Exploring Tropical Biodiversity: Patterns and Factors

1. Chapter 23: Biodiversity Robert E. Ricklefs The Economy of Nature, Fifth Edition

2. Biodiversity • The tropics are a storehouse of biological diversity unrivalled anywhere on the planet: • this fact was brought to light by the work of the great explorer-naturalists of the nineteenth century -- Darwin, Bates, Wallace, and others • estimates of global biological diversity range from 10 to 30 million species or more: • most of these species occur in the tropics (many are small insects!) • thus far, fewer than 2 million species have been catalogued worldwide

3. Tropical Biodiversity Within most groups of organisms, numbers of species increase markedly toward the equator: consider the ants: • at 60oN, we might find 10 species in a small region • at 40oN, between 50 and 100 species • within 20o of the equator, between 100 and 200 species consider breeding birds: • in Greenland, we find 56 species • in New York state, 105 species • in Guatemala, 469 species • in Colombia, 1,395 species

4. Contour lines on the map indicate the # of species of nearshore and continental-shelf bivalves found at locations w/ in the contour intervals

6. Why so many in the Tropics – and so few toward the poles? • Biologists hold two views on the subject of high biodiversity in the tropics: • diversity increases without limit over time: • tropical habitats, being older than temperate and arctic habitats, have had time to accumulate more species • But with integration of population ecology into community theory… diversity reaches an equilibrium at which factors adding species balance factors removing species: • factors adding species weigh more heavily, or factors removing species weigh less heavily, in the tropics

7. Patterns of Diversity Large-scale patterns of diversity vary on a regional basis according to several key factors: • these factors can be seen to operate on the number of mammals and other animals in 150 x 150 mile blocks in North America: • suitability of physical conditions (fewer amphibians in the xeric west) • heterogeneity of habitats (more mammals in the western mountains than in eastern North America) • isolation from centers of dispersal (fewer mammals as one moves down the Baja California peninsula)

11. Vegetation structure determines local diversity. • The number of bird species tends to increase somewhat with increasing productivity within a temperate zone region: • however, the principal determinant of bird diversity seems to be structural diversity of vegetation: • 6 species in grasslands (areas of 5-20 ha) • 14 species in shrublands • 24 species in floodplain forests • the MacArthurs quantified this observation by relating bird diversity to foliage height diversity in the 1960s

13. Marshes are very productive but are structurally uniform and have relatively few species of plants • Desert vegetation is less productive than marsh vegetation but its greater variety of structure makes room for more diversity • == inverse relationship between productivity and species diversity

14. Vegetation structure may be more important than primary productivity in determining diversity Sonoran Desert of Baja California 

15. Vegetation structure may be more important than primary productivity in determining diversity Salt marsh in MA == inverse relationship between productivity and species diversity

16. Birds. Birds. • Structural complexity and diversity have always gone together for bird-watchers and naturalists • the MacArthurs quantified this observation by relating bird diversity to foliage height diversity in the 1960s • Plotted diversity of birds observed in different habitats according to diversity in foliage height, a measure of the structural complexity of the vegetation

18. Diversity is correlated with overall energy input • Potential evapotranspiration (PET) is a good predictor of diversity over large regions: • PET is the amount of water that could be evaporated from the soil and transpired by plants under prevailing conditions of temperature and humidity: • this index integrates temperature and solar radiation (energy input) for a given system • Why this relationship holds is poorly understood: • sharing of more energy by more species? • larger populations less likely to go extinct?

21. Diversity has both regional and local components. • Diversity can be measured at a variety of spatial levels: • local diversity (alpha diversity) is the # of species in a small area of homogeneous habitat • regional diversity (gamma diversity) is the total # of species observed in all habitats within a barrier-free geographic area

22. Beta diversity measures turnover in species composition. • Consider two extremes of compositional segregation by habitat: • if each species occurred in each habitat with a region, local diversity would equal regional diversity • if each habitat had a unique biota, then regional diversity would be the sum of all local diversities • Beta diversity measures turnover in species composition from one habitat to the next within a region.

24. Computation of Beta Diversity • One measure of beta diversity is the # of habitats within a region divided by the average number of habitats occupied per species: • thus, regional diversity = local diversity x beta diversity • consider the island of St. Lucia, West Indies: • 9 habitats (grassland, scrub, lowland forest, mangroves, etc.) • 15.2 species of birds/habitat (local diversity) • each species occupies on average 4.15 of the 9 habitats • beta diversity = 9 habitats/4.15 habitats = 2.17 • regional diversity = 15.2 species x 2.17 = 33 species

25. Local communities contain a subset of the regional species pool. • The species that occur within a region are referred to as its species pool: • each local community is a subset of the species pool • what determines whether a given member of the species pool can be a member of a given community? • the species must be able to tolerate the conditions of the environment and find suitable resources (these conditions must fall within the fundamental niche of the species) • the species must also be able to persist in the face of competitors, predators and parasites (where the species is successful in this respect constitutes the realized niche)

26. Species Sorting • The membership of a species within a local community is determined: • partly by its adaptations to conditions and resources • partly by competitive and other interactions with species • Thus species from the regional pool are sorted into different communities based on their adaptations and interactions, a process called species sorting: • this process may be demonstrated experimentally (read ecologists in the field)

27. Ecological Release For a given range of habitats, species sorting (and beta diversity) should be greatest where the regional species pool is largest: • when the species pool is smaller (perhaps because of geographic isolation) • competition should be relaxed • species should expand into habitats normally filled by other species, a process called ecological release

28. Ecological release • Comparing levels of diversity in islands and neighboring continental regions • Islands: • fewer species • island species have greater densities • Island species expand into habitats normally filled by other species on the mainland • Collectively referred to as: ecological release

29. Evidence for Ecological Release • Evidence from 7 continental areas and islands of various sizes in the Caribbean basin illustrate the process of ecological release: • as regional numbers of birds increased: • habitat breadth and local abundance decreased • local diversity and turnover of species between habitats (beta diversity) increased

32. Diversity & Niche Relationships • A niche represents the range of conditions and resource qualities within which an individual or species can survive and reproduce: • the niche is multidimensional • overlap of niches of two species determines how strongly the two species might compete with each other

33. Diversity & Niche Relationships • Every community can be thought of as having a total niche space within which the niches of all species must fit: • adding or removing species may result in compression or expansion of the realized niches of other species • communities with different numbers of species may differ with respect to: • total community niche space • degree of niche overlap among species • niche breadths of individual species

34. Diversity reflects the relationship between species niche and total community niche spacehorizontal: an ecological resource that defines the total niche space (eg: average size of prey items); height of curve – intensity of use of resource

35. How could more species be added to a community? • (how to move from condition A) • A community could accommodate more species by: • an increase in total niche space (with no change in breadth or overlap) (condition B) • Niche space refers to variety of resources and not amount of resources • an increase in niche overlap (with no change in breadth or total space) (condition C) • Average productivity of each species would decline due to increased sharing of resources – all things equal • a decrease in niche breadth (with no change in total space or overlap) (condition D) • Average productivity would decline since each species would have access to a narrower range of resources

36. Each curve represents a species’ nichehorizontal: an ecological resource that defines the total niche space (eg: average size of prey items); height of curve – intensity of use of resource

37. Competition, Diversity & the Niche • intense competition leads to exclusion of species from the community • Thus many ecologists have argued that in communities with high diversity, competition must be weak: (condition D: narrower niche + reduced niche overlap) • what mechanisms might lead to reduced interspecific competition? • greater ecological specialization (narrower niches) • greater resource availability (greater niche space) • reduced resource demand (smaller populations) • intensified predation (populations below carrying capacity)

38. Are there more ecological roles in the tropics? • More ecological roles in the tropics could be the result of greater niche space: (condition B) • greater niche space could result from increase in the number of niche axes as well as the length of each: • increase in bird species in the tropics is related to nectar-feeding and insectivory from fixed perches (both rare in temperate zone) • tropics are rich in mammals primarily because of the number of flying mammals (bats), less common at higher latitudes • Nonflying mammals =ly diverse in tropics and temperate • epiphytes and lianas are tropical plant forms generally absent or uncommon in forests at higher latitudes

39. Thus: variation in species diversity is generally paralleled by variation in the functional (or niche) diversity of species

40. One way to assess niche diversity: use the morphology of a species as an indicator of its ecological role In other words: assume that differences in morphology among related species reveal different ways of life Example: size of prey captured varies in relation to body size of the consumer As species diversity increases: total variety of morphology increases

41. Species diversity parallels niche diversity. • Evidence for increasing species diversity with increasing niche diversity comes from a study of morphological diversity in bats: (compared bat communities in temperature and tropical localities) • this diversity was determined using ratios (morphological axes) reflecting type and location of prey utilized and flight characteristics: • the less diverse bat communities in Canada (all small insectivores) had relatively limited variation along these morphological axes • the more diverse bat communities of Cameroon, tropical West Africa, occupied much greater range of morphological space

44. Species diversity parallels niche diversity. • Evidence for increasing species diversity with increasing niche diversity also comes from a study of fish in the Rio Tamesi drainage of east central Mexico: • headwater springs and small streams had relatively few fish representing few niches • farther downstream, additional species were added, increasing the diversity of niches • lower reaches added still more species and diversity of niches

46. Predation and Diversity • Diversity generally appears to increase with higher productivity: • higher productivity results in more energy reaching higher trophic levels, thus supporting larger populations of predators: • increased predation pressure should reduce competition among prey and permit more prey to coexist • increased predation should also promote diversification in mechanisms of prey escape

47. Evidence for Predation Effects on Diversity • Do predators play an indirect role in promoting diversity among prey? • when predators are removed from a community, a common consequence is loss of prey species • the variety of color patterns and resting positions among moths is much higher in diverse tropical communities than it is in temperate latitudes • Predators may play an important role in shaping niche relationships and regulating diversity.

48. Diversity of adaptations for escaping predators is high among moths in the Tropics (moths from the Amazon basin in Ecuador)

49. Equilibrium Theories of Diversity: how do we explain patterns of diversity? • Most ecologists now believe that diversity achieves an equilibrium value at which processes that add species and those that subtract species balance each other: • species are added by: • production of new species • movements of individuals between habitats and regions • species are removed by: • competitive exclusion, efficient predation, bad luck • equilibrium concepts can be applied to islands...

50. Each type of community has an equilibrium number of species, often referred to as the saturation number Similar to how a habitat has a carrying capacity for the population of a particular species This view helps explain what was known about species diversity within local habitats and its places at least part of the problem of species diversity within the domain of ecology