Introduction to disturbance: lecture topics

1. Introduction to disturbance: lecture topics • What is stability, how is it defined, and what kinds of stability are there in biological communities? • How is stability related to diversity of ecological communities? • How do ecologists reconcile divergent views of the relationship between diversity and stability? • What is the intermediate-disturbance hypothesis, and how is it related to community diversity?

2. David Tilman’s Minnesota grassland study is one of most celebrated tests of relationship between diversity & stability…what does it tell us? • Experimental study of grassland (prairie) plots • Replicated, well designed, 2m2 plots • Controlled numbers and types of species planted • Followed plots for long time periods (years) • Results? (See Figure 17-10, Stiling text, next slide; & more recent work) • Total community biomass fluctuation decreased with diversity (thus diversity promotes biomass stability) • Fluctuations in population (biomass): no connection stability with diversity (fluctuations increase with div.)

3. What is community stability? • Intuitively, stability means that the population sizes and number of species remain constant over time (“equilibrium”) • Components of stability • Resistance defined as the force needed to change the community • Resilience defined as the ability of the community to return to prior state (equilibrium) after perturbation • Elasticity = how quickly community returns to equilibrium • Amplitude = how much disturbance community can tolerate, and still return to some kind of equilibrium • Stability may be global (applies to entire system or community) or local(applies to response to low-amplitude disturbances); confer prior discussions

4. Are community diversity and stability related? • Charles Elton (1958) proposed that the diversity of biological communities promotes their stability • Accordingly, impact of disturbance would be cushioned by large number of species • This is a “balance of nature” view, held widely among lay people • Elton’s arguments in favor? • Island communities (not very diverse) often vulnerable to invasions, a kind of instability • Invasive exotic species (pests) often become abundant in disturbed (e.g., cultivated) habitats • Tropical rainforests (extremely diverse) tend to have few insect outbreaks, few exotic species

5. However, Elton’s argument of diversity-stability link has been challenged: • Many examples of exotic species known today that invade continents, national preserves, even forests • Alternative explanations for invasion of cultivated habitats by exotics: Biological control agents not present (& other simple communities--salt-marshes, mangroves--not susceptible to invaders) • Even tropical communities experience biological disturbances • Herbivore pest outbreaks have been observed • Disease epidemics can wipe-out trees, other spp. • Locust (& other insect) outbreaks well known • Mathematical studies of food webs (Robert May): no relationship of stability to diversity

6. Some support for idea that diversity promotes stability exists: • Addition of species via addition of trophic levels tends to destabilize populations in experimental (and natural) communities • Effects of buffalo grazing in E. Africa savanna (biomass reduction) greater the less the diversity, supporting diversity->stability link • Study of Yellowstone grassland communities (Frank and McNaughton) showed that resistance increased with diversity (next slide; Fig. 17-9, Stiling) • Resistance defined as inverse of summed component population changes, before versus after a drought • Diversity measured by Shannon index (Hs)

7. Resistance to change as a function of species diversity in Yellowstone National Park grassland ecosystem

8. Result of Tilman’s (Minesota grassland) study: Ecosystem stability increases with species richness, but not population processes

9. More recent results of Tilman’s (et al.) work (next slide): • Aboveground grassland plant biomass (per plot) and total biomass both increase with the number of species in the plot, and the trend becomes stronger with time (2001 Science paper--see reference next slide) • Multiple species plots have more biomass than the most successful monoculture plots • Thus ecosystem (community-wide) characteristics of plots do appear to depend on the number of species present, thus diversity does seem to affect ecosystem-level measures of stability • No evidence that diversity influences stability of individual species (which compete, & are independent)

10. (Tilman et al., 2001, Science 294: 843-846)

11. Intermediate-disturbance hypothesis • Hypothesis proposed by Joseph Connell (1978): • At high disturbance levels only a few species can persist, which have special adaptations to survive • At low levels of disturbance, again a few species predominate in community, namely those that can out-compete all the others in a stable environment • At intermediate levels of disturbance, both kinds of species exist, leading to higher levels of diversity Hi Species diversity Lo Disturbance frequent-------------------------> infrequent Soon after disturbance-------------------------> long after Disturbance large-------------------------------> small

12. Tests of intermediate-disturbance hypothesis • Sousa (1979), intertidal invertebrates on boulders, CA coast • Small boulders (frequently disturbed) had mean of 1.7 spp. (disturbance tolerant) • Intermediate-sized boulders mean of 3.7 spp. • Large, sessile boulders dominated by 2.5 K-selected species • Hiura (1995) beech forests, Japan (next slide): species diversity (Hs) highest at intermediate levels of disturbane (interval between windstorms) • Work of Reice (see Kareiva text reading, discussed in class)

13. Test of Intermediate-disturbance hypothesis (Hiura 1995)

14. Doesstabilitypromotediversity?? • Maybe there is a relationship, but it is not cause-and-effect, as Elton suggested, but rather relatively stable environments (like tropical rainforests, coral reefs, deep ocean trenches) allow the evolution of ecological specialists, which allows greater number of species coexisting • Jury is out on this (see lecture on latitudinal diversity gradients) • Work of Sherry (1984) on tropical rainforest flycatchers: Insectivores are far more specialized ecologically than any temperate insectivores yet studied; this would promote small niches, many spp. • Long-tailed flycatcher = specialist on stingless bees (67%), ruddy-tailed flycatcher specialist on fulgoroid Homoptera • Homogeneous diets among individuals suggests environment is predictable from perspective of these rainforest interior animals

15. A topic related to community stability is concept of keystone species • Define keystone species as any species whose impact on, or role in a community is proportionally greater than its actual relative abundance, biomass, or energy flow • Distinguish keystone species from dominant species, which (latter) is simply a particularly important species in community as measured by relative abundance, biomass, or energy flow

16. Examples of different types of keystone species • Keystone predator (e.g., sea otter “O”, starfish Pisaster “P”, bass “B”); keystone parasite such as distember virus (“VD”), possibly nuclear polyhedrosis virus • Keystone prey (e.g., tropical fruit such as figs & palm nuts that sustains animals in dry season) • Keystone habitat modifier (e.g., beaver, prairie dog, gopher tortoise)

17. The importance of keystone species from perspective of community stability is that they can stabilize or destabilize community out of proportion to their abundance.

18. Conclusions: • Intermediate disturbance hypothesis is reasonably well supported in a number of species, and emphasizes role of disturbance in promoting and maintaining species diversity in ecological communities • Relationship between diversity and stability not well established • Species diversity does not generally promote greater species and population stability • Species diversity does promote ecosystem stability (at least in grassland communities) • Stability may promote diversity more than vice versa, but this idea is not well tested yet, not yet widely accepted (evolutionary ecological idea)