What questions do ecologists ask about communities? Structure Dynamics Function How many species? How do they compare in abundance? Who eats who? How do changes in abundance of one species translate into changes in other species? 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 Trophic structures ● Relative abundances Relative abundances ● Species numbers 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 Tertiary consumers Secondary consumers Primary consumers Primary producers Carnivore Herbivore Plant Carnivore Zooplankton Phytoplankton A terrestrial food chainA marine food chain Figure — Link the trophic levels from producers to top carnivores

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

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

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 High (control) Medium Low Productivity No. of species No. of trophic links Number of species Number of trophic links Figure 53.15

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 Figure 53.16a,b (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. With Pisaster (control) Without Pisaster (experimental) Number of species present ´64´65 ´66 ´67 ´68 ´69 ´70´71 ´72 ´73 (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 Figure Food chain before killer whale involve- ment in chain (a) Sea otter abundance (b) Sea urchin biomass (c) Total kelp density Number per 0.25 m Grams per 0.25 m 2 Otter number (% max. count) Year Food chain after killer whales started preying on otters — Shows the effect the otters have on ocean communities

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 Figure Rainfall (mm) Percentage of herbaceous plant cover

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

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 Community 1 A: 25%B: 25%C: 25%D: 25% Community 2 A: 80%B: 5%C: 5%D: 10% D C B A Figure 53.11

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