Saving Species, Sustaining Life

Highly topical and timely, this week’s Saving Species program on Radio 4 focuses on the impact of humans on the planet. The panel; Jacqueline McGlade from the European Environment Agency, Aubrey Manning - University of Edinburgh, Jon Bridle - University of Bristol, and celebrated environmentalist Vandana Shiva, discuss some of the areas in which human influence has caused significant ecological damage.

Fishing

Demand for fish has increased considerably of late; directly, for human protein, and indirectly, as food for other fish. Vandana Shiva observes that where fish was once a luxury item, it is now considered acceptable for the rich to consume it on a regular basis. Large-scale fishing leads to the disruption of ecosystems. The removal of sharks and large predators is a prime example of this. Without top predators, a large ecosystem cannot be established, and the intricate ocean food webs cannot be maintained.

Jon Bridle explains that the root of the problem is governance. Decision makers and the small communities that manipulate the oceans have very limited understanding of nature. Hence, fisheries are a classic case of the ‘commons’ (Hardin 1968), where decisions are made at the expense of what is out of site, and therefore, out of mind. Further to this, subsidies distort ecosystem value, and the economy needs to be changed in order to use the planet more sustainably to support a given number of people.

Predators

Where once humans were both prey and predator, we are now very much at the core of habitat destruction. The Amazon rainforest, ‘the lungs of the world’, is vital to production of oxygen, carbon dioxide neutralisation, and is rich in highly valuable medicinal species. This unique resource is in jeopardy because of the unrealistic demands of the West.  Vast areas of forest have been cleared to make way for cheap meat production, and soya and palm oil plantations, a topic which is explained in detail here by a fellow blogger.

Too many people

Vandana Shiva describes the current inefficient resource intensive system in fishing, forestry and food production as ‘unsustainable and unjust’. She notes that the human species must recognize that it is our own abundance that is causing such damage to biodiversity. Shiva calls for an increase in ecological justice, restoration of love and appreciation for the planet, and a healing of the deep rift between humans and nature that was caused by the Cartesian revolution in science. There is talk of the need to banish the illusion that we are separate from nature, and recognize that we are part of the community of the earth. The program ends with the thought that the more we save species, the more we save ourselves.

Listen here

Case Study: Amphibians

INTRODUCTION

There is little doubt that human activity is causing loss of species through considerable damage to natural environments, and amphibians have suffered the most. Indeed Sodhi et al (2008) observe that due to their major declines in population, susceptibility to disease, and morphological deformities, amphibians epitomise the modern biodiversity crisis.

Amphibians are cold-blooded vertebrate animals, which metamorphose from water-breathing juveniles to air-breathing adults (Stuart et al. 2004).The current extent of amphibian extinction is perplexing, because during their 350 million year period of evolution, they have managed to overcome mass extinctions similar to that of the present. The exact characteristics enabling these creatures to survive extreme conditions have been the subject of much research (Wake and Vredenberg 2008).

Deep-sea sediment cores suggest that over the past 65 million years, global climate change has been almost continuous, but the recent rapid warming is of particular concern (Carey and Alexander 2003). By comparing the current amphibian extinction rate with background fossil rate, we can improve our understanding of the magnitude of the current biodiversity crisis and the extent to which humans are responsible (McCallum 2007).

Figure 1. Relative percentages of species loss from different altitudes. Source: Pounds et al. (2006).

STATISTICS

Recent research has shown that the current extinction rate exceeds that of both the 1500 and 1980 level, rates of amphibian declines are catastrophic, and projected losses for the future are even more intense.

The 1989 Congress Of Herpetology marked the start of scientific concerns about amphibian population declines, and the IUCN GAA (Global Amphibian Assessment) was conducted in order to gauge the severity of these declines. Results from this indicate that 43.2% of amphibian species are declining, 122 are possibly extinct, and up to a third are at risk of extinction (Stuart et al.2004, Pounds et al. 2006, Wake and Vredenberg 2008, McCallum 2007).

CAUSES

Colins and Storfer (2003) note that there are six underlying hypotheses explaining amphibian declines. Three of these; the invasion of alien species, overexploitation and excessive harvesting of amphibian populations, and land use change, are relatively well understood. By removing, introducing, or changing constants, it is likely that amphibians will suffer. The remaining hypotheses involve complex interactions between global change, infectious diseases and trends amongst amphibian populations.

·      Global Warming

Humans influence on the earth’s climate, large scale warming, and aerosol formation intensify the hydrological cycle and shift the balance between ecological conditions, in turn leading to threats to species survival (Walther et al. 2002, Pounds et al. 2006).

In the Cascade area of Western North America, shallow lakes and ponds provide an ideal location for Western Toads, Bufo boreas, to lay their eggs. Recently, though, the embryos of these toads have begun to die before becoming properly developed. Climate change induced reductions in pond level are thought to be to blame. Shallower water overlying eggs means that protection against exposure to UV-B is reduced, for instance, where water depth is less than 20cm, 80% of toad embryos are killed by Saprolegnia, whereas this figure is only 12% in water depth of 50cm (Pounds2001).

The indirect effects of global climate change include changes to the phenology of breeding, in the case of amphibians, the seasonal variation in timings of egg laying. Evidence suggests that breeding seasons mirror climatic conditions; this puts those hatching early at a disadvantageous risk of mortality due to cold temperatures.

Figure 2. Relationship between climate change, amphibian declines, and extinctions. Source: Pounds (2001).

·      Characteristics of amphibians

Amphibians are directly influenced by temperature and moisture. Their cellular and physiological processes are controlled by heat exchange with air, water, and solar heat. Hence, severe daily temperature fluctuations are dangerous for bodily function.

Most amphibians are found in specific tropical geographic ranges, and aren’t capable of adapting well to different environmental conditions. It is to be expected, therefore, that further habitat changes will accelerate species loss, particularly of those dwelling in ecologically pristine areas. The disappearance of the Monteverde toad and Harlequin frog during unusually warm years illustrate this (Daszak et al. 1999, Wake and Vredenberg 2008, Pounds et al. 2006).

In reproductive terms, water is vital for amphibian existence. Eggs and larvae are deposited in standing water, and so annual variation in rainfall will influence both the number of eggs laid and hatched (Carey and Alexander 2003).

The permeable skin and hormonally regulated development of amphibians makes them highly vulnerable to endocrine disruption. For instance, the herbicide Atrazine, a common contaminant of ground and surface water where amphibians breed, is highly active at low concentrations. This compound chemically castrates and feminizes male amphibian larvae, retards development and growth, and is the cause of unusual behaviour and immunosuppression (Hayes et al. 2006). Although now banned in the EU, Atrazine is still widely used in the USA.

·      Disease

Chytridiomycosis is a panzootic fungal amphibian disease, caused by Batrachochytrium. It develops and spreads in moist aquatic habitats, particularly during the winter. The disease is persistent at low densities, and attacks the moist skin of amphibians by degrading cellulose, chitin and keratin, and producing zoospores. The discovery of a new form of chytrid fungus, Batrachochytrium dendrobatidis, coincided with the observation that amphibian declines were taking place, and sporangia of this fungus were found within mouthparts of tadpoles, particularly from montane habitats (Daszak et al. 1999, Carey and Alexander 2003, Pounds et al. 2006, Pounds 2001). Similarly, the Saprolegnia ferax fungus has caused mortality of amphibian eggs in the Pacific North West, and loss of the Western toad Bufo boreas. Ranaviruses are another concern, and spread of these has increased due to humans (Collins and Storfer 2003, Carey andAlexander 2003).

In addition to these problems, limited knowledge of the true numbers of creatures no longer existing on earth presents further challenges to ensuring the survival of those remaining (McCallum 2007, Stuart et al. 2004).

References

Carey C. and Alexander M.A. (2003) ‘Climate change and amphibian declines: is there a link?’, Diversity and Distributions, 9, 111-121.
Collins J.P. and Storfer A. (2003) ‘Global amphibian declines: sorting the hypotheses’, Diversity and Distributions, 9, 89-98.

Daszak P., Berger L., Cunningham A.A., Hyatt A.D., Green D.E. and Speare R. (1999) ‘Emerging infectious diseases and amphibian population declines’, Emerging infectious diseases, 5, 6, 735-748.

Hayes T.B., Case P., Chui S., Chung D., Haeffele C., Haston K., Lee M., Mai V.-P., Marjuoa Y., Parker J., and Tsui M. (2006) ‘Pesticide Mixtures, Endocrine Disruption, and Amphibian Declines: Are we underestimating the impact?’, Environmental Health Perspectives, 114, 1, 40-50.

McCallum M.L. (2007) ‘Amphibian Decline or Extinction? Current Declines Dwarf Background Extinction Rate’, Journal of Herpetology, 41, 3, 483-491.

Pounds J.A. (2001) ‘Climate and amphibian declines’, Nature, 410, 369-340.

Pounds J.A., Bustamante M.R., Coloma L.A., Consuegra J.A., Fogden M.P.L, Foster P.N., La Marca E., Masters K.L., Merino-Viteri A., Puschendorf R., Ron S.R., Sanchez-Azofeifa G.S., Still C.J. and Young B.E. (2006) ‘Widespread amphibian extinctions from epidemic disease driven by global warming’, Nature, 439, 161-167.

Sodhi N.J., Bickford D., Diesmos T.M.L., Lian P.K., Brook B.W., Sekercioglu C.H. and Bradshaw C.J.A. (2008) ‘Measuring the meltdown: drivers of global amphibian extinction and decline’, Public Library of Science, 3, 2, 1-8.

Stuart S.N., Chanson J.S., Cox N.A., Young B.E., Rodrigues A.S.L., Fischman D.L. and Waller R.L. (2004) ‘Status and trends of amphibian declines and extinctions worldwide’, ScienceExpress, [www] available from: http://people.nnu.edu/jocossel/Stuart%20et%20al%202004.pdf, [19/12/2011].

Wake D.B. and Vredenberg V.T. (2008) ‘Are we in the midst of the sixth mass extinction? A view from the world of amphibians’, 105, 1, 11466-11476.

Walther G.-R., Post E., Convey P., Menzel A., Parmesan C., Beebee T.J.C., Fromentin J.-M., Hoegh-Guldberg O. and Bairlein F. (2002) ‘Ecological responses to recent climate change’, Nature, 416, 389-395.

Ecosystem Services

The Biodiversity Crisis

As discussed previously, humans have a notable impact on the Earth. An estimated 83% of the global terrestrial biosphere is under human influence, and perhaps as much as 36% of the bioproductive surface of the Earth is controlled exclusively by man (Harbel and Krausmann 2010). Species diversity represents a dynamic equilibrium between extinction and speciation. Since human colonization, however, this delicate balance has been upset. Evidence from marine ecosystems demonstrates the impact of humans over the past century. During this time 15% of Pacific Island birds have gone extinct, 20 of 297 mussel and clam species and 40 of 950 fishes have perished in North America, amounting to 1 extinction every 20 minutes. The current level of species loss has been compared to that of the late Cretaceous extinction 65mya, in which the dinosaurs and two thirds of species on earth were killed off, possibly due to asteroid impact (Karieva and Marvier 2003).

Anthropogenic activity has a marked influence on trophic skew. By removing species through hunting, fishing down of food webs, elimination of prey, and altering biophysical conditions, dramatic shifts in vegetation composition may occur, causing alterations to trophic levels (Novacek and Cleland 2001).

Another leading cause of biodiversity loss is habitat fragmentation. This is due to both climate change and population expansion and the resulting resource exploitation and alteration of land use patterns. Fragmentation increases local rates of extinction by reducing species population sizes and colonization from similar habitats, eliminating keystone predators or mutualists, enhancing genetic bottlenecks, promoting edge effects, and interrupting landscape-scale processes (Singh 2002). In the future habitat fragmentation is likely to reduce opportunities for speciation and restrict gene flow between species groups. This may be particularly acute amongst larger species such as primates, which are already prone to high rates of speciation and extinction (Levin and Levin 2002).

Severe habitat destruction, overexploitation of populations, freak meteorological events, or the emergence of new disease often results in direct and abrupt species loss, with small populations more likely to go extinct due to these freak events. The final descent into extinction, however, is often driven by synergistic processes that are disconnected from the original cause of species decline (Karieva and Marvier 2003). Habitat degradation and species extinction taking place over short timescales are likely to reset the future evolution of earth’s biota. Evidence from the fossil record suggests that the recovery of global ecosystems takes place over tens of millions of years (Novacek and Cleland 2001).

Ecosystem services

Ecosystem services are benefits to humans from resources and processes that are supplied by natural ecosystems. This definition was formalized in 2004 following the Millennium Ecosystem Assessment. Society is highly dependent on ecosystem products and services for food, shelter and healthcare. As human populations grow, resource demands imposed on ecosystems increase, and the environmental impacts of human ecosystem exploitation; overfishing, deforestation, industrialisation, and landscape degradation become more evident. The relationship between species and the services they provide needs to be understood in order to assess the implications of population change on humanity’s life support systems (Kareiva and Marvier 2003).

Biodiversity is of immense value to human health as ecosystem function and stability are reliant on it. Healthy functioning ecosystems provide humankind with a multitude of economic benefits including timber and fibre whilst being  essential for human survival. Constanza et al.(1997) have estimated the value of ecological services to be between $16 and 54 trillion per year. On a global scale, biodiversity represents a balance between rates of speciation and extinction, with greater biodiversity resulting in access to more resources (Singh 2002). If only a few individuals of an endangered species remain, however, they are unlikely to be able to make any meaningful contribution to ecosystem function (Balmford et al. 2003) and will be of considerably lower value.

How to measure ecosystem services

Prediction of extinction risk is dependent upon environmental and biological setting. Diversity is not uniformly distributed on earth, and typically increases from poles to equator (Brook 2008, Singh 2002). Complications arise in estimating the value of ecosystems when unidentified species become extinct. Under such conditions, ecosystem value may be reduced if the loss of these unknown species has a detrimental impact on ecosystem function (Duffy 2003).

Biodiversity hotspots

Tropical ecosystems are home to more unique species than any other habitat, and are hence considered to be biodiversity hotspots - areas containing high concentrations of endemic species. Since the coining of the term ‘hotspots’ by Norman Myers in 1988, research and conservation funding has been focused on these areas, neglecting other species-poor regions such as the Arctic. This is not necessarily an optimal approach, however, as the biological value of ecosystem services should also be considered when deciding which areas are deserving of more attention. Species that are of high value to humans are not found solely in biodiversity hotspot areas, so conservation efforts should perhaps be focused on ensuring that no major ecosystems suffer anything greater than a given percentage of biodiversity loss. It may be equally important to save higher taxonomic groups under threat than areas rich in endemic species, as by only conserving the species in a small area, evolutionary patterns may be altered (Kareiva and Marvier 2003).

Figure 2. Biodiversity Hotspots. Source: Myers et al. (2002). http://se-server.ethz.ch/staff/af/Fi159/M/My042.pdf

Measurement of biodiversity

In measurement of biodiversity, there are 4 key indicators.

·   Population richness. The number of populations of a species in a given area.

·  Population size. Number of individuals per population, which indicates the frequency distribution of population sizes. It is necessary to understand the contribution of each population to functioning ecosystems.

·   Population distribution. The spread of populations relative to their maximum possible extent within an area.

·   Genetic differentiation. More genetic variation within populations may provide better resilience to environmental change.

In order to fully understand population change and biodiversity decline, all four indicators must be considered. Biodiversity loss needs to include both species and population-based approaches (Luck et al. 2003).

Human Appropriation of Net Primary Productivity – HANPP, is an indicator that is used to estimate the relative scale of human activities and natural processes. Net primary production – NPP, is the net biomass produced by plants on an annual basis. It can be lost due to human-induced changes in ecosystem productivity, and provides a good indication of trophic energy flows within ecosystems. HANPP represents the extent to which land conversion and biomass harvest change the availability of NPP in ecosystems. Exact definitions of HANPP vary, but Harbel and Krausmann (2010) regard it as being the difference between the amount of NPP available in an ecosystem in the absence of human activities and the amount of NPP remaining in the ecosystem.

HANPP is a significant and useful indicator of human impact for several reasons. It provides a good measure of the physical size of the economy relative to that of the ecosystem. It gives an estimate of what proportion of the potential trophic energy, that could be used for wild animals and other heterotrophs, is still available and indicates human domination of ecosystems. Further to this, utilizing NPP as a basis for ecosystem functioning, human-induced changes of NPP and the way in which they affect patterns, processes and functions of ecosystems can be understood.

Figure 1. Map of Global HANPP. Source: Harbel and Krausmann (2010). http://www.eoearth.org/article/Global_human_appropriation_of_net_primary_production_(HANPP)

Balmford etal. (2003) suggest that the impact of humanity on nature can be gauged using estimations of extinction rates. Habitat loss data is combined with model predictions of changes in species number according to habitat area. Fossil records are incomplete and biased towards abundant and widely distributed species, this limits the applicability of this approach. Further to this, extinctions are difficult to document as only a small fraction of living systems are being fully monitored, there can also be a large time lag between habitat loss and species disappearance and statistical difficulties arise when combining datasets from different studies.

References

Balmford A., Green R.E. and Jenkins M. (2003) ‘Measuring the changing state of nature’, Trends in Ecology and Evolution, 18, 7, 326-330.

Brook B.W., Sodhi N.S. and Bradshaw J.A. (2008) ‘Synergies among extinction drivers under global change’, Trends in Ecology and Evolution, 23, 8, 453-460.

Constanza R. (1997) ‘The value of the world’s ecosystem services and natural capital’, Nature, 387, 6630, 253-260.

Duffy J.E. (2003) ‘Biodiversity loss, trophic skew and ecosystem functioning’, Ecology, 6, 680-687.

Harbel H., Erb K.-H., Krausmann F. (2010) ‘Global human appropriation of net primary production (HANPP), [www], available from http://www.eoearth.org/article/Global_human_appropriation_of_net_primary_production_(HANPP)

Kareiva P. and Marvier M. (2003) ‘Conserving biodiversity Coldspots’, American Scientist, 91, 4, 344-351.

Levin P. and Levin D. (2002) ‘The real biodiversity crisis’, American Scientist, 90, 1, 6.

Luck G.W., Daily G.C. and Ehrlich P.R. (2003) ‘Population diversity and ecosystem services’, Trends in Ecology and Evolution, 18, 7, 331-336.

Novacek M.J. and Cleland E.E. (2001) ‘The current biodiversity extinction event: Scenarios for mitigation and recovery’, Proceedings of the National Academy of Sciences of the United States of America, 98, 10, 5466-5470.

Singh J.S. (2002) ‘The biodiversity crisis: A multifaceted review’, Current Science, 82,6, 638-647.