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Chapter 6: ECOSYSTEMS: NICHES, SPECIES INTERACTIONS, SUCCESSION, AND STABILITY

6 - 1 ROLES OF SPECIES IN ECOSYSTEMS

What Role Does a Species Play in an Ecosystem ?

Ecological niche - a species way of life or functional role in an ecosystem (habitat food, what eats it, how it reproduces, it's nutrient needs, and range of tolerance for physical and chemical factors how it interacts with living and nonliving components and the role it plays in the flow of energy and cycling of matter in an ecosystem).

Fundamental niche - the full potential range of physical, chemical and biological conditions and resources it could use if there were no competition from other species.

Realized niche - part of the fundamental niche that a species actually uses, because species often compete in an ecosystem because their niches overlap.

Why is understanding the Niche of Species So Important?

Scientists are curious to understand nature, but understanding a species niche is vital to preventing it from becoming extinct.

It is also useful in assessing the environmental impact of human changes in ecosystems (clearing a forest, grassland, filling in a wetland).

Unfortunatelv we have little knowledge about the ecological roles of most species because determining the niche of a species is difficult, expensive and time consuming.

Is it Better to Be a Specialist or a Generalist Species?

Generalist species have broad niches: live in many places, eat a variety of foods and tolerate a wide range of conditions (cockroaches, humans).

Specialist species have narrow niches, which make them more prone to become endangered when environmental conditions change (spotted owls, giant pandas).

In a tropical rain forest, diverse species occupy specialized niches in distinct layers of vegetation exposed to different layers of light. The degradation of these forests are dooming large numbers of specialized species to distinction.

When environments are fairly constant (tropical rain forest) specialists have an advantage because they have fewer competitors, but when environments change rapidly, the adaptable generalist is usually better off.

6 - 2 SOME GENERAL TYPES OF SPECIES

How Can We Classify the Roles Vanous Species Play in Ecosystems?

Labels are applied to species to clarify their ecological roles, and any given species may function as more than one of these four types in a particular ecosystem; native, nonnative, indicator, or keystone.

Native species normally live in a particular ecosystem. Others that migrate or are deliberately or accidentally introduced into an ecosystem by humans, are called nonnative species, exotic, or alien species. Some are beneficial (crops, game), but many thrive and crowd out native species by taking over their niches (~'killer bees~').

Indicator species serve as early warnings that a community or ecosystem is being damaged. For example, migratory songbirds in North America are declining in number due to loss of winter habitats in the tropical forests, and summer habitat fragmentation in North America. Trout species require clean water with high levels of dissolved oxygen, so they are indicators of water quality. Amphibians are also indicator species; weather, pollution, increase in ultraviolet radiation, loss of habitat (see Connections).

Keystone species have roles in an ecosystem that are much more important than their abundance of biomass would suggest, because they are critically linked to a large number of other species, although this is controversial In tropical forests, bees, bats, ants and hummingbirds pollinate and/or disperse seeds. Dung beetles remove and recycle animal waste, aerate soil, help establish new plants, and help reduce microorganisms that spread disease. Sea oilers keep sea urchins from depleting kelp beds, which support a diverse community of mollusks, fish and marine mammals, although they are unpopular with people who harvest crabs and abalone (another source of sea otter food). Gopher tortoises of Florida are considered keystone species because the large burrows they dig become cool refuges for some 40 other species (fox, opossum, insects, frogs, snakes and mice). Alligators are another keystone species in Florida. Large gra:ing animals (elephants, rhinoceroses) create forest openings by pushing over, breaking or uprooting trees, which promotes growth of grasses. Beavers build dams that can change a fast-moving stream into a pond, which attracts fish, muskrats, herons and ducks. Top predators (great white shark, wolf; giant anteater, leopard) exert a stabilizing effect on their ecosystems by feeding on and regulating populations of certain species.

The loss of a keystone species can lead to population crashes and extinctions of other species that depend on it for certain services a ripple or domino effect.

6 - 3 SPECIES INTERACTIONS: COMPETITION AND PREDATION

How Do Species Interact? Species may interact if they have activities or resource requirements in common. They may be harmed by, benefit from, or be unaffected by the interaction.

Interspecific competition (among species) - two or more species compete for food, space or any limited resource. It will cause harm in varying degrees, depending on which is the best competitor.

In predation, one species (predator) feed directly on another living species (prey). The predator benefits, while the individual prey is harmed (may or may not die).

Symbiosis (living together) is a long-lasting relationship in which species live together in a more or less intimate association. There are three types: Parasitism - one species (the parasite) feeds on another (the host) by living on or in the host for a large portion of the host's life. The parasite benefits, while the host is harmed.
Mutualism - two species involved in a symbiotic relationship interact in ways that benefit both.
Commensalism - a symbiotic interaction that benefits one species but neither harms nor benefits the other species.

How Do Species Compete with One Another?

Most species face competition (interspecific) from other species for one or more limited resources (food, water, sunlight, nest sites, places to hide), which causes overlap in the fundamental niches of each species. The more overlap, the more they compete with one another.. One of the species may have to migrate to another area, shift its feeding habits or behavior through natural selection and evolution, suffer a decline in population or become extinct in that area. Competition may lead to new adaptations and even new species.

Species compete in two ways. In interference competition, one species may limit another's access to a resource. One form of behavior is establishing a territory they defend against other invading species. Some carry out "aggressive" acts. In coral reefs, tiny coral animals kill nearby coral species by poisoning and growing over them. Plants in desert and grassland habitats release chemicals into the soil that prevent the growth of competing species seeds or reduce their germination rates.

In exploitation competition, competing species have equal access to a resource, but differ in how fast or efficiently they exploit it. One species ends up getting more, which hampers the growth, reproduction or survival of the other.

In competitive exclusion, one species eliminates the other in a particular area through competition for limited resources. In 1934, Russian ecologist G. P. Gause carried out a laboratory experiment in which two closely related species of single-celled, bacteria Paramecium were grown, first separately and then together.

In separate containers (identical conditions) with ample food, the smaller Paramecium aurelia grew faster than the larger Paramecium caudatum, indicating that the former used the food supply more efficiently.

When both species were grown together with a limited amount of food, the smaller P. aurelia outmultiplied and eliminated the P. caudatum.

This research showed that two species with identical fimdamental niches cannot coexist indefinitely in an ecosystem with a limited resource. This competitive exclusion principle can be described as the one-niche, one-species, one-place principle.

Competition among plant species if often more intense than among animals. Some plants have root and leaf systems that let them absorb more sunlight and nutrients (exploitation), and others produce chemicals that inhibit others growth (interference).

Case Study: The Fire Ant

Aggressive red fire ants were accidentally introduced into Mobile, Alabama in the 1930's, perhaps arriving from shiploads of lumber from South America. Without predators, these ants spread throughout the South, and on truckloads of produce to the West. Frost has kept them from the Midwest and Northwest, but this may not last, due to genetic changes in some of their populations. Fire ants reduce access of other ant species to resources (interspecific competition) by direct combat and having 10 times more colonies. Up to 90% of native ant populations, and other species (ladybugs, spiders, ticks, cockroaches) have been reduced or wiped out. Their stings have killed deer fawn, lizards, birds, livestock, pets and 1O0 people. They have disrupted crops, phone service, electrical power, and caused fires and accidents, and made children afraid to play outside. Mass spraying of pesticides temporarily reduced the populations of fire ants, but it also reduced populations of native ant species, and the fire ants developed genetic resistance to the pesticides. Researchers are evaluating the use of a tiny parasitic fly that lays its eggs on the fire ant's body. After the eggs hatch, the larvae eat their way through the ant's head. But, researchers must be sure they will not also cause problems for native ant species.

How Have Some Species Reduced or Avoided Competition ?

Species that compete for the same resources may evolve adaptations that increases biological diversity instead of leading to extinction. Resource partitioning is the dividing up of scarce resources so that species use them at different times, in different ways, or in different places (share the wealth). For example: lions take larger prey, leopards take smaller ones. hawks feed during the day and owls feed at night (same prey). some birds feed on the ground and others feed in trees or shrubs.

Character displacement allows for resource partitioning through the development of physical or behavioral characteristics or adaptations that allow them to use different resources. Bill sizes of birds (similar species) found in the same ecosystem will differ (long and thin, short and thick), while these same bird species that occur alone will often have similar beak sizes.

How Do Predator and Prey Species Interact?

In a predator - prey relationship, members of a predator species feed on members of a prey species. People act as predators when they hunt, fish or pick and eat vegetables. We are indirect predators when we buy meat and vegetables Carnivores are animals that feed on other animals, but animals that feed on plants (herbivores) are also predators. Prey organisms may or may not be killed. Herbivores, hke deer, often harm a plant but don't kill it. At the population level, predation can benefit a prey species by killing sick, weak and aged members (see Case Study; sharks). Reducing the population gives remaining prey greater access to resources, and can improve the genetic stock of the population, enhancing long-term survival.

How Do Predators Increase Their Chances of Getting a Meal?

Herbivores can walk, swim or fly up to plant prey. Carnivores have two options: pursuit and ambush. Pursuit is added by speed (cheetah), keen eyesight (eagle), hunting in packs (wolves), and the use of weapons (humans). Ambush can be aided by camouflage. The flesh flower looks and smells like rotting meat and lures flies.

How Do Prey Defend Themselves Against Predators ?

run, swim, or fly fast acute sense of sight or smell to alert them protective shells, thick bark, spines or thorns camouflage with shapes or colors, or the ability to change color chemical warfare (poisonous, irritating, foul smelling, bad tasting) warning coloration mimicry - looking and acting like a poisonous species behavioral strategies to scare off predators (puffing, appearance, herding)

6 - 4 SYMBIOTIC SPECIES INTERACTIONS

Parasitism is an interaction in which a member of one parasite species obtains its nourishment by living on, in, or near a member of a host species over an extended time. Parasitism is a special form of predation where the parasite:

is usually smaller than its host. is closely associated, receives nourishment and gradually weakens its host. rarely kills its host.

Parasites kill their hosts as part of their life cycle (control pest species). Endoparasites live inside their hosts (tape worms). Ectoparasites attach to the outside of their hosts (lice, ticks mistletoe, fungi).

All species have parasites (hundreds), and most hosts support many species. On a large scale, parasites play important ecological roles. They help control population sizes, which helps promote biodiversity.

Mutualism is a symbiotic relationship where both interacting species benefit, by exploiting the other (not cooperation). This is a win-win situation.

pollination - pollinators get food (pollen) and the flowering plants depend on pollinators for reproduction. lichens - fungi collect moisture and mineral nutrients while the algae provides sugars through photosynthesis. legumes (plants) - RhL-obium bacteria on root nodules convert atmospheric nitrogen into a usable form for the plants, and the plants provide the bacteria with simple sugars. bacteria in your intestines - they gain a safe home while helping digestion and synthesizing vitamin K and the B-complex vitamins. termites use sugar provided by protozoans in their guts that eat the wood. birds ride on large animals and eat parasites on their backs(food supply), whicb benefits the animals. acacia trees - hollow horns provide homes for stinging ants and nectar provides food, while the ants provide protection by eating invading insects and destroying nearby plants, allowing more light for the acacia trees. mycorrhizae (fungi) - get nutrition from plant roots while helping the plant absorb more water and nutrients by sending out hairlike extensions into the soil. Some produce a growth hormone and others protect against toxins and roundworms.

Mutualism seems to increase when resources become scarce. Survival may depend on evolving beneficial relationships with other species.

Secondary succession is more common and begins in an area where the natural vegetation has been disturbed, removed or destroyed, but the bottom soil or sediment remains. New vegetation can usually sprout whithin only a few weeks. Small annual weeds will appear first, followed by perennial weeds and grasses, shrubs young trees, and eventually a mature forest. Changes in vegetation in turn affect food and shelter for animals. As succession proceeds the numbers and types of animals and decomposers also change. Various stages of succession have different patterns of species diversity, trophic structure, niches nutrient cycling and energy flow and efficiency

How Do Species Replace One Another in Ecological Succession ?

Three factors affect how and at what rate succession occur. Facilitation is the influence of pioneer species that gradually make the soil more suitable for other plants found at a later stage of succession. This is little importance in secondary succession, since soil is already present. Inhibition occurs when early species resist the invasion of later species through interference an~or exploitation competition. Succession can proceed only when disturbances remove these existing species. Tolerance occurs when late successional plants are not effected by the development of plants at earlier stages. Late successional plants can thrive in mature communities without eliminating earlier plants.

There is no consensus whether the stages of secondary succession occur because of inhibition, tolerance, or some combination of both.

What is the Role of Disturbance in Ecological Succession?

Disturbances play a major role in ecological succession by converting a particular stage of succession to an earlier stage. A close look at an ecosystem reveals that it consists of an ever-changing, irregular mosaic of patches at different stages of succession - the result of a variety of small and medium-sized distwbances. This increases diversity of plant and animal life and provides sites for early successional species. Fires, most set by lightning, keep some ecosystems at a certain stage of succession (savanna, chaparral, forests). Occasional fires burn away low-living vegetation and a burst of new vegetation follows. In forests, most of the large trees are fire resistant. Some conifers (cone-bearing) have seeds that are released only after being exposed to intense heat, which ensures regeneration after other competitors have been destroved bv fire Suppressing fires in fire-maintained communities alters their structure and function. Lack of fire can allow build up of underbrush, which allows a relatively harmless fire to burn so intensely it destroyed the larger fire-resistant species. An entirely different ecosystem then can develop.

Secondary succession is more common and begins in an area where the natural vegetation has been disturbed, removed or destroyed, but the bottom soil or sediment remains. New vegetation can usually sprout within only a few weeks. Small annual weeds will appear first, followed by perennial weeds and grasses, shrubs, young trees, and eventually a mature forest. Changes in vegetation in turn affect food and shelter for animals. As succession proceeds the numbers and types of animals and decomposers also change. Various stages of succession have different patterns of species diversity, trophic structure, niches, nutrient cycling and energy flow and efficiency.

How Do Species Replace One Another in Ecological Succession?

Three factors affect how and at what rate succession occur. Facilitation is the influence of pioneer species that gradually make the soil more suitable for other plants found at a later stage of succession. This is little importance in secondary succession, since soil is already present. Inhibition occurs when early species resist the invasion of later species through interference andlor exploitation competition. Succession can proceed only when disturbances remove these existing species. Tolerance occurs when late successional plants are not effected by the development of plants at earlier stages. Late successional plants can thrive in mature communities without eliminating earlier plants.

There is no consensus whether the stages of secondary succession occur because of inhibition, tolerance, or some combination of both.

What is the Role of Disturbance in Ecological Succession ?

Disturbances play a major role in ecological succession by converting a particular stage of succession to an earlier stage. A close look at an ecosystem reveals that it consists of an ever-changing, irregular mosaic of patches at different stages of succession - the result of a variety of small and medium-sized disturbances. This increases diversity of plant and animal life and provides sites for early successional species. Fires, most set by lightning, keep some ecosystems at a certain stage of succession (savanna, chaparral, forests). Occasional fires burn away low-lying vegetation and a burst of new vegetation follows. In forests, most of the large trees are fire resistant. Some conifers (cone-bearing) have seeds that are released only after being exposed to intense heat, which ensures regeneration after other competitors have been destroyed by fire. Suppressing fires in fire-maintained communities alters their structure and function. Lack of fire can allow build up of underbrush, which allows a relatively harmless fire to burn so intensely it destroyed the larger fire-resistant species. An entirely different ecosystem then can develop.

6 - 6 BIODIVERSITY AND ECOSYSTEM STABILITY

Stability is the ability of a living system to withstand or recover from external imposed changes or stresses. By complex networks of interconnected negative and positive feedback loops. This is maintained through dynnamic change. In a rain forest, some trees will die and be replaced, but if not disturbed we will still recognize it as a rain forest 50 or 100 years from now

There are three aspects of stability: Inertia or persistence is the ability to resist being disturbed or altered. Constancy is the abiliw to maintain a certain size within the limits of a resource. Resilience is the ability to bounce back after a disturbance that is not too drastic.

Ecologists have the understanding of how ecosystems maintain stability however they have learned the signs of ill health from stress include a drop in primary productivity increased nutrient losses decline or extinction of indicator species larger populations of insect pests or disease organisms a decline in species diversity the presence of contaminants

Does Species Diversity lncrease Ecosystem Stability?

in the l960's, it was believed that the greater the species diversity, the irreat-~r the stability because of more ways to respond to stress. There are many exceptions. There is a minimum threshold of species diversity. No ecosystem can function without soil, plants and decomposers. It is difficult to identify that threshold.

Research indicates that ecosystems with more species tend to have higher net primary productivity and can be more resilient. But how much biodiversity insurance is needed? It seems that 1040 producer species produce the peak for production in any ecosystems contain more, but we can't distinguish between those that are essential for productivity.

Ecologists disagree on how to define sustainability and diversity. It is not clear whether an ecosystem needs both high inertia and high resilience to be stable. Rain forests have high diversity and inertia but if severely degraded it's resilience is degraded. Grasslands by contrast, are less diverse and have low inertia (burn easily, but because their matter is stored in roots, they recover quickly) high resilience.

Another problem is that populations are rarely at equilibrium. Nature is constantly in a state of change. Disturbances are integral spans of nature.

Experiments at the Hubbard Brook Experimental Forest revealed that nutrient losses from a cleared area in a pioneer stage (deforestation) were much higher than from an undisturbed plot. However, if succession was allowed to proceed in the cleared area, nutrient losses dropped sharply.

Agriculture replaces species rich late successional communities (grasslands, forests) with an early successional community, often consisting of a single crop. Herbicides then keep out other opportunistic species.

Timber companies attempt to increase productivity by replacing diverse forests with farms of a single, fast-growing species. Species rich tropical forests have been replaced with single-species plantations of rubber, banana and cocoa plants. Homeowners keep monoculture grass lawns at an early stage of succession.

The intermediate disturbance hypothesis states that communities that experience frequent, moderate disturbances have the greatest diversity of species.

How Predictable is Succession?

A classical view of succession states that it proceeds until an area is occupies by a predictable type of climax community dominated by a few long-lived plant species ("nature's long-distance runners").

But, research has shown that types of species and communities that appear during succession can be highly variable, chaotic and unpredictable. It is not a certainty that the same species will return to an area after a disturbance.

Deforestation over large areas may cause soil erosion that could take hundreds to thousands of years to rebuild. The local climate may become drier due to loss of water in the atmosphere from transpiration by trees.

The chaos theory tells us that random events that magnify small changes in environmental conditions can play key roles in the nature and rate of succession.

Succession reflects the ongoing struggle by different species for enough resources to give them a reproductive advantage over other species; each species trying to occupy as much of it's fundamental niche as possible.

Our knowledge of ecosystems is so limited we could not predict the order of a given succession. Many ecologists prefer terms such as vegetation or community(development or biotic change instead of succession, and have replaced the term climax community with terms such as a relatively stable or mature community).

What Determines the Number of Species in an Ecosystem ?

Two factors affecting species diversity (richness) of an ecosystem are it's size and degree of isolation. Large islands tend to have more species of a certain category.

According to the species equilibrium model, or the theory of island biogeography, proposed by Robert MacArthur and Edward 0. Wilson, the number of species on an island is determined by a balance between: the immigration rate of new species, and the extinction rate of species that are established on the island. At some point the model predicts the immigration and extinction rates will reach an equilibrium point that determines the island's average number of different species.

The model also predicts that the rates (and thus species diversity) are affected by two variables: the size of the island the island's distance from a mainland source of immigrant species. (an island closer to the mainland will have a higher rate of immigration)

A small island has lower diversity because it is harder for immigrants to find (a lower immigration rate) and usually has fewer resources and less diverse habitats (increased extinction rate due to competitive exclusion).

MacArthur and Wilson's model has been tested and supported by field experiments. It has been accepted and elevated to the status of a scientific theory, and has even been applied to the protection of wildlife on land (see Connections).