Individual differences determine the strength of ecological interactions

Abstract

<jats:p>Biotic interactions are central to both ecological and evolutionary dynamics. In the vast majority of empirical studies, the strength of intraspecific interactions is estimated by using simple measures of population size. Biologists have long known that these are crude metrics, with experiments and theory suggesting that interactions between individuals should depend on traits, such as body size. Despite this, it has been difficult to estimate the impact of traits on competitive ability from ecological field data, and this explains why the strength of biotic interactions has empirically been treated in a simplistic manner. Using long-term observational data from four different populations, we show that large Trinidadian guppies impose a significantly larger competitive pressure on conspecifics than individuals that are smaller; in other words, competition is asymmetric. When we incorporate this asymmetry into integral projection models, the predicted size structure is much closer to what we see in the field compared with models where competition is independent of body size. This difference in size structure translates into a twofold difference in reproductive output. This demonstrates how the nature of ecological interactions drives the size structure, which, in turn, will have important implications for both the ecological and evolutionary dynamics.</jats:p>

An ecological explanation for hyperallometric scaling of reproduction

“…of ecological interactions (Bassar et al, 2016;Coulson, 2020;Griffiths et al, 2020;Laskowski et al, 2021).…”

“…A central assumption of the physiological model – that individuals (of different sizes) experience the same environmental conditions (Marshall and White, 2019) – is unlikely to hold true in natural systems, with respect to a key environmental variable: food availability. In wild populations, individuals are rarely equal in their ability to accrue resources, and such differences can drive divergence in realised life histories, generate population dynamics, and determine the strength of ecological interactions (Bassar et al, 2016; Coulson, 2020; Griffiths et al, 2020; Laskowski et al, 2021).…”

Taming fitness: Organism‐environment interdependencies preclude long‐term fitness forecasting

“…To this day, fitness is defined in different and even inconsistent ways. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] In light of this, it is not at all uncommon for prominent evolutionary biologists to concede that "Unfortunately, fitness is difficult to define more specifically so that it can be measured and understood more clearly." [16] Routine concessions like this suggest that little has changed since Stephen Stearns proposed the following satirical definition: "Fitness: something that everyone understands but no one can define precisely."…”

“…To this day, fitness is defined in different and even inconsistent ways. [ 1–15 ] In light of this, it is not at all uncommon for prominent evolutionary biologists to concede that “Unfortunately, fitness is difficult to define more specifically so that it can be measured and understood more clearly.” [ 16 ] Routine concessions like this suggest that little has changed since Stephen Stearns proposed the following satirical definition: “Fitness: something that everyone understands but no one can define precisely.” [ 17 ] Such quips and concessions belie a fundamental challenge: If evolutionary theorists are entitled to deploy inconsistent definitions and corresponding measures of fitness on pragmatic grounds, then it becomes possible for two (or more) evolutionary biologists who observe an evolving population and have all the relevant information about the system in hand to reach incompatible conclusions about whether or to what extent natural selection occurs. [ 18 ] We find such a possibility deeply disconcerting and suspect that others share this unease.…”

Size-dependent eco-evolutionary feedbacks in fisheries

“…While size-selective effects of exploitative competition are dependent upon the allometric scaling exponents of intake and maintenance rates (see above), interference competition almost universally brings an advantage to large-sized individuals in contests for food (Persson 1985; Post et al 1999). In fish, dominance hierarchies are highly size-dependent both among and within species (Griffiths et al 2020; Fausch et al 2021). In experimental populations of the springtail Folsomia Candida , interference favours large-sized individuals that can monopolize resources (Le Bourlot et al 2014).…”