall average phenotype of that species (sometimes slowly, sometimes rapidly).

While we usually talk about the environment in terms of abiotic conditions such as temperature or climate, biotic factors are equally important: that is, the parts of the environment which are themselves also alive. Because of this, changes in one species can have profound repercussions on other species linked within the ecosystem. Thus, the evolution of one species is intrinsically linked to the evolution of other relevant species within the ecosystem: often, these connected evolutionary pathways battle with one another as each one changes. Let’s take a look at a few different examples of how evolution of one species may impact the evolution of another.

Predator-prey coevolution

One of the most obvious ways the evolution of two different species can interact is in predator and prey relationships. Naturally, prey species evolve to be able to defend themselves from predators in various ways, such as crypsis (e.g. camouflage), toxicity or behavioural changes (such as nocturnalism or group herding). Contrastingly, predators will evolve new and improved methods for detecting and hunting prey, such as enhanced senses, venom and stealth (through soft-padded feet, for example).

There are millions of possible examples of predator-prey coevolution that could be used as examples here, based on the continual drive for one species to get the upper hand over the other. But one that comes to mind is of a creature that I learnt about while on holiday in Scandinavia: the pine marten, and how it affects squirrels.

38542167_10216809232693743_2189871337374220288_o.jpg
This photo is one that I took whilst on a lunch break at a bakery in the Norwegian mountains, of a small critter running among the rocks by the lakeside. Not sure exactly what species it was, I asked the tour director who excitedly told me that it was a pine marten. After doing a bit of research on them (and trying to figure out what the difference between a pine marten, a stoat, and a weasel is), I’ve discovered that it’s actually more likely to be a stoat than a pine marten, based on size and colour. But pine martens are still an intriguing species in their own right (and also found in Norway, so the confusion is understandable).

The pine marten is a species in the mustelid family, along with otters, weasels, stoats, and wolverines. Like many mustelids, they are carnivorous mammals which feed on a variety of different prey items like rodents, small birds and insects. One of the more abundant species that they prey upon are squirrels: both red squirrels and grey squirrels are potential food for the cute yet savage pine marten.

However, within the distribution of pine martens (across much of Europe), red squirrels are the native species and grey squirrels are invasive, originating from North America. Because of the long-lasting relationship between red squirrels and pine martens, they’ve co-evolved: most notably, by red squirrels changing to a mostly arboreal lifestyle and avoiding the ground as much as possible. Grey squirrels, however, have not had the evolutionary history to learn this lesson and are easy food for a smart pine marten. Thus, in regions where pine martens have been conserved or reintroduced, they are actively controlling the invasive grey squirrel population, which in turn boosts the native red squirrel population by reduction of competition. The coevolutionary link between red squirrels and pine martens is critical for combating the invasive species.

 

Martens and squirrels figure.jpg
The relationship between pine marten abundance and the abundance of both red (native) and grey (invasive) squirrels. On the left, without pine martens the invasive species runs rampant, outcompeting the native species. However, as pine martens increase in the ecosystem, the grey squirrels are predated on much more than the red squirrels due to their naivety, leading to the ‘natural’ balance on the right.
Martens and squirrels stats.jpg
A diagram of how the abundance of squirrels changes relative to the number of pine martens. The invasive grey squirrels are significantly depleted by pine marten presence, which in turn allows the native red squirrels to increase in population size after being freed from competition.

Host-parasite coevolution

In a similar vein to predator and prey coevolution, pathogenic species and their unfortunate hosts also undergo a sort of ‘arms race’. Parasites must keep evolving new ways to infect and transmit to hosts as the hosts evolve new methods of resisting and avoiding the infecting species. This spiralling battle of evolutionary forces is dubbed as the ‘Red Queen hypothesis’, formulated in 1973 by Leigh Van Valen and used to describe many other forms of coevolution. The name comes from Lewis Carroll’s Through the Looking Glass, and one quote in particular:

‘Now, here, you see, it takes all the running you can do, to keep in the same place’.

The quote references how species must continually adapt and respond to the evolution of other species just keep existing and prevent extinction. Species that remain static and stop evolving will inevitably go extinct as the world around them changes.

Mimicry

Plenty of other strange and unique mechanisms of coevolution exist within nature. One of them is mimicry, the process by which one species attempts to look like another to protect itself. The most iconic group known for this is butterflies: many species, although they may be evolutionarily very different, share similar colouration patterns and body shapes as mimics. Depending on the nature of the copy, mimicry can be classified into two broad categories. In either case, the initial ‘reference’ species is toxic or unpalatable to predators and uses a type of colour signal to communicate this: think of the bright yellow colours of bees and wasps or the red of ladybirds. Where the two categories change is in the nature of the ‘mimic’ species.

Müllerian mimicry

If the mimic is also toxic or unpalatable, we call this Müllerian mimicry (after Johann Friedrich Theodor Müller). By sharing the same colouration patterns and both being toxic, the two mimicking species boost the potential for the signal to be learnt by predators. If a predator eats either species, it will associate that colour pattern with toxicity and neither species are as likely to be preyed upon in the future. In this sense, it is a cooperative coevolutionary relationship between the two physically similar species.

Mullerian mimicry figure
A (somewhat familiar) example of Müllerian mimicry with two species of butterflies, the monarch and the viceroy. Although this has traditionally been thought of as a textbook case of Batesian mimicry (see below), the toxicity of both species likely makes it a scenario of Müllerian mimicry instead. Since both butterflies share the same pattern and both are toxic, it sends a strong signal to predators such as wasps to avoid them both.

Batesian mimicry

In contrast, the mimic might not actually be toxic or unpalatable, and simply copying a toxic species. This is referred to as Batesian mimicry (after Henry Walter Bates), and involves a mimic species relying on the association of colour and toxicity to have been learnt by predators through the ‘reference’ species. Although the mimic is not toxic, it is essentially piggy-backing on the hard evolutionary work that has already been done by the actually toxic species. In this case, the coevolutionary relationship is more parasitic as the mimic benefits from the ‘reference’ but the favour is not returned.

Batesian mimicry figure
An example of Batesian mimicry, with hoverflies and wasps. Hoverflies are not at all toxic, and are generally harmless; however, by mimicking the clear bright yellow warning systems of more dangerous species like wasps and bees, they avoid being eaten by predators such as birds.

Coevolution of species and the importance of species interactions

There are countless of other species interactions which could drive coevolutionary relationships in nature. These can include various forms of symbiosis, or the response of different species to ecosystem engineers: that is, species that can change and shape the environment around them (such as corals in reef systems). Understanding how a species evolves within its environment thus needs to consider how many other local species are also evolving and responding in their own ways.

 

 

Notes from the Field: Octoroks

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Scientific name

Octorokus infletus

Meaning: Octorokus from [octorok] in Hylian; infletus from [inflate] in Latin.

Translation: inflating octorok; all varieties use an inflatable air sac derived from the swim bladder to float and scan the horizon.

Varieties

Octorokus infletus hydros [aquatic morphotype]

Octorokus infletus petram [mountain morphotype]

Octorokus infletus silva [forest morphotype]

Octorokus infletus arctus [snow morphotype]

Octorokus infletus imitor [deceptive morphotype]

All octoroks.jpg
The various morphotypes of inflating octoroks. A: The water octorok, considered the morphotype closest to the ancestral physiology of the species. B: The forest octorok, with grass camouflage. C: The deceptive octorok, which has replaced its tufted vegetation with a glittering chest as bait. D: The mountainous octorok, with rock camouflage. E: The snow octorok, with tundra grass camouflage.

Common name

Variable octorok

Taxonomic status

Kingdom Animalia; Phylum Mollusca; Class Cephalapoda; Order Octopoda; Family Octopididae; Genus Octorokus; Species infletus

Conservation status

Least Concern

Distribution

The species is found throughout all major habitat regions of Hyrule, with localised morphotypes found within specific habitats. The only major region where the variable octorok is not found is within the Gerudo Desert, suggesting some remnant dependency of standing water.

Octorok distribution.jpg
The region of Hyrule, with the distribution of octoroks in blue. The only major region where they are not found is the Gerudo Desert in the bottom left.

Habitat

Habitat choice depends on the physiology of the morphotype; so long as the environment allows the octorok to blend in, it is highly likely there are many around (i.e. unseen).

Behaviour and ecology

The variable octorok is arguably one of the most diverse species within modern Hyrule, exhibiting a large number of different morphotypic forms and occurring in almost all major habitat zones. Historical data suggests that the water octorok (Octorokus infletus hydros) is the most ancestral morphotype, with ancient literature frequently referring to them as sea-bearing or river-traversing organisms. Estimates from the literature suggests that their adaptation to land-based living is a recent evolutionary step which facilitated rapid morphological radiation of the lineage.

Several physiological characteristics unite the variable morphological forms of the octorok into a single identifiable species. Other than the typical body structure of an octopod (eight legs, largely soft body with an elongated mantle region), the primary diagnostic trait of the octorok is the presence of a large ‘balloon’ with the top of the mantle. This appears to be derived from the swim bladder of the ancestral octorok, which has shifted to the cranial region. The octorok can inflate this balloon using air pumped through the gills, filling it and lifting the octorok into the air. All morphotypes use this to scan the surrounding region to identify prey items, including attacking people if aggravated.

inflated octorok
A water morphotype octorok with balloon inflated.

Diets of the octorok vary depending on the morphotype and based on the ecological habitat; adaptations to different ecological niches is facilitated by a diverse and generalist diet.

Demography

Although limited information is available on the amount of gene flow and population connectivity between different morphotypes, by sheer numbers alone it would appear the variable octorok is highly abundant. Some records of interactions between morphotypes (such as at the water’s edge within forested areas) implies that the different types are not reproductively isolated and can form hybrids: how this impacts resultant hybrid morphotypes and development is unknown. However, given the propensity of morphotypes to be largely limited to their adaptive habitats, it would seem reasonable to assume that some level of population structure is present across types.

Adaptive traits

The variable octorok appears remarkably diverse in physiology, although the recent nature of their divergence and the observed interactions between morphological types suggests that they are not reproductively isolated. Whether these are the result of phenotypic plasticity, and environmental pressures are responsible for associated physiological changes to different environments, or genetically coded at early stages of development is unknown due to the cryptic nature of octorok spawning.

All octoroks employ strong behavioural and physiological traits for camouflage and ambush predation. Vegetation is usually placed on the top of the cranium of all morphotypes, with the exact species of plant used dependent on the environment (e.g. forest morphotypes will use grasses or ferns, whilst mountain morphotypes will use rocky boulders). The octorok will then dig beneath the surface until just the vegetation is showing, effectively blending in with the environment and only occasionally choosing to surface by using the balloon. Whether this behaviour is passed down genetically or taught from parents is unclear.

Management actions

Few management actions are recommended for this highly abundant species. However, further research is needed to better understand the highly variable nature and the process of evolution underpinning their diverse morphology. Whether morphotypes are genetically hardwired by inheritance of determinant genes, or whether alterations in gene expression caused by the environmental context of octoroks (i.e. phenotypic plasticity) provides an intriguing avenue of insight into the evolution of Hylian fauna.

Nevertheless, the transition from the marine environment onto the terrestrial landscape appears to be a significant stepping stone in the radiation of morphological structures within the species. How this has been facilitated by the genetic architecture of the octorok is a mystery.

 

Notes from the Field: Cliff racer

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Scientific name

Cinis descendens

Meaning: Cinis: from [ash] in Latin; descendens from [descends] in Latin.

Translation: descending from the ash; describes hunting behaviour in ash mountains of Vvardenfell.

Common name

Cliff racer

cliff racer
A cliff racer hovering above a precipice on Vvardenfell.

Taxonomic status

Kingdom Animalia; Phylum Chordata; Class Aves; Subclass Archaeornithes; Family Vvardidae; Genus Cinis; Species descendens

Conservation status

Least Concern [circa 3E 427]

Threatened [circa 4E 433]

Distribution

Once widespread throughout the north eastern region of Tamriel, occupying regions from the island of Vvardenfell to mainland Morrowind and Solstheim. Despite their name, the cliff racer is found across nearly all geographic regions of Vvardenfell, although the species is found in greatest densities in the rocky interior region of Stonefalls.

Following a purge of the species as part of pest control management, the cliff racer was effectively exterminated from parts of its range, including local extinction on the island of Solstheim. Since the cull the cliff racer is much less abundant throughout its range although still distributed throughout much of Vvardenfell and mainland Morrowind.

Morrowind
The province of Morrowind, which largely contains the distribution of the cliff racer. The island of Solstheim is found to the northwest of the map (the lower half of the island can be seen in brown).

Habitat

Although, much as the name suggests, the cliff racer prefers rocky outcroppings and mountainous regions in which it can build its nest, the species is frequently seen in lowland swamp and plains regions of Morrowind.

Behaviour and ecology

The cliff racer is a highly aggressive ambush predator, using height and range to descend on unsuspecting victims and lashing at them with its long, sharp tail. Although preferring to predate on small rodents and insects (such as kwama), cliff racers have been known to attack much larger beasts such as agouti and guar if provoked or desperate. The highly territorial nature of cliff racer means that they often attack travellers, even if they pose no immediate threat or have done nothing to provoke the animal.

Cliff_Racer_(Online).png
A cliff racer descends upon its prey.

Despite the territoriality of cliff racers, large flocks of them can often be found in the higher altitude regions of Vvardenfell, perhaps facilitated by an abundance of food (reducing competition) or communal breeding grounds. Attempts by researchers to study these aggregations have been limited due to constant attacks and damage to equipment by the flock.

Demography

Prior to the purging of cliff racers in the early 4E by Saint Jiub, the cliff racer was overly abundant throughout its range and considered a pest species by native peoples. Although formal studies on the population structure of the species was never conducted due to their aggressive nature, suppositions of migratory rates, distances and geographies suggested that potentially three major (ESUs) populations existed; one of Solstheim, one of Vvardenfell, and another of mainland Morrowind.

Following the control measures implemented, the population size of these populations of cliff racers declined severely; however, given the survival of the majority of the population it does not appear this bottleneck has severely impacted the longevity of the species. The extirpation of the Solstheim population of cliff racers likely removed a unique ESU from the species, given the relative isolation of the island. Whether the island will be recolonised in time by Vvardenfell cliff racers is unknown, although the presence of any cliff racers back onto Solstheim would likely be met with strong opposition from the local peoples.

Adaptive traits

The broad wings, dorsal sail and long tail allow the cliff racer to travel large distances in the air, serving them well in hunting behaviour. The drawback of this is that, if hunting during the middle hours of the day, the cliff racer leaves an imposing shadow on the ground and silhouette in the sky, often alerting aware prey to their presence. That said, the speed of descent and disorienting cry of the animal often startles prey long enough for the cliff racer to attack.

The plumes of the cliff racer are a well-sought-after commodity by local peoples, used in the creation of garments and household items. Whether these plumes serve any adaptive purpose (such as sexual selection through mate signalling) is unknown, given the difficulties with studying wild cliff racer behaviour.

Management actions

Although suffering from a strong population bottleneck after the purge, the cliff racer is still relatively abundant across much of its range and maintains somewhat stable size. Management and population control of the cliff racer is necessary across the full distribution of the species to prevent strong recovery and maintain public safety and ecosystem balance. Breeding or rescuing cliff racers is strictly forbidden and the species has been widely declared as ‘native pest’, despite the somewhat oxymoron nature of the phrase.

Notes from the Field: Nugs

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Scientific name

Nuggula minutus

Meaning: Nuggula from [nug] in Dwarven; minutus from [smaller] in Latin.

Translation: smallests of the nugs; the smallest species of the broader nug taxonomic group.

Common name

Common nug

Nug creature
A wild nug.

Taxonomic status

Kingdom Animalia; Phylum Chordata; Class Mammalia; Order Eulipotyphyla; Family Talpidae; Genus Nuggula; Species minus

Conservation status

Least concern

Distribution

Throughout the underground regions of Thedas; full extent of distribution possibly spans the full area of the continent.

Thedas Map.jpg
The continent of Thedas. The nug is likely distributed across much of the subterranean landmass, although the exact distribution is unknown.

Habitat

Nugs are primarly subterranean species, largely inhabiting the underground tunnels and cave systems occupied by Dwarven civilisation. However, nugs can be found on the surface predominantly in forested regions with accessible passageways into the subterranean realm.

Behaviour and ecology

Nugs are non-confrontational omnivorous species, preferring to hide and delve in the dark underground systems below the world of Thedas. Thus, nugs will typically avoid contact with people or predators by hiding in various crevices, using their pale skin to blend in with the surrounding rock faces. Reports of nugs in the wild demonstrate that nugs are remarkably inefficient at predator avoidance, despite their physiology; however, nug populations do not appear to suffer dramatically with predator presence, suggesting that either predators are too few to significantly impact population size or that alternative behaviours might allow them to rapidly bounce back from natural declines.

Given the lack of consistent light within their habitat, nugs are effectively blind, retaining only limited eyesight required for moving around above the surface. Nugs feed on a large variety of food sources, preferring insects but resorting to mineral deposits if available food resources are depleted. Their generalist diet may be one physiological trait that has allowed the nug to become some widespread and abundant historically.

Demography

Although the nug is a widespread and abundant species, they are heavily reliant on the connections of the Deep Roads to maintain connectivity and gene flow. With the gradual declination of Dwarven abundance and the loss of entire regions of the underground civilisation, it is likely that many areas of the nug distribution have become isolated and suffering from varying levels of inbreeding depression. Given the lack of access to these populations, whether some have collapsed since their isolation is unknown and potentially isolated populations may have even speciated if local environments have changed significantly.

Adaptive traits

Nugs are highly adapted to low-light, subterranean conditions, and show many phenotypic traits related to this kind of environment. The reduction of eyesight capability is considered a regression of unusable traits in underground habitats; instead, nugs show a highly developed and specialised nasal system. The high sensitivity of the nasal cavity makes them successful forages in the deep caverns of the underworld, and the elongated maw of the nug allows them to dig into buried food sources with ease. One of the more noticeable (and often disconcerting) traits of the nug is their human-like hands; the development of individual digits similar to fingers allows the nug to grip and manipulate rocky surfaces with surprising ease.

Management actions

Re-establishment of habitat corridors through the clearing and revival of the Deep Roads is critical for both reconnecting isolated populations of nugs and restoring natural gene flow, but also allowing access to remote populations for further studies. A combination of active removal of resident Darkspawn and population genetics analysis to accurately assess the conservation status of the species. That said, given the commercial value of the nug as a food source for many societies, establishing consistent sustainable farming practices may serve to both boost the nug populations and also provide an industry for many people.