Understanding Species Diversity: Patterns and Determinants in Community Ecology, Study notes of Ecology and Environment

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Lecture 10 outline -- Community Ecology -- Part I Part I I. Introduction

• A. Overview
• B. Goal for the remainder of the course II. Biodiversity
• A. Spatial patterns: local, regional and global diversity
• 1. Definitions based on spatial scale
• 2. Determinants of latitudinal gradients in α - and β -diversity. Part II
• B. Temporal patterns: Succession
• C. Spatial and temporal patterns: Island biogeography and species turnover
• D. Stability and complexity in food webs III. Applications
• A. World Biomes revisited
• B. Conservation planning revised March 3, 2008 Molles 4th^ edition pages Lecture 10 -- Community Ecology -- Part I Readings : Molles Chapters 16, 17, 20, 21, 22 for all parts of Lecture 10 I. Introduction: A. Overview. In the lecture on competition, we gradually widened our view from two to many species at a time. Focusing on just two species at a time is often a good way to understand what happens with many species: Quite a few patterns can be understood by adding the subpatterns together. There are other patterns, however, which can only be viewed and understood by looking at many species at a time. This is the realm of " community ecology ". Community ecology examines patterns only visible by considering many species at a time. We have already examined some community parts by focusing on parts of communities, for example, many competitors (one type of population interaction) on 1 trophic level. Now we will focus on all of the species that occur at one location, including all trophic levels and all types of possible interactions. For example, community ecology can examine the relationships between different types of two-species interactions in food web--what happens when competitors are preyed upon? Or how many interactions do we expect to see in a food web with a certain number of species. Will all species be interacting? Will some kinds of interactions be more

common than others? And so on. Here is a list of patterns and topics that can only be understood, and defined, taking many species at a time: (Molles Ch. 16.0, 16.1, 16.2)

• 1. species diversity --We can count the number and relative abundance of species and try to explain what determines these patterns worldwide. We now call this BIODIVERSITY.
• 2. community structure -- a general term for properties of food webs, such as a. Complexity: This component of community structure refers to the numbers and types of species in the community, and the numbers and types of interactions (connections, links) between each pair of species. These properties can be descriptors of the complexity of food webs and communities
  • number of trophic levels (length of food chain)
  • number of species on each trophic level and their similarity,
  • numbers of interactions, including # of species in the food web that interact with 1, 2, 3 .... n species (connectance), ratio of different types of interactions (+-,++,-- etc.), b. Stability: This component of community structure refers to the dynamical properties of the community, in other words, the behavior of interacting populations (refer to Lecture 6 if you need to remind yourself of our definition of population behavior). These properties are components of the stability of food webs and communities
• time to return to equilibrium (resilience)
• resistance to disturbance
• probability of extinction of any of the component populations
• 3. succession and community development --regular or predictable changes in communities with time. We can view this as predictable responses of communities to disturbance.
• 4. species-area relationships -- (island biogeography) how number of species changes with increasing area. This topic also emphasizes the turnover of species in time.
• 5. ecosystem and landscape ecology -- study of the community and all the physical forces impinging on it at one site, including energy and nutrient flow, and the links between different ecosystems occupying a landscape. In this course we will skip study of ecosystems. It is impossible to cover the field of ecology in only 10 weeks. Rather than give you a superficial understanding of everything, I have tried to go into depth in a coherent set of subdisciplines of ecology.

B. Goal of the rest of the course: We will wrap up the course by considering community-level patterns. First let's take a moment to reflect on what we have covered so far, so that we can use this section to pull many concepts together. What we have emphasized in this course is evolutionary ecology and population-based ecology. Those two viewpoints are highly related, as the deme is the setting for natural selection. To unify the course, the theme of this last unit is will be the topic of biological diversity : We want to ask " what determines the number of species found in one location? " which is a rephrasing of how we defined the subject of ecology at the beginning of the class " what determines the distribution and abundance of organisms? " We can also call this "biodiversity" or "species diversity". Conservation Biology is a new field of study which is developing as an applied area that combines ecology and population genetics. Many students want to pursue this new field because of their desire to help stem the growing "biodiversity crisis". In other words, the activities of humans are causing the extinction of other species on a scale not seen since the giant bolide collided with the Earth 65 million years ago. Any viable applied field and its practitioners require a solid founding in the principles of the basic sciences most closely allied to this applied field. For the rest of the course, we will investigate the ecological determinants of biodiversity as a function of spatial and temporal variation. We have gotten many of the building blocks of to address the question of biodiversity already. In this section we will cover some more concepts and tie things together. II. What determines biological (=species) diversity? Because this is an ecology course, we will primarily emphasize the ecological aspects of biodiversity. A. Spatial patterns: local, regional and global diversity

1. Exactly what do we mean by biological=species diversity? ( We will restrict our view of biological diversity to mean species diversity and not genetic diversity). Species diversity is: (Molles 16.2) - 1. the number of species (species richness) and - 2. the relative abundance of those species at one time and one place. (species evenness)

There are 4 categories of species diversity that each depend on spatial scale:

• α - diversity (alpha diversity) is strictly species diversity within one habitat --"local"

• β -diversity (beta diversity) , is species diversity in one place with more than one

habitat type (sum of α-diversity in a place that is spatially heterogeneous).

• γ - diversity (gamma diversity) is "regional" species diversity

• global diversity -- all of the species on the planet "Spatial scale" is somewhat arbitrary here--what's “one habitat” and what’s a “region”? The best way to view these definitions is as working definition s. In local, or alpha-, diversity , we are considering populations in a location that is spatially fairly homogenous , with population densities and interactions being pretty much the same in the entire are. In beta-diversity , we are conceding that populations occur over spatial scales in which the environment varies and so do the other populations that a focal population can interact with. In gamma-diversity , we are talking about something less well defined, but in general we can think of it as a group of populations unified over a larger spatial scale by something ecologically important, for example, a common biogeographic history or a common biome type. By definition a region is somehow spatially contiguous but arbitrarily large. A region might be a watershed, a mountain range, an island archipelago, a small continent, or part of a large continent. In community ecology especially, we enter a realm of terms that are potentially "buzz words," that is, words for which the definition takes on a life of its own. We will want to keep grounded solidly in our pursuit of understanding of ecological processes. In defining words, we want to focus on a set of ecologically interesting processes. With this view, we can define a level of species diversity according to what kinds of processes occur on each spatial scale that we want to understand. Finally we can talk about global diversity , or all of the species on the planet. In both gamma- diversity and global diversity, there is a significant evolutionary component to the determinants of species diversity, whereas only ecological factors are considered to determine local diversity (alpha- and beta-diversity). Thus regions are large enough that speciation has occurred in the region and can be considered a source of species diversity. Indeed, speciation is the original source of species diversity. 2. What determines global and gamma diversity? (a mix of evolutionary and ecological processes)
• Global and gamma diversity could be determined by a macroevolutionary process, for example: global and γ-diversity = [speciation minus global extinction] or the available "pool" of species at any one time. However, these levels of diversity are NOT independent of all the local events and processes that determine α-diversity; that is, it depends very much on local events: (e.g., Molles Ch. 22.2, "Differences in speciation

and extinction rates".

• Gamma diversity and global diversity also result from adding up many, many local processes over the entire region or planet. For the rest of this course we are going to focus on these local processes.

3. What determines local, α and β -diversity? (primarily ecological processes)

• [invasion minus local extinction] which gives you the total number of species in one location
• But we also want to know what determines exact population sizes to understand what determines the relative abundance of species (i.e., species diversity ). We need to look at the ecological processes at work that determine which invasions succeed and when species will persist or go extinct and finally what determines the size of any given population. Summary: Local diversity is the number of species weighted by population density (relative abundance or evenness of densities of all species combined. If you go on in ecology, you will learn about a number of species-diversity indices.) Obviously, if a species is very rare in a local area, it is more likely to go extinct. Conversely, a species can be so abundant that its impact dominate the characteristics of the community at that site. So we have to consider not only presence-absence of species but also the density of individuals representing that species. 4. Patterns and determinants of local species diversity: latitudinal gradients [Molles Ch. 22.3] We can deduce many of these from the latitudinal gradients in species diversity : A conspicuous pattern is the change of species diversity with latitude (e.g., Figs. 22.15-17). There is a gradient in which species diversity is highest in the tropics (astounding numbers of species) and species diversity gradually gets less as you go to higher latitudes, and is lowest at the poles. You can see the gradient even within segments of narrower ranges of latitudes, such as lizards, or breeding birds of North America, or United States etc. For example, one hectare in Peru has over 600 breeding bird species, nearly as many as in all of North America. This incredible diversity applies also to insects, many times over. No one knows or will ever know the full diversity of arthropods in the tropics, because of the tragedy of deforestation at these latitudes. Whatever causes the latitudinal gradient of species diversity should give us a clue as to the determinants of species diversity at any given location. We know the things that vary with latitude, so we can start there. Unfortunately, this gives us a lot of non-mutually exclusive-- competing hypotheses--but that is the way ecology is. Patterns at the community level are obviously determined by several to many interacting factors. I have arranged the factors determining local diversity in order of building complexity, starting with mechanisms that "stand

alone" and going to mechanisms that depend on previous ones: (Molles Ch. 22.3). 1-2. Obviously, the time scale is important: Mechanisms (i.e., how does this factor determine species diversity?) are:

• 1. Evolutionary: With enough time, species can evolve in situ. Some say that tropics have been undisturbed by extinctions and major environmental disasters (e.g., as caused by ice ages) for longest time. But this is controversial: the tropics were indeed affected strongly by the ice ages, even though there were no glaciers there.
• 2. Ecological: have population dynamics had enough time to go to equilibrium, i.e.. will we see more extinctions or invasions in the future? In the tropics, where there are relatively fewer disturbances (see below), populations have had enough time to go to equilibrium--whatever that is. There are two types of situations to consider:
     • If populations are vulnerable, for example if they are small or if they have a strong Allee effect, then such populations are less likely to go extinct where there are fewer disturbances. These populations could contribute to higher species diversity in the tropics, if we assume that there are fewer disturbances there.
     • If the outcome of a population interaction (competition or predation, in particular) is extinction, then disturbances can prevent this outcome. For such populations, species diversity will be higher where there is more disturbance (see below). 3. Climatic "stability " By climatic stability, we generally mean the constancy and predictability of climate and weather, which is higher on average in the tropics. Changes in weather can disturb populations. Also, we think of " benign " climates such as found in the tropical latitudes as being less disturbing to populations. Mechanism:
• Presumably in areas with more constant and predictable climates, there are fewer perturbations and fewer disturbances affecting populations, meaning that populations are less likely to go locally extinct.
• Also, specialists can be supported, because of predictability of their narrower range of resources; conversely, if a population cannot count on a narrow or special array of resources, individuals in the population should be more flexible in their use of resources (food etc.)--such individuals would be " generalists ". But, the major climatic features of the tropics leads to other major factors.

4. Productivity. We might combine this with the energy hypothesis. There is higher productivity in the tropics in terms of fixing more energy by primary "producers" (plants), because of higher solar input and higher humidity that favors greater rates of potential evapotranspiration, which favors higher rates of photosynthesis. Mechanism:

• With more total resources, more total individuals possible, to be divided among more species; rarer species then can be supported.
• Also, with more resources around, specialization is favored (for example, in optimal foraging theory), so this factor and 3) climatic constancy both favor specialists. Your book, page 517-8, lists apparent contradictions to this hypothesis, but we need to keep in mind an ‘all else equal’ perspective. Remember that these hypotheses are NOT mutually exclusive, and areas such as the tropical latitudes have more factors favoring higher diversity, for example. Higher productivity and climatic stability leads eventually to: 5. " Habitat complexity " Plants are 'habitat' for animals. Thus factors that favor plants produce environmental complexity for animals. This is where animal ecologists start: features of the physical environment and plant community provide resources for animal populations. Mechanism: The more "resources" i.e., more resource "types " [(as opposed to amounts) be they food, food species, nest sites, shelters from microclimate or predators: i.e., from 1. absolute amount of a resource (productivity) AND 2. variety of resources,] the more populations and larger populations are able exist in a location. The more resources available, the higher the species diversity. [e.g., Molles Fig. 16.9) Bird species diversity and foliage height diversity] The more resources vary, the more species can partition resources (again favoring specialists) and in particular avoid competitive exclusion. 6. Spatial heterogeneity If the physical environment is more variable, then we might except more species. This is in part a companion to the above, habitat complexity, but it is more general. Species are adapted to local physical environments, but because there are more of them in a

given area, then there are more species in a given area. This mechanism is actually the "β-

diversity" we discussed above--a somewhat larger spatial scale relative to the amount of spatial variability. It is not clear whether the tropics should be greater in spatial heterogeneity compared to other latitudes. The specific mechanisms could include the accumulation of species with differing ecophysiological adaptations and (as above) the ability to avoid interspecific competition.

7. Competition (p. 518 under "niche breadths and interspecific interactions, see also ch. 16.3) Competition as an interspecific interaction can cause extinction and lower fitness. The ways in which it influences species diversity depends very much on the time scale :

Mechanism:

• A. In evolutionary time , competition can increase the number of species by favoring specialists through ecological character displacement (ECD). While competition does not result directly in speciation, ECD can reduce extinction, i.e., competitive exclusion. And, it is possible that specialization could favor speciation, by predisposing populations to diverge in sympatry and thus differ from the same species somewhere else. For example, in the Darwin's finches, we saw populations of the same species evolving different bill sizes on different islands, depending on what other species they shared a given island with. Such differences might eventually lead to speciation.
• B. In ecological time, competition can decrease the number of species through competitive exclusion. Competition puts an upper limit on the number of species that can live in one location. If resources are limiting, then species can competitively exclude other species. The limit to similarity means that niches are "incompressible" beyond a certain point. So there is an upper limit to the number of competing species that can coexist on a given amount or variety of resources. We have already discussed factors (above) that can interact with competition as mechanisms for determining species diversity. For example, in the tropics, with benign climate, high productivity and high habitat complexitiy, specialists can be supported. While competition may be intense, especially high in food webs, specialists can do well and so resource partitioning is favored and competitive exclusion is decreased. 8. Predation Predation also can affect species diversity in two ways: Mechanism:
• intense predation can lead to extinction of prey species. We expect this type of extinction more in simple food webs and in settings were not much evolutionary and ecological time has gone by (i.e., new predators on a particular prey). Thus in some settings, predation can decrease species diversity. Because the tropics are more complex, we might expect there to be fewer extinctions based on this mechanism.
• predation on top of competition can prevent competitive exclusion. (chs 16.4; 17.2): intermediate disturbance hypothesis). We saw how predators can hold prey below their carrying capacity. When prey species are potential competitors (which should happen relatively often), a predator or set of predators will hold them all below their carrying capacities. That is, predators, not interspecific competition, regulates the prey populations; recall how we defined "regulation" of prey by predators. What does this mean for species diversity? If competition leads to competitive exclusion when species are too similar, then predators that keep species from competing permit more and more similar species to coexist.

Summary: Predation on communities of potentially competing prey increases species diversity, by reducing competition and reducing the incidence of competitive exclusion. In the tropics, predation is often intense also, depending on the trophic level. This principle has been termed the " KEYSTONE PREDATOR " effect. Keystone predators are seen most often in space-limited communities, which tend to have competitive hierarchies (superior vs. inferior competitors) more do than food-limited communities. (Food limited communities tend to avoid competition by resource partitioning, i.e., constriction of the fundamental niche to the realized niche). The term "keystone" implies that a single species has particularly strong effects on all of the others. Is there a keystone species in the kelp food web? Examples include:

• • Pisaster the predatory starfish eats a competitive hierarchy of species competing for space in the rocky intertidal. The hierarchy from superior to inferior competitors goes like this: mussels → barnacle species 1,2,3 → algae in the rocky intertidal. The starfish prefers the mussels as prey, so the starfish opens up space for the other, less able competitors, to occupy. (Molles: Fig. 17.7-9).
• -Darwin’s lawnmower. (generalist grazing “herbivore”) Darwin noticed that when he mowed his lawn regularly, he could find more species of plants in it. Again, we are talking about species competing for space. Darwin didn't follow up on his experiment, so we don't know if there was a competitive hierarchy, but that is likely based on what is known of other pasture lands.
• -- Grazing herbivores may be similar to a lawnmower but unlike a lawnmower, some herbivores are specialists: they are likely to select species rather than graze all species equally. In some settings, i.e., on grasslands that coevolved with grazing herbivores, some studies show that grazing herbivores increase species diversity. However, in others, species diversity decreases in the face of grazing. Reasons for this contraction may include 1) these examples are humans grazing rangeland with domestic livestock which did not coevolve with plant species in those settings and 2) humans over grazing rangelands, in which case a similar principle to (below) the "intermediate disturbance hypothesis" may apply: extreme disturbance (see below) or extreme grazing pressure necessarily lowers species diversity by driving some species to extinction. (Molles: Fig. 16.20, p. 386; Fig 17. 9, p. 401)
• --. Herbivorous insects on trees (leaf hoppers, histidine beetles). (work of Professor Strong and others) This is an example that does not involve competition for space. The hypothesis is that so many species of insect species, all of which depend on eating leaves of any given tree species, can coexist. Most astonishing is that one species of tree may have a large number of herbivorous insects that specialize on that species, to the exclusion of all other species of trees. There is little partitioning of resources within that

community. Thus, if competition for resources is occurring, how these species coexist would be a mystery. The alternative (and favored) hypothesis is that the populations of herbivorous insects are regulated by predators, mostly the specialized insect 'parasitoids'. These ideas are part of the famous hypothesis of Hairston, Smith and Slobodkin (HSS) who asked "why is the world green", on the observation that not all plants are eaten up by herbivores. The idea is that the determinant of species diversity within a trophic level alternates between trophic levels: plants are controlled by competition , because their herbivores are controlled by predators , so that these predators are controlled by competition , etc. 9. Disturbance on top of competition. In space-limited communities, disturbance acts like a predator. It can literally knock individuals out of space and clear space for other individuals. When disturbance is indiscriminate (like Darwin's lawnmower), it necessarily knocks out more of the superior competitors. Imagine a log floating in the tide and smashing against the rocks with each wave (this is what happens). This disturbance is known to free up space and to increase diversity of sessile species attaching on rocks in the intertidal zone (Figs. 16.17-18; pp. 384-5). In some ways, Darwin's lawnmower might be more like a log in the intertidal, and indeed the mechanism is so similar that ecologists often do not discriminate between disturbance caused by a predator and disturbance caused by a non-living factor in space limited communities. (In communities limited by food, predation might have quite difference effects than non-living disturbances). Fire is another major type of disturbance capable of increasing species diversity in space limited communities. This type of phenomenon led to the proposal of the: INTERMEDIATE "DISTURBANCE" HYPOTHESIS (Fig. 16.18). This is not really an hypothesis anymore: This pattern has been well demonstrated in some systems: includes both disturbance and predation: The intermediate disturbance hypothesis:

• 1. has been demonstrated in the rocky marine intertidal numerous times with both predatory starfish and snails and disturbance caused by floating logs (Professor Quinn in our Department of Environmental Science and Policy); and

• 2. is the leading hypothesis for the astounding tropical tree-species diversity. Shallow roots and frequent treefalls are the source of disturbance. There is also evidence that seed predation acts as a source of “disturbance” in this space-limited community. Summary: Spatial variation in biodiversity We have considered in this section, the determinants of local diversity, i.e., in one relatively limited location in space. To propose hypotheses, we have examined spatial variation in biodiversity, to identify characteristics of one location with more species compared to another location with fewer species. We have not considered any temporal variation in biodiversity or other characteristics of communities, but rather we've considered either the equilibrial state of the community at that location or at least the average number of species that occur in that location over time. Next we will consider the temporal variation in biodiversity and community structure at one location, the topic of succession