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This case study explores the decline and recovery of Southern Sea Otters, an important keystone species. It discusses population dynamics, carrying capacity, and the impacts of environmental resistance on population growth. The study also examines different population change curves and reproductive patterns.
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Environmental Science Population Ecology
Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? • They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback. Figure 8-1
Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? • Sea otters are an important keystone species for sea urchins and other kelp-eating organisms. Figure 8-1
POPULATION DYNAMICS AND CARRYING CAPACITY • Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2
(a) Clumped (elephants) Fig. 8-2a, p. 162
(b) Uniform (creosote bush) Fig. 8-2b, p. 162
(c) Random (dandelions) Fig. 8-2c, p. 162
Limits on Population Growth: Biotic Potential vs. Environmental Resistance • No population can increase its size indefinitely. • The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. • Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.
Exponential and Logistic Population Growth: J-Curves and S-Curves • Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8-4
Exponential and Logistic Population Growth: J-Curves and S-Curves • As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8-4
Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size • Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8-6
Density Dependent Population Control This occurs when the density of the population controls the total population of individuals in a species. • Stress • Disease • Competition for resources • Lack of space
Density Independent Population Control • This is when a population is controlled by natural events other than population density. • fires • floods • earthquakes • hurricanes • volcanoes • drought
Types of Population Change Curves in Nature • Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. • Stable: exhibits dynamic equalibrium. • Irruptive: when populations explode and crash. • Cyclic: populations fluctuate up and down • Irregular: erratic changes.
Types of Population Change Curves in Nature • Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7
Reproductive Patterns:Opportunists and Competitors • Large number of smaller offspring with little parental care (r-selected species). • Fewer, larger offspring with higher invested parental care (K-selected species). Figure 8-9
r-Selected Species Cockroach Dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Fig. 8-10a, p. 168
K-Selected Species Elephant Saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Fig. 8-10b, p. 168
Survivorship Curves: Short to Long Lives • The populations of different species vary in how long individual members typically live. Figure 8-11
