LIFESCI 7B Lecture Notes - Lecture 9: Herbivore, Population Ecology, Heterotroph
10 Jun 2018
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Week 9
25.3: The Carbon Cycle; Ecology, Biodiversity, and Evolution
Food webs trace the cycling of carbon through communities and ecosystems.
● community The set of all populations found in a given place.
○ All of the organisms within a given habitat (e.g. woodland pond)
○ Autotrophs (“self feeders”) that synthesize the organic molecules needed for growth and
reproduction from CO2 are considered primary producers (e.g. plants/algae)
■ Convert inorganic molecules to organic molecules w/ photosynthesis
○ Heterotrophs (other feeders) obtain the carbon they need for growth and reproduction from the
foods they eat, and they also gain energy by respiring food molecules consumers
■ Primary consumers (herbivores/grazers) consume primary producers (heterotrophs)
● Return some carbon back into environment via respiration
■ Secondary consumers: predators/scavengers that feed on primary consumers
● Carnivores: A monophyletic group of animals that consume other animals.
● Also return some carbon back into the environment via respiration
○ Eventually, having passed from one consumer to another, the carbon originally fixed by
photosynthesis is returned to the atmosphere by the respiration of decomposers that break
down dead tissues (e.g. fungi, bacteria)
● Food chain: transfer of carbon from one organism to another; track passage of carbon atoms through
biological carbon cycle
○ carbon cycle underpins the ecological structure of biological communities
○ food web A map of the interactions that connect consumer and producer organisms within the
carbon cycle; the movement of carbon through an ecosystem.
■ provides a better sense of the complexity of biological interactions within the carbon
cycle; defines interactions between organisms within a habitat
● By continually recycling materials, biogeochemical cycles, which involve both biological and physical
processes, sustain life over long intervals.
○ In their absence, life could hardly have persisted for 4 billion years
Biological diversity reflects the many ways that organisms participate in the carbon cycle.
● biodiversity=product of evolution, but shaped/sustained by ecological interactions among organisms
and between organisms and the physical environment
● immense diversity of photosynthetic organisms found today does not reflect evolutionary variations in
the biochemistry of photosynthesis so much as it does structural and physiological adaptations
○ adaptations allow the effective gathering of light, nutrients, and—critical to life on land—water, in
widely varying local environments
● As photosynthetic organisms have adapted structurally and physiologically to local environments
across the globe, consumers have adapted by means of locomotion, mouth and limb specialization,
perception, and behavior to obtain their food
The carbon cycle weaves together biological evolution and environmental change through Earth history.
● algae gained an ecological foothold as nutrients such as nitrate became more widely available
● single-celled eukaryotic heterotrophs expanded their reach by gathering food in ways not possible for
bacteria, ingesting cells and other types of particulate food.
● Photosynthesis and respiration not only cycle carbon, they cycle oxygen (and water) as well.
○ the history of atmospheric oxygen is closely tied to the workings of the carbon cycle through
time.
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○ Burial of some organic carbon generated by photosynthesis in sediments O2 accumulate in
oceans and atmosphere facilitate increase in O2 through time
● Rather than responding passively to environmental change, LIFE can drive environmental chagnes!
○ E.g. evolution of woody plants large drop of atmospheric CO2
■ Evolution of trees increased size of carbon resovoir on land and ushered in new
mechanism for removing carbon from air and ultimately to sedimentary rocks (burial of
plant material on land and forming coal)
○ Humans adding carbon to atmosphere at high rate, warming earth (greenhouse effect);
● life’s diversity in terms of function, phylogeny, and ecology are inextricably intertwined
○ Earth’s remarkable biodiversity reflects the ecology and evolution of populations that interact to
cycle carbon through the biosphere
47.1: The Niche
● a population persists only in places where its members can tolerate the physical environment
● The habitat occupied by a species usually contains other species, and the distribution of each species
will also reflect interactions with the others
○ Competition between species can occur when different species use the same resources in the
same place
○ Distinct species can coexist within shared habitat only if they use different resources, if they are
active at different times, or if the local populations of one or both have so few individuals that
resources are not limiting to their survival and reproduction
■ species persist only when other species that benefit their growth or reproduction are
also present
The niche is a species’ place in nature.
● niche The combination of traits and habitat in which a species exists; combo of physical habitat and
ecological role within said habitat
○ multidimensional habitat that allows a species to practice its way of life--combo of habitat’s
requirements for survival and reproduction as well as role a species plays in its community
○ Determined by abiotic and biotic factors, which lead to adaptations (products of natural
selection), which enable different species to coexist by exploiting different combinations of
resources
● Physical (abiotic): climate, soil chemistry
● Biotic: interaction with other species
■ At the same time, species can also affect resources and other species around them
● Niches are NOT habitats (not interchangeable)
○ Niches have a dual nature: They reflect both where organisms occur and what they do there.
■ Also refers to the ways that organisms respond to, and also affect, the resources and
other species found in the habitat
The realized niche of a species is more restricted than its fundamental niche.
● fundamental niche The full range of climate conditions and food resources that permit the individuals
in a species to live.
○ In nature, however, many species do not occupy all the habitats permitted by their anatomy and
physiology because of interactions with other species that can reduce the range actually
occupied through competition or predation (and other types of interaction between species).
○ E.g. tree frogs can reproduce in ponds near forests all throughout eastern NA
● realized niche The actual range of habitats occupied by a species.
○ E.g. tree frogs CAN reproduce in ponds near forests all throughout eastern NA, but are
excluded from ponds w/ bigger frogs that take up more food resources
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■ Realized niche=ponds near forests WITHOUT bigger frogs (realized niche determined
by competition w/ other species)
○ When species compete, the fitness of BOTH species declines, as resources used by
competitors are not available to help other individuals’ fitness
Niches are shaped by evolutionary history.
● Niches of species are narrow in at least one or often several ways
○ E.g. diets of lizard and bird species are specific despite larger range of resources available in
habitats
● Usually, some aspect of the niches of closely related species is similar evolutionarily conserved
○ E.g. diets of Anolis lizard alike in that they all prey on small live prey
○ phylogenetic niche conservatism The observed similarity in closely related species of some
aspect of their niches, indicating its presence in their common ancestor.
■ commonly seen and speaks to the strong influence of evolutionary history in shaping at
least some aspects of the niches of organisms.
47.2: Antagonistic Interactions Between Species
● interactions with other species play key roles in the distribution and abundance of populations (see
fundamental vs. realized niches)
○ Interactions narrow geographic boundaries of species and help shape realized niches
● All interactions combine costs (often in resources) and benefits (usually increases in reproduction)
○ Direct interactions, species physically interact (e.g. plants and pollinators)
○ Indirect interactions, species influencing eachother through competition for a shared resource
(e.g. food)
○ Benefits usually one sided, where at least one participant loses more than it gains.
■ E.g. one organism consumes the other or competes for resource
Limited resources foster competition.
● Fewer resources than individuals seeking them competition
● Comeptition: an interaction in which the use of a mutually needed resource by one individual or group
of individuals lowers the availability of the resource for another individual or group
○ Competition for space, food, mates
○ intraspecific competition Competition within (individuals of a single) species.
■ Results from intrinsic growth rate and environmental carrying capacity
● Increased population density increased intraspecific competition=main reason
why population growth slows as a species’ environment appraoches carrying
capacity
○ interspecific competition Competition between individuals of different species.
● Whether competition occurs between individuals of one species or two, it is a lose–lose situation: Each
side spends energy it would not spend in the absence of the other
○ “Struggle for existence” (between or within species) as primary driver of natural selection
evolution of traits that aid survival and reproduction by reducing competition in one way or
another
Competitive exclusion can prevent two species from occupying the same niche at the same time.
● In general, when two species have overlapping niches, one will either become extinct in that place or
change its niche
○ competitive exclusion The result of an antagonistic interaction in which (competition results in)
one species is prevented from occupying a particular habitat or niche.
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