Historical patterns of biodiversity

• Geological (1 millon to 4.5 billion years)
• Evolutionary (100k to 1m years)
• Glacial-interglacial (10k to 100k)
• Species range changes (100 to 1k years, but sometimes decades and shorter)
• Secondary succession (on previously vegetated terrain)
• Primary succession (on newly exposed terrain)
• Response to short-term climatic fluctutations e.g. global warming
• Annual changes
• Seasonal (phenological) changes i.e. the timings of recurring natural phenomena e.g. bud burst, usually in response to seasonal weather

Geological time and evolution of life:

•  Age of Earth = 4.5 billion years
• Life on Earth first evolved around 3.5 billion years ago
• Complex (multi-celled) organisms evolved around 540 million years ago during - The Phanerozoic Eon (the Cenozoic is part of this)
• During the Cambrian there was a 'burst' in diversification

• Source: Elias (2010)
• Ordovican period (510My) - first primitive land plants
• Silurian (445My) - first vascular plants and arthropods on land
• Devonian (410My) - first trees and forests, first land vertebrates
• Carboniferous (360My) - tree ferns increase, ancestors of birds, mammals and reptiles
• Permian (286My) - mass extinction event, rise of gymnosperms (gave rise to the Mesozoic, dominated by dinosaurs)
• Triassic (245My) - rise of modern conifers and cycads, first dinosaurs
• Jurassic (208My). - dinosaur. diversity increases, first winged reptiles and. first winged birds
• Cretaceous (146My) - rise of modern insect orders, flowering plants and mammals, extinction event ends age of dinosaurs (although not all dinosaurs are extinct as studies show that birds are dinosaurs
• Tertiary (65My) - increasing diversity of flowering plants and mammals
• Quaternary (2My) - glaciations, extinction events from c. 50,00 years BP

• Some 90% ofspecies that have ever existed have become extinct
• But overall, a pattern of diversity increase is apparent through time
• It is not continuous - there have been periods of rapid speciation and periods of stabilisation
• Punctuated equilibrium - relatively little extinction and speciation. The equilibrium. is punctuated by extinction events which have a significant impact on background record of extinctions
• At any one time, the bulk of biodiversity is contributed by only a relatively few taxonomic groups, most groups  not being especially diverse
• Extinction isn't all bad - without it, we wouldn't have speciation as it provides new space, ecological release and allows new species to evolve
• Scientists have created an animal thatwas possibly our ancestor using more than 4500 phenotypic characteristics e.g. comparative anatomy of skeletalstructure, fur, diet, genetic information. They integrated this data from both fossil and living specimens to come up with the hypothetical placental ancestor, weighing between 6 and 245g and being insectivorous.
  • Today, we have around 5,100 different living placental mammals, showing huge diversity of size and a variety of locomotor diversity.

• All invertebrates - 11My
• Cenozoic mammals - 1-2My
• Marine invertebrates - 5-10My
• Diatoms - 8My
• Marine animals - 4My
• Dinoflagellates - 13My
• All fossil groups - 0.5-5My
• Planktonic foraminifera - 7My
• Mammals - 1My
• Cenozoic bivalves - 10My
• The smaller you are, the longer the lifespan of the species. More complex species tend to survive for less time.

• Tectonic events caused changes in the climate which made an impact on biomes
• Notably...
  • 1st legumes - c. 70My
  • 1st grasses - c. 55My
  • 1st compositae - c. 22My
  • 1st C4 plants - c. 15My

• In pre-human times there would have been a complete animal community comprised of multiple levels of both food limited and predator limited species
• Until the last 50,000 years, megafauna (animals weighing more than 1000kg) have been abundant. in all landscapes on Earth, including survival through glacial and interglacial transitions.
  • These have now been lost, except in Africa (4/7 species remaining), Europe (4/12) and Asia (4/13)
  • There is a theory that many of these extinctions are associated with hunting by humans, or by climate
  • Homo sapiens are effective and generalist omnivorous super-predators that hunted animals previously facing little predation pressures
  • We used domesticated dogs which competed with endemic carnivores for food (dogs were domesticated around 15,000 years ago, now a lot more animals are domesticated such as cattle, sheep, goats and pigs which spread diseases to wild animals)

• Triggered cascades of extinction through vegetation changes, changing fire regimes and loss of prey for megacarnivores
• Analogies from modern ecosystems such as wolves in Yellowstone National Park help us to understand these impacts.
• Megafaunal herbivores' large size makes them effective against predators. They can achieve high population biomass meaning they can alter vegetation at a landscape scale.
• They destroy woody vegetation and consume large amounts of foliage.
• Their population numbers are therefore limited by food availability (bottom-up), but they also exert a strong top-down control on vegetation structure and composition.
• They also regulate the abundance of smaller herbivores (45-999kg) either by direct consumption or inducing behavioural changes
• Carnivores (greater than 100kg) are also limited by food supply
• Carnivores smaller than 21.5kg generally feed on small prey

e.g. megafauna such as elephants in Kruger Park in South Africa reduce 15-95% of woody species in the parl, having negative effects on juvenile plants but also indirect positive effects on woody wegetation through enhanced nutrient cycling, diminished competition with herbaceous vegetation and associated reduced rodent densities and fire frequency.

OVERALL IMPACT - impact the abundance and behaviour of grazing animals, creating simpler ecosystems and shorter food chains

• Biogeochemical cycling of nutrients occurs via leaf consumption, digestion, defecation, urination. Large animals today such as elephants play a key role in this.
• Therefore, post megafaunal extinctions might lead to nutrients being locked into slowly decomposing plant matter, making ecosystems more nutrient poor.

• Several megafauna were seed eaters responsible for dispersing seeds via the gut, facilitating long distance seed dispersal and gene flow
• Therefore, it's likely that changes in the ecosystem biomass and carbon stocks after extinction, and less trampling of vegetation

• Theory of human driven extinctions is known as "Pleistocene overkill"
• Hypothesis first proposed by Martin & Wright (1967), still the subject of debate e.g. Faith et al (2018)
  • Noted that there has been a long term steady decline in megaherbivores beginning 4-5 million years ago, long before the emergence of human species capable of exerting top down control on large mammals and pre dating their interactions with megaherbivore prey
  • An alternative explanation is declining CO2 and expansion of C4 dominated grasslands

• Increasing due to human activity e.g. hunting, habitat change/fragmentation
• Also deliberate/accidental introductions of invasive species, some of which become destructive pests or 'naturalised'
• Risk is higher on islands as endemics can evolve in absence of competition. Populations here are also generally small and spatially restricted, so there is a disproportionate number of extinctions