Take away the deer and more subtle effects on plant communities become apparent

A herd of deer in a forest glade may seem idyllic, but it’s not entirely clear how deer and forests mix. Deer certainly affect tree regeneration through eating saplings. However, they can also affect understorey plant and animal diversity and even litter decomposition. Simon Chollet and colleagues examined how deer interact with abiotic effects, like soil water availability and soil fertility. They found that herbivory by deer overwhelms abiotic factors in structuring plant communities, as experiments where deer are excluded from a site, might not capture the broader range of factors at play in regulating biodiversity.

The study examined the role of Sitka black-tailed deer (Odocoileus hemionus sitchensis) on the plant understorey of the Haida Gwaii archipelago of western Canada. Chollet and others had already found that deer affected tree regeneration, understorey plant communities, bryophyte communities, and effects on insects and birds. Chollet and colleagues examined twenty exclosures on the largest island over twenty years. Surprisingly, only one exclosure was broken by storms, leaving nineteen intact for the survey.

As you might expect, the team found that deer did have a significant effect on plant communities. Deer exclusion resulted in a clear increase in vascular plant richness, diversity and cover, and caused a decline in bryophyte cover. They also found that, with the deer gone, it was possible to discern the effects of local abiotic factors. However, they did not see an increase in beta diversity. Instead, the exclosures started to converge on similar plant communities.

“This pattern of dominance of some species inside exclosures could reflect the fact that large herbivores, by reducing the dominance of the more competitive species (browsing effect), or by creating microhabitat (trampling effect), or by moving seeds (dispersal effect), create heterogeneity that could lead to increased beta diversity,” write Chollet and colleagues. However, they prefer a different explanation.

“An alternative hypothesis is that the dominance of competitive species observed inside the exclosures was partly an artifact linked to past overbrowsing (i.e. legacy effect). Under this hypothesis, species dominance could be understood as the result of the better recovery of a minority of species that had a higher ability to withstand the prolonged presence of deer. This would confer them an initial advantage once protected, and limit the increase of poor colonizing species initially absent or more suppressed.”

“Despite consistency with other studies on the archipelago using different methodologies, this study based on a twenty-year exclosure experiment provided results not entirely in agreement with those observed at more broader and more realistic spatial and/or temporal scales. The less dramatic increase in species richness or the decrease in beta diversity we observed after (only) 20 years of excluding deer from small sections of forests (25 m²) could be patterns sensitive to study length or scale,” conclude Chollet and colleagues.