What questions do ecologists ask about communities Structure

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

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

Measures of community structure ● ● ● Trophic structures Relative abundances Species numbers

Trophic Structure ● Trophic structure — — Is the feeding relationships between organisms in a community Is a key factor in community dynamics

● Food chains Quaternary consumers Carnivore — Link the trophic levels from producers to top carnivores Tertiary consumers Carnivore Secondary consumers Carnivore Primary consumers Zooplankton Herbivore Primary producers Plant Figure 53. 12 A terrestrial food chain Phytoplankton A marine food chain

Food Webs ● Smaller toothed whales Baleen whales A food web — Humans Is a branching food chain with complex trophic interactions Crab-eater seals Birds Sperm whales Elephant seals Leopard seals Fishes Squids Carnivorous plankton Copepods Euphausids (krill) Phytoplankton Figure 53. 13

● Food webs can be simplified — By isolating a portion of a community that interacts very little with the rest of the community Juvenile striped bass Sea nettle Fish larvae Figure 53. 14 Fish eggs Zooplankton

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

● 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

Most of the available data — Support the energetic hypothesis Number of species 6 No. of species 5 No. of trophic links 4 5 4 3 3 2 2 1 1 0 0 High (control) Medium Productivity Figure 53. 15 6 Low Number of trophic links ●

Keystone Species ● Keystone species — — Are not necessarily abundant in a community Exert strong control on a community by their ecological roles

● Field studies of sea stars Exhibit their role as a keystone species in intertidal communities Number of species present — 20 With Pisaster (control) 15 10 Without Pisaster (experimental) 5 0 1963 ´ 64 ´ 65 ´ 66 ´ 67 ´ 68 ´ 69 ´ 70 ´ 71 ´ 72 ´ 73 (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. Figure 53. 16 a, b (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.

● Observation of sea otter populations and their predation 80 60 40 20 0 (a) Sea otter abundance 400 Grams per 0. 25 m 2 Shows the effect the otters have on ocean communities 300 200 100 0 (b) Sea urchin biomass Number per 0. 25 m 2 — Otter number (% max. count) 100 10 8 6 4 2 0 1972 1985 1989 1993 1997 Year Figure 53. 17 Food chain before killer whale involvement in chain (c) Total kelp density Food chain after killer whales started preying on otters

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

● 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

● Long-term experiment studies have shown — That communities can shift periodically from bottom-up to top-down Percentage of herbaceous plant cover 100 75 50 25 0 0 Figure 53. 20 100 200 Rainfall (mm) 300 400

Species in communities vary widely in abundance One or a few common species with 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

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

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

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

● 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

● Two different communities — Can have the same species richness, but a different species diversity A B C D Figure 53. 11 A: 25% Community 1 B: 25% C: 25% D: 25% A: 80% Community 2 B: 5% C: 5% D: 10%

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