New paper: feral honey bees and competition for natural cavities

Our new paper is out in Frontiers in Ecology and the Environment (open access). We used a combined search of peer-reviewed literature and iNaturalist observations to determine what evidence is available on the use of natural cavities and hollows by feral (wild) western honey bees (Apis mellifera). Our paper addresses an important knowledge gap on how invasive honey bees compete with native species in their introduced range.

The western honey bee (A. mellifera) is one of the world’s most successful invasive species. It has spread far and wide beyond its home range in Europe and the Middle East, and is found on every continent (and most islands) except Antarctica.

As with any invasive species, the honey bee can have detrimental ecological effects in places where it’s been introduced. In a recent assessment of some of the top invasive insects in the world, we assessed the honey bee as potentially having the highest level of environmental impact (Massive) in its introduced range, based on the evidence that is available.

Unfortunately, there is very limited evidence of actual impacts from most parts of the world. Most studies use correlative measures, for example showing that presence/abundance/richness etc. of native species declines as honey bee abundance increases. But very few studies have measured actual impacts, in terms of health or fitness effects in other species.

This type of evidence is important, because correlation doesn’t mean causation. To know if an invasive species is potentially threatening the persistence of a native species, we need to know if it has having direct or indirect effects on the health or fitness of the native species – this means things like direct predation, spreading disease/pathogens that increase death rates, impacting reproductive rates, removing availability to food or nesting resources etc.

Occurrence of two species in the same place at the same time doesn’t necessarily mean they are having long-term effects on each other. Even anecdotes of one-off interactions between individuals that seem negative may not always represent actual competition effects at the species level.

Introduced honey bees potentially have many effects on native species, including competition for floral resources, effects on plant reproduction, competition for nesting sites, parasite spread etc. There is evidence for some of these effects in some places, and it’s likely that many of these impacts are happening in other places, but we need more quantitative research that actually shows these effects.

One impact that is commonly speculated on is the capacity for wild honey bees to take over natural cavities, like tree hollows, and out-compete native birds and other animals.

Many of these cavities are also critical for other animals to use for nesting and roosting sites: birds, arboreal mammals, reptiles, other insects. There is valid concern that wild honey bees are out-competing these other species, and reducing their ability to survive and reproduce. This is exacerbated by the fact that natural cavities are declining globally as we continue clearing forests and woodlands.

We wanted to understand the available evidence for this potential competition effect, and identify priorities for more research. We conducted a structured search of the peer-reviewed literature to find published studies that discussed or mentioned the use of cavities by wild honey bees. Surprisingly, we found very little quantitative evidence of competition with other species over cavity occupation. Most studies provided anecdotes and observations of cavity occupancy, and the authors speculated that competition was occurring. A few studies followed cavities over multiple years, and found that honey bee colony occupancy was generally temporary, and in most cases other species moved into the cavities soon after the bees left. Some studies noted that small animals, including reptiles and other invertebrates, co-habited with the colony with no apparent conflict.

All of the studies were subject to selection bias. This means that authors documented nest occupancy based on known locations, or sites that were observed as part of experimental treatments (e.g. nest-box deployment). Therefore, there is still limited understanding of natural occupancy rates and the ecological processes involved in colony occupancy and interaction with other species.

We also conducted a search of iNaturalist to find global observations of wild honey bee colonies. This database has some biases, in that most observations will be of visible colonies, and countries with low proportions of iNaturalist users will be underrepresented. The observations we collected showed colonies using a range of sites and substrates, including trees (internal and external), cliff faces, rock walls, buildings, light poles etc. This information is useful to identify potential patterns and form new hypotheses.

Our paper discusses some of the important issues around cavity occupancy and competition between introduced honey bees and native species, and we identify some priority research questions that need to be addressed to understand more about these important ecological processes.

Importantly, we are not arguing that competition does not occur, or that honey bees have no negative effects on native birds and other species. We are highlighting the lack of evidence available on this important topic, and the need for more rigorous research that clearly identifies and quantifies competition effects.

© Manu Saunders 2021

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