Population density and sociality Flashcards
(32 cards)
Does density always indicate habitat quality?
No. Three possible reasons:
- Winter habitat use is critical for some species, and population surveys are generally only done in one season
- Multi-annual variability in local population density can reflect small-scale food source variability, predators, or abiotic environmental factors
- Social interactions - subdominants may not be able to access high-quality habitat, which suppresses reproduction in high-quality habitat
What does habitat quality mean from a population context? What is habitat quality the product of?
habitat quality is the product of density, mean individual survival probability, and mean expectation of future offspring
What other parameters could be used to improve density-based estimates of dispersal?
Can be improved by paying attention to immigration rates and patterns, adult survival, and the production of juveniles that survive to reproduce
What is the theoretical model of habitat occupancy introduced by Fretwell and Lucas (1969)?
the movement of individuals into poor habitat is a reflection of individual fitness maximization
The per-individual probability of success for unestablished immigrants may be higher in low-quality habitat than in high-quality habitat, because high densities in the high-quality habitat promote a high probability of failure to reproduce successfully and a high mortality rate among the unestablished immigrants
What species characteristics are associated with habitat quality-density decoupling?
- The species should have a social pattern of dominance interactions where it is found in stable populations in high-quality habitat
- High reproductive capacity - this can allow sink populations to reach high densities when the environment becomes temporarily favourable
- This decoupling should be most characteristic of habitat generalists - this is particularly important as such generalists may be used as indicators of habitat quality for a variety of species in those cases where habitat-quality ratings are based on actual survey/census data (Generalists are relatively easy to survey)
- These three characteristics are more closely associated with small rather than large body size
It is likely that for rare species, density may remain a reasonably good indicator of habitat quality if seasonal changes in habitat use are taken into account and if habitat is not patchy
What is the difference between group size being adaptive or emergent?
Adaptive - encoded in the individual
Emergent - groups are non-permanent units fusing and splitting up and an increase in the distance which animals perceive each other increases group fusion/group size
In large mammalian herbivores, the increase of group size with habitat openness was first assumed to be an adaptive response, encoded in the individual. However, it could be an emergent property
What are some general trends identified with herd size in large herbivores?
- Group size tends to increase with population density (emergent - increased density increases rate of ground meeting, therefore size)
- Herd size increases with habitat openness (biological adaptation/encoded - can reduce detection probability by predators in closed habitats by being in small groups, can increase survival in open habitat by being in large groups)
What are the two general types of models that attempt to model how groups are formed and their size?
Optimum-size-seeking models - fitness is maximized for a given group size, and individuals behave as if they know which group size will give them better fitness
Fusion-fission models - groups of large herbivores were non-permanent units that often fused and split up - suggesting that any increase in population density should increase the rate of group meeting, and thus the average group size
What is the optimum-size seeking model, and who first proposed a model?
Sibly 1983 had the idea to develop and examine the properties of a dynamical model of group formation formalizing the idea that individuals exhibited preferences for group sizes shaped by natural selection
Fitness is maximized for a given group size, and individuals behave as if they know which group size will give them better fitness
What are some of the problems with optimum-size seeking models?
There are several problems with the modeling proposed here, as these assumptions and results do not reflect what is actually seen in groups
- mean group size at equilibrium is larger than optimum group size
- At equilibrium, no group size shows a size lower than the optimum size - the distribution of the model does not reflect what is actually seen
- Model does not depend on population density - contrasts with the effect recorded in the populations of large herbivores
- Model treats groups as permanent units at equilibrium - inconsistent with the high lability of groups revealed in an increasing number of large herbivore species
Initial model ignored kin selection
What are fusion-fission models, and who first proposed a model?
Caughley (1964) stated that an increase in population density should increase the mean size of groups that frequently fuse and split up, by enhancing the rate of group encounter and thus fusion. It could be hypothesized that any increase of habitat openness, and thus of the distance at which groups can perceive one another, should increase the rate of group fusion, and thus mean group size
Fusion-fission models can be the mechanism behind both the increase of group size with population density, and increase of group size with habitat openness
This is a purely mechanistic approach - group size is emergent, resulting from multiple fusion/fragmentation events
Properties of these models:
- Group size distributions obtained at equilibrium resemble those ordinarily recorded in large herbivore populations
- Group size distribution at equilibrium for any given value of the parameters is independent of the initial group size distribution, provided population density is left unchanged
- Any increase in population density entails, at equilibrium, not only an increase in mean group size, but also an increase in group density (the number of groups per unit area)
- Any increase in the distance at which groups can perceive one another increases mean group size at equilibrium
What are the effects of habitat openness on group sizes?
Estes 1974 and Jarman 1974 hypothesized that in closed habitat, a herbivore can easily reduce the probability of being detected by predators by being discreet and, especially, by living in small groups.
In open habitat, it is more difficult to escape noticed - being surrounded by many conspecifics should then ensure the best protection against predators
How do fusion-fission models contrast with Sibly’s optimum-size seeking model?
These models contrast with Sibly’s model because groups are assumed to fuse and split up without any group size being preferred by the individuals
Can group size undergo adaptive modification?
Yes. If an individual had less tendency to spontaneously leave the group it is in than did other members of the population, it would be at the origin of fewer group-splitting events and thus would find itself in groups on average a little larger
Such an adaptive modification might occur only if the parameters involved really are affected by mutations (and, if possible, mutations not having effects on other aspects of the phenotype)
What is the resource dispersion hypothesis?
The resource dispersion hypothesis (RDH) asserts that if resources are heterogeneous in space or time, group living might be less costly than was previously thought, regardless of whether individuals gain direct benefits from group membership
where resources are patchily distributed over space/time, the economics of exploiting these patches enables several individuals to share resources over a common area, satisfying their resource needs without imposing large costs on each other
RDH can be generalized to predict spacing behaviour in non-social animals and utilization of resources other than food
RDH could explain social organizations across the animal kingdom, including species that are not normally gregarious, and in those that live in large non-territorial congregations
RDH holds without any assumptions about dominance or other relationships between animals
What are some advantages and disadvantages to animals living in groups?
Advantages:
- cooperative hunting
- predator avoidance
- alloparental care
- reduced predation risk
- maintenance of forage quality
- better access to information about resources
- better defence of resources
- reduced risk of infanticide
Disadvantages:
- elevated parasite burden
- increased infanticide risk
- increased feeding competition
- conspecific aggression
Why do species partition available space? Is this process random?
They partition available space to defend exclusive territories. The size and shape of these territories represent an economic optimum, which cannot be randomly constructed because resources are not randomly distributed through space
How does the pattern of resource availability in space and time influence group size?
Increases in overall resource abundance lead to increases in habitat quality per unit area, resulting in higher animal densities, and typically smaller ranging areas. But resource abundance itself does not necessarily affect group size, however, because rich territories are typically contracted, such that individuals maintain the former per-capita intake rate
But if resources are heterogeneous in space or time, this is no longer necessarily true - proportional increases in average resource abundance might not enable the territory to shrink, because larger areas are still needed to encompass the temporal and spatial variability of these resources. Virtually no resource in nature is homogeneously distributed, therefore the uneven distribution of resources should be an important factor in how animals use their environment
What is density-dependence?
density-dependent processes occur when population growth rates are regulated by the density of a population
What mechanisms can cause density-dependent dispersal?
Competition inducing individuals to emigrate (positive density-dependence)
Social crowding effects impeding free movement (negative density-dependence)
Is positive or negative density-dependence more common in birds and mammals?
The general pattern that emerges is that density-dependent dispersal is fairly common in birds and mammals (nearly half of all studies) and predominantly positive. This is in agreement with most evolutionary models that predict an increase in dispersal because of competitive interactions at high densities - suggesting that conspecific attraction and/or social fence mechanisms are much less common
Bird studies more often reported DD for emigration rates or long-distance recoveries than for average distances within finite study areas
Experimental density manipulation studies consistently generated positive DD, typically showing reduced emigration in response to partial population removal
Studies that compared dispersal between sites different in density also show a mixture of positive and negative DD - suggesting that dispersal changes in a more complex way with seasonal and spatial density variation than with annual densities, or these results are confounded by other factors different between seasons and sites, such as habitat quality
What factors are confounded with intra-annual changes in density
- timing of reproductive events
- changes in age composition
- general environmental conditions
- (particularly in life-cycles with multiple generations per year) also with phases of population growth and decline
What is positive density-dependence? What are the prevailing hypotheses?
Most widespread hypothesis on DD is that competition increases the likelihood of dispersing because individuals have better fitness prospects by leaving high-density sites. The increase in dispersal may be mediated through dominance interactions or outright aggression, or by deteriorating environmental conditions as a consequence of crowding
McCarthy (1997, 1999) predicted that competition would lead to increased dispersal but only up to a point; above a critical ratio of dispersers to vacancies, realized dispersal will decrease again because vacancies are increasingly filled by short-distance dispersers, while long-distance dispersers increasingly fail to settle
What is negative density-dependence? What are the prevailing hypotheses?
The “social fence” and related hypotheses predict that high densities actually lead to reduced dispersal (particularly reduced immigration) because of the increasing likelihood of aggressive encounters. The social fence will act more strongly if residents are not only intolerant of new settlers, but also of transient individuals not (yet) attempting to settle
Negative DD can also be caused by conspecific attraction (a tendency for individuals to be attracted to areas with many conspecifics, or inversely to emigrate out of low-density areas which may induce local Allee effects. Conspecific attraction can be explained by the benefits of social aggregations (anti-predator behaviour, social foraging) or by the use of conspecific density as a cue for availability of resources or mates
