Ecological data
To characterize the environment that stickleback occupy, we focused on a
set of ecological variables which represent putative selective agents.
First, we used the same five habitat classifications (warm, mined,
pondweed, cladophorales, and rocky shore) previously described in (see
supplement for details). We also collated data on ecological variables
likely to reflect selective agents. These were: water temperature, water
depth, stickleback CPUE, piscivorous bird density, and zooplankton
abundances and community composition. These were chosen because
temperature can affect metabolic processes, development, tolerance to
parasite infections , as well as key life history traits , whilst depth
can affect sensory processes invertebrate availability, and stickleback
visibility to predators . Stickleback CPUE was used as a measure of
intraspecific competition , piscivorous bird density as a measure of
predation pressure , and invertebrate data as a measure of prey
abundance and composition . It should be noted that our measures are
only proxies for selection imposed by correlated ecological factors.
Temperature and water depth of each site were used as per . Average
temperature at each site was measured between 30 June 2011 and 18 August
2011 with a temperature logger (iButton Maxim Integrated Products, San
Jose, CA, USA), placed at mid-depth and recording at three-hour
intervals. CPUE for each site was estimated using count data from the
long-term monitoring study from June 2012. To measure piscivorous bird
density, we used data collected during the waterfowl census conducted
each year at Mývatn, during which all waterfowl observed from
pre-determined vantage points with known survey areas are counted . We
used count data collected between 15th May and 10th June 2012 on the
following species known to predate on stickleback: horned grebe
(Podiceps auritus ), red-breasted merganser (Mergus
serrator ), great northern diver (Gavia immer ), red throated
diver (Gavia stellata ) and goosander (Mergus merganser ).
Note that the Arctic tern (Sterna paradisaea) is abundant at
Mývatn, and predates on stickleback, but this species is not counted
during the bird census. We calculated the density of piscivorous birds
(summed across all taxa) in each surveyed segment of the lake
(number/m2) (Figure S1).
We used invertebrate data from , which were collected by conducting
surveys of the epibenthic and zooplanktonic community. Crustaceans
(incl. Daphnia , copepods and epibenthic cladocerans) as well as
rotifers are important food sources for stickleback in Mývatn (e.g. ).
Although chironomid larvae are a main food source for Mývatn
stickleback, data on midge larval abundance were not of sufficient
spatial resolution to be used (see ). However, the benthic community is
spatially correlated with the epibenthic and zooplankton community in
the South basin , suggesting that measures of pelagic and epibenthic
zooplankton may serve as a proxy measure for the benthic community in
Mývatn. Briefly, three transects were conducted between June - July
2012, during which integrated vertical tows of the whole water column
were made at each of 31 sites, spaced 500 – 600m apart (see Figure S2
for distribution, and for more details on sampling and sample
processing). Each pooled sample of 15 L was filtered through 63-μm mesh
and counted in entirety under a binocular microscope. We used data from
the 2nd transect (25th July 2012) as the spatial resolution in this
transect was the greatest. We used data collected from the closest site
to each stickleback sampling site (distance to closest zooplankton site:
min = 290 m, max = 1365 m). All stickleback sites were within 2.55 km of
the nearest site used to collect invertebrate data, which was the
distance at which zooplankton communities were found to be spatially
autocorrelated. We used number per litre (n/L) of each taxon at the
sites closest to stickleback sites.
To summarise variance between sites for use in downstream analyses, we
ran a principal components analysis (PCA) using the native stats package
in R version 4.1.2 . This summarised the invertebrate data in four
general axes, described in detail in Table S1. PC1 (zPC1) described the
overall abundance of crustaceans and rotifers, and explained 29% of the
variation; PC2 (zPC2) described the negative covariance of rotifer sp.
and Alona sp. with planktonic and epibenthic crustaceans, which
explained 17% of the variation; PC3 (zPC3) described the negative
covariance of the rotifer Keratella and the cladoceransAcroperus harpae and Chydorus sphaericus withDaphnia longispina , and explained 15% of the variation;
and PC4 (zPC4) described the negative covariance of the cladoceransEurycercus lamellatus and Macrothrix hirsuticornis withDaphnia longispina , Cyclops abyssorum andAsplancha , explaining 11% of the variation in the data. Overall
invertebrate abundance (described in zPC1), was highly negatively
correlated with stickleback CPUE. We therefore used only CPUE and not
zPC1 for downstream analyses.