Reduction of potential food interference in two sympatric carnivores by sequential use of shared resources

1. What they did

The common genet and the stone marten are similar in size, live in the same areas across southwest Europe, and both eat small mammals and fruit. That makes them likely to compete for food. The authors asked how the two manage to live side by side in the same Mediterranean scrubland in Central Spain, and in particular whether they use the same foods at different times of year.

• Collecting scats (October 1998 to December 2000). Every two weeks they collected genet scats from nine monitored latrines and stone marten scats along four transects, each 3 to 5 km long and 2 to 8 km apart. From the larger pool, they randomly picked 152 genet and 142 stone marten scats for analysis, keeping the seasonal and spatial proportions.
• Analysing the scats. Scats were soaked, washed through a sieve, and the remains identified under the microscope against reference collections of seeds, feathers and skins. Teeth, beaks and stings were used to count individual prey. Seeds were counted and compared with seed counts from 25 fruits of each species to estimate how many fruits were eaten. Rabbit teeth were used to estimate rabbit age and size.
• Working out what mattered. Each food was converted to biomass, capped at 200 g per scat because these animals can eat only about that much at once. A “key resource” was any food making up more than 10% of the biomass in any season. Carrion was left out of the key resources because the authors considered it incidental.
• Comparing the two species season by season, measuring how broad each diet was and how much the two diets overlapped.

2. What they found

Key foods

• Six key resources in total. Strawberry tree fruits were key only for genets. Fungi, blackberries and rabbits were key only for stone martens. Figs and wood mice were key for both.
• Wood mice were the genets’ main food (54.7% of mean biomass), followed by figs (15.2%). Strawberry tree fruits made up only 3.7% over the year but 14.6% in autumn.
• Fungi were the stone martens’ main food (21.9%), eaten mostly in autumn and winter and not at all in summer. Figs (15.7%), wood mice (15.8%), rabbits (mostly in spring) and blackberries (15.6% in summer) were also important.
• Genets ate far more wood mice than stone martens did in every season except summer, when stone martens ate more.

Diet breadth and overlap

• Stone martens had a broader diet than genets in every season except autumn.
• Both diets were broadest in summer and narrowest in winter.
• Diet overlap was low (0.19 to 0.33 on a scale of 0 to 1) except in spring (0.58), when wood mice reached their highest share of the stone marten diet (24.4%).

Taking turns

• For figs, strawberry tree fruits, wood mice, blackberries and fungi, the two species differed in when they ate each food.
• Genets ate figs mainly in summer; stone martens ate them mainly in autumn.
• Genets ate strawberry tree fruits in autumn, when stone martens ate none; in winter, stone martens ate some and genets ate none.
• In summer, when wood mice are scarce, genets switched to figs and stone martens ate more wood mice than genets did.

3. What they concluded

• Diet overlap between the two species is low compared with other carnivore communities.
• Using some key foods only one species relies on, and using the shared ones at different seasons, appears to be an ideal way to reduce competition for food.
• The authors argue the patterns support food interference between the two: for example, stone martens seem to avoid figs in summer, when genets eat them heavily, and turn to blackberries instead.
• Genets seem to prefer wood mice, switching to figs in market gardens only when wood mice are scarce in summer.
• Changed patterns of resource use do not prove competition. Studies of how the two species use space, and of whether one excludes the other from food patches, are needed.

4. How tracking was or was not used

The whole study rests on collecting scats and assigning them to genet or stone marten. Genet scats came from nine monitored latrines and stone marten scats from four walked transects. The paper does not say how scats were identified to species in the field. No other tracks or signs and no trailing are reported.

5. Tracker or observer certification

Not reported. The paper does not say who collected the scats or how they were told apart, although the diet comparison depends on assigning each scat to the right species.