Species richness is a straightforward count of the number of different species in a specific ecological community or region. For example, a local park with squirrels, robins, pigeons, and oak trees has a species richness of four. Another larger park across town might have those same four species plus blue jays, cardinals, and maple trees, giving it a species richness of seven.
This count is a fundamental measure used by scientists to compare ecosystem complexity. An area with a higher number of species is considered more “species-rich.” This measurement is purely about variety, not the population size of any single species. One hundred robins in the first park and only two in the second would not change the richness value for either location.
Distinguishing Richness from Evenness and Biodiversity
Species richness is only one piece of a larger puzzle. To get a more complete understanding of an area’s biological makeup, scientists also look at species evenness, which measures how close in numbers the populations of different species are. An ecosystem can have high richness but low evenness if one or two species are dominant. For instance, two forests could each contain ten different tree species, giving them identical species richness.
In the first forest, nine species might be represented by only a few trees, while a tenth pine species makes up 90% of the population. This forest has low evenness. In the second forest, all ten tree species are present in roughly equal numbers, giving it high evenness. Although their richness is the same, the second forest’s structure is more balanced.
Richness and evenness are components of the broader concept of biodiversity. Biodiversity is a comprehensive term that encompasses the variety and relative abundance of species, the genetic diversity within each species, and the diversity of entire ecosystems. It considers everything from the genes in a frog population to the variety of habitats across the planet.
How Scientists Quantify Species Richness
Determining the exact species richness of an area is a significant challenge, as it is often impossible to find and count every organism, especially in large or complex ecosystems. Scientists have identified approximately 1.6 million species, but estimates suggest many more are unidentified. To overcome this, ecologists rely on sampling methods to estimate richness.
A common technique involves using quadrats, which are square frames of a set size. Researchers place these frames in various locations within a habitat and count the number of species inside. This method is effective for studying plants and slow-moving animals. For larger areas, scientists may use a transect, a line laid across the area where they record every species that touches it.
Data from these samples helps scientists understand the distribution and abundance of species. To estimate the total number of species in an area, they use a species accumulation curve. This graph plots the number of species found against the amount of sampling effort. As researchers collect more samples, the curve rises and eventually levels off, suggesting that most of the species in the area have been found.
Global Patterns and Influential Factors
One of the most consistent global patterns is the latitudinal diversity gradient. This describes the trend of increasing species richness as one moves from the poles toward the equator. For example, Colombia and Kenya each host over 1,000 breeding bird species, whereas the forests of Great Britain have fewer than 200. The reasons for this gradient are still debated, but leading hypotheses point to factors like a more stable climate and greater solar energy input in the tropics.
Another principle is the species-area relationship, which states that larger areas tend to contain more species. This is because larger areas can support larger populations, making them less vulnerable to extinction, and they often contain a wider range of environments. A small island, for instance, will have fewer species than a continent of the same habitat type.
Habitat heterogeneity, or the variety of micro-habitats within a larger landscape, also plays a significant role. A forest with diverse topography, including hills and streams, will provide more niches and support a greater number of species than a flat forest. Conversely, habitat fragmentation from roads or development breaks continuous habitats into smaller patches, reducing the effective area available to species and leading to a decline in species richness.
Impact on Ecosystem Stability and Productivity
The number of species in an ecosystem has direct consequences for its functioning, especially its stability and resilience. The “insurance hypothesis” suggests that a higher level of species richness acts as a buffer against disturbances like drought or climate change. With a greater variety of species, it is more likely that some will withstand a disturbance and continue their functions, compensating for the species that are lost.
Species richness is also linked to ecosystem productivity, the rate at which an ecosystem generates biomass. Studies show that communities with a higher number of plant species are often more productive. For example, grasslands with a greater variety of plant species produce more total plant matter. This increased productivity can ripple through the food web, supporting a greater abundance of herbivores, predators, and decomposers.