Species distributions

Transcription

1 Species distributions

2 What determines the distribution and abundance of a species in space and time?

3 Mapping and measuring the Range 3 types of range maps 1. Outline maps Display range as an irregular area Limitations: highly generalized, ignore gaps 2. Dot maps Plot points on a map where species have actually been found. Limitations: very limited inference/spatial coverage 3. Contour maps Indicate variation in density and abundance within a range. Limitations: underlying data, interpolation, temporal

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6 The Distribution of Populations Population Growth and Demography Malthusian Theory All organisms have the inherent potential to increase their numbers exponentially. However, because resources limit growth, and many environments are unsuitable, no organisms actually increase indefinitely.

7 Ecological niche Describes the ecological space occupied by an organism Resource space Environmental tolerance Ecological role Helps to understand 1. main factors (environmental, competitive) which limit a population s growth 2. mechanisms/factors that influence species geographic distribution

8 Grinnell s niche (1917) Habitat concept; emphasis on where the species lives

9 Elton s niche (1927) Functional concept; emphasis on the trophic position Black rhino White rhino

10 N-dimensional Hutchinsonian niche Multi-dimensional space of resources that a species needs to survive and reproduce Functionalized the niche concept

11 Fundamental vs. realized niche Fundamental niche : species niche in the absence of any interspecific competition and predation Realized niche: species niche in the presence of competing and/or predator species Reflected in geographic distributions of species

12 The niche concept Useful conceptual framework; but Niche alone cannot explain all patterns of species distribution and abundance: Source sink populations Unsuitable habitat Suitable habitat Sink population Source population

13 The niche concept Also, some favorable habitats may be uninhabited just by chance (historical factors)

14 The niche concept Additionally, temporal fluctuations in the environment can determine species presence/absence

15 What limits a species geographic range? What restricts a species fundamental niche to the realized niche? Liebig s law of the minimum Originally developed for agriculture yield is proportional to the amount of the most limiting nutrient Population is limited by single most limiting factor Oversimplification Interaction among multiple factors

16 What limits a species geographic range? What restricts a species fundamental niche to the realized niche? Physical environment Disturbance Biological interactions

17 Tolerance curve and gradients Gradients can represent physical, disturbance, or biological factors

18 Temperature Physical limiting factors Saguaro distribution limited by low temperatures

19 Temperature Physical limiting factors Bark beetles overwintering temperature limits Climate change

20 Temperature Physical limiting factors Eastern phoebe winter range (food limited)

21 Physical limiting factors Precipitation (moisture) Orographic precipitation and vegetation gradients

22 Soil Moisture Xerophytes Physical limiting factors Full sunlight in dry soils Mesophytes Wetter and more shaded environments

23 Physical limiting factors Solar radiation light saturation S

24 Physical limiting factors Solar radiation light limitation

25 Physical limiting factors Wind Timberline and wind Determined by temperature at broad scales Wind critical local limiting factor

26 Disturbance Fire, volcanism, floods, hurricanes, etc. Great impact on species distributions by killing individuals, changing resource availability & environmental conditions, altering species interactions Also biotic disturbances: insect outbreaks, pathogens, herbivore grazing, etc.

27 T.T. Veblen Disturbance Example: 1896 Fire suppression in the Patagonian steppe allows Austrocedrus chilensis expansion F.P. Moreno 1986

28 Some definitions Biological interactions Competition (negative-negative) Exploitative : using resources and therefore making them unavailable for other species Interference: aggressively denying the use of resources to other species Diffuse competition: one species is affected by multiple species that collectively diminish a shared resource

29 Competition Biological interactions Kangaroo rats in the southwest

30 Biological interactions Competition Black walnut Plant allelopathy (interference) Most commonly influences fine scale distributions

31 Biological interactions Predation (positive-negative) Broad definition also includes herbivory, parasitism

32 Biological interactions Predation - herbivory Predator distribution influenced by prey (resource)

33 Predation Biological interactions Prey distribution influenced by predator/s Yellowstone Lake cutthroat trout (native) & lake trout (invasive) Cutthroat trout populations diminish with introduction of lake trout Brown trout Population structure of cutthroat shifts few reach adult stage

34 Predation Parasitism Biological interactions Alcon blue butterfly larva Fools ants with chemistry by mimicking the surface chemicals that the ants have on their own brood Reduces ant reproductive output

35 Biological interactions Mutualism (positive-positive) Can be obligate or nonobligate Acacia-ant mutualism Obligate mutualisms have stronger effects on species distributions

36 Biological interactions Mutualism: mistletoe and its seed dispersers Yellow fruits disperser: mockingbird Green fruits disperser: marsupial

37 Biological interactions Facilitation (positive-zero or positive-positive) Presence of one species aids another Silene acaulis increases plant species richness & abundance Molenda et al. 2012

38 Biological Interactions Principle of Competitive Exclusion (Gause s Principle) Species cannot coexist in a community if their niches are identical Exclusion takes time Only applies if the ecological factors are constant Many exceptions observed in natural systems

39 The paradox of the plankton, Hutchinson 1961: Phytoplankton communities reveal an astonishing biodiversity, whereas classical competition theory suggests that only a few competing species can survive on a fixed abundance of resources.

40 Resolution of the Paradox Non-equilibrium conditions Temporal variation in environmental conditions Disturbances Spatial heterogeneity of resources Biological interactions Complex webs of biological interactions

41 Interaction among factors Interacting factors may have more extreme consequences on species distributions than any factor alone Most commonly the interaction among multiple factors determine species distributions (and not a single factor alone)

42 Interaction among factors Temperature, precipitation, solar radiation influencing the life zones on a mountain environment

43 Interaction among factors Predator mediated coexistence Keystone species (Paine 1969)

44 Interaction among factors Relative importance of physical and biological limiting factors On harsh physical environments competition is less important than on resource rich environments as a factor affecting species distributions.

45 Michalet et al 2006 Relative importance of facilitation versus competition is dependent on environmental setting