The difference between generalist and specialist: the effects of wide fluctuations in main food abundance on numbers and reproduction of two co‐existing predators

Abstract

Specialist individuals within animal populations have shown to be more efficient foragers and/or to have higher reproductive success than generalist individuals, but interspecific reproductive consequences of the degree of diet specialisation in vertebrate predators have remained unstudied. Eurasian pygmy owls (hereafter POs) have less vole‐specialised diets than Tengmalm's owls (TOs), both of which mainly subsist on temporally fluctuating food resources (voles). To test whether the specialist TO is more limited by the main prey abundance than the generalist PO, we studied breeding densities and reproductive traits of co‐existing POs and TOs in central‐western Finland during 2002–2019. Breeding densities of POs increased with augmenting densities of voles in the previous autumn, whereas breeding densities of TOs increased with higher vole densities in both the previous autumn and the current spring. In years of vole scarcity, PO females started egg‐laying earlier than TOs, whereas in years of vole abundance TO females laid eggs substantially earlier than PO females. The yearly mean clutch size and number of fledglings produced of both POs and TOs increased with abundance of voles in the current spring. POs laid large clutches and produced large broods in years of both high and low vole abundance, whereas TOs were able to do so only in years of high vole abundance. POs were able to raise on average 73% of the eggs to fledglings whereas TOs only 44%. The generalist foraging strategy of POs including flexible switching from main prey to alternative prey (small birds) appeared to be more productive than the strictly vole‐specialized foraging strategy of TOs. In contrast to earlier studies at the individual‐level, specialist predators at the species level (in this case TOs) appear to be less effective than generalists (POs), but diet specialisation was particularly costly under conditions when scarcity of main foods limited offspring production.

Habitat degradation increases interspecific trophic competition between three spiny lobster species in Seychelles

“…The presence of a diverse number of FA trophic markers and variability in isotopic values in this study suggest that Seychelles spiny lobsters are generalist feeders, which is in accordance with previous studies on spiny lobsters' diet (Williams 2007). This could be an advantage for these species' survival, as dietary plasticity would help them overcome fluctuations in food availability (Korpimäki et al 2020). However, we suggest that J o u r n a l P r e -p r o o f competition between spiny lobsters could occur in this area or could emerge in the near future, if shared resources (e.g., prey, habitat) become limited.…”

Interactive influences of fluctuations of main food resources and climate change on long-term population decline of Tengmalm’s owls in the boreal forest

“…The relative importance of food and weather for reproduction of birds has rarely been investigated in the same study (but see [22][23][24][25] ). Birds of prey are commonly studied for food limitation on population densities, breeding performance and reproductive success because the abundance and availability of their main foods can be relatively accurately estimated in the field 26 . Firm evidence for food limitation on population densities and reproductive success of birds of prey subsisting on fluctuating food resources (e.g., small rodents and lagomorphs) have been demonstrated by several research groups in boreal and arctic regions [26][27][28][29][30][31][32] .…”

“…Birds of prey are commonly studied for food limitation on population densities, breeding performance and reproductive success because the abundance and availability of their main foods can be relatively accurately estimated in the field 26 . Firm evidence for food limitation on population densities and reproductive success of birds of prey subsisting on fluctuating food resources (e.g., small rodents and lagomorphs) have been demonstrated by several research groups in boreal and arctic regions [26][27][28][29][30][31][32] . Climate-induced changes in species distribution and population densities are attributable to changes in the demography of local population, i.e.…”

“…Secondly, the start of egg-laying could be earlier if connected with higher temperatures and less snow cover due to global warming in boreal regions (as found, for instance, in the common buzzard Buteo buteo and the goshawk Accipiter gentilis 23,51 ). On the other hand, the timing of egg-laying could also be delayed during 1970s-2010s, if the abundance of main foods has declined during the study period because the initiation of the egg-laying is delayed by one month in poor vole years 26,28 . We analysed (4) whether there are long-term trends in the initiation of egg-laying during the 46-year study period and whether they are related to variation in main food abundance and/or weather conditions.…”

Estimating the long-term repeatability of food-hoarding behaviours in an avian predator

“…Pygmy owls are commonly found in boreal forests from Northern Europe to Siberia and their population dynamics are tightly linked to the abundance of a few species of small mammals (voles), which fluctuate widely in multi-annual cycles [10,36,37]. They are considered generalist predators [38] that respond to decreases in vole abundance by targeting other species [12,13] and adjusting their hunting behaviour accordingly [39]. Pygmy owls accumulate prey items in food stores during autumn and consume them in winter, when the availability of small mammals can be affected by the snow cover and is hence uncertain.…”