Jouni Laakso, Janne Salminen, Heikki Setälä
Department of Biology, University of Jyväskylä,
Finland
Acta. Zool. Fenn. 1995, 196:162-167
Two experiments were established to compare the relative importance of biotic (predation) versus abiotic factors affecting the soil animal community structure and the nutrient dynamics in microcosms simulating coniferous forest soil. The experiments had either a diverse or greatly reduced assemblage of soil fauna as prey community, both with or without mesostigmatid mites as predators. The experiments were carried out in the field, and under two different laboratory conditions: 1) in constant temperature and 2) in varied temperature and illumination regimes simulating field conditions. Two destructive samplings were performed, including analysis of numbers and composition of the soil fauna and measurement of KCl-extractable NH4+-N and PO43--P in the test materials. In the second experiment soil respiration was measured weekly, as well.
Abiotic conditions had stronger effects on
modifying the structure of soil animal communities and nutrient dynamics
than predators. Nevertheless, mesostigmatid mites as predators proved to
be an important group affecting the arthropod community in each abiotic
environment, although the impact of predators differed in each abiotic
conditions. The results show abiotic and biotic factors to possess complex
mutual interrelationships, emphasizing the difficulty of applying laboratory
data to field conditions. Laboratory experiments with too simple faunal
composition may overemphasize the role of small group of soil animals in
ecosystem functioning.
Impact of soil faunal structure on decomposition and N-mineralisation in relation to temperature and moisture in forest soil
Huhta, V., Sulkava, P. and Laakso, J.
University of Jyväskylä, Department of Biological and Environmental
Science,
P.O. Box 35, FIN-40351, Jyväskylä,
Finland
Pedobiologia 1996, 40:505-513.
H. Setälä*, J. Laakso, J. Mikola and V. Huhta
Applied Soil Ecology 1997, 9:25-31.
The term `biodiversity' is claimed to lack connections to a serious scientific background. In this work, we approached the concept of biodiversity from a functional point of view by asking: "At what level of the ecological organization (species, trophic species/feeding guilds, trophic levels etc.) should reduction in biodiversity matter to bring about visible changes in ecosystem performance?" We investigate the concepts of `functional diversity' and `ecosystem performance' in relation to feeding habits (such as fungivory, detritivory etc.) of soil fauna and plant growth. After analysing the results of a number of microcosm studies, we came into the following conclusions: (i) trophic level diversity has clear impacts on primary production ¯ more so when the number of trophic levels is low; (ii) manipulation of large predators at the top of the food chain had no influence on plant growth, whereas smaller mesostigmatid predators generally showed a negative impact on plant growth and nutrient uptake; (iii) declining species diversity alters carbon mineralization in heterotrophic systems due to changing interactions between trophic levels; (iv) species composition within a functional group can affect biomass production of plants; (v) inclusion of ectomycorrhizal fungi in model ecosystems may be of fundamental importance to understanding the most significant interactions between primary producers and organisms in detrital food webs.
Jouni Laakso* and Heikki Setälä
Department of Biological and Environmental
Science, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä,
Finland
Oecologia 1997, 111:565-569.
A previously undocumented association between
earthworms and red wood ants (Formica aquilonia Yarr.) was found
while investigating the influence of wood ants on the distribution and
abundance of soil animals in boreal forest soil. We took samples from ant
nest mounds and from the surrounding soil of the ant territories. The ant
nest mound surface (the uppermost approx. 5 cm layer) harboured a much
more abundant earthworm community than the surrounding soil; the biomass
of the earthworms being about 7 times higher in the nests than in the soil. Dendrodrilus
rubidus dominated the earthworm community in the nests. In
soils, Dendrobaena octaedra was more abundant. Favourable temperature, moisture
and pH (Ca content), together with abundant food supply (microbes and decomposing
litter) is likely to make a nest mound a preferred habitat to earthworms,
provided that they are not preyed upon by the ants.
We also conducted laboratory experiments
to study antipredatory mechanisms of earthworms against ants. The
experiments showed that earthworms do not escape predation by avoiding
contact with ants in their nests. The earthworm mucus repelled the ants,
suggesting a chemical defence mechanism against predation. Earthworms probably
prevent the nest mounds from becoming overgrown by molds and fungi indicating
mutualistic relationships between the earthworms and the ants.
Composition and trophic structure of detrital food web in ant (Formica aquilonia) nest mounds and in the surrounding forest soil
Jouni Laakso and Heikki Setälä
University of Jyväskylä, Department
of Biological and Environmental Science,
PO Box 35, FIN-40351, Jyväskylä,
Finland
Oikos 1998, 81:266-278
Community composition and food web structure
of soil decomposer biota in relation to various habitat properties were
compared between upper parts of red wood ant (Formica aquilonia) nest mounds
and the adjacent forest soil. For a description of trophic structure of
the decomposer community in the two habitats soil decomposers were classified
into 14 trophic groups. Classification of the taxa into three habitat preference
categories resulted in a clear division of the fauna into either soil or
nest specialists, relatively few taxa falling in between these two groups.
A large majority of the nest specialists belonged to a non-myrmecophilous
soil decomposer fauna so far largely overlooked in studies on ant - invertebrate
associations. Trophic organisation of the nest mound community differed
clearly from that in the soil by having considerably larger biomass at
the base of the food web, and less large predators -- other than ants --
at the top of the web. Contrary to forest soils, the clear dominance of
bacterial feeding microfauna over the fungal feeding microfauna in the
nest mounds suggests that most of the energy passing through the food web
is channeled through a bacterial-based food-web compartment in the nest
mounds. Relatively constant temperature and moisture in the nest surface,
continuous energy input by the ants to the nests, and ant-induced reduction
in predation pressure on macropredators are suggested to be responsible
for the development of the typical decomposer community structure in the
nest mounds. Thus, the food-web dynamics in ant nest mounds represent an
interesting case in which the behaviour of an invertebrate species (i.e.
the ant) has a potential to control the development of a system-level organisation.
The high biomass of microbi-detritivorous animals, especially earthworms,
in the nest mounds suggests that the activities of the decomposer fauna
may feed back to the structure of nest mound and indirectly alter the performance
of the ant colony.
Sensitivity of ecosystem functioning to changes in the structure of soil food
webs
(Thesis)
Laakso, J., University of Jyväskylä, 1998, 28
p.
(Biological Research Reports from the University of Jyväskylä, ISSN 0356-1062;
66)
ISBN 951-39-0226-9
The main objective of this thesis was to assess the functionally important features in the detrital food web of coniferous forest soil by studying the role of top-down influences and species composition of trophic groups in the functioning of the food web. I manipulated food web structure in microcosms containing defaunated humus and litter, a birch seedling infected with ectomycorrhizal fungi, and a diverse microbial community. The faunal composition was also manipulated under N-poor and N-enriched conditions to assess whether the outcome of trophic interactions is sensitive to the availability of N. The role of wood ant Formica aquilonia Yarr. in the soil food web and system functioning was studied using field samplings and manipulations of wood ant density. In the laboratory experiments the biomasses of microbivorous and microbi-detritivorous grazers were effectively controlled by their predators. The influence of animal grazers on microbial biomass and respiration was weak and predators of grazers could exert only a weak positive influence on microbial biomass and activity. Field studies revealed that wood ants are negatively associated with other invertebrate predators and positively associated with earthworms. Feeding trials showed that earthworms have antipredatory defence against wood ants. However, the overall effect of wood ants on soil food web was weak outside the nest mounds. The presence of a diverse microbivore and microbi-detritivore community, and maintenance of nest mounds by wood ants increased species richness of soil fauna. N mineralisation, and plant growth and N uptake increased in the presence of microbivorous and detritivorous fauna but their predators generally had a weak influence on these processes. N-enrichment magnified the influence of food web structure on microbial biomass and plant N uptake, and modified the outcome of animal-microbial interactions. Wood ants did not influence N dynamics and plant growth outside the nest mounds. The weak propagation of top-down influences and high number of functionally redundant species suggest that ecosystem functioning is robust against changes in the structure of below-ground food webs.
Jouni Laakso
University of Jyväskylä, Department
of Biological and Environmental Science, P.O. Box 35, FIN-40351, Jyväskylä,
Finland
Soil biology and Biochemistry 1999, 31:337-343.
Wood ants are known to affect the structure
of invertebrate community in tree canopies but it is not clear whether
wood ants can affect the structure of soil animal communities. To isolate
the direct impact of wood ants on soil fauna, I manipulated wood ant density
in a six-week mesocosm experiment by placing mesocosms (diameter 14 cm, height
13.5 cm) permeable to ants either on or outside the ant trails. The mesocosms contained litter, humus, and a diverse soil fauna typical of
coniferous forest. I controlled earthworm species composition and biomasses
by inoculating 0.47 g fresh mass of epigeic lumbricid Dendrobaena octaedra
(Sav.) or Dendrodrilus rubidus (Sav.) to the mesocosms.
A 15-fold difference in ant density
affected both taxonomic and trophic structure of the soil invertebrate
community. The biomass of epigeic arthropod predators, Arachnida and Opiliones,
was reduced in the high ant density treatment, whereas the biomass of mesofauna
predators increased in the presence of ants. Although total earthworm biomass
was not affected by the wood ants, reproductive output of epigeic earthworm
Dendrodrilus rubidus increased under high wood ant density, while
reproduction of Dendrobaena octaedra was indifferent. The overall impact
of wood ants on soil fauna was relatively weak: despite the 15-fold difference
in wood ant density between the low and the high density treatments, only
some taxa were affected. Consequently, the direct impact of wood ants on
soil animal food web is likely to be small.
Population and ecosystem level consequences of predation on microbial feeding nematodes
Jouni Laakso and Heikki Setälä
University of Jyväskylä, Department of Biological and Environmental
Science,
P.O. Box 35, FIN-40351, Jyväskylä,
Finland
Oecologia 1999, 120:279-286.
Microbial feeding nematodes are often considered
as one of the most influential groups of soil fauna in decomposition processes.
We studied the role of nematode predation in the functioning of detrital
food webs in autotrophic miniecosystems simulating boreal forest floor.
An increasing gradient of top-down control upon microbivorous nematodes
was created by assembling food webs either without predators, or in various
combinations with a specialist and a non-specialist predatory mite belonging
to Mesostigmata. The prey community was composed of either (i) three species
of bacterivorous or (ii) three species of fungivorous nematodes, or both
groups together. The miniecosystems contained defaunated humus and litter
materials, a diverse microbial community with bacteria, fungi and protozoa,
and a birch (Betula pendula) seedling infected with mycorrhizal fungi.
After two growing periods for the birch (38 weeks), the miniecosystems
were destructively sampled for animal and microbial biomasses, concentration
of mineral N in the soil, plant biomass and N and S uptake.
The specialist predator reduced nematode
biomasses in both bacterial and fungal energy channels by more than 50%,
whereas the non-specialist had a weak positive effect on fungivorous nematodes.
Thus, under high predation pressure, the biomass of microbivores behaved
as predicted by trophic dynamic models assuming strong top-down control
and uniformly behaving trophic levels. Despite this, microbial biomass
was unaffected by the predators. However, microbial respiration generally
increased in the presence of predators. Assuming that microbial respiration
correlates with microbial productivity, the increase in microbial respiration
indicates an occurrence of cascading productivity regulation. The composition
of microbivore community had only a minor effect on the outcome of top-down
control on microbes. The predators reduced nematode respiration and biomass
by more than 50% whereas the secondary influence of this reduction on microbial
biomass and respiration was on average less than 2 and 16%, respectively.
This implies that the propagation of top-down influences in microbivore
food webs is strongly limited.
Presence of the specialist predator
reduced concentration of mineral N in soils at week 38, but this did not
reflect in plant growth or N uptake. Apart from plant S uptake, variation
in the vertical and horizontal structure of the food web had only weak
control over ecosystem processes.
Jouni Laakso and Heikki Setälä
University of Jyväskylä, Department
of Biological and Environmental Science, PO Box 35, FIN-40351, Jyväskylä,
Finland
Oikos 1999, 87:57-64
The proposed mechanisms for the species diversity–function
relationship in plant communities stress the recognition of functional
properties of species, and interactions between plants and soil processes.
As resource availability to plants is influenced by the architecture of
decomposer food webs, it has been hypothesised that the diversity of decomposers
can also control ecosystem processes, including primary production.
We manipulated the complexity of soil
animal communities in a miniecosystem experiment in which a boreal forest
floor with birch seedlings infected with mycorrhizal fungi was created.
The soil animal diversity ranged from zero to typical species richness
of soil fauna (approximately 50 taxa) in coniferous forests. Between these
extremes was a nested factorial design with manually assembled communities
consisting of two sets of one-species, and two sets of five-species animal
communities within fungivorous and microbi-detritivorous trophic groups.
To investigate the role predators in system functioning, the miniecosystems
with fungivorous and microbi-detritivorous fauna were established either
with or without mesostigmatid mites as top predators. The miniecosystems
were incubated in a climate chamber with varying illumination and temperature
regimes for 40 weeks.
Our experiment provides evidence that
primary productivity is generally insensitive to variation at the species
level or even at the level of trophic groups. Although top predators generally
reduced prey population size, no effect was found on system functioning.
However, the removal of microbe- or detritus-feeding fauna, especially
the microbi-detritivore Cognettia sphagnetorum reduced plant N uptake and
accumulation of plant biomass. The functional importance of soil fauna
was inversely related to the trophic position of the group. Our results
suggest that ecosystem functioning is robust against species extinctions
in belowground food webs, and that primary production is predominantly
controlled by organisms at low trophic positions in the decomposer food
web.
Jouni Laakso and Heikki Setälä
University of Jyväskylä, Department
of Biological and Environmental Science, P.O. Box 35, FIN-40351, Jyväskylä,
Finland
Annales Zoologici Fennici 2000, 37: 93–100.
We studied the impact of wood ants on the structure and functioning of the invertebrate food web of boreal forest during a three-year period. Special emphasis was put on the detritus-based food web. The density of wood ants was reduced > 90% by eliminating whole wood ant colonies from five areas in Central Finland, and leaving five areas as controls. We monitored the growth of spruce, pine and birch seedlings planted on homogenised plots, and development of the invertebrate community in soil and vegetation. The ant removal had little effects on the structure of the invertebrate food web. However, biomasses of predators caught in pitfall traps were 60% smaller under high ant density than under low ant density. Also, at the end of the experiment the biomass of the lumbricid earthworm Dendrobaena octaedra was reduced by 54% in the homogenised plots without ant nests. Removal of wood ants had no influence on soil microbial biomass, concentration of soil mineral nitrogen, plant N uptake, or plant growth.
Juha Katajisto, Veikko Huhta and Jouni Laakso
University of Jyväskylä, Department
of Biological and Environmental Science,
P.O. Box 35, FIN-40351, Jyväskylä,
Finland
European Journal of Soil Biology 1999, 35:17-21.
An experiment was carried out in microcosms for testing the hypothesis that a higher level of primary production should maintain a decomposer community with higher biomass and activity. Microcosms with coniferous forest humus and a diverse microbial and faunal community were divided into three sets: (1) control without plants, (2) with birch seedlings in full illumination, and (3) with birch seedlings, shaded to reduce the net primary production. During 16 weeks’ incubation at +16°C no treatment effects were found in numbers or biomasses of taxonomic or functional groups of soil organisms, nor in the system respiration in darkness. The community structure of the shaded systems differed from that of the other two treatments (CCA analysis). High organic content of the soil and long time lag in the system response were considered the main reasons for the small difference between the treatments.
Résumé Une experience a ete conduite dans des microcosmes pour tester l´hypothese considerant que le niveau eleve de la production primaire devrait maintenir une communaute decomposante avec une activite et biomasse elevee. Des microcosmes avec humus de foret de conifere et une diverse communaute de microbe et faune etaient divises en 3 series: (1) controle sans plantes, (2) avec bouleau comme plante en plein eclairage, et (3) avec bouleau comme plante, ombrage pour reduire la production primaire nette. Durant les 16 semaines d´incubation a +16ºC aucun effet de traitement n´etait observe ni en nombre ou biomasse taxonomique ou dans le fonctionnement de groupes d´organisme du sol, ni dans le systeme de respiration dans l´obscurite. La structure de la communaute du systeme ombrage differait de celle des deus autres traitements (CCA analyses). Le contenu organic eleve du sol et le long delai dans le systeme de reponse etaient consideres comme les principales raisons de peu de difference entre les traitements.
Jouni Laakso1, Heikki Setälä1 and Ansa Palojärvi2
1Department of Biological and Environmental
Science, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä,
Finland
2Agricultural Research Centre of Finland,
FIN-31600 Jokioinen, Finland
Plant and Soil 2000, 225:153-165
We studied the sensitivity of soil microbial
communities and ecosystem processes to variation in the vertical and horizontal
structure of decomposer food web under nitrogen poor and N-enriched conditions.
Microcosms with humus and litter layer of boreal forest floor, birch seedlings
infected with mycorrhizal fungi, and decomposer food webs with differing
trophic group and species composition of soil fauna were constructed. During
the second growing period for the birch, we irrigated half of the microcosms
with urea solution, and the other half with de-ionised water to create
two levels of N concentration in the substrate. During the experiment night
time respirations of the microcosms were measured, and the water leached
through the microcosms was analysed for concentration of mineral N, and
nematode numbers. The microcosms were destructively sampled after 37 weeks
for plant biomass and N uptake, structure of soil animal and microbial
community (indicated by PLFA profiles), and physical and chemical properties
of the humus and litter materials.
Predatory mites and nematodes had
a negative influence on the biomass of their microbivorous and microbi-detritivorous
prey, and microbi-detritivores affected the biomass and community structure
of microbes (indicated by PLFA-analysis). Moreover, predatory mites and
nematodes increased microbial biomass and changed the microbial community
structure. The decomposer food web structure affected also N uptake and
growth of plants. Microbi-detritivorous fauna had a positive effect, whereas
predators of microbial and detritus feeding fauna exerted a negative influence
on plant N uptake and biomass production. The impact of a trophic group
on the microbes and plant was also strongly dependent on species composition
within the group.
Nitrogen addition magnified the influence
of food web structure on microbial biomass and plant N uptake. We suggest
that addition of urea-N to the soil modified the animal-microbe interaction
by increasing microbial growth and altering community structure of microbes.
The presence of microbi-detritivores
and predators reduced loss of carbon from the microcosms, and the food
web structure influenced also water holding capacity of the materials.
The changes in plant growth, nutrient cycling, size of N and C pools, and
in the physical properties of the soil emphasize the importance and diversity
of indirect consequences of decomposer food web structure.
Ranta, E., Lundberg, P., Kaitala, V. and Laakso, J.
Proceedings of the Royal Society B 2000, 267:1851-1856.
Characterizing population fluctuations and their causes is a major theme in population ecology. The debate is on the relative merits of density-dependent and density-independent effects. One paradigm (revived by the research on global warming and its relation to long-term population data) states that fluctuations in population densities can often be accounted for by external noise. Several empirical models have been suggested to support this view. We followed this by assuming a given population skeleton dynamics (Ricker dynamics and second-order autoregressive dynamics) topped off with noise composed of low- and high-frequency components. Our aim was to determine to what extent the modulated population dynamics correlate with the noise signal. High correlations (with time-lag 1) were observed with both model categories in the region of stable dynamics, but not in the region of periodic or complex dynamics. This finding is not very sensitive to low-frequency noise. High correlations throughout the entire range of dynamics are only achievable when the impact of the noise is very high. Fitted parameter values of skeleton dynamics modulated with noise are prone to err substantially. This casts doubt as to what degree the underlying dynamics are any more recognizable after being modulated by the external noise.
Ylikarjula, J., Alaja, S., Laakso, J. and Tesar, D.
Journal of Theoretical Biology 2000, 207:377-387.
In this paper, we examine the consequences of the number of spatially scattered local populations by investigating the effects of different global dispersal patterns on the dynamics of two and a larger number of coupled populations and on the level of synchrony between them. We also consider asymmetrical dispersal and the presence of environmental heterogeneity. According to our results, both population dynamics and the level of synchrony differ markedly between two and a larger number of local populations. For two patches different dispersal rules give very versatile results. However, for a larger number of local populations the dynamics are similar irrespective of the dispersal rule. For example, for the parameter values yielding stable or periodic dynamics in a single population, the dynamics do not change when the patches are coupled with dispersal. High intensity of dispersal does not guarantee synchrony in uctuations of local populations. The level of synchrony depends also on dispersal rule, the number of local populations and the intrinsic rate of increase. In our study, the effects of density-independent and density-dependent dispersal rules do not show any consistent difference. The results call for caution when drawing general conclusions from models of only two interacting populations and cause doubts about the applicability of a large number of theoretical papers dealing with two local populations.
Laakso, J. and Kaitala, V. and Ranta, E.
Oikos 2001, 92:119-122.
Birth and death rates, as so many other biological processes, are usually not linearly related to environmental variation. Common examples of non-linear response forms include unimodal "optimum-type" responses and various saturating responses. These responses filter the signal coming from the environment to a corresponding biological process. We explored how different types of environmental signal may be transformed to a biological processes. We were interested on the effect of the filter to modulate (1) variance of the signal, (2) variance-covariance structure between the signal and filtered signal, and on (3) the match between power spectra of the signal and the filtered signal. We found that the filters will change frequency distribution (mean, variance, modality) of the signal. Especially symmetric filters that have a single peak of optimum will change signal structure so that there either exist or does not exist correlation between the signal and the filtered signal. When the correlation exists it may be either positive or negative depending on the signal's mode relative to the filter structure. Also, the power spectrum properties of the signal may be dramatically transformed after passing the filter, e.g., blue noise can turn to red noise. Our results strongly suggest that the studies on the influence of external signal on biological processes, such as population dynamics, should explicitly consider how the signal is transferred to biological processes.
Sulkava, P., Huhta, V., Laakso, J. and Gylén, E-R.
Oecologia 2001, 129:133-138.
We tested 1) how the presence of a diverse soil faunal community affects ecosystem carbon balance, and 2) whether habitat patchiness modifies the influence of soil fauna on plant growth and carbon dynamics. We constructed cylindrical microcosms that contained coniferous forest humus and different litter materials either mixed or in separate patches, and in the presence or absence of diverse soil mesofauna. A birch seedling was planted in the centre of each microcosm. The experiment continued for two growing periods during which the net carbon assimilation was measured continuously. At the end of the experiment the microcosms were destructively sampled for plant biomass, soil fauna, and soil physical and chemical properties. All systems, independently of treatment, were net CO2 producers in the beginning. In the presence of diverse fauna the growth of plants was drastically increased, and the mixed litter systems respired more than the patchy ones. During the second season the patch effect disappeared, while the birch seedlings and mosses continued to grow better in the microcosms with diverse fauna. In the long term, patchiness did not modify the effect of fauna on plant growth or carbon balance. By the end of the experiment the carbon balance approached zero in the refaunated microcosms, while it remained negative in the “simple” systems. The weak impact of patchiness in comparison to the faunal effect may be due to homogenising role of plant roots and progressing decay of the substrates.
Nonlinear biological responses to disturbance: consequences on population dynamics.
Laakso, J. Kaitala, V. and Ranta, E.
Ecological Modelling 2003, 162: 247-258.
We assessed how nonlinear biological responses to environmental noise, or "noise filtering", impact the spectra of density-dependent population dynamics, and the correlation between noise and population dynamics. The noise was assumed to affect population growth rate in a discrete-time population model by Hassell (1975) where the population growth rate was linked to the environment with an optimum type filter. When compared to unfiltered noise, the filtered noise can distort the stationary distribution of population values. The optimum type filter can make cyclic population dynamics more regular and low population values can become more frequent or rare depending on the strength of density dependence. Filtering can cause blue shifted and red shifted population dynamics and determine the strength of correlation between environmental noise and population size. In most cases, optimum type filtering makes linear correlation between population dynamics and noise weaker. The filter effect on population spectra and noise vs. population correlation is sensitive to changes in population model parameters, the location where noise hits the filter, and noise colour.
Gårdmark A., Enberg, K., Ripa, J., Laakso, J. and Kaitala, V.
Ann. Zool. Fenn. 2003 40: 131-144.
The current high rate of population declines and attempts to `manage' their recovery, call for a better understanding of recovery dynamics of populations. In many cases, recovery of a population may primarily be determined by a single life history property or ecological interaction, allowing for straightforward management actions. For example, a generalist predator may prevent the recovery of its prey, and populations with sex-biased dispersal are particularly vulnerable to demographic stochasticity. However, linking life history with intra- and interspecific population dynamics is needed to assess the relative importance of these factors. A clear example is depensatory dynamics that can be caused either by e.g., mutual predation or cooperative breeding. Moreover, dynamics of a recovering population can alter both its physiological and behavioural traits, affecting its interspecific interactions. Here we review life histories (reproduction, resource use and dispersal) and species interactions affecting recovery processes, and discuss their implications for management.
Environmental noise and population dynamics in ciliated protozoa Tetrahymena thermophila in aquatic microcosms
Laakso, J., Löytynoja, K., and Kaitala, V.
Oikos 2003, 102: 663-671
Population theory predicts that the reddened environmental noise, especially in combination with high population growth rate, reddens population dynamics, increases population variability and strengthens environment-population correlation. We tested these predictions with axenic populations of ciliated protozoa Tetrahymena thermophila. Populations with low and high growth rate were cultured in a stable environment, and in environments with sublethal temperature fluctuations that had blue, white and red spectra (i.e. negatively autocorrelated, uncorrelated, or positively autocorrelated, respectively). Population size and biomass of individuals were determined at three-hour intervals for 18 days.
Dynamics of all populations were reddened, suggesting that internal mechanisms can redden the population spectra. However, population dynamics were reddest, variability highest, and environment-population correlation strongest in the red environment as predicted. Contrary to theoretical predictions and previous empirical findings, population growth rate (rmax being equal to 0.05 and 0.3 h-1) had no effect on population dynamics.
Mean cell size and variability of cell size were affected by the presence and type of environmental noise suggesting that the physiological consequences of variability depend on colour. Environmental variability decreased mean population size and biomass and the decrease was strongest in rapidly fluctuating blue and white environments. The latter finding implies that rapid fluctuations are physiologically stressful, an effect that is not accounted for in the basic population models.
Nonlinear in biological responses to disturbance affect population extinction risk.
Laakso, J. and Kaitala, V., Ranta, E.
Oikos 2004, 104: 142-148.
Nonlinearities are commonly observed in the responses of organisms to environment. They potentially modify the qualitative and quantitative properties of population dynamics. We studied how nonlinear responses to environment, or "noise filters", influence population variability and extinction risk by introducing coloured noise to the growth rate in the Hassell (1975) single-species model. The consequences of noise filtering were analysed by comparing the model dynamics with linearly filtered and nonlinearly filtered noise that have the same mean. Three biologically plausible filters we used: saturating, unimodal optimum type, and sigmoid responses. Filtering can either decrease or increase population variability when compared to linear noise response. The effect of noise filtering on variability is most pronounced with stable population dynamics and the outcome depends on the filter type, population growth rate, and noise colour. Nonlinear noise filtering predominantly increases extinction risks when population growth rate is low (R < 5). As an exception, saturating filter has a window of decreased risk at very low growth rate and reddened environment. In the unstable range of the dynamics (15 < R < 25), the optimum and saturating type responses can decrease the extinction risk whereas the sigmoid response increases extinction risk. Noise filtering can also alter the route to extinction in slowly growing populations from slow to rapid decline towards extinction in optimum and saturating responses. These results suggest that accounting for the nonlinear responses to environment should be considered when estimating extinction risks and population variability. Moreover, the nonlinear responses make noise colour a more important factor in these analyses.
Evolution of Hsp90 expression in Tetrahymena thermophila (Protozoa, Ciliata) populations exposed to thermally variable environment.
Ketola, T., Laakso, J., Kaitala, V. and Airaksinen, S.
Evolution 2004, 58: 741-748.
Evolutionary consequences of thermally varying environments were studied in the ciliated protozoan Tetrahymena thermophila. Replicated lines were propagated for 60 days, a maximum of 500 generations, in stable, slowly fluctuating ('red spectrum') and rapidly fluctuating ('blue spectrum') temperatures. The red and blue fluctuations had a dominant period length of 15d and 2h, respectively. The mean temperature of all time series was 25°C and the fluctuating temperatures had the same minimum (10°C), maximum (40°C) and variance. During the experiment, population sizes and biomasses were monitored at three day intervals. After the experiment, carrying capacity and maximum growth rate were measured at low (15°C), intermediate (25°C) and high (35°C) temperatures for each experimental line. Physiological changes in the lines were assessed by measuring the expression of stress-induced heat shock protein Hsp90 at 25, 35, and 39°C. Population sizes and biomasses showed no differences between stable, blue or red temperature treatments during the experiment. Also, after the experiment mean carrying capacities and maximum growth rates were comparable in the stable, blue and red temperature treatments. The expression of Hsp90 was higher in lines from the blue environment than in lines from the stable environment. Lines from the red environment had an intermediate level of Hsp90 expression. This supports the hypothesis that inducible thermotolerance and canalizing genes can evolve in response to rapidly varying environments. Furthermore, we found correlative evidence of benefits and disadvantages of high Hsp90 expression. Lines with high expression of Hsp90 had an increased growth rate at the highest temperature when food resources were not limiting growth. At low and intermediate temperatures the same lines had the lowest carrying capacities.
Population dynamic consequences of adaptive growth rate in fluctuating environment
Jouni Laakso, Veijo Kaitala and Esa Ranta
Ecological Modelling 2005, in press
Linkage between environmental variability and population renewal processes is assumed invariable in standard population models. We explored adaptive, density-dependent population dynamics in a setting where population consists of individuals differing in location of their optimum performance relative to a stochastic and autocorrelated environmental variable, such as temperature, humidity or salinity. Relative growth rates of phenotypes are determined by a Gaussian unimodal function of the environmental variable, which also has a unimodal distribution. Adaptive dynamics causes a significant increase in mean population growth performance and decreases its variance. Population growth fluctuates more rapidly, i.e. has a blue-shifted spectrum. This also causes a blue shift in density-dependent population dynamics in a wide range of population growth rates and autocorrelation of the environment. The blue-shift is most pronounced when the breadth of environmental response is moderately smaller that the breadth of environmental fluctuations. The causes of the blue shift are discussed. The blue shift caused by adaptive dynamics is in contrast with views that evolutionary processes redden population time series, i.e. cause slow fluctuations, due to slow changes in population properties. Mean genetic variance is maximised in rapidly fluctuating blue and white noise environments, and when genotype performance breadth is small. In contrast, genotype diversity is maximised in slowly fluctuating red environments and in populations with large performance breadth. Genetic variance and genetic diversity also have marked temporal fluctuations. Temporal variability of genetic variance increases with noise redness and small response breadth. However, genetic diversity is maximised in slightly reddened environments and narrow performance breadth populations. That genetic variance and diversity are affected by the spectrum of environmental noise and also fluctuate temporally, suggest that models assuming constant amount of genetic variability (equilibrium variance) may not be successful in predicting the consequences of environmental changes.
Interactions between environmental variability and immigration rate control patterns of species diversity
Teppo Hiltunen, Jouni Laakso and Veijo Kaitala
Ecological Modelling 2005, in press
Theories focussing on local competition processes predict that species diversity is maximised with high-frequency environmental variability or alternatively with intermediate frequencies. The models coupling regional processes to local explanations of diversity patterns predict that the immigration rate from a regional species pool can strongly increase diversity. However, the interaction between local and regional processes in temporally fluctuating environment has received little attention.
We explored in a simulation study how the patterns of species diversity are affected by the frequency spectrum of stochastic environmental variations and density independent immigration rate in a competition model. Environmental variations were assumed to affect the growth rate of competing species. The growth rate of a species had a unimodal response to a continuous environmental variable, such as salinity and temperature. The location of optimum and breadth of the growth response in the environmental niche axis is treated as a species-dependent trait. Consequently, environmental variations can affect competitive ranking of the species.
Decreasing the frequency of environmental variations produced monotonically decreasing or unimodal “intermediate disturbance type” diversity patterns in the competition model. The level of diversity and type of the pattern was determined by immigration rate, and the number and traits of the species initially placed along the environmental niche axis. Immigration generally increased diversity, but the amount of increase in intermediate immigration rates was strongly modified by the frequency spectrum of the environmental variation. Decreasing the frequency of the environmental variation always produced a monotonically decreasing diversity pattern in a two-species system where the growth optima are positioned symmetrically around the mode of the environmental noise. However, increasing the species number and asymmetry of optima makes the diversity pattern sensitive to immigration rate: diversity can be very low and unaffected by the frequency of environmental variation in the absence of immigration, peak in low-frequency environment with small immigration rate (i.e. produce the IDH type pattern), or decrease monotonically with high immigration rate. Diversity is most sensitive to small changes in the initial species number and position of the species growth optima when environmental fluctuations have high frequency.
That immigration can change the impact of the frequency of the environmental variation on diversity from monotonically decreasing to unimodal (IDH type) pattern emphasizes that the regional process can be important for explanations of the diversity patterns, especially when the immigration rates are uncorrelated to the local conditions. The strong interactions between community properties, immigration rate, and the spectrum of environmental variation suggest that small changes in community properties or in dispersal rates can have a large impact on species diversity.
Last modified 01.02.2006 /Jouni Laakso