Unit 4. Biodiversity: From Production
to Conservation
Why
is primary production essential to life?
The majority of energy used by
organisms on Earth originates from the sun. Autotrophic organisms or primary
producers hold the key to changing the radiant energy from the sun into a
usable form of chemical energy that other heterotrophic organisms (including
ourselves) can use. Autotrophic organisms, including plants and certain types
of bacteria and protists, are known as primary
producers because they can manufacture complex organic molecules such as
sugars, starches, proteins, lipids, and nucleic acids from simple inorganic
compounds like water, CO2, and nutrients, with the use of radiant
energy from the sun or chemical energy from ocean floor vents. Heterotrophic
organisms on the other hand, cannot produce their own photo-chemical energy and
need to consume the organic material in net primary producers or other
heterotrophic organisms to survive. Heterotrophs
include some plants, the majority of animals, and most microbes. In addition to
consumers of living organisms, heterotrophs include detritivores (decomposers), or organisms that break down
dead organic material and convert it to its inorganic precursors, such as CO2
and nitrogen. This breakdown provides more inorganic compounds for primary
producers to assimilate, and thus completes the cycle of elements from
producers to consumers to decomposers.
Primary producers (autotrophs) capture solar energy through photosynthesis and
use much of that solar energy for their own growth, maintenance, and
reproduction. When sunlight hits a leaf,
the plant uses the radiant energy of light photons, along with CO2
from the atmosphere to produce sugar, or chemical energy, and oxygen. Primary
producers usually produce more chemical energy than is needed for their own
life processes, and this is the energy that is available to heterotrophic
organisms. This chemical energy is stored by the plant and used for maintenance
respiration or for plant growth, resulting in an increase in biomass of
individuals and populations (Figure 4.1). The total amount of energy that
plants produce, without accounting for any losses, is known as gross primary
production or GPP.
Figure 4.1
Image from
Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (www.sinauer.com) and
WH Freeman (www.whfreeman.com)
Yet, the photosynthetic process
that generates this gross primary production is itself energetically expensive;
therefore, as plants are taking up CO2 for photosynthesis, they are
concurrently releasing a smaller amount of CO2 from respiration,
which produces the internal energy needed for maintenance of plant metabolic
activities. In addition to the CO2 lost through respiration, plants
lose heat energy and water through the photosynthetic process. What remains
from the gross photosynthetic process after the respiratory losses associated
with plant construction and maintenance are accounted for, is considered the
net primary production or NPP (Figure 4.2). Therefore, gross primary production
(GPP) minus respiratory losses (R) is equal to the amount of net primary
production (NPP).
NPP = GPP – R
The amount of NPP produced on
Earth is strongly correlated with the amount of water available and
temperature, with warm tropical rainforests producing in excess of 30 metric
tons of NPP per hectare every year (see terrestrial biomes below). Conversely,
cold and hot deserts can produce zero to only 2.5 tons of NPP per hectare per
year due to the unfavorable conditions for primary producers in these
ecosystems. This dramatic difference in the availability of NPP strongly
dictates the number of organisms that different ecosystems can support.
Figure 4.2
How
does energy flow through ecosystems?
Trophic levels are roughly defined by the way that the organisms in the trophic pyramid acquire energy in a food chain. For example, plants and other autotrophic organisms are usually found at the base of a trophic pyramid and are considered the primary producers, which convert radiant energy from the sun and inorganic compounds into the chemical energy that can be passed on to other trophic levels (Figure 4.3). The first level of the trophic pyramid generally belongs to the primary consumers or herbivores that feed directly on the primary producers (or photosynthetic organisms) in the base of the pyramid. In our example, this second trophic level belongs to herbivorous insects such as grasshoppers. The third level of the pyramid belongs to the secondary consumers or carnivores. Carnivore consumers feed directly on the herbivores in the system. In our example, the mouse that eats the grasshopper is considered a secondary consumer. The tertiary consumer in our system is the snake, which eats the mouse, which eats the grasshopper, which eats the primary producers. This might remind you of the nursery song about the old lady that swallowed the fly!
Figure 4.3
Trophic pyramid
with efficiency of energy transfer
http://www.anselm.edu/homepage/jpitocch/genbios/54-11-NetProductPyramid-L.jpg
Trophic pyramids and food chains are a great way to understand the dynamics of how energy flows through an ecosystem; however, they are very simplified compared to the reality of how ecosystems function. Most ecosystems actually have more of a web-like structure, wherein omnivores such as the snake in our system may not only eat the mouse, but may also eat the grasshopper. In addition the mouse might eat the flower seeds as well as the grasshopper; thus the system is not really a linear chain and has more connections, like in a spider web. On the other hand, the linear nature of a food chain or food pyramid facilitates understanding of the efficiency of energy transfer that occurs in ecological systems.
You may recall from
thermodynamics that energy transformations are never 100% efficient (e.g.,
there is no perpetual motion machine). This physical principle determines how
many trophic levels a closed, simplified ecological
system can support. In our example, the sun provides 1,000,000 Joules (J) of
sunlight for the primary producers to convert to chemical energy. However, the
primary producers can only convert approximately 10% of that available radiant
energy into chemical energy that other trophic levels
can consume. This is because useable energy is lost through the system, often
as heat, to support and maintain each trophic level.
In general, the efficiency that trophic levels
convert energy from previous levels ranges from 5% to 20% efficiency. This
amount, however, is highly dependant on the ecosystem type. In our example, we
will use 10% as the mean efficiency of energy transfer between trophic levels.
In Figure 4.3, only 10% of the energy that the sun provides (or 10,000 J) is converted to useable net primary production for the grasshopper to eat. The grasshopper eats the available net primary production (10,000 J) and then can only convert 10% of that energy consumed to available energy (or 1,000 J) for the next trophic level, which is the mouse. The mouse again only has 10% efficiency of energy transfer so now only 100 J of energy are available for the snake to eat. The number of trophic levels that an ecosystem can support depends on the availability of energy. In our example, since the snake also only has 10% energy efficiency, only 10 J of energy would be available for a quaternary consumer like a hawk, which might swoop down to eat the snake. In this system, available energy is depleted as we move up the trophic pyramid, meaning that this system probably could not support many more trophic levels because there is too little energy remaining for subsequent trophic levels.
Does
biodiversity matter?
The amount of biodiversity on
Earth is the result of speciation over very long time periods, countered by
natural extinctions. The term biodiversity
refers to the diversity of life in all forms, and at all levels of organization
from genes to ecosystems, but the most widely accepted definition is the total
number of species on the entire planet. Approximately 1.8 million species have
been described so far on Earth; however, experts believe that the actual number
of species on the planet could range from 10 to 30 million species, and even
these estimates are provisional. Biodiversity can also apply to the biological
diversity at local, regional, or global scales. For example, diversity can
refer to genetic diversity or the genes in a population, or it can refer to the
populations or species themselves. On the other hand biodiversity can also
encompass ecological diversity, which refers to groups of interacting organisms
and the physical environment in which they inhabit. This type of diversity can
range from local populations in niches and habitats to large-scale ecosystems,
landscapes, or biomes.
Increasing habitat destruction,
biodiversity loss, land-use change, and issues related to global change, have
prompted scientists to determine how species composition, abundance, richness
and distribution (all aspects of biodiversity) affect ecosystem functioning.
Species composition is defined simply as, which species are in an
ecosystem at a given time. Species richness is the number of species in
the ecosystem. Species abundance is how many of each species are in the
ecosystem, and species distribution
is how the species are distributed spatially.
Ecosystem function is a broad term that reflects how an ecosystems works and
can refer to ecosystem productivity, biogeochemical cycling, or resistance and
resilience of a system to environmental perturbations.
One idea is that ecosystems with
high biodiversity, or a greater number of species, have greater productivity
than species-poor ecosystems. Certainly there are some highly productive
ecosystems, such as agricultural fields or eutrophic
ponds that contain very few species.
However, over long time periods and in natural systems, more species
seems to lead to more consistent productivity. This may be the result of niche complementarity, in which a greater number of species
results in more complete use of resources due to greater partitioning of those
resources. This is because the niches of species (that is, the specific habitat
best suited for them) must differ, resulting in better use of resources in
complex environments. However, studies have also shown that there seems to be a
saturation effect after a certain level of species richness (Figure 4.4).
Figure 4.4
Relationship between
biodiversity and ecosystem function
High species diversity is also important because a rich
biodiversity buffers ecosystems against perturbations or stress, thus making
them more stable when facing environmental change. As biodiversity increases,
ecosystems become more stable, resistant to disturbance and able to recover
more rapidly after a stressful event. Biodiversity and ecosystem stability are
the basis of the insurance hypothesis of biodiversity, which suggests that
resistance and resilience of ecosystems should increase with species richness.
The insurance hypothesis suggests that enhanced species diversity will increase
the likelihood that species will compensate for each other under perturbations.
If there were multiple species in a grassland ecosystem for instance, and the
system experienced a devastating drought, it is likely that some species that
need very little water would continue to perform and compensate for the species
that were hindered by the drought. Conversely, if the system had low-species
diversity, a disturbance such as this could hinder all of the species in the
system, which could have far reaching consequences for the other organisms that
depend on the productivity of the grassland.
Another way that biodiversity affects ecosystem function
is that high diversity may make ecosystems more resistant to invasive species.
This is the diversity resistance hypothesis, which suggests that diverse
communities are highly competitive and thus resistant to invasion. The idea
here is that in a diverse system, there are less open niches for invasive
species to occupy. This idea has been supported at the local scale and suggests
that local biodiversity may represent an important barrier for preventing the
spread of exotic species. Overall, the effects of diversity on ecosystem
function may vary across ecosystems due to differences in species composition,
as well as biotic and abiotic conditions. However,
various components of biodiversity affect ecosystem function in a variety of
ways. On average greater diversity results in greater productivity in
communities. Furthermore, increased diversity leads to greater nutrient
retention, ecosystem stability, and resistance to invasion by exotic pests
Why is Earth’s
biodiversity on the decline?
Scientists estimate that
currently there may be between 10 and 30 million species on Earth (Figure 4.5).
Unfortunately, according to the Millennium Ecosystem Assessment
(www.millenniumassessment.org), human activities are significantly and
permanently altering Earth’s biodiversity. The major drivers of biodiversity
loss are habitat alteration, species overexploitation, pollution, invasive and
introduced species, and climate change. In the past 50 years alone, the Earth
lost 300,000 species
to extinction, meaning that biodiversity loss was faster than at any other time
in human history. Species ranges and population sizes are currently declining
in many systems, with 40%
of the world’s coral reefs lost or degraded, 35% of mangrove area lost, and
more than 80% of tropical forests destroyed. In addition, 12% of Earth’s bird
species, 23% of mammals, 32% of amphibians, and 25% of conifers are currently
threatened with extinction. Humans have increased the species extinction
rate by approximately 1,000 times greater than historically typical rates.
Experts predict that these rates will continue, or accelerate, in the future.
Figure 4.5
Earth’s biodiversity by type
Currently, the greatest driver
for biodiversity loss is from habitat destruction, habitat fragmentation, and
land transformation, mostly from land use change associated with agriculture.
Agricultural lands now cover ¼ of Earth’s land surface and are expected to
increase to continue to feed Earth’s growing population (Figure 4.6). Since
1960, Earth’s human population has doubled, reaching 6 billion in 2000, and our
population is projected to increase further to 9 billion by 2050. Growing
consumption of ecosystem products and the accelerated use of fossil fuels are
increasing pressure on ecosystems and threatening biodiversity. Tropical
deforestation is one of the primary concerns for biodiversity loss because
rainforests house approximately two-thirds of all species living on Earth.
Another important driver for
biodiversity loss is overexploitation of commercial species of plants and
animals for consumption. This is especially the case for over-fishing with many
marine fish species and fisheries collapsing due to over-consumption.
Globalization, trade, and travel are another driver for species loss around the
globe. Increased travel and trade have introduced many exotic and invasive
alien species of plants, animals, and pests into ecosystems around the world.
In addition, increased globalization has caused the spread of many infectious
diseases, which have had negative impacts on biodiversity.
Changes in Earth’s
biogeochemical cycles are another significant cause for the decline in
biodiversity. Anthropogenic increases in chemical fertilizers, including nitrogen,
phosphorus, sulfur, and other nutrients that are pollutants in high
concentrations, are significantly impacting terrestrial, freshwater, and
coastal ecosystems. This type of pollution is especially detrimental to
amphibian species, which are critically endangered around the globe.
Unfortunately, this type of nutrient pollution is expected to increase
substantially as population growth continues in the future.
Human caused climate change is
increasingly driving biodiversity loss on Earth. Warmer regional temperatures
and increased temperature extremes are causing changes in species
distributions, population sizes, the timing of reproduction or migration
events, and ecosystem stability. For example, many of the world’s coral reefs
have been bleaching and showing signs of decline due to warmer sea surface
temperatures. In addition, climate changes are increasing the frequency of pest
and disease outbreaks, such as many mosquito-borne pathogens. There is evidence
that this global decline in biodiversity is making ecosystems more vulnerable to ecological
disturbance, thus decreasing the ability of the ecosystem to supply products
and services that humans need.
Figure 4.6
Human population growth versus species loss
http://www.msu.edu/course/isb/202/ebertmay/predicting_change/diversity_loss.jpg
What
are some ways to conserve biodiversity?
In addition to the inherent
importance of biodiversity to ecosystem functioning, biodiversity provides the
majority of ecosystem services to humans around the world. For example, all of
our food and most medicines are developed from plants, bacteria, and fungi. In
addition, there are many other products and ecosystem services that
biodiversity provides, or can provide in the future, assuming that it is
adequately protected today.
To achieve greater progress
toward biodiversity conservation we need to address many of the direct and
indirect causes of biodiversity loss and ecosystem change. Some of the direct
drivers of biodiversity loss and ecosystem change are from habitat loss and
land use change, generally from conversion of forested and wild-lands to
agricultural land. In addition impacts from land use change, over-exploitation
and over-consumption of biodiversity, pollution, introduction of alien species,
and human-induced climate change all represent significant direct drivers of
biodiversity loss.
Population growth is one of the
major indirect drivers of habitat destruction and biodiversity loss as human
populations increase, habitats are destroyed, and organisms are often displaced
to make room for human development. In addition to population growth, the
growing divide between the rich and poor, as well as global trade and increased
consumption of resources, all have strong indirect impacts on biodiversity
loss. Empowering women, indigenous people, and young people, who are often
especially dependent on ecosystem services, may be a method for improving
biodiversity and the lives of people that strongly depend on their services.
Policy changes on a local,
regional, and global scale can partially mitigate some of the negative
consequences of the growing pressures on ecosystems if lawmakers make
biodiversity conservation and ecosystem health a priority. Policy changes, such
as investment in public education, infrastructure, and poverty reduction, can
make major strides toward protection and conservation of ecosystems and
biodiversity. In addition, considerable investment in the development of new
technologies to increase energy efficiency, increase efficiency of ecosystem
service use, and reduce greenhouse gases through policy changes could have
profound impacts on biodiversity. For example, promotion of, and investment in,
technologies that produce increased crop yields without harmful side effects
could help pave the path to ecological conservation and sustainability.
Parks, large ecological
reserves, and wildlife corridors are key tools to protect habitat and conserve
biodiversity. By creating reserves and corridors, threatened plants and animals
have refuge from habitat loss and fragmentation that occurs due to human
development. In addition to large-scale ecological reserves, restoring degraded
ecosystems and protecting endemic organisms are other ways to protect
biodiversity. Endemic plants and animals are those species that only grow in a
particular area or region. Changes to endemic species habitat can have profound
effects on those species because they are so rare. Keystone species, or those
species that many other species depend upon for survival, are also particularly
important to protect because the loss on one keystone species may directly
impact the survival of many other species.
Conservation and protection of
biodiversity is not an easy or straightforward task. It involves understanding
the direct and indirect causes of species loss. Conservation of biodiversity
also involves commitments from businesses, governments, and communities to
change policies and behaviors that threaten ecosystems and species. Although,
conservation is a difficult and multi-faceted task, there are many methods that
can be employed to help conserve habitats and ecosystems and make their use
sustainable, so that future generations can enjoy the priceless ecosystem
services that healthy biodiversity and natural ecosystems provide.
Last updated: 10/23/2006 6:11 PM