Friday, December 14, 2007

Biological diversity; is it merely a result of simple evolutionary biology?

Introduction

Charles Darwin and his colleagues opened the door on our understanding of evolutionary biology. Their insights have revolutionized the modern view of both the biological and ecological sciences. Interestingly though, as science has progressed, the somewhat linear theories that arose from Darwin and his counterparts may explain but just a small piece of the evolutionary tale. Systems approaches to understanding ecological processes have identified complex and fascinating interrelationships between the living and non-living world.

The basic evolutionary model assumes that the environment changes randomly, independent of the biological evolution. Accordingly, through a process known as ‘natural selection’, species evolve by means of a number of mechanisms including developing and expressing new beneficial, or the transfer of pre existing ancestral (genetic) traits and passing these on to subsequent generations (Anonymous, 2007). In this manner it is concluded that species, over time, either adapt or perish as their environmental changes. However, application of systems thinking to ecological and evolutionary processes at various (macro) ecological scales raise some intriguing questions about this assumption. For example:

· Have ecological systems evolved to reset the clock backwards so-to-speak across landscapes in order to maintain higher states and rates of biodiversity?

· Do species adapt to their changing environment, or can they change their environment to suit their evolutionary needs and hence evolutionary direction?

Although a strong temporal dimension is embedded in the science of evolutionary biology (Campbell and Reece, 2005) this perspective assumes that biological change occurs in a linear fashion and in response to the environment as an independent variable (Anonymous, 2007). Using examples of ecological processes and species commonly found here in British Columbia I challenge this simple evolutionary model by showing that the processes at play suggest greater complexity in the evolutionary pathways than originally conceived.

Small disturbances; a great diversifier?

Disturbance is a natural phenomena and takes place at many scales. Cleary, the palaeontological record shows great episodes of species evolution and radiation occurring after global catastrophic disturbance (Hlodan, 2007). However, it appears disturbance at localized scales and temporal time frames also plays a unique evolutionary role in maintaining and enhancing biodiversity.

Species diversity in of itself is thought to provide ecological stability and resiliency (The ‘natural range of variability’ (NRV), and the disturbances which take place within this range, help maintain ecosystem health by providing “a temporal dimension to biodiversity where the range of changes that occur in (the numbers and) assemblages of species, site conditions, and ecological relationships, in any one place, and across the landscape, increase an ecosystem’s ability to recover from perturbation” (Reese-Hansen, 2004).

In different ways disturbances such as fire, insect infestations, extreme weather conditions, etc., alter the landscape at various scales creating what landscape ecologists call ‘patches’. In these patches successions of new assemblages of different flora and fauna reestablish themselves on the disturbed site. Normally, each subsequent assemblage leads to a new assemblage of plants and animals, a process known as ‘succession’, and if left undisturbed, the process leads to greater stability with in the system (e.g. old growth forests, which can persist in some ecosystems for thousands of years). However, no ecological system is immune to disturbance and at some point it will be altered. Successional processes which help reestablish a disturbed site and lead to more stable states create greater resiliency (e.g. ability to recover from disturbance), a process that is achieved through spatial and temporal species diversity greater than what occurs in any given site at any one point in time. In fact, where natural processes such as succession and NRV are simplified or interrupted, as is typical of many forest management practices, the stability of the ecosystem can be compromised (Wong and Iverson, 2004).

Disturbance therefore, in a complex fashion involving mutually beneficial interrelationships of assemblages of spatially and temporally interdependent species, takes advantage of disturbance processes by setting the successional ‘clock’ backwards periodically in response to perturbation. In this way, dynamic temporal processes associated with disturbance, and the biological response to these events, have lead to increased species diversity which act together over the landscape and through time increasing system stability and resiliency to perturbation.

What do beavers and salmon have to do with influencing evolutionary direction?

It’s true that the environment is stochastic, changing in ways that can dramatically influence the evolutionary direction of life on the planet. But it also appears to be true that, in some instances and scales, there are overriding biological controls that directly influence the condition of the physical and biological environment in favour of the species making the change. This is certainly true of Homo sapiens, and although there is ample evidence of our significance in exercising this trait, we are not entirely unique. Two examples include beavers and pacific salmon.

Beavers: Number of years ago I set out to investigate links between anthropogenic land uses (a form of disturbance) and increased beaver (Castor canadensis) activity. While conducting the research I was astounded to discover the spatial and temporal magnitude to which beavers change their environment to suit their needs (Reese-Hansen, 2004). For example it became apparent to me that beavers modify the environment at very large scales by creating and maintaining (perhaps for centuries or longer) extensive valley bottom wetlands. In similar example but at a much greater scale, Hey (2001) convincingly argues that prior to the massive conversion of hydrological processes in the Mississippi basin post European contact, beaver played a important role in mitigating flooding through storage of water in vast networks of ponds and wetlands!

Given the beaver’s ability to dramatically alter or ‘engineer’ the environment to suit its specific needs (Haemig. 2007), it seems reasonable to conclude that this species may be inadvertently, but beneficially creating conditions which modify its own (and other species) evolutionary direction.

A small beaver created wetland. (Photo credit: Bonnie Bowin, http://www.flickr.com)

Salmon: The basic life histories of Pacific salmon are well known to British Columbians. For instance, most know salmon spend their adult lives in the marine environment and return to their natal streams as sexually mature adults where they spawn and die. Typically, after the new generation emerges from their gravel nests where they were spawned, they spend the early stages of their lives in the fresh water environment. Eventually, they return to the sea where they complete their journey to adulthood and the cycle is repeated.

To increase survival likelihood during the fresh water segment of the salmon’s life history they rely on some key habitat attributes linked to riparian forests. In a myriad of ways, large standing and downed trees (called large woody debris or LWD) create habitat complexity along the stream channel enhancing a variety of important habitat characteristics which help enhance or ensure survival of salmon populations (and a variety of other aquatic species).

Riparian forests, the LWD they contribute to the stream, and the hydraulic interaction between LWD and flowing water, improve important stream habitat conditions by creating: cover protection and refuge from predators in shallow waters; deep pools for rest and refuge during migration and spawning; sorted and stable substrates (gravel) suitable for egg laying and incubation; stable stream channels and stream banks through extensive riparian forest root networks and LWD which act to armour and hold stream banks and stream channels together; increased volumes of terrestrial organic material improving trophic levels and food availability for rearing slamonids; plus a variety of other enhancements.

Recently, our understanding of how salmon influence the ecological process taking place in salmon natal streams has expanded considerably uncovering a complex and fascinating interrelationship between the ocean and terrestrial riparian environment via pacific salmon. Instrumental in uncovering this relationship was tracing basic elements such as nitrogen (N), which exists as a unique isotope in the marine environment, and which is essential to the health and survival of plants and other organisms. In fact, N is a well known limiting factor for productivity in many ecosystems (Campbell and Reece, 2005).

By comparing the overall nitrogen content of riparian tree and plant species along both salmon and non-salmon streams researchers discovered a strong correlation between high nitrogen content in plants associated with salmon natal streams (Helfield and Naiman, 2007; Naiman et al. 2005; Mathew et al., 2003). By differentiating marine derived from other sources of N in both types of streams it was shown that the additional inputs of marine derived sources were responsible for increases in the productivity of plants and trees in the riparian ecosystem as a whole (Naiman et al. 2005; Mathew et al., 2003). Essentially, salmon streams tend to improve the productivity of riparian ecosystems, and in turn these areas grow trees faster and larger compared to other areas.

The transfer of marine derived nutrients is complex. Facilitated by bears (Helfield and Naiman, 2007) and a host of other species, the marine-terrestrial relationship comes a full circle when we consider one aspect of this relationship: the importance of riparian forests and their LWD contribution toward stabilizing and enhancing the aquatic conditions critical to salmon survival during adult migrations to natal streams, and juvenile rearing in the freshwater environment.

Bears, an important vector for the distribution of marine derived nutrients to the riparian environment. ((Photo credit: Steve Henderson, http://www.flickr.com)

While, given the level of our understanding at this time, it is difficult to know precisely what the evolutionary implications of this complex cycle are, it seems plausible that the interrelationships are not entirely coincidental. I would suggest that it is conceivable that natural selection maybe acting in a fashion such that salmon, which are instrumental in this complex web of interrelationship, may in their evolutionary path, be defining beneficial environmental characteristics essential to their survival. Through this process they may be influencing more than their specific geno and phenotypes, generation to generation, and shaping the environmental character of their natural natal environments which supports an essential part of their life histories’.

Conclusion

The examples I’ve discussed here highlight complex interactions between biological evolutionary processes and the environment. In this discussion I have tried to emphasise through these examples that species, assemblages of species, and ecosystems have responded to a stochastic environment in ways resulting in evolutionary processes that are more complex than a simple linear evolutionary model. By adapting complex ecological mechanisms which utilize biological diversity to take advantage of environmental stochasticity, or by altering the environmental conditions in a manner that favours a species (and often greater diversity), ecosystems and species alike may have the ability to, in complex set of interactions with the physical and biological environment, influence evolutionary direction, plus create greater ecosystem stability and resiliency through creation of greater species diversity.

References

Anonymous. 2007. Gene transfer. Wikipedia Foundation Inc. http://en.wikipedia.org/wiki/Gene_transfer Accessed: Dec 8, 2007.

Campbell and Reece. 2005. Biology (7th Edition). Pearson.

Chapin III, F.S., Zavaleta, E.S., Eviner, V.T., Naylor, R.L., Vitousek, P.M., Reynolds, H.L., Hooper, D.U., Lavorel, S., Sala, O.E., Hobbie, S.E., Mack, M.C., and Diaz, s. 2000. Consequences of changing biodiversity. Nature, 405: 234-242.

Hlodan. 2007. Macroevolution: evolution above the species level. BioScience, 57(3): 222-225.

Reese-Hansen. 2004. Beaver (Castor canadensis) patch-creation in response to land development in the Kitimat Valley, B.C (MSc. thesis). Royal Roads University. http://web.mala.ca/gilchrisa/PDF/LARS%20BEAVER%20Thesis%20-%20V%20Final.pdf Accessed:07/12/08

Haemig. 2007 Ecosystem Engineers: wildlife that create, modify and maintain habitats. (Ecology Information #12) http://www.ecology.info/ecosystem-engineers.htm Accessed: 07/12/08

Hey. 2001. Modern drainage design: the pros, the cons, and the future. Paper presented to the Annual Meeting of the American Institute of Hydrology, Minnesota, October 14-17, 2001. 7p. Wetlands Initiatives Website. http://www.wetlands-initiative.org. Accessed 10/01/03.

McCann, K.S. 2000. The diversity–stability debate. Nature, 405: 228-233.

Mathewson, Hocking, and Reimchen. 2003. Nitrogen uptake in riparian plant communities across a sharp ecological boundary of salmon density. BMC http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=156615 Accessed 07/12/13.

Naiman, Decamps, and McClain. 2005. Riparia: Ecology, Conservation, and Management of Streamside Communities. Academic Press

Wong and Iverson. 2004. Range of natural variability: Applying the concept to forest management in central British Columbia. BC Journal of Ecosystems and Management. Forrex. http://www.forrex.org/publications/jem/jem.asp?issue=21 Accessed: 07/11/12.

1 comments:

Dominic B. said...

VERY VERY interesting....I will have to read more about that fascinating subject!