Chapter 4: Ecosystems and Communities

4.1 Climate

Weather and Climate

• Weather is the day-to-day condition of Earth’s atmosphere
• @@Climate@@ @@refers to the average year-to-year conditions of temperature and precipitation in an area over a long period of time@@
• Environmental conditions can vary over small distances, creating microclimates - environmental conditions within a small area that differ significantly from the climate of the surrounding area

Factors that Affect Climate

• @@Global climate is shaped by many factors, including solar energy trapped in the biosphere@@
  • The greenhouse effect is the process in which certain gases (carbon dioxide, methane, and water vapor) trap sunlight energy in Earth’s atmosphere as heat
  • Greenhouse gases in the atmosphere allow solar radiation to enter the biosphere, but they slow down the radiation of Earth’s heat back to space
• @@Latitude and the transport of heat by winds and ocean currents also help shape global climate@@
  • Earth’s climate zones are produced by unequal distribution of the sun’s heat on the earth’s surface
  • The tropical zone includes the areas near the equator and receives nearly direct sunlight all year
  • Temperate zones and polar zones receive very different amounts of solar energy at different times of the year
    • This variation is because Earth’s axis is tilted, causing solar radiation to strike the Earth at an angle that varies from summer to winter as the Earth revolves around the sun
• @@Earth’s winds and ocean currents interact to help produce climate patterns@@
  • The paths of winds and currents are the result of heating and cooling, Earth’s rotation, and geographic features

4.2 Niches and Community Interactions

The Niche

• Each species has a range of conditions under which it can grow and reproduce
• @@Every species has its own range of tolerance - the ability to survive and reproduce under a range of environmental conditions@@
  • A species has an upper and lower limit of tolerance for every environmental factor for which it cannot survive beyond
  • A species’ habitat - the general place where an organism lives - is determined by its tolerance for environmental conditions
• A niche is the range of physical and biological conditions in which a species lives and the way in which the organism uses those conditions
  • A niche includes the way a species obtains resources (any necessity of life, such as water, nutrients, light, food, and space)

Competition

• @@Competition@@ @@occurs when organisms try to use a limited resource in the same place at the same time as other organisms@@
  • Ex: Plant roots compete for water and nutrients in the soil and animals compete for food, mates, and space
  • Competition can occur among members of the same species (intraspecific competition) and between members of different species (interspecific competition)
• @@The competitive exclusion principle states that no two species can occupy the same niche in the same habitat at the same time@@
  • Direct competition between different species almost always produces a winner and a loser, with the losing species dying out
  • Species usually divide similar resources instead of competing for them

Predation, Herbivory, and Keystone Species

• Predation is an interaction in which one animal (predator) captures and feeds on another animal (prey)
  • Predators can affect the size of prey populations in a community
• Herbivory is an interaction in which one animal (herbivore) feeds on producers (such as plants)
  • Herbivores can affect the size and distribution of plant populations in a community
• Sometimes a population change in a single species can cause dramatic changes in its community; this species is called a keystone species

Symbioses

• Any relationship in which two species live closely together is called symbiosis
• Mutualism is a symbiotic relationship in which both species benefit from the relationship
• Parasitism is a symbiotic relationship in which an organism lives on or inside another organism and harms it
  • Ex: ticks feeding on a mammal’s blood and skin
• Commensalism is a symbiotic relationship in which one organism benefits and the other is neither helped nor harmed
  • Ex: Barnacles attached to, but not harming, a whale

4.3 Succession

Primary and Secondary Succession

• @@Ecological succession@@ @@is a series of gradual changes that occur in a community following a disturbance; as succession proceeds, the number of different species present typically increases@@
• Succession that begins in an area that has no existing community is called primary succession
  • Ex: volcanic explosions and retreating glaciers leave only bare rock - no soil or plant life
• The first species to colonize barren areas are called pioneer species
  • Ex: lichen is a pioneer species that grows on bare rock
• Secondary succession occurs in disturbed areas where parts of previous ecosystems remain
  • Often follows wildfires, hurricanes, or even human activities like logging and farming

Climax Communities

• @@Succession doesn’t always follow the same stages; the end result - the climax community - is not always uniform and stable@@
• In healthy ecosystems, secondary succession often reproduces the original climax community after a natural disturbance
  • However, ecosystems may or may not recover from widespread disturbances caused by humans

4.4 Biomes

The Major Biomes

• Regions with similar latitudes and prevailing winds may have different climates and biological communities due to factors including an area’s nearness to an ocean or mountain range
• Earth’s terrestrial ecosystems are classified into ten groups of regional climate communities called biomes
  • Biomes are described in terms of abiotic factors like climate and soil type, as well biotic factors like plant and animal life
• @@Tropical Rain Forest@@
  • Tall trees form a dense, leafy covering called a canopy, which shades a layer of shorter trees and vines called the understory
  • Abiotic factors: hot and wet year-round; thin, nutrient-poor soils subject to erosion
  • Biotic factors:
  • Plant life: understory plants use large leaves to compete for limited light
  • Animal life: animals are active all year. Many use camouflage to hide from predators
• @@Tropical Dry Forest@@
  • Tropical dry forests grow in areas where rainy seasons alternate with dry seasons
  • Abiotic factors: warm year-round; usually, a period of rain is followed by a long period of drought
  • Biotic factors:
  • Plant life: deciduous plants lose leaves to survive the dry season
  • Animal life: many animals enter long periods of inactivity called estivation (similar to hibernation) to reduce their need for water
• @@Tropical Grassland/Savanna/Shrubland@@
  • Receives more seasonal rainfall than deserts but less than tropical dry forests; mostly grass, with isolated trees and small groves of trees and shrubs
  • Abiotic factors: warm, seasonal rainfall; compact soils, frequent fires set by lightning
  • Biotic factors:
    • Plant life: plant adaptations include waxy leaf coverings and seasonal leaf loss
    • Animal life: many animals migrate during the dry season to find water; some smaller animals burrow and remain inactive during the dry season
• @@Desert@@
  • Deserts have less than 25 centimeters of precipitation annually
  • Conditions vary greatly, depending on elevation and latitude; extreme daily temperature changes
  • Abiotic factors: low precipitation; variable temperatures
  • Biotic factors:
  • Plant life: many plants store water in their tissues and minimize leaf surface area to cut down on water loss
  • Animal life: Many desert animals get water from the food they eat and are active at night to avoid the heat
• @@Temperate Grassland@@
  • Plant communities are mostly grasses maintained by periodic fires and heavy grazing
  • Since their soils are fertile and ideal for growing crops, most have been converted for agriculture
  • Abiotic factors: warm to hot summers; cold winters; moderate seasonal precipitation; occasional fires
  • Biotic factors:
  • Plant life: grassland plants are resistant to grazing and fire
  • Animal life: predation is a threat for smaller animals because of the open environment; thus, camouflage and burrowing are common adaptations
• @@Temperate Woodland and Shrubland@@
  • Large areas of grasses and wildflowers mixed with oak and other trees
  • Fire is a constant threat because of dense, low, oily plants
  • Abiotic factors: hot dry summers; cool moist winters, thin, nutrient-poor soils; periodic fires
  • Biotic factors:
  • Plant life: plants have adapted to drought and have tough, waxy leaves that resist water loss
  • Animal life: animals tend to be browsers that eat varied diets of grasses, leaves, shrubs, and other vegetation
• @@Temperate Forest@@
  • Mostly made up of deciduous and evergreen coniferous trees
  • Soils are fertile and often rich in humus, a material formed from decaying leaves and other organic matter
  • Abiotic factors: cold to moderate winters; warm summers; year-round precipitation; fertile soils
  • Biotic factors:
  • Plant life: deciduous trees drop their leaves in autumn and go dormant in winter
  • Animal life: animals must cope with changing weather; bare trees leave animals exposed in winter
• @@Northwestern coniferous forest@@
  • has a variety of conifers, flowering trees, and shrubs; moss often covers tree trunks and the forest floor
  • Sometimes called a “temperate rain forest” because of its lush vegetation
  • Abiotic factors: mild temperatures; abundant precipitation in fall, winter, and spring; cool, dry summers
  • Biotic factors:
  • Plant life: less diverse than a tropical rain forest, but has ample water and nutrients to support their lush, dense, plant growth; trees here are among the world’s tallest
  • Animal life: camouflage helps insects and ground-dwelling mammals avoid predation
• @@Boreal Forest@@
  • Boreal forests, or taiga, are dense forests of coniferous evergreens found along the northern edge of the temperate zone
  • Abiotic factors: long cold winters, short mild summers; moderate precipitation; high humidity; acidic, nutrient-poor soils
  • Biotic factors:
  • Plant life: a conifer’s shape allows it to shed snow, and its wax-covered, needlelike leaves reduce water loss
  • Animal life: to stay warm, most animals have small limbs and ears, and fat or downy feathers for insulation
• @@Tundra@@
  • The tundra is identified by permafrost, a layer of permanently frozen subsoil
  • In summer, the ground thaws to a depth of a few centimeters and becomes soggy; In winter, the top layer of soil freezes again
  • This cycle of thawing and freezing rips and crushes plant roots; the cold temperatures, high winds, a short growing season, and humus-poor soils also limit plant height
  • Abiotic factors: strong winds; low precipitation; short and soggy summers; long, cold, dark winters; permafrost
  • Biotic factors
  • Plant life: mosses and other plants are low-growing to avoid damage from frequent strong winds
  • Animal life: many animals migrate to avoid long harsh winters, while animals that live in the tundra year-round have adaptations such as natural antifreeze to limit heat loss

Other Land Areas

• @@Some land areas, such as mountain ranges and polar ice caps, are not easily defined and, thus, remain unclassified@@
  • Mountain ranges exist on all continents and in many biomes; conditions such as temperature and precipitation vary with elevation
  • Polar regions border the tundra, are cold year-round, and feature very little plant and animal life

4.5 Aquatic Ecosystems

Conditions Underwater

• Almost three-fourths of Earth’s surface is covered with water
• @@Underwater organisms are affected by water depth@@
  • The short distance that sunlight penetrates through water is called the photic zone, ranging from 200 meters deep (tropical seas) to less than a few meters (swamps)
  • Below the photic zone is the dark aphotic zone, where photosynthesis cannot occur
  • Organisms called benthos live on, or in, rocks and sediments on the bottoms of lakes, streams, and oceans; their habitat is the benthic zone
• @@Underwater organisms are affected by temperature and currents@@
  • Aquatic habitats are warmer near the equator and colder near the poles
  • Temperature also varies with depth
  • Currents can dramatically affect water temperature by carrying water that is noticeably warmer or cooler than normal into a given area
• @@Underwater organisms are affected by nutrient availability@@
  • The type and availability of dissolved substances, such as oxygen, nitrogen, potassium, and phosphorus, varies within and between bodies of water and greatly affects the types of organisms that can survive there

Freshwater Ecosystems

• Only 3 percent of Earth’s surface water is fresh water
• @@Freshwater ecosystems can be divided into three main categories: rivers and streams, lakes and ponds, and freshwater wetlands@@
  • Rivers, streams, creeks, and brooks often originate from underground water sources in mountains or hills
  • Plant and animal life are mainly found downstream
  • Food webs in lakes and ponds often are based on plankton (a general term that includes both phytoplankton and zooplankton) and attached algae and plants
  • Water’s circulation between the surface and the benthos during some seasons distributes heat, oxygen, and nutrients
  • A wetland is an ecosystem in which water either covers the soil or is present at or near the surface for at least part of the year
  • Water may flow or remain still; wetlands help to prevent flooding by absorbing and slowly releasing water
  • Wetlands are often nutrient-rich, highly productive breeding grounds
  • Three main types of freshwater wetlands are freshwater bogs, freshwater marshes, and freshwater swamps

Estuaries

• @@Estuaries are wetlands that form where a river meets the sea@@
• They contain a mixture of fresh water and salt water and are affected by ocean tides
• Many are shallow and support an astonishing amount of biomass
  • Estuaries serve as breeding and nursery grounds for many ecologically and commercially important fish and shellfish species

Marine Ecosystems

• @@Ecologists divide the ocean into zones based on depth and distance from shore.@@

• Organisms in the @@intertidal zones@@ are subjected to regular and extreme changes in temperature

  • At high tide, they are submerged in seawater and often battered by waves and currents
  • At low tide, they are exposed to air and sunlight

• The @@coastal ocean@@ extends from the low-tide mark to the outer edge of the continental shelf (the relatively shallow border that surrounds the continents)

  • Water here is brightly lit and often supplied with nutrients by freshwater runoff from land, making coastal oceans highly productive
  • Kelp forests and coral reefs are two important coastal communities

• The @@open ocean@@ begins at the edge of the continental shelf and is divided into the photic zone and the aphotic zone

  • The photic zone typically has low nutrient levels and supports only the smallest species of phytoplankton; however, most photosynthesis on Earth occurs in the sunlit top 100 meters of the open ocean
  • The permanently dark aphotic zone includes the deepest parts of the ocean and its food webs are usually based on organisms that fall from the photic zone