Professor Timothy Schowalter explains why protecting biodiversity is important, not least in the context of human health.
The Earth is currently experiencing a dramatic reduction in global biodiversity, caused primarily by climate change and habitat destruction. Nearly 200 species have disappeared in the past century, compared to a background extinction rate of two species per century over the past 2 million years. Most people do not appreciate the importance of protecting biodiversity because most species are unfamiliar and, after all, why should the survival of an obscure fish, bird or beetle species stand in the way of ‘progress’? However, the risk of extinction for some iconic species, such as lions, elephants and butterflies has raised awareness.
Why is protecting biodiversity important, and why should we care if some species disappear?
Currently, Earth supports about 1.5 million known species, including one million insects, most of which occur in relatively small areas of the globe. For example, some species occur in only a single cave, hot spring, or valley as a result of specific adaptations and isolation. Such species are particularly vulnerable to extinction if environmental changes or human activities eliminate their small habitat.
Natural ecosystems generally have higher biodiversity than do managed ecosystems, where undesirable species (‘pests’) are minimised. However, all ecosystems include dozens (managed or harsh systems, such as deserts) to thousands (favorable systems, such as estuaries and tropical forests) of species. The species that characterise each ecosystem are adapted to survive recurring environmental changes or disturbances. For example, plant and animal species in temperate forests and grasslands must be adapted to survive harsh winters. Desert plant and animal species are adapted to conserve water. Underground rhizomes and insulating bark are common adaptations in fire-prone ecosystems.
No species can adapt to all conditions that can arise in an ecosystem. Disturbances are characteristic of all ecosystems and often eliminate many species. A particular assemblage of ‘pioneer’ species becomes abundant after a disturbance, trading places, as it were, with those ‘later successional’ species that characterised the ecosystem prior to the disturbance.
Furthermore, different combinations of species become dominant following a fire, a storm, or a drought, given that different adaptations are required to survive heat, wind, or desiccation, or the altered environmental conditions following each of these disturbance types. Consequently, the particular sequence of environmental changes occurring over a time period will produce a unique assemblage of species. A fire followed by a storm will favor a different combination of species than will a storm followed by a fire, because of the different filtering of surviving species by the preceding event.
Pioneer species are typically fast-growing, short-lived, widely-dispersing species (commonly known as ‘weeds’) that become dominant after a severe disturbance. These species thrive on the reduced competition and greater resource availability following loss of the previously dominant species. These pioneer species are essential to the eventual recovery of the ecosystem. The pioneer species cover and stabilise the substrate and conserve nutrients that are necessary for community recovery but would be lost in the absence of the pioneer species. However, pioneer species typically become rare as soil stabilisation and nutrient conservation allow more competitive, longer-lived species to recover and shade out the pioneers.
Where do these critical pioneer species come from following disturbance?
Pioneer species are often present in seed banks in the soil or may colonise from disturbed sites over long distances. If pioneer species were to disappear, the ecosystem would lose their contributions to recovery from future disturbances. Conversely, if species characterising more mature ecosystems disappeared, the ecosystem might fail to recover.
The multitude of species in an ecosystem ensures that ecosystem functions will be maintained under varying environmental conditions (the insurance hypothesis), although the particular species that become abundant may change. Pioneer species stabilise soils and conserve nutrients in different ways and with different efficiencies than do later successional species. Several studies have demonstrated that experimental plots with more plant species maintain plant production at higher levels during droughts or other environmental changes than do plots with fewer plant species.
More diverse plots are better buffered against environmental changes because species tolerant of the change can compensate for decline of intolerant species, whereas less diverse plots may not include tolerant species.
The ‘balance of nature’
Consequently, what is commonly called the ‘balance of nature’ is the product of many species responding to changes in ecosystem conditions, such as the rapid increase in abundance of a particular species, in ways that reduce deviations in ecosystem function.
The cumulative effect of biodiversity is the regulation of ecosystem processes, including plant productivity, decomposition and nutrient cycling, that ensure consistency in ecosystem functions. These processes produce the harvestable resources (food, pharmaceutical compounds, wood) and other ecosystem services on which we depend.
Vegetation diversity reduces the likelihood of insect outbreaks because the host plants available to insects are more limited in diverse vegetation. Predator diversity maintains ‘trophic cascades’ that regulate herbivore populations and promote plant growth. When predators are eliminated, as in the case of wolves, their prey (deer or elk) multiply, overeat their food resources and starve en masse. Similarly, the loss of migratory insectivorous birds allows insect herbivores to increase and reduce plant growth.
However, herbivore and predator abundances are regulated by host plant or prey availability respectively, so as resource abundance declines, the herbivore and predator populations decline. The combination of many overlapping trophic cascades in diverse ecosystems maintains balance among plant production, herbivory, and predation. High diversity of plant and predator species reduces the likelihood of pest outbreaks.
Reducing diversity, such as in agricultural landscapes, increases the likelihood of outbreaks. Birds and predaceous invertebrates are important regulators of insect pests in agricultural crops but often are eliminated by lack of habitat. However, agricultural pests are important regulators in their native habitats, preventing plant growth exceeding soil nutrient supply and maintaining plant diversity. Humans must expend much labour and resources to maintain artificially high abundances of crop plants against natural processes that favor diversity. Crop diversification can reduce the severity and extent of pest problems.
Many species appear to have the same function (and often are considered to be ‘redundant’) but have different traits that change the rate or timing of their effects (making them unique). Some species are innocuous and their effects on ecosystem processes poorly known. However, several studies have shown that even apparently redundant species respond in different, and unique, ways to environmental changes. If species that would be critical to maintaining ecosystem functions under future climate conditions are eliminated now, then those functions may be threatened in the future.
The consequences of biodiversity loss
The decline of many insect pollinators threatens 35% of our global food supply, warning of the consequences of biodiversity loss. The recent widespread reduction in honey bee numbers has received much public attention. Honey bees have been transported world-wide for honey production and pollination services. Decline in honey bees has jeopardised both services and generated much research to discover the cause. However, a large number of other pollinating insects, including bumble bees, orchard bees and other bee, fly, butterfly, and moth species, are perhaps even more critical to pollination services world-wide, generating additional research and efforts to preserve natural habitats that support these species.
Some human diseases are likely to increase as biodiversity declines. For example, bird species diversity is the key to limiting the spread of West Nile virus. Birds are the primary reservoir hosts for this disease, which can be transmitted to humans by some mosquito species. High diversity of resistant bird species reduces the incidence of this disease in susceptible bird species, thereby reducing transmission to humans. Ironically, the most susceptible bird species, such as crows and jays, are typically most abundant in human-dominated ecosystems. Similarly, tick abundance and the spread of Lyme disease depend on the diversity of rodent and other small mammal species. High diversity of resistant mammal species reduces the incidence of this disease in susceptible hosts, thereby reducing transmission to humans.
We don’t know which disappearing species might be crucial to our survival. One frequently-used analogy compares species to the rivets connecting parts of an airplane. How many rivets could be lost before the airplane is no longer safe to fly?
In summary, biological diversity ensures that important ecosystem processes and services are maintained. Loss of species that may be necessary to maintain critical ecosystem processes under future conditions threatens the sustainability of ecosystem services necessary for human survival. Therefore, we should all be concerned about species loss and support environmental policies that protect biodiversity.
Professor Timothy Schowalter
Department of Entomology
Louisiana State University
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This article will appear in SciTech Europa Quarterly issue 30, which will be published in March, 2019