Sunday, February 24, 2013

Scales of diversity: from genes to global

Think about the sheer number of living things.

It’s impressive, even if you just consider the Big Things – the mammals, birds, and reptiles. Yet even when you also consider plants, fungi, and the millions of insect and other invertebrate species, the number of species in a given area or ecological system is just one way of measuring life’s diversity.

flower diversity in western Australia
how many flower species can you count in this
square meter of western Australian desert?

There is variety at many levels in the natural world. Differences exist inside individual genes among various frogs, spiders and chili peppers, as well as people, even within a single family. The organisms that live in the soil of your city park differ from those in the trees, and the birds, lizards and the trees they hide in near London or San Francisco are different from those that live near Cairo or Bangkok.

Biodiversity at different scales

Biologists use the term biodiversity, which is short for biological diversity, to represent the variety of living things in a particular ecosystem, region, or the world, and they typically divide biodiversity into 3 general categories: genetic, species, and ecosystem.

What do these divisions mean? Read on!


1. Genetic diversity: this is the variation in genes within individuals of a species, such as differences between a Maasai warrior in Kenya and an Inuit in northern Canada, or between you and your brother or sister.

maasai warriors
photo: Helga76, Wikimedia
Inuit walrus hunters
photo: Ansgar Walk, Wikimedia

Humans in sunny climates tend to have more melanin, a pigment that darkens skin and protects it from sun damage. Makes sense! For snakes or chameleons that sneak up on their prey, individuals thriving in dense vegetation might have darker coloring than a neighboring population of the same species living in adjacent grasslands. Different traits will have an advantage under different conditions, so maintaining genetic diversity in your species means that some individuals will adapt more readily to changing conditions.

2. Species diversity: this is where we consider differences between species, or how are humans similar to or different from chimpanzees? How are chimpanzees similar to or different from gorillas, or other African forest primates?

And what allows many of these species to coexist in the same place? Species respond to the distribution of resources and competitors around them in different ways - they might fight, run, or hide; they might eat large or small seeds, or leaves, or nectar; they might sleep alone in tree holes or in a large group out in the open -- in other words, they each exploit different ecological niches.

The most famous example of this is the radiation of a group of about 15 bird species living on some remote islands called the Galapagos, otherwise known as "Darwin's finches". The birds are all between 10 and 20 cm (4-8 in) long and relatively plain in color. The differences in the size and shape of the beaks of the different species reflect how they have adapted to feeding on different foods.

Darwin's finches, their beak shapes, and feeding niches; figure by N. Patel,
"Evolutionary biology: how to build a longer beak" in the journal Nature

These beak differences in birds that were otherwise closely related to each other, yet distinct from birds found elsewhere, helped Charles Darwin formulate his theory of evolution by natural selection. In 1845, Darwin wrote, "Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends". In other words, selection for certain traits that thrived under different conditions led to the evolution of many species from a few wayward birds blown by winds or waves to the Galapagos.

3. Ecosystem/ecological diversity: this broad level considers the differences in the patterns and distribution of whole communities of species in their environment, as well the species' respective roles, interactions, and functions in those communities.

In other words, how do the canopy animals and plants in the interior of a rainforest differ from those in neighboring forest at the edge of a lake or river, where more sunlight and heat penetrate through the trees and create a slightly drier and sunnier environment?

Or how different are the soil microbe and plant communities in a grassland on a more acidic type of soil from those in the savanna on a more alkaline soil?

A diversity example in the Year of the Snake

How about an example? At the broadest level, we know that the Earth supports biomes, such as deserts and wetlands, ice fields and forests, grasslands and coral reefs. And within each of these biomes, we can find multiple ecological regions: some forests are relatively dry and trees lose their leaves, while others are wet and lush all year.

dry forest, western Australia
wet forest, Costa Rica

The animals and plant communities of a dry forest will, as you may have guessed, differ from those in a rainforest - the plants, insects, reptiles, birds, and mammals will all be better competitors in an environment where water is scarce, sun is strong, and vegetation is relatively open than their rainforest cousins.

Within each of these groups, there is variety. Snakes, for example, can be poisonous or non-poisonous; some bite their prey while others strangle their prey; and some wear bright colors while others are super camouflaged, as you can see in these photos from the Amazon rainforest in southeastern Peru.

mellow redtail boa constrictor shows off its colors
anaconda digesting - also a constrictor, just really big
fer-de-lance: camouflaged + aggressive + poisonous = watch out!

Even within a single snake species, some individuals might have larger eyes, darker stripes or spots, or more toxic venom, due to the variability of the genes from one snake to the next. Yet the genetic variability between snakes of a particular species will be less than that between snakes of two different species.


And, as you can see among your family, friends, and colleagues, individuals of a single species, such as humans, can vary quite a bit, yet we are all more similar to each other than any of us is to a chimpanzee, gorilla, or snake!



Learn more: biodiversity terms for different spatial scales

For all you studious types, some definitions of several key biodiversity levels (in increasing size order):

Gene = The unit of heredity, transferred from parent to offspring, which determines some characteristic in the offspring. It is normally a segment of DNA responsible for the physical and inheritable characteristics of an organism.

Chromosome = a threadlike strand of DNA in the nucleus of most living cells that carries the genes. We humans have 23 pairs of chromosomes, each with a characteristic length and banding pattern. Chromosomes normally occur in pairs: one from the mother and the other from the father.

Individual organism = Living things that are capable of reacting to stimuli, reproduction, growth, and homeostasis. Includes plants, animals, fungi, and microorganisms. All consist of cells, either one, like bacteria, or many, like a person, a plant, or a poodle.

Population = A group of organisms of one species that interbreed and live in the same place at the same time (e.g. a deer population).

Species = A group of organisms having common characteristics and (usually) are capable of mating with one another to produce fertile offspring. The most basic unit or category of biological classification. Every unique type of animal or plant you can think of is probably a species.

Ecosystem = The interaction of a community of living organisms with their physical environment: in other words, the plants, animals, microorganisms, soil, rocks, minerals, water sources and the local atmosphere all interacting with one another.

Biome = A major, naturally-occurring community of plants and animals occupying a major habitat and climate regime and classified by the climate and dominant vegetation, e.g., forest or tundra.

Learn even more: biodiversity resources

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