Lecture 27 Populations, Communities, & Ecosystems

What is Ecology? Evolution and ecology are two key concepts Evolution: Changes that occur in organisms’ traits over time Ecology: How organisms live in their environment The great diversity of life on earth is the result of evolution And evolution can be said to be the consequence of ecology over time The term was coined by Ernst Haeckel (1866) Thus, ecology is the study of how organisms interact with their environment

5 Levels of Ecological Organization Populations Individuals of the same species living together Communities Populations of different species living together Ecosystems Combination of communities and associated non-living factors Biomes Major terrestrial assemblages of organisms that occur over wide geographical areas The Biosphere All biomes together with marine and freshwater assemblages

Population Growth A population is a group of individuals of the same species living together Critical properties of a population include Population size The number of individuals in a population Population density Population size per unit area Population dispersion Scatter of individuals within a population’s range Population growth How populations grow and the factors affecting growth

The Exponential Growth Model Assumes a population is growing without limits at its maximal rate Rate is symbolized by r and called the biotic potential The actual rate of population increase is

The Logistics Growth Model No matter how fast populations grow, they eventually reach a limit This is imposed by shortages of important environmental factors Nutrients, water, space, light The carrying capacity is the maximum number of individuals that an area can support It is symbolized by k As a population approaches its carrying capacity, the growth rate slows because of limiting resources The logistic growth equation accounts for this A graphical plot of N versus t (time) gives an S-shaped sigmoid growth curve History of a fur seal population on St. Paul Island, Alaska

The Influence of Population Density Density-independent effects Effects that are independent of population size but still regulate growth Most are aspects of the external environment: Weather: droughts, storms, floods Physical disruptions: Fire, road construction Density-dependent effects Effects that are dependent on population size and act to regulate growth These effects have an increasing effect as population size increases

The Influence of Population Density Maximizing population productivity The goal of harvesting organisms for commercial purposes is to maximize net productivity The point of maximal sustainable yield or optimal yield lies partly up the sigmoid curve

Life History Adaptations Life history = The complete life cycle of an animal Life histories are diverse, with different organisms having different adaptations to their environments r-selected adaptations Populations favor the exponential growth model Have a high rate of increase Followed by rapid decrease K-selected adaptations Populations experience competitive logistic growth Favor reproduction near carrying capacity Most natural populations exhibit a combination of the r/k adaptations

Survivorship Curves Provide a way to express the age distribution characteristics of populations Survivorship is the percentage of an original population that survives to a given age Type I Mortality rises in postreproductive years Type II Mortality constant throughout life Type III Mortality low after establishment

Population Demography Greek demos, “people” Demography is the statistical study of populations Greek graphos, “measurement” It helps predict how population sizes will change in the future Growth rate sensitive to: Age structure Sex ratio Cohort = A group of individuals of the same age with a characteristic Birth rate or fecundity Number of offspring born in a standard time Death rate or mortality Number of individuals that die in that period The relative number of individuals in each cohort defines a population’s age structure The proportion of males and females in a population The number of births is usually directly related to the number of females

Population Pyramids A population’s age structure and sex ratio can be used to assess its demographic trends

Human Populations Throughout most of our history, human populations have been regulated by Food availability Disease Predators Two thousand years ago, the human population was ~ 130 million It took one thousand years for it to double And another 650 years for it to double again Starting in the 1700s, technological changes gave humans more control over their environment These changes allowed humans to expand the carrying capacity of their habitats Currently, the human population is growing at a rate of ~ 1.3% annually Doubling time at this rate is only 54 years! Human population growth is not uniform

Communities All organisms that live together in an area are called a community The different species compete and cooperate with each other to make the community stable A community is often identified by the presence of its dominant species The distribution of the other organisms may differ a good deal; however, the ranges of all organisms overlap

The Niche and Competition A niche is the particular biological role of an organism in a community Habitat  place Niche  pattern of living Competition is the struggle of two organisms to use the same resource Interspecific competition occurs between individuals of different species Intraspecific competition occurs between individuals of a single species Because of competition, organisms may not be able to occupy their fundamental (theoretical) niche Instead, they occupy their realized (actual) niche

Competitive Exclusion In the 1930s, G.F. Gause studied interspecific competition among three species of Paramecium P. aurelia; P. caudatum; P. bursaria All three grew well alone in culture tubes However, P. caudatum declined to extinction when grown with P. aurelia The two shared the same realized niche and the latter was better! P. caudatum and P. bursaria were able to coexist The two have different realized niches and thus avoid competition

Resource Partitioning Gause formulated the principle of competitive exclusion No two species with the same niche can coexist Gause’s principle of competitive exclusion can be restated No two species can occupy the same niche indefinitely When niches overlap, two outcomes are possible Competitive exclusion or resource partitioning Persistent competition is rare in natural communities Either one species drives the other to extinction Or natural selection reduces the competition between them

Resource Partitioning Sympatric species occupy same geographical area Avoid competition by partitioning resources Sympatric species tend to exhibit greater differences than allopatric species do Character displacement facilitates habitat partitioning and thus reduces competition Allopatric species do not live in the same geographical area and thus are not in competition

Coevolution and Symbiosis Coevolution is a term that describes the long-term evolutionary adjustments of species to one another Symbiosis is the condition in which two (or more) kinds of organisms live together in close associations Major kinds include Mutualism – Both participating species benefit Parasitism – One species benefits while the other is harmed Commensalism – One species benefits and the other neither benefits nor is harmed

Mutualism Symbiotic relationship in which both species benefit Ants and Aphids Aphids provide the ants with food in the form of continuously excreted “honeydew” Ants transport the aphids and protect them from predators Ants and Acacias Acacias provide the ants with food in the form of Beltian bodies Ants provide the acacias with organic nutrients and protect it from herbivores and shading from other plants

Parasitism Symbiotic relationship that is a form of predation The predator (parasite) is much smaller than the prey The prey does not necessarily die External parasites Ectoparasites feed on the exterior surface of an organism Parasitoids are insects (wasps) that lay eggs on living hosts Endoparasites live within the bodies of vertebrates and invertebrates Marked by much more extreme specialization than external parasites Brood parasites (birds) lay their eggs in the nests of other species Brood parasites reduce the reproductive success of the foster parent hosts

Commensalism Symbiotic relationship that benefits one species and neither harms nor benefits the other Clownfishes and Sea anemones Clownfishes gain protection by remaining among the anemone’s tentacles They also glean scraps from the anemone’s food Cattle egrets and African cape buffalo Egrets eat insects off of the buffalo Note: There is no clear distinction between commensalism and mutualism Difficult to determine if second partner benefits at all Indeed, the relationship maybe even parasitic

Predator-Prey Interactions Predation is the consuming of one organism by another, usually of a similar or larger size Under simple laboratory conditions, the predator often exterminates its prey It then becomes extinct itself having run out of food!

Predator-Prey Interactions In nature, predator and prey populations often exhibit cyclic oscillations The North American snowshoe hare (Lepus americanus) follows a “10-year cycle” Two factors involved Food plants: Willow and birch twigs Predators: Canada lynx (Lynx canadensis) Predator-prey interactions are essential in the maintenance of species-diverse communities Predators greatly reduce competitive exclusion by reducing the individuals of competing species For example, sea stars prevent bivalves from dominating intertidal habitats Other organisms can share their habitat Keystone species are species that play key roles in their communities

Plant Defenses Plants have evolved many mechanisms to defend themselves from herbivores Morphological (structural) defenses Thorns, spines and prickles Chemical defenses Secondary chemical compounds Found in most algae as well Mustard oils Found in the mustard family (Brassicaceae) Mustard oils protected plants from herbivores at first At some point, however, certain insects evolved the ability to break down mustard oil These insects were able to use a new resource without competing with other herbivores for it Cabbage butterfly caterpillars

Animal Defenses Some animals receive an added benefit from eating plants rich in secondary chemical compounds Caterpillars of monarch butterflies concentrate and store these compounds They then pass them to the adult and even to eggs of next generation Birds that eat the butterflies regurgitate them Cryptic coloration: Color that blends with surrounding Aposematic coloration: Showy color advertising poisonous nature Chemical defenses Stings – Bees and wasps Toxic alkaloids – Dendrobatid frogs

Mimicry Many non-poisonous species have evolved to resemble poisonous ones with aposematic coloration Batesian mimicry A harmless unprotected species (mimic) resembles a poisonous model that exhibits aposematic coloration If the mimics are relatively scarce, they will be avoided by predators Müllerian mimicry Two or more unrelated but protected (toxic) species come to resemble one another Thus a group defense is achieved

Self Mimicry Involves adaptations where one animal body part comes to resemble another This type of mimicry is used by both predator and prey Example: “Eye-spots” found in many butterflies, moths and fish

A Closer Look at Ecosystems Ecosystems: the fundamental units of ecology All organisms in an ecosystem require energy Almost all energy comes from the sun Energy flows Energy is lost at each step of the food chain This limits the number of steps

Nutrients & Chemicals Cycle Raw materials are not used up when organisms die They are recycled back into the ecosystem for use by other organisms

Biomes Rainfall and temperature are the two most important factors limiting species distribution These physical conditions with their sets of similar plants and animals are called biomes

Ecological Succession Succession is the orderly progression of changes in community composition that occur over time Secondary succession: Occurs in areas where an existing community has been disturbed Primary succession: Occurs on bare lifeless substrates, like rocks The first plants to appear from a pioneering community The climax community comes at the end

The Process of Succession Succession involves three dynamic critical concepts Tolerance First to come are weedy r-selected species that are tolerant of the harsh abiotic conditions Facilitation Habitat changes are introduced that favor other, less weedy species Inhibition Habitat changes may inhibit the growth of the species that caused them As ecosystems mature, more K-selected species replace r-selected ones Species richness and total biomass increase However, net productivity decreases Thus, agricultural systems are maintained in early successional stages to keep net productivity high

Biodiversity Biologically diverse ecosystems are in general more stable than simple ones Species richness refers to the number of species in an ecosystem It is the quantity usually measured by biologists to characterize an ecosystem’s biodiversity Two factors are important in promoting biodiversity Ecosystem size Larger ecosystems contain more diverse habitats and therefore have greater number of species A reduction in an ecosystem size, will reduce the number of species it can support Faunal collapse (extinction) may occur in extreme cases Latitude The number of species in the tropics is far more than that in the arctic region Two principal reasons Length of growing season Climatic stability

Island Biodiversity In 1967, Robert MacArthur and Edward O. Wilson proposed the equilibrium model The species richness on islands is a dynamic equilibrium between colonization and extinction Two important factors Island size Larger islands have more species than smaller ones Distance from mainland Distant islands have less species than those near the mainland