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.
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.
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!
Trophic pyramid with efficiency of energy transfer
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).
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.
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.
Human population growth versus species loss
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.