Ecological Communities

1. Ecological Communities We wish to learn: • What is an ecological community and what kinds of interactions take place within it? • How important are the various categories of species interactions, including mutualisms, commensalisms, competition, and predation? • What kinds of interactions among species become important when many species affect one another ? • What consequences do these interactions have for the maintenance of biodiversity?

2. Types of Species Interactions • An ecological community is a group of actually or potentially interacting species, living in the same place • A community is bound together by the network of influences that species have on one another. • There are four main classes of two-way interactions, and many possible pathways of indirect interaction.

3. Types of Species Interactions Type of interaction Sign Effects mutualism +/+ both species benefit commensalism +/0 one species benefits, one is unaffected competition -/- each is neg. affected predation +/- each is pos. affected (includes herbivory, parasitism)

4. Food Webs This river food web illustrates the complex inter-connections between producer organisms at the bottom, and consumers progressively higher in the web. Note there are different organisms at the same feeding or “trophic” level

5. Food Webs and Species Interactions • A food web reminds us of the many ways that species interact with one another. • In addition to the direct, two-way interactions already defined, there are ample opportunities for more subtle, indirect effects, as when species A affects species C via an intermediary, species B. • Such effects may cascade from higher to lower trophic levels (a “top-down” effect”), or from lower to higher (“bottom-up effect”).

6. Keystone Species Concept In ecological communities there are little players and big players. The biggest players of all are referred to as keystone species. A keystone species may be defined as one whose presence/ absence, or increase/decrease in abundance, strongly affects other species in the community. Evidence usually comes from addition or removal experiments. Removal of the keystone in the arch will cause the structure to collapse.

7. Starfish as Keystone Predators Photo by Raymond Seed In the rocky intertidal zone, mussels are superior are occupying space, crowding out other species. Starfish are effective predators of mussels, and thereby make room for other species. http://life.bio.sunysb.edu/marinebio/rockyshore.html

8. Starfish as Keystone Predators A starfish removal experiment in Washington State demonstrated that the mussels are competitive dominants. Starfish predation opens up enough space for various barnacles, echinoderms and other marine invertebrate species to maintain a presence. When starfish are experimentally removed, mussels take over and other species are excluded. A hoard of hungry starfish converge on a mussel bed. A starfish curls its body around the mussel, using its tube feet to pry apart the valves enough to insert its extensible stomach. Digestive enzymes break down the mussels’ muscles, and the starfish consumes its prey.

9. Other Indirect Species Interactions • The keystone species effect is the best-known example of indirect interactions. One cannot have a keystone species effect without the presence of indirect interactions. • Keystone species produce strong indirect effects, out of proportion to their abundance. • If a predator strongly suppresses its prey (e.g., herbivores), one expects the trophic level below (e.g., plants) to benefit. Such top-down trophic cascades are well-known in lakes.

10. Domino Effects - A Conservation Issue • Removal of one species causes other species to be lost from the system • Aka “ripple effect” • When the dodo (a 25 kg pigeon) was exterminated on Mauritius, the tree Calvaria major ceased to recruit. Its seeds needed to be abraded in the dodo’s gizzard to germinate.

11. Mutualistic Interactions • A mutualism is an interaction where both species benefit • facultative mutualisms are beneficial but not essential to either species’ survival • obligate mutualisms are essential to the survival of one or both species Pollination is a classic mutualism. The plant gains through gamete transfer. The pollinator receives a reward of nectar and pollen.

12. The Boran - Honeyguide Mutualism • Example of a facultative mutualism between an African people, and a bird, Indicator indicator. • Borans find bee colonies more quickly when guided by the bird • Honeyguides get larvae, wax, and are in less danger from bees The bird informs on direction, distance to, and arrival at bee colony. Search time is reduced from 8.9 to 3.2 hrs

13. The Ant - Acacia Mutualism • Example of an obligate mutualism between ants and a thorny, small tree of dry tropical forests. • Acacia provides shelter (hollow thorns) and food (nectar, protein bodies) to ant. Ants remove herbivorous insects, vines, others plants, and leaf litter (which reduces risk of fire).

14. The Ant - Acacia Mutualism An acacia ant (Pseudomyrmex ferruginea) sipping nectar from the petiolar nectary of a swollen thorn acacia (Acacia collinsii) in Costa Rica. http://waynesword.palomar.edu/acacia.htm#antleafb.gif

15. The Ant - Acacia Mutualism Plants also supply ants with protein and fat-rich food in the form of “Beltian bodies”, shown here being harvested by ants (arrows) from the tips of newly expanding leaflets of Acacia cornigera (Photo by T.W. Sherry)

16. The Ant - Acacia Mutualism • Newly developing bull’s horns (evolutionarily enlarged thorns) • Filled with a pith that ants easily remove, creating hollow interiors • Ants chew small hole into each thorn for use as home

17. The Ant - Acacia Mutualism • Pseudomyrmex ants provide two services to Acacia trees: • 24-hour patrolling of leaves for protection against insects and mammals • Clearing of plants from ground and from Acacia trees themselves as protection from competitors and fire

18. Commensalism • When one species benefits, and the other neither benefits nor is harmed. • A “+/0” interaction Cattle egrets follow cattle, consuming the insects disturbed by the cows’ movements. It is unlikely that the cows benefit.

19. Summary • Species interactions are complex. • Four main categories of two-way interactions include mutualism, commensalism, competition and predation • Indirect, multi-way interactions include keystone species effects and trophic cacades • Forthcoming lectures on competition and predation will add detail to the rich complexity of interactions • Human-driven extinctions of individual; species can ramify to affect many others.