Exam Thursday If you did not do as well as you hoped on Exam #1: 1. Make sure to work through the practice problems, lecture problems, problems from the first exam, and lab problems 2. Make an appointment to meet with me and come prepared with specific questions What determines a species geographic range? Geographic range of the Crenulated grasshopper Cordillacris crenulata (Bruner) Geographic range of the coastal redwood (Sequoia sempervirens)
What determines a species geographic range? Hypotheses Acting Over Ecological Time Scales 1. Dispersal limitation 2. Tolerance to environmental variables Hypotheses Acting Over Evolutionary Time Scales 3. Availability of genetic variation 4. Degree to which gene flow swamps adaptation 5. Speciation Hypothesis 1: Dispersal limitation Although the species could live here, it simply
cannot get here New species forms here Can colonize here Hypothesis 2: Tolerance to environmental variables New species forms here Although the new
species can easily disperse to this habitat, it simply cannot survive when it gets there The second hypothesis has been formalized by the concept of the Ecological Niche Niche The total range of conditions and resources under which the individual or population lives and replaces itself. (Hutchinson, 1957) Niche Performance
of species 1.2 1 0.8 Reproduction 0.6 Individual growth 0.4 Survival
3 30 Real niches are multi-dimensional Niche Minimum Temperature Precipitation The species niche defines a set of suitable habitats Minimum Temperature
Niche Precipitation Summarizing the two ecological hypotheses Although the species could live here, it simply cannot get here New species forms here Can
colonize here Although the new species can easily disperse to this habitat, it simply cannot survive when it gets there How can we distinguish between them?
Possibility 1: Experimental manipulation Possibility 2: Statistical estimation Experimental manipulation Experimental manipulation What experiments would you conduct? How would you interpret the possible outcomes? Statistical estimation Statistical estimation Frog Present?
Precipitation Minimum Temp. 0 5mm 2C 0 6mm 3C
0 4mm 1C 0 5mm 2C 0 4mm
3C 0 7mm 4C 1 11mm 10C 1 12mm
8C 1 11mm 7C 1 13mm 9C 1
12mm 8C 1 10mm 8C How could you use this data to gain insight into the frogs niche? Using the t-test to identify the niche Frog Presen t?
Precipi tation Minim um Temp. 0 5mm 2C 0 6mm
3C 0 4mm 1C 0 5mm 2C 0
4mm 3C 0 7mm 4C 1 11mm 10C 1
12mm 8C 1 11mm 7C 1 13mm 9C
1 12mm 8C 1 10mm 8C Lets first focus on precipitation 0=5.17 1=11.50 20 =1.37 2
)( 6+ 6 66 ) Because the value of our test statistic, t, is greater in magnitude than the critical value, 2.228, areas where the frog lives are significantly wetter than areas where the frog does not live. Thus, we tentatively conclude that precipitation is an important part of the frogs niche Limitations of this simple statistical approach Does not produce a predictive model Sensitive to correlations among environmental variables
We can improve our ability to predict and understand the niche by using more sophisticated statistical approaches Even more sophisticated statistical approaches can be misled in three ways Problem #1 for the statistical approach: Non-equilibrium Perhaps the frogs have not yet colonized all available (and accessible) habitat Problem #2 for the statistical approach: Dispersal limitation Perhaps suitable habitat is available but dispersal into this habitat is impossible
Problem #3 for the statistical approach: Population sinks Source population Environmental variables fall within species niche Sink population Environmental variables fall outside species niche r<0 r>0
With recurrent dispersal, could a species persist in habitats outside of its niche? A model of sources and sinks The question: Could a population persist outside of its niche? Source (r > 0) Sink (r < 0) dN1 N r1 N1 (1 1 ) mN1 dt K dN 2
r2 N 2 mN1 dt What is the population size in the sink at equilibrium? What determines how abundant a species is in a sink? We can ignore density dependence in the sink, because population size should remain small A model of sources and sinks The question: Could a population persist outside of its niche? The equilibrium is:
r1 m N1 K r1 mK (m r1 ) N2 r1r2 N 2 Will be positive, and thus a sink population maintained, any time: m < r1 This makes sense, because only when this condition holds does the source population grow rapidly enough to replace the individuals emigrating to the sink.
Summary of species ranges over ecological time scales A species may be present in a particular geographic region because: 1. The geographic region is part of the species niche 2. The geographic region is outside of the species niche but recurrent immigration occurs from a region within the species niche 3. Non-equilibrium. The population is going extinct but has not yet done so A species may be absent from a particular geographic region because: 4. The geographic region is outside of the species niche 5. The geographic region is inside of the species niche but is beyond the dispersal distance of the species 6. Non-equilibrium. Some suitable habitat has not yet been colonized, but it will be given sufficient time. The evolution of species geographic ranges Why dont species evolve to live everywhere? Or, put differently, why
dont species evolve an infinitely wide niche? r r Rainfall Rainfall Trade-offs are ubiquitous Performance at high temperature Performance at low temperature
Trade-offs: an extreme example White Sands National Monument Holbrookia maculata ruthveni Bleached Earless Lizard Pinyon juniper woodlands Holbrookia maculata Common Lesser Earless Lizard Trade-offs: an extreme example Lizard Performance
White Sands National Monument Pinyon-Juniper woodland It is simply not possible to perform well in both habitats The evolution of species geographic ranges Even if trade-offs exist, why dont species simply adapt to local environmental conditions and thus increase their geographic range? Dark populations on dark habitats White populations on white habitats
Why can range not always expand through adaptation? Reason 1: Lack of genetic variation Frequency Dark Color White Why can range not always expand through adaptation? Reason 1: Lack of genetic variation Increasing thermal stress
Why can range not always expand through adaptation? Reason 1: Lack of genetic variation Drosophila birchii Hoffman et al 2003. Science. Can Drosophila birchii expand its geographic range into regions of greater thermal stress? To this end, studied whether increased desiccation resistance could evolve in Drosophila birchii Why can range not always expand through adaptation? Reason 1: Lack of genetic variation Why can range not always expand through adaptation?
Reason 1: Lack of genetic variation Other Drosophila species Finch Hatton Selected for increased desiccation resistance in laboratory cultures derived from the most resistant population Found no response to selection over 50 generations!!! Why can range not always expand through adaptation? Reason 1: Lack of genetic variation Estimated heritability of desiccation resistance in most resistant (Finch Hatton) population Found NO additive genetic variance for desiccation resistance Together, these results suggest that the geographic range of D. birchii is limited by a lack of genetic variation
Why can range not always expand through adaptation? Reason 2: Gene flow swamps adaptation Source population Sink population Why can range not always expand through adaptation? Reason 2: Gene flow swamps adaptation Because the ancestral source population has a vastly greater population density, genes flow primarily from source to sink The consequence of this is that adaptation to the sink is swamped Source Optimum phenotype in source population
Sink Optimum phenotype in sink population Why can range not always expand through adaptation? Reason 2: Gene flow swamps adaptation Optimum phenotype in source population Optimum phenotype in sink population Why can range not always expand through adaptation? Reason 3: Speciation Even if a species has sufficient genetic variation and local adaptation is not swamped by gene flow, adapting to new habitats/niches may lead to the formation
of a new species rather than a broader niche! Current Habitat Novel Habitat Hot climate Cool climate Favors early flowering Favors late flowering Why can range not always expand through adaptation? Reason 3: Speciation J. Antanovics Studied populations of A. odoratum growing on
tailings of the Trelogan Mine, UK and adjacent populations not on tailings Sweet Vernal Grass Anthoxanthum odoratum Why can range not always expand through adaptation? Reason 3: Speciation Plants on mine tailings flower significantly earlier This difference in flowering time results in a 43% reduction inter-population mating Suggests that adaptation and range expansion is causing speciation Summary of geographic ranges
Over ecological time scales, species ranges are determined by dispersal and the niche Over evolutionary time scales, species ranges evolve in response to trade-offs, genetic variation, gene flow, and speciation
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