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.
There’s tons of useful info in this piece.
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.
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
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.
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.
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)
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.
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.
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.