Plant-Animal Interactions: An Evolutionary Approach (2)
This is my second post about the book – you can read my first post about the book here; that post includes some more general comments and observations. In this post I’ll cover plant-insect interactions and mammalian herbivory.
“Herbivory, which is the consumption of plants by animals, encompasses many different types of interactions that differ in their duration and deadliness to the plant. Insect herbivores, like mammals, feed on plants in numerous ways. Seed and seedling herbivory are predatory interactions because herbivores immediately kill individuals in the plant population. Insect herbivores that feed on leaves and other parts of mature plants typically do not cause plant mortality. In the rare cases when they do, it usually requires much time to kill the host plant. Such relationships are closer to parasite-host than predator-prey relationships. […] Insect herbivores differ from mammalian herbivores in their size, numbers, and the kinds of damage they inflict. Because of their small size, insects often have an intimate, lifelong association with the host plant. Moreover, while their associations are lifelong, often their lives are rather short, predisposing them to rapid rates of evolution. On average, insect herbivores are much more specialized than their mammalian counterparts. […] There has long been debate over why specialist feeding habits are widespread in herbivorous insects. […] There are clearly a number of hypotheses, each with some empirical support […] Because specialization is a complex trait, we don’t necessarily expect a single hypothesis to explain the phenomenon.”
“Insect populations frequently fluctuate in size, and this fact has prompted a good deal of speculation as to what factors limit the size of herbivore populations. Hairston, Smith and Slobodkin (1960) reasoned that, since herbivores rarely consume all of their plant resources (the world is green), herbivore populations are likely to be limited by parasites and predators, but not by resource abundance […] However, whether herbivorous insect populations are limited by food (bottom-up forces in a food web) or by predators (top-down forces) remains a hotly debated topic […], and it is unlikely that either force dominates all insect populations”
“The first obstacle that an insect faces is the fact that, on average, only about 10% of the energy available to one trophic level makes it to the next trophic level. Sources of energy loss include the fact that not everything ingested can be assimilated (e.g. lignin, cellulose). […] the chemistry of plant and animal tissues is very dissimilar. Liebig’s law of the minimum states that growth is possible to the extent determined by the nutrient that is in shortest supply. For herbivores, one such nutrient is protein. Because nitrogen is relatively easy to measure and protein is not, protein content is often estimated by assaying organic nitrogen, which comprises from 15 to 18% of plant proteins […] sap-feeding insects, like cicadas and other homopterans, often eat 100 to 1000 times their body weight per day because amino acids make up only a tiny proportion of the sap […] In general, both micro- and macronutrients can limit the growth rate of insect herbivores.”
I want to interpose an observation here – I find it quite interesting how seemingly unrelated fields can so often become related in ways you do not expect them to. I’m currently reading Mary Barasi’s Nutrition at a glance (which despite its low page count is actually quite a bit of work, as I’ve found out..). It makes sense in retrospect that some things overlap here, but when I started reading Barasi I did not expect stuff covered in this book to be relevant to the coverage in that book (she only deals with humans). It turns out that the stuff above – and some other stuff covered elsewhere in the book as well – is quite relevant to Barasi’s coverage; I’d probably have been somewhat confused by the focus on nitrogen in the protein chapters of Barasi if I had not read the stuff covered in chapter three of this book. When you’re about to learn some new stuff you never really know how that new stuff you’re about to learn may relate to stuff you already know, or for that matter how it may relate to stuff you’ll learn later on. I always love making new connections like these and connect dots I didn’t even know could be connected.
Okay, moving on…
“Aside from nutritional hurdles and the limited availability of some plant parts, herbivores may also be prevented from feeding as a result of plant defences. […] Adaptions include physical barriers, toxins, anti-feedants, decoys and even other organisms [ants!]. Some defences are always present on the plant; we call these constitutive defences. Many others, including thorns and spikes, are inducible, that is, they are augmented only after the plant is attacked […] The list of chemicals that owe their defensive value to their ability to interfere with insect physiology or behaviour is a very long one. While the elaboration of thorns, spines and hairs is restricted largely to their size and shape, the number of possible combinations, principally of carbon, oxygen, hydrogen, nitrogen and sulfur, is enormous. […] These plant constituents are commonly referred to as ‘secondary’ compounds. […] When the role of a secondary compound is defensive, it is commonly referred to as an ‘allelochemical’. […] Synergists are chemicals that enhance the toxicity of chemicals with which they are mixed. […] Our current understanding is that the presence of secondary compounds can deter many herbivores from using plants, but that almost every plant species has a suite of specialized herbivores that are adapted to use these compounds as attractants, as feeding stimulants or as a source of toxins for use in defence against their enemies. […] As many means as plants have to deter insects, insects have ways of circumventing them. […] The overall responses of plants subjected to herbivory may be viewed as a tradeoff between growth and defence.” [my bold, US]
“As a group, insect herbivores tend to have larger effects than mammalian herbivores on plant growth and reproduction […] when a plant is attacked by one herbivore it may become more or less vulnerable to attack by others. […] the degree to which plants can evolve to become better defended, might be constrained by the preferences of beneficial pollinators. […] While it is clear that herbivores can affect plant community composition and species distribution, the reciprocal effect also exists: plant community composition affects insect herbivore loads. […] The ‘resource concentration hypothesis [states that] herbivores are more likely to find hosts that are concentrated, and herbivores remain longer on hosts growing in dense or pure stands. […] The ‘enemies hypothesis’ [states that] increased diversity of predators and parasitoids in diverse stands may limit population densities of herbivores in these stands. The idea that diverse plant community composition may result in reduced attack by herbivores has been called ‘associational resistance’. […] both community composition and the dispersal abilities of herbivores in relation to the scale of community diversity will affect the degree to which plants receive damage from herbivores.”
“In summary, insect herbivores respond to selection by plant defences and nutritional status. Plants strongly affect insect fitness so that, in general, insect herbivores are relatively specialized with respect to their diet breadth (in comparison with mammalian herbivores). […] Plants affect insect abundance through their defences, which often entail the actions of other species, such as predacious and parasitic enemies of herbivores.
Insects in turn affect plant fitness, and may exert selection on plant defences, both physical and chemical. There is a growing body of evidence suggesting that these defences come at some cost to the plant. On a larger ecological scale, insects affect plant distribution and abundance, as well as the species diversity of plant communities. Frass, honeydew and greenfall from insect outbreaks also alter nutrient cycling regimes in the soil and the availability of nutrients to plants.
Finally, many of the adaptions and counter-adaptions of plants and their insect herbivores support the idea that much of the biodiversity of the earth is a result of the arms race between insect herbivores and their host plants.” [my bold, US]
“The amount of food differs between biomes. The tundra has a primary production of only about 140 g m−2 yr–1, while swamps and marshes reach about 3000 g m−2 yr–1, i.e. a 20-fold difference between the extremes […] The plant biomass, or standing crop, shows an even greater range between the least and most productive biomes, i.e. a 75 fold difference from about 600 g m−2 in the tundra to 45 000 in tropical rainforests. Estimates of food resources are vital for understanding the relations between plants and herbivores […] and [there is a] need for estimates that capture both the static and dynamic situations of the food resources. […] Given the large spatial and temporal variation in food abundance and quality, mobility is a valuable trait and the migratory habits of many ungulates represents an adaptive response. There are no strictly sedentary herbivores […] Herbivores have the advantage of feeding on objects that cannot escape, but on the other hand plant food has low nutritive value (it is low in nitrogen and must be digested slowly). […] Diet composition is commonly used to classify animals into functional groups, e.g. predators, omnivores and herbivores. Mammals, like all other living organisms, have a perverse tendency to defy exact classification […] Sixteen different categories of dietary specialization have been proposed, and seven of them refer to herbivores […] a large majority of the [mammalian] herbivores have quite a mixed diet and also feed on animal matter. [my bold, US] […] It is increasingly clear that mammalian herbivory on a given plant species can result in a continuum of responses, depending on the characteristics of the plant, the type of herbivory and the environment. […] there is no simple typical response for a given plant species.”
“The metabolic requirements of mammals increase with (body mass)0.75 (Kleiber 1932), but the capacity of the gastrointestinal tract with (body mass)1.0 […] Smaller animals thus have higher mass-specific food requirements without any accompanying proportional increase in the gut capacity, which limits the volume of digesta retained and its passage […] There is a tradeoff between the rate of intake and the time allowed for chewing. […] The theory of optimal foraging is based on the assumption that an animal would forage in such a way that it optimizes its fitness […] Food, in terms of quantity or quality, is usually highly variable and is sometimes distributed in more or less discrete patches. Therefore, one crucial point in the optimal foraging concept will be the criteria for when to leave a feeding patch and move to another. The ‘marginal value theorem’ states that a herbivore should stay as long as the extraction rate is above the average for the environment as a whole. […] Understanding the decision rules used by a herbivore requires an understanding of its behavioural responses on various time-scales. It is less probable that an animal optimizes its diet at each bite, but rather that it bases future decisions on an integration over longer periods.” [I found these observations rather funny in a way – some of this stuff is a lot like microeconomic theory, it’s just that in this case the hypotheses made relate to the behaviours of non-human organisms, rather than humans..]
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