DOI: 10.5061/dryad.76hdr7stj
title: Individual differences determine the strength of ecological interactions

description: 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.
description: 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).
description:   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.