Citations:
Porter, W.P. and Gates, D.M. 1969. Thermodynamic equilibria of animals with environment. Ecological Monographs 39: 227-244.
Deutsch, C.A., et al. 2007. Impacts of climate warming on terrestrial ectotherms across latitude. PNAS 105 (18): 6668-6672.
Blog author: David Nguyen
Author backgrounds:
Porter is currently a professor in the Dept. of Integrative Biology at the University of Wisconsin – Madison where he has continued studying how animal body size and shape influences energetics, behavior, ecology and evolution.
Deutsch is a professor in the School of Oceanography at the University of Washington where he studies the impact of climatic variability on the biodiversity and adaption of terrestrial ectotherms.
Porter and Gates 1969
The premise of Porter and Gates (1969) is that animals must be in energetic equilibrium with their environments to survive. This means that the energy input that an animal receives must equal the amount of energy lost by an animal over a sufficiently long-time scale. If this was assertion was untrue, animals would exhibit trends in cooling or heating over time which has not been observed. Hence, quantifying the set of environmental conditions in which an animal can maintain energetic equilibrium would help us define the fundamental niche of animals.
Porter and Gates (PG) developed mathematical and physical models to quantify how animals gain and lose energy, since modeling is more feasible than attempting to quantify this experimentally over regions in climate space. In their models, PG considered four environmental variables: radiation, air temperature, wind and humidity. Together, these variables constitute the “climate space” animals will experience. The climate space will impact three modeled animal characteristics: body temperature, rate of moisture loss, and metabolic rate.
The climate space that an animal can survive in can be calculated given information on the body temperature ranges of an animal, metabolic rate, water loss, absorptivity (of radiation), diameter and thickness of insulation. PG demonstrate this analysis for a variety of ecto/endothermic animals of varying body size and insulation. In this way, the fundamental climatic niche of an animal can be estimated without having to experimentally assess animal survival across the entire range of climatic conditions we are interested in.
Deustch et al. 2007
Deustch et al. use data on ectotherm performance varies with temperature to make projections about how climate change will impact fitness. In their main analysis, they use experimental data population growth rate over different temperatures to compute thermal performance curves for insects across a wide latitudinal range. From the thermal performance curves and climate projections for 2100 they find that insects in low latitudes (tropics) will experience decreases in population growth rate while insects in higher latitudes will experience increase in population growth rate. These results are explained by their findings that the “warming tolerance,” the difference between the current temperature and maximum tolerable temperature is smaller for low-latitude vs higher latitude insects, which means that low-latitude insects are more sensitive to changes in temperature. Furthermore, the “thermal safety margin,” the difference between the optimal and current temperature for insect population growth is smaller for low-latitude species than higher-latitude species. This means that increased temperature will decrease the performance of low-latitude species whereas it will enhance the performance of higher-latitude species, since the thermal optima of higher-latitude species tends to be higher than current temperatures.
My thoughts
These papers both focus on how animal performance varies depends on abiotic factors. Porter and Gates seemed to be more focused on the basic task of defining and quantifying the fundamental climate niche of animals, whereas Deutsch et al. were more focused on applying what we now know about thermal performance to make forecasts about the impact of climate change. To me the most interesting aspects of these papers were their integration of existing or (somewhat) easily measurable physiological data with models to make predictions about niche requirements and performance. I think this approach highlights how models and data can be used to more mechanistic and, hopefully, more accurate predictions.
I went into reading the Porter and Gates (1969) paper, worried it was going to be another paper with a great assortment of equations like we have read in the past. The paper did have (some) equations, but it seemed different than the others. I really enjoyed this paper and liked the concept. Porter and Gates presented an interesting idea about the existence of an energy equilibrium between animals and their environments. The amount of data collection for this paper must have been enormous. My only complaint was I wish they have included other animals at the end of the paper (where they laid out which organisms could live in what environment). In particular, I wish they had included aquatic/marine organisms (although they would have to account for the environmental variables affecting these organisms such as tidal currents over air speed (potentially)).
ReplyDeleteThe Deustch et al. (2007) paper was a great follow-up to the 1969 paper. The repercussions of climate change on organisms (a modern idea) were discussed. It was found that tropical organisms will most likely have a reduced fitness first before organisms (primarily insects) at higher latitudes. I particularly liked that this paper (initially dealing with insects) also looked at the effects on other organisms such as lizards and frogs. Unlike the Porter and Gates paper, it did not account for any endotherms (i.e. mammals) which would have been nice to see the full picture. Regardless of this, the paper made for a good read.
I found both of these papers to be a nice pairing for each other. Porter and Gates conducted a very thorough study into modeling thermal performance of animals and laid the groundwork for modern-day applications like Deutsch et al's study. Similar to Kat's point in her comment before mine, I was very impressed at the sheer scale of the amount of data they were drawing from for a paper like this!
ReplyDeleteThe 2007 paper was a very topical application of thermal performance modeling by looking at how temperature changes resulting from climate change could have an effect on ectotherms' performance and broadly how this could impact global biodiversity. Figures 1 & 2 provide a great synthesis showing the latitudinal differences in the fitness curves and projected future change in fitness between temperate vs tropical species -- very readable and showing quite a stark picture of the differences they found. Although it is out of the scope of this paper, I am interested to see what similar studies on thermal performance & climate change have been done on endotherms as well.
- Elizabeth
The Porter and Gates’s paper is really interesting. I cannot imagine that there could be such detailed mathematical model that illustrate the thermodynamics between animals and the surrounding environment. The symbols and equations are clear, and I like that the authors discussed some case studies at the end, especially the comparison between some homeothermic animals and poikilothermic animals. I understand that a model must be simplified, but I am curious how could the model be adjusted in order to consider different shapes of animals, given the original model considered all animals had a cylinder body. In nature, animals have different shapes and many animals change their shapes (huddle up or stretch out) in order to reduce temperature loss or increase the reception of radiation. Also, different body parts/organs of homeotherms have different temperatures, which can affect heat convection and conduction.
ReplyDeleteThe companion paper covers a trending topic – the effect of global warming on animals’ fitness. I have heard about this result before, that tropical animals would suffer more than temperate animals. When I read this paper, I still find it amazing that tropical and temperate insects have identical critical thermal maximum (I thought that CTmax for tropical insects might be higher).
The Porter paper provided a useful (although simplistic) model of animal energy equilibriums. It was nice to see that Porter accounted for both endotherms and ectotherms in his model organisms, but I did have a few questions about some of his methods and assumptions. Porter’s model accounted for fur but did not really look at differences in types of fur that might affect energy. Some animals, like polar bears, have hollow fur that allows for additional insulation, and some animals, like certain breeds of dog, have an undercoat or multiple layers of fur. There were also some unclear assumptions in the methods. When considering water loss from masked shrews, Porter stated that the “values used here are assumed,” but did not explain the rationale or reasoning behind how the assumptions were made. I found some of Porter’s figures very information-heavy and difficult to read (Figures 9 and 10 especially). Overall though, the ideas behind the model are relevant and applicable to many animal systems.
ReplyDeleteThe Deutsch paper was a good complement to Porter’s work. Deutsch examined energy dynamics in insects, which is a system that Porter ignored, and focused specifically on the effects of increased temperature on insects. In the time since the Deutsch paper was published, there was an empirical study looking at the effects of increased temperature on tropical insect biomass, but the results of this experiment are up for debate. A review of the original article concluded that there was “no evidence” to support that warming temperature are causing the significant decrease in arthropod biomass, and the reviewers suggested that other disturbances such as hurricanes and droughts may have affected the insect population more than just temperature alone (https://www.pnas.org/content/115/44/E10397 ; https://www.pnas.org/content/116/25/12143).
It bothered me that the Porter paper clumped all animals together rather than breaking them into endotherms and exotherms. They are just so different in how they regulate their body temperature. I think it would have helped me understand the concepts a bit more. They also included reflected light as a possible source of radiation, which I don't think is particularly ecologically relevant.
ReplyDeleteI really liked the Deutsch paper. As someone who works extensively with TPCS and thermal performance in general, this paper made perfect sense. Even if it goes against what simple intuition may predict.
-Miranda
I found the Porter and Gates paper to be thought-provoking. The ideas presented are especially relevant now in the face of climate change. The companion paper was a good fit for this same reason, and I agree with David that the models benefit from being more mechanistic and hopefully therefore more accurate. However, I wish Porter and Gates had included a more diverse set of animals, including more aquatic animals and more ectotherms. Aquatic animals have entirely different rates of heat loss/gain because water and air have different specific heat capacities. I also think that behavior - which was discussed some - should be considered more. Behavior plays a critical role in determining the local climate an animal is exposed to, especially for ectotherms, and so these animals can manipulate their climate to their advantage.
ReplyDeleteThe Porter and Gates paper was quite thorough in its description and incorporation of the parameters relevant to the energy budget of organisms. While for plants it is fundamental to consider the amount of direct vs. indirect light that they are exposed to in order to determine their photosynthetic rates and energy production, I had never fully considered the effects of incident radiation/absorptivity of “animal surfaces” on the overall energy budget for animals. When the authors described energy transfer between an animal and its environment via convection, it made me fully appreciate the roles that fur, and different skin types/textures have in heat transfer for animals (which is probably obvious to a wildlife biologist!).
ReplyDeleteI thought that, although the paper is already over a decade old, the Deutsch et al. paper is now more than ever a timely and important study. As Arthropoda is the most numerous phyla in the animal kingdom and the species within it are fundamental to most terrestrial food webs, the fitness and survival of individuals within this group is paramount to prevent bottom-up trophic cascades. Although the primary finding of the paper at first could seem counterintuitive, we are already witnessing the phenomena of warmer temperatures positively affecting life histories of insects across temperate biomes. Particularly during the winter months, less extreme cold temperatures have facilitated range expansions in some insects as well as the lack of an overwintering diapause (halt in life cycle). This has allowed for increases in reproduction, resulting in more generations of insects per year, which in turn can lead to more host plant herbivory and subsequent damage/mortality over large geographic scales. It is very troubling to think about the shifts in biodiversity/species composition that are likely in the tropics because organisms are already near their thermal optima and ultimately, their energetic optima, prior to the projected warming.
While I appreciate the rigorous efforts of Porter and Gates to scale from individual physiology to species-level distributions, I find their basic premise odd on two points:
ReplyDelete1. The assertion that animals should be in thermodynamic equilibrium with their environments. I find this assertion strange because it requires that we consider thermal energy to be categorically different from chemical energy, which does not strike me as particularly reasonable. If we agree that physical and chemical energy are both energy, which is obvious for a number of reasons, not least the conversion of food into body heat, then animals are energy sinks for their entire lives by virtue of having physical bodies, taking in chemical energy and holding onto more of it than they release until their deaths. Even then, it takes time for them to decay or be digested. So even after the animal has ceased to be an animal, it is still not at equilibrium with the environment.
2. The viewpoint that " One of the ultimate goals for the energy budget analysis of animals is to predict the climate space which any given animal must occupy in order to survive." This seems quite backwards to me, given that climates impose selection pressures on animals, giving rise to animals that evolve to meet the demands of climates. It also seems to be a viewpoint deeply grounded in an equilibrium understanding of nature, not acknowledging that some relationships might be inherently non-equilibrial.
I thought the Deutsch et al. paper was an interesting approach to the problem of predicting population dynamcics at large scales under changing climate conditions. I found it fascinating that the focused not only on the magnitude of the predicted change but on the relationship between predicted climate change and existing temperature sensitivity. The paper raises a similar question to the Porter and Gates paper in that it leads us to question what, if any, changes in selection pressures might occur as a result of a changing climate.
I really liked the Porter and Gates paper. I was nodding along the entire time I read because it so clearly matched up with how thermodynamics and energy exchange was taught to me in physiology courses. I thought his assumptions were reasonable for qualifying what was essentially the fundamental climatic niche for a species based on simple properties like temperature differentials. It was interesting that he used the volume of cylinders to understand the relationships between climatic factors and body size of animals. From what I understand, this assumption monumentally simplifies the math and gives a close enough approximation that more complex body shapes do not necessarily need to be considered. However, Yuguo brought up an interesting point about some animals' ability to change their body shape to alter heat exchange via behavior. Porter and Gates state that their equations should find the limits of climatic space that may not be fully used by an animal but cannot be exceeded.. It would have been interesting to see if climatic space could be expanded based on an animal's ability to manipulate their surface area to volume ratio. Another factor I was curious about was other physiological adaptations like the special "antifreeze" proteins in Antarctic ice fish.. do these animals now exceed their theoretical climate space, or do we just need to change the scale to look at thermodynamics intracellularly? My guess would be the latter (proteins still follow the laws of thermodynamics) but I wonder how difficult it would be to scale molecular effects up to an individual or a population's climatic space.
ReplyDeleteThe Deutsch et al. paper was an interesting expansion on Porter and Gates because it brings thermodynamics into a modern context where we can ask questions about how species either change their historical ranges or change their thermally optimal space to cope with climate change (or die/go extinct). The thermal performance curves very neatly outlined how fitness would change for insects at a wide range of temperatures. The conclusion of the paper was very sobering, that the largest biodiversity hotspots are also the most vulnerable areas because species there have less plasticity and narrower ranges of thermal tolerance. One question I had (that was outside the scope of this paper) was whether the shape of a thermal performance curve changes dramatically between endo- and ectotherms, particularly at the lower end of temperature ranges.