BIOL203 Lecture 11: Population Distributions, Fall 2024 PDF

Reminders No seminars next week (Remembrance Day) Poster presentation – week of Nov. 25th 1  Lecture 11 Population Distributions BIOL 203 November 8th, 2024 2 Learning objectives 1. Explain why the dis...

Reminders No seminars next week (Remembrance Day) Poster presentation – week of Nov. 25th 1  Lecture 11 Population Distributions BIOL 203 November 8th, 2024 2 Learning objectives 1. Explain why the distribution of populations is limited to ecologically suitable habitats 2. Give the five important characteristics of population distributions 3. Describe how distribution properties of populations can be estimated 4. Recognize that population abundance and density are related to geographic range and adult body size 5. Explain why dispersal is essential to colonizing new areas 3  The distribution of Key Concept populations is limited to ecologically suitable habitats 4 The distribution of populations is limited to ecologically suitable habitats A habitat is a place or physical setting in which an organism lives Determined by an organism’s niche = the range of abiotic and biotic conditions in can tolerate Abiotic factors include temperature, humidity, and pH Biotic factors include predators, parasites, and competitors 5 The distribution of populations is limited to ecologically suitable habitats Two types of niche: Fundamental niche = range of abiotic conditions that a species can live in Realized niche = includes biotic factors which might limit a species distribution 6 Determining suitable habitats Patterns of distribution suggest that only certain habitats are suitable E.g.) Monkeyflowers 7 Ecological niche modelling Generally, the more suitable the habitat, the larger a population can grow within that habitat Allows us to predict actual or potential species distributions (why might this be useful?) Can use historical data/data from other populations to determine suitable habitat conditions = ecological niche modelling 8 Modelling the spread of invasive species Ecological niche modelling can be useful to predict spread of invasive species in new areas E.g.) Chinese bushclover (Lespedeza cuneata) Introduced in late 1800s, displaced native plants Collected data from Asia, used to predict potential distribution in North America 9 Difficulties with ecological niche modelling When using niche models to predict range of introduced species, based on historical environmental conditions Assume that these historical conditions can accurately predict suitable habitat in new environment (not always the case) E.g.) raccoon (Procyon lotor) Native to North America, introduced to Caribbean and Europe Vastly different realized niche in all three locations, means difficult to predict species future range from historical niche 10 Habitat suitability and climate change We can also use current suitable habitats to predict species responses to climate change E.g.) fish in the North Sea Climate change increased water temperature (~4°C) Led to increase in species richness 11 Concept check What is a species’ fundamental niche? What is its realized niched? Why might these be different? 12  Population Key Concept distributions have five important characteristics 13 Population distributions have five important characteristics Population distributions look at the spatial structure of an organism = the pattern of density and spacing of individuals in a population Described using five characteristics: 1. Geographic range 2. Abundance 3. Density 4. Dispersion 5. Dispersal 14 Geographic range Geographic range is the total area covered by a population Individuals often do not occupy every location within their geographic range Climate, topography, soils, vegetation, other factors influence range Includes all the areas individuals occupy through their life (e.g., anadromous salmon, migratory species) 15 Geographic range Geographic range is an important measure because it tells us the size of the area a population occupies If restricted to small area, more susceptible to natural disasters Challenge for endemic species = species that live in a single, often isolated location Populations with larger ranges less vulnerable Species with ranges that span multiple continents = cosmopolitan 16 Abundance Abundance is the total number of individuals of a species that exist within a defined area E.g.) Number of rainbow trout in a lake, number of coconut trees on an island Can tell us whether a population is thriving or on the brink of extinction 17 Density Density is the number of individuals in a unit of area or volume Tells ecologists how many individuals are packed into a given area If a habitat can support a higher density of individuals, population will grow If population density too great, either individuals will leave or will experience reduced growth/survival 18 Density Although individuals only live in suitable habitats, not all habitats are equal quality Environment is variable Typically, density is highest near centre of geographic range Peripheral populations less dense because conditions aren’t as favourable E.g.) Baltimore oriole 19 Dispersion Dispersion describes the spacing of individuals with respect to one another in a geographic range Three patterns: Clustered Evenly-spaced Random 20 Dispersion In clustered dispersion, individuals aggregate in discrete groups Could arise due to social groups (e.g., flocks of birds) Could be clustered near resources (e.g., bugs aggregating under logs) Could arise if offspring stay near parents (e.g., stands of trees) 21 Dispersion In evenly spaced dispersion, each individual maintains a uniform distance between itself and its neighbours Territories (e.g., pika) Competition (e.g., plants) 22 Dispersion In random dispersion, the position of each individual is independent of the position of other individuals in the population Not common in nature (why?) 23 Dispersal Dispersal is the movement of individuals from one area to another Distinct from migration = seasonal movement of individuals back/forth between habitats With dispersal, individuals typically do not return to origin Mechanism for how individuals colonize new suitable habitats Can be used to avoid high competition or predation risk 24 Concept check Why aren’t all locations in a species’ geographic range occupied by a species? What is the difference between dispersal and dispersion when describing population distributions? 25  The distribution of Key Concept properties of populations can be estimated 26 The distribution of properties of a population can be estimated One way to determine the number of individuals in an area is to conduct a census For most species, not feasible, must conduct a survey = counting a subset of the population Can use this data to estimate abundance, density, geographic range, and distribution of a population Often conduct multiple surveys to estimate population mean 27 Area- and volume-based surveys In area- and volume-based surveys, scientists define boundaries of an area/volume and count all individuals in that space = quadrat Size of area/volume sampled depends on size and density of organism E.g.) a few cm3 of soil for bacteria, 1 m2 for estimating corals on a reef, etc. For larger animals, might need to conduct surveys using aerial methods 28 Line-transect surveys To conduct line-transect surveys, scientists count the number of individuals observed as they move along a line Many variations Length of line, how many transects, buffer zone? 29 Mark-recapture surveys Some species elusive, hard to find, can lead to underestimates of population metrics using area-based or line-transect surveys (better for stationary/sedentary species) In a mark-recapture survey, ecologists collect individuals and mark them, return them to the population, and a second collection occurs at a later date The proportion of marked/unmarked individuals captured in the second collection can be used to estimate population size 30 Estimating population size with mark-recapture Marked individuals (M) represent a fraction of total population (N): M/N For second capture, we examine the number of recaptures (R) over the total number of captures (marked + unmarked = C): R/C This second fraction is should be equivalent to the first fraction, so: M/N = R/C N = (M x C)/R 31 Indirect counts of individuals For some species/locations, not feasible to directly count (or capture) individuals E.g.) Difficult to count number of deer in dense forest, number of rabbits in a field with tall grass Indirect counts can act as an index of how many individuals are in an area E.g.) Counting deer tracks, rabbit droppings, etc. Gives us a relative measure of whether one area has more/less individuals than another, or through time 32 Quantifying the dispersal of individuals Quantifying dispersal requires you know the source of individuals If known single source, can see how far individuals spread from that source Could also use mark-recapture: Ear tags, radio transmitters, wing/leg bands Fluorescent dye on pollen 33 Lifetime dispersal distance Common measure of dispersal is lifetime dispersal distance = average distance individuals move from where they are born to where they reproduce Allows us to estimate how rapidly a population can increase its range E.g.) Songbirds can typically spread about 1 km per generation, should take thousands of generations to cross an entire continent However, not always the case 34 Lifetime dispersal distance E.g.) European starling Introduced to NYC in 1890s, found across country within 60 years Average travel of 67 km per year! 35 Concept check Why are surveys rather than censuses used to quantify the abundance of many animals? 36  Population abundance and Key Concept density are related to geographic range and adult body size 37 Population abundance and geographic range Populations with high R2 = 0.13 abundance usually have large geographic ranges Observed in plants, mammals, birds, and protists E.g.) birds in North America Lots of unexplained variation 38 Population abundance and geographic range Cause of relationship widely debated Maybe due to resource availability? If species relies on resource with small range, then will only inhabit that small area Change in habitat use? Maybe more marginal habitats used in years or high reproduction/survival Relationship between abundance-range suggests that reducing range will reduce population size, and that reducing population size will reduce range 39 Population density and adult body size Generally, density of a population is negatively correlated with body mass of an individual Related to space and resources 40 Concept check What is the relationship between population abundance and geographic range across many species? What is the relationship between population density and adult body size across many species? 41  Dispersal is essential Key Concept to colonizing new areas 42 Dispersal limitation Dispersal is essential for colonizing new areas For some species, not enough time has passed for dispersal to occur Substantial barriers could also prevent dispersal = dispersal limitation Common limitation is presence of large expanses of inhospitable habitat that an organism cannot cross (e.g., desert, cities, roads, oceans) 43 Dispersal limitation Sometimes, organisms manage to cross these barriers (e.g., isolated islands) Could be via humans = commensal species 44 Habitat corridors A strip of favourable land that aids dispersal between two habitats is called a habitat corridor Can be natural or made by humans 45 Testing the importance of habitat corridors Source E.g.) pine forests in South Carolina Storms/fires often create clear areas that are suitable habitat for many plants/animals Researchers cleared patches of forest, measured dispersal Central patch was source 46 Testing the importance of habitat corridors Source Measured dispersal of: Common buckeye butterfly (Junonia coenia) Pollen (winterberry, Ilex verticillata) Seeds/plants (holly and wax myrtle) Replicated 8 times 47 Testing the importance of habitat corridors Connected patch had highest level of dispersal 48 Testing the importance of habitat corridors Connected patch had highest level of dispersal 49 Ideal free distribution among habitats If differences in habitat quality and easy dispersal, selection favours movement to habitat which offers most energy per individual = per capita benefit Individuals should choose highest quality habitat However, more individuals means fewer resources 50 Ideal free distribution among habitats When per capita benefit of high- quality habitat reduces, favours movement back to low-quality habitat When all individuals distribute themselves among habitats such that they have equal per capita benefit = ideal free distribution 51 Ideal free distribution E.g.) three-spine stickleback (Gasterosteus aculeatus) Created low/high quality habitats by adding Daphnia Predicted IFD = 5:1 ratio Arranged themselves in 4:1 ratio 52 Ideal free distribution IFD tells us how individuals should distribute themselves if habitat quality differs, but rarely the case Individuals might not know of other habitats Fitness not only affected by resources Other factors can influence habitat use (e.g., predators, occupied by a territorial individual) 53 The effects of habitat quality Often, individuals in high quality habitats have the most reproductive success As a result, high quality habitats often act as a source of dispersal to low quality habitats 54 The effects of habitat quality E.g.) blue tit (Parus caeruleus) Live in downy oak (lots of caterpillars) and holm oak (fewer caterpillars) forests More breeding pairs + more fledglings in downy oak forests 55 The effects of habitat quality Based on typical survival rates, would expect to see 9% growth in downy oak population year over year 13% decline in holm oak (should go extinct) Both population persist because birds move from high to low quality habitat (rescue effect) 56 Concept check Why are many species absent from continents that contain suitable habitat? How do habitat corridors facilitate dispersal? In the ideal free distribution, what are individuals trying to optimize? 57 Next class Population growth and regulation (Chapter 11) No seminars next week (Remembrance Day) Poster presentation – week of Nov. 25th 58