Author: lizmorris66

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The Conversation: ‘Let’s get real’: scientists discover a new way climate change threatens cold-blooded animals

This article is republished from The Conversation under a Creative Commons license. Read the original article.

All animals need energy to live. They use it to breathe, circulate blood, digest food and move. Young animals use energy to grow, and later in life, to reproduce.

Increased body temperature increases the rate at which an animal uses energy. Because cold-blooded animals rely on the thermal conditions of their environment to regulate their body temperature, they’re expected to need more energy as the planet warms.

However, our new research, published today in Nature Climate Change, suggests temperature is not the only environmental factor affecting the future energy needs of cold-blooded animals. How they interact with other species will also play a role.

Our findings suggest cold-blooded animals will need even more energy in a warmer world than previously thought. This may increase their extinction risk.

Young animals use energy to grow, and later in life, to reproduce.

What we already know

The amount of energy animals use in a given amount of time is called their metabolic rate.

Metabolic rate is influenced by a variety of factors, including body size and activity levels. Larger animals have higher metabolic rates than smaller animals, and active animals have higher metabolic rates than inactive animals.

Metabolic rate also depends on body temperature. This is because temperature affects the rate at which the biochemical reactions involved in energy metabolism proceed. Generally, if an animal’s body temperature increases, its metabolic rate will accelerate exponentially.

Most animals alive today are cold-blooded, or “ectotherms”. Insects, worms, fish, crustaceans, amphibians and reptiles – basically all creatures except mammals and birds – are ectotherms.

As human-induced climate change raises global temperatures, the body temperatures of cold-blooded animals are also expected to rise.

Researchers say the metabolic rate of some land-based ectotherms may have already increased by between 3.5% and 12% due to climate warming that’s already occurred. But this prediction doesn’t account for the animals’ capacity to physiologically “acclimate” to warmer temperatures.

Acclimation refers to an animal’s ability to remodel its physiology to cope with a change in its environment.

But rarely can acclimation fully negate the effect of temperature on metabolic processes. For this reason, by the end of the century land-based ectotherms are still predicted to have metabolic rates about 20% to 30% higher than they are now.

Having a higher metabolic rate means that animals will need more food. This means they might starve if more food is not available, and leaves them less energy to find a mate and reproduce.

As climate change raises global temperatures, the body temperatures of cold-blooded animals are also expected to rise.

Our research

Previous research attempts to understand the energetic costs of climate warming for ectotherms were limited in one important respect. They predominantly used animals studied in relatively simple laboratory environments where the only challenge they faced was a change in temperature.

However, animals face many other challenges in nature. This includes interacting with other species, such as competing for food and predator-prey relationships.

Even though species interact all the time in nature, we rarely study how this affects metabolic rates.

We wanted to examine how species interactions might alter predictions about the energetic costs of climate warming for cold-blooded animals. To do this, we turned to the fruit fly (from the genus Drosophila).

Fruit fly species lay their eggs in rotting plant material. The larvae that hatch from these eggs interact and compete for food.

Our study involved rearing fruit fly species alone or together at different temperatures. We found when two species of fruit fly larvae compete for food at warmer temperatures, they were more active as adults than adults that didn’t compete with other species as larvae. This means they also used more energy.

From this, we used modelling to deduce that species interactions at warmer global temperatures increase the future energy needs of fruit flies by between 3% and 16%.

These findings suggest previous studies have underestimated the energetic cost of climate warming for ectotherms. That means purely physiological approaches to understanding the consequences of climate change for cold-blooded animals are likely to be insufficient.

Previous studies have underestimated the energetic cost of climate warming for ectotherms.

Let’s get real

Understanding the energy needs of animals is important for understanding how they’ll survive, reproduce and evolve in challenging environments.

In a warmer world, hotter ectotherms will need more energy to survive and reproduce. If there is not enough food to meet their bodies’ energy demands, their extinction risk may increase.

Clearly, we must more accurately predict how climate warming will threaten biodiversity. This means studying the responses of animals to temperature change under more realistic conditions.
The Conversation

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Interspecific interactions alter the metabolic costs of climate warming

Authors: Lesley A Alton and Vanessa Kellermann

Published in: Nature Climate Change

Abstract

Climate warming is expected to increase the energy demands of ectotherms by accelerating their metabolic rates exponentially. However, this prediction ignores environmental complexity such as species interactions.

Here, to better understand the metabolic costs of climate change for ectotherms, we reared three Drosophila species in either single-species or two-species cultures at different temperatures and projected adult metabolic responses under an intermediate climate-warming scenario across the global range of Drosophila.

We determined that developmental acclimation to warmer temperatures can reduce the energetic cost of climate warming from 39% to ~16% on average by reducing the thermal sensitivity of metabolic rates. However, interspecific interactions among larvae can erode this benefit of developmental thermal acclimation by increasing the activity of adults that develop at warmer temperatures.

Thus, by ignoring species interactions we risk underestimating the metabolic costs of warming by 3–16% on average.

Alton LA, Kellermann V (2023) Interspecific interactions alter the metabolic costs of climate warming. Nature Climate Change DOI

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Can we apply theory developed to understand hermaphroditism in plants, to animals?

Hermaphroditism — a condition where an organism can produce both male and female gametes — is very common in plants and many animals. While the ability to self-fertilise seems like an evolutionary ‘cunning plan’, there are drawbacks. If hermaphrodites only self-fertilise, they run the risk of inbreeding depression, but where survival or mating opportunities are slim, then self-fertilisation is better than not reproducing at all.

Theory does a good job of predicting when hermaphroditism will benefit plants. Interestingly, animals have not received the same attention. George Jarvis and his PhD supervisors Craig White and Dustin Marshall have found that the same theories predicting hermaphroditism in plants can also be applied to animals.

Essentially, strong competition among siblings for resources or among gametes for fertilisation may drive the evolution of hermaphroditism in both plants and animals.

Highly competitive environments can tip the balance towards hermaphroditism, because hermaphrodites can reallocate resources from male to female function (or vice versa) to increase their competitive advantage.

George and his supervisors considered the accepted evolutionary drivers for hermaphroditism in plants and looked for analogues in marine animals.

In plants, species with limited seed dispersal can experience strong competition among siblings for resources, and are more likely to be hermaphrodites. In marine animals, larval dispersal provides an analogous model to test if the same forces are at play.

Larvae released into the plankton travel further than non-planktonic larvae and feeding larvae travel even further than their non-feeding counterparts. Plant theory predicts that hermaphrodites are more common in species where local competition is greatest; in marine animals then, hermaphroditism should be more common in species with non-planktonic larvae.

Similarly, plants pollinated by specialist insects are more likely to be hermaphrodites. In animals, internal fertilisers are akin to plant species with specialist pollinators; competition between gametes is fierce and so an extra guarantee in the form of hermaphroditism should be more common.

George and his supervisors found other analogues between plant and animal theory as well. Smaller plants and plants at higher latitudes are more likely to be hermaphrodites due to increased competition between gametes and rarer mating opportunities respectively. If plant theory holds, the same patterns between hermaphroditism, size and biogeography will be true for animals as well.

The research team set about compiling data for reproductive mode, larval development mode, fertilisation mode, latitude and adult size for 1,153 species of marine annelids, echinoderms and molluscs.

They found theory developed to understand hermaphroditism in plants, predicts many patterns of hermaphroditism in marine animals. Overall, hermaphroditism is generally associated with limited offspring dispersal, internal fertilisation and small body size.

But just to complicate things, George and his supervisors also found that in annelids and molluscs, species that are external fertilizers are more likely to be hermaphrodites when large, but in internal fertilizers the opposite is true.

George, Craig and Dustin feel their results support Darwin’s supposition from over a century ago: hermaphroditism evolves in response to an organism’s life history and ecology —  not the other way around.

This research was published in the journal Evolution.

Theory developed to understand hermaphroditism in plants, predicts many patterns of hermaphroditism in marine animals. In these taxa (top to bottom: annelids, echinoderms and molluscs) hermaphroditism is generally associated with internal fertilisation, small body size and, with the exception of molluscs, limited dispersal. In each graph, the bars represent average (± SE) predicted probabilities of hermaphroditism from standard regressions, and numbers represent the number of species for each mode. Note that scales differ among taxa.
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Changing lanes: can we reconcile the ways we measure reproduction so we can make meaningful comparisons across animal species?

Reproduction is perhaps the only truly unambiguous measure of fitness and yet we measure it in different ways. Biologists working on birds tend to measure clutch size as number of eggs per clutch, while mammal biologists focus on litter size measured in mass. These differences only become obvious when researchers want to move out of their accustomed lanes and ask broader questions applicable to a wide range of animal species. One unequivocal measure of reproduction is reproductive mass per year but how often do researchers measure this?

Reproductive mass per year combines the number of offspring per reproductive bout with the mass of the offspring and, importantly, the number of reproductive bouts per year. We know that some species can have a few large offspring and only reproduce once per year whilst other species can produce many small offspring numerous times per year. So which species puts in the most resources to reproduction? Only by combining measures of offspring mass over time can we really compare reproductive effort across species.

How often are all three components of reproductive mass per year – number of offspring per reproductive bout, offspring mass, and the number of reproductive bouts per year – provided for animals?

PhD student, Sam Ginther, is interested in the energetic costs of reproduction and wondered how feasible it would be to collate reproduction data for a wide range of species. Could he translate the existing data into a consistent and biologically relevant measure of reproductive mass per year?

Sam and supervisors Dustin Marshall, Craig White and Hayley Cameron created a ‘systematic map’ of reproductive trait data that exist in online databases. They used this unbiased approach to collate and describe:

  1. how common is the measure reproductive mass per year in databases, and
  2. how well did more ambiguous reproductive measures (i.e., fecundity per bout, fecundity per year, and reproductive mass per bout) represent a truly comparable measure of reproductive effort – reproductive mass per year.

So, can we use other measures as proxies for reproductive mass per year? While most reproductive measures are poor predictors of reproductive effort, reproductive mass per bout is the exception.

Reproductive databases are amazing resources and represent centuries of work in the field of reproductive biology. However, to unlock their full potential Sam and his colleagues feel that the best way forward is to encourage researchers to measure reproduction in a way that allows us to reconstruct reproductive mass per year; that is, tie reproductive measures to temporal- and volumetric-dimensions. But where this is unrealistic in terms of time and effort then measuring reproductive mass per bout is the next best thing.

This research is published in the journal Global Ecology and Biogeography.

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When is a change for the better?

Metamorphosis — the process by which animals undergo substantial changes to become an adult — can involve a complete redesign of the body plan. While not all transitions are as dramatic as the metamorphosis from larvae to adult, animals that undergo transitions as they move toward the adult stage are described as having complex life cycles. But why did animals evolve such complex life cycles?

In 1986 a scientist named Earl Werner proposed an explanation that has been widely cited since. Werner said that as an individual grows, energy demands increase faster than energy uptake. But, switching body plans (which usually means better access to food sources) enables individuals to continue growing and to reach the reproductive stage more efficiently and in better health. Werner’s model predicts the size at which an individual will transition as the size that minimises the chance of dying compared to its growth rate. Werner assumed the mass of the individual won’t change as a result of transition to the new phase of the life cycle.

Emily Richardson and her supervisors Dustin Marshall and Craig White wondered if this was true. If not, and there was a cost/benefit of transition, then the optimal size to change body plan might differ from Werner’s predictions.

Emily set about reviewing the literature to find out if there were any changes in mass that related to transitioning from one life stage to the next. Emily found data for 100 species and 343 life stages where she was able to record changes in growth rate and mass.

It turned out that across all taxa, as Werner predicted, growth rates were maintained or increased after switching to a new phase. But Werner hadn’t accounted for the change in mass that Emily and her supervisors observed in most taxa. On average amphibians lost 28%, insects 32% and crustaceans 8% of their mass during metamorphosis. During changes from one larval stage to another, fish and crustaceans actually gained mass and fish gained even more mass during metamorphosis. These increases in mass are likely to reflect more subtle life-history transitions where feeding is possible and transitioning is not as energetically costly.

These plots show Werner’s hypothetical predictions for the mass at which an individual should switch body plans when (A) growth rate is included and (B) when mortality is also included – note the optimal size at switching is larger when mortality is included. The dashed green curve in (C) represents one possible outcome if Werner’s model incorporates change in mass during transition to a new phase—in this example, the new curve shifts to the right and size at switching predicted by the new model is larger than predicted by Werner’s original model.

Either way, the team found that when changes in mass during transition are accounted for, the optimal size for transition will deviate from Werner’s predictions. For species that gain mass during a transition to a new phase, individuals should switch at a smaller size, while for species that lose mass, Emily and her supervisors predict they will transition at a larger size compared to Werner’s current theory.

To better understand the optimal size where transition will maximise fitness, we need to incorporate the change in mass that happens during this transition. And to test Werner’s theory further, the team highlight the need to estimate mortality rates in the field, including the risk of dying when transitioning to a new stage.

This research is published in the journal Functional Ecology.

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Travelling in time: an experimental evolution experiment challenges what we thought we knew about size and the cost of production

Time travel has been made possible by a long-term evolution experiment with the bacteria Escherichia coli. In 1988 a biologist at Michigan State University, Richard Lenski, set up 12 flasks of E. coli and his group has maintained and followed their evolution ever since. Periodically, subsamples are frozen enabling scientists to compare the bacteria at different points in time by bringing them back to life.

Over time, the evolving E. coli have grown bigger; after 60,000 generations, cells are roughly twice the size of their ancestors. But has this increase in size been accompanied by changes we expect to see in metabolism and population size and growth rates? Researchers at the Centre for Geometric Biology have collaborated with Richard Lenski to find out.

Metabolism dictates the rate at which organisms transform energy into maintenance and production. While larger species have higher metabolic rates, they are actually more efficient and so have lower metabolic rates relative to their size. So, while smaller species have higher population densities and can reach those densities faster, total population mass is greater in larger species (think mice and elephants).

But does the above hold true within a species? Often the size range within a species isn’t particularly large, making inferences about size difficult to test. The aptly named ‘Lenski Lines’ circumvent this problem. Richard’s lab sent frozen samples of the original E. coli – the ancestors, plus samples from 10,000 and 60,000 generations of evolution. Project leads at Monash University, Dustin Marshall and Mike McDonald, set about reviving the cells and measuring cell size, metabolism, population size and population growth.

The team found that as the cells grew bigger through evolutionary time, metabolic rates increased but were lower relative to their size, as predicted by theory. Also anticipated by theory, populations of larger cells had lower population densities but higher biomass’ than their smaller ancestors. The big surprise and in stark contrast to theory, was that populations of larger cells, despite their relatively lower metabolism grew faster than smaller cells.

The research team found that, as expected, larger cells had lower population densities (b) but greater biomass (c and d) but to their surprise larger cells also had a faster rate of population growth than the smaller cells (a).

We often assume that the energy required to produce a new individual is directly proportional to its mass but as this experiment has shown it is not necessarily the case. Why then, would a larger cell be cheaper to build and maintain

E. coli cells use up a lot of energy maintaining ion gradients across cell membranes. As larger cells have smaller surface areas relative to mass they should also have lower maintenance costs than smaller cells. The evolved cells also have slightly smaller genomes than the smaller ancestral cells so the costs of genome replication are lower for larger cells. What is more, the evolved cells have fine-tuned their genetic components in this highly predictable environment, reducing the costly expression of unneeded transcripts and proteins.

Remarkably, it seems evolution can decouple the costs of production from size; there is no downside to increasing growth rates for the larger evolved cells in terms of yield.

This research is published in Proceedings of the National Academy of Sciences of the United States of America.

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A comparative analysis testing Werner’s theory of complex life cycles

Authors: Emily L Richardson, Craig R White, and Dustin J Marshall

Published in: Functional Ecology

Abstract

A popular theoretical model for explaining the evolution of complex life cycles (CLCs) was provided by Earl Werner. The theory predicts the size at which an individual should switch stages to maximise growth rate relative to mortality rate across the life history.

Werner’s theory assumes that body size does not change during the transition from one phase to another (e.g. from larva to adult) — a key assumption that has not been tested systematically but could alter the predictions of the model.

We quantified how growth rate and mass change across larval stages and metamorphosis for 105 species of fish, amphibians, insects, crustaceans and molluscs. Across all taxonomic groups, we found support for Werner’s assumption that growth rates are maintained or increase around transitions. We found that changes in growth and mass were greatest during metamorphosis, and change in growth correlated with development time. Importantly, most species either gained or lost mass when switching to a new stage — a direct contradiction of Werner’s assumption. When we explored the consequences of energy loss and gain in a numerical model, we found that individuals should switch stages at a larger and smaller size, respectively, relative to what Werner’s standard theory predicts.

Our results suggest that while there is support for Werner’s assumption regarding growth rates, mass changes profoundly alter the timing of transitions that are predicted to maximise fitness, and therefore the original model omits an important component that may contribute to the evolution of CLCs. Future studies should test for conditions that alter the costs of transitions, so that we can have a better understanding of how mass loss or gain affects fitness.

Richardson EL, White CR, Marshall DJ (2022) A comparative analysis testing Werner’s theory of complex life cycles. Functional Ecology PDF DOI

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Winners and losers: how does metabolic rate affect the outcomes of competition?

An individual’s success in competitive environments is often closely aligned with its metabolic rate. When resources are scarce, individuals with lower metabolic rates are expected to grow larger and dominate while individuals with higher metabolic rates will struggle if their energy demands cannot be met. Increasing population density can increase competition for a finite pool of resources and so lower metabolic rate individuals may do better in more competitive environments.

Lukas Schuster and supervisors Craig White and Dustin Marshall noted that investigations into the relationships between metabolic rate and competitive interactions have mainly taken place in the laboratory. They wanted to know how metabolic rate affected competitive interactions in a more realistic field situation. So, they designed an experiment using the model species Bugula neritina, a colonial marine animal that, crucially, does not move allowing survival, growth, and reproductive output to be easily measured in the field.

Lukas settled Bugula larvae on to acetate sheets and after two weeks of growth in the field he brought them back to the lab to measure metabolic rates. Each colony with a known metabolic rate was then assigned to become either a ‘focal’ colony or a ‘neighbour’ colony. Colonies were glued on to small plates and focal colonies either had a neighbour colony placed 1 cm away, or were left alone on the plate to determine the baseline relationship between metabolic rate and performance. Plates were distributed across 5 panels and returned to the field site and Lukas monitored focal individuals bi-weekly for survival, growth and reproductive output.

To the teams surprise they found a range of responses on the different panels despite their relative proximity. They concluded that each panel experienced a different microenvironment that, in turn, influenced the effects of metabolic rates on competitive outcomes. While the presence of a neighbour did reduce performance of focal colonies on most panels, the effects of metabolic rates of both focal colonies and neighbours were much more complex.

Bugula neritina individuals were attached to small plates either alone or with a neighbour and these plates were then attached to larger panels and hung in the marina. Despite their proximity, the team suspect differences in microclimate around each panel contributed to the variable outcomes of metabolic rate on competitive interactions.

Low metabolic rate colonies were larger overall, presumably because of their lower maintenance costs but, in general, the metabolic rate of the neighbour seemed more important to performance of the focal colony than its own metabolic rate. Lukas and his supervisors speculate that focal colonies benefited from being adjacent to fast-growing, low metabolic rate neighbours on panels where flow was higher because the larger neighbours slowed down currents allowing greater access to resources for the focal individual. In low flow environments the opposite may be true; resources are not replaced quickly enough and so low metabolic rate larger neighbours reduce resource access.

Lukas, Craig and Dustin recommend that future studies on the ecological effects of metabolism look at competition both within and among species and are field experiments wherever possible.

While they can’t say for sure, they suspect the variable outcomes of metabolic rate on competition relate to differences in current regimes and the delivery of resources.  Future studies manipulating food availability along with metabolic rates will help address this possibility.

This research is published in the journal Ecology and Evolution.

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Biodiversity increases energy and biomass production but only in younger communities

Preserving biodiversity is important because species diversity affects the productivity of biological communities. Diverse communities can better use available resources and, thus, produce more biomass than species-poor communities. When diversity is high, communities are also more likely to contain very productive species which further increase biomass production.

While these positive biodiversity effects are seen across diverse ecosystems, from tropical forests to agricultural fields, the general mechanism through which biodiversity increases biomass production remains unclear. Energy is what fuels biological production but very few studies have directly measured energy fluxes and even fewer the effects of biodiversity on energy production. Furthermore, biodiversity effects are not fixed but change as communities grow older. So how does diversity affect the relationship between energy and biomass production over time?

We answer this question using marine phytoplankton in a laboratory study. Phytoplankton are an extremely diverse group of unicellular algae of great ecological importance because they sustain 50% of global oxygen production and carbon uptake. Using five phytoplankton species with different characteristics, we set up a total of 50 cultures across three levels of biodiversity (species alone, in pairs and in communities with all five species) and compare their energy and biomass production for ten days. Since phytoplankton reproduce daily, our experiment covers roughly ten generations.

Diversity initially boosts both energy and biomass production, so that five-species communities produce and accumulate more biomass than species alone or in pairs. But as biomass grows, energy production is limited by competition. This limitation occurs in all cultures but is stronger in diverse communities. Therefore, the positive effects of biodiversity decline over time as communities grow older, see below.

Diverse communities (solid lines) produce energy (magenta) and biomass (green) faster than low-diversity communities (dashed lines), thus accumulating more total biomass. But as biomass accumulates, species compete more intensely limiting energy and thus biomass production. These effects are stronger in more diverse communities so that the positive effects of biodiversity progressively reduce as communities grow older. (Image credit: Giulia Ghedini)

In nature, the positive effects of biodiversity might be sustained over much longer periods of time than what we observe because ecosystems are continuously disturbed by storms, arrival of new species or changes in nutrient availability. Since disturbances are so widespread our results help to compare the functioning of ecosystems of different age and with different levels of diversity.

This research was published in the journal Functional Ecology.

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Challenging assumptions: how well do we understand how climate change will affect vector-borne diseases?

Diseases such as malaria and dengue fever are spread by intermediaries, in this case, mosquitoes. The health and economic burdens of such mosquito-borne diseases are enormous. We know that mosquitoes are expanding their ranges and invading new habitats in response to warmer temperatures. Accurately predicting changes in both the size and spread of mosquito populations is essential for anticipating changes in disease dynamics.

To model how changing environments will affect mosquito populations, we need to know how quickly a population can grow under different scenarios. To estimate changes in population growth rate scientists input measures of development time, survival, body size and reproductive output into their models. Body size and reproductive output are particularly difficult to measure directly in mosquito populations so researchers traditionally rely on the relationship between wing length, which is easier to measure, body size and reproductive output.

These are the relationships that the Centre for Geometric Biology are challenging. Underlying most models of mosquito distributions is the assumption that there is a directly proportional relationship between wing length, body size and reproductive output, or in other words, wing length and reproductive output increase at the same rate.

Scientists from the Centre analysed a range of existing data and found that this wasn’t true for most mosquito species.

In fact, explains Dr Louise Nørgaard, lead author on the study, larger females contribute disproportionately more to the replenishment of the population so it is not a straight-line relationship. And surprisingly, when we factor in this non-linear relationship smaller females are also contributing more to population replenishment than is assumed in current models.

This is important because increasing temperatures result in smaller females. So, temperatures where populations have been considered unviable, will, in fact, persist.

There is an additional complication when dealing with underlying assumptions of linearity; Jensen’s Inequality. This relates to a counter-intuitive mathematical rule that in non-linear relationships, such as this one, you can’t predict the mean reproductive output from the mean wing length in the same way you can for linear relationships. In fact, reproductive output in warmer climates will be even greater than predicted without accounting for Jensen’s Inequality.

This figure shows how reproductive output changes when the relationship between wing length and reproductive output is modelled as a isometric / linear relationship (blue) or hyperallometric / non-linear relationship (orange) (graph A). In this scenario a 15% reduction in wing length result in a 40% reduction in reproductive output when you consider both the shape of the relationship and Jensen’s Inequality (graph B). This contrasts to the 90% reduction in reproductive output that is predicted from an isometric / linear relationship and the 70% reduction in reproductive output if you don’t also account for Jensen’s Inequality (graph C).

There is another application of population models that will also be affected by these underlying assumptions. In the fight against Dengue fever, mosquitos that carry a bacteria called Wolbachia are bred in the lab and released into the wild to reduce the transmission of the dengue virus. Females released from the lab are bigger than their wild counterparts and so will contribute disproportionately more to the population when they breed. We are likely underestimating the impact of releasing Wolbachia-infected mosquitos in tackling this disease.

The authors conclude that to predict the response of disease vectors like mosquitos to global change we need to better represent the relationship between size and reproductive output.

This research was published in the journal Global Change Biology.