18.3 Biodiversity and Species Interactions

Biodiversity Measures

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Defining and Quantifying Biodiversity

Biodiversity refers to the variety of life on Earth at all levels, from genes to ecosystems. Two key metrics are used to measure it: species richness and species evenness. Both matter, and they tell you different things.

Species richness is simply the number of different species in a given community, landscape, or region. The Amazon rainforest, with tens of thousands of plant species alone, has extremely high species richness. The Arctic tundra, by contrast, supports far fewer species and has low richness.

Species evenness describes how close in abundance each species is to the others. A community can have high richness but low evenness if one species dominates.

• High evenness: species are present in similar proportions. A healthy coral reef, for example, might have dozens of fish species each represented by roughly similar numbers.
• Low evenness: one or a few species vastly outnumber the rest. An ecosystem invaded by a dominant invasive species often shows this pattern, with native species pushed to very low numbers.

A truly biodiverse community scores high on both richness and evenness.

Ecosystem Stability and Biodiversity

Ecosystem stability is the ability of an ecosystem to maintain its structure and function over time, even when disturbed. Biodiversity directly supports this stability through a concept called functional redundancy: if one species is lost, others that fill a similar ecological role can compensate. For instance, if one plant species that herbivores depend on disappears, other plant species in a diverse community can still provide food.

Why does higher biodiversity make ecosystems more resilient to disturbances like climate change or natural disasters?

• Greater genetic diversity within species gives populations more raw material for adaptation to changing conditions.
• Higher species richness means more "backup" options for essential ecosystem functions (pollination, decomposition, nutrient cycling) if some species decline or vanish.

The takeaway: diverse ecosystems don't just look richer, they function more reliably under stress.

Species Interactions

Symbiotic Relationships

Symbiosis is any close, long-term interaction between two different species. There are three main types, defined by who benefits and who is harmed.

Mutualism (+/+): Both species benefit.

• Oxpeckers eat ticks off the backs of rhinos. The bird gets food; the rhino gets pest removal.
• Flowering plants provide nectar for pollinators like bees, while the pollinators transfer pollen between flowers, enabling reproduction.

Commensalism (+/0): One species benefits; the other is neither helped nor harmed.

• Remora fish attach to sharks using a suction-cup structure on their heads. The remora gains transportation and access to food scraps, while the shark is unaffected.
• Epiphytic orchids grow on tree branches to reach more sunlight. The orchid benefits from the elevated position, and the tree is not significantly impacted.

Parasitism (+/−): One species (the parasite) benefits at the expense of the other (the host).

• Tapeworms live inside the digestive tracts of animals, absorbing nutrients directly from the host's food and causing malnutrition over time.
• Mistletoe plants grow on trees and tap into the tree's vascular tissue, stealing water and nutrients. Heavy infestations can stunt the tree's growth.

Community Dynamics

Predation and Its Effects

Predation is an interaction where one organism (the predator) hunts, kills, and consumes another (the prey). Beyond its direct effect on prey numbers, predation plays a major role in shaping entire community structures.

A classic example: wolves in Yellowstone National Park help regulate elk populations. Without wolves, elk overgrazed streamside vegetation, degrading habitat for other species. When wolves were reintroduced in 1995, elk numbers dropped and vegetation recovered.

This illustrates a trophic cascade, where effects ripple across multiple trophic levels. Another well-studied example: sea otters prey on sea urchins, keeping urchin populations in check. Without otters, sea urchin populations explode and devour kelp forests, collapsing the habitat that hundreds of other species depend on.

Competition and Resource Partitioning

Competition occurs when two or more organisms use the same limited resource, whether that's food, water, space, or sunlight.

• Interspecific competition happens between different species. Over time, this pressure can lead to resource partitioning, where species evolve to use slightly different portions of a shared resource. Robert MacArthur's classic warbler study showed that five warbler species coexist in the same spruce trees by foraging at different heights and on different parts of branches.
• Intraspecific competition happens within a single species. Individuals compete for mates, food, or territory, and this competition can limit population growth as resources become scarce.

Resource partitioning is a key concept because it explains how similar species can coexist rather than one driving the other to extinction through competition.

Trophic Levels and Food Webs

Trophic levels describe the position an organism occupies in a food chain based on its energy source:

1. Primary producers (plants, algae) convert sunlight into chemical energy through photosynthesis.
2. Primary consumers (herbivores) eat producers.
3. Secondary consumers (carnivores) eat herbivores.
4. Tertiary consumers (top predators) eat other carnivores.

A food web is a more realistic model than a simple food chain. It maps out the complex, interconnected feeding relationships across an entire community. Food webs are useful because they help predict cascading effects: if a top predator is removed, herbivore populations may spike, which can then reduce plant populations. That's the trophic cascade concept again, showing up at the community level.