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 mea- sures 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.

We studied the size structured demography of the Trinidadian guppy in four streams draining the southern slope of the Northern Range Mountains of Trinidad (West Indies). These streams contain guppies that were introduced in March 2008 (UL and LL) or March 2009 (TY and CA) from downstream communities containing the predatory fish species such as the pike cichlid (Crenicichla alta) and the wolfish (Hoplias malabaricus). Prior to introduction, the only other fish in the streams were Hart’s killifish (Rivulus hartii), which occasionally preys upon smaller guppies. Within each stream, we introduced approximately 50 males and 50 females, as part of a larger experiment on the evolution of the guppy life histories. Habitat quality varies along and between the four streams. Two of the four streams (Upper Lalaja (UL) and Taylor (TY)) have higher primary productivity as a result of bimonthly canopy thinning manipulations. The remaining two streams (Lower Lalaja (LL) and Caigual (CA)) are neighbouring control streams. At each of the sites, barrier waterfalls restrict the movement of individuals out of the study region. Long-term individual-based monitoring programmes were established in 2008 and 2009, to undertake monthly censuses of the streams. Every month since the introductions, we have returned to the introduction streams to collect, mark and measure the individuals within populations. Guppies are collected using butterfly and aquarium nets and placed in sex-specific nalgene bottles for transportation back to the lab. In the lab, each captured fish is identified by a unique combination of coloured elastomer (Northwest Marine Technologies) marks that are injected under the skin. Newly recruited individuals that are greater than 14mm standard length (SL) are marked. All fish are weighed and photographed at each capture. The fish are then returned to the same locations in the stream where they were captured. Individual guppies are born around 6-7 millimetres SL in length and can reach 14mm SL by one month of age. Females continue to grow for most of their lives with the rate of growth declining with increasing size. Males grow until they reach maturity (about 15-18mm SL). Full details can be found in: Travis, J. et al. Do eco-evo feedbacks help us understand nature? Answers from studies of the Trinidadian guppy. Adv. Ecol. Res. 50, 1–40 (2014).

stream = which stream the individual is from

sampling  = which month of the study is the observation.

ID = raw individual identifier

individual_id = curated identifier code (each individual is associated to an number)

upstream= The highest part of the stream that was being searched when a fish was located.

downstream= The lowest part of the stream that was being searched when a fish was located (individuals were therefore known to be in between the upstream nd downstream locations, when a capture occured).

distance = length of stream  in meters between the midpoint of the section of stream sampled and the ustream or the downstream end of the search region ((downstream-upstream)/2).

sex = If the individual is male (M) or female (F)

weight = Individual weight when captured in mg.

Note: Missing values are noted by NA symbols and occur when an individual fish is not captured on one sampling occassion, but is later observed.