Imagine you’re hiking by the Snake River in Wyoming, enjoying the swishing sound of water riffling by. The river flows past you, carrying nutrients downstream. When you come around a bend, you encounter the scene in the image below (Figure 1). A small, neat dam across the river creates a still, upstream pond. Why do you suppose this river was dammed and who dammed it?
The image shows the result of construction work by beavers—a still pool of ponded water in the background above flowing water in the foreground. Beavers use the mud from the bottom of ponded water to build their lodges, with the pond serving as a moat to protect them from predators.
Every animal species on Earth uses and changes its environment to meet its needs, interacting with resources in ways that support survival and reproduction. Humans are the most extreme example, with our capacity to harness natural resources and convert them into manmade things, resulting in radical transformations of habitats. The study of animals in their natural environments reveals a set of guiding principles that apply across the Animal Kingdom.
The ecological niche concept
Early 20th-century American naturalist Joseph Grinnell introduced the concept of an ecological niche (or Grinnellian niche) to describe how animals interact with their environments in ways that allow them to survive (Grinnell 1917). The Grinnellian niche includes everything that allows a species to exist at a particular location—including living resources like food and competitors and nonliving resources like sunlight, water, or rocks.
A decade later, British zoologist Charles Elton wrote the first textbook on animal ecology (Elton 1927), which helped popularize the niche concept in describing the place of an organism in its ecosystem. Elton’s ideas included the notion that animal species occupy parallel niches in different geographic places. In other words, equivalent niches are filled by entirely different animals in different parts of the world.
Based on the image below (Figure 2), what could we say about the ecological niche of the North American beaver?
Figure 2 shows resources that define a beaver’s ecological niche, including fresh water and sticks of various sizes, which beavers harvest from trees to feed on the living tissue layers as well as build lodges and dams. But what happens if other animals are using the same resources in the same area? Interactions between animals that live in close association (or symbiosis) may dramatically affect their ecological niches. In common language, “symbiosis” usually means two things working together cooperatively. However, in biology, symbiotic relationships include competition, predation, parasitism, and cooperative relationships (mutualism).
Now, consider Figure 3. It shows what looks like a North American Beaver, right? Actually, it’s a related “nutria,” native to South America (CABI 2019). Nutrias were cultivated for fur in the 20th century, and many escaped to the wilds of North America, Europe, and Asia, invading habitats occupied by the native beavers.
What do you think happens when two species attempt to live in the same niche at the same place?
Following up on Elton’s ideas, Russian biologist G. F. Gause (1934) proposed that two species with similar resource needs cannot live together in the exact same place. By 1944, scientists were actively debating what would later be called the “Competitive Exclusion Principle.” The Principle states that two types of animals with similar ecological niches cannot coexist for very long without one eventually outcompeting the other.
Applying the principle to the beaver and nutria, these animals had originally evolved to occupy similar niches on separate continents, but they now overlap in North America and Europe. Ecologists are concerned that nutria in Texas are competing with populations of an endangered beaver subspecies, the Mexican beaver (Millholland 2010), because they occupy the same niche. Evidence includes nutria often found occupying abandoned beaver lodges (Sheffels 2013).
In the 1950s, British ecologist G. Evelyn Hutchinson found a way to visualize competitive overlap in niches using graphs. Hutchinson plotted the range of resources used on the X axis and the amount of use by a species on the Y axis. The result yielded a shape whose area represents an ecological niche. (Hutchinson 1957). Since a given species requires multiple types of resources, the shape would actually be multi-dimensional. The area under the curve represents the species’ “fundamental niche,” which includes all the needed resources. Hutchinson’s model helps visualize how a species fundamental niche contracts to a smaller “realized niche” because of competition with other species (Figure 4).
When a population of animals loses their competitors or is introduced to a new environment without strong direct competitors, the reverse of the Competitive Exclusion Principle often occurs. This so-called “competitive release” is evidenced by many instances around the world in which humans accidentally or purposefully brought animals with them as they traveled. Portuguese biologist Maria João Verdasca and colleagues studied the niches of yellow-legged hornets that were accidentally introduced to Europe in 2004. In their native habitats in Asia, these hornets competed with six other hornet species that do not live in Europe. By tracking variables like land cover and water availability, the researchers found that without competition from other hornets, the introduced yellow-legged hornets shifted to occupy a wider range of environmental conditions in Europe. (Verdasca 2022).
"Food is the burning question in animal society, and the whole structure and activities of the community are dependent upon questions of food supply."
- Charles Elton, 1927
Besides competition for resources like the area of suitable habitat, animals compete directly for food, a central feature of niches. In contrast to a plant, which can make its own food, how an animal feeds has implications for all other aspects of its biology. Animal ecology can be understood in the context of balancing the tradeoffs animals face in securing food resources to grow and reproduce, while avoiding lethal dangers, such as predators. An animal’s feeding niche supports its “biological fitness” or the chance of it surviving to reproductive age and producing offspring (see our Adaptation module).
Animals can be categorized by how they ingest food - their feeding modes. If you look at a top-level taxonomic category (see our modules Taxonomy I and Taxonomy II), such as birds or fish, you’ll find a variety of feeding modes within the group. For example, mammals range from bulk-feeding, meat-eating carnivores, such as lions, to plant-eating constantly grazing herbivores, such as sheep, and everything in between. However, if you zoom in to the Family level, feeding modes are often a shared defining characteristic of the group. Canids (family Canidae, including dogs, wolves and coyotes) are all carnivorous, while Cervids (family Cervidae, including deer, elk and moose) are all herbivorous grazers.
Animals with similar diets will run into the problem of competition cited above. In other words, their feeding niches may overlap. Field evidence suggests that animals with similar diets often avoid competition by dividing the resources in a phenomenon called “resource partitioning.” Canadian biologist Robert H. MacArthur first described this phenomenon in the 1950s during his study of how species of warbler birds divide up resources in conifer forests. He observed that each of the five warbler species specialized on different parts of a tree, showing that “the birds behave in such a way as to be exposed to different kinds of food,” even while feeding on the same trees. (MacArthur 1958). The birds establish a truce of sorts, avoiding direct competition by occupying distinct sub-niches within their shared habitat.
Evidence for resource partitioning in nature continues to grow. Italian evolutionary biologist Elisa Torretta and colleagues recently studied how golden jackals and red foxes manage to coexist in the wilds of north-eastern Italy, despite their similar carnivorous diets. By collecting field observations and analyzing their waste (feces), Torretta found that the jackals and foxes hunted at the same time in the evening and overnight but selected different prey. The golden jackal preyed mostly on hoofed animals like deer, while the red fox preyed on small mammals, a difference that likely evolved over many generations. “Compromise” was achieved via natural selection for behavior that promoted better survival outcomes for both species. Their distinct diets reduced food competition by dividing up the carnivore feeding niche (Torretta et al. 2021).
Think about human behavior. When we designate a natural area to support wildlife, we are also resource partitioning, dividing up the space on Earth to allow for the continued survival of other species. We have shared “our” niche with other species to differing degrees over time, with population growth and consumptive practices making it increasingly difficult.
Regardless of its own feeding mode, every animal is likely to be prey for something else. Only apex predators—those at the very top of the food web—have no natural predators. Apex predators include crocodiles, lions, great white sharks, and polar bears. Still, with humans in the mix, there are arguably no animals that are totally exempt from predation. Additionally, although most apex predators are unbothered by predators in adulthood, they can be victims of predation in earlier developmental stages, such as eggs, hatchlings or newborns, and juveniles. Therefore, dealing with predators is a central challenge of animal survival. It is also worth noting that apex predators do not necessarily have it easy. They are among the most vulnerable during challenging ecological times because disturbances anywhere in the food web reverberate to the top level. Great White Sharks mitigate this risk by preying on a very wide variety of food sources.
What options do you imagine this lizard had to dodge this unfortunate encounter with a bird? (Figure 5)
One option for survival is simply to avoid predators by staying away from the areas they frequent. However, animals face tradeoffs when they avoid predators, as their ranges may also be the best areas to feed. UK ornithologist Alex Sansom studied these tradeoffs in Redshanks, birds that live in estuaries in Scotland where they are vulnerable to sparrowhawk and peregrine falcon predators. She observed Redshank feeding behavior, noting how much time they spent in the saltmarshes where prey is more nutritious but predators are common versus in the mudflats where prey is less nutritious, but predators are rarer. She found that the best-case scenario for survival was avoiding the saltmarshes where predators are common. But resource scarcity during cold weather requires feeding in the dangerous saltmarshes. Redshanks that show more vigilant behavior, such as lifting their heads to scan for predators, experience better survival rates. (Sansom 2009).
Hiding and showing
“In the animal kingdom, one of the keys to survival is to outwit your enemies. And when you're surrounded by carnivores, one of the best strategies is to fade into the background and disappear.”
– American Astrophysicist Neil DeGrasse Tyson
Camouflage is rampant in the Animal Kingdom—white Arctic Hares against the snow, warty toads in a carpet of dead leaves, a moth invisible against tree bark. Coloration that matches an animal to its background has been honed by natural selection over and over for its life-saving outcomes. And it works the same for predators as it does for prey. Just as the Arctic Hare is adapted to hide from its predators, the polar bear has adapted to avoid being spotted by its prey. The bottom line is that there is often a strong advantage for an animal to blend in with its background. How many animals do you see in Figure 6?
The flip side of the coin to camouflage is adaptations that make organisms more visible. Why would an animal want to be more visible to predators? Noticing the bright coloration of some butterflies, bees, and tropical frogs, 19th-century naturalists proposed that the coloration served as a warning that these animals were toxic. This is called “aposematic coloration,” typified by the dramatic coloration of tropical poison dart frogs. This adaptation works well because a predator either dies from eating one or is sickened, reducing predation on others of its species. Over time, predators evolve the instinct to avoid the poisonous prey altogether.
“To these creatures it is useful to be seen and recognised, the reason being that they have a means of defence which, if known, will prevent their enemies from attacking them, though it is generally not sufficient to save their lives if they are actually attacked.”
– English Naturalist Alfred Russell Wallace, 1877
Imagine that you are a behavioral ecologist. Someone shows you the image below (Figure 7) and asks you to interpret what you see. How would you respond? What is this animal doing?
This frog (Figure 6) inhabits grassland habitats in Brazil, Bolivia, and Paraguay and is hiding in plain sight. On its rear end are black markings that look like eyes and can be flashed at an approaching predator. The frog lifts its rear end to display the eyespots, which, in the best-case scenario, fool a predator into thinking a bigger, potentially dangerous animal is staring at it. Bluffing behavior is one form of what English naturalist Henry Walter Bates dubbed “mimicry” as early as the mid-19th-century after noticing the way harmless butterflies in the Amazon mimicked the appearance of toxic species, seemingly to warn off predators (Bates 1862).
Scientists continue to refine our understanding of mimicry and other behaviors as tools for survival. Brazilian biologist Julio M.G. Segovia studies spider behavior. In an ironic twist of roles, some spiders gain protection from predators by mimicking toxic, stinging, or spiny ants. Segovia’s work shows that the more harmful the ant species, the less accurate the spiders mimic them because even a poor copy of a dangerous ant deters predators. Further, Segovia’s work on harvestmen spiders illuminates another survival behavior—faking death—which they use against daytime predators that prefer a live meal (Segovia and Pekár 2021; Segovia et al. 2019).
Imagine you’re a hungry predator and come upon a flock of Murres, like those pictured in Figure 9. It’s a mix of parents and their offspring. How easy do you think it would be to single out a juvenile and snap it up?
A survival tactic that animals employ in situations where they cannot avoid predators is to group up. American Zoologist Warder C. Allee was inspired by observations of how little crustaceans found in freshwaters (isopods called Asellus) tended to group up. In his 1931 book, Animal Aggregations: A Study in General Sociology, Allee proposed that grouping up benefited animals, whether flocks of birds, schools of fish, packs of wolves, or prides of lions (Allee 1931). Allee sorted the benefits of grouping into categories like water conservation, protection from weather, socialization, and predator vigilance that trade off against downsides of crowding like disease transmission.
Animal ecologists continue examining animal grouping behavior using math models and other data science techniques. Mexican Ph.D. student Ana Sofía Guerra studied whether modern fishing techniques are changing how fish are schooling. Grouping behaviors evolved to make each individual less likely to be picked out by a predator. However, when the predators are humans fishing with large nets, schools of fish are disadvantaged relative to individual fish that are less likely to be targeted. Guerra’s evolutionary model predicts that selection pressure against schools of fish over time may change behavior away from grouping up as the individuals that go it alone will have higher survival rates. (Guerra et al. 2020).
The defensive behavior of animals has been observed for centuries, perhaps because it’s one of the most noticeable aspects of animal behavior. One does not forget an encounter with a defensive opossum, a stinging hornet, or a spitting cobra. Active defenses are as varied as the animals that wield them but can be broadly categorized as biting, poking, clawing, stinging, kicking, choking, gooing, regurgitating, and spraying. Consider the lizard in Figure 8. How is it defending itself?
In an 1892 paper, American ichthyologist (a marine biologist who studies fish) Oliver Perry Hay marveled at the defensive liquid squirting from horned lizard eyes. Hay noted, “a discharge of blood into the eyes of some pursuing bird or snake might so seriously interfere with its clearness of vision that the lizard might make its escape while the enemy was wiping its eyes.” (Hay 1892). The blood-squirting behavior had supposedly been known to indigenous people in pre-Colombian times, and modern herpetologists continue to collect observations to make sense of this horned lizard’s behavior. Mexican zoologist Aldo Gómez-Benitez reports from an ecological study in Mexico that juveniles also squirt blood from their sinuses, suggesting it might be a lifetime defensive strategy (Gómez-Benitez 2021).
More than a century after Hay’s publication, today’s behavioral ecologists are still discovering defensive secretions in other organisms. For example, Japanese researchers reported that the larvae of net-winged insects (Order Neuroptera) spray a clear liquid from their anal openings when approached by predators. The secretions repel biting ants and cause frogs to regurgitate net-winged insects that they have swallowed (Iwanami 2021). Given the negative consequences for the predators, they learn to avoid eating these larvae and, if this selective pressure is applied long enough, may evolve to avoid them instinctively.
Facilitation and cooperation
While competition and predation are widespread in the Animal Kingdom, survival for some species increases in the presence of others. So, in some circumstances, niche overlap may improve the “biological fitness” (chance of surviving to reproduce) of one or both species.
“If you're an animal that hangs out with others, then there's clearly an advantage in being smart enough to work out the intentions of the guy sitting next to you (before he takes your mate or your meal).”
– American Astronomer Seth Shostak
In 2003, American marine ecologist John F. Bruno and colleagues proposed that overlapping niches may not always result in niche partitioning or exclusion. There are occasions of niche overlap that benefit biological fitness. For example, on rocky shorelines, dense coverings of seaweed provide refuge for animals like snails. The coverage allows the snails to live higher on rocks than they would otherwise, given the risks of drying out at low tides. The realized niche of the snails is expanded through their association with the seaweed using the same habitat (Bruno 2003). In this “commensal relationship” (where one species benefits and the other is not affected), the snails benefit, and the seaweed appears unaffected.
Examples of such close associations between organisms (also known as “symbioses”) have been noted by naturalists for centuries. They include “parasitic relationships,” where one species benefits at the expense of another. In Aristotle’s book, History of Animals, he noted that cuckoo birds lay eggs in the nests of other birds who are tricked into raising their offspring (Aristotle 350 B.C.E.). Sometime during its evolution, the cuckoo’s niche expanded to include nests of other birds, then further evolved into a parasitic symbiosis so complete that cuckoos no longer built nests of their own (see our Adaptation module). Still, it’s often difficult to classify a relationship as commensal or parasitic. For example, fish lice that hang out inside dolphin mouths to steal their food scraps, a phenomenon also observed by Aristotle.
Who benefits in the image below (Figure 9), the bee or the flower?
In Figure 9, the bee benefits from the meal of nectar and pollen, while the flower benefits from the bee carrying pollen to other flowers. The downside of the bee consuming some of the plant’s pollen is outweighed by the bee’s valuable role in pollination. This scenario where both parties benefit (also called a “mutualism”) is a cooperative evolutionary arrangement in which the ecological niche of each species includes the other.
Cooperative behavior within a species may also evolve to reduce competition and increase individual fitness. In animals, cooperative behavior within a species is coordinated through social behavior. Whether it’s humans sharing food with neighbors or lions hunting in a group, cooperative behavior may convey long-term benefits that outweigh short-term costs. For example, Swiss biologist Ramona Rauber studied the cooperative behavior of meerkats, which live colonially in southern Africa. Individual meerkats take turns as sentinels who watch for predators while the other meerkats feed. The sentinel meerkat gives up feeding opportunities while on duty but benefits from the vigilance of other sentinels when it’s his turn to eat. Rauber’s observations showed that meerkat sentinels coordinate with their group through six different sentinel calls. The sentinels adjust the calls depending on dangers to reflect the tradeoff between looking for food and guarding against predators (Rauber 2020).
Cooperative behavior also shows up in reproduction. Under certain conditions, animals ranging from insects to fish to birds and mammals may breed cooperatively, sharing in the production and raising of offspring. Belgian evolutionary biologist Serge Aron and colleagues examined the dynamics behind ant queens co-founding a new ant colony. They examined how colonies were co-founded by studying the black garden ant and noting that new ant colonies with one queen have a high rate of failure. The researchers gave ant queens the option of having their own nesting chambers. However, they found that sharing promoted faster development of ant workers, boosting the workforce’s size and protecting against raiding by other ant nests (Aron and Deneubourg 2021). In this example, ant queens with identical ecological niches effectively expanded their resource access by cooperating.
Changing habitats, shifting niches
Animals rely on the specific conditions and resources that define their ecological niches. When the resources and conditions in an animal’s environment change, its ecological niche requirements may no longer be met in its habitat. Monarch butterflies have a niche that includes summer breeding grounds in the U.S. and Canada and wintering in the high-altitude fir forests of Mexico. Their cycle of migration between habitats is closely tied to climate. However, changing climate has altered the monarchs’ ecological niche by affecting their main food source (milkweed plants), the timing of their migrations, and the seasonality of weather. The result is massive declines in Monarch populations as their ecological niches become misaligned with available resources (Zylstra 2021). Time will tell if the monarchs are able to adapt by adjusting their migratory patterns.
Species are increasingly rescued from habitats too degraded to support their needs through captive rearing programs. Ecological niches are also key to reintroducing these animals to the wild. English biologists Jackie Chappell and Susannah Thorpe studied captive-reared orangutans. In the wild, orangutans live in Asian tropical forests where their ecological feeding niche is fruits that suddenly become available all at once, followed by long periods with none. Thus, orangutans are adapted to travel long distances in search of whichever trees are currently fruiting. The researchers found that raising captive orangutans for release carries the risk of not exposing them to this key aspect of their niche—the fruiting cycles. It’s vital that orangutans understand the cycle of feasting and then famine to which they’ve adapted through fat storage and temporary switches to other foods (Chappell and Thorpe 2021).
Ecological niches are shaped by millions of years of evolution. As the environments of monarch butterflies, orangutans, and other species continue to change, mismatches will continue to emerge between their ecological niches and the available resources. Sudden changes to habitats can be catastrophic because adaptation is a slow process that takes many generations. A species’ variation in behaviors determines whether a particular individual survives to reproduce. That individual variation is the raw material for natural selection (see our Adaptation module). Ultimately, the behaviors determining an individual’s fitness will depend on its genes and its interactions with the environment it encounters.
The sum of individual behaviors directs the outcome for a species. Whether environments change due to human habitat modifications, harvesting, or global climate change, a species’ collective ability to adjust its ecological niche is key to its survival (see our Factors that Control Earth's Temperature module). Scientists and resource managers are using ecological niche models to understand how habitats are changing and to predict how species may or may not be able to adapt to the changes through shifts in their niches.
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