AP Bio Chapter 54

Limits on Food Chain Length

The length of most food chains in a food web is only five or fewer links. The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain. Only about 10% of the energy stored in the organic matter of each trophic level is converted to organic matter at the next higher trophic level. Thus, a producer level consisting of 100 kg of plant material can support only about 10 kg of herbivore biomass (the sum weight of all individuals in a population) and 1 kg of carnivore biomass. The energetic hypothesis predicts that food chains should be relatively longer in habitats that have higher photosynthetic productivity.

A high level of disturbance is generally the result of...

a high intensity and high frequency of disturbance, whereas a low level of disturbance can result from either a low intensity or a low frequency of disturbance.

The niche is...

an organism's ecological role—how it "fits into" an ecosystem. For example, the niche of a tropical tree lizard includes the temperature range it tolerates, the size of the branches it perches on, the time of day when it is active, and the sizes and kinds of insects it eats.

Prey animals with effective chemical defenses often exhibit bright warning...

aposematic coloration. Predators often avoid potential prey with bright coloration. One prey species may gain protection by mimicking the appearance of another prey species.

Large-scale disturbances...

are also a natural part of many communities.

Keystone species

are not necessarily abundant in a community, but they influence community structure by their key ecological niches. If keystone species are removed, community structure is greatly affected.

Small-scale disturbances...

can create patches of different habitats across a landscape, which help maintain diversity in a community.

However, ecologically similar species can...

coexist in a community if significant differences in their niches arise over time.

Concept 54.4 Biogeographic factors affect...

community diversity

In the traditional view, biological communities are at...

equilibrium, a more or less stable balance, unless seriously disturbed by human activities. This "balance of nature" view focused on interspecific competition as a key factor determining community composition and maintaining stability in communities.

Solar energy input and water availability can be combined as a measure of a community's rate of...

evapotranspiration, evaporation of water from soil plus transpiration of water from plants.

When competition between two species with identical niches does not lead to the local extinction of either species, it is generally because...

evolution by natural selection has resulted in the modification of the resources used by one of the species.

Two key factors contributing to latitudinal gradients are...

evolutionary history and climate. Over the course of evolutionary time, species diversity may increase in a community as more speciation events occur.

The stresses associated with high levels of disturbance...

exceed the tolerances of many species, reducing species diversity as slowly colonizing or slowly growing species are excluded.

Ectoparasites

feed on the external surfaces of the host.

Ecologists define the boundaries of a particular community to...

fit their research questions. An ecologist might study the community of decomposers living on a rotting log, the benthic community in Lake Superior, or the community of trees and shrubs in Shenandoah National Park.

Intermediate levels of disturbance...

foster greater species diversity by opening up habitats for occupation by less competitive species yet not creating conditions so severe that they exceed the environmental tolerances or rates of recovery of potential community members.

A classic experiment of this type, performed in the rocky intertidal zone of Scotland, showed that competition...

from one barnacle species kept a second barnacle species from occupying part of its fundamental niche.

Communities are characterized by...

general attributes, including their species diversity and the feeding relationships they contain.

Concept 54.1 Community interactions are classified by whether they...

help, harm, or have no effect on the species involved

In contrast...

oxygen is rarely in short supply. Although most species use oxygen, they rarely compete for it.

Exploitative interactions include

predation, herbivory, and parasitism.

Stability is...

the tendency of a community to reach and maintain a relatively constant composition of species despite disturbance. Early ecologists thought that the community of plants at a site had only one state of equilibrium that was controlled solely by climate. F. E. Clements, of the Carnegie Institution, argued that mandatory biotic interactions caused the species in this climax community to function as an integrated unit—in effect, as a superorganism. His argument was based on the observation that certain species of plants are consistently found together, such as the oaks, maples, birches, and beeches in deciduous forests of the northeastern United States.

In Müllerian mimicry...

two or more unpalatable species resemble each other. Each species gains an additional advantage because predators are more likely to encounter an unpalatable prey and learn to avoid prey with that appearance. The shared appearance thus becomes a kind of aposematic coloration.

The competitive exclusion principle can be restated to say that...

two species cannot coexist permanently in a community if their niches are identical.

Measuring species diversity is essential for...

understanding community structure and conserving biodiversity.

Ecological Niches and Natural Selection

A species' ecological niche is the sum of its use of abiotic and biotic resources in its environment. In the analogy stated by ecologist Eugene Odum, an organism's habitat is its "address," and the niche is the organism's "profession."

Food webs can be simplified in two ways....

The species in a given community can be grouped into broad functional groups. For example, phytoplankton can be grouped as primary producers in an aquatic food web. A portion of the web that interacts little with the rest of the community can be isolated.

Adaptive coloration...

has evolved repeatedly in prey animals.

Large-scale biogeographic factors contribute to...

the tremendous range of diversity observed in biological communities.

Overview: Communities in Motion

Giant Moray eels allow bluestreak cleaner wrasse to pick out and eat tiny parasites living inside the eel's mouth and on its skin- a mutually beneficial relationship.

Herbivory

Herbivory is a +/− interaction in which an herbivore eats parts of a plant or alga. Terrestrial herbivores include large mammals, such as cattle and water buffalo, and small invertebrates, such as grasshoppers. Aquatic herbivores include snails, sea urchins, some tropical fishes, and mammals like the manatee.

Mutualism

Mutualism is an interspecific symbiosis in which both species benefit from the interaction (+/+). The interaction between termites and the microorganisms that live in their digestive system is an example of obligate mutualism, in which at least one species has lost the ability to survive without its partner. Typically, both partners in a mutualism incur costs as well as benefits. For an interaction to be considered a mutualism, the benefits to each partner must exceed the costs.

Predation

Predation is a +/− interaction between species in which one species, the predator, kills and eats the other, the prey. The term predation applies to a broad range of interactions, including a lion eating an animal.

Natural selection favors adaptations of predators and prey....

Predators have many feeding adaptations, including acute senses and weaponry, such as claws, teeth, fangs, stingers, or poison to help them catch and subdue prey. Predators that pursue prey are generally fast and agile; those that lie in ambush are often camouflaged.

Concept 54.2 Diversity and trophic structure characterize...

biological communities

The common spiny mouse

(Acomys cahirinus) and the golden spiny mouse (A. russatus) share similar microhabitats and food sources. Where they co-occur, A. cahirinus is nocturnal (active at night), while A. russatus is diurnal (active during the day). Laboratory researchers have found that A. russatus is naturally nocturnal. To be active during the day, it overrides its biological clock in the presence of A. cahirinus. When field researchers removed all A. cahirinus individuals from a site in Israel, A. russatus became nocturnal, consistent with the laboratory results. This change in behavior suggests that competition exists between the species and that partitioning of their active time helps them coexist.

Ecosystem engineers

(or foundation species) exert their influence by causing physical changes in the environment that affect community structure. An example of such a species is the beaver, which transforms landscapes by felling trees and building dams. By altering the structure or dynamics of the environment, foundation species act as facilitators, having positive effects on the survival and reproduction of other species.

Parasitoid insects...

(usually small wasps) lay eggs on or in living hosts. The larvae feed on the body of the host, eventually killing it. Some ecologists believe that at least one-third of all species on Earth may be parasites. Many parasites have complex life cycles involving a number of hosts. For example, the life cycle of the blood fluke, which infects approximately 200 million people around the world, requires two hosts: humans and freshwater snails. Some parasites change the behavior of their hosts in ways that increase the probability of the parasite being transferred from one host to another. For instance, the presence of parasitic acanthocephalan (spiny-headed) worms leads their crustacean hosts to move into the open, where they have a greater chance of being eaten by the birds that are the second host in the parasitic worm's life cycle. Parasites can have significant direct and indirect effects on the survival, reproduction, and density of their host populations. Ticks that live as ectoparasites on moose weaken their hosts by withdrawing blood and causing hair breakage and loss, thus increasing the chance that the moose will die from cold, stress, or predation by wolves. Some of the declines of the moose population on Isle Royale, Michigan, have been attributed to tick outbreaks.

Characterizing Disturbance

A disturbance is an event that changes a community by removing organisms or altering resource availability. Storms, fires, floods, droughts, frosts, human activities, and overgrazing are disturbances. The types of disturbances and their frequency and severity vary among communities. Storms disturb almost all communities, even those in the oceans. Fire is a significant disturbance in most terrestrial communities. Chaparral and some grassland biomes require regular burning to maintain their structure and species composition.

Exploitation

All non photosynthetic organisms must eat, and all organisms are at risk of being eaten. As a result, much of the drama in nature involves exploitation, a general term for any +/- interaction in which one species benefits by feeding on the other species, which is harmed by the interaction.

Ecologists have conducted extensive research on primary succession on glacial moraines at Glacier Bay in southeastern Alaska, where glaciers have retreated more than 100 km since 1760.

By studying the communities on moraines at different distances from the mouth of the bay, ecologists can examine different stages in succession. The exposed moraine is colonized first by pioneering species, including liverworts, mosses, fireweed, and scattered Dryas (a mat-forming shrub), willows, and cottonwood. After about three decades, Dryas dominates the plant community. A few decades later, the area is invaded by alder, which forms dense, tall thickets. During the next two centuries, these alder stands are overgrown first by Sitka spruce and then by a combination of western hemlock and mountain hemlock. In poorly drained areas, the forest floor of this spruce-hemlock forest is invaded by sphagnum mosses, which increase soil moisture and acidify soil, killing the trees. Later, 300 years after glacial retreat, the vegetation consists of sphagnum bogs on the poorly drained flat areas and spruce-hemlock forest on the well-drained slopes.

Species with a Large Impact

Certain species strongly affect community structure because they are highly abundant or because they play a pivotal role in community dynamics. The exaggerated impact of these species occurs through their trophic interactions and their influences on the physical environment.

Character Displacement

Character displacement is the tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species. An example of character displacement is the variation in beak size between different populations of the Galápagos finches Geospiza fuliginosa and Geospiza fortis.

Commensalism

Commensalism is an interaction that benefits one species but neither harms nor helps the other (+/0). Commensal interactions are difficult to document in nature because any close association between species likely affects both species, if only slightly. Some commensal associations involve one species obtaining food that is inadvertently exposed by another. For instance, cowbirds and cattle egrets feed on insects flushed out of the grass by grazing bison, cattle, horses, and other herbivores. The birds benefit, increasing their feeding rates when they follow the herbivores. The herbivores may derive some benefit because the birds are opportunistic feeders that occasionally remove and eat ticks and other ectoparasites from the herbivores. The birds may also warn the herbivores of a predator's approach. This example illustrates another key point about ecological interactions: their effects can change over time. In this case, an interaction whose effects are typically + / 0 (commensalism) may at times become + / + (mutualism). All types of interactions can have significant influence on the structure of ecological communities.

Positive Interactions

Ecological communities are also heavily influenced by positive interactions, a term that refers to a + / + or + / 0 interaction in which at least one species benefits and neither is hard. Positive interactions include mutualism and commensalism.

Ecological Succession

Ecological succession is the transition in species composition in disturbed areas over ecological time.

Latitudinal Gradients

In the 1850s, both Charles Darwin and Alfred Wallace pointed out that plant and animal life was more abundant and varied in the tropics than in temperate and polar areas. A study found that a 6.6-hectare plot in tropical Malaysia contained 711 tree species, while a 2-ha plot of deciduous forest in Michigan typically contained 10 to 15 tree species. All of Western Europe north of the Alps has only 50 tree species. Many groups of animals show similar latitudinal gradients. For instance, there are more than 200 species of ants in Brazil but only 7 in Alaska. Such observations suggest that biogeographic patterns in biodiversity conform to a set of basic principles.

Food Webs

In the 1920s, Oxford University biologist Charles Elton recognized that food chains are not isolated units but are linked together into food webs. A food web uses arrows to link species according to who eats whom in a community. A given species may weave into the web at more than one trophic level.

Diversity and Community Stability.

Manipulation of plant diversity in plots at Cedar Creek Natural History Area in Minnesota has found that high-diversity communities are more productive, more stable in their productivity, and better able to recover from environmental stresses, such as droughts. In one decade-long experiment, researchers at Cedar Creek created 168 plots, each containing 1, 2, 4, 8, or 16 perennial grassland species. The most diverse plots were 70% more stable than the single-species plots in the amount of plant mass produced each year. Higher-diversity communities tend to be more resistant to invasive species. Scientists working in Long Island Sound, Connecticut, created communities of different diversity consisting of sessile marine invertebrates, including tunicates. They examined how vulnerable these communities were to invasion by an exotic tunicate, and found that the exotic tunicate was four times more likely to survive in lower-diversity communities than in higher-diversity ones. Relatively diverse communities capture more of the available resources, leaving fewer for the invader and decreasing its survival.

Herbivores have many specialized adaptations...

Many herbivorous insects have chemical sensors on their feet to recognize appropriate food plants that are nutritious and nontoxic. Some mammalian herbivores, such as goats, use their sense of smell to examine plants, rejecting some and eating others. Mammalian herbivores have specialized dentition and digestive systems to process vegetation.

Parasitism

Parasitism is a +/− symbiotic interaction in which a parasite derives its nourishment from a host, which is harmed in the process.

One widely used index is...

Shannon diversity (H), where S is species richness and pi is the proportion of each species in the community: H = -(pA ln pA + pB ln pB + pC ln pC + . . .) A, B, C... are the species in the community p is the relative abundance of each species And ln is the natural log of each value The higher H is the more diverse a community is.

Species Diversity

Species diversity of a community, the variety of different kinds of organisms that make up the community, has two components.

In Batesian mimicry...

a harmless or palatable species mimics a harmful or unpalatable one.

Competitive Exclusion

Strong competition between two species for limited resources can lead to the local elimination of one of the two competing species. When Russian ecologist G. F. Gause cultured two species of Paramecium together, one species was driven to extinction in the culture. Gause concluded that two species competing for the same limiting resources cannot coexist in the same place. One species will always use the resources more efficiently, gaining a reproductive advantage that will eventually lead to local elimination of the inferior competitor. This outcome is called competitive exclusion.

When individuals of two or more species live in direct and intimate contact with one another, they have a symbiotic relationship....

Symbiosis includes all direct and intimate relationships between species, whether harmful, helpful, or neutral.

How is succession on glacial moraines related to environmental changes caused by transitions in the vegetation?

The bare soil after glacial retreat is low in nitrogen content. Most pioneer plant species begin succession with poor growth and yellow leaves due to an inadequate nitrogen supply. However, Dryas and alder have symbiotic bacteria that fix atmospheric nitrogen, and soil nitrogen content increases rapidly during succession. By altering soil properties, pioneer plant species permit new plant species to grow, and the new plants in turn alter the environment in different ways, thus contributing to succession.

Predators may also use mimicry...

The mimic octopus can take on the appearance of more than a dozen marine animals to catch prey and ward off predators.

Humans Disturbance

The strongest agent of disturbance today is human activity. Agricultural development has disrupted the vast grasslands of the North American prairie. Logging and clearing for urban development have reduced large tracts of forest to small patches of disconnected woodlots throughout North America and Europe. Tropical rain forests are disappearing due to clear-cutting for lumber, grazing, and farmland. Centuries of overgrazing and agricultural disturbance have contributed to famine in parts of Africa by turning seasonal grasslands into vast barren areas. Humans disturb marine ecosystems just as extensively as terrestrial ones. The effects of ocean trawling, where boats drag weighted nets across the seafloor, are similar to the effects of clear-cutting a forest or plowing a field. Human-caused disturbances usually reduce species diversity in communities.

Trophic Structure

The trophic structure of a community is determined by the feeding relationships between organisms. The transfer of food energy up the trophic levels from its source in plants and other autotrophs (primary producers) through herbivores (primary consumers) to carnivores (secondary and tertiary consumers) and eventually to decomposers is called a food chain. The position an organism occupies in a food chain is called its trophic level.

Competition

When individuals of different species engage in competition for a resource that limits their growth and survival, the result is detrimental to both species (−/−). For example, grasshoppers and bison in the Great Plains compete for grass.

A biological community is defined as...

a group of populations of different species living close enough to interact.

At the other extreme, low levels of disturbance...

allow competitively dominant species to exclude less competitive species.

Dominant species

are the species in a community that are most abundant or collectively have the highest biomass. There is no single explanation for why a species becomes dominant in a community. One hypothesis suggests that dominant species are competitively successful at exploiting limiting resources. Another hypothesis suggests that dominant species are most successful at avoiding predation or disease. This hypothesis could explain why invasive species, such as kudzu\ can achieve such high biomass in their new environments, in the absence of their natural predators and agents of disease.

Primary succession

begins in a lifeless area where soil has not yet formed, such as a volcanic island or the moraine left behind as a glacier retreats. Initially, only autotrophic prokaryotes, heterotrophic bacteria, and protists are present. Next, mosses and lichens colonize by windblown spores. Soil develops gradually as rocks weather and organic matter accumulates from the decomposed remains of the early colonizers. Once soil is present, grasses, shrubs, and trees sprout from seeds blown or carried in from nearby areas. Producing such a community through primary succession may take hundreds or thousands of years.

Evapotranspiration is much higher in...

hot areas with abundant rainfall than in areas with low temperatures or low precipitation.

Other ecologists questioned whether most communities were at equilibrium or functioned as...

integrated units. A. G. Tansley, of Oxford University, challenged the concept of a climax community, arguing that differences in soils, topography, and other factors created many potential communities that were stable within a region. H. A. Gleason, of the University of Chicago, saw communities as chance assemblages of species found together because they happen to have similar abiotic requirements for temperature, rainfall, and soil type. Gleason and other ecologists also realized that disturbance keeps many communities from reaching a state of equilibrium in species diversity or composition.

Relative abundance

is the proportion each species represents of all individuals in the community.

Species richness

is the total number of different species in the community.

Two key factors correlated with a community's biodiversity (species diversity) are..

its latitude and the area it occupies.

Endoparasites

live within the body of the host; ectoparasites live and feed on the external surface of the host.

Tropical communities are generally...

older than temperate or polar communities. The growing season in the tropics is about five times longer than that in a tundra community. Biological time thus runs five times faster in the tropics. Repeated glaciation events have eliminated many temperate and polar communities.

The intermediate disturbance hypothesis is supported by...

many terrestrial and aquatic studies. Ecologists in New Zealand compared the richness of invertebrate taxa living in the beds of 27 streams exposed to different frequencies and intensities of flooding. When floods occurred either very frequently or rarely, invertebrate richness was low. Frequent floods made it difficult for some species to become established in the streambed, and rare floods resulted in some species being displaced by superior competitors. Invertebrate richness peaked in streams that had an intermediate frequency or intensity of flooding, as predicted by the intermediate disturbance hypothesis.

Plants...

may produce chemical toxins, which may act in combination with spines and thorns to prevent herbivory. Plants' chemical weapons include strychnine, nicotine, and tannins. The flavors of cinnamon, cloves, and peppermint are distasteful to many herbivores. Some plants produce chemicals that cause abnormal development in insect herbivores.

Some prey animals have...

mechanical or chemical defenses. Chemical defenses include odors and toxins, synthesized by the prey species or passively acquired from the food the prey eats.

Determining the number and relative abundance of species in a community can be difficult. Because...

most species in a community are relatively rare, it may be hard to obtain a sample size large enough to be representative. It is difficult to census highly motile or less visible or accessible members of communities, such as mites, nematodes, microorganisms, or nocturnal species. The small size of microorganisms makes them particularly difficult to sample, so ecologists now use molecular tools to help determine microbial diversity. DNA sequencing and molecular tools can be used to count organisms of a particular species.

Secondary succession

occurs where an existing community has been removed by a disturbance, such as a clear-cut or fire, but the soil is left intact. Herbaceous species grow first, from windblown or animal-borne seeds. Woody shrubs replace the herbaceous species, and are in turn replaced by forest trees. Early arrivals and later-arriving species are linked in one of three key processes. Early arrivals may facilitate the appearance of later species by changing the environment. For example, early herbaceous species may increase soil fertility. Early species may inhibit the establishment of later species. Early species may tolerate later species but neither hinder nor help their colonization.

Small and large disturbance...

often have important effects on community structure.

Cryptic coloration...

or camouflage makes prey difficult to see.

One way to investigate the impact of a dominant species is to...

remove it from the community, an experiment has been carried out many times by accident. The American chestnut was a dominant tree in deciduous forests of eastern North America before 1910, making up more than 40% of mature trees. The fungal disease chestnut blight killed all the chestnut trees in eastern North America between 1910 and 1950. The removal of American chestnut trees had a relatively small impact on some species but severe effects on others. Oaks, hickories, beeches, and red maples increased in abundance and replaced the chestnuts. No mammals or birds were harmed by the loss of the chestnut, but seven species of moths and butterflies that fed on the tree became extinct.

Ecologist Robert Paine of the University of Washington first developed the concept of keystone species when he...

removed the sea star Pisaster ochraceous from rocky intertidal communities. Pisaster is a predator on mussels such as Mytilus californianus, a superior competitor for space in the intertidal areas. After Paine removed Pisaster, the mussels were able to monopolize space and exclude other invertebrates and algae from attachment sites. When sea stars were present, 15 to 20 species of invertebrates and algae lived in the intertidal zone. After the experimental removal of Pisaster, species diversity declined to fewer than 5 species. Pisaster is a keystone species, exerting an influence on community structure that is disproportionate to its abundance.

Ecologists can test whether a potential competitor limits a species' realized niche by...

removing the competitor and seeing whether the first species expands into the newly available space.

Species can partition their niches not just in...

space, but also in time.

Concept 54.3 Disturbance influences..

species diversity and composition

Ecologists calculate indexes of diversity based on...

species richness and relative abundance so they can quantitatively compare the diversities of communities across time and space.

Ecologists can identify a species' fundamental niche by...

testing the range of conditions in which it grows and reproduces in the absence of competition.

Another factor that may limit the length of food chains is...

that animals tend to be larger at successively higher trophic levels. The size of a carnivore places an upper limit on the size of the food it can take into its mouth. Most large carnivores cannot live on very small food items because they cannot procure enough food in a given time to meet their metabolic needs. An exception is the baleen whales, huge suspension feeders with adaptations that enable them to consume enormous quantities of krill and other small organisms.

The nonequilibrium model proposes...

that communities constantly change following a disturbance.

Prey animals have evolved adaptations...

that help them avoid being eaten. The behavioral defenses of prey animals include fleeing, hiding, and forming herds and schools. Active self-defense is less common, although large grazing mammals may vigorously defend their young from predators.

Interspecific interactions...

that link the species of a community include competition, predation, herbivory, and symbiosis (including parasitism, mutualism, and commensalism), and facilitation.

The intermediate disturbance hypothesis suggests...

that moderate levels of disturbance foster greater species diversity than do low or high levels of disturbance.

Yellowstone National Park is dominated by lodgepole pines, a tree whose cones remain closed until exposed to intense heat. When a forest fire burns the trees...

the cones open and the seeds are released, allowing a new generation of lodgepole pines to thrive on nutrients released from the burned trees, in sunlight no longer blocked by taller trees. In the summer of 1988, extensive areas of Yellowstone burned during a severe drought. By 1989, the burned areas in the park were largely covered with new vegetation, suggesting that the species in this community are adapted to rapid recovery after fire. Large-scale fires have swept through the lodgepole pine forests of Yellowstone and other northern areas for thousands of years, while more southerly pine forests were historically affected by frequent but low-intensity fires. Human fire suppression has allowed an unnatural buildup of fuels and elevated the risk of large, severe fires to which the species are not adapted. The Yellowstone forest community is a nonequilibrium community, continually changing because of natural disturbances.

Resource partitioning...

the differentiation of niches that enables two similar species to coexist in a community is "the ghost of competition past"— indirect evidence of earlier interspecific competition resolved by the evolution of niche differentiation.

Most of the available data support...

the energetic hypothesis. For example, ecologists have used tree-hole communities in tropical forests as experimental models to test the energetic hypothesis. Many trees have small branch scars that rot, forming small holes in the tree trunk. The tree holes hold water and provide a habitat for tiny communities consisting of decomposer microorganisms and insects that feed on leaf litter as well as predatory insects. Researchers manipulated productivity (leaf litter falling into the tree holes). As predicted by the energetic hypothesis, holes with the most leaf litter, and hence the greatest total food supply at the producer level, supported the longest food chains.

0 indicates that a population is not affected by...

the interaction in any known way.

As a result of competition, a species' fundamental niche...

the niche potentially occupied by that species, may differ from its realized niche, the niche a species actually occupies in a particular environment.

Nonequilibrium conditions resulting from disturbance appear to be...

the norm for most communities.

Interspecific interactions can be symbolized by...

the positive (+) or negative (−) effects they have on the survival and reproduction of the two species engaged in the interaction.

Climate is likely...

the primary cause of latitudinal gradients in richness and biodiversity.