Unformatted text preview: Chapter 4
How ecosystems change How ecosystems change Ecosystems: How They Change Ecosystems: Dynamics of natural populations Evolution as a force for change Ecosystem response to disturbance Jcurve = (exponential growth curve) Jcurve = (exponential growth curve) Scurve = (logistic growth curve) Population growth curves Population Equilibrium Population
A dynamic balance between births and deaths.
Births Deaths Jcurve or Scurve Jcurve or Scurve Population growth curves Is it an issue of Scale? Population equilibrium Carrying capacity: maximum population the environment can sustain without resource degradation over the longterm. Density Dependence and Critical Numbers Numbers Factors of environmental resistance are either: Critical number: the lowest population level – densityindependent: effect does not vary with population density; e.g., adverse weather – densitydependent: effect varies with population density; e.g., infectious disease for survival and recovery Population Dynamics Population Environmental resistance: combination of biotic and abiotic factors that limit population increase. Biotic potential: combination of biotic and abiotic factors that enhance population increase. Reproductive strategies rselection unpredictable environments, not much emphasis on competition because the envir. is likely to change traits of rselection species: high fecundity, small body size, early maturity onset, short generation time, and the ability to disperse offspring widely Reproductive strategies Kselection stable or predictable environments, populations are very constant and close to carrying capacity (K), fecundity, large body size(?), late maturity onset, longlived, and more limited disperse more parental care traits of Kselection species: low Mechanisms of Population Equilibrium (a trophic cascade) Topdown: population size is controlled by predation, higher trophic levels regulate lower. Bottonup: population size is controlled by resource scarcity, lower trophic levels (resources) regulate higher trophic levels Predator-prey Balance: Wolves and Moose (top-down) and Plant-Herbivore Dynamics (bottomPlant-Herbivore up)Reindeer on St. Matthew Island
No regulatory control (predation) on herbivores Exponential growth pattern Overgrazed habitat (resource depletion) Massive dieoff of herbivores Lessons To be Learned About Predator-Prey Balance Predator-Prey Absence of natural enemies allows a herbivore population to exceed carrying capacity which results in overgrazing of the habitat. The herbivore population subsequently crashes. The size of the herbivore population is maintained so that overgrazing or other overuse does not occur. The Human Presence The Three Human revolutions populations increased – Paleolithic (early hominids) tools fire 2.5 million yrs BCE 10,000 yrs BCE – Neolithic agriculture – Industrial mechanics and fossil fuels Other biotic interactions and population dynamics
Parasites and disease can lower a prey’s condition Parasites and disease can lower a prey’s condition and increase predation – Largely density dependent Keystone species: single species that maintains ecological structure through a critical interaction Keystone Species Keystone Predation – Population regulation Ecological engineers – species presence alters conditions or creates resources necessary for other species to occur Crayfish/Herpetofauna Commensalism Commensalism Important fossorial engineers
– 2949 km/ha – 82 m tons/ha/yr Herpetofaunal refugia 1m Competition Interspecific competition: competition among individuals of different species Intraspecific competition: competition among individual of the same species Competition: Interspecific Competition:
Grassland contain plants with both fibrous and tap roots Coexist by accessing resources from different soil levels What happens when we move species (introduce species)? species
A new player in an old game Think competition, predation, disease and parasites in the longterm Think in terms of environmental resistance and biotic potential Rabbits Overgrazing in Australia Rabbits Phillip Island 1978 Largely devoid of vegetation Phillip Island 1988 After Rabbit Removal Phillip Chestnut blight in United States Chestnut Zebra Mussels Zebra Water Hyacinths Water Kudzu Kudzu Introduced Species: Introduced
Biological control: using biotic interactions to control invasive species. Native predators, disease. Unforeseen consequences – Bart the Mother – Episode Number: 206 Season Num: 10 First Aired: Sunday September 27, 1998. Evolution as a Force for Change Evolution Natural selection: Charles Darwin & Alfred Russell Wallace
– More offspring are produced than survive (Differential reproduction) – Advantage to individuals with better fit to the environment. – Over time this will lead to changes in species – Selection for breeds (e.g., dogs) Mechanisms of Species Adaptation Adaptation Change through natural selection. – Selective pressure determines which organisms survive and reproduce and which are eliminated Recipe for Change Recipe
+ IRO ENV ENT NM ADAPTATIONS NATURAL SELECTION: For? or Against? Adapt natural selection (time) Limits of Change (powerful selective pressure, e.g., habitat degradation) degradation) Migration move to a location where there is less pressure Extinction/Extirpation Vulnerability of different organisms to environmental changes. Which Species Will Survive In A Rapidly Changing Environment? Rapidly Panda Narrow Distribution Small population Low genetic variation Large size Small # of offspring Limited migration Mouse Wide distribution Large population High genetic variation Small size Large # of offspring High migration Speciation (allopatric speciation) Geographic separation: population populations that do not interbreed with one another. Separated populations are exposed to different selective pressures. Reproductive isolation Reproductive isolation Reproductive Premating isolating mechanisms – Temporal isolation. Time of reproductive activity is different – Ecological isolation. Mate in preferred habitat which is different from other populations – Behavioral isolation. The wrong signals are given – Mechanical isolation. Copulation is attempted, but transfer of sperm does not take place Speciation: Foxes Speciation: Speciation: Galapagos Finches Speciation: Adaptive radiation: rapid evolutionary
radiation characterized by an increase in the morphological and ecological diversity of a single, rapidly diversifying lineage rapidly Drifting Continents: Pangea 225 Million Years Ago Million Earth’s Crust: 14 Tectonic Plates Plates ...
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