Population dynamics of coral-reef fishes: Controversial concepts and hypotheses

Abstract Knowledge of processes that drive the local population dynamics of coral-reef fishes is important for managing reef fisheries, and for using these species as models for understanding the ecology of demersal marine fishes in general. However, the reef-fish literature is replete with poorly defined concepts and vague hypotheses regarding the issue of population dynamics. Dichotomous arguments, such as whether or not recruitment drives population dynamics, are misdirected because they fail to incorporate several important concepts. First, changes in local population size are driven by four demographic rates (birth, death, immigration and emigration), all of which must be studied to understand population dynamics. Second, all populations that persist do so because at least one of these demographic rates operates in a density-dependent way that is both sufficiently strong and appropriately time-lagged. Therefore, identifying the source(s) of direct density dependence is critical for understanding the limits to variation in population size (i.e. population regulation). Third, regulation does not imply a simple point equilibrium in population size; density dependence in populations of reef fishes is bound to lie within a field of stochastic variation, and thus be difficult to detect. Since its formal origin in 1981, the ‘recruitment limitation’ hypothesis for explaining local population dynamics in reef fishes has undergone ambiguous changes in definition that threaten its usefulness. ‘Recruitment, ‘originally defined as the appearance of newly settled fish on a reef, more recently is often measured months after settlement, thus confounding pre- and post-settlement processes. ‘Limitation, ‘ which originally referred to recruitment being so low as to preclude local populations from reaching densities where resources were limiting, is more recently defined as an absence of any form of density dependence after settlement. The most effective means of testing whether post-settlement mortality is in fact density-independent is to examine patterns of mortality directly, rather than indirectly by interpreting the shape of the relationship between initial recruit density and subsequent adult density within a cohort (the recruit-adult function). Understanding the population dynamics of coral-reef fishes will require a more equitable focus on all four demographic rates, be they density dependent or not, as well as greater attention to identifying sources of density dependence. Such a pluralistic focus necessitates integrated studies of both pre- and post-settlement processes conducted at multiple spatial and temporal scales. For example, recent evidence suggests that density-dependent pre-dation on new recruits that have settled among reefs at different densities may prove to be an important source of local population regulation, especially via the aggregative response of transient piscivores.     

Fluctuations in food supply drive recruitment variation in a marine fish

Reproductive rates and survival of young in animal populations figure centrally in generating management and conservation strategies. Model systems suggest that food supply can drive these often highly variable properties, yet for many wild species, quantifying such effects and assessing their implications have been challenging. We used spatially explicit time series of a well-studied marine reef fish (black surfperch Embiotoca jacksoni) and its known prey resources to evaluate the extent to which fluctuations in food supply influenced production of young by adults and survival of young to subadulthood. Our analyses reveal: (i) variable food available to both adults and to their offspring directly produced an order of magnitude variation in the number of young-of-year (YOY) produced per adult and (ii) food available to YOY produced a similar magnitude of variation in their subsequent survival. We also show that such large natural variation in vital rates can significantly alter decision thresholds (biological reference points) important for precautionary management. These findings reveal how knowledge of food resources can improve understanding of population dynamics and reduce risk of overharvest by more accurately identifying periods of low recruitment.     

Crypsis in a heterogeneous environment: relationships between changeable polymorphic colour patterns and behaviour in a galaxiid fish

1. The ability to achieve optimal camouflage varies between microhabitats in heterogeneous environments, potentially restricting individuals to a single habitat or imposing a compromise on crypsis to match several habitats. However, animals may exhibit morphological and behavioural attributes that enhance crypsis in different habitats. 2. We used an undescribed fish species, Galaxias‘nebula’, to investigate two objectives. First, we examined two potential methods of enhancing crypsis: change in colour pattern and selection of a suitable background. Second, we characterised the colour pattern of this unstudied fish and assessed its capacity for crypsis. 3. No background selection was apparent but the area of dark pigment expressed varied between backgrounds, which may negate the requirement to be choosy about habitats. The capacity to change colour and selection of a background that maximises crypsis are most likely separate, non‐mutually exclusive strategies. 4. Galaxias‘nebula’ exhibits polymorphic, non‐interchangeable colour patterns that have elements of both background pattern matching and disruptive colouration. This, coupled with habitat characteristics, suggests a combination of generalist and specialist strategies of habitat use. The fish’s camouflage strategy and air‐breathing ability may be key to survival under increasing pressure from habitat degradation and invasive predators.     

Ontogenetic and spatial variation in size‐selective mortality of a marine fish

Although body size can affect individual fitness, ontogenetic and spatial variation in the ecology of an organism may determine the relative advantages of size and growth. During an 8‐year field study in the Bahamas, we examined selective mortality on size and growth throughout the entire reef‐associated life phase of a common coral‐reef fish, Stegastes partitus (the bicolour damselfish). On average, faster‐growing juveniles experienced greater mortality, though as adults, larger individuals had higher survival. Comparing patterns of selection observed at four separate populations revealed that greater population density was associated with stronger selection for larger adult size. Large adults may be favoured because they are superior competitors and less susceptible to gape‐limited predators. Laboratory experiments suggested that selective mortality of fast‐growing juveniles was likely because of risk‐prone foraging behaviour. These patterns suggest that variation in ecological interactions may lead to complex patterns of lifetime selection on body size.     

Recruitment variability of resident brown trout in peripheral populations from southern Europe

1. Population regulation was studied for seven consecutive years (1992–98) in five rivers at the periphery of the distribution of Salmo trutta, where the fish were living under environmental constraints quite different from those of the main distribution area. 2. Recruitment is naturally highly variable and the populations had been earlier classified as overexploited. Thus we expected that densities of young trout in most populations would be too low for density‐dependent mortality to operate. We tested this by fitting the abundance of recruits to egg densities over seven consecutive years (stock–recruitment relationship), and used the results to judge whether exploitation should be restricted in the interests of conserving the populations. 3. The density of 0+ trout in early September, as well as the initial density of eggs and parents, varied greatly among localities and years. The data for all populations fitted the Ricker stock–recruitment model. The proportion of variance explained by the population curves varied between 32% and 51%. However, in most cases the observations were in the density‐independent part of the stock–recruitment curve, where densities of the recruits increased proportionally with egg densities. 4. Our findings suggest that recruitment densities in most rivers and years were below the carrying capacity of the habitats. Although density‐dependent mechanisms seemed to regulate fish abundance in some cases, environmental factors and harvesting appeared generally to preclude populations from reaching densities high enough for negative feedbacks to operate. The findings thus lend support to Haldane’s (1956) second hypothesis that changes in population density are primarily due to density‐independent factors in unfavourable areas and areas with low density due to exploitation. Exploitation should be reduced to allow natural selection to operate more effectively.     

Recruitment as a driver of production dynamics in stream-resident brown trout (Salmo trutta)

1. The objective was to assess the role of recruitment as a determinant of the production dynamics of stream-resident brown trout ( Salmo trutta ) across replicate habitats of contrasting quality and population attributes. A total of 128-year-classes (YC) at 12 stream sites were examined along four tributaries of the Rio Esva drainage (northwestern Spain).
2. A meta-comparison revealed that growth, density, mortality and production were essentially site-specific. However, when all data were pooled, recruitment (as a delayed density-dependent process) affected both growth and mortality in a way such that individuals in YC with high recruitment grew less and had higher mortality.
3. The value of total YC production recorded covered the global range of variation in the production of stream salmonids reported in the literature. Linear regressions of log-transformed data revealed that 89.0%, 58.9% and 70.7% of the variation in YC density, biomass and production, respectively, were explained by variations in recruitment.
4. The inclusion of growth and mortality, together with recruitment, into a multiple regression increased the variance explained of the total YC production by 13.3%, from 70.7% to 84.0%.
5. The functional relationships between recruitment and the population attributes elucidated in this study appear to provide a useful tool for management applications, including forecasting population status.     

Body condition dependent dispersal in a heterogeneous environment

We find the evolutionarily stable dispersal behaviour of a population that inhabits a heterogeneous environment where patches differ in safety (the probability that a juvenile individual survives until reproduction) and productivity (the total competitive weight of offspring produced by the local individual), assuming that these characteristics do not change over time. The body condition of clonally produced offspring varies within and between families. Offspring compete for patches in a weighted lottery, and dispersal is driven by kin competition. Survival during dispersal may depend on body condition, and competitive ability increases with increasing body condition. The evolutionarily stable strategy predicts that families abandon patches which are too unsafe or do not produce enough successful dispersers. From families that invest in retaining their natal patches, individuals stay in the patch that are less suitable for dispersal whereas the better dispersers disperse. However, this clear within-family pattern is often not reflected in the population-wide body condition distribution of dispersers or non-dispersers. This may be an explanation why empirical data do not show any general relationship between body condition and dispersal. When all individuals are equally good dispersers, then there exist equivalence classes defined by the competitive weight that remains in a patch. An equivalence class consists of infinitely many dispersal strategies that are selectively neutral. This provides an explanation why very diverse patterns found in body condition dependent dispersal data can all be equally evolutionarily stable.     

Giving‐up densities and ideal pre‐emptive patch use in a predatory benthic stream fish

1. We used observational and experimental field studies together with an individual‐based simulation model to demonstrate that behaviours of mottled sculpin (Cottus bairdi) were broadly consistent with the expectations of Giving‐Up Density theory and an Ideal Pre‐emptive Distribution habitat selection model. 2. Specifically we found that: (i) adult mottled sculpin established territories within patches characterised by significantly higher prey densities and prey renewal rates than patches occupied by juveniles or randomly selected patches; (ii) patches abandoned by adult sculpin possessed significantly lower prey densities than newly occupied patches, although this was not true for juveniles; (iii) the observed giving‐up density (GUD) for adult sculpin (i.e. average prey density in patches recently abandoned) increased linearly with increasing fish size up to the average prey density measured in randomly selected patches (i.e. 350 prey items per 0.1 m²) and decreased with increasing sculpin density and (iv) juveniles rapidly shifted their distribution towards the highest quality patches following removal of competitively dominant adult sculpin. 3. These results provide the first evidence of the applicability of GUD theory to a stream‐dwelling organism, and they elucidate the underlying factors influencing juvenile and adult sculpin habitat selection and movement behaviours. Furthermore, optimal patch use, ideal pre‐emptive habitat selection and juvenile ‘floating’ provide behavioural mechanisms linking environmental heterogeneity in the stream benthos to density‐dependent regulation of mottled sculpin populations in this system.     

Patterns of seasonal and yearly mass variation in West African tropical savannah birds

Birds in the northern hemisphere usually increase mass reserves in response to seasonal low temperatures and shorter day length that increase foraging unpredictability and so starvation risk. In the lowland tropics, relatively low temperatures and short day lengths are absent and so the risk of starvation may be reduced, leading to much smaller seasonal effects on mass. Nevertheless, other factors such as high temperatures and water and food availability may vary greatly between tropical wet and dry seasons, leading to variable starvation risk and seasonal mass effects. Using data collected from 47 species of birds caught over a 10‐year period in a tropical savannah region in West Africa we tested for seasonal variation in mass in response to a predictable, strongly seasonal tropical climate. Many species (91%) showed seasonal variation in mass, and this was often in a clear annual pattern that was constant across the years. Many species (89%) varied their mass in response to seasonally predictable rainfall. Annual variation in mass was also important (45% of species). Relatively few species (13%) had a seasonal pattern of mass variation that varied between years. Feeding guild or migratory status was not found to affect seasonal or annual mass variation. Seasonal mass change was on average 8.1% across the 21 species with a very large sample size and was comparable with both northern and southern temperate species. Our study showed that biologically significant consistent seasonal mass variation is common in tropical savannah bird species, and this is most likely in response to changing resource availability brought about by seasonal rainfall and the interrupted foraging response due to the constraints of breeding.     

Density-dependent population dynamics in clonal organisms: a modelling approach

1. Density dependence may act at several stages in an organisms life-cycle (e.g. on mortality, fecundity, etc.), but not all density-dependent processes necessarily regulate population size. In this paper I use a density manipulation experiment to determine the effects of density on the transition rates between different size classes of the clonal zoanthid Palythoa caesia Dana 1846. I then formulate a density-dependent matrix model of population dynamics of Palythoa , and perform a series of sensitivity analyses on the model to determine at what stage in the life-cycle regulation acts.
2. Seven of the 16 transition probabilities decreased with density, most of them being shrinkage (due to loss of tissue or fission) and stasis (the self–self transition) of medium and large colonies. The only probability to increase was for the stasis of large colonies. Recruitment was quadratically dependent on density, peaking at intermediate densities.
3. Equilibrium cover in the model was 84% and was reached in ≈40 years. To determine which density-dependent transitions were involved in population regulation, the strength of density dependence was varied in each independently. This sensitivity analysis showed that only changes in the probabilities of large colonies remaining large and producing medium colonies, were regulating.
4. These results suggest that regulation is primarily acting on fission of large colonies to produce intermediate-sized colonies, in combination with size specific growth rates. Fission rates decrease greatly with density, resulting in a greater proportion of large colonies at high densities and large colonies grow more slowly than small. Overall, this behaviour is very similar to that of clonal plants which have a phalanx type life history.     

Population size,habitat fragmentation,and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect adaptive genetic variation and adaptive population differentiation are largely unexplored. Owing to pronounced genetic drift, small, fragmented populations are thought to exhibit reduced adaptive genetic variation relative to large populations. Yet fragmentation is known to increase variability within and among habitats as population size decreases. Such variability might instead favour the maintenance of adaptive polymorphisms and/or generate more variability in adaptive differentiation at smaller population size. We investigated these alternative hypotheses by analysing coding-gene, single-nucleotide polymorphisms associated with different biological functions in fragmented brook trout populations of variable sizes. Putative adaptive differentiation was greater between small and large populations or among small populations than among large populations. These trends were stronger for genetic population size measures than demographic ones and were present despite pronounced drift in small populations. Our results suggest that fragmentation affects natural selection and that the changes elicited in the adaptive genetic composition and differentiation of fragmented populations vary with population size. By generating more variable evolutionary responses, the alteration of selective pressures during habitat fragmentation may affect future population persistence independently of, and perhaps long before, the effects of demographic and genetic stochasticity are manifest.     

Influence of activity in a heterogeneous environment on the dynamics of fish growth: an individual-based model of roacl

A spatio-temporal individual-based model (IBM), including bioenergetic principles, is used to investigate how energy costs coupled to activity represent a variable fraction in a heterogeneous environment and how this is related to another energetic criterion, the potential individual growth, under the conditions of a structured environment. With this approach, it is possible to connect simulated spatio-temporal activity patterns with the energetic needs required for these activities. By using simple foraging rules and a modified random walk model it is possible to reproduce spatial distributions and length frequency distributions. The simulated spatial distribution of roach Rutilus rutilus in Lake Belau, Germany, results in a mean weekly activity multiplier [(standard metabolic rate + activity costs) (standard metabolic rate)⁻¹] of 1–9 with deviations >100% during a simulated year. These deviations are of key importance to differences in the growth rate of individual simulated roach.     

Climatic drivers of pinyon mouse Peromyscus truei population dynamics in a resource-restricted environment

Highly variable patterns in temperature and rainfall events can have pronounced consequences for small mammals in resource-restricted environments. Climatic factors can therefore play a crucial role in determining the fates of small mammal populations. We applied Pradel's temporal symmetry model to a 21-year capture–recapture dataset to study population dynamics of the pinyon mouse (Peromyscus truei) in a semi-arid mixed oak woodland in California, USA. We examined time-, season- and sex-specific variation in realized population growth rate (λ) and its constituent vital rates, apparent survival and recruitment. We also tested the influence of climatic factors on these rates. Overall monthly apparent survival was 0.81 ± 0.004 (estimate ± SE). Survival was generally higher during wetter months (October–May) but varied over time. Monthly recruitment rate was 0.18 ± 0.01, ranging from 0.07 ± 0.01 to 0.63 ± 0.07. Although population growth rate (λ) was highly variable, overall monthly growth rate was close to 1.0, indicating a stable population during the study period (λ ± SE = 0.99 ± 0.01). Average temperature and its variability negatively affected survival, whereas rainfall positively influenced survival and recruitment rates, and thus the population growth rate. Our results suggest that seasonal rainfall and variation in temperature at the local scale, rather than regional climatic patterns, more strongly affected vital rates in this population. Discerning such linkages between species' population dynamics and environmental variability are critical for understanding local and regional impacts of global climate change, and for gauging viability and resilience of populations in resource-restricted environments.     

Population size is weakly related to quantitative genetic variation and trait differentiation in a stream fish

How population size influences quantitative genetic variation and differentiation among natural, fragmented populations remains unresolved. Small, isolated populations might occupy poor quality habitats and lose genetic variation more rapidly due to genetic drift than large populations. Genetic drift might furthermore overcome selection as population size decreases. Collectively, this might result in directional changes in additive genetic variation (V_A) and trait differentiation (Q_ST) from small to large population size. Alternatively, small populations might exhibit larger variation in V_A and Q_ST if habitat fragmentation increases variability in habitat types. We explored these alternatives by investigating V_A and Q_ST using nine fragmented populations of brook trout varying 50‐fold in census size N (179–8416) and 10‐fold in effective number of breeders, N_b (18–135). Across 15 traits, no evidence was found for consistent differences in V_A and Q_ST with population size and almost no evidence for increased variability of V_A or Q_ST estimates at small population size. This suggests that (i) small populations of some species may retain adaptive potential according to commonly adopted quantitative genetic measures and (ii) populations of varying sizes experience a variety of environmental conditions in nature, however extremely large studies are likely required before any firm conclusions can be made.     

Impact of male alternative reproductive tactics on female costs of sexual conflict under variation in operational sex ratio and population density

Sexual conflict over mating rate is both pervasive and evolutionarily costly. For females, the lifetime reproductive fitness costs that arise through interactions with potential mates will be influenced by the frequency of such interactions, and the fitness cost of each interaction. Both of these factors are likely to be influenced by variation in operational sex ratio (OSR) and population density. Variation in OSR‐ and density‐dependent male alternative reproductive tactics (ARTs) may be particularly important if the fitness costs that females experience vary with the reproductive tactics that males express. Using a simple model, we consider several examples of OSR‐ and/or density‐dependent variation in male ARTs and the frequency of male–female interactions, and find that variation in the expression of male ARTs has the potential to augment or diminish the costs of frequent male interactions for females. Accurately documenting variation in the expression of male ARTs and associated female fitness costs will benefit future work in this area.