Researcher at Rethink Priorities with expertise in ecology and mathematical modelling. She holds advanced degrees in philosophy and biology and is an academic researcher in theoretical and population ecology.
kcudding
Karma: 300
r-K selection theory suggests that some particular life history characteristics, such as short lifespan and many offspring, are tied together by selection forces. This is not true for some large groups.
Just because you belong to a group with high fecundity does not mean that either 1. total expected adult lifespan is relatively short or 2. that the highest mortality occurs at the youngest ages. For example, see our post on insect life history and in particular our reply to Tomisk regarding variability in survivorship curves.
I think we need to dig into the data to get stats on things like number of offspring. We are saying that not all species that people might describe as r-selected have many offspring, and it would be better to look at the data for different species or species groups than to use life history generalizations.
There’s tons of useful info in this piece.
:) Thank-you!
I take it that your “Life span” section refers to adult lifespans?
No, this section refers to total lifespan, except where specifically noted. So 20 days is maturity AND death in 20 days (some species of aphids at very warm temperatures).
Do you have estimates for life expectancy at birth (maybe ignoring egg mortality, assuming eggs aren’t sufficiently sentient to warrant concern)?
No, this is hard. Average life expectancy at birth will vary wildly between species of terrestrial insect herbivores because of large variation in both maximum lifespan and survivorship.
Your sections on “Predators” and “Parasitoids” gave some point estimates based on when predation and inoculation by parasitoids often occur. Maybe those are reasonable approximations for life expectancy at birth.
These are lifespan expectations for individuals killed by predators or parasitoids only (and are really only gross generalizations), so they don’t represent average lifespans.
On the other hand, isn’t survivorship almost always “concave upward”, with most deaths occurring quite early? This figure is one random example, showing that most of the insects are dead before the second instar. And because of the concave-upward shape, the average age of death should be pretty young.
In short, no. As we state, the survivorship curves are wildly variable depending on the species, the location and the year. For examples of the variability please see Fig 2 from Cornell & Hawkins 1995 and Fig 1 from Hunter 2000. Different groups have different major causes of mortality, which lead to different curves. For example, endophytic species that are not leaf miners have very low mortality at the youngest ages, and experience the most loss at late juvenile or pupal stages.
I tend to assume that insects in diapause have relatively little subjective experience, such that those periods of time “don’t count” very much if we’re using lifespan as a measure of how long the animal experiences pleasure and pain. Of course, if the insect is minimally sentient during that time, then maybe deaths occurring during that time aren’t that bad.
I would be uncomfortable making this generalization. There is a gradient from simple behavioral inactivity to deep diapause, and the mechanisms of diapause are quite variable even within species groups (e.g., Hand et al. 2016).
Extending this idea, it seems plausible that ectotherms that mature slowly in cool climates have less sentience and less hedonic experience per day than those in warm climates, because biological activity is generally slowed down in cool climates. So maybe the difference in total amount of life experiences is less than one might assume between longer-lived slow-developing insects in high latitudes vs fast-developing insects at low latitudes.
This is almost certainly incorrect. A species that lives in a cool climate does not necessarily have an average experienced daily temperature that is less than a species in a warmer climate, except for really extreme cases (e.g., like comparing yearly average of species in the arctic to those at low elevation in the tropics). The temperatures experienced by insects are determined by their microclimate, which will vary with species and habitat type even on vanishingly small scales (e.g., Pincebourde & Casas 2019. It might be better to attempt such generalizations by species groups, but even that is not going to be easy (e.g., soil insects in closed canopies will experience cooler temperatures on average in the summer than leaf feeders open canopies, but warmer temperatures in the winter).
If we imagine only two species of insect—one with lifetime fecundity of 2 and one with 4000 -- and if each species has equal numbers of egg-laying mothers, then the ratio of (total offspring)/(total mothers) will still be very high: (2 + 4000)/(1 + 1) = 2001. When we make assessments about the net hedonic balance of an entire ecosystem containing multiple species, it’s this average value that seems most relevant. (Of course, this number is only one heuristic. A full evaluation has to consider the sentience of each organism, the cause of death, lifespan, etc.)
Please note that these values are extrema and are not a good representation of the distribution. The median fecundities are reported and are the best representation of the central tendency (overall 138), since the medians are seriously left-shifted from the max. Therefore, the majority of individuals will be born to parents with median fecundity.
References Cornell, H. V., & Hawkins, B. A. (1995). Survival patterns and mortality sources of herbivorous insects: some demographic trends. The American Naturalist, 145(4), 563-593. https://www.journals.uchicago.edu/doi/abs/10.1086/285756
Hand, S. C., Denlinger, D. L., Podrabsky, J. E., & Roy, R. (2016). Mechanisms of animal diapause: recent developments from nematodes, crustaceans, insects, and fish. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 310(11), R1193-R1211. https://www.physiology.org/doi/full/10.1152/ajpregu.00250.2015
Hunter, A. F. (2000). Gregariousness and repellent defences in the survival of phytophagous insects. Oikos, 91(2), 213-224.https://doi.org/10.1034/j.1600-0706.2000.910202.x
Pincebourde, S., & Casas, J. (2019). Narrow safety margin in the phyllosphere during thermal extremes. Proceedings of the National Academy of Sciences, 116(12), 5588-5596. https://www.pnas.org/content/pnas/116/12/5588.full.pdf
The position of this piece is that, given we have no knowledge regarding the central tendency and distribution of rodent well-being, all we can do realistically is compare the quality of deaths. Whether you view an earlier death as a positive or a negative depends on both whether you assume rodent lives are on average net positive or net negative, and whether you assume the subsequent numerous offspring are likely to experience net positive or net negative lives. We are agnostic on this, but do point out that the length of time suffering due to poisoning or starvation will be longer than the suffering due to cat predation. However, one point we raised regarding outdoor cat presence is of course that fear-based vicinity control could reduce total deaths due to both poisoning and predation (although we certainly accept that this is this is not a simple approach Bedoya-Perez et al 2019, Krijger et al. 2017).
These points aside, the reason that we frame cat predation and rodenticide use as alternatives is that if you prioritize human interests and welfare above wildlife interests (and again we are agnostic here) then you will absolutely need to come up with an alternative method of control. Commensal rodent populations have large economic and human health impacts (e.g. Meerburg et al. 2009, Stenseth et al. 2003). (As an aside, given the tremendous reproductive output, you will also need controls if you consider rodent starvation something to be averted). With respect to downstream effects, cat predation and fear generation are methods of control that may be under appreciated and which at least don’t have the far-reaching chemical contamination issues that rodenticides do.
Lit cited
Bedoya-Perez, M. A., Smith, K. L., Kevin, R. C., Luo, J. L., Crowther, M. S., & McGregor, I. S. (2019). Parameters that affect fear responses in rodents and how to use them for management. Frontiers in Ecology and Evolution, 7, 136. https://doi.org/10.3389/fevo.2019.00136
Krijger, I. M., Belmain, S. R., Singleton, G. R., Groot Koerkamp, P. W., & Meerburg, B. G. (2017). The need to implement the landscape of fear within rodent pest management strategies. Pest management science, 73(12), 2397-2402. https://doi.org/10.1002/ps.4626
Meerburg, B. G., Singleton, G. R., & Kijlstra, A. (2009). Rodent-borne diseases and their risks for public health. Critical reviews in microbiology, 35(3), 221-270. https://doi.org/10.1080/10408410902989837
Stenseth, N. C., Leirs, H., Skonhoft, A., Davis, S. A., Pech, R. P., Andreassen, H. P., … & Zhang, Z. (2003). Mice, rats, and people: the bio‐economics of agricultural rodent pests. Frontiers in Ecology and the Environment, 1(7), 367-375. https://doi.org/10.1890/1540-9295(2003)001[0367:MRAPTB]2.0.CO;2
I have added some context at the beginning of the piece, since you are quite right that it might not be obvious that this post is about determining the grounds for some wild animal welfare claims.
It would be under competition in the pie chart. It is a lowish percent of total mortality.
Yes, I hope I made it plain that r- and K- classification is still in use, and that there were a variety of critiques, not just the fact there were are exception to the generalization.
I’m curious tho, some of Pianka’s associated traits have opposite relationships to those stated in big taxonomic groups. Notably for insects, reptiles and fish, “generally” reproductive output increases with body size as compared to mammals and birds where it decreases. As an evolutionary biologist what is your take here? I can think of half a dozen explanations, but never found a literature consensus (e.g., Pianka just got the traits wrong for r—and K and now we use...., r and K- not good for these groups, body size relationships within groups not so important etc etc)
Very interesting about warm-weather diapause and metabolic rate for mosquitoes. I’ll agree that during deep cold-weather diapause insects are reducing metabolic rate (goodness, but maybe not when REALLY cold??). A quick lit search turned up seasonally variable brain size and cognitive abilities in shrews (Lázaro et al. 2018)!
No idea how this relates to lived experience tho. Extending this argument, would you also claim that species with slower metabolism have less lived experience than those with faster metabolism (e.g., “less sentience and less hedonic experience per day”), because then comparing between species with different metabolic rates is going to be quite difficult. In fact I think it quite likely that those species with faster metabolic rates have different lived experience rates than species such as humans, e.g., Healy et al. 2013.
Healy, K., McNally, L., Ruxton, G. D., Cooper, N., & Jackson, A. L. (2013). Metabolic rate and body size are linked with perception of temporal information. Animal Behaviour, 86(4), 685-696. https://doi.org/10.1016/j.anbehav.2013.06.018
Lázaro, J., Hertel, M., LaPoint, S., Wikelski, M., Stiehler, M., & Dechmann, D. K. (2018). Cognitive skills of common shrews (Sorex araneus) vary with seasonal changes in skull size and brain mass. Journal of Experimental Biology, 221(2), jeb166595. https://jeb.biologists.org/content/jexbio/221/2/jeb166595.full.pdf
Gilbert and Manica (2010) list Kheper nigroaeneus as the dung beetle with the lowest median lifetime fecundity, but do not in their manuscript or supplemental materials attach a reference to the individual data points in their meta-analysis. I suspect the source for this data point is Edwards & Aschenborn (1989). In the supplemental data Eurysternus balachowskyi is also listed with lifetime fecundity of 2. My best guess at a reference here is Halffter et al (1980).
Edwards & Aschenborn (1989) report that female Kheper nigroaeneus lay one egg per year, but will lay another if the larvae is unsuccessful early on. The authors further note that that “To ensure population replacement, females must live at least into a second year. We have indirect evidence (laboratory studies; tibial wear measurements) that some adults survive into a third year.” In other words, occasionally a female will produce 3 offspring.
On the other hand, there are a fair number of dung beetles listed as endangered (at least in 1st world countries, e.g. references in Buse et al. 2015, Carpaneto et al. 2007) due to habitat loss, fragmentation and reduction or extinction of associated mammal species.
Sure, so included/excluded unless there is a specific statement otherwise WRT to a particular study: 1. strictly terrestrial species, no aquatic juveniles, 2. lots of species with winged adults, 3. nectavores in included. 4. As far as I know no species which switch between carnivorous and herbivorous.… I’m struggling to think of examples, but my guess would be that these belong to eusocial species, which are not really dealt with here.
Parental care in a particular group does reduce mortality in general compared to the average rates in that group. I can’t speak to comparisons across groups tho. So for example, don’t know, but don’t think that, in general, parental care in exophytics decreases mortality below that of endophytics.
I totally get where you are going with this.… but do want to offer the following observation regarding the context dependence of life-stage associated welfare. Many of these species have tight seasonal links for development that in turn are related to factors like plant phenology. So.… just to make it more specific, for the things like juvenile leaf miners (ample food, fairly but not completed protected from predators and parasitoids,) you might “lock” individuals into a declines in both food quality and quantity, and increases in predation and parasitism by preventing them from emerging from the leaf as the the season advances. Thus decreasing the welfare of those “locked” individuals.
I found this comment interesting “Ideally, a population should be managed in such a way that maximizes its total welfare expectancy.” and wonder how you envision that proceeding? For example, for some large groups of herbivorous insects, the juvenile life stage is probably the best in terms of welfare Cuddington 2019 (and in some cases, the longest stage), but an optimal strategy might therefore imply that we should prevent these individuals from maturing and experiencing the harms associated with death by exposure etc etc. But of course, result of that strategy would be that a given species would become extinct in about one generation. Related to this, I also wonder how one reconciles managing a given population for optimal welfare given the downstream welfare effects on other species (mutualists, competitors, predators, amenalists, keystone species etc etc).
Indeed! If anyone has a way to do add superscript in this version of markdown, I would be grateful to know!!
We’ve tried to keep it clean with respect to data on herbivores, and in most cases have noted where the reference deals with other data as well.
For example, we do not include dung beetles in our definition, and you’ll see above where we note the fecundity of these reported by Gilbert and Manica (2010a), but then ferret out the herbivore-specific extremes from their supplemental data (their medians and stats DO include non-herbivores tho). (Note: my bad, I just spotted that sentence didn’t make the final cut… I’ve added it back into the post for greater clarity).
Dust mites would not be included here since we deal only with Insecta, although certainly some of the lifespan papers that are referenced in a more general way (e.g., Convey 1997, Danks 2006) also have data on mite species.
Not so sure that some of the edge cases really constitute a deviation from herbivory unless there is a significant portion of the diet involved (e.g., cows eat a lot of insects). However, Hawkins et al 1997 and Cornell & Hawkins 1995 claim all the species they examine are terrestrial herbivores as classified by larval stage. I can’t speak to these specific examples for bees, other than to note that the percentage of Hymenoptera in the review papers is small (mortality:6%; fecundity: ~12%).
On the other hand, I’ve started arguing in the scientific lit that this type of trophic classification is always context-dependent anyway.
I think its going to differ between species groups and habitats within the classification of detritivore. The fact that plants are not significantly consumed by terrestrial herbivore suggests that there is lots of dead and decaying material about for terrestrial detritivores. Moreover, the plants keep producing more, so its not like it runs out (at least not unless there is a major perturbation). Of course there are issues like C:N ratios but most (all?) detritivores consume microbes and fungi as part of their diet as well. The situation may be different for species that focus on other resources (dung beetles), and certainly in aquatic systems there is often no significant standing stock of primary producers, so there could be limitations there (e.g., studies on stream leaf shredders often point this way), although in open pelagic systems the “standing stock” is really secondary production, and that is what detritivores would focus on.
Very interesting… particularly this point “The impacts of harvest on seasonal and annual variation in survival likely differ among populations and animal species exposed to different sets of environmental conditions”. That is, whether mortality is compensatory or additive depends on context (as always.… ecology is just not easy to generalize).
I’m also really curious as to number of natural predators in areas were we expect high outdoor cat presence (e.g., low density housing) and how that plays out re compensatory and additive mortality.
Essentially, the life histories of most species are not well captured by the classification schemes used in animal welfare arguments (e.g., most species are neither r- nor K-selected). As a result, it seems much more difficult to argue that the welfare generalizations based on these schemes correspond to the actual affective experiences of most individuals.