What questions do ecologists ask about communities?

1. What questions do ecologists ask about communities? How many species? How do they compare in abundance? Who eats who? Structure How do changes in abundance of one species translate into changes in other species? Dynamics How does energy flow through trophic levels? How are nutrients cycled and retained? Function

2. What are communities? • Set of all populations in an enclosed area • Movement of plants and animals and multiple scales of organization complicate definition

3. Measures of community structure • Trophic structures • Relative abundances • Species numbers

4. Trophic Structure • Trophic structure • Is the feeding relationships between organisms in a community • Is a key factor in community dynamics

5. Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Zooplankton Herbivore Primary producers Plant Phytoplankton Figure 53.12 A terrestrial food chain A marine food chain • Food chains • Link the trophic levels from producers to top carnivores

6. Humans Smaller toothed whales Baleen whales Sperm whales Elephant seals Leopard seals Crab-eater seals Squids Fishes Birds Carnivorous plankton Copepods Euphausids (krill) Phyto-plankton Figure 53.13 Food Webs • A food web • Is a branching food chain with complex trophic interactions

7. Juvenile striped bass Sea nettle Fish larvae Fish eggs Figure 53.14 Zooplankton • Food webs can be simplified • By isolating a portion of a community that interacts very little with the rest of the community

8. Limits on Food Chain Length • Each food chain in a food web • Is usually only a few links long • There are two hypotheses • That attempt to explain food chain length

9. The energetic hypothesis suggests that the length of a food chain • Is limited by the inefficiency of energy transfer along the chain • The dynamic stability hypothesis • Proposes that long food chains are less stable than short ones

10. 6 6 No. of species 5 5 No. of trophic links 4 4 Number of species Number of trophic links 3 3 2 2 1 1 0 0 Low Medium High (control) Productivity Figure 53.15 • Most of the available data • Support the energetic hypothesis

11. Keystone Species • Keystone species • Are not necessarily abundant in a community • Exert strong control on a community by their ecological roles

12. With Pisaster (control) 20 15 Number of species present 10 Without Pisaster (experimental) 5 0 1963 ´70 ´71 ´73 ´64 ´65 ´69 ´66 ´72 ´67 ´68 (b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity. (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. • Field studies of sea stars • Exhibit their role as a keystone species in intertidal communities Figure 53.16a,b

13. 100 80 60 Otter number (% max. count) 40 20 0 (a) Sea otter abundance 400 300 Grams per 0.25 m2 200 100 0 (b) Sea urchin biomass 10 8 6 Number per 0.25 m2 4 2 0 1972 1985 1989 1993 1997 Year (c) Total kelp density Food chain after killerwhales started preyingon otters Food chain beforekiller whale involve-ment in chain Figure 53.17 • Observation of sea otter populations and their predation • Shows the effect the otters haveon ocean communities

14. Bottom-Up and Top-Down Controls • The bottom-up model of community organization • Proposes a unidirectional influence from lower to higher trophic levels • In this case, the presence or absence of abiotic nutrients • Determines community structure, including the abundance of primary producers

15. The top-down model of community organization • Proposes that control comes from the trophic level above • In this case, predators control herbivores • Which in turn control primary producers

16. 100 75 50 Percentage of herbaceous plant cover 25 0 0 100 200 300 400 Rainfall (mm) Figure 53.20 • Long-term experiment studies have shown • That communities can shift periodically from bottom-up to top-down

17. Species in communities vary widely in abundance One or a few common species with many many rare species Important concept: Rare species can be important in communities: many weak interactions can lend stability Important concept: Some species there by accident

18. Patterns of Rarity • Most species common somewhere • Source-sink dynamics lead to “spill-over” into nearby habitats and communities • Some species rare in all environments • Low growth rate or highly specialized life history

19. Species numbers • The species number of a community • Is the variety of different kinds of organisms that make up the community • Has two components

20. Species numbers vary widely across communities Forest birds Vascular plants in deciduous forests Vascular plants in fir forests

21. Species richness • Is the total number of different species in the community • Species diversity • Is the total number of different species weighted by their relative abundance

22. A B C D Community 1 A: 25% B: 25% C: 25% D: 25% Community 2 Figure 53.11 A: 80% B: 5% C: 5% D: 10% • Two different communities • Can have the same species richness, but a different species diversity

23. A community with an even species abundance • Is more diverse than one in which one or two species are abundant and the remainder rare