Impact On The Environment: The Loss Of Functional Tree Species

Introduction

Ellison et al. (2005) present a series of case studies which chronical the loss of three tree species, which they term “foundational”, from forests the world over due to things like non-native pest and pathogen outbreak, human over-exploitation and habitat alteration. According to the authors, this loss is “unfortunate” and can have profound effects on typical forest function and structure (e.g., altered rates of decomposition, nutrient flux, energy flow).

However, the situation has afforded the researchers opportunities to study the impacts of such loss and to develop models and metrics that will help them to identify foundational species prior to local extirpation; with earlier identification comes time to determine how a trees loss will alter their specific forest ecosystem; and thus provides the context necessary to implement appropriate conservation and management strategies designed to mitigate the impact of said loss or, hopefully, to avoid it entirely. I do not see why I would disagree or contend the position that Ellison et al. (2005) have taken, in that it makes sense that we should want to understand how the loss of foundation species impact the environment in which they are found.

With this knowledge we have the potential to better understand the “cascade” of changes to follow and an enhanced ability to manage against or avoid change. In the following synthesis we will discuss the loss of three foundational species identified by the researchers, what their loss means functionally/structurally, the divergence in function and structure in the ecosystems that result from this loss, and how management can be developed from this newfound understanding and insight.

Three Foundational Species Highlighted: A Case Study

To illustrate how the loss of foundational species can impact the environment they are found in the researchers chose three species they have determined to be foundational and have chronicled the impacts to ecosystem structure and functions that have resulted as a consequence. We quickly introduce the species, means of loss and alterations that followed. Hemlock (Tsuga canadensis) is one such foundational species that greatly influences forested ecosystems where it occurs in nearly pure stands; soil pH, rates of decomposition, nutrient cycling, the interception of moisture, its flow, and stream temperature are all mediated by it.

Unfortunately, the combined impacts of human land conversion from forest to agriculture, increased rates of natural disturbance, and most notable, an insect outbreak traced back to the 1980’s has resulted in the dramatic decline of hemlock. It is believed that the species could go functionally extinct within the next several decades. This reality is problematic as the loss of the species is posited to result in a replacement forest dominated by hardwoods (e.g., Betula spp., Quercus spp., Acer spp.) which function much differently, supporting higher annual thermal maximums and minimums, lower levels of available soil nutrients (e.g., C:N:P) and metals, with high light penetration; and associated streams that are more variable, with elevated rates of transpiration and a flashier response to seasonal precipitation.

Similarly, to hemlock, white-bark pine or Pinus albicaulis, is presented as a foundational species that has experienced dramatic losses due to the combinatory effect of a series of non-native pathogen (Cronartium ribicola) outbreaks, predation by native bark beetles (Dendroctonus ponderosae) and a significantly human altered fire regime.

White pine is a high-elevation species growing as a dense krummholz, or a prostrate, cushiony, mat of trees. In this form, whitepine retains snow, retards snowmelt and thus modulates the flow of water. In turn, the loss of and replacement by other pine species has resulted in a flashier hydrologic system, altered wildlife species assemblage and succession rates.

The last foundational species covered by the researchers is the American chestnut. Again, as is the requirement of foundational species, it strongly influences both ecosystem structure and function. This species contains a high amount of secondary compounds (e.g., tannins) and a relatively low C:N ratio, meaning that it breaks down or decomposes rather slowly, the tannins further mitigating its breakdown and thus mobilization of nutrients into the soil.

There is evidence to suggest that chestnut utilized allelopathy to exclude other species from certain habitat types where they might otherwise dominate. Just as has been the case with the other foundational species, chestnut populations have been dramatically reduced by a pathogen, Cryphonectria parasitica, or chestnut blight. The adjacent streams and macroinvertebrate communities in forests formerly dominated by chestnut were dependent upon its nutrient dense leaves as an allochthonous input. It is likely that the species assemblage within stream have diverged compositionally and surely functionally as a result. Clearly, a species must be considered foundational with such perverse ecosystem alterations solely from their loss. What information can be learned from this loss and how can it be utilized to manage for resilience in the face of such change?

Knowledge Gained?

Our current level of understanding of foundational species and the consequences of their loss to ecosystem structure and function is scant and relies on just a few case studies like those of the previous section. The authors suggest that the inability to determine what species are in fact foundational until it is too late, that is only after they have dramatically declined, is partly to blame. In addition to early detection, long-term monitoring of the decline of foundational species populations and the resulting biotic assemblages, their trajectory, structure and function are needed. They see these losses as an opportunity, albeit unfortunate, to study the effects on dependent species and ecosystem processes.

Another problem brought up by the authors is a general lack of understanding of the natural histories of the very species being lost. They argue that this lack of detailed knowledge means that we cannot accurately characterize the role(s) and influence foundational species have on the biotic and abiotic systems they are found within. Their loss not only represents a tangible alteration to ecosystem structure and function but also a missed opportunity to understand and characterize their influence. Without this information managers are ill-equipped to deal with the cascade of changes that will follow as new replacement ecosystems emerge.

Conclusion

A foundation species is a given species that disproportionally influences the structure and function of an ecosystem. Due to a variety of extraneous variables (e.g., climate change, native- and non-native insects and disease, human habitat alteration and resource exploitation) foundation species are being lost at an ever-increasing pace without evidence of slowing down. In many cases, while species are actively declining, we have been unable to determine whether they serve as foundation for the ecosystem they reside within, and thus have yet to accurately describe their many influences on ecosystem structure and function and what their loss means for the future of their former ecosystem.

What is needed is a renewed effort and method for the early detection of candidate foundation species and a thorough characterization of the functional/structural role that they play within the ecosystem they reside. With this information researchers, managers and citizen-scientists will know what is at stake when we speak of potential losses and to what effort need be invested to steer the course of succession from complete loss to something more desirable. This will only be possible with good data. A review of existing case studies would be enlightening and provide the basis for understanding necessary to begin the process of early determination and possible management strategies.

Here I propose a simplistic model to be applied to identified candidate foundation species that are amid decline:

• the longest portion of the model, should include as many of the influences the species has on its immediate environment (e.g., soil pH and flux, energy flux, soil moisture, stream inputs, thermal influence, transpiration, cover, food) that time, money and human understanding can afford;
• identification of the extraneous variables that are causing said decline should be listed in descending order in respect of their impact;
• the development of or utilization of pre-existing state and transition models to better understand potential successional trajectories and what they would mean in the context of ecosystem structure and function;
• the analysis of the above findings and the creation or consideration of a suite of management actions to be utilized to mitigate loss or to be used to steer successional trajectory to another stable state community that serves similar, acceptable, structural and functional roles;
• the implementation of a robust monitoring program appropriate for tree species.While the implementation of such a framework is a tall order, it is ultimately necessary if we are to maintain the health and integrity of our natural ecosystems as we know them.

Theodore Roosevelt said it best in that, “We must dare to be great; and we must realize that greatness is the fruit of toil and sacrifice and high courage.” We must have the courage to toil and sacrifice our time away to save foundation species and the ecosystems they create in the face of seemingly insurmountable challenges like a changing climate and human exploitation. I ask, if not us, then who?