Species diversity

What determines: 1) Species distributions? 2) Species diversity? Patterns and processes At least 120 different (overlapping) hypotheses explaining species richness ... We are going to think about general principles and the most obvious hypotheses

Hierarchy ... and some terms

• Species – species richness, species diversity

• Populations • Communities

– ”community assembly”

• Ecosystems and biomes

• Environmental variation ... ”Habitat”

• What explains the distribution of plant species (and thus the local species richness) in Skåne?

• Why are there no lions in Skåne?

Floran i Skåne Tyler et al. (2007)

”Drivers” of diversity

■ What controls species distributions and therefore levels of species diversity??

Species diversity

Ecology and/or chance?

Scale

• The same general processes operate on all spatial and temporal scales

Adaptation?

Habitat and adaptation ”Adaptation” Process of evolution by natural selection

• Basic physiological and morphological

characteristics of a species – determine whether a species can grow in a particular

place

• Large scale patterns – often determined by physiological constraints. e.g.

climate

• Fine scale patterns – Determined by e.g. local variation in soil properties

Adaptation

• Natural selection

• Operates on individuals

• Better relative survival and reproductive fitness

... depends on available genetic variation

... large scale adaptation within species

... speciation

Modern natural distribution of Fagus sylvatica according to Atlas Florae Europaeae (Jalas & Suominen, 1972–99), bold black line; and areas where F. sylvatica would be the dominant tree under natural conditions (Bohn et al., 2000), shaded areas.

Large scale distribution (European beech)

Large scale patterns often determined by physiological constraints (e.g. climate)

Fine scale patterns; often determined by e.g. local variation in soil properties

Fine scale distribution (mine tailings with very high levels of lead and zink)

Why may there be problems with predicting future distributions from present distributions and climatic limits?

Why may there be problems with predicting future distributions from present distributions and climatic limits? • Other factors are involved

– Purely spatial processes (”false correlations”)

– Historical processes (present distribution may not be in equilibrium with present-day climate)

• The distributions and species diversity that we observe at the present day are a complex reflection of processes that operate/have operated on different spatial and temporal scales

• Adaptation and ecology

• Non-adaptive processes: history, chance

[The impacts of these two types of driver will be a central thread in my lectures and in the exercise at the end of the biodiversity theme]

History

• Immigration patterns

• Barriers

• Chance

[we have already discussed how ancient chance events may still

be evident in patterns of present day genetic variation within species...]

(Dispersal)

Horse chestnut

Cameraria ohridella

Big seeds!

Dispersal

Species pool

• Regional species pool

• Species within a region (scale!)

– The species in the pool are a subset of the species that could potentially exist under the particular climatic and environmental conditions in that particular region

– Filtered by historical processes: migration, dispersal, previous regional processes

Habitat heterogeneity

Adaptation

Different environmental conditions within a particular area (applies at different scales)

Land-cover heterogeneity

Habitat heterogeneity

Scale!

Fig. 1. The multivariate and interacting nature of farming practices and some of the routes by which farming practice impacts on farmland birds. Arrows indicate known routes by which farming practices indirectly or directly affect farmland bird demography and therefore local population dynamics and finally total population size.The goal of manipulating farming practice is to impact on population size. Rather than identifying key routes through this web to change in a piece-wise fashion (e.g. insecticide usage), we suggest that management designed to increase habitat heterogeneity is likely to benefit the organisms in such a way as to meet the management goals

Benton, Vickery & Wilson (2003) Trends Ecol Evol

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Competition

Competition

• “Complete competitors cannot coexist"

• Competition for light (shading): above ground

• Competition for nutrients: below ground

• Competition can lead to exclusion of most species OR to the coexistence of many species

Competitive exclusion

”complete competitors cannot coexist”

Fertile (= high levels of nutrients) and moist conditions mean that vegetation grows vigorously and there is high biomass (productivity is high)

There is competition for light and no room for non-vigorous species

”Niche”

• Species’ ecological situation

• Abiotic variables (e.g. soil moisture or nutrients)

• Biotic interactions (e.g. competition or interactions with

predators, mycorrhizal fungi etc. etc.)

• [gaps for establishment ... ”regeneration niche”]

Niche separation • “Complete competitors cannot coexist"

• Low-nutrient and dry conditions are stressful

• Competetion for nutrients leads to ...

• Co-existence of functionally different species through niche separation: different ways of acquiring the scarce nutrients (functional diversity)

• Highly diverse systems are often nutrient-poor

[In the short term, sorting of existing species and, in the long term ”character displacement” and speciation]

[Fundamental and realized niches]

Functional diversity What kind of things do organisms do within a community

or ecosystem? ”Traits” (functional or otherwise) = particular

characteristics of an organism (influence a species’ fundamental niche)

§ Rooting depth, ways of acquiring nutrients, leaf area, plant height, leaf thickness, how seeds are dispersed etc. etc.

■ adaptation to different environments

■ competition ■ dispersal

Communities can be characterized in terms of the diversity of functional traits (”functional diversity”)

Functional differentiation Functional diversity

Intermediate disturbance hypothesis Highest diversity at ”intermediate” levels of disturbance ”Level” can refer to the intensity, frequency or size of the

disturbance ... or the time since disturbance Mixture of different processes (dispersal and competition) e.g. Gentle (”intermediate”) disturbance keeps fine scale habitat

heterogeneity (microhabitat diversity) e.g. Open ground. Initial colonization by a few species more

species arrive and the first ones hang on the environment changes as succession proceeds (soil changes that may be influenced by the plants themselves). the vegetation becomes closed and there are less gaps for establishment

e.g. Productivity: (disturbed) low productivity intermediate high productivity (competitive exclusion)

Connell 1978: Science (Connell 1978)

• Intuitively sensible hypothesis, but can be explained, or viewed in a range of different ways

• Has been a lot of recent controversy about this hypothesis

• Different ways of looking at ecology: ”empirical” (based on data and observations) and ”logical” (based on theories about how nature should work)

• Interplay between productivity and disturbance

Huston 2014 Ecology

Species pool

• If we have a set of species within a region or local area, what determines whether they will occur in a particular patch of apparently suitable habitat?

• Dispersal gives access to the habitat • History of loss plus fragmentation coupled with a poor

dispersal ability that stops recolonization...

• Real habitat specialists ... Habitat may not actually be suitable

• Local extinction ... lose species from the regional or local pool

Regional Species Pool

Zobel 1997 Trends Ecol. Evol.

Landscape and habitat fragmentation

• ”Landscape modification and habitat fragmentation are key drivers of global species loss” (Fischer & Lindenmayer 2007: Global Ecol. Biogeogr.)

• Connectivity, patch size, edge effects, habitat heterogeneity, local extinction

Fragmentation

Patches of habitat are large and close to each other: they support large populations Patches become smaller and more sharply separated: they support smaller populations The distance between fragments increases: the smallest patches can no longer support populations: dispersal and gene flow are limited

Edge effects

• Edge effects: fertilizer and nutrients spread in from adjacent habitats

• Unexpected effects of fragmentation on species diversity?

Edge effects

• Edge effects: fertilizer and nutrients spread in from adjacent habitats

• Unexpected effects of fragmentation ...

• High diversity in small fragments as a result of edge effects

• What ”kind” of diversity are we interested in?

• Habitat specialists

Spatial structure in a fragmented landscape

• Fragment size

– Habitat heterogeneity

– Probability of colonization

– Reduced edge effects (habitat quality)

• Connectivity

– Dispersal

– Realized connectivity

• Surrounding habitat matrix

– Local species pool

Fragmentation and stochastic processes

• As the area of habitat become smaller and fragments of habitat become more and isolated

• Edge effects may reduce the suitability of the habitat

• Species will be lost (increasingly randomly) from small habitat patches

Extinction

• Local extinction (loss of a species from a habitat fragment)

• Then regional extinction • Gradual process: not just all individuals that die

suddenly • Fragmentated habitats: what influences the

probability of local extinction? – dispersal, connectivity, landscape context

• Loss of genetic variation contributes to the problem of small populations. Genetic diversity is important!

Extinction debt

• Habitat fragmentation ... fragments were once part of a larger, more connected area of habitat

• ”Inertia” .. the properties of the communities in the fragments still reflect earlier times

• What will happen to species richness if there is no further habitat fragmentation in the future?

Grasslands

• ”Truncated succession” • Grazing • Low levels of nutrients • Intermediate disturbance • Fine-scale habitat heterogeneity ... gaps (”regeneration

niche”) • Fine scale niche-differentiation (competition for nutrients)

[habitat fragmentation and history]

• Grassland specialists and habitat generalists

Dark diversity

• Regional species pool contains many species that are ecologically suited to a particular habitat ...

• But samples from plant communities typically innclude only a proportion of the species that could be there: the missing species = the ”Dark diversity”

– Functional traits that explain the dark diversity?

Neutral model

• Everything depends on neutral (not influenced by ecological processes) processes such as dispersal and species’ physiological tolerances. Competition unimportant

• The model depends on the assumption that dispersal is neutral and that dispersal traits are non-adaptive (i.e. not subject to natural selection)

Dispersal

But neutral (i.e. non-adaptive and random) processes are contributing to patterns of distribution and diversity on all spatial and temporal scales. Examples Large scale: immigration history and chance Very fine scale: the processes involved in fine-scale establishment in grassland. Small gaps and the probability of a seed from a particular species finding a gap.

The carousel model

Van der Maarel, & Sykes 1993. Small-scale plant species turnover in a limestone grassland: the carousel model and some comments on the niche concept. Journal of Vegetation Science 4: 179-188.

5-year study, grasslands on Öland “Species mobility on the scale of the small [0.001 m2 and 0.25 m2]quadrats used implies both appearance and disappearance [of species] from these quadrats.” "We postulate that in homogeneous, grazed, nutrient- and water-deficient environments many species can reach virtually all microsites within the plot, which we express through the idea of the ‘carousel model’. We also question the usefulness of the niche concept and re-interpret it by stating that all species of this plant community have the same habitat niche, while most of them are short-lived and have the same regeneration niche. The essential variation amongst the species is their individual ability to establish or re-establish by making use of favourable conditions appearing in microsites in an unknown, complex spatio-temporal pattern.

Characterizing species diversity

• Alpha and Beta diversity

• Within and between ”samples” or habitats

• Alpha diversity = ”Species diversity”

• Beta diversity = ”Species turnover”

[”Quadrats” = samples]

■ Range expansion; immigration history (a lot of chance events (non-adaptive processes))

■ Basic ecological amplitude: can the species grow in a region? Abiotic environmental factors (adaptive variation)

■ ■ Get the species into the regional species pool (chance + general ecology and availability of suitable habitats)

■ Don’t lose species as a result of habitat fragmentation (stochastic processes)

■ Habitat heterogeneity

■ Biotic interactions (competition)

■ All this goes on on different spatiotemporal scales!

Farmland biodiversity: is habitat heterogeneity the key? Benton, Vickery & Wilson 2003 TRENDS in Ecology and Evolution: 18

Agricultural intensification & Habitat heterogeneity

Land Mosaics R.T.T. Forman (in the library)

General text book

Dark diversity: shedding light on absent species Pärtel, Szava-Kovats & Zobel 2011 TRENDS in Ecology and Evolution: 26

”Dark Diversity”