Interspecific Interactions and the Ecology of Communities Chapter 57

1. Interspecific Interactions andthe Ecology of CommunitiesChapter 57

2. Biological Communities • Community • Sets of species that occur together in repeated units through time and space. • Typically characterized by dominant vegetation. • Vary in ecological attributes: • Species richness - Number of species present • Species diversity • Primary productivity - Amount of energy produced • others Cypress Swamp Pine flatwoods Freshwater marsh

3. Biological Communities • Two views of structure and functioning of communities • Individualistic concept: a community is nothing more than an aggregation of species that happen to occur together at one place. Same combination of abiotic factors results in presence of same set of species. • Holistic concept: a community is an integrated unit; superorganism – more than the sum of its parts. Biotic interactions more important. If one species is present, it will increase enhance options for others. • Most ecologists today favor the individualistic concept • In communities, species respond independently to changing environmental (=abiotic) conditions • Community composition changes gradually across landscapes

4. Biological Communities • Ecotones: places where the environment changes abruptly – from one community (and ecosystem) to another. • Ecotones can be areas of higher diversity and higher density of some species. Species use edges to take advantage of attributes of adjacent communities. Attributes may be biotic (trees to perch in) or abiotic (water for osmoregulation).

5. Ecological Niche • Niche: the “ecological space” occupied by a species in the community. The sum total of all the ways an organism uses the resources of its environment • Space utilization (height off ground) • Food consumption (size) • Temperature range • Appropriate conditions for mating • Requirements for moisture and more • “an n-dimensional hypervolume” + + y x - + z (Three dimensional graph) - -

6. Resource partitioning among sympatric lizard species Use different niches to avoid direct competition 6

7. Ecological Niche • Interspecific competition • Occurs when two species attempt to use the same resource and there is not enough resource to satisfy both (resource is limited) • Interference competition • Physical interactions over access to resources • Exploitative competition • Consuming the same resources

8. Ecological Niche • Fundamental niche • Entire niche that a species is capable of using, based on physiological tolerance limits and resource needs • Realized niche • Actual set of environmental conditions, presence or absence of other species, in which the species can establish a stable population • Other causes of niche restriction • Predator absence or presence • Absence of pollinators • Presence of herbivores

9. Ecological Niche J.H. Connell’s classical study of barnacles

10. Ecological Niche • Competitive Exclusion Principle • If two species are competing for a limited resource, the species that uses the resource more efficiently will eventually eliminate the other locally • G.F. Gause’s classic experiment on competitive exclusion using three Paramecium species shows this principle in action

11. Resource partitioning is often seen in similar species that occupy the same geographic area • Thought to result from the process of natural selection • Character displacement • Differences in morphology evident between sympatric species • May play a role in adaptive radiation What interspecific interaction is occurring?

12. Species Interactions: Predator–Prey • Predation • Consuming of one organism by another • Predator strongly influences prey populations • And vice versa!

13. Plants adapt to predation (herbivory) by evolving mechanisms to defend themselves • Chemical defenses: secondary compounds • Oils, chemicals to attract predators to eat the herbivores, poison milky sap, and others • Herbivores coevolve to continue eating the plants Poison-dart frogs of the family Dendrobatidae produce toxic alkaloids in the mucus that covers their brightly colored skin

14. Mimicry allows one species to capitalize on defensive strategies of another • Resemble distasteful species that exhibit warning coloration • Mimic gains an advantage by looking like the distasteful model • Batesian mimicry • Mimics look like distasteful species • Müllerian mimicry • Several unrelated but poisonous species come to resemble one another

15. More Species Interactions • Symbiosis • 2 or more kinds of organisms interact in more-or-less permanent relationships • Potential for coevolution • Three major types of symbiosis • Commensalism • Mutualism • Parasitism

16. Species Interactions • Ecological processes can have unexpected interactive effects • Predation can have a positive impact on populations -- keeps population away from carrying capacity -- reduces competition that can increase diversity • Predator choice depends partly on relative abundance of the prey options • Superior competitors may be reduced in number by predation • This allows other species to survive when they could have been outcompeted Starfish eat barnacles, allowing other species to thrive instead of being crowded out by the explosive population of barnacles

17. Species Interactions • Keystone species • Species whose effects on the composition of communities are greater than one might expect based on their abundance • Sea star predation on barnacles greatly alters the species richness of the marine community • Keystone species can manipulate the environment in ways that create new habitats for other species • Beavers • Gopher tortoise

18. Succession • Communities have a tendency to change from simple to complex over time • Primary succession occurs on bare, lifeless substrate (new beach in FL) • Secondary succession Occurs in areas where an existing community has been disturbed but organisms still remain • Field left uncultivated • Forest after a fire • Organisms gradually move into an area and change its nature

19. Succession and Disturbance • Three dynamic concepts in the process • Tolerance: early successional species are characterized by r-selected species tolerant of harsh conditions • Facilitation: early successional species introduce local changes in the habitat. K-selected species replace r-selected species later in succession • Inhibition: changes in the habitat caused by one species inhibits the growth of the original species. K-selected plant species can out compete r-selected species for light.

20. Succession and Disturbance • Animal and plant species at a disturbed site can also change over time • Krakatau island • Volcanic eruption • Faunal changes occur in synchrony with the vegetation • Changes in animals affect plant occurrences; pollination, animal dispersion

21. Succession and Disturbance • Intermediate disturbance hypothesis • Communities experiencing moderate amounts of disturbance will have higher levels of species richness than communities experiencing either little or great amounts of disturbance • Patches of habitat will exist at different successional stages • May prevent communities from reaching the final stages of succession • Disturbance (fire, hurricanes etc.) is common, rather than exceptional in many communities • Understanding the role that disturbances play in structuring communities is an important area of ecology • In many cases, disturbance resets succession back to an earlier state.