Relationships in nature are rarely as clear-cut as they seem. Take for example, the relationship between the red squirrel (Sciurus vulgaris) and its predator the pine marten(Martes martes). One would naturally assume that the recent increase in the predator’s population would spell bad news for the squirrel, but it is actually a blessing in disguise. You see the pine marten isn’t the only species giving red squirrels a hard time; invasive grey squirrels (Sciurus carolinensis) are actually proving to be a much bigger threat. The greys outcompete the reds and infect them with squirrelpox, a deadly disease that the greys carry asymptomatically but which proves fatal to red squirrels. The increase in pine marten numbers has been met by a crash in the population of grey squirrels where the two co-occur. This decrease in grey numbers may be as a result of direct predation, but it may also be a by-product of behavioural responses to trying to share a habitat with an abundant predator. The stress of being perpetually alert to potential attacks may impact on foraging behaviours as well as limiting breeding efforts. Whatever the cause, pine martens have proven to be an unlikely saviour for the red squirrel in Ireland.
Similar relationships have been observed in Australia, where the culling of dingoes has led to a reduction in small mammal numbers. In the absence of dingoes, larger mammals feed more effectively, removing the understory shrub and vegetation that the smaller mammals utilise as habitat. The non-native fox also thrives in the absence of the dingo and predates on Australia’s native small mammals.
A new study published in Science looks at another relationship that is more complicated than we originally thought. Greater spotted cuckoos (Clamator glandarius )parasitise the nest of carrion crows (Corvus corone corone) , leaving their eggs to be cared for by the impromptu foster parents. Greater spotted cuckoos are a non-evicting brood parasite which means they don’t evict the hosts eggs and young from the nest, but instead leave them to be raised alongside its own offspring. This still infers a cost on the host species, as the young cuckoo will put pressure on their own offspring through competition for food. Indeed, the study found that nests which had been parasitized fledged fewer crows than those that were not. The benefits didn’t all go one way however, as nests containing a parasitic cuckoo were less susceptible to predator-induced failure than those without a cuckoo.
So what was behind this unexpected drop in predation? Well, cuckoos secrete a noxious substance when threatened that seems to deter predators such as birds of prey and feral cats. It almost operates like a protection racket. In exchange for tolerating the burden the cuckoo has placed on the crows, the cuckoo offers protection against other antagonists. The author concludes the publication with a reminder that we should not be so quick to pigeonhole relationships as strictly parasitic or mutualistic, as these relationships can be context dependant.
So next time you feel like strangling that freeloading housemate who steals your food and contributes nothing beyond noise pollution, consider for a moment that his offensive behaviour may actually be keeping bigger turds from your door…Probably not though. Screw that guy.
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