Populations and Communities

1. Populations and Communities September 11, 2013

2. Tell me what you know about… • Populations • Communities • Feeding relationships within a community • Roles organisms play in their communities

3. Organismal ecology: habitat • Habitat = the environment in which an organism lives • Includes living and nonliving elements • Scale-dependent: from square meters to miles • Habitat use = each organism thrives in certain habitats, but not in others • Habitat selection = the process by which organisms actively select habitats in which to live • Availability and quality of habitat are crucial to an organism’s well-being • Human developments conflict with this process

4. Organismal ecology: niche • Niche = an organism’s use of resources and its functional role in a community • Habitat use, food selection, role in energy and nutrient flow • Interactions with other individuals • Specialists = species with narrow niches and very specific requirements • Extremely good at what they do, but vulnerable to change • Generalists = species with broad niches that can use a wide array of habitats and resources • Able to live in many different places

5. Which one is a generalist? A specialist?

6. Levels of ecological organization • Population ecology = investigates the quantitative dynamics of how individuals within a species interact • Community ecology = focuses on interactions among species • Ecosystem ecology = studies living and nonliving components of systems to reveal patterns

7. Describe the levels you see here!

8. Species interactions • Species interactions are the backbone of communities • Most important categories: • Competition = both species are harmed • Predation, parasitism, and herbivory = one species benefits and the other is harmed • Mutualism = both species benefit

9. Competition • Competition = relationship where multiple organisms seek the same limited resources they need to survive: • Food - Water • Space - Shelter • Mates - Sunlight • Intraspecific competition = between members of the same species • High population density = increased competition • Interspecific competition = between members of 2 or more species • Leads to competitive exclusion or species coexistence

10. Results of interspecific competition • Competitive exclusion = one species completely excludes another species from using the resource • Species coexistence = neither species fully excludes the other from resources, so both live side by side • This produces a stable point of equilibrium, with stable population sizes • Fundamental vs. realized niche • Species adjust to minimize competition by using only a part of the available resource – resource partitioning

11. Population characteristics All populations show characteristics that help scientists predict their future dynamics

12. Population characteristics • Population size • Population density = the number of individuals within a population per unit area • High densities make it easier to find mates, but increase competition, and vulnerability to predation • Low densities make it harder to find mates, but individuals enjoy plentiful resources and space

13. Population characteristics • Population distribution (dispersion) = spatial arrangement of organisms within an area • Random – haphazardly located individuals, with no pattern • Uniform – individuals are evenly spaced due to territoriality • Clumped – arranged according to availability of resources • Most common in nature

14. More characteristics of populations • Sex ratios • Moon snails (!) • Age structure (distribution) • Birth & death rates • Survivorship curves • Type I = humans • Type II = birds, iguanas • Type III = fish, toads

15. Some organisms play big roles • Keystone Species =has a strong or wide-reaching impact far out of proportion to its abundance • Removal of a keystone species has substantial ripple effects • Alters the food chain

16. Species can change communities • Trophic Cascade = predators at high trophic levels can indirectly affect populations of organisms at low trophic levels by keeping species at intermediate trophic levels in check • Extermination of wolves led to increased deer populations, which led to overgrazed vegetation and changed forest structure • Ecosystem engineers = physically modify the environment • Beaver dams, prairie dogs, fungi

17. Communities respond to disturbances • Communities experience many types of disturbance • Removal of keystone species, spread of invasive species, natural disturbances • Human impacts cause major change • Resistance = community of organisms resists change and remains stable despite the disturbance • Resilience = a community changes in response to a disturbance, but later returns to its original state

18. Duckweed lab • Question: How does the presence of fertilizer affect the growth rate of duckweed (Lemna minor)? • Materials you may use are on the front table; please plan to use milliliters of fertilizer! • Today you and one partner will develop a plan to answer the question; next class you will set up – greenhouse is available to house plants; microscopes are useful to look at them!