Arxiu d'etiquetes: ecological niche

The living space of organisms

We all have our own living space, the place where we feel comfortable, like we were at home. We also have our routines, habits and that list of preferences that make us unique. Each of us, ultimately, have our own ecological niche, an extensive concept for each species that share the Earth with us. From it comes an important ecological processes such as competition or speciation, a key concepts for understanding the assembly and dynamics of natural ecosystems.

INTRODUCTION

When you are asked how you would describe close people, the first thing that comes to your mind is their way of being when you’re with them and what they loves to do. We know what is the first thing they always ask in a restaurant, what annoys them, what sites they like to frequent, what they like to do when they have free time and even how they behave when they like someone. If we have also lived with them, we could guess almost their daily routine since they wake up until they go to bed. Although we do not always have the same behaviour, there are many traits, hobbies and routines that characterize and differentiate us. Each of us have our comfort zone, our hobbies, food preferences and people with whom we love spending our free time.

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The dietary preferences of each of us and our routines and hobbies serve as a comparison to illustrate the diversity of ecological niches in the natural world. Source: Flickr, George Redgrave.

THE ECOLOGICAL NICHE OF A SPECIES

This “living space” that all of us have and in which we feel identified, is also comparable to the ecological niche of the organisms. The ecological niche of a species is a concept that always has been presented us as the “occupation”, “profession” or “work” that an organism carries up in the place where it lives (Wikipedia or CONICET), but the definition includes more than that. Hutchinson (1957) defined it as: ” n-dimensional hypervolume, where the dimensions are environmental conditions and resources, that define the requirements of a species to persist over time.” Despite the confusing definition, it is interested to point out the term “n-dimensional” as the ecological niche is based on this idea. An ecological niche is nothing more than all those multidimensional species requirements. In other words, the ecological niche of a species would be everything that involve the species and make it to prosper and survive where it is. Refers, ultimately, to all those variables that affect them in their daily lives, both biological variables -the contact with other species- and the physical and chemical ones-the climate and the habitat where they live-. An ecological niche of a species would be the spectrum of food it eats or can consume, the time of the day in which it is active to perform its functions, the time of the year and the way it carries out the reproduction, the predators and preys, the habitat it tolerates and all those physical and chemical factors that allow this species to remain viable.

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These 5 species of warblers of North America seem to occupy the same habitat (the fir), but actually not. The truth is that each warbler occupies a different position in the tree. Source: Biology forums.

To give an illustrative example, let us place ourselves in the African savannah. The main grazing ungulates and those which perform mass migrations are compound by zebras, wildebeest and Thomson’s gazelles. At first glance, you might think that their ecological niche is very similar: same habitat, same routine, same predators and same food. The same food? Absolutely not. During migration, zebras go ahead, devouring tall grass, which is the worst quality. They are followed by wildebeest, which eat what remains standing, and these are followed by Thomson gazelle, which eat the high-quality grass, which is starting to grow again.

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Although at first glance it may seem that feed on the same food, each species focuses on a different part of the plant. Source: Abierto por vacaciones.

CAN TWO SPECIES LIVE TOGETHER WITH THE SAME NICHE IN THE SAME PLACE?

The competitive exclusion principle, proposed by Gause (1934), states that two species occupying the same niche can not coexist in the long term as they come into competition for resources. Thus, in a competitive process for the same ecological niche, there is always a winner and a loser. In the end, one of the competitors is imposed by another, and then two things can happen: the extinction of the loser one (image A) or a traits displacement in order to occupy another niche (image B). In fact, the competitive exclusion principle is behind the current problems with invasive species. Invasive species niche is very similar to native species niche and, when they converge in the same habitat, the invasive species end up displacing native species, as they are better ecological competitors. It also often happens, of course, the opposite: the exotic species is worse than its counterpart and the competitor fails to thrive in the new environment.

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Image A | This study was conducted in order to observe the effect of competitive exclusion in two species of protists. Both species occupy almost identical ecological niches, but they are not living together in nature. The density of one falls sharply when they are forced to share the same space, until it eventually disappears. This same process occurs with invasive species. Source: Jocie Broth.
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Image B | When the 3 species of Darwin’s finches (in different colors) coexist on the same island, a trait displacement occurs by competitive exclusion. Individuals from the ends tend to have very similar bill depths to those of the other species, resulting in a niche overlap and subsequent competition. The final boundaries are established thanks to this process. Source: Nature.

THE FUNCTIONAL EQUIVALENCE

We have seen that to share ecological niche is synonymous of having conflict between species. However, there is a situation in which problem do not take place. The hypothesis of functional equivalence proposed by Hubbell proclaims that if the niches are identical and the species life parameters (fertility, mortality, dispersion) are also the same, none of them has a competitive advantage over the other, and the battle ends in tables. This fact seems to occur only in a very stable ecosystem in a Panama rainforest island (Barro Colorado). Different species of trees, as having almost identical parameters of life, do not compete between them and are distributed randomly, as if the individuals of different species belong to the same species. Furthermore, it seems that speciation in this kind of rainforest could also occur by chance, which would have caused the high density of species that harbor these forests.

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Tropical forests have a tree species density unique in the world. One hectare of tropical forest may contain up to 650 tree species, more than the number of tree species present in both Canada and continental US. Will Hubbell’s functional equivalence theory be behind the explanation for this curious fact? Source: Flickr, Jo.

NEW NICHES, NEW SPECIES

Speciation, or the creation of new species, usually occurs when new ecological niches are created or the existing become unoccupied. In both cases, to occupy a new ecological niche imply a gradual differentiation from the initial population to become a genetically distinct species. As an example of formation of new ecological niches we have the case of the emergence of angiosperms. Their booming opened many new possibilities, thanks both to increasing diversity of seeds and fruits (which, in turn, increased the number of specialized species) and the emergence of complex flowers, which allowed the explosion of many pollinators (facilitating the emergence of new insectivores). As an example of unoccupied niche, there is the famous case of the extinction of non-avian dinosaurs. Dinosaurs dominated a lot of niches, from land to air ecosystems, and even the aquatic environment. Those empty niches was occupied by many mammals, thanks to their high fertility and plasticity (flexibility to adapt into different habitats). That eventually led large ratios of speciation in a short time, what is known as adaptive radiation.

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This is Eomaia scansoria, an extinct species of mammals that lived at the same time as the dinosaurs. The extinction of the dinosaurs opened up a wide range of possibilities to mammals, which, although they were expanding, remained in the background. Their great plasticity led them to colonize many habitats, by occupying the free ecological niches left by the dinosaurs. Source: Wikipedia.

ASSEMBLY OF COMMUNITIES

As we have seen, the ecological niche is behind fundamental ecological and evolutionary processes. All living communities today have been formed thanks to the niches of different species. Through competition, species niches were overlaping, and the communities were assembled like a puzzle. When a piece disappears, another takes its place, playing the role that the other had in the community. However, knowing the whole ecological niche of a species is arduous and, in most cases, impossible. As in human relationships, an exhaustive knowledge of everything that influences the life of a species (or the living space of a person) is of great importance in order to ensure their long-term preservation.

REFERENCES

Ricard-anglès

Synapsids: Before dinosaurs ruled the Earth

Before dinosaurs ruled the Earth, at the end of the Palaeozoic Era, the land was dominated by the synapsids. The synapsids (the amniote line that includes mammals) were a highly successful group which occupied most niches during the late Carboniferous and the Permian periods, but at the end of the Palaeozoic Era most families were extinguished by the Permian-Triassic mass extinction (around 252 million years ago) with only the mammalian line surviving to the present day. In this entry we’ll look at some of the more peculiar synapsid groups, which have led to the evolution of mammals like us.

CHARACTERISTICS AND EVOLUTIONARY TRENDS

The clade Synapsida includes mammals and all other amniotes more closely related to them than to reptiles. The synapsids were the first amniotes to diversify and appeared about 320 million years ago, at the middle of the Carboniferous period. These first synapsids were characterized by the presence of only one temporal fenestra behind each orbit through which the jaw muscles pass. Synapsida literally means “fused arches” referencing to the zygomatic arches (because in the past scientist believed that synapsids had evolved from diapsid reptiles and so their arches were thought to be “fused”).

Archaeothyris.svgDrawing of Archaeothyris’s skull, in which we can see some of the characteristics of the synapsids, like the temporal fenestrae and caniniform teeth. Drawing by Gretarsson.

Other characteristics that appeared through their evolution were:

  • Differentiation of differently-shaped (heterodont) teeth.
  • Lower jaw formed by fewer bones.
  • Acquisition of a more erect posture and an endothermic metabolism.

The first groups of more primitive or “reptile-like” synapsids are informally called pelycosaurs, while the latter more advanced forms are called therapsids (clade Therapsida, which in fact derived from pelycosaurs). As we will see, the evolution of synapsids is of the kind of “one group, which includes the next group, which includes the next group”.

Synapsid treeModified evolutionary tree of the amniotes by Kenneth D. Angielczyk (2009).

THE ORIGIN OF SYNAPSIDS, THE PELYCOSAURS

CotylorhynchusDB2Reconstruction of Cotylorhynchus, a caseasaurian that grew up to 3 metres long. Drawing by Dmitry Bogdanov.

The first synapsids had a sprawling limb posture, low slung bodies and were probably ectothermic. If we look at the skull morphology, the earliest groups of synapsids were the caseasaurians (clade Caseasauria) characterized by their small heads, an overdeveloped snout and their huge bodies (they were probably ectothermic and slow-moving creatures). Yet, if we look at the postcranial skeleton, the earliest synapsids were two groups called the varanopids and the ophiacodontids (Varanopidae and Ophiacodontidae families) which were similar to varanids (through convergent evolution), and while the former were quite small and agile creatures, the latter developed bigger forms with huge heads.

varanopid ophiacodontidDrawings of the varanopid Varanodon (top) and the ophiacodontid Ophiacodon (bottom). Both drawings by Dmitry Bogdanov.

Just before the appearance of the more advanced therapsids, the last two groups of “pelycosaurs” evolved and occupied most land ecosystems. Both groups shared a tall sail along their backs (similarly to Spinosaurus) formed by tall neural spines. In life, this sail probably was covered in skin and had plenty of blood vessels. Although it’s believed that these two groups were still ectothermic, this sail was probably used to gain or lose heat more easily.

Ianthasaurus_species_DB15_2Reconstruction of different species of edaphosaurids of the genus Ianthasaurus, showing their characteristic sail. Drawing by Dmitry Bogdanov.

The first of these groups is the Edaphosauridae family. Unlike most basal synapsids, the edaphosaurids were herbivorous and, along with the caseasaurians, they were among the first large amniotes to adopt a vegetarian lifestyle. The sails of edaphosaurids were covered with spiny tubercles, of which their function is still debated.

EdaphosaurusSkeleton of Edaphosaurus from the Field Museum of Chicago, where the tubercles on its spines are shown. Image by Andrew Y. Huang (2011).

The other group, the Sphenacodontidae family, were the sister group of the therapsids, inside the clade Sphenacodontia. While all other pelycosaur clades had their teeth loosely set in the jaw, the sphenacodontians had their teeth set in deep sockets. Most sphenacodontids were carnivorous, with strong jaws and well-developed caniniform teeth. Some species became the top predators on land before the apparition of the therapsids.

Dimetrodon_gigashomog_DBReconstruction of the sphenacodontid Dimetrodon, by Dmitry Bogdanov.

THE FIRST THERAPSIDS

Biarmosuchus_tener_skeleton_234Skeleton of Biarmosuchus, a basal therapsid in which we can see its more erect posture. Image by Ghedoghedo.

The therapsids (clade Therapsida, “beast arches”) appeared around 275 million years ago and replaced the pelycosaurs as the dominant land animals in the middle Permian. Early therapsids already had a more erect posture, unlike the sprawling limbs of the pelycosaurs. Also, their temporal fenestrae were larger, which made their jaws more powerful.

Estemmenosuchus_uralensisReconstruction of Estemmenosuchus, a dinocephalian from which fossil skin imprints have been found and so it’s known that it was covered in smooth, glandular skin without scales. Drawing by Mojcaj.

The therapsids diversified greatly and developed some extraordinary adaptations. The dinocephalians (clade Dinocephalia, “terrible head”) developed bony head knobs which are believed to be involved in some kind of head-butting behaviour. Another group, the anomodonts (clade Anomodontia, “abnormal teeth”) were characterized by having no teeth except for a pair of upper canines (which were probably covered by a beak). The anomodonts were the sister group of theriodontians.

Placerias1DBReconstruction of Placerias, an anomodont which could weigh up to one tonne. Drawing by Dmitry Bogdanov.

THERIODONTIA AND THE FIRST SABER-TEETH

Theriodontians (clade Theriodontia “beast teeth”) became the most successful group of synapsids. The three main groups probably looked pretty mammal-like, with fully-erect posture, a secondary bony palate which allowed them to breathe while swallowing or holding a prey and heterodont teeth (incisiviform, caniniform and molariform teeth). The most primitive theriodontian group were the gorgonopsians (clade Gorgonopsia, Gorgonopsidae family). All members of this group were carnivorous and active predators, as revealed by their sabre-toothed teeth. Although most of them were of a modest size, the larger ones reached up to 3 metres long and had canines of up to 15 cm long.

Inostrancevia_4DBReconstruction of Inostrancevia, the largest gorgonopsid genus, preying upon Scutosaurus, a parareptilian. Drawing by Dmitry Bogdanov.

A second group, the therocephalians (clade Therocephalia, “beast head”), were pretty more advanced than the gorgonopsians, although they didn’t reach their cousins’ size. Their feet resembled those of early mammals, they presented small pits on their bones which probably supported whiskers on fleshy lips, and most evidence suggests that they were already endotherms.

Pristeroognathus_DBReconstruction of a pair of Pristerognathus, a therocephalian genus in which we can see some more mammalian characteristics. Drawing by Dmitry Bogdanov.

Both gorgonopsians and therocephalians disappeared at the end of the Permian. The only therapsid group that survived through the Mesozoic period and that coexisted with the dinosaurs were the cynodonts

SMALL CYNODONTS

The cynodonts (clade Cynodontia “dog teeth”) appeared at the late Permian and diversified greatly along with the archosaurs. Although it is not really proven, most paleoartists represent cynodonts covered in fur, as evidence suggests an endothermic metabolism. Some characteristics of the cynodonts were:

  • Lower jaw formed only by the dentary bone, while the other jaw bones became the ossicles of the middle ear (the articular, the quadrate and the angular bones evolved into the malleus, the incus and the stapes).
  • Complex teeth: incisors to hold, canines to pierce, and premolars and molars to chew.
  • Only two sets of teeth (diphyodonts), instead of constantly-renewing teeth (polyphyodonts like most reptiles).
  • Large brain cavities. Some fossil burrows of different cynodonts have been found, revealing complex social behaviours.
thrinaxodon_by_ntamuraReconstruction of Thrinaxodon, a burrowing cynodont with whiskers and hair. Image by Nobu Tamura.

Even if they competed with archosaurs, some early forms became quite large. For example, some carnivores, like Cynognathus, had a large head and measured 1 metre long, while Trucidocynodon was about the size of a leopard. Yet, the evolutionary trend would make the cynodonts smaller, like the Brasiliodontidae family which, like most cynodonts, lived in the shadow of dinosaurs and other bigger reptiles. Brasiliodontids are thought to be the sister group of the Mammaliaformes (mammals and their most recent relatives).

Brasilitherium_riograndensisReconstruction of Brasilitherium, one of the most advanced non-mammalian cynodonts, which was only 12 cm in length. Drawing by Smokeybjb.

Finally, mammals appeared at the end of the Triassic period around 225 million years ago. The first mammaliaforms were probably, insectivorous, nocturnal shrew-like animals. It is thought that this nocturnal lifestyle is what actually propelled the development of fur coats, because in therapsids endothermy appeared before fur did. These mammaliaforms probably had mammary glands to feed their young when they had no teeth, but they probably had no nipples like current monotremes.

MegazostrodonLive reconstruction of Megazostrodon, a small mammaliaform which represents very well the transition from cynodonts to modern mammals. Image by Udo Schröter.

After the extinction of most archosaurs at the end of the Cretaceous period, the surviving synapsids took over the empty ecological niches. Mammals have ruled the world since then, conquering the land, the sea and even the air, but it wouldn’t have been possible without all the different adaptations acquired by early synapsids throughout their evolution. Thanks to them, humans and all other mammals are currently the dominant animals on the planet.

REFERENCES

The following sources have been used during the elaboration of this entry:

Difusió-anglès