Population Growth Curves and Mechanisms

Species and Population Organism Species: genetically similar organisms that reproduce Population: all members of a species in an area

Characteristics of populations: Same species living in same area

Age structure: Prereproductive Reproductive Postreproductive

What limits population growth?

Biotic Potential vs. Environmental Resistance

Biotic Potential an organism’s maximum reproductive rate, provided: unlimited resources are available (food, water, shelter, etc.) freedom from inhibiting factors (no predators or disease, etc.) Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Table 6.2

Biotic potential – capacity for growth Intrinsic rate of increase (r)

Environmental Resistance – Biotic and Abiotic factors limiting population size

Recruitment - ability to survive environmental resistance and become part of the breeding population

Reproductive strategies High biotic potential and low recruitment Low biotic potential and high recruitment

r - strategists K – strategists

Transparency

R strategists: Many small offspring Little /no parental care Early reproductive age Most offspring die Small adults High growth rate (r) Population fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species

K strategists: Fewer, larger offspring High parental care Later reproductive age Larger adults Lower population growth rate (r) Population size fairly stable and close to carrying capacity (K) Specialist niche High ability to compete Late successional species

Growth Curve - Result of biotic potential and environmental resistance.

Exponential growth - (J-curve) before environmental resistance factors kick in to reduce population size

Population explosion

Logistic growth - (S - curve) Balance between environmental resistance and biotic potential

Carrying Capacity - The maximum population a habitat can support without being degraded over the long term.

Carrying Capacity Population Size maximum # individuals Time maximum # individuals of a given species that the environment can support Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.4

Carrying Capacity maximum # individuals of a given species that the environment can support J-curve Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.4

Critical number – minimum population required to provide protection and support for members of population

Calculating Fertility Rates and Doubling Times (CBR - CDR)/10 = Rate of Increase in percentage To get r (intrinsic growth rate), divide by 100 70/ Rate of Increase = Doubling Time

A population had a growth rate of 1. 7 in 2000 A population had a growth rate of 1.7 in 2000. In what year would the population be double its current population?

70 / 1.7 = 41 years 2000 + 41 = 2041

Calculating Fertility Rates and Doubling Times: Practice

Threatened – species whose populations are declining rapidly Endangered – population is near the critical number

Mechanisms of population equilibrium:

Predator – prey dynamics

Snowshoe hare Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.9

Predator-prey Balance:Wolves and Moose

Isle Royale

Isle Royale As Wolves decreased Moose increased. As Wolves increased the Moose decreased. Wolves kept the Moose population stabilized until canine parvovirus probably killed off large #’s of Wolves. Moose prospered until overgrazing of Ash and Aspen caused mass death.

Plant Herbivore dynamics: Herbivores are the natural enemies of plants Overgrazing

Mechanisms of Population Equilibrium: Plant-Herbivore

Introduced Species: Rabbits in Australia Introduced into Australia from England in 1859 No natural enemies – rabbit population exploded Overabundant herbivore population devastated natural vegetation (see Fig. 4-11 in text). Using disease as control measure – why will this procedure fail in the long term?

Eliminating predators upsets basic plant – herbivore relationships.

Competition Interspecific Intraspecific

Density dependent factors affecting population growth: Increasing effect as population increases Examples Competition Disease Parasites Stress due to crowding

Density independent factors controlling population: affect all populations, regardless of density Most are abiotic Forest fires Floods