Populations & Communities

1. Populations & Communities • Population Ecology • Density • Factors affecting density • Community Ecology • Trophic Relationships • Trophic pyramid • Food web • Structure & Local Species Assemblages • Species Interactions

2. Population Ecology • Population density = # of individuals of a species per unit area or volume Intrinsic rate of increase = maximum rate at which a population of a given species can increase under ideal conditions

3. Factors affecting density • Density dependent factors • Density independent factors • Governing forces: • Species interactions • (comp/pred) • Abiotic factors • (tolerances) • Historical factors ~ • dispersal 2-5x increase in densities of Batrachoseps attenuatus on islands in SF Bay over those on mainland due to lower predation pressure on the islands

4. P. metcalfi P. teyahalee

5. Chihuahuan Desert - precipitation ranges from 50 to 400 mm Lizard species’ densities studied over 5 year period, exhibited fluctuation correlated with precipitation.

6. Schall & Pianka 1978 • Compared Aus & N.A. • Turtle & frog richness pos. correlated to annual rainfall, neg. mean annual hrs sunshine • Lizards had opposite pattern • Snakes positively correlated w/ mean annual temp & rain

7. Community ecology Community = All of the organisms that inhabit a particular area; an assemblage of populations of different species living close enough for potential interaction • Trophic Relationships • Trophic pyramid • Food web • Structure & Local Species Assemblages • Species Interactions

8. Structuring forces Gause’s Principle 1960’s & ’70’s: Competition structures communities Ecological niche Current view: physical environment predicts composition

9. trophic pyramid quaternary trophic structure trophic levels tertiary secondary consumer primary consumers primary producers Detritovores - decomposers • Guild =

10. energy flow through ecosystems:much is lost at each trophic level

12. 2) food web:

13. The structure of a community & assemblages • Structure = species diversity, species richness • Diversity = • Richness = • Species assemblages - Subset of species being considered in a community • General trends

14. Gradients in Species Richness Latitude/ Altitude – more species found lower (taxon. widespread) Tropical lowland herp diversity peaks around 150-200 • Some exceptions (salamanders, turtles, lizards(!))

17. Why the trends? • Climactic features vary with increases in latitude • Habitat heterogeneity

18. HABITAT HETEROGENEITY • The greater the range of environmental conditions, the more kinds of topography, soil conditions, microclimate and habitat – the greater the heterogeneity, the greater the biodiversity of a landscape • Physical or Biotic • Spatial or Temporal • Fixed or Dynamic • Higher heterogeneity =

20. What drives these gradients? Many proposed mechanisms: Productivity Historical (lag) Structural diversity Climate (seasonality)

21. Pattern, Process, Mechanism Mechanism- factors affecting individuals (e.g. competition) Process- population-level effects of individual interactions (neg b/w species) Pattern- community level (presence/ absence, resource/microhabitat use, etc.)

22. PPM, cont’d • Cause and effect relationships among these 3 levels often assumed • Attribute negatives between 2 spp to using same prey (interference) or aggression (exploitative) However, different mechanisms can produce indistinguishable patterns

23. PPM, cont’d • Differences in prey use b/w spp • competition, or • independent evolution? • Process may be difficult to detect • Resource-limited years (Dunham 1980) • ‘Ghost of competition past’ • Differentiation, displacement

24. Species Interactions - S1 S2 • Competition • parasitism predation • mutualism - + S1 S2 - + S1 S2 +

25. 1) Interspecific Competition for limited resource • Interactions lead to either decrease in abundance or some component of fitness • the 2 species diverge in their use so the co-existence is possible

26. Interspecific competition results in microhabitat differences • Microhabitat parapatry results from intense competition between P. cinerus & P. shenandoah • P cinerus, more aggressive – P. shenanadoah becoming restricted to dry slopes

27. Species Interactions: Competition • Williams (1983) examined microhabitat use of 9 Anolis • Sun/shade • Perch diameter • Perch height • Reduces interactions – but a result of comp?

28. Ex: Anolis lizard sp. perching sites in the Dominican Republic

29. Alternative explanations? • Environmental tolerances (T, water loss) • Preferred prey • Other researchers have shown determinism in Anolis composition on islands

32. Determinants of Community Structure • Often many interacting factors both abiotic and biotic Competition in Caribbean Anolis: 2-9 species on islands; few predators, lizards abundant- resources limiting Interspecific competition may be alleviated by resource partitioning

33. Anolis competition • Removal of an aggressive species showed habitat expansion by another (Jenssen 1973) • Enclosure experiments revealed greater partitioning b/w a species pair from a more resource-limited island (Pacala & Roughgarden 1985)

34. Predation • Negatively affects prey (they are consumed) • Most predators feed on more than one prey species (dependent upon abundance)

35. Predator and prey populations follow a series of synchronized fluctuations • The prey population grows exponentially, and reproduction in the predator population is a function of the number of prey consumed • As a single predator population increases, the single prey population decreases to a point at which the trend is reversed • The two populations rise and fall, oscillating in a predictable manner

36. Lotka-volterra model -simplest model of predator-prey interactions, predict greater stability w/more “links”

37. Species Interactions: Predation Predation- easier to establish than c Often, more abundant prey species are taken – modify community structure Submergent behavior- prey respond by reducing activity Restrictive activity times, places promote prey-switching in predator mediate competitive coexistence

38. Species Interactions:Parasitism • ~1/2 all animals are parasites often specialized; differential effects • Shift P-P, competitive interactions • Rarely studied at the community level • Pathogens may cause local extinction: • B. boreas in Colorado by Aeromonas

39. Abiotic factors: Habitat complexity • Pianka (1967) Plant height diversity best predicted species richness of flatland desert lizards

40. Abiotic factors: Physiological tolerances • Each species has different tol/ pref • Trade-offs • Distinct vs. shared preferences • Temp, precipitation, ET are often correlated with a species’ range, and its abundance therein

41. Weather-induced community changes • Whitford & Creusere (1977) – fecundity and composition in Chihuahuan Desert lizard community enhanced in wet years • Pechmann et al. (1989) # species metamorphosing from ponds related to hydroperiod • Longer-lived spp’s may endure poor weather conditions better

42. Anthropogenic effects • Dispersal: • Polynesians– gecko introduction • Anglers– introduced salamander bait • Seri- Sauromalus Landscape changes: Maya cultivated much of Yucatan, favoring open-habitat species Local scale: Single-tree harvest in Amazonia promoted heliothermic lizards; forest species retreated (Vitt et al 1998)

43. Prey of Brazilian Colubridae • Vitt & Vangilder (1983) explained resource-use with present-day ecological factors • Cadle & Greene (1993) assert that much of observed prey use was due to ancestry

44. No invertebrate specialists Vitt & Vangilder (1983) Too much competition w/ insectivorous mammals, too many small snake predators, suitable microhabitat lacking Cadle & Greene (1993) Insect eating snakes are rare/ absent in neotropics

45. Anuran-eating snakes are speciose V&V: Convergence on this resource due to frog abundance and year-round availability C&G: Large proportion of the frog-eaters have common ancestry

46. Salamander competition • Hairston (1949) Distributional patterns in GSM suggest inter-specific comp Density manipulations confirmed geog variation in degree of competition

47. Larval amphibian assemblages • Extremely dynamic systems • Size-dependent predation/competition • Composition turnover • Hydroperiod; nutrient flux

48. Spatial partitioning In Thailand, different tadpole species use distinct aquatic zones Heyer (1973)

49. Trade-offs- puddle vs. pond • Selective forces differ depending on larval environment • Fast-drying ponds- promote quick development • Permanent ponds- predatory contingent promotes submergent behavior

50. Predation & Competition • 6 tadpole species at different newt densities (Morin 1983) • Low predation, 4 comp dominant species suppressed 2 inferior tadpoles • High predation, inferiors survived better • 3 species developed faster with heavy predation