Trophic Cascade

The Critical Value of Hunters:  The Necessary Apex Predator of Suburban and Rural Woodlands

A trophic cascade is an ecological phenomenon triggered by the addition or removal of top predators and involving reciprocal changes in the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling.

Historically, humans have modified many temperate ecosystems by decimating native animal populations and often substituting domesticated stock, thereby influencing food webs and simplifying interactions among species.  Large predators, especially, have been subject to persecution.  The profound ecological implications of losing top predators are only now beginning to be understood. Across a variety of environments, predator extirpation can lead to a trophic cascade” regarding habitat degradation at multiple trophic levels, species loss, and even ecosystem collapse.

Researchers investigating why invasive species are so successful often highlight the absence of natural enemies.  Within its native range, any species of deer contends with a suite of factors such as predators, hunters, pathogens, primary productivity, and others that can limit population densities.  When released in a new region without its co-evolved competitors and disturbance regimes, a species can reach unprecedented densities.  The “enemy release hypothesis” appears to accurately characterize such invasions and represent a specific case of trophic cascades.

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By the mid-20th century our local forests have regrown and deer populations had recovered to levels that support recreational hunting in most eastern states.  However, since recent times deer populations have continued to increase suggesting that contemporary hunting pressure (combined with other sources of mortality, such as vehicular collisions) may be insufficient to keep irrupting deer populations in check or to preserve biodiversity.

Increasing evidence suggests that deer are pushing local forests into an “alternative stable state” – one characterized by a dense cover of regionally common grasses and ferns that inhibits germination and establishment of tree seedlings.  In the long-term, these forests could become similar to a savannah or European woodlot, consisting of an overstory and then not much else other than a browse tolerant herb/grass ground layer.

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Wildlife managers are increasingly recognizing that control of deer numbers through hunting is limited by declines in hunter numbers, increases in areas off-limits to hunting, and a reluctance of hunters to engage in antlerless harvests or other population control strategies.  Indeed, white-tailed deer harvest rates have increased dramatically in the eastern United States over the past 30 years, even with a decline in the number of deer hunters over the same interval.  In addition, hunting by humans is typically not functionally equivalent to predation by large carnivores due to factors involving risk and the ecology of fear.

In each of these cases, large mammalian herbivores in the absence of strongly – interacting predators have been found to have major impacts on vegetation, reducing and skewing tree recruitment in forests, transforming understory herbaceous communities, and denuding riparian vegetation.  These profound and potentially devastating effects of large herbivore overabundance are little understood or appreciated by much of the public and many practicing biologists.

Deer hyper-abundance is only one, but perhaps the most obvious, consequence of large predator loss.  Others include deer behavioral changes, meso-predator release (as top predators decline in an ecosystem, an increase in meso-predators occurs, which are middle trophic predators such as fox, raccoon and skunk), negative indirect effects on small vertebrates, changes in the composition and/or structure of plant communities, altered hydrologic interactions in riverine systems, facilitation of the invasion of exotic plants, etc. In the face of so many deleterious consequences of predator loss, there is no simple solution.  Simply liberalizing hunting regulations will not suffice to mitigate or reverse so many adverse trends including the loss of natural predator/prey behavioral dynamics.

Wherever deer have been released from historic predation pressure (truncated trophic cascade), we observe the same general pattern – high population densities and loss of predation risk accompanied by intensive browsing that begins to shift plant community composition towards domination by browse-tolerant and resistant plants.  Even after an irruptive peak, persistently high foraging pressure continues to drive changes in plant community composition.  Over time, the ecological integrity of terrestrial ecosystems, including native plant communities and wildlife species dependent upon them, are invariably affected.

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In forested ecosystems, declining species diversity first becomes apparent in understory vegetation, as less palatable ferns or grasses crowd out more palatable herbaceous plants and inhibits the establishment of tree seedlings.  Suppressed tree recruitment opens gaps in the age structure of palatable shrub/tree species, possibly hindering recovery, even when a natural predation regime even when a natural predation regime is restored.  With continued intensive browsing, plant communities shift towards an alternative stable state composed of species resistant or resilient to herbivore pressure.  Decreased standing biomass and decreased diversity of native species may, in turn, trigger additional ecosystem effects, such as increased soil and streambank erosion, altered terrestrial and aquatic food webs, and changes in nutrient cycling.  Thus, the removal or significant reduction of large predators sets in motion a chain of events that initiates a downward spiral toward ecosystem simplification.  Although partial or complete loss of forest ecosystems is a possible, though still unproven, endpoint preventing and/or reversing the impacts of large herbivores so that diverse native plant communities and dependent wildlife can thrive represents a daunting but pressing challenge for the scientific community and society as a whole.

Source Material (PDF Articles for downloading):

LargePredators

Trophic Cascade Effects of Deer Overabundance on Connecticuts Na

Mark’s Rant:  I have never hunted deer, but all four of my son’s are deer hunters, primarily with cross-bows.  I do however very much enjoy upland game bird and waterfowl hunting.  Two, three or four days a week, I am in the woods for work or for pleasure, and I can almost always find significant evidence of deer browse, deer over-abundance, and degradation caused by deer in the form of alien invasive plants and the loss of structural habitat. 

I care deeply about the ability to see native wildflowers, plant species diversity, species richness and abundance.  I relish a forest that has all structural matrices intact, an over-story, associate trees, suppressed trees, tree saplings and seedlings, a healthy shrub layer with multiple species, and then a gracious herb layer of mixed forbs, grass, sedge, rush and the like.  I love to see quality examples of natural ecological community structure.  But more often than not, and especially in urbanizing woodlots, we see a total lack of diversity, lots of alien invasives, and little to no opportunity for sustained natural regeneration. 

I predict that in the near future we may have classifications of ecological communities for impaired urbanized woodlands degraded by deer as being similar to European woodlots or savannahs, where we have a relatively open understory and a grass/fern and invasive-dominated, or at least only non-palatable forest types (i.e. poplar/spicebush).  Sad commentary.  We must continue to promote hunting and keep deer in a range of 8 to 12-deer per square mile, to really cut back on herbivory pressure.  It’s a trophic cascade, and we can help play the roll of Apex Predator!  More coyotes too.

 

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