Consumption of fleshy fruit: Are central European carnivores really less frugivorous than southern European carnivores?

1. What they did

Many researchers have said that carnivores in southern Europe eat more fruit than those further north, because the south has more fruiting plants and longer fruiting seasons. The author, a botanist, suspected that fruit-eating by carnivores in Central Europe had been underestimated, and set out to check this against the published diet studies.

He tested three ideas:

• Carnivores in Central Europe eat about as much fleshy fruit as the same species in southern Europe, when the same season is compared.
• The only difference is the variety of fruit species eaten, which should be greater in the south.
• Wherever they live, carnivores that eat more kinds of fruit also eat more fruit overall.

He examined 158 published diet studies of European carnivores from 1970 to 2010, and chose 78 records on the three most generalist groups for analysis: badgers, red foxes and martens (stone and pine martens). The data came from scat (faeces) and stomach analyses between southern Spain and central Scandinavia. To make the studies comparable he kept only sites up to 800 m above sea level, kept summer and autumn data separate from year-round data, left out studies where dry fruit (such as hazelnuts, walnuts, chestnuts, maize and acorns) made up more than 5% of the “fruit”, and used only studies reporting frequency of occurrence (O%) or percentage of biomass (B%). He took each site’s latitude and longitude from the study or from Google Earth, and tested for links with geography.

2. What they found

Fruit eaten in Central Europe

• Carnivores in Central Europe ate 35 types of fleshy fruit, 25 identified to species. Pine martens and red foxes ate 18 species each, badgers 16 and stone martens 15.
• Important fruits were cultivated and wild Prunus (cherries, plums, sloes and bird cherry), apples and pears. Brambles, rowan, alder buckthorn, strawberries, roses and bilberry were also eaten.

How much fruit (summer and autumn, Central Europe)

• Martens: fruit in 41.8% of samples on average (up to 100%), 30.6% of biomass.
• Badgers: fruit in 38.0% of samples (up to 82.5%), 16.3% of biomass.
• Foxes: fruit in 27.8% of samples (up to 81.0%), 14.3% of biomass.
• These differences between the three groups were not significant, either for summer and autumn or for the whole year. Each group ate on average about 7 fruit species.

Geography

• How often fruit appeared in summer and autumn diets was significantly linked to latitude (less fruit further north), but not to longitude. Fruit biomass showed no significant link.
• With year-round data, there was no significant link with latitude or longitude.
• Southern sites had the most fruit species in the diet, but the decline northward was not significant. Sites at almost the same location could differ by as many as 12 fruit species between studies.
• The number of fruit species in the diet and how often fruit appeared in summer and autumn were closely correlated (r = 0.912), but this fell just short of statistical significance (p = 0.088). There was no significant correlation for biomass or year-round data.

3. What they concluded

• The author concludes that fruit-eating by these carnivores does not clearly vary across Europe by latitude or longitude, and that only weak trends exist between southern Spain and the Baltic states. This contradicts earlier work.
• One reason is that southern researchers often count dry fruit, such as nuts and acorns, as “fruit”, while northern researchers usually mean only fleshy fruit. Year-round data also favour the south, where fruit is available for longer. The author suggests dry and fleshy fruit should be counted separately.
• When the same season is compared, carnivores at different latitudes appear to use fruit equally where it is available.
• How much fruit carnivores eat depends mostly on local fruit availability and plant diversity, which vary widely within short distances. Scrubby edges between forest and open land are especially rich in fruit species.
• Fruit in carnivore diets may be partly a result of human changes to the landscape, such as orchards and mosaics of forest and open land. In Central Europe, carnivores in natural forest ate less fruit than those in more human-changed places.
• The author treats hypothesis 3 as confirmed, although the correlation was not statistically significant (see the detailed keywords).

6. Other technologies, skills or innovations

• A standardised way of pooling published diet data: same season, same measures, altitude limit and a rule for dry fruit.
• Google Earth was used to find coordinates when a study did not give them.

7. Implications for conservation (from the paper)

• The paper does not discuss conservation directly. It notes that fruit is an important food for these opportunistic carnivores almost everywhere, and that orchards, scrubby edges and other human-changed landscapes can shape how much fruit they eat.