Deer as horticulturists?

Right now, my whole hometown is ‘hunt crazy’; most men (and some women) have hunting on their minds and cannot focus on anything else.  November 1st to 13th is the rifle deer hunt, so everyone – including myself – is chatting with their hunting group; discussing which days, times, places, and ammo that might provide the best chances in bagging a big buck.



Frank the deer at QUBS

 For these two weeks, deer are viewed as nothing but a potentially delicious dinner, via sausages, pepperettes, steaks, and roasts.  Unfortunately, most hunters don’t stop to think about the importance of these creatures in the local ecosystem.  Deer can actually alter the forest composition at many levels (i.e. populations, communities) throughout their range; they act as vectors of seed dispersal of many plants.  This can occur in several ways: epizoochory (i.e. seeds get caught and transported in fur), zoochory (i.e. seeds move along animal pathways or are trampled into the dirt), and endozoochory (i.e. seeds are ingested and eliminated somewhere else). 

These phenomena often go unnoticed; if the seeds being transported belong to a locally common plant species, changes in surrounding distributions will not be noticeable (although genetic diversity can increase).  However, if the transported seeds belong to a new and/or invasive species, deer can facilitate frequency and distribution growth quickly.

This was exactly the case for Cynoglossum germanicum (green hound’s tongue) in the Arcen-Barrois forest in north-eastern France.  C. germanium is an epizoochorus herbaceous plant that did not exist in these hardwood forests in 1976 when a long-term experiment began; an extensive network plot survey was set up, having 1035 400m2 plots spaced 333m apart.  At this time, C. germanicum was not present in any of the plots.  However, the study site was overrun with roe and red deer, so for each plot a ‘browsing coefficient’ – a relative measure of deer presence – was also recorded.



Figure 1 from Boulanger et al. showing the increased frequency of
C. germanicum over 30 years.



 The forest was re-surveyed in 1981 and found that 20 of 300 subsample plots contained C. germanicum, but browsing coefficients were reduced as a result of changes to hunting regulations (i.e. increased hunted reduced deer populations).  Again in 2006, a subsample of 330 plots included 117 plots containing C. germanicum, all again having reduced browsing coefficients.

So over thirty years, the frequency of the biennial plant skyrocketed despite the decreasing deer population.  In fact, the frequency of C. germanicum in 2006 and 1981 were only significantly (or nearly so) related to the browsing frequency of 1976; i.e. the abundance of the biennial in both 1981 and 2006 could only be attributed to the relative deer population of 1976.  How could this be?  Why did the frequency of the biennial not depend on yearly deer browsing/movement?




Figure 2 from Boulanger et al. demostrating increased frequency of
C. germanicum due to deer browsing in 1976.

Boulanger et al. hypothesize that the initial seed source came from a near-by forest, carried by deer fur.  The soil seed bank is an unlikely source; Cynoglossum seeds are not very persistent in the soil.  Too, endozoochory is an unlikely facilitator because C. germanicum leaves contain toxic pyrrolizidine alkaloids; they are not eaten by deer.  As well, basic seed dissemination is an unlikely travel agent, because no other nearby forests without deer populations have populations of the biennial.  Most likely, the abundant deer walked through the nearby C. germanicum population and carried the seeds into the Arcen-Barrois forest by their fur.

Once in the Arcen-Barrois forest, deer may have helped the seeds germinate in three ways: i) they somehow altered the habitats to favour germination (i.e. high light and high soil nitrogen environments); ii) their trampling of the soil created favourable microsites for germination; iii) or they trampled some of the seeds, removing the seed coat and ending dormancy.  Once established, C. germanicum individuals no longer relied on deer to reproduce and spread; their toxicity eliminated all seed predators and gave the biennial a reproductive competitive advantage in the forest.

Who knew that deer could be responsible – knowingly or not – for the introduction of a species?  Next week when I’m sitting in my deer stand, I’m going to take a minute to appreciate the important role deer play in many aspects of the forest culture; especially their surprising role as horticulturists!

Source:  Boulanger V, Baltzinger  C, Said S, Ballon P, Ningre F, Picard JF, Dupouey JL (2010) .  Deer-mediated expansion of a rare plant species. Plant Ecol. DOI 10.1007/s11258-010-9823-9