Two things

2. Two things • Elephants And • The new book

3. Chapter ( - continued - Population and community structure and food web structure

4. Are there food web structures that are more stable than others? • What is stability? • A resilient vs a resistant community • Resilient: returns rapidly to former state • Resistant: little change • A fragile vs a robust stability • Fragile: remains unchanged but alters completely in large disturbance • Robust: remains roughly the same in the face of larger disturbances • Stability: in the face of disturbance • Disturbance – loss of one or more populations from a community

5. Keystone species • removal of this species would lead to significant changes throughout the food web, producing a community with a very different species composition • one whose impact is disproportionately large relative to its abundance • Can occur at any trophic level (not just predators)

6. Conventional wisdom: increased complexity = increased stability • But mathematical models do not support this argument • Conclusions differ depending on whether we focus on individual populations within a community or on aggregate properties of the community (such as their biomass or productivity)

7. Food webs… • Number of species they contain • Connectance of the web (fraction of all possible pairs of species that interact directly with one another) • Average interaction strength between pairs of species • Increases in # of species, increases in connectance, and increases in average interaction – all tend to decrease the tendency of individual populations within the community to return to their former state following a disturbance (resilience) • Thus – community complexity leads to population instability

8. What is found in real communities – and not in models? • Keep in mind that: • The only communities we can observe are those that are stable enough to exist • Data on interaction strength for whole communities are unavailable – so assume contant • Recent studies: • Connectance may decrease with species number …or… • May be independent of species number … or … • May even increase with species number • Conclusion? • Stability argument does not receive consistent support from food web analyses either

9. Summary of this chapter • there are multiple determinants of the dynamics of populations • Dispersal, patches and metapopulation: movement can be vital factor in determining and/or regulating number • There are temporal patterns in community composition • No predator-prey, parasite-host or grazer-plant pair exists in isolation

10. Chapter 10

11. -- review -- • Every population exists within a web of interactions • with other populations • across several trophic levels • Each population must be viewed in the context of the whole community • Populations occur in patchy and inconstant environments in which disturbance and local extinction may be common

12. …to add… • Need to appreciate that the world’s biological diversity is becoming increasingly important • Need to understand why species richness varies widely across the face of the Earth • Why do some communities contain more species than others? Are there patterns or gradients in biodiversity? If so, why?

13. Species richness… • The number of species in a community • Why is it difficult to count or list the species in a community? • Taxonomic inadequacies • Only a proportion of the organisms can usually be counted • Number of species recorded… • Depends on the number of samples that have been taken or on the volume of the habitat that has been explored • But…the most common species are likely to be represented in the first few samples… more samples -> rarer species will be added • ? – at what point does one stop taking further samples? • Until the number of species reaches a plateau • Species richness of different communities should be compared only if • They are based on the same sample sizes • Area of habitat explored • Time devoted to sampling • Number of individuals included in the samples

14. Diversity indices • Species richness • two communities with ten species…but • Diversity indices • Combines both species richness and evenness or equitability of the distribution of individuals among those species • Calculated by determining for each species, the proportion of individuals or biomass that that species contributes to the total in the sample • Diversity then increases with equitability, and for given equitability, diversity increases with species richness

16. Add to those factors… • (1) interspecific competition • If a community is dominated by interspecific competition, the resources are likely to be fully exploited • Species richness will then depend on the range of available resources (specialists and niche overlap) • (2) predation • Exerts contrasting effects • Predators can exclude certain prey species  community can be less than fully saturated (unexploited resources)  predation may reduce species richness • Or predation may keep species below k – reducing intensity and importance of competition for resources  more niche overlap and a greater richness of species

17. …and other factors… • Factors that vary spatially • Productivity • Predation intensity • Spatial heterogeniety • Environmental ‘harshness’ • Factors that vary with time (temporal variation) • Climatic variation • Disturbance • Evolutionary age

18. Spatially varying factors Productivity and resource richness • For plants: productivity of the environment can depend on most limiting element (nutrient or condition) • Animals: broadly speaking, productivity of environment for animals follows the same trends as for plants • Why? • Due to changes in resource levels at the base of the food chain

19. Productivity and richness • Expect species richness to increase with productivity • Supported by an analysis of species richness of trees in North America in relation to a measure of available environmental energy, potential evapotranspiration (PET) • Amount of water that under prevailing conditions would evaporate or be transpired under a saturated surface • Water and energy • Energy: heat and light • Higher energy inputs lead to more ET and more water • Southern African trees – species richness increased with water availability (annual rainfall) but first increased and then decreased with available energy • What about animals? • Animal species richness positively correlated with crude atmospheric energy

21. Productivity and species richness • Why? • Not sure… but • For an ectotherm (eg – a reptile) – extra atmospheric warmth  increase intake and use of food resources • For an endotherm (eg – bird) – extra warmth  less use of resources in maintaining body temp and more resources for growth and reproduction • In both cases  faster individual and population growth  larger populations • Warmer environments might -> allow species with narrower niches to persist and such environments may support species

22. Productivity and species richness • May also be a direct relationship between animal species richness and plant productivity

23. Strong positive correlations between species richness and precipitation for both seed-eating ants and seed-eating rodents; more species of very large ants and very small ants. Why? • Species richness of fish also increases with lake’s phytoplankton productivity • Note: increase in production does not always lead to increase in diversity

24. Predation intensity • Predation may increase richness • By allowing competitively inferior species to coexist with their superiors (predator-mediated coexistence) • Predation may reduce richness • Intense predation may drive prey species to extinction • Overall: predation intensity and species richness: humped relationship • Greatest richness at intermediate intensities

25. Removed starfish from an 8 m long and 2 m deep shoreline • Other species became dominant (barnacles and then mussels) • Later – reduction in species richness • Predator-mediated coexistence

26. Wolves… • The gray wolf was reintroduced to Yellowstone National Park in the spring of 1995, after a 70-year absence. In the past 6 years, the population has grown from 31 released animals to more than 100 individuals, as wolves have exploited an abundant elk population. Consequently, elk that had previously experienced significant mortality primarily in the late winter because of starvation now face mortality throughout the year. When an elk is killed by wolves, its carcass is partially consumed by the wolves and then is scavenged extensively by eight other carnivore species (coyote, bald eagle, golden eagle, grizzly bear, black bear, raven, magpie, and red fox) and less intensely by up to 20 other species. Field observations indicate that the infusion of wolf-killed ungulate carrion throughout the year has created an abundant and dependable food source for these other carnivores.

27. Spatial heterogeneity • Environments – more heterogeneous – more potential for species richness. Why? • Some studies: positive relationship between plant diversity and heterogeneity • Substrate…slope… drainage … soil pH • Most studies: related species richness of animals to structural diversity of plants in their environment • Due to experimental manipulation of plants • Or through comparisons of natural communities that differ in plant structural diversity • Or plant species richness

28. Environmental harshness • Environments dominated by extreme abiotic factor (harsh environments) more difficult to recognize than apparent • Anthropocentric view. What is cold for us might seem benign to a penguin • ‘let the organism decide’ harshness factor • Environment extreme – if organisms, by their failure to live there, show it to be so. Circular definition. • Environment extreme: one that requires, of any organism tolerating it, a morphological structure or biochemical mechanism that is not found in most related species and is costly, either in energetic terms or in terms of the compensatory changes in the biological processes of the organism that are needed to accommodate it • Example: plants living in highly acidic soil (low pH). Need specific structures/mechanisms • Harsh environments: low species richness • Hot springs. Caves. Highly saline water (Dead Sea) • Each also categorized by low productivity and low spatial heterogenity

29. Temporally varying factors • Climatic variation • Is the variation predictable? (relative time scales) … • temporal niche differentiation • Opportunities for specialization in a non-seasonal environment • Is the variation unpredictable? – climatic instability • Could increase species richness …or not • Does species richness increase as climatic variation decreases? • Disturbance • Environmental age: evolutionary time

31. Temporally varying factors • Disturbance • Reminder: In a dominance-controlled community  community succession • Very frequent disturbances: keep most patches in early stages of succession (few species) + rare disturbances allow patches to be dominated by best competitors (few species) • Conclusion: intermediate disturbance hypothesis • All communities are subject to disturbances that exhibit different frequencies and intensities • Environmental age: evolutionary time

32. Temporally varying factors • Environmental age: evolutionary time • Communities may differ in species richness because some are closer to equilibrium and therefore more saturated than others • Some argued that tropics are richer than temperate in part because tropics have existed over long and uninterrupted periods of evolutionary time (temperate regions – recovering from glaciations) • But tropical areas were also disturbed during the ice ages by associated climatic changes that saw tropic forest limited to a small number surrounded by grassland • Another explanation: species evolve faster in the tropics because of higher rates of mutation in these warmer climes • Rates of evolution of woody plant species twice as fast in the tropical species

33. Gradients of species richness • Habitat area and remoteness: Island biogeography • Latitudinal gradients • Gradients with altitude and depth • Gradients during community succession

34. Island biogeography • Number of species on islands decreases as island area decreases. Species-area relationship • What is an island? • Islands of land in a sea of water • Lakes - Islands in a sea of land • Mountaintops – high altitude islands in a low altitude ocean • Gaps in a forest canopy – island in a sea of trees • Islands of particular geological types, soil types, or vegetation types

35. Island biogeography • Species-area relationships: one of the most consistent of all ecological patterns • Is the impoverishment of species on islands more than would be expected in comparably small areas of mainland? • Why do larger areas contain more species? • Encompass more different types of habitat – not enough of an explanation • Island size and isolation: number of species on an island determined by a balance between immigration and extinction; this balance is dynamic; and extinction rates may vary with island size and isolation

36. Island biogeography • The number of species on an island should eventually become roughly constant through time • This should be a result of continual turnover of species, with some becoming extinct and others immigrating • Large islands should support more species than small islands • Species number should decline with the increasing remoteness of an island - MacArthur and Wilson’s theory’s predictions

37. Latitudinal gradients • Increase in species richness from the poles to the tropics • Why? • No clear explanation. • Latitudinal gradient intertwines components previously discussed…

38. Gradients with altitude and depth • Decrease in species richness with altitude – akin to that observed with latitude - -in terrestrial environments • Other studies: increase with altitude; other studies: hump-shaped patterns • Productivity and temperature? Productivity and growing season? Stress with extremes? • In aquatic environments: change in species richness with depth strongly similar to terrestrial gradient with altitude

39. Skip – patterns in taxon richness in the fossil record

40. Summary • Richness and diversity • Productivity and resource diversity • Predation intensity • Spatial heterogeneity • Environmental harshness • Climatic variation • Disturbance • Environmental age • Island biogeography • Gradients in species richness