population dynamics

Question 1

How does cooperation influence biological organisation?

Answer 1

All biological organisation arises from cooperation - e.g. the cooperation of chromosomes to form genomes is a form of within-organism cooperation. Individual organisms are dependent upon one another within a population, entailing a vulnerability to ‘cheats’ who might exploit cooperative systems through asymmetric contribution.

Question 2

How do individuals behave within cooperative systems and how does natural selection act on this behaviour?

Answer 2

Community structures imply cooperation, which entails the coordination of individual activities. Natural selection thus acts both on a target individual and surrounding individuals within the context of social behaviour.

Question 3

What are some examples of ‘cheating’ across levels of biological organisation?

Answer 3

At the gene level, ‘meiotic drivers’ are alleles which have evolved mechanisms to manipulate meiosis to favour their own transmission into more than half of the gametes produced (e.g. meiotic drivers may employ a ‘poison-antidote’ system whereby they produce a toxin damaging/disabling non-driver sperm, effectively eliminating them from the gamete population - whilst encoding an ‘antidote’ to protect the self-cell). At the cellular level, chloroplasts may suppress male function in plants (as chloroplasts are inherited maternally). The metastasis of cancer cells involves selfish replication at the expense of the organism.

Question 4

Do classical models of natural selection account for cooperation?

Answer 4

If natural selection exclusively followed classical models, species would not display positive social behaviour beyond mating and parental care. If natural selection primarily acted on direct individual fitness, cooperative individuals would be rapidly outcompeted or subverted by selfish individuals, decreasing average population fitness.

Question 5

What is Hamilton’s Rule?

Answer 5

A social action in which an actor reduces their own number of offspring by c and increases a recipient’s number of offspring by b will be favoured by natural selection when rb-c>0 where r is the regression relatedness of the actor to the recipient.

Hamilton’s Rule:

rb > c

Question 6

How does regression relatedness impact the selective advantage conferred by a social behaviour?

Answer 6

If the actor and recipient are identical twins, the action will be selected for if the offspring benefits to the related recipient outweigh the costs. If the recipient is random, there is no net benefit of the action to the actor - and this behaviour confers no selective advantage. When analysing social behaviour, we must regard a possible recipient as a mixture of an identical twin and a random individual, such that the action is selected for when rb > c. This explains altruistic behaviour through kin selection.

Question 7

What is neighbour-modulated fitness?

Answer 7

The neighbour-modulated fitness of an individual refers to the sum of components each attributable to a particular neighbourhood phenotype comprising an individual’s reproductive success (i.e. how the social environment generated by neighbours’ social behaviour impacts individual reproductive success, cooperative or competitive).

Question 8

What is inclusive fitness?

Answer 8

The inclusive fitness of an individual refers to both their own reproductive success and to the reproductive success of their relatives, with whom they share genetic material. An individual can thus increase their inclusive fitness through altruistic action toward related recipients, at the expense of their direct reproductive output.

Question 9

What are some issues associated with measuring inclusive fitness?

Answer 9

Fluctuating paternity in outbred populations can lead to varying patterns of reproductive dominance, impacting social behaviours within communities due to fluctuating patterns of relatedness.

Question 10

What are egalitarian transitions and how do they differ from fraternal transitions?

Answer 10

Egalitarian transitions involve co-operating partners which are distinct, non-fungible entities (e.g. genes at different loci), whereby cooperation arises without direct relatedness (stemming from mutual benefit without competition). An interspecies mutualism is an example of an egalitarian transition (e.g. the interdependence of ants and acacia trees). Fraternal transitions involve co-operative partners of the same kind, with variable relatedness and replaceability (e.g. single cells forming a multicellular organism).

Question 11

What are direct benefits?

Answer 11

Direct benefits accrue directly to the actor and their immediate offspring. The cooperative founding of nests by queen ants is an example of social behaviour driven by direct benefit. The collaboration of two unrelated queens in colony founding is linked to enhanced worker production and colony survivorship, which is particularly beneficial in protecting against brood raiding from other colonies. Initial cooperation is beneficial but often culminates in the queens fighting, resulting in the death of one. Cheating is possible (i.e. a queen could neglect to help the other and reap the benefits of the constructed nest) but outweighed by the scale of the shared benefit associated with two-queen colony founding.

Question 12

What are indirect benefits?

Answer 12

Indirect benefits accrue to others’ offspring (e.g. improving the reproductive fitness of a related individual).

Question 13

What are byproduct benefits?

Answer 13

Byproduct benefits, or unenforced direct benefits, occur when one organism benefits from the behaviour of another organism - a behaviour performed for reasons unrelated to influencing the benefiting organism.

Question 14

How does reciprocity enforce cooperation?

Answer 14

An individual’s actions may appear altruistic, but the sequence of interactions in which they participate culminate in direct benefit. This form of reciprocity requires repeat interactions between the same partners and consequences associated with non-cooperation.

Question 15

What is policing and how does it enforce cooperation?

Answer 15

In some social insects (e.g. honeybees), the queen is the sole reproductive unit within the hive and secretes pheromones suppressing worker ovary development. If there is a temporary window where the queen does not secrete these pheromones, workers might ‘cheat’ and produce male offspring. In arrhenotokous populations (where the male is formed from an unfertilised egg), workers are less related to other worker laid male eggs (~0.125) than queen-laid males. Throughout the hive, cooperative workers police reproduction through the selective removal of worker-laid eggs, reducing cheating, maintaining stable intra-group relatedness and suppressing genetic competition. This occurs in semisocial populations where there is a genetic distinction between siblings (i.e. queen-laid eggs) and nieces/nephews (i.e. worker-laid eggs). The cost of cheating (i.e. wasting reproductive resources on a non-viable egg) outweighs the cost of cooperation. When the effectiveness of policing is high, the percentage of reproductive workers is very low.

Question 16

What is an example of a reciprocal cooperative action?

Answer 16

Cleaner fish (e.g. blue wrasse) derive nutrients without foraging through the removal of ectoparasites from ‘client’ fish. Cheating (e.g. eating the cleaner) provides a short-term advantage but is associated with long-term costs (e.g. difficulty finding another cleaner to remove parasites). Client fish populations decline when cleaner fish are removed, highlighting the importance of the interaction.

Question 17

How do imposed incentives/sanctions make cheating unprofitable?

Answer 17

The imposition of incentives/sanctions can be viewed as policing on an anonymous level. For example, soybeans withhold oxygen from Rhizobium bacteria that cheat by not fixing nitrogen (with high nodule specificity). Experimental evidence (using argon to prevent nitrogen fixation) has shown that non-fixing bacterial populations decline due to oxygen deprivation, with the host regulating the mutualism through the imposition of cost. This imposition has three key phases: target behaviour, punishment behaviour and public good (e.g. the apoptosis of ‘cheating’ cancer cells prevents metastasis and disease progression).

Question 18

What is population viscosity?

Answer 18

Population viscosity refers to the tendency of individuals in a population to remain close to their birthplace, resulting in limited dispersal and higher genetic relatedness amongst neighbours. This increased relatedness can influence the evolution of altruistic behaviours, as individuals are more likely to act to benefit their relatives. Frequent neighbour interaction and the imposition of direct reciprocity leads to cooperation in line with tit-for-tat game theory.

Question 19

What are some examples of kin discrimination in natural populations?

Answer 19

The only cooperatively nesting bird found in England is the long-tailed tit, wherein some individuals - ‘helpers’ work to raise offspring that are not their own (often when their own nests have failed). 96% of helper tits preferentially assist nests containing related chicks, identifying kin by song. Errors in discrimination occur when helpers struggle to identify non-siblings with similar songs (an example of kin discrimination as explaining errant data points in the context of broader kin selection hypotheses). Within slime moulds, individuals preferentially form fruiting bodies with members of their own lineage (implying recognition of relatedness and, therein, kin discrimination).

Question 20

How does density-dependence in a population impact the application of Hamilton’s Rule?

Answer 20

Within density-dependent populations, the benefit (b) to a recipient might be offset by increased competition for resources. If the population is at carrying capacity, or if the benefit to the recipient results in reduced reproductive fitness of other individuals due to density-dependent effects, the overall population benefit for the extra offspring is zero. In these scenarios, the actor might be selected to be purely selfish, as altruistic actions are not associated with a net increase in the number of related individuals in a population. In high viscosity populations, collaboration between genetically similar individuals is offset by competitive interactions.

-(1-r)c>0

Question 21

How is cooperative lekking an example of indirect fitness benefits?

Answer 21

Male wild turkeys gather in ‘lekking arenas’ to display for females who visit and select a mate based on this display. Males may defend single territories or form a collaborative pair, wherein the subordinate brother assists in the dominant male’s display. The subordinate male is helping his brother to mate successfully, increasing the transmission of his genetic material (as his solitary breeding would likely be comparatively unsuccessful). The congregation of brothers in leks is positively selected for via indirect fitness benefits.

Question 22

How does cooperative breeding of superb fairy-wrens positively impact inclusive fitness?

Answer 22

Superb fairy-wrens form small, cooperative breeding groups. A territory is held by a breeding pair, who are joined by non-breeding male helpers (including adult male offspring). This results in a kin-structured social group, wherein helpers feed nestlings, engage in sentinel behaviour and defend the territory from predators; working to increase sibling survival and thus enhancing their own inclusive fitness as r=0.5. Across cooperatively breeding species, helpers are more likely to discriminate in favour of relatives when help improves offspring survival (kin selection theory).

Question 23

How does kin selection contribute to the evolution of eusociality?

Answer 23

From an inclusive fitness perspective, the benefits of a worker investing resources in producing direct offspring or in the same number of full siblings are effectively equivalent, provided that relatedness is equal. Strict monogamy in the ancestor of eusocial lineages ensures equal sibling and offspring relatedness, facilitating conditions where siblings and offspring are genetically equivalent (as monogamy promotes higher relatedness between offspring).

Question 24

What are Hamilton’s ‘greenbeard’ genes?

Answer 24

A ‘greenbeard’ gene produces a detectable trait - allowing the bearer to recognise this trait in others and inducing altruistic behaviour towards conspecifics. The greenbeard is thus a ‘supergene’ - helping self-copies irrespective of host relatedness. This thought experiment was formulated to suggest how a single gene could drive altruistic behaviour independently of kin selection.

Question 25

Are there any real-world examples of greenbeard genes?

Answer 25

Throughout North American populations of the red imported fire ant, polygyny (multiple queens per colony) is common - and associated with the bb genotype at the Gp9 locus. Young BB (monogynous) queens are killed by workers, leaving surviving Bb queens (with the 'b' allele suggested to cause Bb workers to kill non-'b' queens). This would be a real-world example of a greenbeard gene, where the 'b' gene induces killing of non-'b' queens to promote the 'b' allele, but it could be more complex (polygyny could confer a selective advantage to invasive colonies).

Question 26

What is demographic stochasticity?

Answer 26

Demographic stochasticity refers to random fluctuations in population size arising from inherent variability in birth and death rates amongst individuals. The effect of demographic stochasticity on a population results in fluctuations deviating from the expected smooth termination of growth at carrying capacity.

Question 27

What is one primary issue with simple population models?

Answer 27

Within simple model formulations, the use of deterministic numbers to represent growth rates (r) can produce outputs of 'fractional individuals'. More realistic models represent the number of individuals introduced or removed from a population in a future year as a random variable (as an average can be estimated but the individual number of births/deaths will fluctuate randomly).

Question 28

What distribution is preferred for modelling births and deaths in a population subject to demographic stochasticity?

Answer 28

When accounting for demographic stochasticity, Poisson distributions must be used in modelling (not normal distributions, as these treat individual births/deaths as continuous variables). Poisson distributions deliver Nt+1 as a discrete integer - eliminating the need for truncation as fractional individuals are no longer potential outputs.

Question 29

Why is the effect of demographic stochasticity amplified in smaller populations?

Answer 29

As average population size declines, the proportional size of these fluctuations increases - such that a species can be driven to extinction as a result of chance events (e.g. black-footed ferret populations previously became functionally extinct due to the impact of bubonic plague and an extreme stochastic male skew in birth rates - resulting in an absence of viable females).

Question 30

How does intrinsic growth rate impact the recovery rate of populations from stochastic 'unlucky' years?

Answer 30

Species with a high intrinsic growth rate (r) recover more quickly from high environmental stochasticity. Conversely, species with a low r value may retain a low population size and reach extinction if this population size declines below fifty.

Question 31

What is environmental stochasticity?

Answer 31

The term 'environmental stochasticity' describes unpredictable fluctuations in environmental conditions (e.g. temperature, resource availability), resulting in variation in population size and dynamics.

Question 32

What is the intrinsic growth rate of a population?

Answer 32

The intrinsic growth rate of a population represents the maximum potential growth rate of a population under ideal conditions, with no limiting factors (i.e. zero predation, infinite resources) - reflecting the difference between birth rate and death rate when the population is at low density.

Question 33

How do the effects of genetic drift impact small populations?

Answer 33

If a population remains very small for an extended period of time, genetic drift could result in a loss of genetic diversity. This could lead to inbreeding depression, resulting in reduced reproductive health (sterile individuals) and impaired adaptability through genetic erosion. The Florida panther population plummeted to below fifty individuals between the 1970s and 1980s - inducing a genetic bottleneck. This resulted in a significant loss of genetic diversity and increased levels of inbreeding. The offspring of inbred Florida panthers often exhibited heart defects, cryptorchidism (undescended testicles), kinked tails and increased vulnerability to disease.

Question 34

What is the Allee Effect?

Answer 34

The Allee Effect describes a phenomenon where population growth or individual fitness decreases at low population densities. When the Allee Effect is strong, the population growth rate becomes negative below a critical population threshold and the population inevitably declines to extinction. If the Effect is weak, the per capita growth rate remains small but does not necessarily decline to become negative. The absolute population growth falls but the per capita growth rate might still increase.

Question 35

What are some mechanisms potentially underlying the Allee Effect?

Answer 35

In a species reliant on group size for predation defence (e.g. musk oxen), a minimum group size is required for success. Decline below this size can result in dramatic predation increases. This effect is similar in cooperative hunting populations (e.g. African wild dogs), whereby hunting success decreases as pack size diminishes. Decline may also occur at low densities in populations with highly specific mating systems. For example, the female kakapo is attracted by amplified, booming choruses from multiple males, whilst their population relies upon a bumper crop of remu berries every few years. These birds are now critically endangered due to habitat transformation (e.g. diminishing remu berry yields) - a common pattern associated with K-selected animals under globalisation.

Question 36

What is the minimum viable population?

Answer 36

The minimum viable population is the smallest population size which has a high probability of persisting for a specified period (e.g. 95% chance of persistence for over a hundred years - such that the probability of extinction is low enough to be ignored). For many vertebrate species, the median MVP is approximately 4,000.

Question 37

What are the implications of the Allee Effect and demographic stochasticity for conservation?

Answer 37

Understanding of stochastic fluctuations in population dynamics promotes early intervention in conservation, with increased sensitivity to population decline preventing the necessity of extreme conservation efforts (e.g. the intensive kakapo recovery program, which is challenged by aspergillosis vulnerability associated with genetic erosion).

Question 38

What produces chaotic dynamics in discrete-time models?

Answer 38

The deterministic unpredictability of high intrinsic growth rates can produce chaotic dynamics. For example, high r value populations can exceed their carrying capacity within a single generation. This 'boom' will be coupled with a 'bust' of overcompensating density dependence, wherein the next generation is very small. These 'chaotic' systems are highly sensitive to fluctuations in external conditions - causing issues with long-term forecasting.

Question 39

Are there any real-world examples of truly chaotic biological systems?

Answer 39

Extreme fluctuations have been observed in populations of wild soay sheep on the St Kilda island, likely due to environmental stochasticity (with extreme birthing events in summer and harsh, starving winters) and not due to truly chaotic dynamics.

Question 40

What equation underlies the Levins Metapopulation Model?

Answer 40

rate of change of occupied patches = colonisation (c) - extinction (e) dp/dt = cp(1-p)-ep Therefore, a higher rate of colonisation paired with a lower rate of extinction yields a greater proportion of occupied patches.

Question 41

What is the most common structure of populations of individual species and how sensitive is this structure to stochastic disruption?

Answer 41

Populations of individual species are often small, extinction-prone and patchily distributed in space. Population persistence and the dynamics of these populations thus depend on the capacity of individuals to migrate between patches. These populations vary in size around the carrying capacity of the local 'patch' - with more extreme fluctuations arising from the sensitivity of small population sizes to environmental stochasticity. Stochastic events can cause these small, fragmented populations to decline to zero.

Question 42

What is an example of fragmentation arising from human activity?

Answer 42

Throughout the Atherton Tablelands in Australia, sparse rainforest fragments along watercourses punctuate converted agricultural land - resulting in the isolation of forest-dependent species. Habitat fragmentation is increasingly common due to anthropogenic influences

Question 43

What is an example of natural habitat fragmentation?

Answer 43

Acacia trees supporting diverse specialised insect herbivores are dispersed widely across the Kenyan savanna. Whilst this habitat fragmentation occurs naturally, the isolation of smaller individual populations due to reduced patch area similarly renders these populations extinction prone.

Question 44

What is the most significant factor determining population persistence?

Answer 44

The dispersal ability of a species relative to the level of habitat fragmentation determines how the population is structured and whether it can persist in fragmented landscapes (as patches where populations become locally extinct can thus be re-colonised by immigrants from other patches).

Question 45

How should the discrete-time logistic model be altered to account for open populations?

Answer 45

Immigration and emigration must be factored in, such that both within-population and among-population processes are examined.

Question 46

What is a metapopulation?

Answer 46

A metapopulation is a group of spatially separated populations of the same species connected by cross-interaction (migration of individuals from one patch to another, enabling gene flow and impacting the persistence of the species). Within metapopulations, species persist on a balance between extinction and colonisation events in different patches.

Question 47

How can conservationists improve the persistence of metapopulations?

Answer 47

The persistence of metapopulations can be achieved through the minimisation of extinction rates (through habitat management and maintenance of heterogeneity across habitats - as heterogeneous patches provide a wider range of resources). It is also important to maximise colonisation rates (facilitating efficient connection between patches), conserve empty habitat patches (increasing the proportion available for colonisation) and reduce population synchrony (through habitat management, as decoupling population dynamics prevents the whole metapopulation for suffering 'unlucky' years simultaneously - decreasing the likelihood of landscape-level extinction).

Question 48

What is a real-world example of a metapopulation?

Answer 48

Long-term studies of the Glanville fritillary butterfly in Finland have revealed that local populations within meadow fragments often decline to extinction whilst the metapopulation persists as extinction events are balanced through the colonisation of previously unoccupied patches. This extinction-colonisation dynamic is a natural feature of the fritillary butterfly population structure.

Question 49

What factors are associated with high colonisation rates in metapopulations?

Answer 49

Colonisation rates are highest when there is an intermediate number of occupied patches, providing sources for dispersal whilst also maintaining available empty patches for further colonisation. Colonisation rates are also increased when the average distance between patches is comparably low and when patch sizes are larger. This is because larger patches have a larger carrying capacity and thus support larger, more resilient populations.

Question 50

What are panmictic populations?

Answer 50

The metapopulation concept is a single point on a continuum of spatially structured populations. Some species (e.g. holly leaf miners) appear patchily distributed but are so mobile that extinction rates are much lower than colonisation rates. This population structure is panmictic - with all patches actively and frequently interacting. Conservation strategies for panmictic populations concentrate on conserving the entire population, not individual patches.

Question 51

What are mainland-island dynamics?

Answer 51

A stable 'mainland' population serves as a continuous source of immigrants to smaller, extinction-prone 'island' populations. This mainland population is effectively immortal - as local extinctions on islands are offset by mainland recolonisation events. This means that conservation effort should prioritise the core mainland population (as satellite populations are dependent).

Question 52

What are source-sink dynamics?

Answer 52

The 'source' provides a surplus of individuals to be exported to 'sink' patches, wherein death rates exceed birth rates (and would thus decline to extinction without continuous source immigration). Sea rockets on beaches act as a 'source', providing seeds colonising nearby dune 'sinks'.

Question 53

What defines 'antagonistic interactions'?

Answer 53

Within antagonistic interactions, one species benefits at the expense of another. For example, trophic interactions result in nutrition (a positive fitness consequence) for one species and negative fitness consequences for the consumed species.

Question 54

What are the distinctions between parasitism and parasitoidism?

Answer 54

The relationship between a parasite and host is often more specific, with one or few hosts consumed by a single parasite. The parasite lives on or within their host and serves as a drain on host resources. Parasitoidism is distinguished from parasitism as the parasitoid only consumes one host during its lifetime and the development of the parasitoid must kill the host species (e.g. parasitoid wasps lay eggs within the body of a host aphid).

Question 55

What is the distinction between predation and herbivory?

Answer 55

There is a fixed dynamic associated with predation, with the prey always killed by the predator. A predator typically attacks prey indiscriminately. The resource exploited by a herbivore is a primary producer and tissue is consumed by that herbivore, but the whole resource individual is not killed.

Question 56

Do we consider the rainforest weevil to be a herbivore?

Answer 56

No. Certain Curcolionid weevil species are pre-dispersal seed predators - laying their eggs within developing seeds. The larvae develop within, consuming seed contents. The developing seed - an individual organism - is killed by the action of the weevil. This is more similar to parasitoidism.

Question 57

How do antagonistic interactions impact population dynamics?

Answer 57

Population size is defined by the relationship between birth and death rates, with carrying capacity reached when both rates equalise. Birth and death rates are influenced by resource availability, competition and the presence of natural enemies (e.g. predators and parasitoids). The introduction of invasive species, for example, can cause rapid native prey population decline due to an absence of co-evolutionary defences.

Question 58

What is a real-world example of the influence of natural enemies on species abundance?

Answer 58

Across the Pacific Ocean, sea otters serve as a key predator of sea urchins. The success of the fur trade resulted in sea otter population decline and an associated decrease in kelp forests (as sea urchin populations boomed, allowing for more grazing of marine algae). Kelp forests are crucial 'ecosystem engineers', providing habitat and spawning grounds for a broad diversity of marine organisms. Their decline had top-down effects throughout the food chain, with repercussions for fisheries and marine biodiversity.

Question 59

How are natural enemies used to control invasive species?

Answer 59

The deliberate introduction of natural enemies to control invasive species is a form of classical biological control. The introduction of the cactus moth to control the invasive Central American Opuntia prickly pear (an agricultural pest) in Australia was highly successful in reducing prickly pear populations below a damaging threshold through reduction of the ecological carrying capacity of the invaded region.

Question 60

What is the Lotka-Volterra predator-prey model?

Answer 60

The Lotka-Volterra model describes the dynamics of predator-prey populations. The pair of differential equations model the interaction between predator and prey, predicting oscillation population cycles (coupled dynamics). dN/dt = rN-aPN Prey (N) increase exponentially, but are removed from the population at a rate dependent on the predator-prey encounters (PN) and predator attack rate (a). dP/dt = faPN-qp Predators (P) decline exponentially due to starvation at mortality rate q. Births depend on predator-prey encounters (PN), attack rate (a) and the efficiency of turning food into new individuals (f).

Question 61

What does calculating for zero isoclines within the Lotka-Volterra model reveal about predator-prey dynamics?

Answer 61

Within the context of population dynamics, the term 'zero growth isocline' refers to the set of population sizes at which the rate of change for one population in a pair of interacting populations is zero (i.e. the population is stable - therefore we must solve for dn/dt = 0). The prey population is stable when predator population is r/a. The predator population is stable when the prey population is q/fa. Superimposition of the graphs of these respective differential equations results in coupled oscillations - correlating rises and falls in each population. Peaks in prey abundance are often followed by delayed peaks in predator abundance.

Question 62

Do any real-world populations display classical predator-prey dynamics?

Answer 62

The data collected from the Hudson Bay Company's fur trapping records from the mid-19th to early 20th centuries reveal delayed, cyclical oscillations in snowshoe hare and lynx populations. Within Arctic environments, food webs are relatively simple (the lynx has few alternative prey, the hares have few alternative predators), resulting in tightly coupled population dynamics. The complexity of most real ecosystem provides buffering effects preventing the same clear cyclical dynamics.

Question 63

What are the different types of functional response?

Answer 63

A functional response is the intake rate of the consumer as a function of food density. The Lotka-Volterra model assumes a Type I functional response - wherein the rate of prey consumption increases linearly with prey density (implying constant predation and constant prey population growth). A Type II functional response reflects an upper limit on consumption (as predator intake rate plateaus due to satiation). A Type III functional response is sigmoidal - with low consumption at low prey densities (because of predator difficulties in finding prey). At intermediate densities, consumption increases rapidly - and at very high density, satiation occurs. Type III predation is density-dependent at low prey densities, but predation can become inversely density-dependent at very high population levels.

Question 64

Can parasites limit populations in a density-dependent manner?

Answer 64

Yes. For example, cliff swallows nest colonially. The number of ectoparasites per nestling increases with the size of the colony, resulting in a decline in colony size past a certain threshold due to high parasitic burden.

Question 65

What does the Janzen-Connell Hypothesis explain?

Answer 65

This hypothesis posits that pest pressure from specialised herbivores/pathogens results in density-dependent survival of seeds and seedlings. Abundance is associated with high mortality as pathogens can spread easily between high density conspecifics (a negative feedback mechanism). Rare species are released from high pest pressure, allowing for increase in abundance. This hypothesis serves to explain the extreme biodiversity of tropical rainforests as pest pressure promotes coexistence and prevents species monoculture.

Question 66

How do human activities alter predator-prey dynamics?

Answer 66

On small islands formed by anthropogenic damming (e.g. Venezuelan Lago Guri), isolation results in an absence of top down regulation of vertebrates (i.e. through predation). Vertebrate species density (e.g. howler monkeys) can grow several orders of magnitude larger than mainland population, severely reducing seedling/sapling densities and limiting forest regeneration.

Question 67

How does niche partitioning facilitate coexistence?

Answer 67

Robert MacArthur's study of five morphologically similar insectivorous warbler species in North American coniferous forests provides a classic example of niche partitioning. MacArthur notes that the species coexisted through feeding across distinct parts of each tree (e.g. yellow-rumped warblers on lower branches, Cape May warblers on upper branches). This feeding niche separation resulted in strong density-dependence, concentrating competition within species (i.e. high population density increases intraspecific competition for shared resources, reducing growth rate). This mechanism results in population fluctuations around a stable equilibrium, preventing species from outcompeting one another and enabling coexistence - with population rescue effects when species become rare.

Question 68

Does niche partitioning eliminate interspecific competition?

Answer 68

While niche partitioning results in the quasi-independence of each species, interspecific competition still has a negative impact. If one species declines, the response of other populations would, however, be less pronounced due to niche separation.

Question 69

What is an example of a life-history trade-off resulting from interspecific competition?

Answer 69

Interspecific competition fosters specialisation, as plants must trade-off between individual growth rate and investment in defence mechanisms. Therefore, species become differentiated along niche axes. For example, the modification of leaves to spines in cacti compromises photosynthetic capacity and therefore growth rate. Cacti persist within desert niches, where the cost of tissue loss is very high due to resource scarcity (defence prioritisation). Bamboo, on the other hand, grow very rapidly but do not invest in physical defences. The montane forest niches where bamboo thrives are associated with plentiful resources and a high growth rate is key to avoid shading (e.g. shading from the fast-growing balsa tree, with enormous leaves optimised for light capture).

Question 70

What are pioneer trees and how do they differ from shade-tolerant trees?

Answer 70

Pioneer species (e.g. birch and balsa) are fast-growing, produce large volumes of seeds and cannot regenerate in the shade. These species are short-lived as fast growth entails low-density wood production (i.e. with air spaces). Shade-tolerant species (e.g. beech) grow more slowly but can regenerate well in low light. These species are long-lives as their slow growth enables the production of high-density, robust wood.

Question 71

What do long-term studies of tropical rainforest reveal about pioneer/shade-tolerant species dynamics?

Answer 71

The loss of a large tree results in a 'gap' in the rainforest ecosystem and a 'race' to reach canopy sunlight (as around one percent of sunlight reaches the forest floor). Data from Barro Colorado Island in Panama revealed that pioneer species grow rapidly when gaps appear, casting shade. These species are short-lived, enabling shade-tolerant saplings surviving in a seed bank on the forest floor to claim the pioneer gap. Conversely, when a shade-tolerant tree falls, pioneers colonise as shade-tolerant seedlings cannot survive the deep shade cast by mature shade-tolerators. This intergenerational cycling allows coexistence. Pioneers are often rarer due to the longer lifespans of shade-tolerators, but increase in frequency after disturbance events (e.g. hurricanes).

Question 72

Which lizard species serves as a common example of the possibility of repeatable evolutionary cycles resulting from niche partitioning?

Answer 72

Lizards of the Anolis genus, which colonised the Antilles islands from the South American mainland. Different species ('ecomorphs') have evolved distinct morphologies/feeding habits linked to their specific niche within their island habitat (e.g. trunk-crown, trunk-ground). Larger islands support more Anolis ecotypes as a result of habitat diversity. Across different islands, repeated independent evolution of the same ecomorph has occurred despite separate evolutionary histories. This is due to similar ecological pressures resulting in similar trade-offs - demonstrating the possibility of repeatable evolutionary cycles linked to ecological factors.

Question 73

What is Darwin's 'entangled bank'?

Answer 73

The term 'entangled bank' refers to complex ecological networks of antagonistic and mutualistic interactions. Food webs, as pioneered by Charles Elton, describe antagonistic networks of organismal interactions, illustrating the flow of energy through a particular ecosystem. These networks summarise the complexity of community interactions and contribute to understanding community structure and community dynamics (e.g. the effects of interference).

Question 74

What are different ways in which ecological networks can be represented?

Answer 74

An ecological network can be represented through interaction matrices or connectance graphs. Linkage networks, for example, track cross-species interactions. Energy flow networks quantitatively track energy flow across species. Functional/interaction strength networks describe the relative importance of interactions in structuring a community (and require experimental evidence to be sustained).

Question 75

What are some fundamental properties of networks?

Answer 75

Networks describe how nodes (e.g. species) are connected by edges (e.g. feeding links). These connected networks display small-world properties; they contain 'clique-like' sub-networks with a high clustering coefficient, with connections between almost any two nodes within. Within ecological networks, species across a variety of aquatic and terrestrial ecosystems are an average of two linkages apart, with ninety-five percent of species connected within three linkages.

Question 76

What are the implications of the small-world properties of ecological networks?

Answer 76

Changes to abundance of one species can propagate effects quickly as population dynamics are inherently interconnected. For example, over-harvesting of one species could result in knock-on effects for a number of other linked species.

Question 77

How is network connectance measured?

Answer 77

Connectance within a network (e.g. a food web) serves as a measure of interconnectedness amongst nodes, representing the proportion of possible interactions realised within a network. It is calculated as the ratio of actual interactions observed in the network to the maximum possible number of links between species. connectance = actual links/possible links

Question 78

What has observational analysis of food webs revealed about population dynamics?

Answer 78

Whilst there are some issues in using older networks, with many derived patterns thought to be artefacts due to the skewing effect of taxonomic bias or the lumping of similar species as a single node. The analysis of public networks has revealed robust, pervasive patterns, such as the truncated length of most food chains (limited to three or four trophic levels).

Question 79

What is a possible explanation for the truncated length of food chains?

Answer 79

Elton and MacArthur's Energy Attenuation Hypothesis posits that there is a finite volume of energy at the base of the food web, some of which is lost as heat at each trophic level (in accordance with the second law of thermodynamics). Beyond a certain point, there is no longer sufficient energy available in the population to sustain higher trophic levels. Communities with more efficient energy transfer or higher productivity may be defined by longer food chains. However, the instability of long chains hypothesis suggests that chance population variations are amplified up the food web, making longer chains less stable (particularly in unpredictable environments).

Question 80

What has modelling food webs revealed about population dynamics?

Answer 80

The usage of 'real networks' in modelling allowed researchers to measure robustness as secondary extinctions following simulated extinctions. The removal of species with few connections had minimal impact and the robustness of a network increased with connectance. Robert May's extension of the Lotka-Volterra model predicts short food chains, with increased complexity paradoxically reducing stability. This prediction acts in contrast to Elton's findings that complexity promotes stability through buffering effects.

Question 81

What is the implication of May's findings for real networks?

Answer 81

If increased complexity results in decreased population stability, real networks must have non-random structures enabling the persistence of complexity. Extant food webs are thus not a random subset of all food webs, but representative of a stable subset of food webs persisting through evolutionary time (i.e. extant food webs are equivalent to surviving war bombers, which have no bullet marks near to their engines).

Question 82

What have experimental studies revealed about food web dynamics?

Answer 82

Robert Paine's field experiment involving the removal of the starfish Pisaster from rocky shores showed that experimental manipulation can be used to identify keystone species. The removal of the starfish shifted food web structure and reduced species diversity. Laboratory experiments with simple communities (e.g. protists) supported May's models, as greater species richness resulted in higher extinction rates (an increase from 2.5% to 27.5% across two-species and eight-species communities). These experiments suggest that specific weak interactions may be more important to community robustness than overall complexity.

Question 83

How do indirect interactions impact ecological networks?

Answer 83

Indirect interactions can be described as dynamic linkages across species without feeding traction. Trophic cascades are one example, whereby predation at one trophic level indirectly benefits a non-adjacent level (e.g. cat predation of mice increases bee/flower abundance). Apparent competition is another, whereby two species indirectly negatively affect each other through sharing of a common predator (e.g. increases in hare abundance can increase predator numbers, negatively impacting rabbit populations without direct competition).

Question 84

Does grey squirrel outcompetition of red squirrels result from indirect interactions?

Answer 84

Partially. Grey squirrels are larger and more robust, directly competing more effectively for limited resources. These squirrels can also digest seeds with high tannin content (e.g. acorns) more efficiently, increasing survival in nutrient-scarce environments. However, grey squirrels are also carriers of the squirrelpox virus - which is fatal to red squirrels. SQPV transmission is an indirect interaction despite involving direct contact (as grey squirrels can transmit the virus through bites, scratches or bodily fluids). This is because it is non-trophic.

Question 85

What are keystone species?

Answer 85

Keystone species have far greater effects on the structure of their ecosystem than would be expected from their relative abundance/biomass. Their extinction can thus have widespread impacts. The reintroduction of wolves in Yellowstone, for example, reduced elk overgrazing and allowed for the restoration of willows and river habitats (as the river had previously grown shallower and broader in the absence of streamside willows).

Question 86

What is the relationship between ecology and population dynamics?

Answer 86

The study of the distribution and abundance of organisms and the factors driving changes in these variables underlies ecological study. As this is quantitative research, both ecological and population studies rely on analytical and statistical models.

Question 87

Are ecological models correct?

Answer 87

No. Models are inherently wrong as they present a simplified abstraction of ecological networks. However, models are useful in capturing essential features of population dynamics. All populations must increase from low density to persist and their growth must plateau due to environmental limits (carrying capacity). However, populations exhibit seemingly random fluctuations.

Question 88

How is population growth rate linked to population size?

Answer 88

Population growth rate (dN/dt) is intrinsically linked to population size. This correlation is not linear because larger populations have higher birth and death rates, resulting in larger absolute growth rates. The simplest growth model is exponential growth, which assumes infinite growth and a constant per capita growth rate: dN/dt = rN (where r = per capita population growth rate).

Question 89

What is the logistic growth model?

Answer 89

The logistic growth model incorporates carrying capacity (K) as a variable to account for real environmental limits. dN/dt = rN(K-N/K) At low population densities (N < K), the model approximates exponential growth. However, as N approaches K, K-N/K approaches 0; births balance deaths and the population growth rate approaches zero. This logistic function produces a smooth, s-shaped curve, with growth slowing as it approaches the plateau of K (the growth of bacteria in culture tightly correlates with this curve).

Question 90

What are some key features of logistic growth curves?

Answer 90

There is a linear decline from a maximum of r down to zero. This is negative density dependence, regulating population size systematically. The maximum absolute growth rate of the population occurs when N = K/2. The absolute population growth rate plotted against population size forms an inverted parabola, peaking again at N=K/2.

Question 91

What are the implications of logistic growth for resource management?

Answer 91

The concept of maximum absolute population growth rate at N = K/2 is applied in resource management. In fisheries, the reduction of a target population to N = K/2 is thought to maximise sustainable yield. However, these simple models can result in overexploitation/extinction due to stochastic variation.

Question 92

What is the difference between ecological communities and trophic guilds?

Answer 92

The term 'ecological communities' refers to groups of species co-occurring in space and time, competing for the same limiting resources (carbon dioxide, for example, is not a limiting resource as it cannot be meaningfully depleted, whereas the uptake of nitrate molecules depletes local supply). Species within an ecological community form 'trophic guilds' - groups of species sharing similar feeding habits and patterns of resource use.

Question 93

What is Gause's Principle of Competitive Exclusion?

Answer 93

Gause's Principle of Competitive Exclusion states that two species cannot indefinitely coexist on a single limiting resource if other ecological factors remain constant. One species will inevitably outcompete the other (as evidenced within natural monocultures such as palm forests and reed beds).

Question 94

Do Connell's barnacles exemplify competitive exclusion?

Answer 94

The barnacle Chthalamus is competitively excluded from low intertidal zones by the faster-growing, more aggressive Balanus barnacle. Chthalamus persists on the upper shore due to high desiccation tolerance (to which Balanus is sensitive). Therefore, varying ecological conditions allow coexistence despite competitive dominance of one species.

Question 95

Do Gause's paramecium experiments provide evidence of competitive exclusion?

Answer 95

Yes. Paramecium aurelia outcompeted Paramecium bursaria for nutrients in a well-stirred culture. When the mixture was not stirred, P. bursaria persisted at the bottom, tolerating anoxia due to photosynthetic endosymbionts. Non-constant ecological conditions (i.e. oxygen gradients) enabled coexistence. This is a post-hoc explanation, describing why one species persists rather than predicting community outcomes.

Question 96

What is Tilman's R* Theory?

Answer 96

Tilman's R* theory provides a predictive framework for competitive outcomes, particularly for labile resources in well-mixed systems. He studied diatoms (phytoplankton with silica-based cell wall structures) and observed that monocultural growth of each organism resulted in the reduction of the limiting resource (silica) to a specific concentration. The equilibrium resource concentration is R*, representing the minimum resource concentration a species requires for positive population growth. Tilman predicted that the species with the lowest R* for a particular limiting resource will be the superior competitor, as was verified with further experimentation. This predictive model can be used to predict the outcome of two species competition, through the development of a hierarchy based on R* and biomass.

Question 97

What evidence supports the validity of the R* theory over the biomass theory of two-species competition?

Answer 97

The application of both theories to tallgrass prairies in Minnesota, with nitrogen as a limiting resource. The R* theory accurately predicted that Agropyron would outcompete Agrostis after five years, a prediction not reflected by monoculture biomass models.

Question 98

What is the benefit of the Lotka-Volterra competition model over the R* model?

Answer 98

Whilst the R* theory provides strong predictions for a single limiting resource, coexistence of competitors (e.g. Chthalamus and Balanus) occurs when predicted otherwise, with the Lotka-Volterra competition model providing a more general framework. These models account for the density of intraspecific and interspecific competitors. If the competition coefficient (alpha) is more than one for an interspecific competitor, the interspecific competitor has a stronger competitive effect than conspecifics. Analysing the magnitude of the competition coefficient yields outcompetition, stable coexistence or unstable equilibrium (variation in conditions could upset competitive dynamics). Stable coexistence is thus possible when intraspecific competition is stronger than interspecific competition.

Question 99

What is the difference between continuous-time logistic models and discrete-time logistic models?

Answer 99

The continuous-time logistic model (dN/dt = rN(K-N/N)) is a simplified representation useful for the visualisation of baseline population dynamics, assuming continuous births and deaths and a smooth approach to equilibrium (thus applicable for continuously reproducing organisms e.g. bacteria). For species with synchronised reproductive events (e.g. annually calving wildebeests), a discrete-time model is more suitable. Nt+1 = Nt + rNt (K-Nt/K)

Question 100

What do 'r' and 'K' describe in terms of discrete-time logistic models?

Answer 100

'r' describes the intrinsic rate of natural increase of a population and is a life-history trait influencing population demography. 'K' describes the carrying capacity of a given habitat; it is an environmental property not a life-history trait, and is highly dependent on resource availability.

Question 101

How does environmental stochasticity impact the representation of 'K' in discrete-time models?

Answer 101

Some populations (e.g. tawny owls) display strong density dependence, whilst others fluctuate rapidly, appearing unregulated. This may link to environmental stochasticity, causing temporal variance in K (e.g. weather-related fluctuations in acorn abundance are reflected in mouse/vole population dynamics). As environmental variables are inherently unpredictable, we can represent K with a random variable. Long-term studies reveal fluctuations of annual 'K' around a mean 'K' value, reflecting environmental stochasticity. 'K' is thus stochastic and the outcome of one year cannot be predicted with relation to the outcome in another. The use of simplified models can lead to misinterpretation of natural fluctuations of 'K' as indicative of unregulation.

Question 102

How does the r value of a population impact responsiveness to fluctuations in the 'K' value?

Answer 102

Recovery time after catastrophic events (e.g. periodic crashes in grey heron populations due to the freezing over of lakes and inaccessibility of fish in harsh winters) is inversely proportional to the intrinsic rate of population growth (r). Species with high r (e.g. rabbits, which produce large litters at high frequency) recover much faster. Species with low r (e.g. humpback whales, with infrequent, slow-maturing calves) recover slowly from increased periods of adult mortality.

Question 103

Which environments favour r/K selected species?

Answer 103

r-selected species are favoured in unpredictable, hazardous environments where populations spend comparatively more time in recovery from disturbance. These species are often small-bodied, producing large, frequent broods and have early sexual maturity (e.g. blue tits/rodents). K-selected species are favoured in stable, competitive environments and are less resilient to increases in hazard frequency. They are often large bodied, mature late and produce a few, large competitive offspring (e.g. whales, elephants).

Question 104

Do life-history traits evolve in response to the environment?

Answer 104

Yes. Certain species (e.g. guppies) can show substantial trait variation across populations. This forms a fast-slow continuum of life histories (r/K continuum). Sometimes, a positive feedback loop can manifest in hazardous environments, where abiotic factors select for higher r, resulting in the production of smaller, more susceptible individuals, further increasing selection for higher r.

Question 105

What is the issue with simple models with respect to r/K dynamics?

Answer 105

Simple models can reproduce realistic population dynamics (e.g. density dependence) without identification of underlying patterns/complexity. Ecologists should apply Occam's Razor (testing simple explanations first) but also 'seek and distrust simplicity', recognising that apparent simplicity does not guarantee full correctness.