Patterns of Biodiversity

I. Biological Interactions

1. Competition Between Species:

Competition occurs when populations either compete directly for limited resources, such as grazers feeding on the same grass, or compete for ACCESS to the resource...like in white-tailed deer when males battle one another for access to all the females in the area. In either case, available resources are reduced (if another organism is using them) and/or additional energy must be spent to acquire that resource..leaving less for reproduction. So, competition REDUCES the fitness of BOTH competitors. Sure someone 'wins' and gets the resource, but they had to pay for it in additional energy. So, NATURAL SELECTION should favor organisms that eithr reduce resource overlap (use something else) or gain an advantage in acquiring it (are bigger). Obviously, competition occurs between organisms in the SAME species (intraspecific competition) or in DIFFERENT species (interspecific competition). We will focus on interspecific competition because that can affect the number of species in an area and how they use resources.

In the 1930's, Gause studied competition in lab culture of Paramecium species. He found that two species could live just fine by themselves, but that only one species would survive when they wer put together. He resoned that they were both using the same resources, and coined the "competitive exclusion principle", stating that "two species cannot coexist if they use the same resources." So, one outcome of competition is a reduction in diversity: species can be eliminated from a community by competition.

In the 1950's, Brown studied the distribution and beak size of Darwin's finch species in the Galapagos. He found that when species were by themselves on differnt islands, the range in beak sizes were roughly the same. However, on Islands where they co-occurred, one species had smaller beaks and the other had larger beaks...and they fed on different sized seeds. Brown hypothesized that, in the presence of the competing species, each species had been selected to shift their pattern of resource use to seeds not eaten by the other. He coined the term "niche/resource partitioning" to describe this new allocation of resources in the presence of competitors. And, he surmised that--when feeding on these new seeds--each species would be selected to have different sized beaks. Thus, the behavioral change in resource use from competition selcted for a morphological change. He called this change in morphology resulting from competition "character displacement".

At a community level involving many species that feed on similar resources, we often see niche/resource partitioning playing out among several species. Species may be "packed" along one axis of the resources, neatly distributed in how they use the resource. Often this correlates with body size, because differnt sized animals are likely to use different sized prey more efficiently (like seeds). In the Middle East, Dayan showed that weasels have canines that are slightly different sizes and eat differnt prey. Here at Furman, my students and I have shown that male dragonflies that compete for territories along the margins of ponds will perch at different heights and reduce competitive interactions between them. In addition, this pattern correlates with body size; large species use taller perches and, by attacking smaller interlopers, relegate smaller species to shorter perches. So, competition between and among species can lead to resource partitioning, which allows more species to coexist.

When their are more than two competitors, there can be some interesting and unpredictable outcomes. Obviously, it is possible that the weakest competitor is excluded even more swiftly as a consequence of competing with two better competitors. In this case, the negative effects of pair-wise competition are ADDITIVE. However, something else can happen, too; competition among the two strongest competitors can reduce the effect they have on the weakest competitor, allowing it to persist. In this case, competition can promote coexistence through INDIRECT EFFECTS. Let's take a look at a couple examples.

Way back before you were born, I conducted some experiments on competition between fruit fly species (in the genus Drosophila) with Furman student Jennifer Moore. We placed 20 females of each species in a bottle by themselves, with a small mushroom. The adults lay eggs on the mushroom, the larvae eat and compete for the yeasts and bacteria growing on the mushroom, and we count the number of offspring that completed development and emerged as adults. Then, we placed 20 females of two species together, in each pair-wsie combination. Finally, we placed 20 females of all three species together. One species, Drosophila falleni, is affected in an additive way by the other species. The number of offspring is reduced by the presence of each competitor, and reduced even more when both competitors are present. However, another species, Drosophila putrida, exhibits a different pattern. It was the weakest competitor, and was reduce dramatically by each of the other species in pair-wise competition. However, when both competitors wer present, it did better than when it was competing only with Drosophila tripunctata. So, think aboutit this way: adding the third species, which exerted a negative effect on D. putrida in a pair-wise test, actually helped D. putrida when D. tripunctata was also present. Drosophila falleni and Drosophila tripunctata also compete strongly with one another. So, in the three species community, they reduce one another's abundances to the point where the effects on D. putrida are lessened. In a sense, this is like the old adage, "the enemy of my enemy is my friend"... even if that first combatant is my enemy, too. In 1972, Wilbur demonstrated that indirect effects also occur in salamanders. #2 Ambystoma laterale salamanders would grow to an average size of 0.94 grams when reared alone. The presence of 32 individuals from two other species would each decrease the size of A. laterale in pair-wise experiments. But when both competitors were present with A. laterale, they were larger than in either two-species situation. So, through indirect effects, the presence of multiple competitors can actually PROMOTE coexistence and maintain biodiversity.

2. Predation

Like competition, predation can obviously led to the elimination of a species (the prey species that gets eaten). However, selective pressures, and thus the evolutionary results, are much different. BOTH competitors are affected negatively by competition, so within BOTH species, individuals that reduce the interaction (use a different resource) have a fitness advantage over other members of their own species that continue to compete with the competitor. As such, within both species, selection will favor a reduction of the interaction through resource partitioning.. and the interaction weakens. In predation, although the prey is affected negatively by the interaction, the predator is affected POSITIVELY--they get a MEAL! So, although selection favors prey that can reduce the interaction (by escaping, somehow), selection favors predators that promote the interaction (by being even more efficient, faster, or clever). This sets up an "arms race" or evolutionary feedback loop of changes in one species promoting changes in the other.

Gause used his lab system with Paramecium to study predation, too. He added a predatory protist and found that the prey went extinct and then the predator went extinct, too, because they had nothing to eat. this can happen in nature if the predator is a very efficient specialist predator. But ecologists often see something else with specialist predators and thier prey - oscillating population sizes through time. When the prey decline and there is less food, the predaot population declines, too. Now, this relieves pressure on the prey, and it might rebound. Now, with more prey, the predator population can rebound, too, and on it goes.

Since these oscillations ar common, ecologist wanted to determine what caused them. Huffaker set up a lab system with two mite species, a predator and a prey species that feeds on citrus. He created a complex enviroment with many 'islands of habitat' (separated oranges). He found that as long as the prey species could disperse among the oranges faster than the predator, it could maintain itself 'here and there' before the predator could extinguish it on all the islands at the same time. So, with many habitats, the prey could recolonize after both went extinct and 'stay ahead' of the predator, even though the predator would extinguish the prey on a particular island when it found it. So, multiple habitats can promote coexistence between predators and prey, as long as the prey can move among them. We will consider more complex effects later.

3. Mutualisms

A mutualism is an interaction in which the fitness of both interactors increases as a consequence of the interaction. These may be very specific interactions involving only two species, or they may be very diffuse interactions, involving two sets of species. In these interactions (which have a beneficial effect on fitness), individuals within a species that maximize the interaction have higher fitness than other members of that species. Thus, selection within the species favors individuals maximizing that relationship.This happens in both species, so the feedback loops are positive, continually reinforcing a strong relationship

Ecologists (like most folks in Western societies) have focused on the importance of COMPETITION as an interaction that structures the community. Darwin was obviously a proponent of this view, following Malthus. However, cooperation has actually been equally, if not more, important over evolutionary history. The endosymbiotic origin of eukaryotic cells is a mutualistic interaction between aerobic bacteria (that became mitochondria) and larger archaeans that engulfed them. Likewise with the photosynthetic bacteria that were engulfed and became chloroplasts. Mutlicellularity was another act of mutualism, in which cells worked together for the mutual benefit of the the 'collective' rather than competing for resources. A cancer tumor, in many ways, is a group of cells that rebel and begin to compete for resources rather than cooperate with other cells. Indeed, all animals are dependent on gut bacteria for efficient digestion, and many plants have mutualistic relationships with soil fungi. So, cooperative relationships have had a dramatic impact on the evolution of life on Earth.

In trophic mutualisms, both partners help the other get energy. Gut bacteria and soil fungi are examples here. Another are the algae that corals and some molluscs absorb but don't digest. The algae gets nutrients from the animals digestion, while the animal gets sugars produced by the photosynthetic algae. In winter, many differnt species of birds flock together. They use the 'more heads are better than one' strategy to find food.

In defensive mutualisms, one partner provides food whuile the other provides defense. Leaf-cutter ants raise and protect a fungus, which is the only thing they eat. Other ants live on Acacia plants and defende the plant from herbivores, while getting sugars, nectar rich in amino acids, and hollow thorns to nest in from the plant. Other ants tend 'herds' of aphids, eating their sugary secretions and protecting them from predators.

Some birds pick ticks and mites off malls; the bir gets a meal and the mammal gets rid of their parasites.

Lastly, there are dispersive mutualisms: many plants bribe animals with food to disperse their pollen and seeds.

Although mutualisms increase fitness in both species, they may also increase dependencies between the species. This may actually increase the risk of extinction for each species, because now they will only be able to survive in places that can support their partner. This is a godd example of how evolution does not necessarily operate for the long-term survival of the species. Selection favors current reproductive success in this given environment. But if the environment changes (and you lose your mutualist), all bets are off.

4. Interactions in Communities (with competitors, predators, and mutualists!)

a. Mutliple Competitors: We have already seen how the weaker of two competing species can actually be HELPED by the addition of a third competing species through indirect effects.

b. Keystone Predators can have the same effect: they can have a POSITIVEeffect on they species they eat through indirect effects. Consider a classic experiment done in the '60's by Robert Paine. The rocky coast of Oregon is home to several species of animals that settle out of the water and attach to the rocks. They compete for space, and in the absence of a predator the blue mussel outcompetes chitons, limpets, and barnacles. All of these species are eaten by sea stars, so sea stars exerrt a direct negative effect on each species. However, sea stars prefer to eat mussels. So, when predators are present, they graze down the mussles, opening space for the other species that now persist in the community. The sea star is the 'keystone' that holds up the 'arch' of diversity. Lose the predator and you lose all the other species but the mussel. In the 70's Jane Lubchenko showed that the relative competitive ability and the environment affects the outcome. Snails prefer to eat green algae, and green algae is the competitive dominant in tide pools. So, as snail density increases, so does biodiversity--due to a keystone effect. However, densities can get too high, at which point the predator eats everything and diversity declines. Also, on rock oucrops where red algae dominates and is not eaten by predators, the presence of snails decreases diversity because the inferior competitiors are also getting nailed by predation.

c. There are also many other indirect effects that can percolate through complex communities with many species acting in complex ways, creating 'apparent competition and mutualisms'.

d. Lastly, the realtionships among species can change, even as a reuslt fo body size. Species may both compete for resources while also preying upon one another...typically preying on the young of their competitors. Wissinger shwoed that two species of dragonfly larvae compete for the damselfly larvae they both eat. However, they will also eat one another. As a consequence, the prey species has LOWER ortlality when BOTH species of predator are present, because they each one another and thus reduce the predator effects that they each have on the damselfly.

So, in a diverse community with lots of species interacting in complex ways, it is very difficult to predict how the communty might change as a consequence of adding an introduced species or wiping out another species. As a great conservationist once said, "The first rule of the tinkerer is to save all the pieces." We don't know how nature works yet... so as we change this complex system, the only way to insure that it still might work in a manner that supports human life is to "save all the pieces".

II. Time

Biodiversity tends to increase through time, both on a LARGE geologic scale and a smaller ecological scale. On a large scale, diversity increases as species evolve and diverge, creating evolutionary radiations of new types of organisms exploiting new habitats or old ones in new ways. There have been five mass extinction events in Earth's history, and we are curently in the sixth. On a shorter ecological time scale (100's of years), diveristy increases as species find the site and adapt to these habitats. New species don't form on this time scale, the local increase in diversity is due largely to colonization over time. this pattern cause a change in the commuity over time called "succession', like we discussed behind the chapel.

III. Area