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Novel parasite invasion leads to rapid demographic compensation and recovery in an experimental population of guppies

Citation

Bassar, Ron (2020), Novel parasite invasion leads to rapid demographic compensation and recovery in an experimental population of guppies, Dryad, Dataset, https://doi.org/10.5061/dryad.jm63xsj7z

Abstract

The global movement of pathogens is altering populations and communities through a variety of direct and indirect ecological pathways.  The direct effect of a pathogen on a host is reduced survival, which can lead to decreased population densities. However, theory also suggests that increased mortality can lead to no change or even increases in the density of the host. This paradoxical result can occur in a regulated population when the pathogen’s negative effect on survival is countered by increased reproduction at the lower density.  Here we analyze data from a long-term capture-mark-recapture experiment of Trinidadian guppies (Poecilia reticulata) that was recently infected with a nematode parasite (Camallanus cotti). By comparing the newly infected population with a control population that was not infected we show that decreases in the density of the infected guppy population were transient. The guppy population compensated for the decreased survival by a density-dependent increase in recruitment of new individuals into the population, without any change in the underlying recruitment function. Increased recruitment was related to an increase in the somatic growth of uninfected fish. Twenty months into the new invasion, the population had fully recovered to pre-invasion densities even though the prevalence of infection of fish in the population remained high (72%). These results show that density-mediated indirect effects of novel parasites can be positive, not negative, which makes it difficult to extrapolate to how pathogens will affect species interactions in communities.  We discuss possible hypotheses for the rapid recovery.

Methods

Field Data (Individual Data and Population Summaries)

We established four experimental populations of guppies in the upper Guanapo drainage in the Northern Mountain Range of Trinidad, West Indies. All four populations were established from a single source population on the lower Guanapo River where guppies live in diverse fish communities and with other species that frequently prey upon guppies. The guppies from these high predation communities were moved to the experimental streams, which previously lacked guppies, but were otherwise similar to natural upstream locations without predators. Upstream barriers prevent the upstream movement of guppies out of the experimental populations. More details of the experimental translocations can be found elsewhere.

During our monthly recapture, each fish greater than 14mm standard length is measured for mass, photographed and, if not previously marked, marked with a unique color combination of subcutaneous elastomer implants (Northwest Marine Technologies). Each fish receives two colors in two of eight locations on the body allowing us to individually identify 4,032 fish of each sex. Fish are lightly anesthetized with MS-222 for processing, housed overnight in medicated water to prevent infections from marking, and returned to the streams the following day.

We estimated the population size, survival, and recruitment of females and male guppies in the population using the POPAN module of program MARK implemented in Program R. For each population, we fitted models that were fully time dependent (apparent survival and probability of capture) and crossed by sex. Because our populations are introduced and we know exactly how many individuals were released into each stream, we constrained the probably of capture parameter for the first time period to be equal to unity. Other than this parameter, the default settings were used for each model.

Lab Data 

Female guppies (n=104) were collected from the extralimital site in the Caigual in January 2019 (month 131).  All guppies were caught using butterfly nets and transported back to the field station in two-liter Nalgene bottles filled with stream water.  A wide range of guppy sizes were chosen to understand the impact of the parasite in all life stages and sizes of the fish.  The initial standard length (SL, distance from the tip of the snout to the hypural plate) of the fish ranged from 7.99 to 33.35 millimeters. 

In the laboratory, the fish were housed in three-gallon tanks that received constant aeration and were under ambient temperature and light conditions.  The fish were not marked but were housed individually for the duration of the experiment, making them individually identifiable by tank number.  The water was changed every third day.

At the start of the experiment and each week thereafter, the standard length of each fish was measured to the nearest 0.01 mm using digital calipers (Mitutoyo) and the fish was weighed to the nearest 0.001 g on an Ohaus Scout scale.  During these weekly measurements, all fish were individually anaesthetized with 0.2% Tricaine S (MS-222) and screened for visible signs of infection using a stereo-microscope (Carl Zeiss Stemi 305).

Fish were fed twice daily with live, freshly hatched brine shrimp napauli (Artemia spp.).  Fish were fed food rations proportional to their size using a glass microliter syringe (Hamilton 50 μl or 250 μl micropipette).  Food amount was calculated each week according to the fish’s most recent weight, using the equation food= mass*e(4-(0.5*mass)) . Tanks were checked twice daily for offspring and dead fish.  Any offspring born during the course of the experiment were counted and housed separately from the mother.  Fish that died during the course of the experiment were immediately preserved in seven percent buffered formalin and saved for later dissection.

            After four weeks, the fish were euthanized with an overdose of MS-222 and preserved in seven percent buffered formalin or pure ethanol. Only females were selected for dissection because we sought to determine the effect of infection on fecundity (number of embryos) and offspring size. The gastrointestinal tract of each fish was examined for worms and, if worms were present, the number and wet weight of the worms was recorded to quantify parasite load.  The number of embryos and regressors (aborted embryos or unfertilized eggs) was recorded.  The somatic tissue, ovary tissue, regressors, and embryos were dried at 50° Celsius for at least 24 hours in a drying oven (Quincy Lab Model 40) and dry weights were recorded.

 

Usage Notes

Individual Data

individual_id = curated identifier code (each individual is associated to an number)

stream = which stream the individual is from

sampling  = which month of the study is the observation.

sex = If the individual is male (M) or female (F)

weight = Individual weight when captured in g.

parasite = 0 if no parasite observed. 1 if parasite observed.

survival = 0 if individual was not observed in subsequent capture periods. 1 if individual was observed in subsequent capture periods.

F.dens.hat = density of female guppies in the stream (individuals / square meter)

Note: Missing values are noted by NA symbols and occur when an individual fish is not captured on one sampling occassion, but is later observed.

 

Population Summaries

stream = which stream the summary is from

sampling  = which month of the study is the observation.

year = year of the study

month = calendar month

M.dens.hat, M.dens.lcl, M.dens.ucl = mean, lower 95% confidence interval, upper 95% confidence interval of male density

(individuals / square meter)

F.dens.hat, F.dens.lcl, F.dens.ucl=mean, lower 95% confidence interval, upper 95% confidence interval of female density (individuals / square meter)

M.bio.dens.hat = mean grams of male fish per meter square

F.bio.dens.hat = mean grams of female fish per meter square

M.phi, M.phi.lcl, M.phi.ucl = mean, lower 95% confidence interval, upper 95% confidence interval of male survival

F.phi, F.phi.lcl, F.phi.ucl = mean, lower 95% confidence interval, upper 95% confidence interval of female survival

F.rec.dens.hat, F.rec.dens.lcl, F.rec.dens.ucl=mean, lower 95% confidence interval, upper 95% confidence interval of female recruit density (individuals / square meter)

prop.para = proportion of population observed with parasites

 

Lab Data

ID = Numeric indentifier of an individual

Sex = If the individual is male (M) or female (F)

Initial.Length = standard length at the begining of the experiment (mm)

num.embryos = number of embryos being carried at the end of the experiment

embryo.wet.wt = mass in mg of embryos

stage = developmental stage of embryos (0-50)

num.regressors = number of undeveloped eggs.

num.worms = number of parasites in gut

embryo.dry.et = dry mass of all embryos (g)

para.load = wet mass of parasites in gut (g)