Predators have important roles in structuring ecosystems, yet many are critically endangered and their reputation among non-scientists is decidedly mixed.
Humans have a love–hate relationship with predators. On the one hand, many of the iconic species that attract conservation funding are predators. On the other hand, predators are a major source of conflict between humans and wildlife. In addition, predators in the wrong place, as invasive species, are themselves the cause of many conservation problems. While predators feature regularly in our pages, the January and February issues offer a particularly rich celebration of predator ecology and evolution.
Top-down effects are important features of many ecosystems, with predators having major influence on herbivore numbers, nutrient cycling and disease dynamics. O’Bryan et al. review how these effects of predators and scavengers provide benefits to humans in shared landscapes, pointing out that such landscapes greatly exceed the protected areas that are more usually associated with large predators. Predators and scavengers help to control disease, waste and agricultural pests, yet their reputation amongst the public and economic stakeholders is largely based on killing livestock and humans, or harbouring disease. There is therefore a need for education and outreach, but also a need for rethinking the economics. The parties who benefit from predators, such as tourists, governments and arable farmers, are often different to those whom predators can disadvantage, such as livestock farmers and residents of rural–urban frontier landscapes (although O’Bryan et al. point out that there may also be advantages for livestock farmers). Payments to protect predators and compensation for livestock loss are mechanisms to redress this imbalance.
While the O’Bryan Review focuses on terrestrial ecosystems, many of the most iconic, and indeed the world’s largest, predators are marine. Understanding the ecosystem effects of marine predators and their vulnerability depends in part on improving our limited knowledge of their biogeography. van Denderen et al. examine this in bony fish, finding that energy flow from the bottom of food webs explains the latitudinal distribution of teleost predators. In cartilaginous fish, Bird et al. use stable isotope data to show differences in trophic geography between individuals for shelf-dwelling sharks, and a particular dependence on mid-latitudinal carbon sources by oceanic and deep-sea sharks. This kind of functional understanding of biogeography can help to fine tune conservation priorities, such as those suggested on a broader scale by Stein et al. using an evolutionary approach across all chondrichthyans.
Humans have a long history of interactions with carnivores, and somewhere in this lie the complex explanations for why we like to hunt animals that are not sources of food. These include protection of self and livestock, and use as non-food resources. Not all interactions are antagonistic, of course, such as our domestication of dogs and cats. However, our negative, hunting relationship with large predators forms the basis for three recent studies. The thylacine, or Tasmanian tiger, was hunted to extinction in the early twentieth century largely because it threatened livestock. Feigin et al. sequence its genome and suggest that the decline in its genetic diversity actually began before human arrival in Australia. Interestingly, reduced genetic diversity might be responsible for the susceptibility of its extant relative, the Tasmanian devil, to infectious facial tumours. Kardos et al. also take a genomic approach to reduced diversity, documenting the effects of inbreeding on a fine scale across the genome in Scandinavian wolves that have been endangered by hunting. Sticking with Scandinavia, Bischof et al. report that regulated hunting causes substantial life-history changes in a bear population. Regulated hunting is a common but controversial component of apex predator management schemes, and this study shows that we need to look beyond just population numbers when assessing its utility.
Invasive predators are a frequent factor in the decline of native prey species. However, invasive prey can also affect native predators. Cattau et al. show that the endangered snail kite in North America has responded rapidly to the invasive apple snail, which is larger than its native prey, by increasing its beak size. While this effect is largely to do with phenotypic plasticity, the selective pressure is substantial and an evolutionary response will probably follow.
Evolutionary and phenotypically plastic effects of predators on prey feature in two other recent studies that remind us not to focus just on large predators. Aposematic prey display warning colouration to deter predators, but this works as an evolutionary strategy only if predators learn to avoid such prey. If all predators in a population must consume aposematic prey before learning to avoid them, aposematism is unlikely to evolve. Thorogood et al. report that great tits learn to avoid prey by watching the disgust exhibited by conspecifics consuming the prey. This social transmission of avoidance allows aposematism to evolve without unsustainable levels of prey consumption being required. Moving to even smaller predators, Reger et al. show that midge and fish predators can drive local adaptation of phenotypic plasticity in the water flea, Daphnia pulex.
Having surveyed predators from large to small, it is worth thinking about what scaling patterns emerge. Rizzuto et al. address this question by looking at the activity levels of carnivores. They examine how foraging time scales with body size, and find that medium-sized predators, such as the fox depicted on this month’s cover, spend the largest proportion of their time searching for prey. This makes predators of this size particularly vulnerable to environmental change. Foxes may not be the easiest cause to promote to the public, but given the threats to predators of all sizes, in all environments and from all taxonomic groups, we need to make a general case for conserving predators that doesn’t rest just on iconic large carnivores.