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Species distributions What determines the distribution and abundance of a species in space and time? 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 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. 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 Grinnell’s niche (1917) • Habitat concept; emphasis on where the species lives Elton’s niche (1927) • Functional concept; emphasis on the trophic position Black rhino White rhino N-dimensional Hutchinsonian niche • Multi-dimensional space of resources that a species needs to survive and reproduce • Functionalized the niche concept 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 The niche concept Useful conceptual framework; but… Niche alone cannot explain all patterns of species distribution and abundance: • Source – sink populations Unsuitable habitat Sink population Suitable habitat Source population The niche concept • Also, some favorable habitats may be uninhabited just by chance (historical factors) The niche concept • Additionally, temporal fluctuations in the environment can determine species presence/absence 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 What limits a species’ geographic range? What restricts a species’ fundamental niche to the realized niche? • Physical environment • Disturbance • Biological interactions Tolerance curve and gradients • Gradients can represent physical, disturbance, or biological factors Physical limiting factors Temperature • Saguaro distribution limited by low temperatures Physical limiting factors Temperature • Bark beetles overwintering temperature limits • Climate change Physical limiting factors Temperature • Eastern phoebe winter range (food limited) Physical limiting factors Precipitation (moisture) • Orographic precipitation and vegetation gradients Physical limiting factors Soil Moisture • Xerophytes – Full sunlight in dry soils • Mesophytes – Wetter and more shaded environments Physical limiting factors Solar radiation – light saturation S Physical limiting factors Solar radiation – light limitation Physical limiting factors Wind Timberline and wind • Determined by temperature at broad scales • Wind critical local limiting factor 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. Disturbance Example: 1896 • Fire suppression in the Patagonian steppe allows Austrocedrus chilensis expansion F.P. Moreno 1986 T.T. Veblen Biological interactions Competition (negative-negative) Some definitions • 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 Biological interactions Competition • Kangaroo rats in the southwest Biological interactions Competition • Plant allelopathy (interference) • Most commonly influences fine scale distributions Black walnut Biological interactions Predation (positive-negative) • Broad definition also includes herbivory, parasitism Biological interactions Predation - herbivory • Predator distribution influenced by prey (resource) Biological interactions Predation • 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 Biological interactions Predation Parasitism • 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 Biological interactions Mutualism (positive-positive) • Can be obligate or nonobligate • Obligate mutualisms have stronger effects on species distributions Acacia-ant mutualism Biological interactions Mutualism: mistletoe and its seed dispersers Yellow fruits disperser: mockingbird Green fruits disperser: marsupial 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 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 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. Resolution of the Paradox • Non-equilibrium conditions • Temporal variation in environmental conditions • Disturbances • Spatial heterogeneity of resources • Biological interactions – Complex “webs” of biological interactions 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) Interaction among factors • Temperature, precipitation, solar radiation influencing the life zones on a mountain environment Interaction among factors • Predator mediated coexistence • Keystone species (Paine 1969) 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. Relative importance of facilitation versus competition is dependent on environmental setting Michalet et al 2006