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Oaks, Acorns, and Acorn Woodpeckers

Oaks ‘n Folks – Volume 15, Issue 1 – March 2000

No species is more intimately associated with oaks than the acorn woodpecker, a common resident of oak woodlands throughout California. Acorn woodpeckers eat acorns directly off trees in the fall as acorns mature. More dramatically, they harvest acorns in large numbers and store them in special trees in their territories in which the birds have drilled holes, each of which can hold an individual acorn. Holes are used over and over each year and accumulate with time. Indeed, such “storage trees” or “granaries” have been reported to contain up to 50,000 acorns or more!

In good acorn years, acorn woodpeckers not only reproduce extremely well the following spring but may even successfully breed in the fall, fledging young as late as early November. However, in poor years, the birds are forced to abandon their territories and wander off in search of acorns elsewhere. How far they have to go presumably depends on how geographically synchronous the acorn crop is within and between species of oaks. If the acorn crop was poor and synchronous both among individuals of the same species and across species over a large geographic area, there would be few acorns anywhere nearby and the birds might have to travel a very long distance before finding an area where there were enough acorns to survive. Whereas, if the acorn crop is not synchronous within and between species, they may only have to travel a few miles before coming to an area with lots of acorns where they could survive through the winter.

This brings us to the question of geographic synchrony in acorn production within and between species of California oaks. In order to address this issue, Jean Knops of the University of Nebraska and I initiated the California Acorn Survey in 1994. Currently encompassing 16 sites from Shasta to San Diego County, the survey involves visually counting acorns of over 950 trees of 7 species of oaks.

With only six years of data the results are preliminary, but the general conclusion is clear: acorn production is highly synchronous over large geographic areas for at least several of the species. For example, blue oaks, which we survey at 10 localities over a 500-mile transect both in the Sierra and coast ranges, are synchronous throughout the state (i.e., when it’s a good year for blue oaks in Shasta County, it’s also a good year for blue oaks in Santa Barbara County). Blue oaks cover approximately 3 million acres of woodland in California at densities (based on vegetation surveys at Hastings Reservation in Monterey County) of about 173 trees per acre, which means we’re talking on the order of over 100 million trees, each of which produces a crop of acorns synchronously with the others. If we assume an average of 1,000 acorns per tree in a good year, we’re up to 100 billion acorns- enough to support a lot of woodpeckers, as well as anything else that can handle the bitterness of raw acorns in its diet.

The flip side, however, is that in a bad acorn year, few or no blue oak acorns are likely to be found almost anywhere in the state. Fortunately for acorn woodpeckers, as well as other acorn-dependent species, most areas in California contain several species of oaks and synchrony among species of oaks is generally not great. This is especially true between “1-year” species (such as blue and valley oaks) that require only a single season to mature acorns, and “2-year” species (such as California black oak and canyon live oak) that require two years to mature a crop of acorns. Consequently, when it’s a bad year for blue oaks, it’s unlikely to concurrently be a bad year for California black oaks or canyon live oaks present in the same area. Because acorn woodpeckers, like other birds and mammals that eat acorns, tend not to be picky about which species they use, this means that the probability of a total crop failure in a locality is low, despite the large geographic synchrony exhibited by individual species.

These features of the acorn crop have dramatic consequences on the geographical ecology of acorn woodpeckers along the Pacific coast. Most strikingly, the range of acorn woodpeckers generally is restricted to sites containing two or more species of oaks, presumably because the probability of total crop failures in areas with only a single species of oak is too high to maintain populations over the long term. Second, the annual variability of acorn woodpecker population size at a site is less when the number of species of oaks present is greater. That is, when there are more species of oaks in an area, both the overall acorn crop and the population size of acorn woodpeckers are more stable and fluctuate less from year to year. Meanwhile, the absolute size of the acorn woodpecker population in an area appears to depend primarily on the amount of oak woodland present. Thus, the average population size of acorn woodpeckers is determined by oak abundance, while annual variability in population size is determined by resource variability, which is in turn inversely dependent on oak species number.

Put differently, these results demonstrate that greater biodiversity of oaks enhances both the viability and stability of acorn woodpecker populations. But although acorn woodpeckers are an extreme, they are only one of several no-table species dependent on acorns in California. Whether similar kinds of relationships between oak diversity and the geo-graphical ecology of deer, wild pigs, turkeys, jays, or quail exist today, or with grizzly bears and native Americans in the past, remain to be investigated.

prepared and edited by Richard B. Standiford, Justin Vreeland, Bill Tietje

Walt Koenig, Research Zoologist, Hastings Reservation, UC Berkeley