No strong associations between temperature and the host-parasite interaction in wild stickleback

Abstract

As climate change progresses, thermal stress is expected to alter the way that host organisms respond to infections by pathogens and parasites, with consequences for the fitness and therefore population processes of both host and parasite. The authors used a correlational natural experiment to examine how temperature differences shape the impact of the cestode parasite Schistocephalus solidus on its host, the three-spined stickleback (Gasterosteus aculeatus). Previous laboratory work has found that high temperatures benefit S. solidus while being detrimental to the stickleback. The present study sought to emulate this design in the wild, repeatedly sampling naturally infected and uninfected fish at matched warmer and cooler locations in the Baltic Sea. In this wild study, the authors found little evidence that temperature was associated with the host-parasite interaction. Although infection reduced host condition and reproductive status overall, these effects did not vary with temperature. Host fitness indicators correlated to some extent with temperature, with cooler capture sites associated with larger size but warmer sites with improved reproductive potential. Parasite fitness (prevalence or size) was not correlated with temperature at the capture site. These mismatches between laboratory and field outcomes illustrate how findings from well-controlled laboratory experiments may not fully reflect processes in more variable natural settings. Nonetheless, the findings of this study indicate that temperature can influence host fitness regardless of infection, with potential consequences for both host demography and parasite transmission dynamics in this complex system.

Keywords: Gasterosteus aculeatus; Schistocephalus solidus; cestode; climate change; ecological parasitology; host-parasite interaction.

FIGURE 1

The location in Finland (top left map) of the field site (central map) and within that, sampling sites (aerial photographs, with site names). Black squares show areas enlarged in other maps; ellipses show specific sampling areas, with inner locations in black and outer locations in white

FIGURE 2

Associations of temperature (left panels a and c) and S. solidus infection (right panels b and d) with host phenotype (length, top panels a and b) and host fitness as indicated by reproductive status (bottom panels c and d). The points show raw data, with larger points indicating a greater number of individuals, and the fitted lines show predictions with shaded 95% c.i., derived from the minimal models given in Table 2 (fitted without random effects). For clarity, the length model does not include the sex effect; males were overall shorter than females (Table 2)