definition of insurance hypothesis

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• Review Article
• Published: 11 May 2000

The diversity–stability debate

• Kevin Shear McCann 1  

Nature volume  405 ,  pages 228–233 ( 2000 ) Cite this article

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There exists little doubt that the Earth's biodiversity is declining. The Nature Conservancy, for example, has documented that one-third of the plant and animal species in the United States are now at risk of extinction. The problem is a monumental one, and forces us to consider in depth how we expect ecosystems, which ultimately are our life-support systems, to respond to reductions in diversity. This issue — commonly referred to as the diversity–stability debate — is the subject of this review, which synthesizes historical ideas with recent advances. Both theory and empirical evidence agree that we should expect declines in diversity to accelerate the simplification of ecological communities.

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Acknowledgements

This paper benefited from comments by D. Raffaelli. I also thank J. Rasmussen and P. Yodzis for conversations on this issue, and D. Kramer for providing a single comment that led me to a different viewpoint.

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McCann, K. The diversity–stability debate. Nature 405 , 228–233 (2000). https://doi.org/10.1038/35012234

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Testing a biological mechanism of the insurance hypothesis in experimental aquatic communities

Affiliation.

• 1 Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, UK.
• PMID: 19594662
• DOI: 10.1111/j.1365-2656.2009.01586.x

1. The insurance hypothesis predicts a stabilizing effect of increasing species richness on community and ecosystem properties. Difference among species' responses to environmental fluctuations provides a general mechanism for the hypothesis. Previous experimental investigations of the insurance hypothesis have not examined this mechanism directly. 2. First, responses to temperature of four protist species were measured in laboratory microcosms. For each species, we measured the response of intrinsic rate of increase (r) and carrying capacity (K) to temperature. 3. Next, communities containing pairs of species were exposed to temperature fluctuations. Community biomass varied less when correlation in K between species (but not r) was more negative, and this resulted from more negative covariances in population sizes, as predicted. Results were contingent on species identity, with findings differing between analyses including or not including communities containing one particular species. 4. These findings provide the clearest support to date for this mechanism of the insurance hypothesis. Biodiversity, in terms of differences in species' responses to environmental fluctuations (i.e. functional response diversity) stabilizes community dynamics.

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• Research Support, Non-U.S. Gov't
• Bacteria / growth & development
• Biodiversity*
• Ciliophora / physiology*
• Models, Biological
• Population Dynamics
• Temperature

The chytrid insurance hypothesis : integrating parasitic chytrids into a biodiversity–ecosystem functioning framework for phytoplankton–zooplankton population dynamics

• Special Issue: Parasites in Aquatic Ecology
• Open access
• Published: 16 February 2024
• Volume 204 , pages 279–288, ( 2024 )

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• András Abonyi   ORCID: orcid.org/0000-0003-0593-5932 1 , 2 ,
• Johanna Fornberg   ORCID: orcid.org/0000-0001-5350-7401 3 ,
• Serena Rasconi   ORCID: orcid.org/0000-0001-6667-8904 4 ,
• Robert Ptacnik   ORCID: orcid.org/0000-0001-7176-7653 1 ,
• Martin J. Kainz   ORCID: orcid.org/0000-0002-2388-1504 1 , 5 &
• Kevin D. Lafferty   ORCID: orcid.org/0000-0001-7583-4593 6  

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In temperate lakes, eutrophication and warm temperatures can promote cyanobacteria blooms that reduce water quality and impair food-chain support. Although parasitic chytrids of phytoplankton might compete with zooplankton, they also indirectly support zooplankton populations through the “mycoloop”, which helps move energy and essential dietary molecules from inedible phytoplankton to zooplankton. Here, we consider how the mycoloop might fit into the biodiversity–ecosystem functioning (BEF) framework. BEF considers how more diverse communities can benefit ecosystem functions like zooplankton production. Chytrids are themselves part of pelagic food webs and they directly contribute to zooplankton diets through spore production and by increasing host edibility. The additional way that chytrids might support BEF is if they engage in “kill-the-winner” dynamics. In contrast to grazers, which result in “eat-the-edible” dynamics, kill-the-winner dynamics can occur for host-specific infectious diseases that control the abundance of dominant (in this case inedible) hosts and thus limit the competitive exclusion of poorer (in this case edible) competitors. Thus, if phytoplankton diversity provides functions, and chytrids support algal diversity, chytrids could indirectly favour edible phytoplankton. All three mechanisms are linked to diversity and therefore provide some “insurance” for zooplankton production against the impacts of eutrophication and warming. In our perspective piece, we explore evidence for the chytrid insurance hypothesis , identify exceptions and knowledge gaps, and outline future research directions.

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Chytrid parasitism facilitates trophic transfer between bloom-forming cyanobacteria and zooplankton (Daphnia)

Pathways linking nutrient enrichment, habitat structure, and parasitism to host–resource interactions

Exploring the Implications of the Stoichiometric Modulation of Planktonic Predation

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Introduction

The Irish Potato famine is reminder of what can happen when a population relies on a narrow diet. In lakes, generalist grazers like Daphnia push the phytoplankton community towards a low-diversity state dominated by inedible species at the expense of food-chain support for zooplankton. Indeed, the theory of biodiversity–ecosystem functioning ascribes several positive benefits to diverse communities (Table  1 ). A proposed source of food during phytoplankton blooms is the microbial loop (Pomeroy 1974 ) that occurs when bacteria, phagotrophic flagellates, and protozoans decompose organic material and then become a supplemental food source for zooplankton (Porter 1996 ). However, microbes are not always a sufficient replacement diet for edible phytoplankton (Taipale et al. 2012 ).

Like the microbial loop hypothesis, the mycoloop hypothesis aims to explain how zooplankton often persist under bloom conditions (Kagami et al. 2007a ; Kagami et al. 2007b ; Grami et al. 2011 ; Table  1 ). In contrast to the microbial loop, the chytrid fungi (Phylum Chytridiomycota) that infect and kill phytoplankton (not the distantly related chytrids that infect amphibian hosts), form an energetic loop that redirects energy from inedible phytoplankton to zooplankton in the form of edible fungal zoospores (Kagami et al. 2014 ; Frenken et al. 2017 ). This mycoloop provides essential dietary molecules (Taube et al. 2019 ; Gerphagnon et al. 2019 ; Rasconi et al. 2020 ) that benefit zooplankton (Agha et al. 2016 ; Frenken et al. 2020a ; Frenken et al. 2020b ; Abonyi et al. 2023 ) and exceed the carbon cycled through the microbial loop (Abonyi et al. 2023 ). Therefore, the mycoloop is a potentially more important source of food during bloom conditions than the microbial loop (e.g. Kagami et al. 2007a ; Kagami et al. 2007b ; Agha et al. 2016 ; Abonyi et al. 2023 ). But if and how chytrids contribute to pelagic food webs outside bloom events is unclear. In this perspective piece, we consider a new idea called the “ chytrid insurance hypothesis ” that considers a role for parasites in biodiversity–ecosystem functioning.

The biodiversity–ecosystem functioning hypothesis (BEF) argues that diversity can promote stability, production, and other ecosystem attributes (Naeem and Li 1997 ; Hooper et al. 2005 ; Duffy et al. 2017 ). This can occur if having more species in a community makes it easier for the community to respond to changing or unpredictable conditions due to mechanisms like complementarity (species play different roles) or a portfolio/sampling effect (if there are many species, at least one is bound to be successful at a particular moment) (Loreau and Hector 2001 ; Table  1 ). To that end, according to the insurance hypothesis, biodiversity increases the chance that some species maintain functioning even if others fail (Yachi and Loreau 1999 ; Table  1 ). Just as a diverse stock portfolio gives some insurance that a worker’s retirement account makes steady gains even in an uncertain financial market, biodiversity can help insure that an ecosystem will function and provide benefits even in unstable or unpredictable conditions. Thus, it seems plausible that a more diverse diet could help insure zooplankton against population crashes (Striebel et al. 2009 ). As envisioned by Frainer et al. ( 2018 ), there are three ways that chytrid parasites might mediate biodiversity–ecosystem functioning: (1) parasites produce edible stages, (2) infected hosts are altered in a way that adds trait diversity to a host population, (3) parasites help maintain host diversity. We propose that chytrids can do all three: (1) free-swimming chytrid zoospores are nutritious food for zooplankton, (2) chytrid sporangia can make inedible hosts edible by infection of the host, (3) chytrid host specificity maintains edible phytoplankton diversity. If true, chytrids provide insurance to zooplankton against conditions that would otherwise lead to blooms of inedible phytoplankton.

Zooplankton and phytoplankton communities

Phytoplankton community composition can regulate zooplankton growth (Behl and Stibor 2015 ). Zooplankton can be picky eaters, preferring small- to medium-sized diet items, such as Cryptophytes and green algae (Fig.  1 ). Filter-feeding zooplankton, like rotifers, selectively graze on small things < 10 µm (Arndt 1993 ), whereas non-selective cladocerans ( Daphnia ; Burns 1968 ; Carpenter et al. 1993 ) and copepods (Vanderploeg et al. 1984 ; Bern 1994 ) eat small- and medium-sized food (< 30 μm). However, most zooplankton are unable to eat large phytoplankton (> 30 µm). Diverse phytoplankton communities tend to have more edible species. More diverse communities are also better food, because they have more lipids (Stockenreiter et al. 2012 ), including long-chain polyunsaturated fatty acids (LC PUFA; Marzetz et al. 2017 ) that zooplankton need (Tessier and Goulden 1982 ), but cannot synthesise (Gulati and Demott 1997 ; Table  1 ). Under eutrophication and warming conditions (Paerl and Huisman 2008 ; O’Neil et al. 2012 ), the diverse phytoplankton community often shifts from diatoms and chrysophytes to a few green algae or cyanobacteria (Ptacnik et al. 2008 ; Glibert et al. 2018 ; Gobbler 2020 ). Figure  1 illustrates how this compositional shift reduces the availability of LC PUFA and sterols for zooplankton (Taipale et al. 2019 ; Calderini et al. 2023 ), thereby decreasing zooplankton production (Elert et al. 2003 ; Ger et al. 2016 ; Peltomaa et al. 2017 ). Indeed, because phytoplankton vary in their edibility and nutritional value (Taipale et al. 2013 ), phytoplankton community structure can affect zooplankton production more than phytoplankton abundance (Calderini et al. 2023 ).

Phytoplankton vary in size and nutritional quality for zooplankton. High-quality phytoplankton are rich in polyunsaturated fatty acids (PUFA) (green background), such as the small-sized palatable Stephanodiscus (1), Scenedesmus (2), Chlamydomonas (3) or Cryptomonas (4), or, the large-sized inedible Asterionella (5) and Dinobryon (6). Poor-quality phytoplankton lack PUFA and sterols (red background), such as the small-sized edible cyanobacterium Synechococcus (7), the small colonial Aphanocapsa (8), and the inedible bloom-forming cyanobacteria Planktothrix (9), Microcystis (10) or Dolichospermum (11). Rotifers preferentially take up food < 10 µm (e.g. Bdelloidea, 12), whereas copepods (e.g. Cyclops , 13) and cladocerans (e.g. Daphnia , 14) eat things < 30 µm. Green arrows show dietary pathways between edible phytoplankton and zooplankton. Figures 1–11: authors’ drawings, Figs. 12–14: www.phylopic.org

Parasitic chytrids are pervasive and diverse in pelagic food webs

Key to our hypothesis is that chytrids are diverse and their abundance scales with host density. There are many chytrid species, and collectively, they often reach 20% and can reach almost 100% prevalence during phytoplankton blooms (Rasconi 2012 ). Free-swimming zoospores (~ 2–5 µm; Sparrow 1960 ) seek out and, depending on the chytrid species, infect a range of algae and cyanobacteria, mainly inedible taxa (Sparrow 1960 ; Sime-Ngando 2012 ; Money 2016 ). Upon successful infection, chytrids develop sessile sporangia that convert host biomass into spores. This reduces phytoplankton fitness and can be fatal (Frenken et al. 2017 ).

Chytrid zoospores are nutritious food for zooplankton

The mycoloop is an example of how adding parasite species can directly increase ecosystem functioning. Although it might seem as if chytrids simply compete with zooplankton for phytoplankton, zoospores are eaten by both selective (e.g. rotifers, see Frenken et al. 2018 ) and non-selective filter feeders (e.g. cladocerans, see Kagami et al. 2007a ; Kagami et al. 2007b ; Agha et al. 2016 ). Notably, zoospores are nutritious, because they convert short-chain to long-chain PUFA (Taube et al. 2019 ; Rasconi et al. 2020 ) and can produce sterols de novo (Gerphagnon et al. 2019 ). Consequently, chytrids benefit zooplankton by converting inedible poor-quality phytoplankton into nutritious edible zoospores through “trophic upgrading” (Breteler et al. 1999 ; Veloza et al. 2006 ; Bec et al. 2006 , 2010 ; Table  1 ).

The mycoloop hypothesis has been subject to population modelling to evaluate scenarios and explore population dynamics (Grami et al. 2011 ; Miki et al. 2011 ; Kagami et al. 2014 ; Frenken et al. 2020a ; Frenken et al. 2020b ; Thongthaisong et al. 2022 ). In a pioneering paper, Miki et al. ( 2011 ) found that although the mycoloop could have positive effects on zooplankton production, the overall impact could be negative if zooplankton growth efficiency was lower when feeding on chytrids than on small algae (Fig.  4 , left panel). In more recent models, parasitism by chytrids was found to alleviate competition amongst edible phytoplankton, increasing zooplankton production (Kagami et al. 2014 ). High zooplankton densities can also reduce chytrid transmission (through consuming free-living infective zoospores) and overgraze edible algae, favouring the growth of inedible phytoplankton that can then feedback to depress the zooplankton population (Kagami et al. 2014 ; Thongthaisong et al. 2022 ). Although modelling indicates that the mycoloop predominantly influences blooms (Thongthaisong et al. 2022 ), this might relate to modelling assumptions that nutrients are not recycled in the system through zooplankton excretion and death. Model outcomes, thus, may hinge on critical details about how the mycoloop system is defined mathematically.

Chytrid infection helps make inedible hosts edible

In addition to producing edible spores, chytrids diversify zooplankton diets by fragmenting inedible phytoplankton (Gerphagnon et al. 2013 ; Agha et al. 2016 ; Frenken et al. 2020a ; Frenken et al. 2020b ) and increasing their nutritional quality (Gerphagnon et al. 2019 ; Taube et al. 2019 ; Rasconi et al. 2020 ; Table  1 ). Following fragmentation, zooplankton can graze on both chytrid zoospores (Gerphagnon et al. 2019 ; Taube et al. 2019 ; Rasconi et al. 2020 ) and phytoplankton fragments (Abonyi et al. 2023 ), providing a more diverse resource to sustain zooplankton populations. By providing carbon, LC PUFA, and sterols, chytrids may act as a buffer for energetic and essential molecule requirements when they are most needed by zooplankton (Fig.  2 ; Table  1 ).

Eutrophication and warming stimulate blooms of inedible cyanobacteria that lack polyunsaturated fatty acids and sterols. Chytrid fungal parasites primarily infect inedible phytoplankton hosts and facilitate fragmentation, leading to enhanced feeding, including the consumption of edible chytrid zoospores (left). This increased availability of dietary options improves diet quality regarding PUFA and sterols. The PUFA and sterols derived from chytrids act as a buffer, supporting zooplankton nutrition during cyanobacteria blooms (right)

Chytrid host specificity maintains edible phytoplankton diversity

Although it might seem like phytoplankton production alone should drive zooplankton growth rates, fast-growing edible algae in diverse communities can also enhance zooplankton growth (Marzetz et al. 2017 ). Zooplankton growth appears to increase because higher diversity enhances diet quality (Marzetz et al. 2017 ) due to a higher likelihood of encountering high-quality resources (i.e. the sampling effect; Naeem and Wright 2003 ). When ecosystem functions like zooplankton production and water quality are supported by phytoplankton diversity, there is the potential for a positive BEF relationship (Fridley 2001 ; Loreau and Hector 2001 ). Indeed, a positive BEF relationship is a key assumption on which the chytrid insurance hypothesis rests. Specifically, chytrids may diversify phytoplankton communities with complementarity or portfolio effects that benefit zooplankton communities as well as physical processes like nutrient cycling in water.

Succession in plankton can be predictable, with some systems experiencing seasonal blooms (Sommer et al. 1986 ). In early stages, phytoplankton density is low, and reduced competition allows additive community assembly (see Table  1 ), promoting diverse and rapidly growing communities (Weis et al. 2007 ). Edible phytoplankton are expected to particularly benefit zooplankton during early successional stages (Thongthaisong et al. 2022 ). In non-bloom conditions, chytrid prevalence typically remains moderate (ranging between 3 and 20%, Sime-Ngando 2012 ; Gsell et al. 2022 ). However, because most chytrids are host specific (e.g. Rhizophydium planktonicum, Rhizophydium megarrhizum ; Frenken et al. 2017 and references therein), “kill-the-winner” dynamics, where numerically common taxa suffer more from parasitism (Thingstad et al. 2000 ; Table  1 ), might maintain diversity amongst edible phytoplankton. Indeed, the community composition of chytrids often follows the seasonal dynamics of the phytoplankton community, with infection rates reflecting phytoplankton composition and abundance (Rasconi et al. 2012 ). Thus, the interactions amongst chytrids, phytoplankton and zooplankton may vary considerably over succession (Fig.  3 ).

Edible phytoplankton serve as vital sources of energy and essential dietary molecules for zooplankton during early successional stages. A high diversity of edible items enhances zooplankton biomass production and increases the likelihood of encountering high-quality diet items (bottom left, sampling effect). However, as succession or degradation occurs, inedible phytoplankton become more prevalent, hindering edible phytoplankton and zooplankton production (bottom right, weaker sampling effect). Chytrids take advantage of inedible phytoplankton dominance and provide an alternative pathway to buffer energy and essential molecules through zoospore production in the mycoloop and increased feeding (top left, chytrid insurance). Furthermore, chytrids can suppress inedible phytoplankton, indirectly benefiting edible phytoplankton and their diversity (dashed lines). Environmental degradation amplifies inedible phytoplankton blooms, thereby augmenting the buffering effects that chytrids can provide (top right, increased chytrid insurance). Please refer to the colour codes of phytoplankton in Fig.  1 . Black arrows represent positive interactions, red arrows represent negative interactions, and dashed arrows represent indirect interactions. Arrow width indicates relative interaction strength between each scenario

At later successional stages, zooplankton depress the abundance of edible phytoplankton, which releases nutrients and may allow inedible phytoplankton to undergo monospecific blooms (Huisman et al. 1999 ; Prince et al. 2008 ; Chakraborty and Feudel 2014 ). Depending on the presence of generalist grazers like Daphnia (Tessier and Woodruff 2002 ) and the lake’s trophic state, large-sized diatoms, green algae, or cyanobacteria can eventually dominate over others (Sommer et al. 1986 ). Collectively, eutrophication, grazing on edible phytoplankton, and lake warming intensify the dominance of inedible phytoplankton, particularly cyanobacteria (Paerl and Huisman 2008 ; O’Neil et al. 2012 ), resulting in more efficient resource use for phytoplankton but impaired ecosystem functioning for zooplankton (Filstrup et al. 2014 ). Like bloom-forming taxa, chytrid densities increase with lake trophic state and track positively with host dominance (Rasconi et al. 2012 ; Thongthaisong et al. 2022 ). High rates of chytrid infection can suppress host density (Agha et al. 2016 ; Frenken et al. 2018 ; 2020a ; 2020b ; Abonyi et al. 2023 ), suggesting a strong regulatory role for chytrids under bloom conditions. By predominantly infecting most inedible phytoplankton (Van Donk and Bruning 1992 ; Frenken et al. 2017 ; Sassenhagen et al. 2023 ), chytrids might reduce the rate at which inedible species outcompete edible species (Rasconi et al. 2011 ). For instance, phytoplankton diversity increased after a chytrid-induced decline of the dominant inedible diatom, Asterionella formosa (Canter and Lund 1951 ; Van Donk and Ringelberg 1983 ), remaining the best evidence that chytrids can foster phytoplankton diversity.

Although the chytrid insurance hypothesis assumes that chytrids increase phytoplankton community evenness and reduce phytoplankton abundance, the diversity and abundance of phytoplankton communities (and the relationship between diversity and abundance) should feedback to affect chytrid success. Such feedbacks may strengthen or weaken the chytrid insurance hypothesis. For instance, if phytoplankton communities are additive (more nutrients or diverse habitats leads to more phytoplankton individuals per species), then host-specific and generalist chytrids should increase in prevalence with phytoplankton diversity, further increasing phytoplankton evenness (and chytrid insurance) whilst regulating phytoplankton abundance. If phytoplankton communities are substitutive (more diversity leads to fewer individuals per species; Table  1 ), then increased phytoplankton diversity should not affect overall chytrid prevalence, but should increase the proportion of generalist chytrid species, thereby reducing the potential for kill-the-winner dynamics and chytrid insurance. Furthermore, if zoospores do not discriminate amongst host species when attempting to infect, a diversity-dilution effect could result (Frenken et al. 2017 ) whereby increased phytoplankton diversity would decrease host-specific chytrid species, further reducing chytrid insurance. Finally, phytoplankton diversity-abundance relationships might change with succession or environmental conditions, making chytrid insurance context dependent.

Testing the chytrid insurance hypothesis

Field observations, lab experiments, and mathematical models have been used to investigate the mycoloop hypothesis and these approaches could also be used to assess the conditions under which the chytrid insurance hypothesis might hold. For instance, field observations on the mycoloop could be expanded to include more information about the associations between phytoplankton diversity, chytrid prevalence, and zooplankton growth and abundance. Obvious predictions are that zooplankton growth rates increase with phytoplankton diversity and that chytrids are more likely to infect abundant phytoplankton species. One growing source of information is environmental DNA samples that can indicate chytrid and phytoplankton diversity, albeit with limitations related to the paucity of sequence data for parasitic chytrids and the inability to quantify abundance (Frenken et al. 2017 ). This and more traditional information are needed to document when the chytrid insurance hypothesis is important in nature.

Some investigators have manipulated the presence of chytrids in microcosms to test cause and effect relationships. Chytrids can be isolated from field collections and kept in culture for short-term microcosm experiments (e.g. Agha et al. 2016 ; Kagami et al. 2007a ; Kagami et al. 2007b ; Abonyi et al. 2023 ). Future work could manipulate phytoplankton diversity to evaluate the conditions under which chytrids can increase zooplankton growth. Furthermore, infection experiments between different chytrid and phytoplankton species would help evaluate the key assumption that chytrids engage in kill-the-winner strategies that promote phytoplankton diversity. It should also be possible to gauge how diet quality and diversity affect zooplankton growth in synthetic communities. Finally, by manipulating the presence or absence of chytrids, researchers could assess the degree that the mycoloop fosters phytoplankton diversity and delays or constrains succession towards blooms and associated zooplankton crashes. Given that there are many potential reasons for associations between chytrid parasitism, phytoplankton diversity, and zooplankton growth rates, laboratory experiments will be key to deciphering whether chytrids help foster a positive biodiversity–ecosystem functioning relationship or are simply correlated with it.

The chytrid insurance hypothesis could also be subject to population modelling to evaluate scenarios and explore population dynamics difficult to evaluate empirically. Models could be designed to determine the parameter ranges under which insurance occurs and the extent and nature to which insurance affects pelagic food webs. Doing so is not trivial because to identify the range of conditions under which the presence of chytrids facilitates a positive BEF relationship would require modelling food diversity, food quality, and phytoplankton diversity. Although such models have not been attempted, foundational mycoloop models have fallen into two categories: Lotka–Volterra style NPZ models (see Franks 2002 ) like Miki et al. ( 2011 ; Fig.  4 , left panel) and food-web models as pioneered by Grami et al. ( 2011 ). Food-web models may be better suited for encompassing a broader diversity of edible and inedible phytoplankton and their various indirect effects. Alternatively, Smith et al. (2016) provides a model of phytoplankton diversity designed to evaluate BEF that might be altered to include chytrids. Such models are not simple, but seem worth exploring.

Theoretical NPZ model proposed by Miki et al. ( 2011 ), Kagami et al. ( 2014 ) and Thongthaisong et al. ( 2022 ) (left); and extension including the insurance effects of chytrids (right). Resources (R) such as dissolved nutrients support populations of n species of edible (X 1,…,n ) and inedible (N 1,…,n ) phytoplankton. Zooplankton (Z) feed on edible phytoplankton, as well as on n species of fungal chytrid zoospores (F 1,…,n ), i.e. the “F-Z feeding link”. Species of infected inedible phytoplankton (I 1,…,n ) become palatable to zooplankton with time

Regardless of the modelling framework, considering that chytrids have distinct parasitic and dispersal stages might lead to different dynamics than the current modelling approach of treating chytrid spores like predators (Miki et al. 2011 ; Thongthaisong et al. 2022 ). This is because recognising an infected class for phytoplankton will allow chytrids to directly compete with uninfected hosts for resources, and suffer the consequences of intimacy if hosts die (Fig.  4 , right panel). Indeed, models could benefit from more information about the chytrid life cycle, such as which hosts are infected by spores, and the effect of sporangia on infected host reproduction and mortality. Another consideration for modelling is that chytrid resting stages may increase system stability or that environmental factors like light and temperature can constrain infection dynamics (Gsell et al. 2022 ). Finally, allowing nutrient recycling through zooplankton excretion and death (Barranco et al. 2020 ) would reduce phytoplankton competition for nutrients, and this might alter previous modelling conclusions that chytrid effects are limited to bloom periods (Fig.  4 , right panel). These suggestions should be approached with the understanding that incorporating additional complexities in models can reduce tractability and pose challenges in parameterisation. Yet when combined with field and experimental work, modelling will likely be needed to fully understand the conditions under which the chytrid insurance hypothesis applies.

In posing the chytrid insurance hypothesis , we integrate the mycoloop into a biodiversity–ecosystem functioning framework. Biodiversity might benefit zooplankton production in three ways. First, adding diversity in the form of edible chytrid parasites creates potential for the mycoloop to benefit zooplankton directly. Second, chytrid parasites further add to diet diversity by altering the edibility of infected hosts. Finally, if phytoplankton diversity leads to more predictable and higher quality food, then kill-the-winner dynamics that promote phytoplankton diversity could reduce the conditions under which non-edible phytoplankton species bloom. All of these outcomes seem plausible, at least under some conditions, but models, experiments, and observations are needed to test core assumptions and refine predictions.

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Acknowledgements

This work was supported by the Austrian Science Fund (FWF Project P 30419-B29). AA was also supported by FK 142485 (National Research, Development and Innovation Office, Hungary) and by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.

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András Abonyi

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Serena Rasconi

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Abonyi, A., Fornberg, J., Rasconi, S. et al. The chytrid insurance hypothesis : integrating parasitic chytrids into a biodiversity–ecosystem functioning framework for phytoplankton–zooplankton population dynamics. Oecologia 204 , 279–288 (2024). https://doi.org/10.1007/s00442-024-05519-w

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The Insurance Hypothesis

Farming monoculture, such as in this corn field in Marqueseac (France), can lead to decreased net ecosystem CO2 intake, meaning land becomes less useful in the fight against climate change. According to the insurance hypothesis: “Biodiversity insures ecosystems against declines in their functioning because many species provide greater guarantees that some will maintain functioning even if others fail”. Put simply, given the same set of environmental conditions, more diverse plant ecosystems will tend to photosynthesise at a greater rate than ecosystems with fewer species present. Obviously, forests are very diverse, whereas crop fields are usually monocultures, meaning that only the one species of plant is present. In fact, any other plant species present are usually classified as ‘weeds’ and are, hence, destroyed by farmers post-haste.

The idea behind ‘the insurance hypothesis’ is simple: when different species are present, they are able to fulfil a variety of different ecological niches within a given ecosystem. By contrast, with monocultures, all of the individual plants are competing for the resources held within one specific ecological niche. Consequently, the overall rate of photosynthesis in biodiverse ecosystems tends to be much higher than that of monocultures, which means biodiverse ecosystems, usually natural ones, are much better at helping us tackle climate change.

In addition, monocultures are much more susceptible to disease than biodiverse ecosystems are. Plant viruses are usually specialised toward attacking a particular species, genus, or family of plants. Consequently, it is possible for one viral strain to destroy an entire monoculture plantation of biofuel crops, thus reducing the photosynthesis rate of this area of land to zero. In stark contrast to this, should a virus destroy any given type of plant within a biodiverse ecosystem, the gap created will quickly be filled by the other plant species present, meaning that overall rate of photosynthetic carbon dioxide uptake will remain high. Thus, the clearance of forests, in order to make way for crop plantations, is not only detrimental in terms of the animal species lost, for whom this forest was their habitat, but it can also cause a net increase in atmospheric carbon dioxide levels over time.

References:

Naeem, S. & Li, S. (1997). Biodiversity enhances ecosystem reliability. Nature 390 pp 507-509.

Yachi, S. & Loreau, M. (1999). Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis. Proceedings of the National Academy of Sciences of the United States of America 96 (4) pp 1463-1468.

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Definition of hypothesis

Did you know.

The Difference Between Hypothesis and Theory

A hypothesis is an assumption, an idea that is proposed for the sake of argument so that it can be tested to see if it might be true.

In the scientific method, the hypothesis is constructed before any applicable research has been done, apart from a basic background review. You ask a question, read up on what has been studied before, and then form a hypothesis.

A hypothesis is usually tentative; it's an assumption or suggestion made strictly for the objective of being tested.

A theory , in contrast, is a principle that has been formed as an attempt to explain things that have already been substantiated by data. It is used in the names of a number of principles accepted in the scientific community, such as the Big Bang Theory . Because of the rigors of experimentation and control, it is understood to be more likely to be true than a hypothesis is.

In non-scientific use, however, hypothesis and theory are often used interchangeably to mean simply an idea, speculation, or hunch, with theory being the more common choice.

Since this casual use does away with the distinctions upheld by the scientific community, hypothesis and theory are prone to being wrongly interpreted even when they are encountered in scientific contexts—or at least, contexts that allude to scientific study without making the critical distinction that scientists employ when weighing hypotheses and theories.

The most common occurrence is when theory is interpreted—and sometimes even gleefully seized upon—to mean something having less truth value than other scientific principles. (The word law applies to principles so firmly established that they are almost never questioned, such as the law of gravity.)

This mistake is one of projection: since we use theory in general to mean something lightly speculated, then it's implied that scientists must be talking about the same level of uncertainty when they use theory to refer to their well-tested and reasoned principles.

The distinction has come to the forefront particularly on occasions when the content of science curricula in schools has been challenged—notably, when a school board in Georgia put stickers on textbooks stating that evolution was "a theory, not a fact, regarding the origin of living things." As Kenneth R. Miller, a cell biologist at Brown University, has said , a theory "doesn’t mean a hunch or a guess. A theory is a system of explanations that ties together a whole bunch of facts. It not only explains those facts, but predicts what you ought to find from other observations and experiments.”

While theories are never completely infallible, they form the basis of scientific reasoning because, as Miller said "to the best of our ability, we’ve tested them, and they’ve held up."

• proposition
• supposition

hypothesis , theory , law mean a formula derived by inference from scientific data that explains a principle operating in nature.

hypothesis implies insufficient evidence to provide more than a tentative explanation.

theory implies a greater range of evidence and greater likelihood of truth.

law implies a statement of order and relation in nature that has been found to be invariable under the same conditions.

Examples of hypothesis in a Sentence

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'hypothesis.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Greek, from hypotithenai to put under, suppose, from hypo- + tithenai to put — more at do

1641, in the meaning defined at sense 1a

Phrases Containing hypothesis

• counter - hypothesis
• nebular hypothesis
• null hypothesis
• planetesimal hypothesis
• Whorfian hypothesis

Articles Related to hypothesis

This is the Difference Between a...

This is the Difference Between a Hypothesis and a Theory

In scientific reasoning, they're two completely different things

Dictionary Entries Near hypothesis

hypothermia

hypothesize

Cite this Entry

“Hypothesis.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/hypothesis. Accessed 7 May. 2024.

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