Chapter 6: Community and Population Ecology

Chapter 6: Community and Population Ecology

Section 6.1: Species Diversity and Sustainability of a CommunityWhat is a Community?Community: a collection of populations of different species in a given area that can potentially interact with one another.

Community Characteristics:Species Diversity: the number of different species it contains (species richness) combined with the relative abundance of individuals within each of those species (species evenness).Varies with geographical location: Species richness is highest in the tropics and decreases as we move from the equator toward the poles.Niche Structure: how many ecological niches occur, how they resemble or differ from one another, and how the species occupying different niches interact.

What is Sustainability?Sustainability involves resisting or responding to changing environmental conditions.Three aspects of stability or sustainability of a living system:Inertia (Persistence): the ability of a living system to resist being disturbed or alteredConstancy: the ability of a living system such as a population to keep its numbers within the limits imposed by available resourcesResilience: the ability of a living system to repair damage after an external disturbance that is not too drastic

Roles of Species in a CommunityEach species in a community occupies a unique ecological niche that describes its role in a community. This includes the particular habitat in which it lives, the environmental conditions such as temperature necessary for its survival, and the methods it uses to acquire its supply of nutrients.Different ecological niches in communities are played by:Native species: species that normally live and thrive in a particular communityNonnative / Invasive / alien species: other species that migrate into a community, or are deliberately or accidentally introduced; they can spread rapidly if they find new niches that are as suitable as their original niches were; these often do not face predators and diseases as they had before, or they may be able to out-compete some native species in their new niches.Indicator species: Species that provide early warnings of harmful environmental changes taking place in a community or ecosystemDifferent ecological niches in communities are played by:Keystone species: Species that play critical roles affecting many other organisms in an ecosystem; loss of these species can lead to population crashes and extinctions of other species in a community that depends on it for certain ecological servicesPollination of flowering plant speciesTop predator keystone species feed on and help regulate the populations of other speciesDecompose animal wastes and nitrify and aerate soilFoundation species: a species that plays a major role in shaping communities by creating and enhancing their habitats in ways that benefit other species, strengthening and expanding the foundation of its community

Section 6.3: How do Species Interact?Symbiotic Relationships:

These interactions influence the abilities of the interacting species to survive and reproduce, and thus serve as agents of natural selection. Some interactions also help limit population sizes, illustrating one of the four scientific principles of sustainability.

Symbiotic RelationshipsFive basic types of interactions between species:Interspecific competitionPredationParasitismMutualismCommensalismInterspecific CompetitionInterspecific competition: attempts by members of two or more species to use the same limited resources in an ecosystem; with intense competition for limited resources (food, water, sunlight, nesting sites, etc), one of the competing species must either migrate, shift feeding habits or behavior through natural selection, suffer a sharp population decline or become extinct.Interspecific CompetitionResource partitioning: occurs when species competing for similar scarce resources evolve specialized traits that allow them to use shared resources at different times, in different ways, or in different places; Adaptations that allow species to reduce or avoid competition for resources with other speciesEx: Lions and leopards live in the same area, but lions eat larger prey and leopards eat smaller preyEx: Hawks and owls feed on the same type of prey, but hawks hunt during the day, and owls hunt at night

PredationPredation: when a member of one species (predator) feeds directly on all or part of a living organism of another species (prey) as part of food webs, forming a predator-prey relationshipAdaptations that help predators capture prey:Pursuit: adapting an ability to outrun their prey; adapting keen eyesight for spotting prey from long distances; hunting in packs, etcAmbush: camouflage to help predators hide in plain sight to help them ambush their preyChemical warfare: venoms and poisons that help predators catch and kill prey

PredationAdaptations that help prey species avoid predators:Flight: adapting an ability to run, swim or fly fastEnhanced sensitivity: highly developed sense of sight, hearing or smell that alerts them to the presence of predatorsSelf-defense mechanisms: hard outer shells, thick bark, thorns, spines, regenerative body parts (tails, legs, etc.)Camouflage: colors and shapes that help hide them from predatorsPredationAdaptations that help prey species avoid predators:Chemical warfare: some prey discourage predators through the use of poisonous, irritating, foul-smelling, bad-tasting, or staining chemicalsMost of these have evolved a warning coloration to deter predators, flashing a warning signal to experienced predators that Eating me is risky. Two major rules apply:If it is small and strikingly beautiful, it is probably poisonous.If it is strikingly beautiful and easy to catch, it is probably deadlyPredationAdaptations that help prey species avoid predators:Mimicry: nonpoisonous species have developed similar colorings to poisonous species to discourage predators, even though the prey species is not deadly or poisonousA predator such as a praying mantis that blends in with surrounding plants is better able to surprise its target. However, many prey species also engage in mimicry, developing markings similar to those of unpalatable species so that predators avoid them. For example, harmless viceroy butterflies have similar coloration to monarch butterflies, which store toxins in their tissues, so predators avoid viceroy butterflies. Living in large groups or herds: there is power in numbersPredationBehavioral strategies: attempting to scare off predators by making themselves to appear larger or more dangerous than they really areOptimal foraging strategies enable predators to obtain a maximum amount of net energy per unit of time spent foraging. Predators are more likely to survive and reproduce if they restrict their diets to prey that provide the most energy per unit of handling time and focus on areas that are rich with prey or that are close together. PredationAdvantagesDisadvantagesPredators kill the sick, weak, old and least fit members of prey species; supporting natural selection / survival of the fittestFeeding of predator species and familyControlling of prey populationsMembers of prey species are harmed or killedPoor poor bunny! ParasitismParasitism: occurs when one species (the parasite) feeds on the body of, or the energy used by, another organism (the host), usually by living on or in the host. In this relationship, the parasite benefits, and the host is usually harmed, though not immediately killed.Parasite is usually smaller than the host, and rarely kills the host, primarily only weakening the hostParasites can promote biodiversity by increasing species richness and they help control host populationsMutualismMutualism: occurs when two species behave in a way that benefits both by providing each with food, shelter, or some other resource; unintentional cooperation between species as a result of traits obtained through natural selection.Ex: birds that ride on the backs of elephants remove and eat parasites and pests from the animals body and often make noises to warn the larger animals when predators approach. Ex: gut-inhabitant mutualism, where vast armies of bacteria in the digestive system help break down foods, and in return, receive shelter and food from their host.

CommensalismCommensalism: an interaction that benefits one species but has little, if any, effect on the other.Ex: Birds benefit by nesting in trees, generally without affecting the trees in any way.Ex: Plants (epiphytes such as some orchids and bromeliads) that attach themselves to tree trunks in order to anchor themselves closer to sunlight, water, and air, without benefiting or harming the tree to which it is attached.Section 6.4: Community Response to Changing Environmental ConditionsEcological Succession: The gradual change in species composition of a given area in response to changing environmental conditions (fire, climate change, deforestation, etc)Two types of ecological succession:Primary succession: involves the gradual establishment of communities in originally lifeless areas where there is no soil in a terrestrial community or no bottom sediment in an aquatic community.Ex: bare rock exposed by a retreating glacier or sever soil erosion, newly cooled lava, an abandoned highway or parking lot, newly created shallow pond or reservoirUsually takes a long time (thousands or millions of years) because it takes many MANY years for natural processes to produce fertile soilSecondary succession: Occurs when a community has been cleared by a disturbance that has not destroyed the soilEx: abandoned farmlands, burned or cut forests, heavily polluted streams, flooded landUsually only takes a few weeks for existing seeds or newly deposited seeds to germinate and begin to replenish species

Intermediate Disturbance Hypothesis: fairly frequent but moderate disturbances lead to the greatest species richness. WHY???Such disturbances can create new conditions that can harm or eliminate some species, while releasing nutrients and creating unfilled niches for others, promoting population control, and increasing the complexities of food webs for energy flow and nutrient cycling.

Climax Community: the balance of nature; non-existent because mature, late-successional communities are in a constant state of continual disturbance and changePrecautionary Principle: When substantial preliminary evidence indicates that an activity can harm human health or the environment, we should take precautionary measures to prevent or reduce such harm even if some possible cause-and-effect relationships have not been fully established scientificallyCommonsense principles such as, An ounce of prevention is worth a pound of cure, Better safe than sorry, Slow down for speed bumps.Cost-risk analysis: is the risk of ________ worth the price/cost of _____________?

Section 6.5: Limiting Factors of Population GrowthFour Variables that govern population size:BirthsDeathsImmigration: arrival of individuals from outside the populationEmigration: departure of individuals from the population

Population change = (Births + Immigration) (Deaths + Emigration)

Limiting factors of Population Growth:Age structureCarrying capacity Reproductive patternsNatural population controls

Age Structure of a population: the proportion of individuals at various ages that has a strong impact on how rapidly the population increases or decreasesPre-reproductive stage: organisms not old enough to reproduce (majority = future increase in population)Reproductive stage: organisms capable of reproduction (majority = immediate increase in population)Post-reproductive stage: organisms too old to reproduce (majority = decrease in population)Carrying capacity: the maximum population of a given species that a particular habitat can sustain indefinitely without being degraded; as populations near carrying capacity, resources such as food, water and space begin to dwindle

Reproductive Patterns: help ensure the survival of speciesHigh-rate population increase: r-selected species / Opportunists Have many offspring, usually small, with little to no parental care or protection, when conditions are favorable or when a disturbance opens up a new niche or habitat opportunity.Overcome population loss by having large quantities of offspring so that the few that actually do survive will reproduce many more offspring.Ex: algae, bacteria, rodents, annual plants, insectsLow-rate population increase: k-selected species / CompetitorsReproduce later in life and have a small number of offspring with fairly long life spansTypically, offspring develop inside mothers (where they are safe), are born fairly large, mature slowly, and are cared for and protected by one or both parents (and in some cases by families or herds) until they reach reproductive ageEx: Most large mammals, birds of prey, and large long-lived plantsNatural population controls on human species:Potato famine in Ireland (1845), when a fungus destroyed the potato crop, around 1 million people died and 3 million people migrated to other countries.Bubonic plague in Europe (14th century), where rats infested with the epidemic spread through densely populated cities and killed at least 25 million peopleAIDS virus (current), where humans contracting Human immunodeficiency virus (HIV) can no longer naturally fight off infections and die Since 1980, over 25 million people have died from AIDS, and an average of 3 million people per year (6 per minute) die from AIDS.

What do you think?

So far, technological, social, and other cultural changes have extended the earths carrying capacity for the human species. We have increased food production and used large amounts of energy and matter resources to occupy normally uninhabitable areas. As humans spread into other areas, they interact with and attempt to control the populations of other species. Some say we can keep expanding our ecological footprint indefinitely mostly because of out technological ingenuity. Others say that sooner or later we will reach the limits that nature always imposes on populations.